Intro Microwave

Microwave technology is vastly used today especially in broadcast and

telecommunications as relays and satellite. A microwave system is widely used for its

practicality in terms of economic costs, flexibility, and reliability. It is a form of electromagnetic

radiation with a frequency ranges from 300MHz to 300GHz. Due to the high frequency, more

information can be carried making it ideal for high data rate applications. In addition, this high

frequency also limits microwave transmission to a line of sight between the transmitter and

receiver. It is not able to penetrate the earth’s surface requiring the placement of repeaters for

long ranges.

Terrestrial Microwave

Terrestrial microwave communication employs Earth-based transmitters and receivers. The frequencies used are in the low-gigahertz range, which limits all communications to line-of-sight. You probably have seen terrestrial microwave equipment in the form of telephone relay towers, which are placed every few miles to relay telephone signals cross-country.

Microwave transmissions typically use a parabolic antenna that produces a narrow, highly directional signal. A similar antenna at the receiving site is sensitive to signals only within a narrow focus. Because the transmitter and receiver are highly focused, they must be adjusted carefully so that the transmitted signal is aligned with the receiver.

A microwave link frequently is used to transmit signals in instances in which it would be impractical to run cables. If you need to connect two networks separated by a public road, for example, you might find that regulations restrict you from running cables above or below the road. In such a case, a microwave link is an ideal solution.

Some LANs operate at microwave frequencies at low power and use nondirectional transmitters and receivers. Network hubs can be placed strategically throughout an organization, and workstations can be mobile or fixed. This approach is one way to enable mobile workstations in an office setting.

In many cases, terrestrial microwave uses licensed frequencies. A license must be obtained from the FCC, and equipment must be installed and maintained by licensed technicians.

Terrestrial microwave systems operate in the low-gigahertz range, typically at 4-6 GHz and 21-23 GHz, and costs are highly variable depending on requirements. Long-distance microwave systems can be quite expensive but might be less costly than alternatives. (A leased telephone circuit, for example, represents a costly monthly expense.) When line-of-sight transmission is possible, a microwave link is a one-time expense that can offer greater bandwidth than a leased

circuit.

Costs are on the way down for low-power microwave systems for the office. Although these systems don’t compete directly in cost with cabled networks, when equipment frequently must be moved, microwave can be a cost-effective technology. Capacity can be extremely high, but most data communication systems operate at data rates between 1 and 10 Mbps. Attenuation characteristics are determined by transmitter power, frequency, and antenna size. Properly designed systems are not affected by attenuation under normal operational conditions—rain and fog, however, can cause attenuation of higher frequencies.

Microwave systems are highly susceptible to atmospheric interference and also can be vulnerable to electronic eavesdropping. For this reason, signals transmitted through microwave are frequently encrypted.

Microwaves are electromagnetic waves with wavelengths longer than those of terahertz (THz) frequencies, but relatively short for radio waves. Microwaves have wavelengths approximately in the range of 30 cm (frequency = 1 GHz) to 1 mm (300 GHz). This range of wavelengths has led many to question the naming convention used for microwaves as the name suggests a micrometer wavelength. However, the boundaries between far infrared light, terahertz radiation, microwaves, and ultra-high-frequency radio waves are fairly arbitrary and are used variously between different fields of study. The same equations of electromagnetic theory apply at all frequencies. Apparatus and techniques may be described as "microwave" when the wavelengths of signals are roughly the same as the dimensions of the equipment, so that lumped-element circuit theory is no longer accurate. The term microwave generally refers to "alternating current signals with frequencies between 300 MHz (3×108 Hz) and 300 GHz (3×1011 Hz)."

Satellite Microwave

Satellite microwave systems relay transmissions through communication satellites that operate in geosynchronous orbits 22,300 miles above the earth. Satellites orbiting at this distance remain located above a fixed point on earth.

Earth stations use parabolic antennas (satellite dishes) to communicate with satellites. These satellites then can retransmit signals in broad or narrow beams, depending on the locations set to receive the signals. When the destination is on the opposite side of the earth, for example, the first satellite cannot transmit directly to the receiver and thus must relay the signal through another satellite.

Because no cables are required, satellite microwave communication is possible with most remote sites and with mobile devices, which enables transmission with ships at sea and motor vehicles.

The distances involved in satellite communication result in an interesting phenomenon: Because all signals must travel 22,300 miles to the satellite and 22,300 miles when returning to a re-ceiver, the time required to transmit a signal is independent of distance. It takes as long to

transmit a signal to a receiver in the same state as it does to a receiver a third of the way around the world. The time required for a signal to arrive at its destination is called propagation delay. The delays encountered with satellite transmissions range from 0.5 to 5 seconds.

Unfortunately, satellite communication is extremely expensive. Building and launching a satellite can cost easily in excess of a billion dollars. In most cases, organizations share these costs or purchase services from a commercial provider. AT&T, Hughes Network Services, and Scientific-Atlanta are among the firms that sell satellite-based communication services.

Satellite links operate in the low-gigahertz range, typically at 11-14 GHz. Costs are extremely high and usually are distributed across many users by selling communication services. Bandwidth is related to cost, and firms can purchase almost any required bandwidth. Typical data rates are 1-10 Mbps. Attenuation characteristics depend on frequency, power, and atmospheric conditions. Properly designed systems also take attenuation into account—rain and atmospheric conditions might attenuate higher frequencies. Microwave signals also are sensitive to EMI and electronic eavesdropping, so signals transmitted through microwave frequently are encrypted.

Earth stations can be installed by numerous commercial pro viders. Transmitters operate on licensed frequencies and require an FCC license.

By "microwave" we mean the range of radio frequencies between about 1 GHz (one gigahertz, or one billion oscillations per second) and about 300 GHz. For comparison, television transmissions normally occupy frequencies below the microwave region, from about 50 Mhz to 600 MHz (one Megahertz is one million oscillations per second, one GHz is 1,000 MHz). Cellular telephones operate in two bands, one from about 800 to 900 MHz and another around 1.8 to 1.95 GHz, again just below this definition of microwave frequencies.

A 30 centimeter wavelength is equivalent to 1 GHz (to convert from frequency to wavelength, just divide the speed of light 300,000,000 meters per second by the frequency in cycles per second to get meters of wavelength).

Radio Waves

Electromagnetic radiation is a wave that combines electric and magnetic fields, moving out from its source as an expanding sphere and having waves as the fields alternate in value. Its formal name is Transverse Electro Magnetic wave, or TEM. This kind of radiation has different utility as its wavelength changes.

Waves of a very long wavelength, such as thousands of meters, tend to travel along the surface of the earth and even penetrate into the water. These are useful for communication with submarines, and for broadcasting time signals. Broadcast radio, short-wave radio, television, cellular telephones, walky-talkies, 2-way police radios, satellite television, and other such communication/broadcast systems all use electromagnetic radiation, or "Radio Frequency Waves". Each communication service uses a part of the spectrum that is suitable for its needs.

Light, infra-red heat, ultra-violet (black light), and even X-rays and Gamma-rays are all forms of electromagnetic waves. All of these last forms are thousands of times shorter wavelengths than

the shortest wavelengths of microwaves. They also behave in ways where thier particle-like nature becomes apparent. The discovery of the dual nature of light waves has led to significant discoveries in physics (see below: What are photons?).

Early Microwaves Surprisingly enough, some of the first electromagnetic experiments conducted by Heinrich Hertz in 1886 and also by Marconi used frequencies near the microwave region - some around 500 MHz and some even in the multiple GHz (Gigahertz) region. By the way, the current unit frequency "Hz" is the "Hertz" named after Heinrich. GHz is pronounced "Giga Hertz". There were some very interesting experiments conducted in 1895 with frequencies/wavelengths that are even today considered a challenge.

Communication using electromagnetic radiation (except for light) began early in this century, and most early practical systems used very long wavelengths (low frequencies) which traveled great distances. Eventually, electronics were developed, including the vacuum tube (or "valve") which allowed controlled frequencies and modulation. This led to the use of higher frequencies, many channels, and commercial and industrial radio. During the 1930's and 1940's various experimenters discovered that higher frequencies could bring other advantages to communications. Some of these experimenters were government agencies and the military - some were universities, and some were private individuals.

Among these discoveries were that microwaves are easier to control (than longer wavelengths) because small antennas could direct the waves very well. One advantage of such control is that the energy could be easily confined to a tight beam (expressed as narrow beamwidth). This beam could be focused on another antenna dozens of miles away, making it very difficult for someone to intercept the conversation. Another characteristic is that because of their high frequency, greater amounts of information could be put on them (expressed as increased modulation bandwidth). Both of these advantages (narow beamwidth and modulation bandwidth) make microwaves very useful for RADAR as well as communications.

Eventually, these qualities led to the use of microwaves by the telephone companies. They placed towers every 30 to 60 miles each with antennas, receivers and transmitters. These would relay hundreds or even thousands of voice conversations across the country. The ability to modulate with a wide bandwidth permitted so many conversations on just one signal, and the reduction in beamwidth made this reasonably secure. In the 1950s experiments were conducted that showed the potential to connect the two coasts of the US via these microwave circuits to produce television programming on a continental basis, and true television networks were born.

Amateur radio interests in microwaves have mostly been for the challenge of working with such esoteric frequencies that require specialized techniques in design, fabrication and testing. Furthermore, in order to reach beyond LOS (line-of-sight) amateurs have spent countless hours carefully measuring propagation phenomena. Amateurs have carried on conversations using 10GHz well over 1,000 miles, and have bounced signals at that frequency off the moon. For more information about amateur radio uses of microwaves set your browser to www.wa1mba.org, contact a local VHF/Microwave Amateur radio club, or contact the ARRL.

What are photons, and how does this relate to microwaves? Someone recently asked: does the word "photon" only apply to light? And if so, what word should be used when referring to microwave energy?

A photon is a quantum of electromagnetic energy. Physicists think of electromagentic energy as having a "dual nature", in that some experiments reveal its nature as a particle which we call a photon and other experiments reveal its nature as a wave.

When it comes to lower frequencies (longer wavelengths), such as microwaves, VHF, and the like, it becomes much less convenient to think of energy in the form of photons, but there is no specific reason to decide that only one nature exists at these longer wavlengths. Sometimes photons are reffered to when describing an RF interaction with matter. The author does not know of any other word to describe the particulate nature of a propogating RF energy field except "photon". When the interaction with matter converts the energy into a mechanical form, we sometimes refer to the energy packets as "phonons". This is not a propogating Electro-Magnetic (EM) field, but rather a sound wave, and at the most minute level, even mechanical energy is quantized.

In most antenna, transmission line, waveguide, and quasi-optic formulations, the EM field is described according to its wave-like nature. When dealing with the interaction bewteen a microwave field and a molecule of Oxygen (for instance), in order to understand just why there are specific resonant freqencies of the molecule, a quantized nature re-appears, and the notion of the field expressed as photons can make sense.

The interactions between matter and EM fields have clearly different properties when comparing the interaction that causes a change in mechanical vibration with the interaction that causes a change in electron orbital state. The first occurs in the microwave and millimeter wave range - such as the serious absorption of 22 GHz signals by water vapor in the atmosphere. Here the interaction causes vibration and heat. To cause changes in electron orbital states, infrared, visible and UV range wavelengths are involved - such as is evidenced by floresence and lasers. In these cases much more than conversion to heat occurs. We call the second group of wavelengths "light" and the word "photon" is derived from Greek for light.

How are the higher frequencies designated? What is meant by "Millimeter waves"?

People often misuse the term "millimeter waves" to mean any microwaves of higher frequencies than those normally used.

Here are some frequency bands, exact frequencies and approximate wavelengths.

Band Starts at Ends at

VHF 30 MHz 300 MHz

- 10 meters 1 meter

UHF 300 MHz 3 GHz

- 1 meter 10 cm< /TR>

SHF 3 GHz 30 GHz

- 10 cm 1 cm

EHF 30 GHz 300 GHz

- 1 cm 1 mm

Sub Millimeter 300 GHz Infinity

- 1 mm zero

EHF is considered "Millimeter Waves" because the range of wavelengths is between 1 mm and 1 cm. Above 300 GHz is considered "Sub-millimeter waves" because the wavelength is below one millimeter. Frequencies above 1 THz (1000 GHz) are called "Terahertz frequencies" or "Very long wave infrared"

Various designators and names have been associated with the wavelengths between 300 GHz and 300 THz. They usually are called Infrared in one form or another, but as you can see, this range covers three orders of magnitude - or one thousand fold.

300 THz - or 1 um (one micrometer, or 1000 nano-meters, or 10,000 angstroms) is still infrared, because visual sensitivity begins around 480 THz or about 625 nm which is a very very deep red color.

Amateur Radio Bands above 30 MHz

VHF ham bands = 50 MHz, 144 MHz, 222 MHzUHF ham bands = 432 MHz, 903 MHz, 1296 MHz, 2304 MHzHF ham bands =3456 MHz, 5760 MHz, 10368 MHz, 24192 MHzEHF ham bands = 47 GHz, 80 GHz, 120 GHz, 145 GHz, 240 GHz

Sub millimeter wave ham bands = One band defined as all frequencies above 300 GHz. Therefore, a contact at 411 GHz is in the same ham band as a red laser and the same as a blue laser and the same as an X-ray contact.

THE MICROWAVE BANDS...  

The 10GHz BAND.

I have worked many stations on this fine band. In the 1970's I began with simple klystron and Gunn diode transceivers but moved up to ssb/cw in 1991 on acquiring a surplus MA/Comm unit which was easily modified to become a 10GHz transverter. It is driven with just a few milliwatts of 144MHz from an Icom IC202S hand held transceiver. I added  a two stage GaAsFET preamplifier and a 1 watt RF amplifier to this 10GHz transverter. In 2000, after almost ten years of using the MA/Comm surplus unit, I made a new, more powerful (5 watt) transverter based upon a kitset from DB6NT.

Under normal conditions, from a good location, the range of the 1 watt gear is in the order of 400km but much greater distances have been achieved during enhanced conditions. Enhancements occur during rainstorms (= "rainscatter") and tropospheric ducts. In October 1994, there was a period of  outstanding "tropo" during which I was able to work over 600km into Germany. Stations in the Netherlands were also contacted. Other UK amateurs worked much further, many over 1100km! My gear at this time was /portable in my bedroom(!) with the dish just aimed through a double-glazed window!

This photo shows my first 10GHz portable 1 watt ssb/cw transverter with 60cm offset fed dish antenna.

The assembly is mounted on a sturdy tripod which has a graduated compass scale to allow rapid setting of beam headings...important when it is raining and a quick dash from the VW Camper wasneeded to reset the dish heading!

This second photo shows the VW and 10GHz microwave gear set up on the top of Alport Height in Derbyshire, ready for the start of a 10GHz Cumulative Contest. Click on it to get more details and a bigger picture!

 I have plans to set up a dish for home station use but my location is very poor. However, with enhancements such as "rainscatter" I hope to work some good UK or even Eu DX at times.

The most popular microwave band in the UK is possibly 10GHz. Every month (usually on the last Sunday) there is a 10GHz Cumulative Contest. If you are in the UK you can listen to the talkback or liason contacts for this contact as they take place around 144.175MHz (the microwave talkback calling channel). The contest takes place between 0900 and 2100 GMT. Activity is divided approximately 50:50 into portable and home stations. Most portable stations run around 1 watt cw/ssb output to dishes up to 1.2m diameter. Satellite TV dishes (60 and 80cm diam,) are also very popular with portables. Home stations use from 1 watt to as high as 60 or 100 watts in some cases! The essence of 10GHz is HOME BUILT equipment. If you like home brewing then this is the microwave band for you! You can operate most modes here, especially cw, ssb and ATV.

On August 18th, 1995, G3WDG worked VK2ALU in Australia by using 10GHz E.M.E ("Moonbounce") techniques....the ultimate microwave achievement? There is are photos of  both G3WDG's and VK2ALU's dishes in the PHOTO ALBUM.

Another popular band is 1296 MHz.

At the present time there are hundreds of UK amateurs with equipment for this band, much of it commercial, but there is surprisingly little activity. It takes a good "lift" in conditions to tempt people into operating! Coax-fed yagi type antennas with ssb/cw power outputs of dozens and even hundreds of watts are quite common. This is a very good band on which to start microwaving.

24GHz, 47GHz and even higher........

I used wideband FM on 24GHz for some years. The photo shows my wideband FM transmitter/receiver mounted on a 35cm offset fed dish. My best DX with this setup so far is 146km. The UK record is presently held by Petra, G4KGC and her OM Charlie, G3WDG, who, on the 14th January,1997, worked PA0EZ on narrowband over a path length of 391km.

24GHz. In the past few years we have seen UK amateurs develop sophisticated equipment for the 24GHz, 47GHz, 76GHz and 145GHz bands. Originally there was a lot of wideband FM used on these bands but, by 2001 narrowband modes such as cw and ssb became the "modus operandi" right up to 145GHz!

There are surplus, ex digi-comms equipments around from time to time. In early 1996 a large quantity of 22/23GHz transceivers by Digital Microwave Corporation became available to UK amateurs. Many of these  so-called "Grey Cubes" were modified for ssb/cw/nbfm use on the 24GHz band.

Today, however, the main source of equipment for the millimeter bands is the large range of kits and ready made items from DB6NT of Kuhne Electronic in Germany.    

2.3GHz, 3.4GHz and 5.7GHz.......

These are our "middle" microwave bands and a renewed interest is presently being taken in them, especially 2.4GHz as it became a very important part of the recent Phase 3D (AO40) satellite program. Terrestrially, there is relatively low activity on these bands but a number of UK amateurs are now actively developing equipment for them. During 2001 to 2003, surplus solid state power amplifiers for both the 5.7GHz and 3.4GHz became easily available on the UK market. Combined with the excellent DB6NT series of microwave transverters, they make potent equipments for both portable and home station use.

Discovery

The existence of electromagnetic waves, of which microwaves are part of the frequency spectrum, was predicted by James Clerk Maxwell in 1864 from his equations. In 1888, Heinrich Hertz was the first to demonstrate the existence of electromagnetic waves by building an apparatus that produced and detected microwaves in the UHF region. The design necessarily used horse-and-buggy materials, including a horse trough, a wrought iron point spark, Leyden jars, and a length of zinc gutter whose parabolic cross-section worked as a reflection antenna. In 1894 J. C. Bose publicly demonstrated radio control of a bell using millimetre wavelengths, and conducted research into the propagation of microwaves.

Frequency range

Plot of the zenith atmospheric transmission on the summit of Mauna Kea throughout the entire gigahertz range of the electromagnetic spectrum at a precipitable water vapor level of 0.001 mm. (simulated)

The microwave range includes ultra-high frequency (UHF) (0.3–3 GHz), super high frequency (SHF) (3–30 GHz), and extremely high frequency (EHF) (30–300 GHz) signals.

Above 300 GHz, the absorption of electromagnetic radiation by Earth's atmosphere is so great that it is effectively opaque, until the atmosphere becomes transparent again in the so-called infrared and optical window frequency ranges

Devices

Vacuum tube based devices operate on the ballistic motion of electrons in a vacuum under the influence of controlling electric or magnetic fields, and include the magnetron, klystron, traveling wave tube (TWT), and gyrotron. These devices work in the density modulated mode, rather than the current modulated mode. This means that they work on the basis of clumps of electrons flying ballistically through them, rather than using a continuous stream.

uses

A microwave oven works by passing microwave radiation, usually at a frequency of 2450 MHz (a wavelength of 12.24 cm), through the food. Water, fat, and sugar molecules in the food absorb energy from the microwave beam in a process called dielectric heating. Many molecules (such as those of water) are electric dipoles, meaning that they have a positive charge at one end and a negative charge at the other, and therefore rotate as they try to align themselves with the alternating electric field induced by the microwave beam. This molecular movement creates heat as the rotating molecules hit other molecules and put them into motion. Microwave heating is most efficient on liquid water, and much less so on fats and sugars (which have less molecular dipole moment), and frozen water (where the molecules are not free to rotate). Microwave heating is sometimes incorrectly explained as a rotational resonance of water molecules: such resonance only occurs at much higher frequencies, in the tens of gigahertz. Moreover,

large industrial/commercial microwave ovens operating in the 900 MHz range also heat water and food perfectly well.

o A common misconception is that microwave ovens cook food from the "inside out". In reality, microwaves are absorbed in the outer layers of food in a manner somewhat similar to heat from other methods. The rays from a microwave electrically manipulate water particles to cook food. It is actually the friction caused by the movement that creates heat and warms the food. The misconception arises because microwaves penetrate dry nonconductive substances at the surfaces of many common foods, and thus often deposit initial heat more deeply than other methods. Depending on water content the depth of initial heat deposition may be several centimeters or more with microwave ovens, in contrast to grilling ("broiling" in American English), which relies on infrared radiation, or the thermal convection of a convection oven, which deposit heat shallowly at the food surface. Depth of penetration of microwaves is dependent on food composition and the frequency, with lower microwave frequencies being more penetrating.

AT&T Long Lines Microwave Relay Tower, Utah. The lower horn antennas are for TD Radio, 3.7-4.2 GHz, and can simultaneously carry signals in the 6 and 11 GHz bands.

Microwave radio is used in broadcasting and telecommunication transmissions because, due to their short wavelength, highly directive antennas are smaller and therefore more practical than they would be at longer wavelengths (lower frequencies). There is also more bandwidth in the microwave spectrum than in the rest of the radio spectrum; the usable bandwidth below 300 MHz is less than 300 MHz while many GHz can be used above 300 MHz. Typically, microwaves are used in television news to transmit a signal from a remote location to a television station from a specially equipped van.

Before the advent of fiber optic transmission, most long distance telephone calls were carried via microwave point-to-point links through sites like the AT&T Long Lines facility shown in the photograph. Starting in the early 1950's, frequency division multiplex was used to send up to 5,400 telephone channels on each microwave radio channel, with as many as ten radio channels combined into one antenna for the hop to the next site, up to 70 km away.

Radar also uses microwave radiation to detect the range, speed, and other characteristics of remote objects.

Wireless LAN protocols, such as Bluetooth and the IEEE 802.11 specifications, also use microwaves in the 2.4 GHz ISM band, although 802.11a uses ISM band and UNII frequencies in the 5 GHz range. Licensed long-range (up to about 25 km) Wireless Internet Access services can be found in many countries (but not the USA) in the 3.5–4.0 GHz range.

Metropolitan Area Networks: MAN protocols, such as WiMAX (Worldwide Interoperability for Microwave Access) based in the IEEE 802.16 specification. The IEEE 802.16 specification was designed to operate between 2 to 11 GHz. The commercial implementations are in the 2.5 GHz, 3.5 GHz and 5.8 GHz ranges.

Wide Area Mobile Broadband Wireless Access: MBWA protocols based on standards specifications such as IEEE 802.20 or ATIS/ANSI HC-SDMA (e.g. iBurst) are designed to operate between 1.6 and 2.3 GHz to give mobility and in-building penetration characteristics similar to mobile phones but with vastly greater spectral efficiency.

Cable TV and Internet access on coax cable as well as broadcast television use some of the lower microwave frequencies. Some mobile phone networks, like GSM, also use the lower microwave frequencies.

Many semiconductor processing techniques use microwaves to generate plasma for such purposes as reactive ion etching and plasma-enhanced chemical vapor deposition (PECVD).

Microwaves can be used to transmit power over long distances, and post-World War II research was done to examine possibilities. NASA worked in the 1970s and early 1980s to research the possibilities of using Solar power satellite (SPS) systems with large solar arrays that would beam power down to the Earth's surface via microwaves.

A maser is a device similar to a laser, except that it works at microwave frequencies.

Most radio astronomy uses microwaves.

Microwave frequency bands

The microwave spectrum is usually defined as electromagnetic energy ranging from approximately 1 GHz to 1000 GHz in frequency, but older usage includes lower frequencies. Most common applications are within the 1 to 40 GHz range. Microwave Frequency Bands as defined by the Radio Society of Great Britain in the table below:

Microwave frequency bands

Designation Frequency range

L band 1 to 2 GHz

S band 2 to 4 GHz

C band 4 to 8 GHz

X band 8 to 12 GHz

Ku band 12 to 18 GHz

K band 18 to 26.5 GHz

Ka band 26.5 to 40 GHz

Q band 30 to 50 GHz

U band 40 to 60 GHz

V band 50 to 75 GHz

E band 60 to 90 GHz

W band 75 to 110 GHz

F band 90 to 140 GHz

D band 110 to 170 GHz

The above table reflects Radio Society of Great Britain (RSGB) usage. The term P band is sometimes used for Ku Band. For other definitions see Letter Designations of Microwave Bands

MICROWAVE ANTENNAS

INTRODUCTION

Antennas are devices used to radiate electromagnetic energy into space. The characteristics of transmitting and receiving antennas are similar, so a good transmitting antenna is often a good receiving antenna. A single antenna performs both functions in many modern applications.

ANTENNA CHARACTERISTICS

Since the operating principles of low-frequency and microwave antennas are essentially the same, the electrical characteristics are also very similar. You will need a fundamental knowledge of radar and communications antenna electrical theory in your shipboard antenna maintenance work. Antenna theory is primarily a design consideration of antenna size and shape requirements that depend on the frequency used. A brief description of antenna electrical characteristics is sufficient for the needs of most students of electronics.

Antenna Efficiency

The effectiveness of an antenna depends upon its ability to couple or radiate energy into the air. An efficient antenna is one which wastes very little

energy during the radiation process. The efficiency of an antenna is usually referred to as the POWER GAIN or POWER RATIO as compared to a standard reference antenna. The power gain of an antenna is a ratio of the radiated power to that of the reference antenna, which is usually a basic dipole. Both antennas must be fed rf energy in the same manner and must be in the same position when the energy is radiated. The power gain of a single dipole without a reflector is unity (one). An array of several dipoles in the same position as the single dipole, and fed with the same line, has a power gain of more than one.

The effectiveness of an entire transmitting/ receiving system depends largely on impedance matching between the elements of the system. Impedance matching is particularly critical at the antenna connection. If a good impedance match is maintained between the system and the antenna throughout the operating frequency band, power transfer to and from the antenna is always maximum. The transmission line or waveguide used to transport energy to and from the antenna should have a characteristic impedance equal to that of the antenna. A proper impedance match allows all available power to be absorbed and radiated by the antenna without reflections back down the line.

If you have a transmission line or waveguide with an impedance mismatch at the termination, standing waves are set up by the reflections. Standing waves cause losses in the form of unwanted radiations, heat losses in transmission lines, and arcing in waveguides.

The STANDING-WAVE RATIO, abbreviated swr, is a way to measure the degree of mismatch between the transmission line and its load. The swr can be expressed as a ratio of the maximum and minimum values of the current or voltage in the standing waves that are set up on the lines as follows:

A transmission line or waveguide approaches a perfectly matched condition when the swr approaches a value of 1. A ratio that is a little higher than 1 is usually acceptable in practical applications.

Measurement of swr is the only practical method of detecting an impedance mismatch between a transmitting/receiving system and its antenna. As such, the system swr is an important indication of the overall efficiency of the system during operation.

The line impedance can usually be matched to the antenna at only one frequency. However, the swr will NOT become too high if the antenna is used over a small range of frequencies and the line is matched to the center frequency.

Antenna Directivity

You can divide antennas into two general classes based on directivity, omnidirectional and directional. OMNIDIRECTIONAL antennas radiate and receive energy from all directions at once (SPHERICAL WAVEFRONT). They are seldom used in modern radar systems as the primary antenna, but are commonly used in radio equipment and iff (identification friend or foe) receivers. DIRECTIONAL antennas radiate energy in LOBES (or BEAMS) that extend outward from the antenna in either one or two directions. The radiation pattern contains small minor lobes, but these lobes are weak and normally have little effect on the main radiation pattern. Directional antennas also receive energy efficiently from only one or two directions, depending upon whether it is unidirectional or bidirectional.

Directional antennas have two characteristics that are important to you in radar and communications systems. One is DIRECTIVITY and the other is POWER GAIN. The directivity of an antenna refers to the NARROWNESS of the radiated beam. If the beam is NARROW in either the horizontal or vertical plane, the antenna has a high degree of directivity in that plane. An antenna may be designed for high directivity in one plane only or in both planes, depending on the application. The power gain of an antenna increases as the degree of directivity increases because the power is concentrated into a narrow beam and less power is required to cover the same distance.

Since microwave antennas are predominantly unidirectional, the examples you will study in this chapter are all of the unidirectional type.

Reciprocity

You read in this chapter that an antenna is able to both transmit and receive electromagnetic energy. This is known as RECIPROCITY. Antenna reciprocity is possible because antenna characteristics are essentially the same regardless of whether an antenna is transmitting or receiving electromagnetic energy. Reciprocity allows most radar and communications systems to operate with only one antenna. An automatic switch, called a DUPLEXER, connects either the transmitter or the receiver to the antenna at the proper time. Duplexer operation will be. explained in later NEETS modules dealing with radar and communications systems. Because of the reciprocity of antennas, this chapter will discuss antennas from the viewpoint of the transmitting cycle. However, you should understand that the same principles apply on the receiving cycle.

Radar Fundamentals

Radio, television, radar, and the human eye have much in common because they all process the same type of electromagnetic energy. The major difference between the light processed by the human eye and the radio-frequency energy processed by radio and radar is frequency. For example, radio transmitters send out signals in all directions. These signals can be detected by receivers tuned to the same frequency. Radar works somewhat differently because it uses reflected energy (echo) instead of directly transmitted energy. The echo, as it relates to sound, is a familiar concept to most of us. An experienced person can estimate the distance and general direction of an object causing a sound echo. Radar uses microwave electromagnetic energy in much the same way.

Radar transmits microwave energy that reflects off an object and returns to the radar. The returned portion of the energy is called an ECHO, as it is in sound terminology. It is used to determine the direction and distance of the object causing the reflection. Determination of direction and distance to an object is the primary function of most radar systems.

Telescopes and radars, in terms of locating objects in space, have many common problems. Both have a limited field of view and both require a geographic reference system to describe the position of an object (target). The position of an object viewed with a telescope is usually described by relating it to a familiar object with a known position. Radar uses a standard system of reference coordinates to describe the position of an object in relation to the position of the radar. Normally ANGULAR measurements are made from true north in an imaginary flat plane called the HORIZONTAL PLANE. All angles in the UP direction are measured in a second imaginary plane perpendicular to the horizontal plane called the VERTICAL PLANE. The center of the coordinate system is the radar location. As shown in figure 3-1, the target position with respect to the radar is defined as 60 degrees true, 10 degrees up, and 10 miles distant. The line directly from the radar to the target is called the LINE OF SIGHT. The distance from point 1 to point 2, measured along the line of sight, is called TARGET RANGE. The angle between the horizontal plane and the line of sight is known as the ELEVATION ANGLE. The angle measured in a clockwise direction in the horizontal plane between true north and the line of sight is known as BEARING (sometimes referred to as AZIMUTH). These three coordinates of range, bearing, and elevation determine the location of the target with respect to the radar.

Figure 3-1. - Radar target position.

Bearing and elevation angles are determined by measuring the angular position of the radar antenna (the transmitted beam) when it is pointing directly at the target. Range is more difficult to determine because it cannot be directly measured. The radar system is designed to measure range as a function of time. Since the speed of electromagnetic energy is the same as the speed of light, range is determined by measuring the time required for a pulse of energy to reach the target and return to the radar. Because the speed of the pulse is

known, the two-way distance can be determined by multiplying the time by the speed of travel. The total must be divided by two to obtain the one-way range because the time value used initially is the time required for the pulse to travel to the target and return.

The discussion of microwave antennas in this chapter requires only the most basic understanding of radar concepts! Radar fundamentals will be discussed in more detail in a later NEETS module.

Q.1 Microwave antennas and low-frequency antennas are similar in what ways?

Q.2 What term is used to express the efficiency of an antenna? Q.3 What term is used to express the measurement of the degree of mismatch

between a line and its load? Q.4 What type of antenna radiates in and receives energy from all directions

at once? Q.5 What is the term that is used to describe narrowness in the radiated beam

of an antenna? Q.6 What characteristic allows the same antenna to both transmit and receive?

REFLECTOR ANTENNAS

A spherical wavefront (one in which the energy spreads out in all directions) spreads out as it travels away from the antenna and produces a pattern that is not very directional. A wavefront that exists in only one plane does not spread because all of the wavefront moves forward in the same direction. For an antenna to be highly directive, it must change the normally spherical wavefront into a plane wavefront. Many highly directive microwave antennas produce a plane wavefront by using a reflector to focus the radiated energy. The PARABOLIC REFLECTOR is most often used for high directivity.

Microwaves travel in straight lines as do light rays. They can also be focused and reflected just as light rays can, as illustrated by the antenna shown in figure 3-2. A microwave source is placed at focal point F. The field leaves this antenna as a spherical wavefront. As each part of the wavefront reaches the reflecting surface, it is phase-shifted 180 degrees. Each part is then sent outward at an angle that results in all parts of the field traveling in parallel paths. Because of the special shape of a parabolic surface, all paths from F to the reflector and back to line XY are the same length. Therefore, when the parts of the field are reflected from the parabolic surface, they travel to line XY in the same amount of time.

Figure 3-2. - Parabolic reflector radiation.

If a dipole is used as the source of transmission, energy will be radiated from the antenna into space as well as toward the reflector. Energy which is not directed toward the paraboloid has a wide-beam characteristic which will destroy the narrow pattern of the parabolic reflector. However, a HEMISPHERICAL SHIELD (not shown) may be used to direct most of the radiation toward the parabolic surface and thus prevent the destruction of the narrow pattern. Direct radiation into space is eliminated, the beam is made sharper, and more power is concentrated in the beam. Without the shield, some of the radiated field would leave the radiator directly. Since this part of the field that would leave the radiator would not be reflected, it would not become a part of the main beam and could serve no useful purpose.

In figure 3-3 the radiation pattern of a paraboloid reflector contains a major lobe and several minor lobes. The major lobe is directed along the axis of revolution. Very narrow beams are possible with this type of reflector. Figure 3-4 illustrates the basic paraboloid reflector.

Figure 3-3. - Parabolic radiation pattern.

Figure 3-4. - Basic paraboloid reflector.

You may see several variations of the basic paraboloid reflector used to produce different beam shapes required by special applications. The basic characteristics of the most commonly used paraboloids are presented in the following paragraphs.

Truncated Paraboloid

Figure 3-5, view (A), shows a TRUNCATED PARABOLOID. Since the reflector is parabolic in the horizontal plane, the energy is focused into a narrow beam. With the reflector TRUNCATED (cut) so that it is shortened vertically, the beam spreads out vertically instead of being focused. This fan-shaped beam is used in radar detection applications for the accurate determination of

bearing. Since the beam is spread vertically, it will detect aircraft at different altitudes without changing the tilt of the antenna. The truncated paraboloid also works well for surface search radar applications to compensate for the pitch and roll of the ship.

Figure 3-5A. - Truncated paraboloid.

The truncated paraboloid may be used in target height-finding systems if the reflector is rotated 90 degrees, as shown in view (B) of figure 3-5. Since the reflector is now parabolic in the vertical plane, the energy is focused vertically into a narrow beam. If the reflector is truncated, or cut, so that it is shortened horizontally, the beam will spread out horizontally instead of being focused. Such a fan-shaped beam is used to accurately determine elevation.

Figure 3-5B. - Truncated paraboloid.

Orange-Peel Paraboloid

A section of a complete circular paraboloid, often called an ORANGE-PEEL REFLECTOR because of its orange-peel shape, is shown in figure 3-6. Since the reflector is narrow in the horizontal plane and wide in the vertical plane, it produces a beam that is wide in the horizontal plane and narrow in the vertical plane. In shape, the beam resembles a huge beaver tail. The microwave energy is sent into the parabolic reflector by a horn radiator (not shown) which is fed by a waveguide. The horn radiation pattern covers nearly the entire shape of the reflector, so almost all of the microwave energy strikes the reflector and very little escapes at the sides. Antenna systems which use orange-peel paraboloids are often used in height-finding equipment.

Figure 3-6. - Orange-peel paraboloid.

Cylindrical Paraboloid

When a beam of radiated energy that is noticeably wider in one cross-sectional dimension than in another is desired, a cylindrical paraboloidal section which approximates a rectangle can be used. Figure 3-7 illustrates such an antenna. A PARABOLIC CYLINDER has a parabolic cross section in just one dimension which causes the reflector to be directive in one plane only. The cylindrical paraboloid reflector is fed either by a linear array of dipoles, a slit in the side of a waveguide, or by a thin waveguide radiator. It also has a series of focal points forming a straight line rather than a single focal point. Placing the radiator, or radiators, along this focal line produces a directed beam of energy. As the width of the parabolic section is changed, different beam shapes are obtained. You may see this type of antenna system used in search radar systems and in ground control approach (gca) radar systems.

Figure 3-7. - Cylindrical paraboloid.

Corner Reflector

The CORNER-REFLECTOR ANTENNA consists of two flat conducting sheets that meet at an angle to form a corner, as shown in figure 3-8. The corner reflector is normally driven by a HALF-WAVE RADIATOR located on a line which bisects the angle formed by the sheet reflectors.

Figure 3-8. - Corner reflector.

HORN RADIATORS

Like parabolic reflectors, you can use HORN RADIATORS to obtain directive radiation at microwave frequencies. Because they do not use resonant elements, horns have the advantage of being useful over a wide frequency band.

The operation of a horn as an rf radiating device is similar to that of an automobile horn radiating sound waves. However, the throat of an automobile horn usually is sized much smaller than the sound wavelengths for which it is used. The throat of the rf radiating horn is sized to be comparable to the wavelength being used.

Horn radiators are used with waveguides because they serve both as an impedance-matching device and as a directional radiator. Horn radiators may be fed by coaxial and other types of lines.

Horn radiators are constructed in a variety of shapes, as illustrated in figure 3-9. The shape of the horn determines the shape of the field pattern. The ratio of the horn length to the size of its mouth determines the beam angle and directivity. In general, the larger the mouth of the horn, the more directive is the field pattern.

Figure 3-9. - Horn radiators.

LENS ANTENNAS

With a LENS ANTENNA you can convert spherically radiated microwave energy into a plane wave (in a given direction) by using a point source (open end of the waveguide) with a COLLIMATING LENS. A collimating lens forces all radial segments of the spherical wavefront into parallel paths. The point source can be regarded as a gun which shoots the microwave energy toward the lens. The point source is often a horn radiator or a simple dipole antenna.

Waveguide Type

The WAVEGUIDE-TYPE LENS is sometimes referred to as a conducting-type. It consists of several parallel concave metallic strips which are placed parallel to the electric field of the radiated energy fed to the lens, as shown in figure 3-10, views (A) and (B). These strips act as waveguides in parallel for the incident (radiated) wave. The strips are placed slightly more than a half wavelength apart.

Figure 3-10A. - Waveguide lens.

Figure 3-10B. - Waveguide lens.

The radiated energy consists of an infinite number of RADIAL SECTIONS (RAYS). Each of the radial sections contains mutually perpendicular E and H lines and both are perpendicular to the direction of travel. Because each of the radial sections travels in a different direction, the point source, in itself, has poor directivity. The purpose of the lens is to convert the input spherical microwave segment (which consists of all of the radial sections) into parallel (collimated) lines in a given direction at the exit side of the lens. The focusing action of the lens is accomplished by the refracting qualities of the metallic strips. The collimating effect of the lens is possible because the velocity of electromagnetic energy propagation through metals is greater than its velocity through air. Because of the concave construction of the lens, wavefronts arriving near the ends of the lens travel farther in the same amount of time than do those at the center. Thus, the wavefront emerging from the exit side of the lens appears as a plane wave. It consists of an infinite number of parallel sections (with both the E field and H field components) mutually perpendicular to the direction of travel.

Delay lens

Another type of lens that you may see is the DIELECTRIC or METALLIC DELAY LENS shown in figure 3-11. The delay lens, as its name implies, slows down the phase propagation (velocity) as the wave passes through the lens. The delay lens is convex and is constructed of dielectric material. The delay in the phase of the wave passing through the lens is determined by the DIELECTRIC CONSTANT (REFRACTIVE INDEX) of the material. In most cases, artificial dielectrics, consisting of conducting rods or spheres that are small compared to the wavelength, are used. (Artificial dielectrics are of three-dimensional construction and act as a dielectric to electromagnetic waves.) In this case the inner portion of the transmitted wave is decelerated for a longer interval of time than the outer portions. The delay causes the radiated wave to be collimated.

Figure 3-11. - Delay-type lens.

Loaded Microwave Lens

The LOADED MICROWAVE LENS, shown in figure 3-12, is a multi-cellular array of thousands of cells. Each cell contains a slow-wave (delayed), serrated-metal, plastic-supported waveguide element which acts as a phase-controlling device. A loaded lens can focus microwave energy in much the same way as the waveguide type. The reason is that the speed of propagation is higher in the region between parallel plates than in free space. The parallel plates support the cells.

Figure 3-12. - Loaded lens.

The lens shown in figure 3-12 has an egg-crate appearance because it is really two lenses occupying the same volume. Vertical plates make up a lens that focuses a vertically polarized beam, and horizontal plates handle beams which are horizontally polarized. In other words, this type of construction can be used in multiple-beam applications where the polarization of the beams is different.

Q.12 What is the purpose of a collimating lens? Q.13 How does a waveguide-type lens focus spherical wavefront microwave

energy? Q.14 What type of lens decelerates a portion of a spherical wavefront?

ANTENNA ARRAYS

Sharply directive antennas can be constructed from two or more simple half-wave dipole elements. They must be positioned so that the fields from the elements add in some directions and cancel in others. Such a set of antenna elements is called an ANTENNA ARRAY. When a reflector is placed behind the dipole array, radiation occurs in one direction with a pattern similar to the one shown in figure 3-13.

Figure 3-13. - Field pattern of an antenna array.

You will encounter two basic types of antenna arrays, PARASITIC and DRIVEN. Both types of antenna arrays were explained in NEETS, Module 10, Introduction to Wave Propagation, Transmission Lines, and Antennas. Only a brief review is presented in this chapter.

The parabolic reflector antennas previously discussed and the antenna shown in figure 3-13 are examples of parasitic arrays. Notice that the reflector in figure 3-13 is not directly connected to the energy source. Driven arrays, in which all the radiating elements are connected to the energy source, have smaller losses than parasitic arrays while retaining some of the narrow-beam characteristics. Parasitic arrays, such as the parabolic reflector, are used primarily as antennas in fire control radars and other installations, such as microwave communication systems, that require very accurate (narrow) beams. Driven arrays are used primarily as search-radar antennas because extremely narrow beams are less critical than low losses.

If you position a number of driven half-wave antenna elements with respect to each other so that energy from the individual elements will add in certain directions and cancel in other directions, then the antenna system is directional.

Signals from a number of different sources may contribute to or subtract from the overall effect. By properly phasing the energy fed to the antenna elements, and by properly locating the elements, you can control the direction of the energy. You can cause the energy to add in the desired direction and to be out of phase (cancel) in the undesired direction.

Driven arrays are usually made up of a number of half-wave dipoles positioned and phased so that the desired directional pattern will be achieved. Figure 3-14, view (A), shows a simple antenna array consisting of two horizontally mounted elements, each a half wavelength long and fed in phase. The resulting radiation pattern is in a direction at right angles to the plane containing the antenna conductor.

Figure 3-14. - Horizontal array field patterns.

Three- and four-element arrays are shown in figure 3-14, views (B) and (C), respectively. The field pattern of each array is shown beneath it. Note that the beam becomes sharper as the number of elements is increased. If a still-narrower beam is desired, you may add additional elements. The field patterns of the antennas in the figure are bidirectional. Unidirectional patterns may be obtained with a parasitic reflector mounted behind the driven antenna elements.

The BEDSPRING ARRAY (figure 3-15), so called because of its resemblance to a bedspring, is an example of a unidirectional antenna. It consists of a stacked dipole array with an untuned reflector. The more dipoles that are used or stacked in one dimension (horizontal, for example), the more narrow the beam of radiated energy becomes in that plane. Consequently, the size of the antenna is not the same for all installations. Antennas such as the bedspring array are commonly used in TWO-DIMENSIONAL SEARCH RADARS that obtain the range and bearing information of a target.

Figure 3-15. - Bedspring array.

FREQUENCY-SENSITIVE ANTENNA

The radar antenna in figure 3-16 uses a feed section to drive horizontally stacked array sections which radiate the applied rf pulses. The same array sections receive the target returns. Each array contains slots cut to radiate and receive a particular frequency. Bearing data is obtained by mechanically rotating the antenna 360 degrees. Elevation data is obtained by electronic scanning of the beam in elevation. The radar antenna is frequency sensitive and radiates pulses at an elevation angle determined by the applied frequency. When the frequency is increased, the beam elevation angle decreases. Conversely, when the applied frequency is decreased, the beam elevation angle increases. The beam elevation angle is therefore selected by the application of a frequency corresponding to the desired angle of elevation. The physical length of the antenna feed section, called the SERPENTINE SECTION (figure 3-17), in relation to the wavelength of the applied energy determines the direction of the radiated beam. You may understand this more clearly if you consider how the beam is shifted. The shift occurs with a change in frequency because the positive and negative peaks of the energy arrive at adjacent slotted arrays at different times. The change in the field pattern is such that the angle of departure (angle at which the radiated beam leaves the antenna) of the beam is changed. Note that a change in phase of the applied rf energy would cause the same effect.

Figure 3-16. - Frequency-sensitive antenna.

Figure 3-17. - Serpentine feed.

A SLOT ANTENNA exhibits many of the characteristics of a conventional dipole antenna. When arranged in arrays, a high degree of directivity can be obtained. Also, the beam can be caused to scan a volume of space by changing either the frequency or phase of the energy driving the antenna elements.

Basic Slot Antenna and Its Complementary Dipole

The slot antenna consists of a radiator formed by cutting a narrow slot in a large metal surface. Such an antenna is shown in figure 3-18. The slot length is a half wavelength at the desired frequency and the width is a small fraction of a wavelength. The antenna is frequently compared to a conventional half-wave dipole consisting of two flat metal strips. The physical dimensions of the metal strips are such that they would just fit into the slot cut out of the large metal sheet.

This type of antenna is called the COMPLEMENTARY DIPOLE.

Figure 3-18. - Slot antenna and complementary dipole.

The slot antenna is compared to its complementary dipole to illustrate that the radiation patterns produced by a slot antenna cut into an infinitely large metal sheet and that of the complementary dipole antenna are the same.

Several important differences exist between the slot antenna and its complementary antenna. First, the electric and magnetic fields are interchanged. In the case of the dipole antenna shown in figure 3-18, the electric lines are horizontal while the magnetic lines form loops in the vertical plane. With the slot antenna, the magnetic lines are horizontal and the electric lines are vertical. The electric lines are built up across the narrow dimensions of the slot. As a result, the polarization of the radiation produced by a horizontal slot is vertical. If a vertical slot is used, the polarization is horizontal.

A second difference between the slot antenna and its complementary dipole is that the direction of the lines of electric and magnetic force abruptly reverse from one side of the metal sheet to the other. In the case of the

dipole, the electric lines have the same general direction while the magnetic lines form continuous closed loops.

When energy is applied to the slot antenna, currents flow in the metal sheet. These currents are not confined to the edges of the slot but rather spread out over the sheet. Radiation then takes place from both sides of the sheet. In the case of the complementary dipole, however, the currents are more confined; so a much greater magnitude of current is required to produce a given power output using the dipole antenna.

The current distribution of the dipole resembles the voltage distribution of the slot. The edges on the slot have a high voltage concentration and relatively low current distribution; the complementary dipole has a high current concentration and relatively low voltage.

Slot antennas are adaptable for the vhf and uhf ranges. One of their practical advantages is that the feed section which energizes the slot may be placed below the large metal surface in which the slot is cut. Thus, nothing needs to extend from the surface. In addition, the slot itself may be covered by a section of insulating material to provide a seal so that the antenna can be pressurized with dry air. Dry air pressurization reduces moisture in the waveguide and prevents arcing.

Many of the new radar systems reaching the fleet over the next few years will use frequency- or phase-sensitive antennas. Some of the new radars will use antennas that electronically scan the azimuth as well as elevation, eliminating the moving antenna.

The ANTENNA CHARACTERISTICS of microwave and low-frequency antennas are essentially the same. The efficiency of an antenna is expressed as a POWER GAIN or POWER RATIO as compared to a standard reference antenna.

The STANDING WAVE RATIO (swr) is a measurement of the impedance mismatch between a transmission line and its load and is an indicator of overall system efficiency.

DIRECTIVITY refers to the direction in which an antenna radiates and the narrowness of the radiated beam in DIRECTIONAL ANTENNAS.

OMNIDIRECTIONAL ANTENNAS radiate and receive in all directions at once.

RECIPROCITY is the ability of an antenna to both transmit and receive electromagnetic energy.

REFLECTOR ANTENNAS are antennas that use a reflector to focus electromagnetic energy into a beam that is directional in either the vertical plane, the horizontal plane, or both planes at once. The basic PARABOLIC REFLECTOR shown in the illustration, or one of its variations, is most often used.

LENS ANTENNAS use a COLLIMATING LENS to force the spherical components of a wavefront into parallel (focused) paths by delaying or accelerating portions of the wavefronts, as shown in the illustration.

An ANTENNA ARRAY is a set of antenna elements and may be one of two basic types, the DRIVEN ARRAY or the PARASITIC ARRAY.

FREQUENCY-SENSITIVE ANTENNAS use frequency-sensitive slots as radiation sources to achieve directivity. The angle at which the radiated beam leaves the antenna is determined by the frequency of the radiated energy. Currently the most common frequency-sensitive antennas use this feature to achieve elevation coverage while azimuth coverage is achieved by rotating the

antennas. New systems will use stationary frequency-sensitive antennas to achieve both azimuth and elevation coverage.