All the antennas we discussed can also be used at microwave frequencies. However, these antennas will be extremely small. At 5 GHz a half-wave dipole is slightly less than 1 in long and a one-quarter wavelength vertical is slightly less than 0.5 in long. These antennas can, of course, radiate microwave signals, but inefficiently. Because of the line-of-sight transmission of microwave signals, highly directive antennas are preferred because they do not waste the radiated energy and because they provide an increase in gain, which helps offset the noise and distance problems at microwave frequencies. For these important reasons, special high-gain, highly directive antennas are normally used in microwave applications.
At low microwave frequencies, less than 2 GHz, standard antennas are commonly used, including the dipole and its variations such as the bow tie, the Yagi, and the ground-plane antenna. Another variation is the corner reflector shown in Fig. 16-43. This antenna is a fat, wide-bandwidth, half-wave dipole fed with low-loss coaxial cable. Behind the dipole is a reflector made of solid sheet metal, a group of closely-spaced horizontal rods, or a fine-mesh screen material to reduce wind resistance. This arrangement gives better reflection than a simple rod reflector as used in a Yagi, so the gain is higher.
The angle of the reflector is usually 45°, 60°, or 90°. The spacing between the dipole and the corner of the reflector is usually in the range from about 0.25λ to 0.75λ. Within that spacing range, the gain varies only about 1.5 dB. However, the feed point impedance of the dipole varies considerably with spacing. The spacing is usually adjusted for the best impedance match. It is common to use 50- or 75-ohms coaxial cable, which is relatively easy to match. The overall gain of a corner reflector antenna is 10 to 15 dB. Higher gains can be obtained with a parabolic reflector, but corner reflectors are easier to make and much less expensive.
As discussed previously, waveguides are sometimes used with microwave signals. Below approximately 6 GHz, special coaxial cables can be used effectively if the distances between the antenna and the receiver or transmitter are less than 50 ft. In some microwave systems, waveguides are preferred because of their low loss. Microwave antennas, therefore, must be some extension of or compatible with a waveguide. Waveguides are, of course, inefficient radiators if simply left open at the end. The problem with using a waveguide as a radiator is that it provides a poor impedance match with free space, and the mismatch results in standing waves and reflected power. The result is the tremendous power loss of the radiated signal. This mismatch can be offset by simply flaring the end of the waveguide to create a horn antenna, as shown in Fig. 16-44. The longer and more gradual the flair, the better the impedance match and the lower the loss. Horn antennas have excellent gain and directivity. The longer the horn, the greater it’s gain and directivity.
Different kinds of horn antennas can be created by flaring the end of the waveguide in different ways. For example, flaring the waveguide in only one dimension creates a sectoral horn, as shown in Fig. 16-45(a) and (b). Two of the sides of the horn remain parallel with the sides of the waveguide, and the other dimension is flared. Flaring both dimensions of the horn produces a pyramidal horn, as shown in Fig. 16-45(c). If a circular waveguide is used, the flare produced a conical horn as in Fig. 16-45(d).
The gain and directivity of a horn are a direct function of its dimensions; the most important dimensions are horn length, aperture area, and fl are angle (Fig. 16-46). The length of a typical horn is usually 2λ to 15λ at the operating frequency. Assuming an operating frequency of 10 GHz, the length of λ is 300/f = 300/10,000 = 0.03 m, where f is in megahertz. A length of 0.03 m equals 3 cm; 1 in equals 2.54 cm, so a wavelength at 10 GHz is 3/2.54 =1.18 in. Thus a typical horn at 10 GHz could be anywhere from about 2½ to 18 in long. Longer horns are, of course, more difficult to mount and work with, but provide higher gain and better directivity.
The aperture is the area of the rectangle formed by the opening of the horn, and it is the product of the height and width of the horn, as shown in Fig. 16-46. The greater this area, the higher the gain and directivity. The flare angle also affects gain and directivity. Typical flare angles vary from about 20° to 60°. Obviously, all these dimensions are interrelated. For example, increasing the fl are angle increases the aperture area, and for a given aperture area, decreasing the length increases the flare angle. Any of these dimensions can be adjusted to achieve the desired design objective.
Remember that the directivity of an antenna is measured in terms of beam width, the angle formed by extending lines from the center of the antenna response curve to the 3-dB down points. In the example in Fig. 16-47, the beam width is approximately 30°. Horn antennas typically have a beam angle somewhere in the 10° to 60° range.
The signal radiated from an antenna is three-dimensional. The directivity patterns indicate the horizontal radiation pattern of the antenna. The antenna also has a vertical radiation pattern. Fig. 16-48 is a typical plot of the vertical radiation pattern of a horn antenna. On pyramidal and circular horns, the vertical beamwidth is usually about the same angle as the horizontal beamwidth. This is not true for sectoral horns.
The horizontal beamwidth B of a pyramidal horn is computed by using the expression
B = 80/w/λ
where w = horn width
λ = wavelength of operating frequency
As an example, assume an operating frequency of 10 GHz (10,000 MHz), which gives a wavelength of 0.03 m, as computed earlier. If the pyramidal horn is 10 cm high and 12 cm wide, the beam width is 80/(0.12/0.03) = 80/4 = 20°.
The gain of a pyramidal horn can also be computed from its dimensions. The approximate power gain of a pyramidal horn antenna is
G= 4π KA/λ2
where A = aperture of horn, m2
λ = wavelength, m
K = constant derived from how uniformly the phase and amplitude of
electro magnetic fi elds are distributed across aperture
The typical values of K are 0.5 to 0.6.
Let us take as an example the previously described horn (height = 10 cm;
width = 12 cm). The aperture area is
A = height x width = 10 x 12 =120 cm2 = 0.012 m2
The operating frequency is 10 GHz, so λ = 0.03 m or 3 cm. The gain is
G =4(3.14)(0.5)(0.012)/(0.03)2= 0.07536/0.0009 = 83.7
This is the power ratio P. To fi nd the gain in decibels, the standard power formula is used:
dB = 10 log P
where P is the power ratio or gain. Here,
dB = 10 log 83.7
This is the power gain of the horn over a standard half-wave dipole or quarter-wave vertical.
Most antennas have a narrow bandwidth because they are resonant at only a single frequency. Their dimensions determine the frequency of operation. Bandwidth is an important consideration at microwave frequencies because the spectrum transmitted on the microwave carrier is usually very wide so that a considerable amount of information can be carried. Horns are essentially nonresonant or aperiodic, which means that they can operate over a wide frequency range. The bandwidth of a typical horn antenna is approximately 10 percent of the operating frequency.
The bandwidth of a horn at 10 GHz is approximately 1 GHz. This is an enormous bandwidth—plenty wide enough to accommodate almost any kind of complex modulating signal.
Horn antennas are used by themselves in many microwave applications. When higher gain and directivity are desirable, it can easily be obtained by using a horn in conjunction with a parabolic reflector. A parabolic reflector is a large dish-shaped structure made of metal or screen mesh. The energy radiated by the horn is pointed at the reflector, which focuses the radiated energy into a narrow beam and reflects it toward its destination. Because of the unique parabolic shape, the electromagnetic waves are narrowed into an extremely small beam. Beam widths of only a few degrees are typical with parabolic reflectors. Of course, such narrow beamwidths also represent extremely high gains.
A parabola is a common geometric figure. See Fig. 16-49. A key dimension of a parabola is a line drawn from its center at point Z to a point on the axis labeled F, which is the focal point. The ends of a parabola theoretically extend outward for an infinite distance, but for practical applications they are limited. In the figure, the limits are shown by the dashed vertical line; the endpoints are labeled X and Y.
The distance between a parabola’s focal point and any point on the parabola and then to the vertical dashed line is a constant value. For example, the sum of lines FA and AB is equal to the sum of lines FC and CD. This effect causes a parabolic surface to collimate electromagnetic waves into a narrow beam of energy. An antenna placed at the focal point F will radiate waves from the parabola in parallel lines. If used as a receiver, the parabola will pick up the electromagnetic waves and reflect them to the antenna located at the focal point.
The key thing to remember about a parabolic reflector is that it is not two-dimensional. If a parabola is rotated about its axis, a three-dimensional dish-shaped structure results. This is called a paraboloid.
Fig. 16-50 shows how a parabolic reflector is used in conjunction with a conical horn antenna for both transmission and reception. The horn antenna is placed at the focal point. In transmitting, the horn radiates the signal toward the reflector, which bounces the waves off the reflector and collimates them into a narrow parallel beam. When used for receiving, the reflector picks up the electromagnetic signal and bounces the waves toward the antenna at the focal point. The result is an extremely high-gain, narrow-beam-width antenna.
Note that any common antenna type (e.g., a dipole) can be used with a parabolic reflector to achieve the effects just described.
The gain of a parabolic antenna is directly proportional to the aperture of the parabola. The aperture, the area of the outer circle of the parabola, is
A = πR2
The gain of a parabolic antenna is given by the simple expression
G = 6(D/λ)2
where G = gain, expressed as a power ratio
D = diameter of dish, m
λ = wavelength, m
Most parabolic reflectors are designed so that the diameter is no less than l at the lowest operating frequency. However, the diameter can be as much as 10λ if greater gain and directivity are required.
For example, the power gain of a 5-m-diameter dish at 10 GHz (λ 5 0.03, as computer earlier) is
Expressed in decibels, this is
dB = 10 log 166,673 = 10(5.22) = 52.2
The beamwidth of a parabolic reflector is inversely proportional to the diameter. It is given by the expression
B = 70/D/λ
The beam width of our 5-m, 10-GHz antenna is 70(5/0.03) = 70/166.67 = 0.42°.
With a beamwidth of less than 0.5°, the signal radiated from a parabolic reflector is a pencil-thin beam that must be pointed with great accuracy in order for the signal to be picked up. Usually, both the transmitting and receiving antennas are of a parabolic design and have extremely narrow beamwidths. For that reason, they must be accurately pointed if contact is to be made. Despite the fact that the beam from a parabolic reflector spreads out and grows in size with distance, directivity is good and gain is high. The precise directivity helps to prevent interference from signals coming in at angles outside of the beamwidth.
Keep in mind that the parabolic dish is not the antenna, only a part of it. The antenna is the horn at the focal point. There are many physical arrangements used in positioning the horn. One of the most common is the seemingly awkward configuration shown in Fig. 16-51. The waveguide feeds through the center of the parabolic dish and is curved around so that the horn is positioned exactly at the focal point.
Another popular method of feeding a parabolic antenna is shown in Fig. 16-52. Here the horn antenna is positioned at the center of the parabolic reflector. At the focal point is another small reflector with either a parabolic or a hyperbolic shape. The electromagnetic radiation from the horn strikes the small reflector, which then reflects the energy toward the large dish, which in turn radiates the signal in parallel beams. This arrangement is known as a Cassegrain feed.
The Cassegrain feed has several advantages over the feed arrangement in Fig. 16-51. The first is that the waveguide transmission line is shorter. In addition, the radical bends in the waveguide are eliminated. Both add up to less signal attenuation. The noise figure is also improved somewhat. Most large earth station antennas use a Cassegrain feed arrangement.
Many other feed arrangements have been developed for parabolic reflectors. In antennas used for satellite TV reception, a waveguide is not used. Instead, a horn antenna mounted at the focal point is usually fed with microwave coaxial cable. In other large antenna systems, various mechanical arrangements are used to permit the antenna to be rotated or its position otherwise physically changed. Many earth station antennas must be set up so that their azimuth and elevation can be changed to ensure proper orientation for the receiving antenna. This is particularly true of antennas used in satellite communication systems.
A parabolic reflector antenna has a diameter of 5 ft. Calculate (a) the lowest possible operating frequency, (b) the gain at 15 GHz, and (c) the beam width at 15 GHz. (The lowest operating frequency occurs where the dish diameter is l.)
Some sophisticated systems use multiple horns on a single reflector. Such multiple feeds permit several signals on different frequencies to be either radiated or received with a single large reflecting structure.
A helical antenna, as its name suggests, is a wire helix (Fig. 16-53). A center insulating support is used to hold heavy wire or tubing formed into a circular coil or helix.
The diameter of the helix is typically one-third wavelength, and the spacing between turns is the approximately one-quarter wavelength. Most helical antennas use from six to eight turns. A circular or square ground-plane antenna or reflector is used behind the helix. Fig. 16-53 shows a coaxial feed line. Helical antennas are widely used at VHF and UHF ranges.
The gain of a helical antenna is typically in the 12- to the 20-dB range, and beam widths vary from approximately 12° to 45°. Although these values do not compare favorably with those obtainable with horns and parabolic reflectors, helical antennas are favored in many applications because of their simplicity and low cost.
Most antennas transmit either a vertically or a horizontally polarized electromagnetic field. With a helical antenna, however, the electromagnetic field is caused to rotate. This is known as circular polarization. Either right-hand (clockwise) or left-hand (counterclockwise) circular polarization can be produced, depending on the direction of winding of the helix. Because of the rotating nature of the magnetic field, a circularly polarized signal can easily be received by either a horizontally or a vertically polarized receiving antenna. A helical receiving antenna can also easily receive horizontally or vertically polarized signals. Note, however, that a right-hand circularly polarized signal will not be picked up by a left-hand circularly polarized antenna, and vice versa. Therefore, helical antennas used at both transmitting and receiving ends of a communication link must both have the same polarization.
To obtain greater gain and narrower beamwidth, several helical antennas can be used in an array with a common reflector. A popular arrangement is a group of four helical antennas.
Most microwave antennas are highly directional. But in some applications, an omnidirectional antenna may be required. One of the most widely used omnidirectional microwave antennas is the bicone (Fig. 16-54). The signals are fed into bicone antennas through a circular waveguide ending in a flared cone. The upper cone acts as a reflector, causing the signal to be radiated equally in all directions with a very narrow vertical beamwidth. A version of the bicone replaces the upper cone with a flat horizontal disk that performs the same function.
A slot antenna is a radiator made by cutting a one-half wavelength slot in a conducting sheet of metal or into the side or top of a waveguide. The basic slot antenna is made by cutting a one-half wavelength slot in a large metal sheet. It has the same characteristics as a standard dipole antenna, as long as the metal sheet is very large compared to λ at the operating frequency. A more common way of making a slot antenna is shown in Fig. 16-55.
The slot must be one-half wavelength long at the operating frequency. Fig. 16-55(a) shows how the slots must be positioned on the waveguide to radiate. If the slots are positioned on the center lines of the waveguide sides, as in Fig. 16-55(b), they will not radiate.
Several slots can be cut into the same waveguide to create a slot antenna array (Fig. 16-56). Slot arrays, which are equivalent to driven arrays with many elements, have better gain and better directivity than single-slot antennas.
Slot antennas are widely used on high-speed aircraft. External antennas would be torn off at such high speeds or would slow the aircraft. The slot antenna can be integrated into the metallic skin of the aircraft. The slot itself is filled in with an insulating material to create a smooth skin surface.
Dielectric (Lens) Antennas
As discussed previously, radio waves, similar to light waves, can be reflected, refracted, diffracted, and otherwise manipulated. This is especially true of microwaves, which are close in frequency to light. Thus a microwave antenna can be created by constructing a device that serves as a lens for microwaves just as glass or plastic can serve as a lens for light waves. These dielectric or lens antennas use a special dielectric material to collimate or focus the microwaves from a source into a narrow beam. Fig. 16-57 shows how a lens concentrates light rays from a source into a focused narrow beam. A dielectric lens antenna operates in a similar way.
An example of a lens antenna is one used in the millimeter-wave range. The microwave energy is coupled to a horn antenna through a waveguide. A dielectric lens is placed over the end of the horn, which focuses the waves into a narrower beam with greater gain and directivity. In technical terms, the lens takes the microwaves from a source with a spherical wave front (e.g., a horn antenna) and concentrates them into a plane wavefront. A lens like that shown in Fig. 16-58(a) can be used. The shape of the lens ensures that all the entering waves with a spherical wave front are put into phase at the output to create the concentrated plane wave front. However, the lens in Fig. 16-58(a) will work only when it is very thick at the center. This creates great signal loss, especially at the lower microwave frequencies. To get around this problem, a stepped or zoned lens, such as the one shown in Fig. 16-58(b), can be used. The spherical wave front is still converted to a focused plane wave front, but the thinner lens causes less attenuation.
Lens antennas are usually made of polystyrene or some other plastic, although other types of the dielectric can be used. They are rarely used at the lower microwave frequencies. Their main use is in the millimeter range above 40 GHz.
Patch antennas are made with microstrip on PCBs. The antenna is a circular or rectangular area of copper separated from the ground plane on the bottom of the board by the thickness of the PCB’s insulating material (see Fig. 16-59). The width of the rectangular antenna is approximately one-half wavelength, and the diameter of the circular antenna is about 0.55l to 0.59l. In both cases, the exact dimensions depend upon the dielectric constant and the thickness of the PCB material. The most commonly used PC board material for patch antennas is a Tefl on- fiberglass combination.
The feed method for patch antennas can be either coaxial or edge. With the coaxial method, the center conductor of a coaxial cable is attached somewhere between the center and the edge of the patch, and the coaxial shield is attached to the ground plane [Fig. 16-59(a)]. If the antenna is fed at the edge, a length of microstrip is connected from the source to the edge, as shown in Fig. 16-59(b). The impedance of the edge feed is about 120 Ω. A quarter-wave Q section can be used to match this impedance to the 50-Ω impedance that is characteristic of most circuits. When coaxial feed is used, the impedance is zero at the center of the antenna and increases to 120 Ω at the edge. Correctly positioning the coaxial cable center on the patch [dimension x in Fig. 16-59(a)] allows extremely accurate impedance matching.
Patch antennas are small, inexpensive, and easy to construct. In many applications, they can simply be integrated on the PCB with the transmitter or receiver. A disadvantage of patch antennas is their narrow bandwidth, which is usually no more than about 5 percent of the resonant frequency with circular patches and up to 10 percent with rectangular patches. The bandwidth is directly related to the thickness of the PCB material, i.e., the distance between the antenna and the ground plane [h in Fig. 16-59(b)]. The greater the thickness of the PCB dielectric, the greater the bandwidth.
The radiation pattern of a patch antenna is approximately circular in the direction opposite to that of the ground plane.
A phased array is an antenna system made up of a large group of similar antennas on a common plane. Patch antennas on a common PCB can be used, or separate antennas such as dipoles can be physically mounted together in a plane. See Fig. 16-60. Slot antennas are also used. The antennas are driven by transmission lines that incorporate impedance-matching, power-splitting, and phase-shift circuits. The basic purpose of an array is to improve gain and directivity. Arrays also offer better control of directivity, because individual antennas in an array can be turned off or on, or driven through different phase shifters. The result is that the array can be “steered”—i.e., its radiation pattern can be pointed over a wide range of different directions—without physically moving the antenna, as is necessary with Yagis or parabolic dish antennas.
There are two common arrangements for phased arrays. In one configuration, the multiple antennas are driven by a common transmitter or feed a common receiver. A second approach is to have a low-power transmitter amplifier or low-noise receiver amplifier associated with each dipole or patch in the array. In both cases, the switching and phase shifting are under the control of a microprocessor or computer. Different programs in the processor select the gain, directivity, and other factors as required by the application.
Most phased arrays are used in radar systems, but they are finding applications in cell phone systems, wireless local area networks, and satellites.
Because antennas are so small at microwave frequencies, they can be conveniently made right on a printed circuit board that also holds the transmitter and/or receiver ICs and related circuits. No separate antenna structure, feed line, or connectors are needed. The patch and slot antennas discussed previously are examples. But there are a few other types that are widely used. These are the loop, the inverted-F, and the meander line antennas shown in Fig. 16-61.
A loop antenna is just as its name implies, a single closed loop that is usually rectangular but could be round as well. See Fig. 16-61(a). The length of the loop is usually in the 0.1λ to λ at the operating frequency. The loop is usually resonated with a parallel capacitor. Because the characteristic impedance of the loop is very low, about 10 Ω at 0.5λ but as high as 120 Ω for λ, some form of the microstrip transmission line is used to match the impedance to the receiver or transmitter. Loops are relatively inefficient but are effective in short-range applications such as garage door openers and remote keyless entry radios in cars and in pagers.
This unique antenna is a variation of the ground plane as it is designed to work over a conducting ground plane. See Fig. 16-61(b). Note the lengths of the various segments. These are experimented with to get the desired performance as well as the impedance match to the transmission line. A desirable feature is that the radiation pattern is effectively omnidirectional.
The meander line in Fig. 16-61(c) is an attempt to shorten an antenna by bending the conductors back on themselves to save space. The design is essentially a half-wave dipole and performs like one. All sorts of variations have been created by using curved sections and cross patterns in addition to the bow-tie design discussed earlier. The antennas in most cell phones are special variations of the antennas in Fig. 16-61. They are designed not only to fit into the small thin cases of a cell phone but also to have wide bandwidth or to be able to operate on multiple bands.
A dielectric antenna is one whose copper pattern elements are formed on some type of resonant dielectric material such as ceramic or some derivative thereof. The dielectric is designed to be resonant to the frequency of operation and actually contributes to the radiation. Dielectric antennas often use the inverted-F, dipole, or meander line configurations. No ground plane is necessary for operation. Dielectric antennas are small and have a wide bandwidth. These antennas are often available as a component for mounting on a printed circuit board.
Intelligent Antenna Technology
Intelligent antennas or smart antennas are antennas that work in conjunction with electronic decision-making circuits to modify antenna performance to fit changing situations. They adapt to the signals being received and the environment in which they transmit.
Also called adaptive antennas, these new designs greatly improve transmission and reception in multipath environments and can also multiply the number of users of a wireless system. Some popular adaptive antennas today use diversity, multiple-input multiple-output, and automatic beamforming.
Diversity was discussed. It uses two or more antennas that receive the signal from different physical positions. In this way, they get different signals. The antenna with the strongest signal is selected, or the signals are combined to produce a stronger overall signal. More and more, microwave equipment is using diversity simply because of the degrading effects of multiple signal paths, reflections, diffractions, and other conditions that weaken the signal in complex environments. Some systems use three or four antennas with different modes of signal selection to optimize reception.
Multiple-input, multiple-output (MIMO) takes the idea of diversity to a whole new level. It uses two or more antennas for receiving but also uses two or more antennas for transmission. One possible arrangement is shown in Fig. 16-62. The data to be transmitted is divided into two separate streams of bits that are transmitted simultaneously. Because there are two separate data paths, the effect is to double the actual data rate of transmission. For example, in one type of wireless LAN, the maximum data rate is 54 Mbps. With two data streams, the composite throughput is 108 Mbps. The transmit antennas are physically separated by a wavelength or more so that they truly generate different paths to the receiver. The modulation is usually some form of OFDM, and the data is transmitted in the same bandwidth.
The arrangement in Fig. 16-62 is designated 2 x 4 MIMO. The first number is the number of transmitters and the second number is the number of receivers. Common MIMO formats in wireless LANs and cell phone systems are 2 x2, 2 x3, 4 x 4, and even 8 x 8.
At the receiving end, three or more antennas are used. The two transmitted signals take different paths to the four receiver antennas shown in Fig. 16-62. The receiver antennas are separated by a wavelength or so, providing multiple paths for each of the two transmitted signals. The signals experience multipath reflections and other anomalies along the way. The receiver antennas pick up everything. The outputs of the four receivers are then digitized with analog-to-digital converters (ADCs), and their outputs are combined in a DSP.
Special algorithms programmed into the DSP manipulate and combine the signals in different ways to minimize the multipath effects and to create usable signals where none were available with just one transmitter and receiver. MIMO provides an amazing increase in signal gain and reliability. And despite the seemingly high cost and complexity of such a scheme, in reality, the very small low-cost IC receivers and transmitters make this technique very simple and affordable from the hardware point of view. The real complexity and “magic” are in the DSP. MIMO makes a previously impossible wireless application work by providing multiple signals that can be combined and processed to produce a usable signal.
Adaptive Beam Forming
Adaptive antennas are systems that automatically adjust their characteristics to the environment.
They use beam-forming and beam-pointing techniques to zero in on signals to be received and
to ensure transmission under noisy conditions with interference from other sources.
Beam-forming antennas use multiple antennas such as the phase arrays discussed earlier. By using many antennas, the transmit/receive pattern can be adjusted as required by the situation. The beam may be narrowed or widened, and the direction of the beam may be electronically adjusted on the fly thanks to electronic controls. Such directional antennas can pinpoint a specific signal while tuning out interfering signals on the same frequency at nearby locations. Beam-forming antennas also have high gain, which helps boost the signal strength of the desired signal and improves the link reliability because noise and interference have been minimized.
There are two kinds of adaptive antennas, switched beam arrays and adaptive arrays. The radiation pattern of a switched beam antenna looks something like that in Fig. 16-63. The antenna itself is usually multiple phased arrays. For example, there may be four-phased arrays each capable of covering 90° to 100° of azimuth. Multiple beams are formed in that 90° range. The electronics controlling the arrays steers the beam by some predetermined algorithm. The most common arrangement is for the antenna to scan the entire 360° range in seeking a signal. As each beam is switched in, the signal strength is monitored. The beam with the strongest signal is then selected. Signals outside the beam will not be received at all, or at most only a small amount of signal will be present. Such antennas can provide a gain of 20 to 50 dB.
An adaptive antenna can also cover the entire 360° range but uses more sophisticated control algorithms. The adaptive array not only seeks out the strongest signal and adjusts the beam width to enhance it, but also recognizes interfering signals and adjusts the antenna to null out the interfering signal. See Fig. 16-64. Adaptive arrays track signals and then finetune themselves for best reception. All this takes place automatically at electronic speeds.
Both switched beam arrays and adaptive arrays are already being employed in some cell phone systems and in newer wireless LANs. They are particularly beneficial to cell phone systems because they can actually boost the system capacity since they can reuse the same frequencies multiple times and allow the antenna to keep signals on the same frequency from interfering with one another. This concept is known as spatial division multiplexing or spatial division multiple access (SDMA).
Microwave and Millimeter-Wave Applications
The communication applications in which microwaves are most widely used today are telephone communication, computer networking, cell phones, satellites, and radar. However, there are many other significant uses of microwave frequencies in communication. For example, TV stations use microwave relay links instead of coaxial cables to transmit TV signals over long distances, and cable TV networks use satellite communication to transmit programs from one location to another. Communication with satellites, deep-space probes, and other spacecraft is usually done by microwave transmission because microwave signals are not reflected or absorbed by the ionosphere, as are many lower-frequency signals.
Electromagnetic radiation from the stars is also primarily in the microwave region; only sensitive radio receivers and large antennas operating in the microwave region are used to map outer space with far greater precision than could be achieved with optical telescopes. Finally, microwaves are also used for heating—in the kitchen (microwave ovens), in medical practice (diathermy machines used to heat muscles and tissues without causing skin damage), and in the industry for melting material and heat treating.
Fig. 16-65 summarizes the major applications of microwaves. The military uses microwaves for multichannel communication with ranges from 1 km to 160 km for line-of-sight and troposcatter communications. The multiple channels can carry a variety of communication signals.
The electronic communication system is known as radar (radio detection and ranging) is based on the principle that high-frequency RF signals are reflected by conductive targets. The usual targets are airplanes, missiles, ships, and automobiles. In a radar system, a signal is transmitted toward the target. The reflected signal is picked up by a receiver in the radar unit. The reflected or return radio signal is called an echo. The radar unit can then determine the distance to the target (range), its direction (azimuth), and in some cases its elevation (distance above the horizon).
The ability of the radar to determine the distance between a remote object and the radar unit is dependent upon knowing the exact speed of radio signal transmission. In most radar applications, nautical miles are used instead of statute miles to express transmission speeds. One nautical mile is equal to 6076 ft. The speed of a radio signal is 162,000 nautical miles per second. (Sometimes, a special unit known as a radar mile is used. One radar mile is equal to 6000 ft.) It takes a radio signal 5.375 μs to travel 1 mi and 6.18 μs to travel 1 nautical mile.
A radar signal must travel twice the distance between the radar unit and the remote target. The signal is transmitted, a finite time passes before the signal reaches the target and is reflected, and the signal then travels an equal distance back to the source. If an object is exactly 1 nautical mile away, the signal takes 6.18 μs to reach the target and 6.18 μs to return. The total elapsed time from the instant of initial transmission to the reception of the echo is 12.36 μs.
The distance to a remote target is calculated by using the expression
D = T/12.36
where D = distance between radar unit and remote object, nautical miles
T = total time between transmission and reception of signal, μs
In short-distance applications, the yard is the common unit of distance measurement. A radio signal travels 328 yd/μs, so the distance to an object in yards is computed as
D = 328T/2 = 164T
A measured time of 5.6 μs corresponds to a distance of 164 (5.6) = 918.4 yd.
To obtain a strong reflection or echo from a distant object, the wavelength of the radar signal should be small compared to the size of the object being observed. If the wavelength of the radar signal is long with respect to the distant object, only a small amount of energy will be reflected. At higher frequencies, the wavelength is shorter and therefore the reflected energy is greater. For optimal reflection, the size of the target should be one-quarter wavelength or more at the transmitted frequency.
The shorter the wavelength of the signal compared to the observed object, the higher the resolution or definition of the remote object. In most cases, it is necessary only to detect the presence of a remote object. But if very short wavelengths are used, in many cases the actual shape of an object can be clearly determined.
The term cross-section is often used with reference to a radar target. If a target is at least 10 times larger than λ of the radar signal detecting it, the cross-section is constant. The cross-section of a target, a measure of the area of the target “illuminated” by the radar signal, is given in square meters. A target’s cross-section is determined by the size of the object, the unique geometry of the target, the viewing angle, and the position.
The larger the cross-section, the greater the reflected signal power, the greater the distance of detection, and the higher the probability of the signal being greater than the noise. Another factor influencing the return signal strength is the material of the target.
Metal returns the greatest signal; other materials can also reflect radar waves, but not as effective. Some objects will absorb radar waves, making the reflected signal very small. The F117, the U.S. Air Force’s stealth fighter, has a large physical cross-section (about 1 m2), but it is designed to deflect and absorb any radar signal aimed at it. All the plane’s surfaces are at an angle such that radio signals striking them are not reflected directly back to the radar unit. Instead, they are reflected off at an angle, and little if any reflected energy is received. The surface of the aircraft is also coated with a material that absorbs radio waves.
This combination gives the F117 an effective cross section the size of a small bird.
All the most important factors affecting the amount of received signal reflected from a target are summed up in what is called the radar equation:
where Pr =received power
G = antenna gain (product of transmitting and receiving gains)
σ = cross section of target
Ae = effective area of receiving antenna (dish area)
R = range or distance to target
Most of the relationships between the variables are obvious. However, given the fact that the received power is inversely proportional to the fourth power of the distance to the target, it is not surprising that radar range is generally so limited. Keep in mind that the radar equation does not take into account the S/N ratio. Very low-noise receiver front ends are essential for target acquisition.
Because radar uses microwave frequencies, line-of-sight communication results. In other words, radar cannot detect objects beyond the horizon. Objects do not have to be physically visible but must be within line-of-sight radio distance in order for detection to occur.
The relationship between range, azimuth, and elevation can be expressed by a right triangle, as shown in Fig. 16-66. Assume that the radar is land-based and used to detect aircraft. The distance between the radar unit and the remote airplane is the hypotenuse of the right triangle. The angle of elevation is the angle between the hypotenuse and the baseline, which is a line tangent to the surface of the earth at the radar location. The altitude is defined by the angle of elevation. The greater the angle of elevation, the greater the altitude. Knowing the range and the angle of elevation allows the altitude to be computed by using standard trigonometric techniques.
Another important factor in locating a distant object is knowledge of its direction with respect to the radar set. If the radar station is fixed and land-based, the direction (bearing or azimuth) of the remote object is usually given as a compass direction in degrees. Recall that true north is 0° or 360°, east is 90°, south is 180°, and west is 270°.
If the radar unit is located in a moving vehicle, such as an airplane or a ship, the azimuth is given as a relative bearing with respect to the forward direction of the vehicle. Straight ahead is 0° or 360°, directly to the right is 90°, directly behind is 180°, and directly to the left is 270°.
The ability of a radar unit to determine the direction of a remote object requires the use of a highly directional antenna. An antenna with an extremely narrow beamwidth will receive signals only over a narrow-angle. The narrower the beamwidth of the antenna, the more precisely the actual bearing can be determined.
Since most radar systems operate in the microwave region, highly directional antennas are easily obtained. Horns with parabolic reflectors are the most common, and beam widths of less than 1° are readily attainable. These highly directional antennas are continuously rotated 360°. The same antenna is used for transmitting the original signal and receiving the reflected signal.
Circuits within the radar unit are calibrated so that the direction in which the antenna is pointing is accurately known. When the echo is received, it is compared to the calibrated values and the precise direction determined.
The ability of a radar unit to determine the altitude of a remote target depends on the vertical beamwidth of the radar antenna. The radar antenna may scan vertically while measuring the distance of the object during the scan. When the object is detected, the vertical elevation of the antenna is noted and the actual altitude is then computed.
A radar set detects the presence of an aircraft. The time between the radiated and received pulses is 9.2 μs. The antenna is set to an angle of elevation of 20°. Determine (a) the line-of-sight distance to the aircraft in statute miles and (b) the altitude of the aircraft.
There are two basic types of radar systems: pulsed and continuous-wave (CW). There are also numerous variations of each. By far the most commonly used radar system is the pulsed type. Signals are transmitted in short bursts or pulses, as shown in Fig. 16-67. The duration or width W of the pulse is very short and, depending upon the application, can be anywhere from less than 1 μs to several microseconds.
The time between transmitted pulses is known as the pulse repetition time (PRT). If the PRT is known, the pulse repetition frequency (PRF) can be determined by using the formula
PRF = 1/PRT
For example, if the pulse repetition time is 150 μs, the PRF is 1/150 x 10-6 = 666.7 kHz.
The ratio of the pulse width to the PRT is known as the duty cycle. The duty cycle
is normally expressed as a percentage:
Duty cycle = W x 100/PRT
For example, a pulse width of 7 μs with a PRT of 280 μs produces a duty cycle of 7 x 100/280 = 2.5 percent.
It is during the interval between the end of the transmitted pulse and the beginning of the next pulse in sequence in Fig. 16-67 that the echo is received.
The duration of the transmitted pulse and the PRT are extremely critical in determining the performance of a radar system. Very short-range radars have narrow pulses and short pulse repetition times. If the target is only a short distance away, the echo travel time will be relatively short. In short-range radars, the pulse width is made narrow to ensure that the pulse is terminated before the echo of the target is received. If the pulse is too long, the return signal may be masked or blanked by the transmitted pulse. Longrange radars typically have a longer pulse repetition time because it takes longer for the echo to return. This also permits a longer burst of energy to be transmitted, ensuring a stronger return.
If the PRT is too short relative to the distance of the target, the echo may not return during the time interval between two successive pulses, but after the second transmitted pulse. This is known as a double range or second return echo. Naturally, such echoes lead to imprecise distance measurements.
Continuous-Wave (CW) Radar
In continuous-wave (CW) radar, a constant amplitude continuous microwave sine wave is transmitted. The echo is also a constant-amplitude microwave sine wave of the same frequency, but of lower amplitude and obviously shifted in phase. The question is, How is such a signal used to determine target characteristics?
The answer has to do with the object itself. In most cases, the target is moving with respect to the radar unit. The reflected signal from a moving airplane, ship, missile, or automobile undergoes a frequency change. It is this frequency change between the transmitted signal and the returned signal that is used to determine the speed of the target.
The frequency shift that occurs when there is relative motion between the transmitting station and a remote target is known as the Doppler effect. A familiar example of the Doppler effect is the fixed-frequency sound waves emitted by an automobile horn. If the horn sounds when the car is stationary, you will hear a single tone. However, if the car is moving toward you while the horn is on, you will experience a tone of continuously increasing frequency. As the car moves closer to you, the sound waves are compressed, creating the effect of a higher-frequency signal. If the car is moving away from you with its horn on, you will experience a continually decreasing frequency. As the car moves away, the sound waves are stretched out, creating the effect of a lower-frequency signal. This same effect works on both radio and light waves.
In a Doppler system, the transmitter sends out a continuous-frequency signal. If the frequency difference between a transmitted signal and a reflected signal is known, the relative speed between the radar unit and the observed object can be determined by using the formula
where f = frequency difference between transmitted and reflected signals, Hz
λ = wavelength of the transmitted signal, m
V= relative velocity between the two objects, nautical mi/h
Assume, e.g., a frequency shift of 1500 Hz at a frequency of 10 GHz. A frequency of 10 GHz represents a wavelength of 300/f (in MHz) = 300/10,000 = 0.03 m. The speed is therefore (1500)(0.03)/1.03 = 43.7 nautical mi/h. There are 1.15 nautical miles per hour per standard miles per hour, so the speed is 43.7 x 1.15 = 50.56 mph.
This formula assumes that the moving object is moving directly toward or away from the transmitter. If there is an angular offset, the speed should be multiplied by the cosine of the angle between the direct path and the actual path.
In CW radar, it is the Doppler effect that provides frequency modulation of the reflected carrier. For there to be a frequency change, the observed object must be moving toward or away from the radar unit. If the observed object moves parallel to the radar unit, there is no relative motion between the two and no frequency modulation occurs. The greatest value of CW radar is its ability to measure the speed of distant objects. Police radar units use CW Doppler radar for measuring the speed of cars and trucks.
Some radar systems combine both pulse and Doppler techniques to improve performance and measurement capabilities. One such system evaluates successive echoes to determine phase shifts that indicate when a target is moving. Such radars are said to incorporate moving target indication (MTI). Through a variety of special signal processing techniques, multiple moving targets can be distinguished not only from one another but from fixed targets as well.
Block Diagram Analysis
Fig. 16-68 is a block diagram of a typical pulsed radar unit. There are four basic subsystems: the antenna, the transmitter, the receiver, and the display unit.
The transmitter in a pulsed radar system invariably uses a magnetron. Recall that a magnetron is a special high-power vacuum tube oscillator that operates in the microwave region. The cavity size of the magnetron sets the operating frequency. A master timing generator develops the basic pulses used for triggering the magnetron. The timing generator sets the pulse duration, the PRT, and the duty cycle. The pulses from the timing network trigger the magnetron into oscillation, and it emits short bursts of microwave energy. Magnetrons are capable of extremely high power, especially when operated on a pulse basis. Continuous average power may be low, but when pulsed, magnetrons can produce many megawatts of power for the short duration required by the application.
This helps ensure a large reflection. Klystrons and TWTs are commonly used in CW Doppler radars. Low-power radars such as those used by the police for speed detection use Gunn diodes.
In Fig. 16-68, you can see that the transmitter output is passed through a circulator and then applied to the antenna. The circulator is a type of duplexer that allows the transmitter and receiver to share a single antenna and prevents the high-power transmitted signal from getting into the receiver and damaging it.
A radar duplexer is a waveguide assembly containing special devices that prevent interference between the transmitter and receiver. The most commonly used device is a spark gap tube. Spark gap tubes are either TR (transmit-receive) or anti-transmit-receive (ATR) types. TR tubes prevent transmitter power from reaching the receiver. When RF energy from the transmitter is detected, the spark gap breaks down, creating a short circuit for the RF energy. ATR tubes effectively disconnect the transmitter from the circuit during the receive interval. The TR and ATR tubes, when combined with the appropriate one-quarter and one-half wavelength waveguides, provide effective isolation
between transmitter and receiver. In low-power radars, PIN diodes or a standard circulator can be used for this purpose.
The antenna system is typically a horn with a parabolic reflector that produces a very narrow beamwidth. A special waveguide assembly with a rotating joint allows the waveguide and horn antenna to be rotated continuously over 360°.
The same antenna is also used for reception. During the pulse of time, the received signal passes through the antenna, the associated waveguide, and either the duplexer or the circulator to the receiver. The receiver is a standard high-gain superheterodyne type.
The signal is usually fed directly to a mixer in most systems, although some radars use an RF amplifier. The mixer is typically a single diode or diode bridge assembly. The local oscillator feeds a signal to the mixer at the appropriate frequency so that an IF is developed. Both klystrons and Gunn diodes are used in local-oscillator applications. The IF amplifiers provide very high gain prior to demodulation. Some radar receivers use double conversion.
The demodulator in a pulsed radar system is typically a diode detector since only the pulses must be detected. In Doppler and other more complex radars, some form of frequency- and phase-sensitive demodulator is used. Phase-locked loops are common in this application. The output of the demodulator is fed to a video amplifier, which creates signals that are ultimately displayed.
The display in most radar systems is a CRT. Various display formats can be used. The most common type of CRT display is known as a type P display or plan position indicator (PPI). PPI displays show both the range and the azimuth of a target. The center of the display is assumed to be the location of the radar unit. Concentric circles indicate the range. The azimuth or direction is indicated by the position of the reflected target on the screen with respect to the vertical radius line. The targets show up as lighted blips on the screen.
The PPI display is developed by a sophisticated scanning system tied into antenna rotation. As the antenna is rotated by a motor, an encoder mechanism sends signals to the PPI control circuits that designate the azimuth or direction of the antenna. At the same time, the horizontal and vertical deflection coils (the yoke) around the neck of the CRT rotate in synchronism with the antenna. The electron beam in the CRT is swept from the center of the screen out to the edge. The sweep begins at the instant of pulse transmission and sweeps outward from the center. The beginning of each transmitted pulse begins another sweep of the electron beam. As the deflection yoke rotates, the beam moves in such a way that it appears that a radius from the center to the edge is continuously rotating. Target reflections appear as lighted blips on the screen. Calibrations on the screen in the form of graticule markings or superimposed electron beam patterns permit distance and azimuth values to be read directly.
Newer radars use a different system with a LCD or other flat-panel display to show the equivalent of the CRT PPI presentation.
Some PPI radars scan only a narrow range of azimuth rather than the entire 360°. Airplanes with radars in the nose, e.g., only scan a 90° to 180° range forward.
An important high-tech type of radar known as phased array radar provides greater flexibility in scanning narrow sectors and tracking multiple targets. Instead of a single horn and parabolic reflector, multiple dipoles or patch antennas are used. Slots in a waveguide are also used. A half-wave dipole at microwave frequencies is very short. Therefore, many can be mounted together in a matrix or array. The result is a special collinear array above a reflecting surface with very high gain. By using a system of separate feed lines and a variable phase shifter for each antenna, the beam width and directivity can be controlled electronically. This permits rapid scanning and on-the-fly adjustment of directivity. Phase array radars eliminate the mechanical systems needed for conventional radars.
The newest form of radar is called ultra wide band (UWB) radar. It is a form of pulsed radar that radiates a stream of very short pulses several hundred pico seconds long rather than a burst of RF at a specific carrier frequency. The resulting spectrum as determined by a Fourier analysis is very broad, usually several gigahertz wide. This spectrum typically overlaps radio signals within the signal bandwidth, commonly in the 1- to 10-GHz range. To avoid interference with other radio signals, very low power (microwatts) is used. The very narrow pulses give this radar extreme precision and resolution of small objects and details. However, the low power restricts operation to short distances (LESS THAN 100 m). The circuitry used is relatively simple, so it is possible to make inexpensive, single-chip radars. These are used in short-range collision detection systems in airplanes and soon will be in automobiles for automatic braking based upon distance from the vehicle ahead.
Another application of UWB radar is personnel detection on the battlefield. These radars can penetrate walls to detect the presence of human beings.we will provide more details on UWB systems.
One of the most important uses of radar is in weapons defense systems and in safety and navigation systems. Search radars are used to locate enemy missiles, planes, and ships. Tracking radars are used on missiles and planes to locate and zero in on targets. Radars are widely used on planes and ships for navigating blind in fog or bad weather. Radars help ground controllers locate and identify nearby planes. Special radars assist planes in landing in bad weather when visibility is near zero.
In civilian applications, radars are used on boats of all sizes for navigation in bad weather. The police use radar to catch speeders. Small handheld Doppler radar units can also be used in sporting events—to time race cars or determine the speed of a pitched baseball or tennis serves. Some of the newer upper-scale cars such as Lexus and Mercedes Benz have built-in radars that detect the distance between the vehicle ahead and automatically adjust the speed to maintain a safe distance. These short-range radars operate in the 20- to 40-GHz bands. Finally, ground and satellite-based radars are widely used to track clouds, storms, and other phenomena for the purpose of weather forecasting.
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