Space Antennas including Terahertz Antennas€¦ · In particular, horn antennae, either corrugated as spline proﬁles for data downlink and uplink communications and TT&C applications

Space Antennas including Terahertz Antennas

R. Gonzalo*, I. Ederra, J. C. Iriarte and J. TenienteElectrical and Electronic Engineering Department, Public University of Navarra, Pamplona, Navarra, Spain

Abstract

This chapter deals with the analysis of several kinds of space antennae with a special section devoted toTHz antennae. In particular, horn antennae, either corrugated as spline profiles for data downlink anduplink communications and TT&C applications are developed. More innovative antenna designs basedon the use of Electromagnetic BandGap (EBG) or Metamaterial structures (MTM) are included. Theseones exhibit very promising properties to be used in applications such as TT&C or Navigation. Finally,due to the increasing interest in scientific missions operating at THz frequencies, a section including thelast results of using MTM technologies for implementing antennae at THz bands for imaging spaceapplications is presented.

Keywords

Horn antennae; Corrugated antennae; TT&C; Space antennae; Electromagnetic band gap technology(EBG); Metamaterials; Terahertz antennae

Introduction

Following the IEEE standard definitions, an antenna is that part of a transmitting or receiving system,which is designed to radiate or to receive electromagnetic waves. In satellite communications, the use ofantennae is fundamental in order to keep the satellite in operation or to be able to run the differentapplications such as TV broadcasting, data transmission, etc.

Each satellite needs different kind of antennae, and their requirements are very dependent on theapplications: scientific, commercial, civil, etc. In general, the antennae on board of a satellite platform canbe divided into three groups: antennas for telemetry, tracking, and control (TT&C) which control thesatellite operation, antennae for high-capacity transmission, which are mainly in charge of the commu-nications, and finally antennae for the different instruments mounted in the satellite which are fully relatedwith the mission.

For each of the aforementioned groups, there are different antenna technologies, which can be appliedin order to comply with the expected performances of the antenna. In the literature, it is possible to findfrom planar antennae to horn antennae, including arrays or reflectarrays. Anyway, this chapter will focuson two kinds of antenna developments. The first one will be horn antennae, either based on smooth orcorrugated profiles. The second case will be devoted to more innovative antennae, which will followMetamaterial concepts for their implementation.

Horn antennae are well known for many years. They exhibit very good radiation performances whichmake them very suitable to cover most of the needs for the radiation patterns in a satellite. They presentvery good pattern symmetry, very low cross-polar levels, very low sidelobe levels, and a wide bandwidth

*Email: [email protected]

Handbook of Antenna TechnologiesDOI 10.1007/978-981-4560-75-7_123-2# Springer Science+Business Media Singapore 2015

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response. Even in the last years, new profiles have appeared making them very competitive versus otherkind of antennae by reducing their overall size (length). This chapter will mainly present the performancesof a new kind of corrugated horn antennae, named Gaussian Profile Horn Antennae (GPHA), which havebeen used in the last years to comply with the stringent requirements of the radiation performances ofmany different space missions or communication systems. The performances of different GPHA antennaefor TT&C, high-capacity, or space instruments will be included.

On the other hand, Metamaterial-based antennae have been recently appeared as a new approach in thedevelopment of antennae. In particular and focusing on the space sector, there are different proposalswhich make use of Metamaterial concepts for improving antenna performances. This chapter will dealwith the use of metasurfaces in satellite antennae for TT&C and GPS applications. This innovativeconcept is an alternative to conventional technologies which can be applied in those applications forimproving overall performances, including mass and cost aspects.

A special section on THz space antennae is included. This one will mainly be centered in the progress ofMTM antennae operating in this frequency range for space missions.

Data Downlink and Uplink Feed Horn Antennae for SpaceborneCommunications

IntroductionData feed horn antennae on today’s communication satellite systems are of very different types. It is notpossible to cover all feed horn antenna solutions for all types of spaceborne communications missions, butthe following section provides an overview of the most common designs employed nowadays forgeostationary communication satellites.

Not much information can be found from actual satellite programs since trade secrets in this highlycompetitive commercial field protects new developments from being disclosed. However, a lot of usefulinformation from author’s experience has been included, supported with several examples and highlight-ing the main constraints for each antenna type.

Spaceborne communications requirements for data feed horn antennae are always in evolution sincenext generation systems require higher capacity and flexibility complicating the feed horn design.

Geostationary satellites offer Fixed Satellite Services (FSS) and Broadcasting Satellite Services (BSS)that are often used for broadcasting to and from television networks and local stations. They can also beused for video conference, broadband internet, and general commercial telecommunications. The mostcommon frequencies employed in actual commercial geostationary satellite communications are the Kuband from 12 to 18 GHz, and in K and Ka bands together (often referred simply as Ka band) from 18 to40 GHz. Typically, Ku band uses the frequencies from 10.7 to 12.7 GHz for the downlink (Tx from thesatellite) and 12.8–14.5 GHz for the uplink (Rx from the satellite’s view). Note that in some cases, theuplink moves to the 17.3–18.2 GHz range. On the other hand, Ka band usually employs for the downlink(Tx) the 18.3–20.2 GHz range and the 27.2–31.2 GHz range for the uplink (Rx). It is common forFSS/BSS satellites to carry a multitude of feed horn antennae, which are operating at different frequencybands.

Metallic feed horns are the preferred feed elements for most spaceborne communications, because theirsize is reasonably small and because they have great power handling capability. Additionally, horn designtechniques have evolved to the point where very good polarization purity, pattern symmetry, gainperformance, reflector aperture illumination control, and frequency bandwidth are readily achievablethrough the synthesis of the internal horn wall profile. Several horn types are widely used for this type ofantennae, but the most widely used nowadays are high-performance corrugated horns and high-


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performance smooth-walled spline profile feed horns. Advanced synthesis techniques of rather complexhorn wall profiles are able to optimize performance at a number of selected frequencies and thereforesignificantly widen the frequency band or achieve a very good dual-band performance covering both theTx band and the Rx band.

Spaceborne Reflector TechnologyFeed horn antennae for spaceborne communication missions usually employ reflectors to form contouredbeams from the geostationary orbit. There are two main methods to cover the earth’s surface from ageostationary satellite:

• A shaped reflector which consists in shaping the energy radiated by a single feed element (usually ahigh-performance corrugated feed horn) into a contoured beam to form the desired coverage area overthe earth’s surface (Viskum and Sorensen 1994). The shaping is done by appropriate reflector surfaceoptimization.

• Multibeam reflector antennae where the coverage over the earth is divided in beams that cover the earthsurface with cells. This technology employs several reflector surfaces (three or more) which are usuallyoffset parabolic reflectors and a single feed horn per beam (usually high-performance smooth-walledspline profile feed horns) over the earth (Rao 1999). So all the beams must be covered by an array offeed horns.

One of the problems with shaped reflectors, as opposed to multifeed parabolic reflectors, is that onlyone beam can be generated per reflector profile. For higher rate communications and to allow more reuseof frequencies and more reconfigurability, multiple spot beams and contoured beams are actually thepreferred solution.

These systems may use different antennae for Tx and Rx or common Tx/Rx antennae. Mass and cost-saving considerations increasingly drive the design into common Tx/Rx antenna solutions. One challengein this case is to design the antenna for optimal performance in two widely spaced frequency bands overthe same coverage spots on the ground. Designs have evolved to address this challenge. Clearly, thedesign antennae for Tx only or for Rx only is generally much simpler and often leads to slightly better gainperformance across the coverage region. The mass and cost penalties are, however, quite high.

RF Performance Parameter Considerations in Spaceborne AntennaeRF performance parameters include gain (minimum gain across the coverage region as well as peak gain),isolation (gain levels on the predefined isolation areas on the ground) and cross-polarization discrimina-tion (XPD) calculated as the copolarized gain minus the cross-polarized gain in dB.

Minimum gain across the coverage region must be optimized by means of proper reflector and feedhorn design. Feed horn copolar radiation pattern plays a key role in this optimization since it should keep apattern decay at the reflector edge as constant as possible over the whole frequency band as well asminimize spillover radiation to improve the isolation between different areas on the ground.

On the other hand, cross-polarization discrimination is also crucial since it must minimize the crosstalkinterference between transmissions using alternative polarizations. Cross-polarization generation inspaceborne antennae is caused mainly by four factors: feed horn inherent cross-polarization, reflectorsurface curvature, feed horn location away from the focal point and inclination angle of the feed hornrelative to the parabolic main axis. Other secondary order, but potentially important contributors to cross-polarization are scattering effects by support struts and feed lines, reflections of the horn primary patternand the reflector radiated near fields by neighboring structures including the adjacent spacecraft panels.


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These effects should be quantified, analyzed and tested since they can potentially degrade performancesignificantly.

High Performance Corrugated Feed HornsSpaceborne communications usually require the use of corrugated horn antennae to feed the shapedreflector surfaces designed to generate contoured beams that illuminate the desired coverage regions overthe earth. The reason to use corrugated horn antennae and no other type of feeds is that these type ofantennae offer, with improved performance over a quite wide bandwidth; low crosspolar levels, lowsidelobe levels and high return losses as well as nice gaussian-like radiation patterns. Therefore, theperformance of corrugated horns usually maintains high performance for both the Tx and Rx frequencybands.

A disadvantage of corrugated horns is that they are generally bulky with larger aperture diameter thansmooth walled horns for a comparable gain. They are also heavier and more expensive than smoothwalled horns. The basic corrugated horn profile is a linear corrugated flare angle leading to a longcorrugated feed horn, see Fig. 1.

There is a very wide variety of corrugated horn profiles (Rudge et al. 1982; Olver et al. 1994; Tenienteet al. 1999; Granet et al. 2000; Maffei et al. 2000; Hay et al. 2001; Teniente et al. 2002a; Granet and James2005), some of them but not all of them are: linear, sinusoidal, sine-squared, exponential, gaussian,hyperbolic, polynomial, etc. However, new software tools are now available to facilitate and enhance theeffectiveness of corrugated horn profile optimization, making it possible to design this type of horns tomeet an even wider variety of performance requirements and volume constraints. Modern commercialsoftware packages, use efficient full-wave analysis techniques such as mode matching, are usuallyemployed in horn design (http://www.mician.com). The agreement between predictions and measuredresults has become excellent, such that horn prototyping is usually no longer required. Other analysistechniques, such as the finite element method and the method of moments, are also occasionally used, butthey are not as computationally efficient as the mode matching method of analysis.

80

80

40

40

0D

iam

eter

, (m

m)

0 48 96 144 192Length, (mm)

240

d

w p

288 336 384

Fig. 1 Conical corrugated horn antenna


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http://www.mician.com/

After the simple conical corrugated profiles, different profiled corrugated profiled appeared, see Fig. 2.This new designs were shorter and the possibilities of optimization increased with the profile shape.

Nowadays, thanks to the available software tools and the speed of computers, the design of a corrugatedfeed horn for spaceborne communications is usually not any form of profile function. In fact, actualantenna designers leave every corrugation depth and step between corrugations free in the optimizationprocess leading to efficient profiles that cannot be met by simpler profile formulas as the ones usedpreviously.

The shortest corrugated horn profile and indeed very suitable for spaceborne communications as itpresents reduced mass and volume is the horn antenna that combines horizontal corrugations (known alsoas axial corrugations (Bird 2008) or choke horns (Milligan 2005)) for the throat region and verticalcorrugations (known also as radial corrugations) for the flare region, see Fig. 2. These horn antennadesigns have been used in modern communication satellite feeders as well as in other applications,(Gonzalo et al. 2002; Teniente et al. 2005, 2006; Teniente et al. 2009).

This type of horn antenna achieves significant improvements in axial length, return loss over a widebandwidth, computation complexity and manufacture complexity, when compared to a conventionalcorrugated horn with perpendicular corrugations. The only disadvantage is that, depending on the design,the manufacture could require splitting the horn in two pieces, one for the horizontal corrugations andanother for the vertical corrugations. Both sections must be bolted together and aligned properly withspecial care to join them assuring a tight contact, to prevent narrow radiating slots (Granet et al. 2008).

Example of a Ku-band Corrugated Horn to Feed a Shaped Reflector for SpaceborneCommunicationsAs an example, a Ku band communication satellite feedhorn whose specifications are given in Table 1, ispresented here. Such feed horn have more than 50.2 % bandwidth and the partial bandwidths are 22.2 %for Tx and 4.9 % for Rx bands respectively. It can be observed that the overall requirements are extremelysevere, since a return loss value above 32 dB and a crosspolar level below�45 dB for the whole frequencybands, are required. Note that this kind of profile is very short for a corrugated horn antenna.

Anyway, it is important to remark that every satellite has its own frequency plan that usually determinesthe feed horn design. However, all of the horn antennae for spaceborne communications that feed a shapedreflector need usually a simulated crosspolar maximum level below �45 dB and a copolar radiationpattern with a certain variation along the frequency band to assure the footprint of the radiation patternfrom the satellite over the earth to be frequency independent, see Table 1.

Fig. 2 Length comparison between different-profiled corrugated horn antennas (Teniente 2003) of the same directivity


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One of the best possible results to comply with the specifications given in Table 1 was a profile with3 axial corrugations and 44 perpendicular corrugations, see Fig. 3. The resultant length was 20 % shorter(8 � l(fmin1(GHz)) mm), than the maximum specified, so the mass reduction is really remarkable. The restof the specifications were met, see Fig. 4. The optimization process selected individually all thecorrugation parameters (except thicknesses) and the length and angle of the initial linear taper at thethroat. A total of 97 variables were individually optimized for the final design. Final results are shown inFigs. 4 and 5. The results are in good agreement with the specifications.

The resultant radiation patterns can be seen in Fig. 5. The results presented are mode matchingsimulations [1], but this method as it has been said previously is so accurate that the measurements areextremely similar if the feed horn is properly manufactured. This horn antenna was manufactured in asingle piece, see Fig. 6, and is successfully operating onboard a geostationary satellite.

High-Performance Smooth-Walled Spline Profile Feed HornsApplications requiring higher data rates, such as high-speed Internet services for consumers and smallbusinesses, call for much higher antenna gains and wider frequency bandwidths. Large antenna gains areassociated with a small coverage region on the ground and with large antenna aperture diameters in termsof wavelength. Realizing wider bandwidths is increasingly easier as the frequency of operation increases,

Table 1 Example Ku band spaceborne corrugated feed horn specifications

Parameter Value requested

Electrical specifications

Frequency bands Tx: fmin1 to 1.25 � fmin1 GHz

Rx: 1.62 � fmin1 to 1.67 � fmin1 GHz

Maximum crosspolar level < �45 dB in a range of +/� 22 deg

Return loss >32 dB in the whole frequency bands

Taper at 20� ~ �9.5 to �16 dB at Tx

~ �19 dB at Rx

Maximum sidelobe level <23 dB for angles above +/� 20 deg

Mechanical specifications

Total length <10 � l(fmin1(GHz)) mm

Maximum external diameter <3.55 � l(fmin1(GHz)) mm

Fig. 3 Optimized corrugated feed horn for ku band


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since then the bandwidth as a percentage of the center frequency remains relatively small. For thesereasons, this type of service has been typically provided with multiple spot beam antennae (with small

Fig. 4 Results of the optimized corrugated feed horn profile: (a) Return loss (b) Crosspolar level (c) Taper at 20�

Fig. 5 Far field radiation patterns of the optimized corrugated feed horn profile: (a) At fmin1 (b) At 1.25 � fmin1 (c) At 1.62 � fmin1

(d) At 1.67 � fmin1


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circular coverage spots) operating at K/Ka-band (Rao 1999). The coverage spots are typically arranged ina tight hexagonal grid on the Earth, where either all spots have the same diameter or different diameters areused to address different expected traffic density conditions.

These systems may use different antennae for Tx and Rx or common Tx/Rx antennae. Mass- and cost-saving considerations increasingly drive the design into common Tx/Rx antenna solutions and usuallysmooth wall horns are the preferred solution to reduce drastically the mass of the feed horn array since alot of horns must be arranged together.

For these systems, known as spot beams, a high efficiency of the horn aperture is needed to ensurebetter edge of coverage gain (Amyotte andMartins-Camelo 2012; Bhattacharyya and Goyette 2013). Thisaspect also drives to the use of smooth wall feed horns since an aperture efficiency near 90% can be met inTx and Rx bands at the same time with modern spline profiles. On the other hand, if corrugated feed hornsare selected, it is difficult to achieve better than 70% aperture efficiency (aperture efficiency is the relationbetween effective area and physical area at the horn antenna aperture).

Nowadays, for these kinds of applications, the best solution for spot beams is the use of smooth-walledspline profile feed horns (Granet et al. 2004). Spline-profiled feed horns are longer than corrugated horns,their performance regarding crosspolar level and bandwidth is worse but they are low weight (suitablemass reduction in arrays for spaceborne communications) and aperture efficient. Crosspolar level around�30 dB can be achieved in Tx and Rx bands at the same time which for this kind of multiple spot beamswhere the reflectors are not shaped is considered enough to ensure the required crosspolar isolation ordiscrimination (XPI or XPD).

However, simulation of a smooth-walled horn is not as easy as a corrugated horn. A corrugated horn isdiscrete; their profile is well defined by steps, so the number of steps to analyze would be the number ofcorrugations multiplied by two. However, smooth-walled horns must be discretized to be analyzed andsuch discretization must be enough to assure good performance. This issue leads to have above 500 stepsin a normal horn, much more steps than in a corrugated horn. In this situation, the designer has not suchfreedom and must rely on a certain formula to define the profile. Up to now, the best approach is the use ofspline profiles. Some results can be found in for a 6 radii case in (Granet et al. 2004) and for higher numberin (Zeng et al. 2010).

Fig. 6 Manufactured corrugated feed horn of the example (Reproduced with permission from ANTERAL company)


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Example of a Ka-band Smooth-Walled Spline Profile Feed Horn to Feed Multibeam ReflectorAntennae for Spaceborne CommunicationsAs an example, a Ka band communication satellite feedhorn whose specifications are given in Table 2, isdeveloped. Such feed horn has more than 42.3 % bandwidth and the partial bandwidths are 3.5 % for Txand 2.4 % for Rx bands respectively. It corresponds with a very stringent requirement for a smooth-walledhorn, since a return loss value above 40 dB and a crosspolar level below�26 dB for the whole frequencybands, are required. Furthermore, it is expected to present a reduced length.

The design employs 40 radiuses to define the profile, see Fig. 7. The result is rather curved inside for asmooth-walled horn, but with this profile, see Figs. 8 and 9, the requirements can be met.

The resultant radiation patterns are plotted in Fig. 9. This horn antenna was manufactured and it issuccessfully operating onboard a geostationary satellite where it forms part of a feed horn array for spotbeam communications.

Navigation Antennae

Navigation systems are commonly used nowadays. They provide geospatial position with global cover-age to users. There are different navigation systems as GPS fromUSA, GLONASS fromRussia, Compass

Table 2 Example Ka band spaceborne smooth-walled spline profile feed horn specifications

Parameter Value requested

Electrical specifications

Frequency bands Tx: fmin1 to 1.04 � fmin1 GHz

Rx: 1.5 � fmin1 to 1.54 � fmin1 GHz

Maximum crosspolar level < �26 dB in a range of +/� 15 deg

Return loss >40 dB in the whole frequency bands

Taper at 13� Spillover at Tx < 0.5 dB

Aperture efficiency at Rx > 0.85

Mechanical specifications

Total length <14 � l(fmin1(GHz)) mm

Maximum external diameter <4.8 � l(fmin1(GHz)) mm

Fig. 7 Optimized smooth-walled spline profile feed horn


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Fig. 8 Results of the optimized smooth-walled spline profile: (a) Return loss (b) Crosspolar level (c) Aperture efficiency (d)Spillover loss above 13�

Fig. 9 Far field radiation patterns of the optimized smooth-walled spline profile: (a) At fmin1 (b) At 1.04 � fmin1 (c) At 1.5 � fmin1

(d) At 1.54 � fmin1


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from China, and Galileo from the European Union. Their operational frequencies cover two bands:1150–1300MHz and 1559–1611MHz. The frequency band from 1300 to 1560MHz is reserved for otherradio users such as military, telemetry, radio astronomy, Earth-to-space operation, and satellite Internetservices. There are frequency overlaps between systems, creating a compatibility between them andallowing users to obtain their position using several systems at the same time. The working frequencies ofeach system can be seen in Table 3.

The accuracy of these systems needs to be improved. Geosynchronous satellite Navigation antennae(GSNA) are used to provide GPS/GALILEO/GLONASS signal corrections for better position accuracyand to certify signal integrity. The accuracy of Navigation systems has been improved by the used of thesecomplementary systems as the Wide Area Augmentation System (WAAS) in USA, EGNOS in Europe,and MSAS in Japan.

In this section, the Wide Area Augmentation System (WAAS) has been taken as GSNA referenceapplication. WAAS is an extremely accurate navigation system developed to complement the GlobalPositioning System (GPS). One of the important components of this system is the Tx L-Band navigationantennae providing global Earth coverage from geostationary satellites. These on board antennae operatein circular polarization with Axial Ratio (AR) values lower than 1.5 dB and provide gain values around17 dBi over the entire 18� beamwidth of the conical coverage area representing the antenna Field of View(FoV). The intended Navigation Antenna application requires also to achieve a precise and stable antennaphase center location (<20 mm) over both the antenna Field of View (FoV) and the operating frequencybandwidth, to allow for maximum overall system positioning accuracy. The required WAAS channelfractional bandwidth is relatively small and represents about 1.5 % of the positioning system L1 channelcenter frequency in L-Band. The main requirements from antenna point of view are shown in Table 4.

Conventional technology offers different solutions to comply with GSNA specifications. Arrayconfigurations with 9–19 elements size are necessary to obtain the desired gain values in the coveragearea. Different solutions are given using patches or helixes elements to create the array. The number ofelements in these arrays is not such a critical parameter. The main problem in these configurations is theBeam Forming Network (BFN) design, cost, and mass. The fabrication process of the helixes antennaecould also be not either easy.

Table 3 Frequency band for GPS, GLONASS, Galileo and Compass

System Coding Frequencies (MHz) Country

B1: 1559.052–1591.788

Compass CDMA B2: 1162.220–1217.370 China

B3: 1250.618–1286.423

E1: 1575.420

Galileo CDMA E6: 1278.750 European Union

E5b: 1207.140

E5a: 1176.450

L1: 1602 and 1575.420

L2: 1246 and 1242

GLONASS CDMA/FDMA L3: 1202.025 Russia

L5: 1176.450

L1: 1575.420

GPS CDMA L2: 1227.6 USA

L5: 1176.450


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EBG GSNA ConfigurationElectromagnetic Band Gap (EBG) technology offers new alternatives to design competitive solutions toconventional technology. EBG designs can obtain similar and even better, radiation performances(complying with all the radiation specifications) while the cost, dimensions, and manufacturing processcomplexity is minimized. The BFN can be simplified by reducing the number of radiating elements in thearray configuration using the gain enhancement mechanism given by a superstrate EBG configuration.EBG technology can be applied to these applications due to the reduced required bandwidth (ffi1.5 %).

In this section, an alternative to conventional technology based in EBG technology is presented. TheEBG superstrate configuration complies with the RF requirements, while it reduces the cost and heightand simplifies the BFN considerably. The comparison between the prediction and measurements will beshown.

The theory of the EBG gain enhancement phenomenon has its foundation in the pioneering work ofVon Trentini (1956) on the gain enhancement properties of cavity antennae. It consisted in a partiallyreflective surface (PRS) located a quarter wavelength from a ground plane. The structure forms a Fabry-Perot cavity. Power is reflected and transmitted (radiated) in each bounce of the trapped waves. When thepower radiated is in phase over the partially reflective surface, a directivity increase is achieved. Underthis condition, the radiation aperture has been increased. Out of the resonance band of the cavity waves areradiated out of phase, creating destructive interference in the far field. This gain enhancement phenom-enon can also be explained using leaky wave antennae theory (Lovat et al. 2006a, b).

Successively, more studies were done in 1985 (Jackson and Alexopoulos 1985) and 1988 (Jackson andOliner 1988) where the reflective surface was substituted by a quarter wavelength dielectric. Higherdirectivity values can be achieved using several dielectric layers (Jackson et al. 1993; Thèvenotet al. 1999a; Feresidis and Vardaxoglou 2001).

Different techniques have been applied to create partially reflected surfaces. Frequency SelectiveSurfaces (FSS) or EBG with a defect in their periodicity can be used to produce the gain enhancementphenomenon (Biswas et al. 2001; Fehrembach et al. 2001; Cheype et al. 2002; Lee et al. 2005). Theintroduction of a defect in the stacked structure leads to the apparition of a narrow pass band, where thewaves incident to the structure can suddenly be propagated in the now defective crystal. The quality factorof the defective crystal can be adjusted by playing with the permittivity of the slabs, and the quality factordecreases with the decrease of the dielectric permittivity. This quality factor is going to limit the maximumachievable gain and the operational bandwidth of the antenna configuration (Diblanc et al. 2005). Thehigher the Q-Factor, the smaller the operating relative bandwidth of the structure is.

Table 4 WAAS specifications

Center frequency 1575.42 MHz

Bandwidth 24 MHz

Bandwidth at 4GHz 80 MHz

Gain over coverage >16.9 dB

Edge of coverage 8.9�

Polarization Circular RHCP

Axial ratio on coverage <1.5 dB

Phase center collocation <20 mm

3 Standard deviations over complete

FoV and frequency band

Diameter <570 mm

Height <350 mm


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This technology is intrinsically limited to relatively narrow band applications where the operatingrelative frequency bandwidth is limited to a few percents. As the frequency increases from the optimumoperating frequency providing the antenna peak gain, the directivity decreases and the pattern getsdistorted. On the other hand, most of the rays are reflected back by the EBG when the frequency islower than the optimum gain frequency, therefore trapping the RF energy emitted by the source betweenthe ground plane and the reflecting slab, see Fig. 10.

EBG theory has provided a new and powerful framework for the understanding of the gain enhance-ment phenomena and has enabled significant advances in the design of antennae using partially reflectingsurfaces. The gain enhancement phenomenon is now described in terms of wave propagation in 1Ddefective EBGs and it has been shown that the gain enhancement factor and the bandwidth are closelyinterrelated and essentially fixed by the quality factor of the defective 1D crystal resonance (Iriarteet al. 2006). The quality factor of the defective 1D crystal resonance can be adjusted by adjusting thereflectivity of the partially reflective surface and generally increases when the reflectivity of the surface isincreased. Thin metallic sheets with holes or etched metallic patterns on thin dielectric sheets can be usedto produce lightweight 1D defective EBGs for space antenna applications.

One of the main advantages that the EBG technology could bring to navigation antennae is thereduction of the antenna complexity by reducing the number of radiating elements to just a few or evento a single one. It has been shown in Thèvenot et al. (1999b), that a 1D defective crystal with a high qualityfactor and a single element source can meet the directivity and Axial Ratio requirements of the WAASapplication. However, it has also been found that the phase center stability is significantly degraded withthis large gain enhancement configuration. It therefore appears that the phase center stability needs to betraded-off against the quality factor of the defective 1D crystal and the intrinsic directivity of the sourceilluminating the EBG antenna. A quality factor of about 50 combined with a 2� 2 element array excitingthe antenna 1D defective crystal structure was found to be a good compromise in order to meet the desiredgain and phase center stability requirements of the WAAS antenna application.

An EBG design, frequency scaled to C-Band is shown in Fig. 11. The model has been scaled due tomeasurement facilities in this frequency range. The result is completely scalable. It consists of a 2 �2 array of circularly polarized two-port patches fed in quadrature and placed underneath an EBGsuperstrate consisting of an etched metallic pattern of circular holes on a thin dielectric sheet. The circularholes are positioned on a regular square matrix. The radiating elements are sequentially rotated in 90�

steps to improve the Axial Ratio over the antenna Field of View (Iriarte et al. 2009).At the edges of the ground plane, four dielectric threaded rods provide support to the EBG layer and

allow for adjustment of the height of the layer above the radiating elements and antenna ground plane. TheEBG superstrate is etched on a thin 50 mm thick Kapton substrate and tensioned on a fiberglass reinforced

Fig. 10 Depiction of the spatial filtering concept


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epoxy frame. Johannson Blocks were used to set the EBG layer height above the ground plane with anaccuracy better than 50 mm. The height of the EBG layer above the ground plane is about 0.45 l0.

RF PerformancesThe optimum operating frequency band of the frequency scaled-up is from 3,700 to 3,780 MHz. TheRHCP directivity elevation cuts at y = 0�, 45�, 90�, and 135� at 3,740 MHz are provided in Fig. 12, andthe axial ratio cuts the same y� cuts at 3,740 MHz are provided in Fig. 13.

The radiation pattern performance summary of the GSNA configuration is provided in Table 5. Theantenna peak RHCP directivity ranges from 20.5 to 21.2 dBi. The worst RHCP directivity over the FoVranges between 17.9 and 18.3 dBi.

0.47 dB of losses have been estimated for the design. Including the 0.47 dB of losses, the minimumRHCP gain over the FoV is 17.39 over the antenna operating bandwidth. Axial ratio over the FoV is betterthan 1.2 dB, therefore compliant to the targeted 1.5 dB value.

Fig. 11 (a) Patch array configuration and (b) EBG gain enhanced WAAS navigation antenna

Fig. 12 RCHP directivity elevation cut at y = 0�, 45�, 90� and 135� azimuth at 3.74 GHz of GSNA EBG antenna


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Different phase center calculation methods can be used to estimate the value from the antennaparameters. In this case, standard 3s deviation and peak-to-peak methods are used to its calculation.The best phase center collocation properties of the GSNA EBG antenna over both the frequency band andthe antenna FoV, provided in Table 6, are meeting the targeted specification presented in Table 4 usingboth methods.

Flight Implementation ConsiderationsIn order to move towards a fully space qualified Navigation antenna product, several issues linked to boththe mechanic and electrical environments specific to space applications must still be addressed. In thissection, the focus will be on the impact of the thermomechanical distortions on the antenna frequency ofoperation, and on the evaluation of the available margin with respect to multipaction factor.

Fig. 13 Axial ratio elevation cut at y = 0�, 45�, 90� and 135� azimuth at 3.74 GHz of the GSNA EBG antenna

Table 5 Summary results for the GSNA EBG antenna

Frequency RHCP Peak directivity (dBi) Worst RHCP FoV directivity (dBi) Worst FoV axial ratio (dB)

3700 20.51 17.86 1.11

3710 20.67 17.00 0.96

3720 20.80 18.11 0.84

3730 20.92 18.20 0.79

3740 21.01 18.26 0.69

3750 21.08 18.31 0.60

3760 21.14 18.33 0.58

3770 21.18 18.33 0.64

3780 21.21 18.32 0.60

Average over the channel bandwidth

3740 20.95 18.19 0.76

Worst over the channel bandwidth

3740 20.51 17.86 1.11


Page 15 of 39

Impact of Thermomechanical DistortionsThe typical thermal environment of a low-power top-floor antenna with passive thermal control (thermalblankets and sunshields) is stringent when the antenna base is not radiatively coupled to the spacecraftEarth Deck. In this thermal configuration, the antenna can face minimum and maximum temperatures inthe order of �130 �C to +120 �C and must remain operational.

The most critical dimension of the antenna is the separation between the ground plane and thesuperstrate. Any error in this dimension shifts the operational bandwidth of the antenna. In fact, thefractional frequency shift is proportional to the fractional variation of this dimension. Assuming that themanufacturing errors can be leveled out from the design by careful trimming of the antenna structure atambient temperature (20 �C), the antenna will still suffer from some frequency shifts due to thermaldistortions. Although that low Coefficient of Thermal Expansion (CTE) materials can be used in thefabrication of the antenna superstrate structure, such as Kevlar (CTE ~4 ppm/�C) or Invar (CTE ~2 ppm/�C),the fairly large temperature excursions seen by the antenna will lead to some dimensional changes as well asvariations of the effective dielectric constant of the dielectric materials.

The separation between the ground plane and the superstrate is approximately L = l0/2 and thetemperature variation will result in a variation of the electrical length between the ground plane and thesuperstrate. The fractional frequency shift Df and the electrical length fractional variation DLe are givenapproximately by:

Df � DLe � DLp þ Dkeff

where DLp = CTE DT is the fractional variation of the physical separation between the ground plane andthe superstrate and Dkeff is the fractional variation of the effective propagation constant.

The dielectric constant of Kevlar varies by approximately 6 % over the �130 �C to +120 �Ctemperature range. In order to obtain a light low permittivity sandwich supporting the EBG layer, lowdensity Kevlar honeycomb could be used to provide the structural stiffness. A conservative estimate of thefilling factor of the cavity with Kevlar is 5 % resulting to an average low-density sandwich relativepermittivity of eeff � 1.1. This leads to fractional variations of the propagation constant of:

Dkeff ¼ffiffiffiffiffiffiffiffiffiffi

Deeffp

The fractional bandwidth required in the antenna design is therefore given by:

BW

f0¼ Df þ 24

1575:42ffi Df þ 1:53%

which leads to a required design bandwidth of 29.6 MHz at L1. This corresponds to the 80 MHzbandwidth at C-Band (4 GHz) used in the simulations and measurements of the breadboard.

Considerations on MultipactorThe strongest concern about the power handling capability of this type of antenna configurations in aspace environment lies in the field buildup related to the 1D defective photonic crystal quality factor at its

Table 6 Phase center variation over both the antenna field of view and the frequency bandwidth for the GSNA EBG antenna

Channel phase center data (scaled to L1) (mm)

Radial variation 3s dev. 17.55

Radial variation Pk-Pk. 19.83


Page 16 of 39

resonant frequency. A multipaction analysis has therefore been conducted on the EBG gain enhancedantenna configuration to understand the power-handling capability of this structure.

Evaluation of the impact of thermomechanical distortions on the antenna operating frequency bandshows that the proposed solution can be implemented using low CTE materials to make the criticaldimensional parts of the antenna. As well, a preliminary multipaction analysis shows that RF power in the100 W range can be used with this type of antenna in the space environment.

Telemetry and Telecommand Antennae

Telemetry, Tracking, and Control (TT&C) are vital functions for successful operation of all satellites,because they lead the spacecraft management. The main tasks of a TT&C system are, for instance, tomonitor the performance of all satellite subsystems and transmit the monitored data to the satellite controlcenter for routine operational and failure diagnostic purposes. Furthermore, it must represent a trackingplatform to earth stations by accomplishing the determination of orbital parameters to maintain a satellitein its assigned orbital slot and provide look angle information to earth station in the network. Besides, thesystem receives commands from the control center to execute various function of the satellite, forexample, power on/off subsystems, change subsystem operating modes, deploy booms, antennae, solarcell arrays, and so forth. Usually, uplink (UL) and downlink (DL) communications with TT&C satellitesare carried out at S-band (UL: 1.6–2.2 GHz and DL: 2.2–2.3 GHz) such as in case of Inmarsat or Galileo(Galileo System Requirement 2002), C-band (UL: 5.9–6.5 GHz and DL: 3.7–4.2 GHz), and Ku-band(UP: 14–14.5 GHz and DL: 11.7–12.2 GHz).

For the purpose of providing robust communications with the established receivers, the TT&Cantennae located onboard the satellites must provide appropriate earth coverage and work simultaneouslywith right hand (RHCP) and left hand circular polarization (LHCP). Particularly in the systems working atthe C-band, one of the most stringent radiofrequency (RF) requirements regarding the antennae perfor-mance is the necessary gain; the typical value is 16.5 dBi at the edge of coverage (EOC), that is, �9�.Commonly, high bulky horn antennae (up to 4 kg for low C-band frequencies) and 2 kg for high C-bandfrequencies are implemented for TT&C applications. The reduction in the mass and dimensions of suchantennae becomes a major concern for the spatial industry, especially when the carrier satellites are small.Due to this fact, the development of competitive and alternative antenna technologies complying with theoverall RF requirements of conventional antennae but providing at the same time the advantage oflightness and compactness represents an attractive research area. TT&C onboard antenna requirementsare shown in Table 7.

As it has been shown in the GSNA EBG antenna, the incorporation of EBG technology-basedstructures in different emerging communication frameworks consolidate them as feasible solutions fordifferent applications, such as wireless and satellite industries (Galileo 2002; Rodes et al. 2006; Brandet al. 2009; Hajj et al. 2009; Iriarte et al. 2009; Oses et al. 2009; Kanso et al. 2011). Requirement of TT&Con board antennae applications can be covered by the use of EBG gain enhancement configurationantennae.

An EBGmodel which complies with the requirements of the selected TT&C application is presented innext section. These low-mass EBG superstrate antennae are capable to fulfill the given C-band systemrequirements. Characteristics such as the gain over coverage, dual circular polarization, return loss,isolation, and mass levels demanded by this kind of satellite systems are analyzed. The obtained resultsconfirm the viability of the proposed EBG-based antennae for onboard TT&C antenna configurations.

Anyway, for Ku-band, horn antennae can be used for TT&C since the higher frequency reduces sizeand the mass. A very compact horn antenna for TT&C is presented afterwards. This horn antenna was


Page 17 of 39

capable to cover simultaneously downlink and uplink frequencies and was connected to a very compactseptum polarizer to produce the required circular polarization with enough isolation between ports.

EBG TT&C AntennaAs in the case of GSNA navigation antennae, gain increase can be accomplished by the resonance formedin a cavity, which, in the presented case, is created between a metallic ground plane and an EBGsuperstrate that is acting as a partially reflective surface. The latter consists of a metallic sheet with asquare lattice of drilled holes, see Fig. 14.

The EBG superstrate fixes the operational frequency and the final directivity of the antenna. The radiusof the holes determines both the magnitude and phase of its reflection coefficient. Meanwhile the phasedetermines the resonance frequency, the magnitude relates with the field spread on the superstrate surfaceand therefore determines the illumination, fixing the directivity.

The radiating system is composed by a 2 � 2 patch array feeding the resonant cavity created betweenthe ground plane and the EBG superstrate (see Fig. 14a). The distance between patches was optimized inorder to obtain an improved symmetric pattern with low sidelobe levels. Two feed ports are perpendic-ularly located at each radiant patch, which are fed with a phase difference of 90 �. In addition, each patchof the 2 � 2 array is sequentially rotated with respect to its adjacent radiators. Thus, not only circularpolarization at each patch is ensured but also further improvement in the circular polarization of thecomplete feed system.

Fig. 14 Final truncated EBG superstrate configuration. (a) Isometric view. (b) Field confined in the center of the cavity

Table 7 TT&C antenna requirements

Center frequency 3.7GHz

Bandwidth 80 MHz (goal 120 MHz)

Gain over coverage >16.5 dB

Edge of coverage 9�

Polarization Circular (RHCP/LHCP)

Crosspolarization < �25 dB (1 dB AR)

(CP/CX)

Return loss < �18 dB

Isolation >25 dB

Power handling 10 Watt

Mass <800 g

Envelope To be minimized (height < 2.5l)


Page 18 of 39

As the mass is critical in this application, the total dimension of the superstrate is also a criticalparameter in the design process. It must be large enough to comply with the maximum directivityrequirement stated in Table 7, but at the same time, it must be as light as possible. In a preliminarystructure of the proposed configuration, the dimension of the superstrate was set to 365� 365 mm. Sincethis area causes a significant increase in overall system weight, a strategy to overcome this problemconsisting on truncating the corners of the superstrate is undertaken (see Fig. 14a). This solution does notproduce any degradation of the antenna properties as the electromagnetic field is mostly confined in thecenter of the cavity (see Fig. 14b). After an optimization process trying to keep unaltered the radiationperformances of the antenna, the cuts from the corners are stated to L= 110 mm so the resulting total areais 109.025 mm2.

The EBG layer is placed 0.45 l0 at working frequency above the ground plane.The losses of the radiators and BFN have been considered as 0.15 and 0.16 dB respectively, together

with matching and connector losses (0.04 and 0.05 dB). An implementation margin including thermaleffects of 0.10 dB has been added so that the total losses are predicted to be equal to 0.5 dB in the finalsystem. Therefore, the gain of the proposed configuration should be above 16.66 dBi in the desiredworking bandwidth (see Fig. 15).

Figure 16 presents the RHCP and LHCP radiation patterns for 0�, 45�, and 90� phi cuts at 3.66, 3.73,and 3.79 GHz for the EBG-based antenna. Note that the low part of the black rectangular box embeddeddetermines the�25 dB required crosspolar level. It is clear that the designed EBG antenna complies withthis RF performance. Similarly, in Fig. 17, the axial ratio parameter is depicted. The AR value is under1 dB at 3.74 GHz for all the phi cuts. At the same time, the AR for the worst phi cut at EOC is plotted inFig. 17b with a gray dashed line. Analyzing this result, the AR operational bandwidth goes from 3.61 to3.92 GHz, fulfilling the 80 or 120 MHz bandwidth stated at the specifications. The directivity and AR forthe TT&CEBG truncated superstrate antenna are summarized in Table 8. Regarding the isolation betweenports, the appropriate design of the BFN is the key issue. It must provide the optimized amplitude andphase coefficients to the array elements. In this case, uniform excitation has to be settled for bothamplitude and phase. The BFN is composed by 1:2 T-dividers whereby supplied amplitudes and phasesin each port guarantee RHCP.

Fig. 15 Directivity vs. frequency at y = 0� (black solid line) and y = 9� (gray dashed line)


Page 19 of 39

The directivity, axial ratio and requirements are met, together with the input matching below 18 dB andisolation level between ports below 25 dB within the operational bandwidth. The overall mass of theproposed EBG antenna system has been calculated taken into account all the constitutive antenna parts,such as the material used to build the brackets, holders, bolts and nuts, and a certain margin for eachcomponent. The superstrate is placed over a Kevlar layer to ensure its steadiness, which mass is estimatedto be 222 g. The concluding outcome of the overall mass leads to estimation of 870 g. The overweight of70 g is almost negligible.

25

20Cut Phi(0°) LHCPCut Phi(45°) LHCPCut Phi(90°) LHCPCut Phi(0°) RHCPCut Phi(45°) RHCPCut Phi(90°) RHCPMax Specification

15

10

5

0

Dire

ctiv

ity L

HC

P [d

Bi]

−5

−10

−15

−20

−25−90 −75 −60 −45 −30 −15 0

Theta [deg]15 30 45 60 75 90

a

25


15

10

5

0

Dire

ctiv

ity L

HC

P [d

Bi]

−5

−10

−15

−20

−25−90 −75 −60 −45 −30 −15 0

Theta [deg]

15 30 45 60 75 90

c

25


15

10

5

0

Dire

ctiv

ity L

HC

P [d

Bi]

−5

−10

−15

−20

−25−90 −75 −60 −45 −30 −15 0

Theta [deg]

15 30 45 60 75 90

b

Fig. 16 RHCP and LHCP radiation pattern at y(0�), y(45�) and y(90�). (a) 3.66 GHz, (b) 3.73 GHz and (c) 3.79 GHz


Page 20 of 39

EBG superstrate antennae are capable to fulfill the TT&C requirements of a global geostationarytelecommunication system working at C-band (3.7 GHz). Other bands can also be covered with thistechnology as the design is completely scalable. The desired properties in the working bandwidth, as gain

Fig. 17 Axial Ratio. (a) 0� to 180� phi cuts each 15� at 3.74 GHz. (b) AR for the worst phi cut (gray dashed line) in EOC

Table 8 Directivity and axial ratio summary for the TT&C EBG antenna

BW 80 MHz BW 120 MHz

Frequency

Peakdirectivity(dBi)

Worstdirectivity(dBi)

CP/XP (Fov)Worst case Frequency

Peakdirectivity(dBi)

Worstdirectivity(dBi)

CP/XP (Fov)Worst case

3.69 20.13 17.31 25.35 3.67 19.93 17.16 25.12

3.74 20.40 17.47 27.13 3.74 20.40 17.47 27.13

3.77 20.34 17.36 28.44 3.79 20.17 17.16 29.34


Page 21 of 39

over coverage, dual circular polarization, return loss, and isolation values can be achieved with thepresented structure. Although mass of the configuration is not under the established threshold, theoverweight is almost negligible. Anyway the final mass is well below the typical horn antenna designs.In addition, the use of EBG-based technology antennae makes it possible to obtain gain enhancement witha reduced number of radiating elements; in this case just a 2 � 2 array of patches.

EBG-based systems could advantageously replace conventional horn antennae and arrays withcomlicated BFN working at 3.7 GHz for TT&C applications, by reducing the antenna complexity,mass, and cost, similarly to the case of the GSNA application.

Dual Band EBG Antenna ConfigurationTT&C frequency band requirements in C bands usually cover 3.7 GHz and 4.2 GHz bands at the sametime. Two different antennae are used in conventional technology to cover these working bands or largearrays of broadband patches are also used to comply the frequency range specifications. Dual bandantenna configurations are also being designed by the inclusion of another EBG superstrate in theconfiguration complying the TT&C requirement in dual band with a single EBG antenna.

The same configuration presented for the single frequency behavior can be used for the dual band withthe inclusion of another EBG layer. The second layer is based on the combination of Frequency SelectiveSurfaces (FSS) metallic rings with Jerusalem crosses (see Fig. 18) inside them, placed just 1 mm below theEBG layer of circular holes in a metallic sheet.

The directivity versus frequency results are shown in Fig. 19. Maximum directivity of 21.74 dB at3.78 GHz and 21.28 dB at 4.18 GHz is obtained. D(9�) is above 16 dB from 3.7 to 3.8G Hz (2.66 %) and

Fig. 18 Detail of the superstrate formed by two EBG layers


Page 22 of 39

from 4.13 to 4.25 GHz (2.86 %). Therefore, 100 MHz working bandwidth is obtained at both frequencybands, complying requirements.

The directivity patterns at 3.76 and 4.32 GHz are shown in Fig. 20. Symmetric directivity patterns withcross polarization level below 25 dB are obtained, therefore fulfilling the specifications.

The overall mass of the proposed system is similar to the single frequency configuration, 870 g., as thesecond EBG layers is printed in the same sheet used to print the upper layer: one on a the top face of thesheet and the other one on the bottom face. The losses of the configuration have been estimated taking intoaccount the EBG antenna and the feeder to be around 0.5 dB.

Horn Antenna for TT&C at Ku BandFrom the geostationary orbit at about 36,000 km above the earth, the earth subtends an angle of 17.4�.With an increased number of satellites orbiting the earth, reducing the possibility of interference withother satellites is becoming more important than in the past. To minimize this interference, the amount ofsidelobe energy should be as low as possible, both for the principal and cross-polarized signals. Takingthese considerations into account, it is apparent that an ideal full-earth coverage antenna should becircularly symmetric and, therefore, most global coverage antennae are either reflectors or horns.

A corrugated horn can be made very low weight and also space qualified. Materials as carbon fiberreinforced plastic materials are suitable for fabrication of the final corrugated horn for TT&C (Bird andGranet 2008).

One of the first compact corrugated horn designs for low sidelobes and global earth coverage wasdescribed by (Granet et al. 2000). This horn exhibits quite low sidelobes (<36 dB) and it is quite compact5.6�lc but it presents two principal disadvantages: it operates in a narrow bandwidth, (less than 5 % formaximum �30 dB sidelobe level and �30 dB crosspolar level and �20 dB return loss) and it is verysensitive to manufacturing tolerances in the throat region (mode generator) (Bird et al. 2000).

Another wideband solution is the corrugated horn antenna that combines axial and perpendicularcorrugations (Teniente et al. 2002b), shown in Fig. 21. This antenna presents a measured return loss betterthan 28 dB for above 22 % bandwidth. The measured sidelobe level is extremely low, below �35 dB forthe overall operational bandwidth, and presents a crosspolar level below �35 dB (Table 9).

Fig. 19 Directivity vs. frequency for the 0� and 9� cuts of the 2 � 2 array EBG superestrate antenna


Page 23 of 39

THz Space Antennae

In the last years the number of space missions where submillimeter wave and Terahertz onboardinstruments are operating is increasing. Some of the last missions which incorporate such instrumentsare PLANK and HERSCHEL with the HIFI instrument; in addition, in the near future scientific missionwhich are in need of high frequency instruments are expected to be launched, i.e., missions such as SPICAwhere the SAFARI European instrument will operate at THz frequencies (Goicoechea et al. 2009).

All of these instruments implement antennae which operate in bands covering frequencies between100 and 900 GHz. At these THz frequencies there are different effects to be investigated. For example,understanding how the first galaxies were formed or the star formation process. Moreover, on a largerscale in this frequencies the development of galaxies can be monitored and studied; a wide range ofmolecules, including water, absorb radiation; and they can be used to determine exactly what the gases are

Fig. 20 RHCP and LHCP radiation patterns (a) at 3.78 GHz (b) at 4.12 GHz


Page 24 of 39

composed of and their temperature and pressure. Therefore, antenna technologies able to comply with theinstrument requirements need to be developed. Apart from the conventional horn antennae, innovativeconcepts have been proposed, such as integrated antennae, Metamaterial (MTM) antennae or morerecently antenna coupled Kinetic Inductance Detectors (KIDs).

Integrated antennae are the natural solution for the development of large imaging arrays operating inthis frequency range. To this end, they would be placed in the focal plane of a collimating reflector or lens.However, they present several fundamental problems related to the use of electrically thick substrates suchas the excitation of surface waves, which decrease their efficiency and degrade their performance.Solutions to this problem have been found by conforming the substrate creating a lens or by using ametamaterial substrate, which prevents surface wave excitation. Several antennae making use of substrateintegrated extended hemispherical lenses fed by dual-slot antennae, log-periodic and spiral antennae,have resulted in excellent performance at millimeter-wave frequencies. In addition to these, new antennaesuch as integrated horn antennae, dielectric-filled parabolas or Fresnel plate antennae have been proposed.

A large effort has been dedicated to match these antennae to the detectors. These detectors range fromSchottky diodes to bolometers. In principle it should be possible to build planar receivers with perfor-mance comparable to the best waveguide-based systems if efficient planar antennae and matchingnetworks are developed. Recently, efforts have been made to integrate Kinetic inductance detectors(KIDs) (Day et al. 2003; Vayonakis et al. 2008; Baryshev et al. 2011) together with planar antennae.These are among the most sensitive cryogenic sensors available for detection of electromagnetic radiation

Fig. 21 Measured radiation pattern at fc of the corrugated horn antenna that combines axial and perpendicular corrugations forTT&C (Reproduced by permission of ANTERAL)

Table 9 Radiation pattern details of the corrugated horn antenna that combines axial and perpendicular corrugations forTT&C

H plane taper @8.7�

45 deg plane taper @8.7�

E plane taper @8.7�

Maximum sidelobelevel

Maximum crosspolarlevel Directivity

�2.74 dB �2.82 dB �2.9 dB �43 dB �38 dB 20.5 dB


Page 25 of 39

and probably the most promising sensors to be employed in large focal plane array configurations. Theycan be coupled to a single on-chip microwave transmission line enabling the read-out of thousands ofdetectors with just a single coaxial cable pair connecting the on-chip transmission line to room temper-ature read-out electronics. KIDs can be easily integrated with lens antennae and leaky lens antennae. Inparticular this last configuration, allows achieving a very large bandwidth, thanks to the phase stability ofthe leaky lens antenna.

Anyway, this section will focus on the MTM concepts applied to THz antennae. As mentioned, MTMsubstrates are one of the solutions for surface wave reduction and radiation pattern improvement. Thischapter describes in detail the fundamentals behind their use and several applications.

One of the main goals for Terahertz technology is the realization of imaging arrays, which are of greatinterest for space astronomy and for atmospheric research. Particularly in astronomy most of the spectralline emitting regions are usually spatially extended over many observing beams in the sky and mapping isrequired to astrophysically understand the regions under study. In atmospheric research multiple beamobserving systems allow push-broom measurements for limb scanning experiments. As the regions ofinterest in astronomy research are usually distributed over many observing beams, the time needed tobuild an image can be reduced in approximately inverse proportion to the number of elements included inthe array.

THz imaging arrays have conventionally been based on feedhorns and waveguide technology and areusually assembled from discrete elements, some recent examples of missions using horn antennae can befound in PLANCK or HERSCHEL (Doyle et al. 2009). The costs, mass and volume associated with thisapproach may limit the maximum practicable number of pixels. In fact, PLANCK mission implements afocal plane formed by more than 40 antennae ranging from 30 to 857 GHz. On the other hand,HERSCHEL mission, although more focused at Infrared frequencies, also comprises 5 frequencybands covering 480–1,150 GHz with SIS mixers. The SPIRE detectors are spider-web bolometersusing NTD germanium thermometers, which are coupled to the telescope by hexagonally close-packeddiameter single mode conical feedhorns.

In the last time, the advances on lithography techniques and processes have open the possibilities todevelop new THz antenna concepts which can be competitive versus conventional waveguide technol-ogies. Being able to produce lithographically planar receivers with equivalent performance is allowing therealization of much larger two-dimensional arrays. Such an approach is dramatically reducing cost andgreatly simplifying manufacture and assembly.

Although waveguide and feedhorn technology provides a natural isolation between adjoining pixels. Incontrast, the performance of planar antenna arrays can be severely limited by the parasitic cross couplingof the antennae via surface waves (Pozar 1983). Electromagnetic bandgap (EBG) or Photonic Crystal(PC) structures, working as surface wave suppressing substrates, have consequently attracted muchinterest (Joannopoulos et al. 1995; Gonzalo et al. 1999; Sievenpiper et al. 1999; de Maagt et al. 2003;Yang and Rahmat-Samii 2003). EBG structures prevent the propagation of radiation, for frequencieswithin the bandgap, in 1, 2 or 3 spatial directions, the number usually corresponding to the number ofdimensions in which the crystal is periodic (Joannopoulos et al. 1995). Whilst 2-D EBG materials havebeen proven to be an useful substrate for planar antennae (Gonzalo et al. 1999), a 3-D EBG appears to bemore desirable because any antenna fundamentally radiates in 3-D and an antenna array by definitionextends over at least one dimension perpendicular to the boresight.

Within this section, the performance of these structures as substrate for a dipole antenna will be studied.In particular two different single dipole antennae backed by a 3-D EBG structure are presented. Thiscombination exhibited low back radiation and a higher directivity when compared to the equivalent dipolein free space. Two different EBG substrate shave been used for this purpose: the so-called Woodpile andFAN EBG structures (Ho et al. 1994; Sözuer and Dowling 1994; Martinez et al. 2007). In the case of the


Page 26 of 39

Fan structure it was built with a very high dielectric constant material, Zirconium Tin Titanate (ZTT). Thishigh dielectric material can be of interest for miniaturization. However, the excitation of surface wouldlimit its performance as antenna substrate. This problem can be sort out if an EBG structure isimplemented in the substrate. Once the performance of an element is studied a seven element 0.5 THzimaging array will be discussed.

Single Detector THz ConfigurationIn this section, the performance of a dipole antenna on top of an EBG structure operating at THzfrequencies is described. Two cases are discussed. Both of them use as radiating element a printed dipoleantenna and either a woodpile EBG or a FAN EBG structure as substrate.

When a dipole antenna is placed on top of a periodic structure, its radiation pattern and input impedancedepend on the position and orientation of the dipole with respect to the features on its surface. Completestudies for both configurations can be found in (Gonzalo et al. 2001; Ederra et al. 2013). In summary, foreach EBG structure it can be found a polarization dependence of the input impedance. The parallel to themain features (dielectric bars in the woodpile and air channels in Fan’s) polarization presents very lowinput impedance, which can be ascribed to the close to 180� reflection coefficient phase. On the contrary,the perpendicular polarization exhibits higher impedance, easier to match to a CPS line, which would bethe most natural feeding circuit for the dipole antennae. Regarding the radiation features, no significantdifferences are found between positions and polarizations. In all cases, there is an increase of directivity,thanks to the effect of the EBG substrate and coupling between neighboring antennae will be reduced.

For the design of the overall dipole antenna/EBG configuration as a detector, a subharmonicallypumped heterodyne receiver was considered. This incorporates a flip chip mounted Schottky diode inthe center of the dipole; and the local oscillator signal is delivered along a coplanar stripline (CPS). Thelow pass filter is a standard high–low impedance filter, which was constructed by widening the tracks ofthe CPS. Its design was carried out taking into account the loading of the underlying EBG. The filterdimensions were optimized in order to maximize the reflection of RF power at 0.5 THz. The downconverted RF signals also passes along the same stripline. A low pass filter is incorporated to prevent theCPS from loading the dipole at frequencies around 0.5 THz. The design procedure was first to optimizethe impedance matching when the dipole antenna was fed by a CPS and afterwards the RF power coupledto the diode was maximized.

In the first case, the dipole antenna is placed on the FAN EBG structure. The feeding CPS line and thedipole were printed on a 20 mm thick quartz substrate. Details of the dipole and feeding line dimensionscan be found in Ederra et al. (2013). For these dimensions, the impedance of both CPS line and dipolewere similar and close to 170 Ω, which assures a good impedance matching.

The Schottky diode was placed as close as possible to the dipole antenna in order to minimize its effecton the radiation pattern of the dipole. The distance between the diode and the low pass filter was optimizedso that the power transferred from the incoming RF signal to the diode was maximized.

The circuits were patterned in a gold film, which had been deposited on a fused quartz substrate, usingstandard photolithographic techniques. The quartz was then cut to size and lapped to a final thickness of20 mm.

The Fan structure was held to a low loss polymer, TPX high-density polyethylene, support by usingcyanoacrylate adhesive. The quartz substrate was then aligned and fixed using a small quantity of thesame adhesive at its edges. The output of the choke filter was connected to a coaxial line using gold ribbonand Epotek H20E. A photograph of the manufactured antenna and Fan EBG combination is shown inFig. 22.


Page 27 of 39

Radiation Pattern ResultsThe predicted radiation pattern of the complete detector configuration, i.e., dipole, diode, and low passfilter, was computed using Ansoft HFSS. Since the diode and the feeding lines affect the radiation patternsome ripples in the E-Plane are appreciated. The computed directivity, 6.4 dBi, is larger than thedirectivity of the ideal dipole. This higher directivity corresponds to the peaks of the ripples and not toboresight direction.

The radiation pattern was measured at the University of Bern using an AB Millimeter submillimeterwave Vector Network Analyzer (VNA) from ABmm. Both E and H plane patterns were measured at 0.48,0.5 and 0.52 THz. The comparison between the measured patterns and the predicted ones are depicted inFig. 23, where normalization of measured data at boresight has been performed. The above directivityincrease with respect to a dipole on a standard substrate can be noticed.

In the second case, a similar design procedure was followed, but in this design, the substrate was awoodpile EBG structure. More details about the woodpile properties and its manufacturing properties canbe found in (Gonzalo et al. 2001). The dipole antenna was a lithographically patterned 2 mmmicrons thickgold structure. A similar receiver configuration was used, where a Schottky diode was used to convert theincoming RF signal at 500 GHz to DC.

The dipole antenna was fed using a transmission line on a quartz membrane and a simple quarter-wavelength, high-low impedance filter was used in order to isolate the RF from the DC signal. Themembrane was kept thin enough so as not to disturb the performance of the EBG structure. The RF filter isadjoined to two bonding pads.

The fabricated structure of the dipole on top of the woodpile EBG is displayed in Fig. 24. The woodpile,the antenna, the filter, the membrane to support the feeding line, and the Schottky diode are clearly visible.

The measurement system included a Carcinotron operating at 500 GHz, a computer controlledpositioning system with two axes of movement, onto which the configuration is placed and a lock-inamplifier to measure the detected DC signal. The results for the E and H planes are plotted in Fig. 25.

It should be noted that the obtained E-plane beamwidth is y = 60� while it is y = 85� for the H-plane.These values correspond to an estimated effective area much larger than that of a simple stand-alonedipole that has an effective area of 0.13l0

2. This leads to higher directivity values that can be derived to beof the order of 8–10 dB for the EBG backed antenna compared with only 2.14 dB obtained for a simpledipole suspended in air.

Fig. 22 Photograph of the dipole on top of the Fan EBG structure (From reference (Ederra et al. 2013) reproduced bypermission of # 2013 IEEE)


Page 28 of 39

In addition, both E and H plane patterns tend to zero at 90� and 270� confirming that the EBG substrateis suppressing the parasitic surface wave mode excitation.

0330

a b

c

30

300 60

270 90

240 120

E-PlaneH-Plane

E-PlaneH-Plane

E-PlaneH-Plane210 150

180

f = 0.48 THz f = 0.5 THz

f = 0.52 THz

−20 −10 0 10

0330 30

300 60

270 90

240 120

210 150180

−20 −10 0 10

0330 30

300 60

270 90

240 120

210 150180

−20 −10 0 10

Fig. 23 Comparison between the measured (dashed line) and predicted (solid line) radiation pattern of a dipole antenna placedon top of the Fan structure at 0.48 THz (a), 0.5 THz (b) and 0.52 THz (c) (From reference (Ederra et al. 2013) reproduced bypermission of # 2013 IEEE)

Fig. 24 Photographs taken with the electron microscope showing the fabricated dipole on top of the woodpile photonic crystal(From reference (Gonzalo et al. 2001) reproduced by permission of # 2001 IET)


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Array Configuration of Single Dipole THz DetectorsThe enhanced performance of the single EBG backed element shown in previous sections is followed bythe use of this configuration as base for an imaging array. In this case, a seven-element Schottky diode-based heterodyne array for 500 GHz which could be the base for future space instruments in imagingapplications has been studied (Ederra et al. 2008). This design represents a significant step towards thegoal of a planar technology imaging array. The array was designed to match the typical optical system of asubmillimeter wave astronomical instrument: f/25 with a far field beam crossover level of�3 dB. In orderto obtain these beam characteristics, a 90� paraboloidal mirror was used to modify the radiation pattern ofthe detector antennae. The design of the individual receivers was optimized for maximum sensitivity i.e.,for best impedance matching between the Schottky diodes and the planar dipole antenna.

Dipole and Array DesignAn optimized design of the 500 GHz dipole antenna together with the associated RF choke/ IFtransmission line on top of the EBG structure is used for the array design (Fig. 26). Limitations on the

Fig. 25 Simulated and measured radiation patterns (red and black curves respectively) of a dipole on top of a woodpile EBGstructure. 0� and 90� cuts are plotted


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dipole antenna separations arise from the 500 GHz silicon woodpile, which has a 72 mm bar width and232 mmperiod in the substrate plane. As the dipoles need to be placed above intersections of the bars in thetop two layers, the separation between elements is consequently restricted to multiples of this period.

A previous study of the mutual coupling of dipole antennae on EBG substrates showed that a minimumseparation of two periods is necessary to ensure coupling values below �20 dB in both E and H planes.One compact configuration of seven antennae that meets this requirement was developed. Althoughdenser configurations are possible, the selected one offers the advantage that the six outer antennae liebetween 3.6 and 4 woodpile periods from the central element. It will be shown later on that it is possible todesign matching optics for this array that achieve the desired beam properties: the relatively narrow spreadof radii assists in achieving a design that is compliant with the beam crossover point requirement.

The mutual coupling between the array elements was investigated. The results obtained with AnsoftHFSS shows that for the case under study this value is below 40 dB.

Mirror DesignAlthough in principle a lens could have been used, reflective optics exhibit lower losses and standingwaves. The f# requirement on the array imposes the far-field power half- angle, i.e., the directivity of theresulting antenna–reflector combination. To meet the f/25 specification, the beamwidth of the outputbeams should be 2.2�. Therefore, the reflector must provide the required magnification, increasing thedirectivities of the radiating elements and simultaneously separating the beams. However, the further theantennae are from the optical axis of the system, the more degraded the directivity and beamwidths of theirradiation patterns becomes. The central dipole is located at the optical focus of the mirror and theprojection of the mirror on the x-z plane is an ellipse with its major axis in the z direction. To optimizethe illumination of the mirror, the dipole array was also oriented with its major axis along the z-axis.Taking this into account, elevation (azimuth) corresponds to the H (E) -plane of the central beamrespectively.

The mirror dimensions were optimized to match the array described above. The obtained optimumvalues were found to be D = 17.3 mm and f = 25.4 mm.

Physical optics GRASP (http://www.ticra.com/products/software/grasp) software was used to calculatethe far-field radiation patterns. As an input for this program, the radiation of an isolated dipole on a finitesilicon woodpile, containing 11 periods, was computed with Ansoft-HFSS. Predicted individualbeamwidths of (2.1 � 0.1)� were obtained, matching well the desired 2.2� value. GRASP simulationsalso showed that there was negligible impact on the far-field radiation pattern frommounting the woodpileon either a dielectric, TPX, or a metal plate.

Manufacturing of the Imaging EBG Array ConfigurationA monolithic quartz substrate was used for the overall dipole array, this process ensures accuratealignment of all dipoles on the EBG substrate. Standard photolithographic techniques were used topattern the array in a gold film that had been deposited on a fused quartz substrate. The quartz was then cutto size and lapped to a thickness of 20 mm. Separated and substrate thinned Schottky diodes were fixed inposition on the metal using silver loaded epoxy (Epotek H20E). Electrical connections were made fromthe ends of the choke filters to coaxial lines using gold ribbon and silver loaded epoxy.

The characteristic curves of the Schottky diodes were monitored repeatedly during assembly. Nodeviation from the initial manufacturer’s values was found, apart from for one diode. Here an increasein series resistance was localized to either the diode to substrate connection or to within the diode chip.

The mirror was made from a diamond turned, gold coated, parabolic reflector of 12.7 mm focal lengthand a projected diameter of 17.3 mm.


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http://www.ticra.com/products/software/grasp

Experimental ResultsAVNA based measurement technique and apparatus was used for the characterisation of the beam patternof the array at 500 GHz. As a source, the 100 GHz output of a phase locked Gunn diode oscillator wasquintupled in frequency and radiated by a corrugated horn antenna towards the EBG array. A stabilizedDC bias voltage was connected to the diodes through a bias-T in the IF transmission line in order to set theoptimum operating point of the receiver. In each case, the LO power and applied DC bias were adjusted tomaximize the signal to noise ratio of the IF signal.

Fig. 26 Manufactured array configuration of 7 dipole antennas on top of an EBG structure (From reference (Ederra et al. 2008)reproduced by permission of # 2008 IEEE)

Fig. 27 Positioning system used to measure the array configuration


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The imaging array was mounted so that it could be rotated about azimuth and elevation axes through theoptical center of the parabolic mirror: Fig. 27. Beam pattern measurements were made at a feed horn toarray separation of 0.8 m with a dynamic range above 25 dB.

Two dimensional beams scans, normalized to their maxima, are presented in Fig. 28 for some of thereceivers over an angular range of �8� with a step size of 0.5�. To improve the presentation quality,interpolation of the data has been performed and�3 dB,�6 dB,�10 dB and�15 dB contour lines added.

To represent better the overall behavior of the array, a contour plot of the�3 dB signal levels for all thebeams is presented in Fig. 29. In the measurements the absolute pointing of the array had an offset of 1� inelevation and 1� in azimuth from the nominal optical axis, which has been subtracted for this figure. The

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reason for this is probably a slight misalignment of the parabolic reflector and the test setup. Goodagreement between patterns is observed, although the individual measured beams are slightly narrowerthan predicted, which leads to crossover points of around �4 dB. The predicted slightly compressedelevation plane characteristic is also exhibited by the measurements.

Finally, the radiation pattern cuts, azimuth and elevation, of different elements of the array arepresented in Fig. 30. These measured pattern are compared with their counterparts obtained by simulationwith HFSS software. In general, the agreement is good, some discrepancies can be found in the sidelobelevels, in particular, higher sidelobes are appreciated in the elevation cuts which are attributed to the effectof the mirror holder used in the measurement positioning system.

These results constitute the first demonstration of a planar imaging array based on EBG technology.Although the performance was still far from that obtained with more mature technologies it is the first steptowards its application.

Conclusion

This chapter has demonstrated the use of advanced horn antenna profile to comply with the very stringentrequirements of space antennae. Nowadays, the most of the space applications is requiring betterelectromagnetic performances to the antennae; lower cross-polar levels, low sidelobes, higher gains,higher efficiencies, larger operational bandwidths, etc. Conventional designs are not able to comply withthese needs and more sophisticated antennae have to be developed. Several examples have demonstratedthe possibilities of horn antennae to achieve those high performances with reduced lengths. New pro-posals based on the use of horizontal and vertical corrugations together with spline profiles have beenanalyzed and their radiation characteristics have been included.

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Fig. 29 Comparison of the 3-dB contours of the measured (solid lines) after correcting for mirror misalignment and thepredicted patterns (dashed lines). In the measurements the absolute pointing of the array had an offset of 1� in elevation and 1�

in azimuth from the nominal optical axis, which has been subtracted for this figure. Operating frequency 500 GHz


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On the other hand, there are space applications where conventional antennae suffer to obtain the desiredradiation performances, mainly as they are heavy and bulky. In that case, this chapter has presented thepossibilities of using EBG technology to develop innovative antennae. For instance, EBG-enhancedantennae could advantageously replace conventional array antenna in WAAS applications. This EBGtechnology simplifies the antenna configuration and leads to lighter and more cost effective solutions bysimplifying the antenna BFN. The EBG GSN complies with all the requirements of a GSN application,resulting in a real alternative antenna for navigation on board antennae. Furthermore, EBG superstrateantennae fulfill the requirements TT&C application at C-Band in single and dual frequency performance.The system directivity is up to 17 dB in 100MHz around 3.7GHz and 4.2GHz; the cross-polarization levelis lower than 25d.B. Although mass of the configuration is not under the established threshold, theoverweight is almost negligible. Consequently, EBG antennae technology system represents an

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Fig. 30 2-D cuts of azimuth and elevation of beams 2 (a), 7 (b) and 4 (c). Operating frequency 500 GHz (From reference(Ederra et al. 2008) reproduced by permission of # 2008 IEEE)


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interesting alternative to the bulky horn antennae and complicated BFN of array usually used for TT&Capplications in current satellites.

As the last part of this chapter, some results of EBG antennae operating at THz frequencies have beenincluded. In particular, two cases are analyzed. In the first case, the enhanced performance of a dipoleantenna placed on top of an EBG structure is discussed. The reduction of the back and side radiation andthe improvement of the antenna directivity have been reported. These clear advantages have led todevelop an array configuration suitable to be used in imaging THz application for scientific instruments.An array formed by seven elements has been presented. This configuration has been designed to complywith typical requirements in imaging applications by means of the integration together with a parabolicmirror. The radiation performances have been measured and good agreement with simulated results hasbeen obtained. This constitutes the first demonstration of a planar imaging array based on EBGtechnology.

Cross-References

▶Antennas in Radio Telescope Systems▶Commercial Antenna Design Tools▶ Frequency Selective Surfaces▶Metamaterials and Antennas▶Mm-Wave Sub-mm-wave Antenna Measurement▶Radiometer Antennas▶Terahertz Antennas and Measurement

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Space Antennas including Terahertz Antennas€¦ · In particular, horn antennae, either corrugated as spline proﬁles for data downlink and uplink communications and TT&C applications

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