35
CHAPTER
Antennas for RF Communications
WE CAN CATEGORIZE RF ANTENNAS INTO RECEIVING TYPES AND TRANSMITTING TYPES. NEARLY ALL transmitting antennas can receive signals quite well within the design frequency range. Some, but not all, receiving antennas can transmit RF signals with reasonable efficiency.
Radiation Resistance
When RF current flows in an electrical conductor, such as a wire or a length of metal tubing, some EM energy radiates into space. Imagine that we connect a transmitter to an antenna and test the whole system. Then we replace the antenna with a resistance/capacitance (RC) or resistance/inductance (RL) circuit and adjust the component values until the transmitter behaves exactly as it did when connected to the real antenna. For any antenna operating at a specific frequency, there exists a unique resistance RR, in ohms, for which we can make a transmitter “think” that an RC or RL circuit is, in fact, that antenna. We call RR the radiation resistance of the antenna.
Determining Factors
Suppose that we place a thin, straight, lossless vertical wire over flat, horizontal, perfectly conducting ground with no other objects in the vicinity, and feed the wire with RF energy at the bottom. In this situation, the radiation resistance RR of the wire is a function of its height in wavelengths. If we graph the function, we get Fig. 35-1A.
35-1 Approximate values of radiation resistance for vertical antennas over perfectly conducting ground (A) and for center-fed antennas in free space (B).
Now imagine that we string up a thin, straight, lossless wire in free space (such as a vacuum with no other objects anywhere nearby) and feed it with RF energy at the center. In this case, RR is a function of the overall conductor length in wavelengths. If we graph the function, we obtain Fig. 35-1B.
Antenna Efficiency
We rarely have to worry about antenna efficiency in a receiving system, but in a transmitting antenna system, efficiency always matters. The efficiency expresses the extent to which a transmitting antenna converts the applied RF power to actual radiated EM power. In an antenna, radiation resistance RR always appears in series with a certain loss resistance (RL). We can calculate the antenna efficiency, Eff, as a ratio with the formula
Eff = RR/(RR + RL)
As a percentage, we have
Eff% = 100 RR/(RR + RL)
We can obtain high efficiency in a transmitting antenna only when the radiation resistance greatly exceeds the loss resistance. In that case, most of the applied RF power “goes into” useful EM radiation, and relatively little power gets wasted as heat in the earth and in objects surrounding the antenna. When the radiation resistance is comparable to or smaller than the loss resistance, a transmitting antenna behaves in an inefficient manner. This situation often exists for extremely short antenna radiators because they exhibit low radiation resistance. If we want reasonable efficiency in an antenna with a low RR value, we must do everything we can to minimize RL. Even the most concerted efforts rarely reduce RL to less than a few ohms.
If an antenna system has a high loss resistance, it can work efficiently if we design it to exhibit an extremely high radiation resistance. When an antenna radiator measures a certain height or length at a given frequency, and if we construct it of low-loss wire or metal tubing, we can get its radiation resistance to exceed 1000 ohms. Then we can construct an efficient antenna even in the presence of substantial loss resistance.
Half-Wave Antennas
We can calculate the physical span of an EM-field half wavelength in free space using the formula
Lft = 492/fo
where Lft represents the straight-line distance in feet, and fo represents the frequency in megahertz. We can calculate the physical span of a half wavelength in meters, Lm, using the formula
Lm = 150/fo
Velocity Factor
The foregoing formulas represent theoretical ideals, assuming infinitely thin conductors that have no resistance at any EM frequency. Obviously, such antenna conductors do not exist in physical reality. The atomic characteristics of wire or metal tubing cause EM fields to travel along a real-world conductor a little more slowly than light propagates through free space. For ordinary wire, we must incorporate a velocity factor v of 0.95 (95 percent) to account for this effect. For tubing or large-diameter wire, v can range down to about 0.90 (90 percent).
Open Dipole
An open dipole or doublet comprises a half-wavelength radiator fed at the center, as shown in Fig. 35-2A. Each side or “leg” of the antenna measures a quarter wavelength long from the feed point (where the transmission line joins the antenna) to the end of the conductor. For a straight wire radiator, the length Lft, in feet, at a design frequency fo, in megahertz, for a center-fed, half-wavelength dipole is approximately
35-2 Basic half-wave antennas. At A, the dipole. At B, the folded dipole. At C, the zepp.
Lft = 467/fo
The length is meters is approximately
Lm = 143/fo
These values assume that v = 0.95, as we would normally expect with copper wire of reasonable diameter. In free space, the impedance at the feed point of a center-fed, half-wave, open dipole constitutes a pure resistance of approximately 73 ohms. This resistance represents RR alone. No reactance exists because a half-wavelength open dipole exhibits resonance, just as a tuned RLC circuit would if made with a discrete resistor, inductor, and capacitor. In fact, a dipole antenna (as well as many other types) constitutes an RLC circuit of a specialized, unique sort.
Folded Dipole
A folded dipole antenna consists of a half-wavelength, center-fed antenna constructed of two parallel wires with their ends connected together as shown in Fig. 35-2B. The feed-point impedance of the folded dipole is a pure resistance of approximately 290 ohms, or four times the feed-point resistance of a half-wave open dipole made from a single wire. This “resistance-multiplication” property makes the folded dipole ideal for use in parallel-wire transmission lines, which usually have high characteristic impedance (Zo) values.
Half-Wave Vertical
Imagine that we stand a half-wave radiator “on its end” and feed it at the base (the bottom end) against an earth ground, coupling the transmission line to the antenna through an LC circuit called an antenna tuner or transmatch designed to cancel out reactances over a wide range of values. Then we connect the other end of the feed line to a radio transmitter. This type of antenna works as an efficient radiator even in the presence of considerable loss resistance RL in the conductors, the surrounding earth, and nearby objects, because the radiation resistance RR is high.
Zepp
A zeppelin antenna, also called a zepp, comprises a half-wave radiator fed at one end with a quarter-wave section of parallel-wire transmission line, as shown in Fig. 35-2C. The impedance at the feed point is an extremely high, pure resistance. Because of the specific length of the transmission line, the transmitter “sees” a low, pure resistance at the operating frequency. A zeppelin antenna can function well at all harmonics of the design frequency. If we use a transmatch to “tune out” reactance, we can use any convenient length of transmission line.
Because of its non-symmetrical geometry, the zepp antenna allows some EM radiation from the feed line as well as from the antenna. That phenomenon can sometimes present a problem in radio transmitting applications. Amateur radio operators have an expression for it: “RF in the shack.” We can minimize problems of this sort by carefully cutting the antenna radiator to a half wavelength at the fundamental frequency, and by using the antenna only at (or extremely near) the fundamental frequency or one of its harmonics.
J Pole
We can orient a zepp antenna vertically, and position the feed line so that it lies in the same line as the radiating element. The resulting antenna, called a J pole, radiates equally well in all horizontal directions. The J pole offers a low-cost alternative to metal-tubing vertical antennas at frequencies from approximately 10 MHz up through 300 MHz. In effect, the J-pole is a half-wavelength vertical antenna fed with an impedance matching section comprising a quarter-wavelength section of transmission line. The J pole does not require any electrical ground system, a feature that makes it convenient in locations with limited real estate.
Some radio amateurs “hang” long J poles, cut for 3.5 MHz or 1.8 MHz, from kites or helium-filled balloons. Such an antenna offers amazing performance, but it presents a danger if not tethered to prevent it from breaking off and flying away with the kite or balloon. Such an antenna must never be “flown” where it might fall on a utility or power lines. Long, kite- or balloon-supported wire antennas can acquire massive electrostatic charges, even in clear weather—and they literally attract lightning. “Flying” such an antenna in unstable weather invites deadly disaster.
Quarter-Wave Verticals
The physical span of a quarter wavelength antenna is related to frequency according to the formula
Lft = 246v/fo
where Lft represents a quarter wavelength in feet, fo represents the frequency in megahertz, and v represents the velocity factor. If we express the length in meters as Lm, then the formula becomes
Lm = 75v/fo
For a typical wire conductor, v = 0.95 (95 percent); for metal tubing, v can range down to approximately 0.90 (90 percent).
A quarter-wavelength vertical antenna must be operated against a low-loss RF ground if we want reasonable efficiency. The feed-point value of RR over perfectly conducting ground is approximately 37 ohms, half the radiation resistance of a center-fed half-wave open dipole in free space. This figure represents radiation resistance in the absence of reactance, and provides a reasonable impedance match to most coaxial-cable type transmission lines.
Ground-Mounted Vertical
The simplest vertical antenna comprises a quarter-wavelength radiator mounted at ground level. The radiator is fed at the base with coaxial cable. The cable’s center conductor is connected to the base of the radiator, and the cable’s shield is connected to ground.
Unless we install an extensive ground radial system with a quarter-wave vertical antenna, it will exhibit poor efficiency unless the earth’s surface in the vicinity is an excellent electrical conductor (salt water or a salt marsh, for example). In receiving applications, vertically oriented antennas “pick up” more human-made noise than horizontal antennas do. The EM fields from ground-mounted transmitting antennas are more likely to interfere with nearby electronic devices than are the EM fields from antennas installed high above the ground.
Ground Plane
A ground-plane antenna is a vertical radiator, usually ¼ wavelength tall, operated against a system of ¼-wavelength conductors called radials. The feed point, where the transmission line joins the radiator and the hub of the radial system, is elevated. When we place the feed point at least ¼ wavelength above the earth, we need only three or four radials to obtain low loss resistance for high efficiency. We extend the radials straight out from the feed point at an angle between 0° (horizontal) and 45° below the horizon. Figure 35-3A illustrates a typical ground-plane antenna.
35-3 Basic quarter-wave vertical antennas. At A, the ground-plane design. At B, the coaxial design.
A ground-plane antenna works best when fed with coaxial cable. The feed-point impedance of a ground-plane antenna having a quarter-wavelength radiator is about 37 ohms if the radials are horizontal; the impedance increases as the radials droop, reaching about 50 ohms at a droop angle of 45°. You’ve seen ground-plane antennas if you’ve spent much time around Citizens Band (CB) fixed radio installations that operate near 27 MHz, or if you’ve done much amateur-radio activity in the very-high-frequency (VHF) bands at 50 or 144 MHz.
Coaxial Antenna
We can extend the radials in a ground-plane antenna straight downward, and then merge them into a quarter-wavelength-long cylinder or sleeve concentric with the coaxial-cable transmission line. We run the feed line inside the radial sleeve, feeding the antenna “through the end” as shown in Fig. 35-3B. The feed-point radiation resistance equals approximately 73 ohms, the same as that of a half-wave open dipole. This type of antenna is sometimes called a coaxial antenna, a term that arises because of its feed-system geometry, not merely because it’s fed with coaxial cable.
Loops
Any receiving or transmitting antenna made up of one or more turns of wire or metal tubing constitutes a loop antenna.
Small loop
A small loop antenna has a circumference of less than 0.1 wavelength (for each turn) and can function effectively for receiving RF signals. However, because of its small physical size, this type of loop exhibits low radiation resistance, a fact that makes RF transmission inefficient unless the conductors have minimal loss. While small transmitting loops exist, you won’t encounter them often.
Even if a small loop has many turns of wire and contains an overall conductor length equal to a large fraction of a wavelength or more, the radiation resistance of a practical antenna is a function of its actual circumference in space, not the total length of its conductors. Therefore, for example, if we wind 100 turns of wire around a hoop that’s only 0.1 wavelength in circumference, we have a low radiation resistance even though the total length of wire equals 10 full wavelengths!
A small loop exhibits the poorest response to signals coming from along its axis, and the best response to signals arriving in the plane perpendicular to its axis. A variable capacitor can be connected in series or parallel with the loop and adjusted until its capacitance, along with the inherent inductance of the loop, produces resonance at the desired receiving frequency. Figure 35-4 shows an example.
35-4 A small loop antenna with a capacitor for adjusting the resonant frequency.
Communications engineers and radio amateurs sometimes use small loops for radio direction finding (RDF) at frequencies up to about 20 MHz, and also for reducing interference caused by human-made noise or strong local signals. A small loop exhibits a sharp, deep null along its axis (perpendicular to the plane in which the conductor lies). When the loop is oriented so that the null points in the direction of an offending signal or noise source, the unwanted energy can be attenuated by upwards of 20 dB.
Loopstick
For receiving applications at frequencies up to approximately 20 MHz, a loopstick antenna can function in place of a small loop. This device consists of a coil wound on a solenoidal (rod-shaped), powdered-iron core. A series or parallel capacitor, in conjunction with the coil, forms a tuned circuit. A loopstick displays directional characteristics similar to those of the small loop antenna shown in Fig. 35-4. The sensitivity is maximum off the sides of the coil (in the plane perpendicular to the coil axis), and a sharp null occurs off the ends (along the coil axis).
Large Loop
A large loop antenna usually has a circumference of either a half wavelength or a full wavelength, forms a circle, hexagon, or square in space, and lies entirely in a single plane. It can work well for transmitting or receiving.
A half-wavelength loop presents a high radiation resistance at the feed point. Maximum radiation/response occurs in the plane of the loop, and a shallow, rather broad null exists along the axis. A full-wavelength loop presents a radiation resistance (and zero reactance, forming a purely resistive impedance) of about 100 ohms at the feed point. The maximum radiation/response occurs along the axis, and minimum radiation/response (though not a true null) exists in the plane containing the loop.
The half-wavelength loop exhibits a slight power loss relative to a half-wave open or folded dipole in its favored directions (the physical directions in which it offers the best performance). The full-wavelength loop shows a slight gain over a dipole in its favored directions. These properties hold for loops up to several percent larger or smaller than exact half-wavelength or full-wavelength circumferences. Resonance can be obtained by means of a transmatch at the feed point, even if the loop itself does not exhibit resonance at the frequency of interest.
An extremely large loop antenna, measuring several wavelengths in circumference, can be installed horizontally among multiple supports such as communications towers, trees, or wooden poles. We might call this type of antenna a giant loop. The gain and directional characteristics of giant loops are hard to predict. If fed with open wire line using a transmatch at the transmitter end of the line, and if placed at least a quarter wavelength above the earth’s surface, such an antenna can offer exceptional performance for transmitting and receiving.
Ground Systems
End-fed quarter-wavelength antennas require low-loss RF ground systems to perform efficiently. Center-fed half-wavelength antennas do not. However, good grounding is advisable for any antenna system to minimize interference and electrical hazards.
Electrical versus RF ground
Electrical grounding constitutes an important consideration for personal safety. A good electrical (that is, DC and utility AC) ground can help protect communications equipment from damage if lightning strikes in the vicinity. A good electrical ground also minimizes the risk of electromagnetic interference (EMI) to and from radio equipment. In a three-wire electrical utility system, the ground prong on the plug should never be defeated because such modification can result in dangerous voltages appearing on exposed metal surfaces.
A good RF ground system can help minimize EMI, even if it’s not necessary for efficient antenna operation. Figure 35-5 shows a proper RF ground scheme (at A) and an improper one (at B). In a good RF ground system, each device is connected to a common ground bus, which, in turn, runs to the earth ground through a single conductor. This conductor should have the smallest possible physical length. A poor ground system contains ground loops that can act like loop antennas and increase the risk of EMI.
35-5 At A, the correct method for grounding multiple hardware equipment. At B, an incorrect method for grounding multiple equipment, creating RF ground loops.
Radials and the Counterpoise
A surface-mounted vertical antenna should employ as many grounded radial conductors as possible, and they should be as long as possible. The radials can lie on the earth’s surface or be buried a few inches underground. In general, as the number of radials of a given length increases, the overall efficiency of any vertical antenna improves if all other factors remain constant. As the radial length increases and all other factors remain constant, vertical-antenna efficiency improves. The radials should all converge toward, and connect directly to, a ground rod at the feed point.
A specialized conductor network called a counterpoise can provide an RF ground without a direct earth-ground connection. A grid of wires, screen, or metal sheet is placed above the earth’s surface and oriented horizontally to obtain capacitive coupling to the earth’s conductive mass. A vertical antenna is located at the center of the counterpoise. This arrangement minimizes RF ground loss, although the counterpoise won’t provide a good electrical ground unless connected to a ground rod driven into the earth, or to the utility system ground. Ideally, a counterpoise should have a radius of at least a quarter wavelength at the lowest anticipated operating frequency.
Gain and Directivity
The power gain of a transmitting antenna equals the ratio of the maximum effective radiated power (ERP) to the actual RF power applied at the feed point. Power gain is expressed in decibels (dB). It’s usually expressed in an antenna’s favored direction or directions.
Suppose that the ERP, in watts, for a given antenna equals PERP, and the applied power, also in watts, equals P. We can calculate the antenna power gain using the formula
Power Gain (dB) = 10 log10 (PERP/P)
In order to define power gain, we must use a reference antenna with a gain that we define as 0 dB in its favored direction(s). A half-wavelength open or folded dipole in free space provides a useful reference antenna. Power gain figures taken with respect to a dipole (in its favored directions) are expressed in units called dBd. Some engineers make power-gain measurements relative to a specialized system known as an isotropic antenna, which theoretically radiates and receives equally well in all directions in three dimensions (so it has no favored direction). In this case, units of power gain are called dBi.
For any given antenna, the power gains in dBd and dBi differ by approximately 2.15 dB, with the dBi figure turning out larger. That is,
Power Gain (dBi) = Power Gain (dBd) + 2.15
An isotropic antenna exhibits a loss of 2.15 dB with respect to a half-wave dipole in its favored directions. This fact becomes apparent if we rewrite the above formula as
Power Gain (dBd) = Power Gain (dBi) − 2.15
Directivity Plots
We can portray antenna radiation patterns (for signal transmission) and response patterns (for reception) using graphical plots, such as those in Fig. 35-6. We assume, in all such plots, that the antenna occupies the center (or origin) of a polar coordinate system. The greater the radiation or reception capability of the antenna in a certain direction, the farther from the center we plot the corresponding point.
35-6 Directivity plots for a dipole. At A, the H-plane (horizontal plane) plot as viewed from high above the antenna. Coordinate numbers indicate compass-bearing (azimuth) angles in degrees. At B, the E-plane (elevation plane) plot as viewed from a point on the earth’s surface far from the antenna. Coordinate numbers indicate elevation angles in degrees above or below the plane of the earth’s surface.
A dipole antenna, oriented horizontally so that its conductor runs in a north-south direction, has a horizontal plane (or H-plane) pattern similar to Fig. 35-6A. The elevation plane (or E-plane) pattern depends on the height of the antenna above effective ground at the viewing angle. In most locations, the effective ground is an imaginary plane or contoured surface slightly below the actual surface of the earth. With the dipole oriented so that its conductor runs perpendicular to the page, and the antenna ¼ wavelength above effective ground, the E-plane pattern for a half-wave dipole resembles Fig. 35-6B.
Forward Gain
Forward gain is expressed in terms of the ERP in the main lobe (favored direction) of a unidirectional (one-directional) antenna compared with the ERP from a reference antenna, usually a half-wave dipole, in its favored directions. This gain is calculated and defined in dBd. In general, as the wavelength decreases (the frequency gets higher), we find it easier to obtain high forward gain figures.
Front-to-Back Ratio
The front-to-back (f/b) ratio of a unidirectional antenna quantifies the concentration of radiation/response in the center of the main lobe, relative to the direction opposite the center of the main lobe. Figure 35-7 shows a hypothetical directivity plot for a unidirectional antenna pointed north. The outer circle depicts the RF field strength in the direction of the center of the main lobe, and represents 0 dB (relative to the main lobe, not a dipole). The next smaller circle represents a field strength 5 dB down (a radiation/response level of −5 dB) with respect to the main lobe. Continuing inward, circles represent 10 dB down (−10 dB), 15 dB down (−15 dB), and 20 dB down (−20 dB). The origin represents 25 dB down (−25 dB) with respect to the main lobe, and also shows the location of the antenna. In this particular example, we can determine the f/b ratio by comparing the signal levels between north (azimuth 0°) and south (azimuth 180°). It appears to be 15 dB in Fig. 35-7.
35-7 Directivity plot for a hypothetical antenna in the H (horizontal) plane. We can determine the front-to-back and front-to-side ratios from such a graph. Coordinate numbers indicate compass-bearing angles in degrees.
Front-to-Side Ratio
The front-to-side (f/s) ratio provides us with another useful expression for the directivity of an antenna system. The specification applies to unidirectional antennas, and also to bidirectional antennas that have two favored directions, one opposite the other in space. We express f/s ratios in decibels (dB), just as we do with f/b ratios. We compare the EM field strength in the favored direction with the field strength at right angles to the favored direction. Figure 35-7 shows an example. In this situation, we can define two separate f/s ratios: one by comparing the signal level between north and east (the right-hand f/s ratio), and the other by comparing the signal level between north and west (the left-hand f/s ratio). In most directional antenna systems, the right-hand and left-hand f/s ratios theoretically equal each other. However, they sometimes differ in practice because of physical imperfections in the antenna structure, and also because of the effects of conducting objects or an irregular earth surface near the antenna. In the situation of Fig. 35-7, both the left-hand and right-hand f/s ratios appear to be roughly 17 dB.
Phased Arrays
A phased antenna array uses two or more driven elements (radiators connected directly to the feed line) to produce power gain in some directions at the expense of other directions.
End-Fire Array
A typical end-fire array consists of two parallel half-wave open dipoles fed 90° out of phase and spaced ¼ wavelength apart, as shown in Fig. 35-8A. This geometry produces a unidirectional radiation pattern. Alternatively, the two elements can be driven in phase and spaced at a separation of one full wavelength, as shown in Fig. 35-8B, producing a bidirectional radiation pattern. When we design the phasing system (also called the phasing harness), we must cut the branches of the transmission line to precisely the correct lengths, taking the velocity factor of the line into account.
35-8 At A, a unidirectional end-fire antenna array. At B, a bidirectional end-fire array.
Longwire
A wire antenna measuring a full wavelength or more, and fed at a high-current point or at one end, constitutes a longwire antenna. A longwire antenna offers gain over a half-wave dipole. As we increase the length of the wire, making sure that it runs along a straight line for its entire length, the main lobes get more and more nearly in line with the antenna, and their magnitudes increase. The power gain in the major lobes (that is, the strongest lobes, representing the favored directions) in a straight longwire depends on the overall length of the antenna; the longer the wire, the greater the gain. We can consider a longwire as a phased array because it contains multiple points along its span where the RF current attains maximum values. Each of these current loops acts like the center of a half-wave dipole or zepp antenna, so the whole longwire, in effect, constitutes a set of two or more half-wave antennas placed end-to-end, with each section phased in opposition relative to the adjacent section or sections.
Broadside Array
Figure 35-9 shows the geometric arrangement of a broadside array. The driven elements can each consist of a single radiator, as shown in this illustration, or they can consist of more complex antennas with directive properties. In the design shown here, all the driven elements are identical, and are spaced at ½-wavelength intervals along the phasing harness, which comprises a parallel-wire transmission line with a half-twist between each element to ensure that all the elements operate in phase coincidence. If we place a flat reflecting screen behind the array of dipoles in Fig. 35-9, we obtain a system known as a billboard antenna. The directional properties of any broadside array depend on the number of elements, on whether or not the elements have gain themselves, on the spacing among the elements, and on whether or not a reflecting screen is employed. In general, as we increase the number of elements, the forward gain, the f/b ratio, and the f/s ratios all increase.
35-9 A broadside array. The elements all receive their portions of the outgoing signal in phase, and at equal amplitudes.
Parasitic Arrays
Communications engineers use so-called parasitic arrays at frequencies ranging from approximately 5 MHz into the microwave range for obtaining directivity and forward gain. Examples include the Yagi antenna and the quad antenna. In the context of an antenna array, the term parasitic describes the characteristics of certain antenna elements. (It has nothing to do with parasitic oscillation, a phenomenon that can take place in malfunctioning RF power amplifiers.)
Concept
A parasitic element is an electrical conductor that forms an important part of an antenna system, but that we don’t connect directly to the feed line. Parasitic elements operate by means of EM coupling to the driven element or elements. When power gain occurs in the direction of the parasitic element, we call that element a director. When power gain occurs in the direction opposite the parasitic element, we call that element a reflector. Directors normally measure a few percent shorter than the driven element(s). Reflectors are normally a few percent longer than the driven element(s).
Yagi
The Yagi antenna, which radio amateurs sometimes call a beam antenna, or simply a beam, comprises an array of parallel, straight antenna elements, with at least one element acting in a parasitic capacity. (The term Yagi comes from the name of one of the original design engineers.) We can construct a two-element Yagi by placing a director or reflector parallel to, and a specific distance away from, a single half-wave driven element. The optimum spacing between the elements of a driven-element/director Yagi is 0.1 to 0.2 wavelength, with the director tuned 5 to 10 percent higher than the resonant frequency of the driven element. The optimum spacing between the elements of a driven-element/reflector Yagi is 0.15 to 0.2 wavelength, with the reflector tuned 5 to 10 percent lower than the resonant frequency of the driven element. Either of these designs give us a two-element Yagi. The power gain of a well-designed two-element Yagi in its single favored direction is approximately 5 dBd.
A Yagi with one director and one reflector, along with the driven element, forms a three-element Yagi. This design scheme increases the gain and f/b ratio as compared with a two-element Yagi. An optimally designed three-element Yagi exhibits approximately 7 dBd gain in its favored direction. Figure 35-10 is a generic example of the relative dimensions of a three-element Yagi. Although this illustration can serve as a crude “drawing-board” engineering blueprint, the optimum dimensions of a three-element Yagi in practice will vary slightly from the figures shown here because of imperfections in real-world hardware, and also because of conducting objects or terrain irregularities near the system.
35-10 A three-element Yagi antenna. See text for discussion of specific dimensions. The lowercase, italic Greek lambda (λ) means “wavelength.”
The gain, f/b ratio, and f/s ratios of a properly designed Yagi antenna all increase as we add elements to the array. We can obtain four-element, five-element, or larger Yagis by placing extra directors in front of a three-element Yagi. When multiple directors exist, each one should be cut slightly shorter than its predecessor. Some commercially manufactured Yagis have upwards of a dozen elements. As you can imagine, engineers must spend a lot of time “tweaking” the dimensions of such antennas to optimize their performance.
Quad
A quad antenna operates according to the same principles as the Yagi, except that full-wavelength loops replace the half-wavelength elements.
A two-element quad can consist of a driven element and a reflector, or it can have a driven element and a director. A three-element quad has one driven element, one director, and one reflector. The director has a perimeter of 0.95 to 0.97 wavelength, the driven element has a perimeter of exactly one wavelength, and the reflector has a perimeter of 1.03 to 1.05 wavelength. These figures represent electrical dimensions (taking the velocity factor of wire or tubing into account), and not free-space dimensions.
Additional directors can be added to the basic three-element quad design to form quads having any desired numbers of elements. The gain increases as the number of elements increases. Each succeeding director is slightly shorter than its predecessor. Long quad antennas are practical at frequencies above 100 MHz. At frequencies below approximately 10 MHz, quad antennas become physically large and unwieldy, although some radio amateurs have constructed quads at frequencies down to 3.5 MHz.
Antennas for Ultra-High and Microwave Frequencies
At ultra high frequencies (UHF) and microwave frequencies, high-gain, many-element antennas have reasonable physical dimensions and mass because the wavelengths are short.
Waveguides
A waveguide is a specialized RF transmission line comprising a hollow metal “pipe” or “duct” with a circular or rectangular cross section. The EM field travels down the waveguide quite efficiently, provided that the wavelength is short enough (or the cross-sectional dimensions of the pipe are large enough). In order to efficiently propagate an EM field, a rectangular waveguide must have height and width that both measure at least 0.5 wavelength, and preferably more than 0.7 wavelength. A circular waveguide should measure at least 0.6 wavelength in diameter, and preferably 0.7 wavelength or more.
The characteristic impedance (Zo) of a waveguide varies with the frequency. In this sense, a waveguide behaves differently than a coaxial or parallel-wire RF transmission line, whose Zo value remains independent of the frequency over the entire range of wavelengths for which the line is designed.
A properly installed and maintained waveguide acts as an exceptional RF transmission line because dry air has essentially zero loss, even at UHF and microwave frequencies. However, if we expect a waveguide to work properly, we must keep its interior free from dirt, dust, insects, spider webs, and condensation. Even a small obstruction can seriously degrade the performance and cause significant power loss.
The main limitation of a waveguide, from a practical standpoint, is its relative inflexibility, both figuratively and literally. We can’t run a waveguide from one point to another in a haphazard fashion, as we can do with coaxial cable. Bends or turns in a waveguide present a particular problem, because we must make them gradually. We can’t simply “turn a corner” with a waveguide! Another limitation involves the usable frequency range. Waveguides are impractical for use at frequencies below approximately 300 MHz because the required cross-sectional dimensions become prohibitively large.
Horn
The horn antenna has a characteristic shape like a squared-off trumpet horn. It provides a unidirectional radiation and response pattern, with the favored direction coincident with the opening of the horn. The feed line is a waveguide that joins the antenna at the narrowest point (throat) of the horn. Horns are sometimes used all by themselves, but they can also feed large dish antennas at UHF and microwave frequencies. The horn design optimizes the f/s ratio by minimizing extraneous radiation and response that occurs if a dipole is used as the driven element for the dish.
Dish
Most people are familiar with dish antennas because of their widespread use in consumer satellite TV and Internet services. Although the geometry looks simple to the casual observer, a dish antenna must be precisely shaped and aligned if we want it to function as intended. The most efficient dish, especially at the shortest wavelengths, comprises a paraboloidal reflector, so named because it’s a section of a paraboloid (the three-dimensional figure that we get when we rotate a parabola around its axis). However, a spherical reflector, having the shape of a section of a sphere, can also work in most dish-antenna system designs.
A dish-antenna feed system consists of a coaxial line or waveguide from the receiver and/or transmitter along with a horn or helical driven element at the focal point of the reflector. Figure 35-11A shows an example of conventional dish feed. Figure 35-11B shows an alternative scheme known as Cassegrain dish feed. The term Cassegrain comes from the resemblance of this antenna design to that of a Schmidt-Cassegrain reflector telescope. All dish antennas work well at UHF and microwave frequencies for transmitting and receiving. For hobby use, they’re physically impractical at frequencies much below 300 MHz.
35-11 Dish antennas with conventional feed (A) and Cassegrain feed (B).
As we increase the diameter of the dish reflector in wavelengths, the gain, the f/b ratio, and the f/s ratios all increase, and the width of the main lobe decreases, making the antenna more sharply unidirectional. A dish antenna must measure at least several wavelengths in diameter for proper operation. The reflecting element can consist of sheet metal, a screen, or a wire mesh. If a screen or mesh is used, the spacing between the wires must be a small fraction of a wavelength. At microwave frequencies, large dish antennas can have forward gain figures that exceed 35 dBd.
Helical
A helical antenna is a high-gain, unidirectional antenna that transmits and receives EM waves with circular polarization. Figure 35-12 illustrates the construction of a typical helical antenna. The reflector diameter should be at least 0.8 wavelength at the lowest operating frequency. The radius of the helix should be approximately 0.17 wavelength at the center of the intended operating frequency range. The longitudinal spacing between helix turns should be approximately 0.25 wavelength in the center of the operating frequency range. The entire helix should measure at least a full wavelength from end to end at the lowest operating frequency. When properly designed, this type of antenna can provide about 15 dBd forward gain. Helical antennas, like dish antennas, are used primarily at UHF and microwave frequencies.
35-12 A helical antenna with a flat reflector.
Corner Reflector
Figure 35-13 illustrates a corner reflector with a half-wave open-dipole driven element. This design provides some power gain over a half-wave dipole. The reflector is made of wire mesh, screen, or sheet metal. The flare angle of the reflecting element equals approximately 90°. Corner reflectors are widely used in terrestrial communications at UHF and microwave frequencies. For additional gain, several half-wave dipoles can be fed in phase and placed along a common axis with a single, elongated reflector, forming a collinear corner-reflector array.
35-13 A corner reflector that employs a dipole antenna as the driven element.
Safety
All engineers who work with antenna systems, particularly large arrays or antennas involving the use of long lengths of wire, place themselves in physical peril. By observing some simple safety guidelines, the risk can be minimized, but there’s never a complete guarantee of safety. Some basic guidelines follow.
Antennas must never be maneuvered or installed in such a way that they can fall or blow down on power lines. Also, it should not be possible for power lines to fall or blow down on an antenna, even in the event of a violent storm.
Wireless equipment having outdoor antennas should not be used during thundershowers, or when lightning exists anywhere in the vicinity. Antenna construction and maintenance should never be undertaken when lightning is visible or thunder can be heard, even if a storm appears far away. Ideally, antennas should be disconnected from electronic equipment, and connected to a substantial earth ground, at all times when the equipment is not in use.
Tower and antenna climbing constitutes a job for professionals only. No inexperienced person should ever attempt to climb any antenna support structure.
Indoor transmitting antennas can expose operating personnel to EM field energy. The extent of the hazard, if any, posed by such exposure has not been firmly established. However, sufficient concern exists among some experts to warrant checking the latest publications on the topic.
Quiz
Refer to the text in this chapter if necessary. A good score is at least 18 correct. Answers are in the back of the book.
1. An antenna has a radiation resistance of 900 ohms and a loss resistance of 100 ohms. What’s its efficiency to the nearest percentage point?
(a) 66 percent
(b) 75 percent
(c) 90 percent
(d) 93 percent
2. An antenna has a radiation resistance of 10 ohms and a loss resistance of 40 ohms. What’s its efficiency to the nearest percentage point?
(a) 20 percent
(b) 25 percent
(d) 67 percent
3. A circular loop antenna that measures exactly one wavelength in circumference, and that lies entirely in a single plane, radiates best
(a) in all possible directions; it’s isotropic.
(b) in the plane perpendicular to the loop axis.
(c) along the loop axis in both directions.
(d) along the loop axis in one direction.
4. How long is a wavelength in free space at 7.05 MHz? Round your answer to three significant figures.
(a) 279 ft
(b) 140 ft
(c) 69.8 ft
(d) 34.9 ft
5. How long should you cut either side of a straight, center-fed, half-wavelength open dipole wire antenna for operation at a fundamental frequency of 7.05 MHz, taking into account the typical velocity factor for thin wire? Round your answer to three significant figures.
(a) 33.1 ft
(b) 66.2 ft
(c) 132 ft
(d) 265 ft
6. The feed-point impedance of a half-wave, center-fed, two-wire folded dipole is a pure resistance of
(a) eight times that of a half-wave open dipole made from a single wire.
(b) four times that of a half-wave open dipole made from a single wire.
(c) twice that of a half-wave open dipole made from a single wire.
(d) the same as that of a half-wave open dipole made from a single wire.
7. Even if an antenna system has a lot of loss resistance, it can work efficiently if you design it to have an extremely
(a) low reactance.
(b) high reactance.
(c) low radiation resistance.
(d) high radiation resistance.
8. How tall is a ground-mounted, quarter-wavelength vertical antenna designed for 10 MHz? Assume that the tubing used for the radiating element has a velocity factor of 92 percent. Express your answer to two significant figures.
(a) 6.9 ft
(b) 14 ft
(c) 23 ft
(d) 45 ft
9. You want to transmit radio signals with a base-fed, inductively loaded, short vertical antenna over perfectly conducting ground. The inductor comprises a low-loss coil. You feed the antenna with low-loss 50-ohm coaxial cable. Your radio is designed to work into a nonreactive, 50-ohm load. You’ll get the best results if you
(a) make the antenna less than ⅛ wavelength tall.
(b) use thick copper tubing as the radiating element.
(c) install a wide-range antenna tuner at the feed point.
(d) feed the antenna directly with the cable.
10. In terms of physical size, waveguides are practical at all frequencies
(a) below 3 MHz.
(b) below 300 MHz.
(c) above 3 MHz.
(d) above 300 MHz.
11. The feed-point impedance of a Zepp antenna is a pure,
(a) high resistance.
(b) low resistance.
(c) high capacitive reactance.
(d) high inductive reactance.
12. In a three-element Yagi, the director is half-wave resonant at
(a) the same frequency as that of the driven element.
(b) twice that of the driven element.
(c) a slightly lower frequency than that of the driven element.
(d) a slightly higher frequency than that of the driven element.
13. To get reasonable efficiency with a half-wavelength vertical antenna mounted over ground with fair conductivity (using a wide-range antenna tuner at the antenna’s base to match the radiator to the feed line), you should
(a) make the radiating element a thick, solid metal rod.
(b) use a ground rod and a few radials (you won’t need many).
(c) bury at least 100 radials measuring a half wavelength or more.
(d) salt the earth near the antenna to improve the ground conductivity.
14. Imagine a ground-plane antenna measuring ¼ wavelength tall, with four radials each ¼ wavelength long and drooping at 45°. You elevate the feed point at least ¼ wavelength above the surface and let the radials double as guy wires. This antenna has a purely resistive feed-point impedance of approximately
(a) 37 ohms.
(b) 50 ohms.
(c) 73 ohms.
(d) 100 ohms.
15. When the total loss resistance in an antenna equals exactly half the feed-point radiation resistance, the antenna’s efficiency is
(a) 33 percent.
(b) 50 percent.
(c) 67 percent.
(d) 75 percent.
16. In a seven-element Yagi, the longest element is the
(a) driven element.
(b) first director.
(c) last director.
(d) reflector.
17. A small loopstick receiving antenna is most sensitive to signals arriving from
(a) all possible directions; it’s isotropic.
(b) directions in the plane perpendicular to the loop axis.
(c) along the loop axis in both directions.
(d) along the loop axis in one direction.
18. Which of the following transmission-line types is ideally suited for use with a half-wave folded dipole?
(a) Parallel-wire line
(b) Waveguide
(c) Coaxial cable
(d) Any of the above; it doesn’t matter
19. Which of the following antennas works best for satellite TV?
(a) Folded dipole
(b) Ground plane
(c) Loopstick
(d) Dish
20. Which of the following antenna types can provide significant power gain over a half-wave dipole?
(a) Yagi
(b) Dish
(c) Helical
(d) All of the above