Satellite Antennas on Vehicles...Satellite Antennas on Vehicles Stefan Lindenmeier* and Simon Senega Institute of High Frequency Technology and Mobile Communication, Universit€at

Satellite Antennas on Vehicles

Stefan Lindenmeier* and Simon SenegaInstitute of High Frequency Technology and Mobile Communication, Universit€at der Bundeswehr M€unchen, Neubiberg,Germany

Abstract

The mobile reception of satellite services on vehicles places high demands on the antennas in manyregards. Due to the high path loss because of the great distances, low signal levels are experienced on theground.

The following chapter gives an overview on antennas which can be used for the mobile reception onvehicles. The main areas of application in this regard are systems for global positioning and for satelliteradio services. At first an overview of the requirements on the antennas imposed by the different servicesis given. Thereafter some basic antenna types are discussed regarding their advantages and disadvantagesas far as the reception of satellite services are concerned including dipole and ring structures. Moreadvanced antenna designs are also presented which are specifically optimized for different satellitesystems.

In reception scenarios with severe signal impairments like multipath propagation resulting in deepsignal fades, a single antenna is not sufficient for satellite reception. The mechanisms which lead to thesescenarios are shortly introduced followed by a discussion of antenna diversity techniques which are aneffective means to reduce these impairments. Special consideration is given to scan-phase diversity whichefficiently combines the advantages of a simple system design with high signal quality improvements.Measurements obtained in real fading scenarios are presented for single antenna as well as scan-phasediversity systems. They show that antenna diversity can significantly improve the audio availability inadverse reception scenarios compared to single antenna systems. Furthermore, diversity can even allowfor using antenna mounting positions which are unsuitable for single antennas like the dashboard or singleside mirrors while still outperforming a rooftop mounted standard antenna.

Keywords

GNSS; Satellite radio; Ring antenna; Dipole; Scarabeus antenna; Multipath fading; Antenna diversity

Introduction

In this chapter an overview on car antennas and antenna diversity technologies for mobile reception ofsatellite radio signals is given. The special requirements for the design of satellite reception antennas on avehicle are discussed in general and in detail for the very common satellite services for navigation andradio broadcast. Basic antenna types are discussed in comparison with each other with respect to meetingthe requirements for mobile satellite radio reception on vehicles. Together with the well-known commonstructures, for example, patch antennas, new ring types of receiving antennas are explained leading to animproved reception quality and enabling a lean efficient design. Applications are shown for the examples

*Email: [email protected]

Handbook of Antenna TechnologiesDOI 10.1007/978-981-4560-75-7_101-1# Springer Science+Business Media Singapore 2015

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of navigation and satellite radio services. Furthermore, ways of optimizing the reception quality ofsatellite radio significantly via antenna diversity are shown. This includes the discussion of appropriatediversity circuits and antenna structures which may be located together in one mounting volume which isas compact as former single antenna solutions. In real scenarios it is shown by experiment that in criticalfading scenarios the number of mutes is reduced by more than one order of magnitude. The diversitytechnology enables the choice of new antenna mounting positions, which would not be chosen for a singleantenna because of the strong decrease of reception quality. By means of diversity technology, successfulreception of high quality is shown even for the use of inferior antenna positions like in side mirrors or onthe dashboard.

General Requirements on Car Antennas for Satellite Reception

For satellite radio reception in cars, high demands are placed on the antennas with respect to gain,efficiency, volume requirements, and mounting conditions as well as reliable reception in criticalreception scenarios like fading environment. Carrier frequencies to be considered are settled in theL-band and in the S-band. Since the wavelengths at these frequency bands are considerably shorterthan those of the terrestrial radio reception services, only small antenna elements with a footprint ofaround 5 cm by 5 cm (200 by 200) and a height of less than 2.5 cm (100) are needed. In the following, the mainrequirements for mobile satellite receiving antennas on vehicles are listed:

– Lowmounting volume (typical footprint area: around 5 cm by 5 cm (200 by 200); typical height:<2.5 cm(100))

– Low losses (typically 1.5 dB or less), high efficiency– Power matching to 50 Ω– Omnidirectional behavior with respect to azimuth– Radiation pattern in elevation tailored to the satellite position to be expected from point of view of the

receiver– Easy fabrication and high reproducibility– Circular polarization and high axial ratio– Sufficient bandwidth (order of magnitude in MHz between 1 and 10 in case of navigation services and

between 10 and 100 in case of satellite radio and mobile communication)– Mounting position of low influence on the radiation pattern by the car’s structure– Low influence on the antenna performance when placing antennas for other services in close proximity

The angle ranges, in which certain gain values are required, differ very much with the satellite system.At LEO (low Earth orbit) and MEO (medium Earth orbit) systems, there is only a low height of thesatellite above the sea level in comparison with the orbit radius, which means that the direction, in whichthe satellite might be expected from point of view of the receiving antenna, could be nearly arbitrary. Withthat, the radiation pattern of such systems should cover the complete upper half-space. In contrast insystems using a geostationary orbit (GEO) satellite, this satellite can be expected most probably in a rangeof diagonal elevation angles from point of view of the receiving antenna. HEO satellite systems usesatellites where the transmission is active while its direction is of high elevation angle above the horizon.This means that the radiation pattern for such systems should have high gain values around the zenith.Additionally, there is often the requirement for reception of signals in very low elevation since terrestrialrepeaters often support satellite services in urban regions where shadowing of high buildings might harmthe satellite reception. As very common examples for satellite services are using orbits ofMEO, GEO, and


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HEO type, the following sections focus more closely on the requirements of navigation services andespecially on the different aspects of requirements for satellite digital audio radio services (SDARS).

Antenna Requirements for Mobile Reception of Navigation Signals

For navigation, mobile satellite services such as GPS, GLONASS, and GALILEO are nowadays part of atypical equipment of vehicles for civil and military use. For these services the L-band is used with a centerfrequency of 1.575 GHz for GPS and Galileo, 1.65 GHz for GLONASS, and additional possible centerfrequencies at 1.23 GHz and 1.18 GHz for GPS as well as 1.19 GHz and 1.28 GHz for Galileo, each with abandwidth of less than 10 MHz, typically 2–5 MHz (Hegarty and Chatre 2008).

Since the satellite orbits of the navigation systems are MEOs, the angle range of reception antennas forpositioning services is very wide between zenith with theta = 0� and low elevation over ground withtheta = 70�. In this angle range a constant gain is required. Often, a requirement for a certain axial ratio isreported. In some cases an axial ratio of less than 6 dB helps in fact to prevent first-order reflections of theradio waves in the environment since with the reflection the polarization is inverted. The problem is thatfor low elevation angles above the ground, only a high axial ratio is possible since in the horizontal planeabove a very well conducting ground, an electromagnetic wave can only have a vertical polarization. Invertical direction a first-order reflection is only possible on the ground. In such a case it is more of interesthow large the back lobe of the antenna is after being mounted onto a car. So the only case where a lowaxial ratio helps to prevent interference by reflections at high buildings is an elevation angle around 45�.

For these requirements, cheap patch antennas with a footprint area of less than 2.5 cm by 2.5 cm (100 by 100)have been commonly used. Nowadays other structures are considered too which also ensure easy fabricationcombined with high efficiency and well covering of the required angle ranges.

Antenna Requirements for Mobile Reception of Satellite Digital Audio RadioSystems

In the last years, satellite radio systems gained more and more importance also for broadcast reception inthe USA (Davarian 2002). Also for Europe, there have been plans to establish new satellite radio servicesin the S-band (Reding 2007) to follow an earlier system called WorldSpace which operated in L-band(Sallam et al. 2008).

In contrast to terrestrial systems, satellite radio services use satellites to broadcast the radio signals to ahuge area of reception. By using satellites for transmission in satellite radio services, the reception area isextended to cover almost the complete continental USA as well as large parts of southern Canada. Thefrequencies usable in such a system are limited by the available transmission power as well as transmittingantenna size (and of course regulatory restrictions). American SDARS work in a frequency range from2,320 to 2,345 MHz with a bandwidth of 25 MHz (Briskman and Prevaux 2004; Patsiokas 2001). Whilecurrently no European SDARS are commercially available, the frequency band between 2,170 and2,200 MHz has been dedicated to this purpose (Reding 2007) and was auctioned to two buyers(Reding 2009). In contrast to the US systems, these will use a signal coding that is part of the DVBfamily and was standardized as DVB-SH by ETSI (ETSI 2010a, b, c).

An overview of the setup of a satellite radio system is depicted in Fig. 1. The broadcast signal isencoded in the broadcast center using all desired audio and data channels as well as supplementaryinformation. After modulation and up-conversion in frequency, it is transmitted to the satellites. Thesatellites amplify and down-convert the signal to the desired frequency band and retransmit it after further


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amplification (Briskman and Prevaux 2004). Mobile (and also stationary) receivers can then decode thetransmitted information. In areas where a direct signal from the satellite is not available due to signalblocking – which can be the case due to natural or man-made attenuators in the signalpath – supplementary terrestrial transmitters can be used. Of course these increase the overall systemcost and are thus only sparsely used in areas with high numbers of service subscribers (e.g., in big cities).

Different satellite constellations are used by the (formerly two and now one) service provider(s) of USSDARS. One service started with (and still uses) two GEO satellites, while the other complemented itsinitial three HEO satellites with an additional GEO satellite. The HEOs’ ground track – which describesthe position of a satellite projected to Earth’s surface over the course of one revolution – is a lemniscate.The ground tracks of three HEO and two GEO US SDARS satellites are depicted in Fig. 2. The GEOsatellites always stand on the same position in the sky and therefore have an elevation angle that is almostconstant unless the receiving antenna moves over great distances north or south. In most of the continentalUSA, the elevation angle of a GEO satellite is in the range of 30–45� above the horizon. The elevationangle of a HEO satellite strongly changes over time even for a stationary receiver. It can even havenegative values when the satellite is below the horizon. Therefore at least three satellites are needed inorder to ensure sufficiently high availabilities at any given time. The advantage of such a configuration isthat one of the satellites is usually visible in very high elevation angles over ground of 60–90� (e.g., the reddot in Fig. 2) thus lowering the probability of a blocked signal path compared to a GEO transmitter. InFig. 3 the elevation angles of the US SDARS satellites are plotted for a receiver located in a major US cityover a time of 24 h.

The distance of the satellites to Earth is of course much higher than the distance between terrestrialtransmitters and receivers. Due to propagation in the vastness of space, the free-space path loss of thetransmission link gives a very convenient estimation of the attenuation that can be expected between thesatellite and a receiver in line-of-sight (LOS) conditions. Free-space path loss a is calculated according tothe following.

a ¼ l04pd

� �2

(1)

broadcastcenter

satellite

terrestrialtransmitter

mobilereceivers

signalblocking

Fig. 1 SDARS system overview and signal paths (Senega 2013)


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where l0 is the free-space wavelength and d is the distance between transmitter and receiver.Due to the elliptical orbit, changes occur not only in the elevation angle but also in the distance of the

satellite to the receiver. This means that the varying free-space path loss leads to signal levels that aredependent on the satellites position. A calculation of the free-space path loss of the signals from GEO andHEO using Eq. 1 and values given in Briskman and Prevaux (2004) leads to an attenuation in the range of�190 to�193 dB depending on orbit position as it is given in Table 1 (neglecting further attenuation dueto atmosphere, weather, etc.).

Fig. 2 Ground track of the satellites of XMSatellite Radio (GEO, black dots) and Sirius Satellite Radio (HEO, blue, green, andred lemniscate, GEO not depicted) (M€uller 2010)

Fig. 3 Elevation angles over ground of GEO and HEO satellites over New York City, NY, USA, over a time period of 24 h(M€uller 2010)


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The modulation and digital coding of the satellite signals comprise a number of means to ensure highservice availability in most reception scenarios (Briskman and Prevaux 2004). Of course any includedredundant information for error correction or avoidance reduces the available number of informationchannels or the audio quality due to the limited frequency and power resources. Therefore theimplemented measures are always a compromise between service variety, quality, and availability.While frequency, time, and spatial diversity as well as forward error correction schemes ensure highaudio availabilities in many reception scenarios, they cannot avoid all signal outages, for example, insevere multipath fading scenarios.

The measure for quality of reception is the SNR at the receiver. If the SNR falls below a certainvalue – e.g., 5 dB – the decoding of the signal will suddenly be almost impossible due to bit errors. Theexact value of the critical SNR is highly dependent on several parameters which among others include themodulation scheme, the number of bits per symbol, the method and parameters of the forward errorcorrection, the influence of noise and disturbances on the transmission path and the receiver, etc. A criticaldecision during the design of communication systems is that of the link margin in typical receptionscenarios. The link margin describes the typical difference between the signal level at the receiver and itssensitivity limit (the lowest signal power that it can decode quasi error free). In this regard the free-spacepath loss needs to be considered when the minimum transmit power of the satellites is specified. Anyadditional attenuation in the transmission path due to obstacles, weather, or multipath propagation willthen reduce the remaining signal power until the SNR falls below the threshold, and decoding of the signalbecomes impossible.

Special requirements on the radiation characteristics are made for SDARS signal reception (Haller2001). Regarding the reception of US SDARS signals transmitted by HEO satellites, an angle rangebetween zenith direction at theta = 0� and lower elevation at theta = 35� can be expected. In the lastyears transmission via GEO satellites gained more and more importance, so currently antennas arerequired which are appropriate for both the reception of signals from high elevation angles of HEOsatellites and signals of medium elevation angles of GEO satellites. For GEO satellites the relevant anglerange can be expected between low elevation at theta = 70� for northern regions and high elevation attheta = 45� for southern regions up to higher elevations at theta = 37� in a small area at the south bordertoward Mexico (if the receiver position’s longitude is close to that of the GEO satellite). Each of theantennas still has to fulfill additional requirements for reception of terrestrial signals also at very lowelevation angles. In parallel to this development, new ring type antennas have been created which can bemade of panel without using lossy dielectric material. This results in a very high efficiency and hence to astrong improvement in signal to noise ratio.

Basic Antenna Structures for Mobile Satellite Signal Reception

In the following basic antenna structures are considered which are capable for the reception of satellitesignals. All of these structures enable omnidirectional reception with respect to azimuth and a circularpolarized field with a vertical main beam direction for HEO satellite signal reception or a diagonal main

Table 1 Free-space path loss from an SDARS satellite to Earth for different orbital positions

Distance to Earth (km) Free-space path loss (dB)

Perigee 24,469 (�187.55)

HEO 35� elevation 32,750 �190.09

Apogee 47,102 �193.24

GEO 38,192 �191.42


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beam direction for GEO satellite signal reception. All the structures considered fulfill the commonrequirement of a low antenna height in order to allow for easy integration underneath flat antennaradomes. In literature (e.g., Balanis 2005) there can be found a large number of other structures, forexample, helical structures, which require a large antenna height or a large diameter with respect to thewavelength, so that normally such structures cannot be considered for integration in the small mountingvolumes given in cars.

Crossed Dipole AntennasA first very basic and well-known structure consists of crossed horizontal dipoles (Balanis 2005) whichare positioned in a distance d above the ground plane. Figure 4 shows such a structure where the boldblack lines represent the dipoles, while the gray structures represent the feeding network. If the distance dis one fourth of the wavelength, the reflection at the ground plane yields additional 3 dB in Gain in verticaldirection, since a positive superposition of the waves propagating directly in vertical direction togetherwith the waves which are reflected by the ground plane arises. For l/2 dipoles the sinusoidal currentdistributions which are shown above the dipoles in the figure are obtained. In order to excite a circularpolarized wave with vertical main beam direction, the two horizontal dipoles are connected with the feedwith 90� of phase difference. This means that in one moment only the current distribution along one dipoleis developed as shown in solid lines, while in one fourth of the time period, only the current distributionalong other dipole is developed as shown in dashed lines.

If the branches of the dipoles were to be connected each with the same single-ended feed, it is obviousthat a complicated network of phase shifters, power combiners, and baluns is necessary. In Fig. 4 anexample of such a network is shown, where different phase shifters for each of the four branches yield aphase at these branches which is increasing in 90�steps in circular direction around the phase center.

For wideband applications such an antenna is advantageous since it can be expanded to a widebandcrossed butterfly antenna structure as shown in Fig. 5. The wideband capabilities enable a use for severalsatellite services. Depending on the phase shifting and matching circuit, for different frequencies, LHCPand RHCP wave reception is possible at the same time. In Lindenmeier et al. (2001, 2002b),multifunctional antenna structures have been presented for different satellite services and terrestrialservices. While for the satellite services the four vertical lines together with the horizontal elements actlike a crossed butterfly antenna structure, they act in a common phase like a monopole with roof capacitorfor terrestrial services.

matching circuitand phase shifter

Antenna elements

feed ground plane

Fig. 4 Crossed dipole structure


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If further gain in the vertical direction is needed, the crossed horizontal dipoles can be expanded to twoantenna arrays (Balanis 2005) as it is shown in Fig. 6. Off course, the up to 3 dB higher gain yields also alower beam width which makes this structure more capable to reception of GEO satellite signals insouthern regions. Since this structure is rather large, there is a strong requirement to shorten this structure.If the structure should be used for only one service, there is an unnecessary hardware effort to be paid forthe feeding structure, which additionally decreases the antenna efficiency because of its circuit losses.

Patch AntennasAvery similar current distribution is occurring at the well-known patch antennas. Due to their small size,patch antennas are very common for mobile satellite signal reception and have been described in a largenumber of variations in literature, as, for example, in Sharma and Gupta (1983), Herscovici et al. (2003),Nasimuddin et al. (2007), and Pozar and Duffy (1997). As it is well known patch antennas represent amicrostrip waveguide with an effective length of half of the wavelength for the first resonance mode.At such a short microstrip waveguide, there occurs a current distribution of a sinusoidal standing wavewhich has got maximum values along the opposite side edges of the microstrip line.

Patch antennas for a circularly polarized field are of roughly squared shape where the squared patch canbe seen as a microstrip waveguide along two orthogonal horizontal directions. Hence, two orthogonal

matching circuitand phase shifter

Antenna elements

feed ground plane

Fig. 5 Multifunctional crossed dipole structure

Matching circuitand phase shifter feed ground plane

Fig. 6 Crossed pairs of dipoles


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modes of resonance can be excited for the two orthogonal horizontal directions. In Fig. 7 a patch antennais shown with the current distribution along the opposite edges of the patch. In between, the currentdistribution forms a saddle function with a minimum in the center of the patch.

If the two orthogonal resonant modes are excited with a phase distance of 90� between each other,current distributions are obtained at the edges of the patch which are similar to those of the pairs of crosseddipoles. The current distributions on opposite sides which are in phase with each other yield a strong gainin vertical direction, acting similar to a pair of l/2 dipoles. On the other hand, the length of the edges issmaller than the length of the l/2 dipole of Fig. 6 because of the shortening effect of the dielectricsubstrate. If ceramics substrate with high permittivity is used, the patch concept enables a small antennasize and hence an easy integration of the antenna in small mounting volumes.

The resonant modes can be coupled with each other via chamfered corners of the metal patch or thedielectric plate or a slightly rectangular shape together with an asymmetric feed, so that no phase shiftcircuit is needed and an easy fabrication is possible. This is why patch antennas are widely used forreception of GPS signals. The relative bandwidth of patch antennas, which is in a rather low range of a fewpercent, can be increased by the use of additional structures like stacking or insertion of slots in the patch(Herscovici et al. 2003; Nasimuddin et al. 2007; Pozar and Duffy 1997).

For the reception of satellite radio signals, patch antennas are often used either. Though the metal patchis oriented in horizontal plane, the substrate enables weak reception of a vertical field component at verylow elevation which can be used for reception of signals of terrestrial repeaters.

Because of the relatively high gain in vertical direction, a patch antenna is not capable very well forreception of GEO satellite signals at the lower elevation angles. A general disadvantage of patch antennasis the loss in the dielectric material which decreases the signal to noise ratio of the received signal.

Crossed Frame AntennaIn order to obtain a radiation pattern which can be used for lower elevation angles, a crossed frame antennahas been introduced in Lindenmeier et al. (2002a). In the same way as a crossed dipole structure has beenrealized via phase shifters, two vertical frame antennas are crossed and excited with a phase distance of90� to each other.

The basic principle is shown in Fig. 8 together with a sketch of the current distribution. Due to thevertical parts of the frame antennas, a considerable share of vertical polarized waves is received inhorizontal plane and at low elevation angles. This enables also the reception of GEO satellite signals. Dueto series capacitors being inserted in the frame antennas, the antennas are set into resonance and theradiation pattern can be adapted to special requirements. In Fig. 9 an example of such an antenna is showntogether with its matching and phase-shift circuit. Like for the crossed dipole structure, the matching andphase-shift circuit required means an additional hardware effort, harming also the efficiency of theantenna. Nevertheless, due to its capability also for lower elevation angle ranges, this antenna has beenused successfully for SDARS reception and in particular for GEO satellite reception.

ground plane

Fig. 7 Patch antenna


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Loop AntennasIn the following a very efficient antenna concept is shown for satellite signal reception. If a horizontal loopantenna has got a circumference equal to the wavelength, it is a resonant structure. It can be consideredlike an electromagnetic waveguide where the wave is guided in a circle. As long as such a wave is excitedonly for propagation in one direction, there will occur a current distribution which moves along the closedloop in positive direction as shown in Fig. 10. This current distribution yields a very pure circularpolarization of the antenna structure. In Fig. 11 a radiation pattern for ideal conditions is shown for theleft-hand and right-hand circular polarized field (LHCP and RHCP). While for the LHCP a gain of 8 dB invertical direction is obtained, only a gain of �11 dB is occurring for RHCP.

An antenna design which is tailored to high efficiency, a wide angle range, and a small size is the“Scarabeus” antenna (Kammerer and Lindenmeier 2011), which is depicted in Fig. 12. It consists of a loopantenna which is connected to vertical elements. These vertical elements are connected to ground viacapacitors. Due to the vertical elements, two advantageous effects occur: At first, the structure is evensmaller than the loop structure of Fig. 10 because of a shortening effect of the capacitive vertical elements.Secondly, the current distribution along the vertical elements yields a nearly omnidirectional verticallypolarized field in horizontal plane. In Fig. 13 the radiation pattern in the vertical plane is shown for theco-polarization LHCP and the cross-polarization RHCP. In Fig. 14 the radiation pattern is shown in thehorizontal plane for ideal conditions.

ground plane

Fig. 8 Crossed frame antenna

Fig. 9 Hardware example


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Since manufacturing of these antenna structures is easy – the conducting structures are realized in paneland since dielectric material can be prevented – these structures have a very high efficiency with losses ofonly 0.5 dB. This is why even for a simple realization, as shown in hardware samples in Figs. 15 and 16,an antenna characteristic is achieved, which is very close to the ideal case, as is shown in the measureddiagrams in Figs. 17 and 18.

As the radiation diagrams show, the antenna offers an omnidirectional behavior with respect to azimuthand a wide beam width in elevation of more than 90� which enables to use the concept for SDARSservices with HEO and GEO satellites as well as for navigation services. Special characteristics of thisantenna type are as follows:

– High efficiency and low losses of only 0.5 dB since the antenna structure can be realized by panelwithout the need to lead the electric field through material of high permittivity as it is necessary forpatch antennas.

– Low footprint area of around 13 % of the wavelength.– High axial ratio of more than 15 dB in zenith.– Easy combination with terrestrial antennas which can be positioned in the phase center of the Scarabeus

antenna.– The wide bandwidth enables its use both for GPS and for GLONASS global positioning service.

Especially for situations where satellite antennas should be hidden invisibly, it might be necessary toembed the antenna into the skin of the car in a way that the antenna is not sticking out of the surroundingmetal surface. This leads to an increased focus of the radiation pattern to higher elevation angles, whichmight harm its application for GEO satellite signal reception. In case of the Scarabeus antenna, the high

ground plane

Fig. 10 Horizontal loop antenna

ground plane

Fig. 11 Scarabeus antenna


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efficiency nonetheless yields a sufficient reception quality even for SDARS signals as has been shown inKammerer and Lindenmeier (2013b, a) even for an antenna combination of SDARS and GPS, hiddeninvisibly in a squared cavity of 40 � 40 � 10 mm3. In numerous test drives in real reception scenarios,the Scarabeus antenna has proven a significantly larger reception quality due to its high efficiency togetherwith an antenna characteristic which is tailored to the needs of GEO satellite signal reception while stillenabling as well the reception of HEO satellite signals.

Due to its ring shape, Scarabeus antennas can be combined with each other for different services atdifferent carrier frequencies (Kammerer and Lindenmeier 2012, 2013a). The design allows also easycombination with antennas for terrestrial signal reception as, for example, mobile phone antennas or evenantennas for AM, FM, and DAB broadcast reception, which can be placed in the center of the Scarabeusantenna (Kammerer et al. 2012).

Circular Polarized Homogenous Field Loop AntennaIn the following an antenna structure will be considered which is useful for the reception of circularlypolarized signals transmitted via GEO satellites (Saala et al. 2009, 2010; Saala and Lindenmeier 2010).

015

30

45

60

75

90

−40 −20 −10 −5 0 4 7 10

Fig. 13 Radiation pattern of Scarabeus antenna (blue: LHCP; red: RHCP, green: total)

0 15

30

45

60

75

90

−40 −20 −10 −5 0 4 7 10

Fig. 12 Radiation pattern of loop antenna (blue: LHCP, RHCP not visible)


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This structure consists of a ring structure which is combined with a vertically polarized monopolestructure, as shown in Fig. 19.

In contrast to all the other antenna concepts shown here, the current distribution of this antenna ishomogenous with respect to azimuth. While the current distribution of the monopole with its verticaldirection is independent from azimuth anyway, an omnidirectional current distribution on the loop isachieved via insertion of series capacitors.

The monopole yields a vertically polarized field (theta direction) with omnidirectional characteristicsand constant phase in azimuth and a horizontal main beam direction, as shown in Fig. 20a in blue. Theloop antenna yields a horizontal polarized field (phi direction) also with omnidirectional characteristicsand constant phase in azimuth and a main beam direction close to 45� in elevation, as shown in Fig. 20a inred.

105 75

60

45

30

15

0

345

330

315

300

x

1202326 MHz

135

150

165

180

195

210

225

240

10

7

4

0

−5

−10

−20

−30−40

Fig. 14 Radiation pattern in the horizontal plane at 2.3 GHz (total field in vertical polarization)

Fig. 15 Hardware sample of a loop antenna for 2.3 GHz; length and width: 34 mm; height: 10 mm


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Exciting the monopole and the loop antenna together with a chosen weighting and a phase distance of90� to each other, a weighted and phased superposition of its characteristics is obtained. The resultingcharacteristics of a circularly polarized field are shown in the radiation patterns of Fig. 20b. The circularco-polarization (co-pol) of the complete antenna is shown in red, while the cross polarization (x-pol) isshown in blue. It can be seen that the cross-polarization ratio is dependent on the elevation angle.Depending on the power distribution between the horizontal loop and the vertical monopole, a minimum

Fig. 16 Hardware sample of a Scarabeus antenna for 2.3 GHz; length and width: 16 mm; height: 10 mm

7.00

0�

5.00

3.00

1.00

−1.00

−3.00

−5.00

−7.00

−9.00

−11.00

−15�

−30�

−45�

−60�

−75�

−90�

15�

30�

45�

60�

75�

90�

Fig. 17 Measured rad. pattern of loop antenna (LHCP and RHCP)

0�10.00

6.00

2.00

−2.00

−6.00

−10.00

−15�

−30�

−45�

−60�

−75�

−90�

15�

30�

45�

60�

75�

90�

Fig. 18 Measured rad. pattern of Scarabeus (LHCP and RHCP)


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in cross polarization can be shifted to an elevation angle of choice. While in vertical direction there is noreception possible, for the circular co-polarization this antenna structure achieves an especially high gainfor diagonal elevation angles around 45�.

Hence, this structure is ideally tailored to the reception of signals transmitted via GEO satellites. Withinthe required elevation angle range between theta = 65� and theta is 70�, a gain of more than 3 dB can beachieved in ideal conditions by the combination of those two antennas.

For real hardware structures, the efficiency is depending on the quality of the capacitors. For air-filledcapacitors, which can be realized in an air slot technology, losses of less than 0.8 dB are achieved. InFig. 21 a hardware sample of a circular polarized field loop antenna is shown. A typical measuredradiation pattern is shown in Fig. 22. The 10 dB bandwidth achieved is 28 MHz and its efficiency is1.5 dB.

According to the principle of duality, the abovementioned antennas can also be realized as slottedstructures. Dual to the crossed dipole there, one can also have a crossed slot structure in the upper surfaceof a cavity (Balanis 2005). This realization is as well applicable to the ring structure in the form of a slotring. Even the combination of dipoles and slots is possible as the next example will show.

Narrow Slot Dipole AntennaIn the following the combination of a slot antenna with a dipole antenna for mobile reception of circularlypolarized waves is shown, as has been introduced in M€uller et al. (2009). The structure is especiallyadvantageous in mounting situations where the antenna has to be integrated into a very narrow finlikeradome on the roof of a car.

ground plane

Fig. 19 Circular polarized homogenous field loop antenna

0a b15

30

45

60

75

90

6

3

−3

−6

−9

−12

−15

−18

−21

−24

0

6

3

−3

−6

−9

−12

−15

−18

−21

−24

0

015

30

45

60

75

90

Fig. 20 Vertical radiation pattern of circularly polarized field loop antenna; (a) linear polarization vertical (red), horizontal(blue); (b) circular co-polarization (red), x-polarization (blue); total gain (green)


Page 15 of 33

The basic concept is shown in Fig. 23. Like at a crossed dipole structure, two antenna parts yield twoshares of the field with linear polarization which are orthogonal to each other having a phase distance of90� to each other. While in the common crossed dipole structure (see Fig. 4), two dipoles are contributingthese linear polarizations, this is done here by a monopole and a slot antenna. While the dipole yields acurrent distribution corresponding to a field which is polarized in the same direction as the dipole, the slotantenna corresponds to a field with a polarization which is orthogonal to the slot direction. In order toobtain the required phase distance between the excitation of the two orthogonal field shares, the dipole isfed with that phase delay via a feed line with a length of l/4.

In Fig. 24 a hardware sample of this antenna is shown for SDARS at 2.3 GHz with a height of aroundl/4 (32 mm), a similar length of 30 mm, and a thickness of 1.5 mm. Figure 25 depicts a typical radiationpattern achieved by this structure.

For the first applications in SDARS signal reception, crossed frame antennas and patch antennas havebeen commonly used. Especially for the transmission via HEO satellites, patch antennas have proven tofulfill the requirements quite well. This led to the situation that the most common antennas for thisapplication were patch antennas which have been used in different variations. For combined GEO andHEO satellite systems, loop antennas like the Scarabeus antenna are increasingly used because of its highefficiency in combination with an easy fabrication. This is also true for the positioning services wherethese antennas offer a wide bandwidth, which offers its use for more than one service, as, for example,GPS and GLONASS, being covered by the same antenna.

Fig. 21 Hardware sample of homogenous field loop antenna (length and width: 32 mm; height: 20 mm)

0�5.00−15�

−30�

−45�

−60�

−75�

−90�

5 1 −3 −7 −11 −15 51−3−7−11−15

15�

30�

45�

60�

75�

90�

Fig. 22 Gain in dBic of the homogenous field loop antenna


Page 16 of 33

Environmental and Mounting Aspects, Antenna Combinations, andMultifunctional Structures

In the following the environment of satellite reception antennas is considered with respect to commonintegration technologies, common packaging and housing of multiple antennas and front-end parts,integration into narrow environments, and the investigation of coupling effects. In mobile applicationsthe close integration of front-endmodules and antennas in a narrow environment with a variety of possibleelectric and electromagnetic interferences is of high challenge. The small size of microwave antennas withlow directivity enables a common integration and close packaging of antennas and circuitry for ampli-fication of antenna signals or signal processing of multiple antenna signals in antenna diversity and smartantenna systems. In Fig. 26 a typical antenna combination is shown, in which terrestrial antennas for AMand FM broadcast and cell phone are combined with satellite antennas for SDARS and GPS signalreception underneath one common roof radome.

Coupling effects are prevented by inserting band-stop filters into a cell phone antenna. If terrestrialantennas can be positioned in the phase center of a satellite antenna, the modes of oscillation are

ground plane

λ/4

feed

Fig. 23 Slot dipole antenna concept

Fig. 24 Slot dipole antenna hardware sample


Page 17 of 33

decoupled from each other. This is possible with the Scarabeus loop antenna. As has been shown inKammerer et al. (2012) even for the combination of Fig. 26, the radiation characteristics of two Scarabeusantennas – one for SDARS and one for GPS –whose phase centers are coincident with the phase center ofa large AM-FM antenna are not harmed.

If a satellite antenna is integrated into plastic parts where no large ground plane is available and only asmall plate can be added with a diameter of 7.5–10 cm (300 to 400), a ripple will occur in the verticalradiation diagram, as can be seen for different examples in Fig. 27. While the dashed line shows thediagram for a large ground plane in the solid blue lines, a superimposed ripple is observed whose envelopeis mainly depending on the diameter of the ground plate. The number of ripples is depending on the heightof the antenna position over ground.

In Fig. 28 a result is shown for a small ground plate with diameter of only 65 mm with a high ripple,leading to deviations from required gain by around 3–4 dB. Since the demands on the antenna gain arevery close to that which is achievable for a large ground plane, it is obvious that in a situation with a smallground plate, the antenna will not be capable any more for SDARS satellite signal reception.

A similarly inferior situation where a single antenna would not fulfill the gain requirements in theregarded angle ranges also occurs if a satellite antenna is integrated in mounting positions, for example,inside the side mirror or on the dashboard of a car. In Fig. 29 the radiation pattern of an antenna is shownwhich is integrated in one side mirror. Not even on the right side of the diagram the required gain isfulfilled in all the angle ranges since values of gain below 0 dB at an elevation of theta = 60� are notacceptable. Because of shadowing and reflection effects, the gain is even further decreased on the left sideof the diagram in the direction toward the car.

Generally spoken in situations of special mounting positions –where the antenna cannot be mounted ontop of a large metallic plane – there are problems to keep the required reception quality of a single antenna.

0�6.00−15�

−30�

−45�

−60�

−75�

−90�

15�

30�

45�

60�

75�

90�

Fig. 25 Measured radiation pattern of the slot dipole antenna

Fig. 26 Antenna combination of satellite antennas for SDARS and GPS with terrestrial antennas for AM, FM broadcast, andphone underneath one common roof radome (Kammerer et al. 2012)


Page 18 of 33

In this case the use of antenna diversity is of great help, where critical mounting positions and changingenvironmental conditions can be treated by the use of alternative antennas in an adaptive way. In the nextsection antenna diversity for satellite radio reception is explained, which enables to keep a high receptionquality also in critical mounting situations and also for critical environmental situations around the car.

Antenna Diversity for Mobile Satellite Reception

The reception of satellite radio signals using single antennas generally works flawless in conditions withan LOS propagation path from the satellite transmitter to a stationary or mobile receiver. Still it can beseverely impaired in non-LOS scenarios (Parsons et al. 1975). The reasons for these impairments are acombination of attenuation due to signal shadowing and reflection, refraction, and scattering caused byobjects in and close to the direct transmission path. This leads to a received signal which is a superpositionof a multitude of waves with different directions, levels, as well as phases and causes statistically

010

7

4

0

−5

−10

−20

−30−40

90

75

60

45

30

Vertical plane15

ZZa b010

7

4

0

−5

−10

−20

−30−40

90

75

60

45

30

Vertical plane15

Fig. 27 Vertical radiation diagram of SDARS satellite antenna with small ground plane of 85 mm diameter for different height hover ground: (a) h = 1,5 m, (b) h = 0.5 m

0 Z Vertical plane10

7

4

0

−5

−10

−20

−30−40

90

75

60

45

30

15

Fig. 28 Vertical radiation diagram of SDARS satellite antenna with small ground plane of 65 mm diameter at 0.5 m heightover ground


Page 19 of 33

distributed deep signal fades along the driving paths (Parsons et al. 1975; Brennan 1959). An example ofsuch a scenario and the resulting signal levels is depicted in Fig. 30.

The probability of the deep fades is lower with a strong LOS component and increases with increasingattenuation of the direct signal. A very high probability of deep fades exists in the absence of an LOSsignal component. Mathematically the probability distribution in a fading scenario can be described with aRice distribution where an LOS component is present. In non-LOS conditions this reduces to a Rayleighdistribution with a much higher probability of low signal levels. The Rayleigh distribution of theprobability fR of signal amplitudes r in such a scenario can be described as

f R r,Oð Þ ¼ 2r

O� e�r2

O

whereO denotes the total power of the signal. The resulting distribution in dependence of the signal powerO is displayed in Fig. 31. The area below the probability distribution is 1 in all cases. When the total signalpower is high (Ω = 4), the probability distribution is flat meaning that a broad variety of signalamplitudes is experienced at the receiver. With smaller total signal power (e.g.,Ω = 0.5), the probabilityof high signal amplitudes is very low, whereas small amplitudes are very common. Assuming a signaloutage when the amplitude is below rcrit (and amplitudes in the positive range including 0), the probabilityfor outages is calculated using the cumulative distribution function

pr�rcrit ¼ðrcrit0

f R r,Oð Þdf ¼ 1� e�rcrit

2

O

Resulting values for pr�0.5 for different values of Ω are given in Table 2.Every single antenna will experience such a distribution of signal levels, and thus, deep signal fades

along a driving path through this kind of fading scenario. This can be seen in Fig. 32 which shows tworecorded antenna signal levels relative to the LOS level in a Rayleigh fading scenario. The signals havebeen recorded during test drives in a real fading scenario in the USA.

−105° 105°

−90° 90°

−75° 75°

−60° 60°

−45° 45°

−30° 30°

−15°0°

15°

Fig. 29 Vertical radiation diagram of SDARS satellite antenna with small ground plane of 65 mm diameter at 0.5 m heightover ground


Page 20 of 33

A possibility to avoid the deep fades and therefore increase the reception quality significantly is givenby antenna diversity systems (Brennan 1959; Parsons et al. 1975). Assuming two antennas are completelyuncorrelated and both show a mute probability of pm, then the probability of a mute on both antennassimultaneously can be calculated as pm

2. Therefore if each single antenna has a mute probability of 0.1,this means that two independent antennas have a mute probability of 0.01 and three antennas only have0.001. Of course the reduction of the probability of mutes with an increasing number of antennas alsoincreases the cost and mounting volume of the system so that a compromise has to be made.

The complexity of antenna diversity systems ranges from a lean switching diversity to the mostcomplex form which is given by a maximum ratio combining diversity system. All systems have incommon that their effectivity increases with antenna signals that are less correlated. This decorrelationcan, for example, be ensured by a significant dislocation of the antennas’ phase centers (Parsonset al. 1975). In practical applications it is much preferred to ensure decorrelated signals by using differentpolarizations, antenna patterns, or other means that do not necessitate in multiple mounting positions tobe used.

Antenna Diversity Basic ConceptsSwitching or scanning diversity simply avoids deep fades by switching between the available antennasignals (Barié et al. 2010; Lindenmeier et al. 2007; Lindenmeier 2007). A block diagram is depicted inFig. 33. Switching is either initiated if the signal level of the currently selected antenna falls below athreshold or if another signal level is higher. In the case where always the best signal is selected, thediversity signal follows the highest of all input signal levels as it is depicted in Fig. 37.

Due to the phase modulation of the satellite radio signals and the decorrelation of the received antennasignals phase, differences occur between the antennas. Without further measures this would lead to biterrors and thus audio mutes after switching. In order to avoid these impairments, switching needs to be

0

S/N

x

Fig. 30 Signal reception in multipath fading scenarios; red and orange: reception signals of two diversity antennas; blue:improved signal level with diversity


Page 21 of 33

synchronized to the phase reference symbols that are transmitted regularly (Barié et al. 2010). These aremade to inform the receiver of the current absolute phase of the received signal thus allowing for thecorrect decoding of the transmitted information.

Overall, switching diversity can be realized in a quite lean diversity system, as it is shown in Fig. 33.Due to the frequency dependence of the transmission path, a channel-selective level detection circuitry isnecessary. The signal does not need to be decoded in order to achieve this information; thus, a simple local

0 0.5 1 1.5 2 2.5 30

0.5

1

1.5

2

signal amplitude

prob

abili

ty d

ensi

ty

Ω = 0.5

Ω = 1

Ω = 2

Ω = 3

Ω = 4

Fig. 31 Rayleigh distribution of signal amplitudes

150 150.05 150.1 150.15 150.2 150.25 150.3−20

−15

−10

−5

0

Time (s)

Sig

nal l

evel

rel

ativ

e to

LO

S (

dB)

Fig. 32 Signal levels of two antenna signals in a Rayleigh fading scenario relative to the LOS signal level

Table 2 Probability of signal outages assuming a Rayleigh distribution. The critical amplitude rcrit = 0.5

O pr�rcrit in %

0.5 39.4

1 22.1

2 11.7

3 8.0

4 6.1


Page 22 of 33

oscillator suffices without the need of phase and frequency locking to the signal carrier. But due to thenecessity of synchronizing the switching activity to the phase reference symbol, a cooperative receivermust deliver the respective timing information.

The ability of the diversity circuit to select another antenna if one of the antennas is in a deep fadingleads to the prevention of the main adverse effect of Rayleigh fading. This is why without a phasealignment and superposition of antenna signals, the pure switching between the antenna signals andchoosing the best antenna signal in each moment yield a considerable improvement in reception quality,as it has been shown in Lindenmeier et al. (2007), Lindenmeier (2007), and Barié et al. (2010).

Another diversity concept which is well known is phase diversity (or equal gain diversity), where thereis no selection of antenna signals. Instead they are continuously superimposed after phase alignment.A block diagram is depicted in Fig. 34. Phase shifters in all antenna paths are needed to allow for thesuperposition of the signals. Also a channel-selective phase detection must be done for each pathseparately. Due to the complexity of this component, it is commonly situated in the receiver. Thereforeseparate RF cables and tuners are necessary for each antenna. Due to this restriction not more than twoantennas have been usually considered for such a radio reception system. In cases where two inputantenna signals have equal levels (in signal and noise, respectively), a phase diversity system allows for animprovement of the SNR of 3 dB. Lower improvements are achievable with signal levels differingstronger. A disadvantage of such a system is that the SNR of the combined signal can even be lower thanthat of the better input signal if the other signal has a very low signal level but a similar noise level (e.g., ina deep fade). This means that despite the very high effort to realize such a system, the improvement isstatistically not better than the one of a switching diversity, which will be shown by example of signallevels depicted in Fig. 37.

A phase diversity system with maximum ratio combining phase-aligns the antenna signals beforesuperimposing them. A (continuously valued) weighting factor is applied to each signal which is derivedfrom its SNR. A block diagram is depicted in Fig. 35. By using phase alignment as well as signalweighting, negative contributions to the combined SNR by input signals with a low SNR are avoided.This allows for the highest possible SNR of the combined signal using the given input signals and explainsthe name of this system. Figure 37 shows signal levels of single antennas as well as of several diversitysystems one of which is an MRC system. The signal levels of an (ideal) MRC diversity system are thehighest to be achievable.

Nevertheless, the difference between the results of MRC and the other diversity systems is very low incritical moments of fading at an antenna. This means that the effect of improvement of reception quality isnot much higher than the one of the other diversity concepts.

diversityantenna module

RF -cable

receiver

tuner

sync

decoder

channel-selective level

detection

Fig. 33 Switching diversity system


Page 23 of 33

On the other hand, a great disadvantage of this system is its very high complexity. Phase shifters as wellas variable gain or attenuation stages are needed in every signal path. Also a complex SNR as well asphase measurement is needed for each signal independently. This leads to separate RF cables and tuners aswell as an increased requirement in signal processing power in such systems.

A scan-phase diversity system combines the advantages of a simple switching diversity with thecapabilities of a phase diversity (Lindenmeier et al. 2008; Barié 2008). This combination almost reachesthe performance of an MRC diversity but with a much reduced hardware effort. The block diagram isgiven in Fig. 36. It shows the phase shifters and switches as well as the signal combining. In contrast to theMRC system, simple switches deactivate signals with a negative contribution to the SNR of the combinedsignal. Also phase shifters with discreet phase steps are sufficient due to the low order of the satellitesignal’s PSK modulation (usually QPSK is used) (Senega et al. 2010). Only a minor degradation ofperformance results from these steps that on the other hand significantly decrease the overall systemcomplexity. The selection of antenna signals and the calculation of phase differences can be based onsignal levels to further reduce the complexity of the detection circuitry by avoiding SNR and phasemeasurements.

The overall complexity is low enough to allow for the implementation of the complete system in theantenna module itself (Senega and Lindenmeier 2012). Only a single RF cable connects the antennadiversity module to the receiver. The diversity circuit is run independently of the receiver, requiring nofurther cooperation. Hence, regarding the interface to an SDARS receiver, it behaves like a single standardantenna. Therefore any receiver can be connected to such an antenna systemwithout adaptation in order toimprove the reception quality significantly compared to a single antenna.

antenna-module

RF-cable

receiver

tuner

tuner

decoder+

ϕ

ϕ

channel-selective phase

detection

Fig. 34 Equal gain phase diversity system

antenna-module

RF-cable

receiver

tuner

tuner

decoder+

ϕ

ϕ


and phasedetection

Fig. 35 Maximum ratio combining diversity system


Page 24 of 33

Signal levels of two single antennas as well as of a scan-phase diversity system are depicted in Fig. 37.The signal levels achieved with the scan-phase diversity system are approximately of the same values likethose of an MRC diversity system with a very little average deviation of less than 0.4 dB.

Comparison of Diversity SystemsThe antenna diversity systems described above show significant differences in their complexity. Table 3gives a short summary of some of the most important and complex subcircuits which are needed in thedifferent antenna diversity systems.

Regarding the improvements of signal reception which are possible using the presented diversitysystems, Fig. 37 shows a short level history of recorded antenna signals as well as of simulated signallevels of the diversity systems. All of them avoid most of the deep signal fades because they use more thanone antenna. Only in cases where all antennas experience a deep signal fade at the same time a deep fade inthe diversity signal occurs.

Scanning diversity always shows the signal level of the best single antenna. Deep fades are avoidedunless all input antenna signals fade at the same time (a total signal blockage). Due to phase differences ofthe uncorrelated antenna signals, a synchronization to the transmitted reference phase is mandatory insuch a system which necessitates in a modified SDARS receiver. In contrast to a scanning diversitysystem, equal gain diversity can improve the signal level up to 3 dB above that of the two input signals ifthey show similar levels (e.g., around sample 10,150 in Fig. 37). But due to the lack of signal weighting,the SNR is lower than that of the best input signal in cases where one signal level is much lower than theother. In these cases an equal gain diversity shows worse output SNR than even a simple switchingdiversity system (e.g., around sample 10,125 in Fig. 37).

An ideal MRC system of course shows the highest improvement of all diversity systems – as it can beseen in Fig. 37 – due to its ability to weight the antenna signals before their phase-aligned combination.But although the hardware complexity of anMRC diversity is much higher, the improved signal quality ofthe scan-phase diversity system is comparable to that of an MRC diversity system. An analysis ofrecorded signal levels with calculated ideal diversity systems shows only a small deviation of less than0.4 dB in average between the MRC system and a scan-phase diversity system (Lindenmeier et al. 2013).

While the results shown in Fig. 37 are true for the simulated diversity systems in reality, the describedsystems react to changing signal conditions on very different time scales. While switching and scan-phasediversity provide almost immediate responses to deep fades, the phase and SNR estimation invoked inequal gain and MRC diversity usually take much longer and therefore impede fast reaction times. Thisbecomes increasingly problematic when higher driving speeds are considered. Investigations onswitching speed and the resulting diversity efficiency show the importance of fast reaction times (Bariéet al. 2010). Figure 38 shows the degradation of the effective number of antennas for switching time

RF-cable

frequencyconversion

receiver

tuner

diversity antenna module

decoder+

ϕ

ϕ


detection

Fig. 36 Scan-phase diversity system


Page 25 of 33

periods in the range of 137 ms to 2.2 ms. Up to a time period of 275 ms, almost constant numbers ofeffective antennas are achieved, while for 2.2 ms the diversity system shows no improvement of receptionquality at all. This investigation was done for a speed of 104.6 km/h (65 mph) so that a distance of l atSDARS frequencies is covered in 4.4 ms. Therefore reaction times equivalent to distances of less thanl/8 at the highest driving speed considered are necessary to ensure sufficient efficiency of diversityreception systems.

The necessity of fast reaction times to changing signal conditions is obvious from the signal levelhistory depicted in Fig. 37 as well as by the effective number of antennas depicted in Fig. 38. A logicalconsequence of this fact is that slow beam steering of an antenna combination will not yield noteworthyimprovements of signal levels in a fading scenario. The maximum gain which is possible by such ameasure of course is 3 dB. While this might seem like a huge improvement at first glance, the fast-changing phases of the individual signals in such a scenario prohibit the combination of the antennas withslow reaction times so that the combined antenna will itself behave like a single antenna and experiencedeep signal fades which by far are much worse than the 3 dB gained by combination.

In contrast to a slow combination, the fast selection and superposition of antenna signals that is done ina scan-phase diversity system will significantly increase the SNR of the combined signal even in fadingscenarios. While this gain in SNR could be seen as a bonus that improves the audio availability, anotherpossibility is that such a gain could be considered in the system design because it is equivalent to anincreased link margin of the transmission path. This increased link margin allows for the reduction ofother error-correcting redundancies in the signal so that more information could be transmitted instead.Users would then benefit in the form of additional audio channels or higher audio quality.

Table 3 Number of a selection of subcircuits needed in different diversity systems with n antennas (Senega 2013)

Scanning div. Equal gain div. MRC div. Scan-phase div.

Ant.-LNAs n n n n

RF cables 1 n n 1

Tuners 1 n n 1

Frequency conv. – – – 1

Measured parameter Level Phase Level + phase Level

1.005 1.01 1.015 1.02

x 104

−21

−18

−15

−12

−9

−6

−3

Samples

Rel

ativ

e S

igna

l Lev

el (

dB)

Ant 1Ant 2MRCScan DivScan−Phase DivPhase Div

Fig. 37 Signal levels of two single antennas and different simulated ideal diversity systems


Page 26 of 33

Satellite Antenna Structures for DiversityIn the following concepts for compact antenna, diversity structures are considered, in which the RF signalprocessing units and antennas structures are integrated together in a common mounting volume at onesingle position at the vehicle. Particular attention has to be paid to the decorrelation of the antenna signals.In the following examples for antenna, diversity structures are shown, which are advantageous for aphased superposition of their antenna signals with an optimized signal to noise ratio in order to increasethe reception quality significantly in comparison to a common single antenna.

New fabrication technologies enable the integration of three antenna structures in one part in amounting volume of 29 � 29 � 17 mm3 as shown in Fig. 39. The three antennas which are printedonto a common carrier consist of a horizontal loop antenna type together with a circular polarizedhomogenous field loop antenna and a vertically polarized monopole structure which yield radiationpatterns being orthogonal to each other.

Such a set of three antennas being also presented in M€uller et al. (2010a) provides three highlydecorrelated reception signals for the diversity processor. After phase alignment of the antenna signalsand superposition, a combined radiation pattern of the three antenna array (blue line) can be seen in Fig. 40in comparison to the gain of a standard patch antenna (dashed black line). It shows that in LOS situationsthe three antennas together yield a gain which is overall around 2–3 dB higher than the one of a commonpatch antenna.

In a Rayleigh fading scenario, this additional gain would not help to increase the SNR if the main beamdirection of the common antenna pattern would be only directed toward the satellite. Together with thediversity circuits shown above, the alternative antenna signals are combined in a way that even in a deepfading scenario – which is most critical for reception quality – the fading can be prevented. This is onlypossible if the antenna signals are strongly decorrelated, so that fading effects would not occur at the sametime at all the alternative antennas of the diversity set. The radiation patterns of the different antennasshow a sufficient orthogonality to each other, and hence, the antennas are decoupled from each other byvalues of more than 25 dB.

Similar values have been achieved with an antenna diversity structure as it is presented in M€ulleret al. (2010b) requiring a mounting volume of only 29 � 29 � 11 mm3 and consisting of a circularpolarized homogenous field loop antenna together with a patch antenna. In Fig. 41 this antenna set is

Fig. 38 Diversity efficiency is high for fast-switching systems


Page 27 of 33

shown in a mounting position on the dashboard of a car. Figure 42 shows the measured radiation pattern ofthe combined diversity antennas in LOS conditions, which occurs depending on the phase constellationbetween the two antennas. The blue line shows the main lobe which can be steered onto any angle inazimuth via the phase distance between the two diversity antennas.

Measurements of Scan-Phase Diversity in Fading ScenariosTest drives have been done in Rayleigh fading scenarios in the USA in order to assess the functionality ofthe scan-phase diversity system. For these test drives a hardware demonstrator was created and connectedto a multitude of different antenna diversity sets located in various mounting positions. Also the receptionof HEO and GEO satellites has been evaluated separately in order to show that both can benefit from sucha diversity reception system. Pictures of two of the demonstrators used in these measurements are shownin Fig. 43.

Measurement results using a diversity antenna set to receive GEO satellite signals on the rooftop of acar show that the scan-phase diversity system with two antennas significantly improves the availability ofthe audio signal compared to a single standard patch antenna. Recorded audio levels of a typical result aredepicted in Fig. 44. During a test time of 430 s, the mute time of 67.1 s with the single antenna is reducedto only 5.1 s using the scan-phase diversity system. Longer test drives over a duration of 15 min evenshow mute times as low as 10.9 s resulting in an availability of 99.1 % (Senega and Lindenmeier 2011).

Fig. 39 Three-antenna diversity set (29 � 29 � 17 mm3) fitting into an ESD cover

10 1090

75

60

45

30

15Z

7 74 40 0−5 −5−10 −10

Fig. 40 Radiation pattern of the combined diversity antenna set (blue line) in comparison with the pattern of a typical singleantenna for SDARS reception (dashed black line)


Page 28 of 33

Measurements of a single HEO satellite show an improvement of availability from 86.8 % with a singleantenna to 97.8 % with diversity (Senega et al. 2009).

Apart from the significant improvement of audio availability when rooftop mounting is considered,diversity allows for the consideration of mounting positions that would be unthinkable of using a singleantenna. A typical application of such mounting positions would be in convertibles (where rooftopmounting is impossible) or cars where the rooftop mounting space is otherwise restricted or unavailable.Test drives with a diversity set mounted on the dashboard of a car (see Fig. 41) show that diversity caneven outperform a rooftop mounted single antenna. The availability is improved from 84 % with thesingle antenna to 86 % although the reference antenna was placed on the metallic roof of the same test car(Senega and Lindenmeier 2012).

Even mounting the antenna set in a single automotive side mirror has been investigated showing againeven better results than a rooftop mounted reference antenna in the reception of a GEO satellite. The testdrives have been conducted on a straight road in both directions east to west and west to east so that also inthis test drive, the shadowing of the satellite by the car itself has been considered. The availability isimproved from 96.7 % with the single antenna to 98.7 % with the diversity system (Senega et al. 2014).

Fig. 41 Diversity antenna function demonstrator in a mounting volume of 29 � 29 � 11 mm3 on dashboard of a test car

−10° 0° 10°−20° 20°

−30° 30°−40° 40°

−50° 50°

−60° 60°

−70° 70°

−80° 80°

−90° 90°

Fig. 42 Measured radiation pattern of combined diversity antennas; blue line: main lobe which can be steered onto any anglein azimuth via the phase distance between the two diversity antennas


Page 29 of 33

Conclusion

Together with classical antenna concepts, new antenna types for reception of signals of HEO, MEO, andGEO satellite systems have been explained which yield high efficiency, low mounting volume, tailoredradiation patterns, and easy manufacturing to meet the special requirements for satellite signal reception ina vehicle. Especially, loop antennas like the Scarabeus antenna yield an easy combination with terrestrialantennas in close proximity, which can be set into its phase center. For satellite digital radio services,where gain requirements are relatively high, diversity concepts have been shown which help to improvethe reception quality in critical reception scenarios considerably. This is especially achieved with aconcept of a scan-phase diversity which can be realized easily and which has got a simple interface tothe radio which is equivalent to one of a common single antenna.Within a similar mounting volume as it isalready used for single satellite reception antennas and its amplifiers, the complete diversity system can beintegrated, consisting of up to three antennas and the diversity circuit. The use of diversity systems alsoenables the use of inferior mounting positions in the car, where no single antenna could achieve therequired reception quality.

Fig. 43 Scan-phase diversity system demonstrators for up to three antenna signals: (a) first demonstrator board; (b) test boardof actual 4 � 4 mm2 diversity IC

0 50 100 150 200 250 300 350 4000

0.5

1

Time (s)

Ant. 1

Mute time: 67.1 sAvail.: 84.8 %

0 50 100 150 200 250 300 350 400

0

0.5

1

Time (s)

Div

Mute time: 5.1 sAvail.: 98.8 %

Fig. 44 Audio signal levels recorded in a Rayleigh fading scenario using a single antenna reception system (red) compared toaudio levels from a scan-phase antenna diversity system (green)


Page 30 of 33

Cross-References

▶Antenna Design for Diversity and MIMO Application▶Circularly Polarized Antennas▶Loop Antennas▶Low-Profile Antennas▶Microstrip Patch Antennas

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Satellite Antennas on Vehicles...Satellite Antennas on Vehicles Stefan Lindenmeier* and Simon Senega Institute of High Frequency Technology and Mobile Communication, Universit€at

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