Antennas

One of the most common types of transducers is the radio antenna. An antenna performs either of two functions: conversion of an alternating electric current into an electromagnetic field, or vice versa. For such a simple function, antennas can get awfully complicated. We can only outline some of the most often-used kinds of antennas in this section; whole volumes have been devoted to the subject. For more information an antenna text should be consulted .

RECEIVING ANTENNAS

A receiving antenna converts an electromagnetic field into an alternating current having the same frequency characteristics. Note that we say characteristics; an electromagnetic field rarely has just one frequency. The simplest form of receiving antenna is a length of conducting material, such as wire or metal tubing. Theoretically, every piece of conducting material (or even semi-conducting) has currents flowing in it as the result of electromagnetic energy at radio frequencies. Even your own body is affected to some extent. (In recent years there has arisen some concern that man-made electromagnetic fields might have some adverse health effects because of this fact.)

If you have engaged in shortwave listening, then you know that (in general) the physical size of a receiving antenna makes a difference in how well it will perform. it's not as simple a proposition as “the longer the better,” though, except at low and very low frequencies. The important consideration is resonance. The antenna, in order to work at its best, must display a purely resistive impedance without reactance. That is, it must exhibit no capacitance or inductance at the receiver input. The resonant condition occurs at a certain frequency, normally that at which the antenna is one- half wavelength long, and at all integral multiples of that frequency. For a given frequency in megahertz, the length of a resonant antenna in feet is given approximately by

L = 468/f

where L represents the physical length of a wire antenna. If metal tubing is used, then L is somewhat shorter than would be indicated by the above formula; a typical value might be

L = 450/f

Radiation Resistance

When an antenna is operating at resonance—the absence of reactance—the impedance R + jX is purely resistive. That is, X = 0, and R has a certain finite value. For a half-wave antenna, as given by the above equations, the value of R is about 73 Ohm for a thin wire in free space. If tubing is used, or if there are objects in the vicinity of the antenna that might affect the impedance, or if the antenna is not straight, then the value of R is somewhat different from the nominal 73 Ohm. The value of R tends to be increased by the presence of objects, especially near the ends of the antenna. The value of R goes down if the apex angle is much less than 180 deg. (ill. 24-1).

ill. 24-1. Impedance dipole.

If an antenna is operated at an odd harmonic of the fundamental frequency (that is, a multiple of 3, 5, 7, and so on), then the radiation resistance will be fairly close to the value at the fundamental frequency. But the situation is much different at even harmonics (multiples of 2, 4, 6, and so on). Resonance occurs at these frequencies also, but the radiation resistance is many times higher; theoretically, it's infinite, but in practice it can be hundreds or thousands of ohms.

Radiation resistance is important because we must know its value in order to obtain the optimum impedance match between an antenna and the feed line. Ideally, the feed line should have a characteristic impedance that is identical to the radiation resistance of the antenna at resonance. Common coaxial feed lines are available with characteristic impedances of about 50 Ohm and 75 Ohm, which closely approximates the radiation resistance of a half-wave antenna in free space. For second-harmonic operation of a half-wave antenna, or for operation at any even harmonic, common feed lines don't work well because the radiation resistance is so high that practical feed-line design will not provide a high-enough value of characteristic impedance. If the feed line is designed for extremely low loss, however, a fairly severe impedance mismatch can be tolerated.

Reactance

At most frequencies an antenna is not resonant but shows capacitive or inductive reactance as well as resistance. Just below the resonant frequency of a half-wave antenna, the reactance is capacitive; that is, X is negative in the complex impedance R + jx. A perfect impedance match between an antenna and the feed line is impossible when capacitive reactance is present, unless it's tuned out by means of a series inductance, lithe antenna is a little too long for resonance—that is, the frequency is slightly above the resonant frequency of the antenna—there will be inductive reactance, and X will be positive. A perfect match can't be obtained if inductive reactance is present in the antenna, unless it's tuned out by means of a series capacitance. Figure 24-2 shows examples of inductive loading (A) and capacitive loading (B) for tuning out reactances in an antenna system.

ill. 24-2. Inductive loading (A) and capacitive loading (B) in a dipole antenna.

The above discussion might make it appear as if a perfect impedance match is absolutely necessary between an antenna and the feed line. This is not really true; a little reactance can be tolerated, and it's not essential that the radiation resistance of an antenna exactly match the characteristic impedance of the feed line. A near-perfect match is more important in some situations than in others. Generally, as the frequency of operation increases, the importance of a perfect match becomes greater. At low and very- low frequencies, large mismatches may be of no consequence. But at very-high, ultrahigh, and microwave frequencies, a good match is very important.

TYPES OF RECEIVING ANTENNAS: EXAMPLES

A “random wire” is the simplest form of receiving antenna:

In conjunction with a tuning network at the receiver input, such an antenna can provide good reception over a wide range of frequencies. The tuning network eliminates the reactance at the feed point and matches the radiation resistance of the antenna to the characteristic impedance of the receiver input (ill. 24-3).

ill. 24-3. A matching/tuning network for a random-wire antenna.

A random wire is an unbalanced form of antenna and requires a good earth ground for optimum functioning. In general, the wire should be as high as possible above the ground and should be made as long as possible, especially for reception at low or very-low frequencies.

A specialized form of wire antenna is a single wire having a length that is a multiple of one-quarter wavelength. This kind of antenna, like the random wire, requires a good earth ground and often will also need a matching system. The advantage of the resonant single wire is that the matching network can be made simple, because there is no reactance at the receiver input. In fact, if the wire is an odd multiple of one-quarter wavelength, it may not be necessary to use a matching network at all. The chief disadvantage of the resonant wire is that performance is optimum only at the resonant frequency or frequencies.

A simple half-wave dipole antenna is shown in ill. 24-4. This is a balanced antenna, although it can be fed with unbalanced coaxial line with little degradation of performance as compared with a balanced parallel-wire line. The half-wave dipole presents a good match for either 50 Ohm or 75 Ohm feed lines. The antenna will operate well at all odd harmonics of the fundamental frequency and reasonably well at even harmonics or frequencies not related to the fundamental, provided the feed line is not too long and a matching network is employed at the receiver input.

ill. 24-4. A half-wave dipole can be fed with coaxial line as shown here.

A quarter-wave vertical antenna is shown in ill. 24-5. This antenna requires a good earth ground for effective performance. It presents a reasonable match for 50 Ohm coaxial cable. The ground- mounted vertical relies so heavily on an excellent ground that radial wires are recommended to be installed in a manner similar to that shown. The radials enhance the conductivity of the earth in the vicinity of the antenna, minimizing loss and thereby maximizing efficiency.

The vertical antenna will function well at odd harmonics of the resonant (quarter-wave) frequency without the use of a matching network. At even harmonics, if the feed line is short, a matching network at the receiver input will provide good performance. If the matching network can tune out reactance, a vertical antenna will work well at any frequency down to about the one-eighth wave value.

ill. 24-5. A ground-mounted quarter-wave vertical antenna.

A vertical antenna may be elevated above ground and the radial system replaced by three or four horizontal or slightly drooping “spokes” as shown in ill. 24-6. This kind of antenna is called a ground plane. The fundamental resonant frequency is that frequency at which the vertical element and the radial “spokes” all measure one-quarter electrical wavelength. Performance is similar to that of a ground-mounted vertical antenna with an extensive radial system.

ill. 24-6. A ground-plane antenna.

Both of the above-described vertical antennas are unbalanced systems and should, ideally, be used with unbalanced feed lines such as coaxial cable. it's possible to use parallel-wire feed lines with unbalanced antennas, but there will be some sacrifice of feed-line efficiency.

Receiving antennas enjoy a certain advantage over transmitting antennas in that physical size is not that important, provided the design is optimized. (We will have more to say about this later.) The radiation resistance of a receiving antenna can be extremely low and performance quite good nonetheless. This is not the case with a transmitting antenna. As a result of this fact, tiny receiving antennas can be designed and made quite sensitive. By “tiny” I mean as small as your little finger for operation at low and medium frequencies. Most small transistor radios employ antennas of this kind. The device is called a ferrite loopstick.

Figure 24-7 is an illustration of a ferrite loopstick antenna employing a single stage of amplification. The amplifier uses a dual- gate MOSFET, providing about 15 to 20 dB of gain. The resonant frequency of the antenna is determined by means of a variable capacitor. The wire coil, wound around a small solenoidal piece of powdered-iron or ferrite material, acts as a much larger antenna when the capacitor is tuned so that parallel resonance occurs. The impedance then becomes very high—on the order of thousands of ohms—and the tiny coil acts as a much larger resonant system. This kind of antenna works best at frequencies below about 20 MHz.

ill. 24-7. Example of a ferrite-loopstick antenna with one stage of amplification.

Another small receiving antenna, also employing an amplifier stage, is shown in ill. 24-8. This antenna uses a short “whip,” perhaps 2 feet in length, and a parallel-resonant tuned circuit. This kind of antenna will function very well into the very-high range, or up to about 300 MHz, provided good circuit design is employed in the amplifier.

TRANSMITTING ANTENNAS

Many types of receiving antennas will also work for transmit ting. A transmitting antenna converts an alternating current into an electromagnetic field having the same frequency characteristics. The random-wire, tuned-wire, vertical, ground-plane, and various other types of receiving antennas can be effectively used to transmit signals. But the ferrite loopstick and whip antennas (shown in Figs. 24-7 and 24-8) will not work well for transmitting. Their radiation resistance is very low, and most of the transmitter output power would be wasted as loss if these types of antennas were used for transmitting.

ill. 24-8. Example of a whip-antenna system employing one stage of amplification.

Radiation resistance is a direct function of the physical size (in terms of the wavelength) of an antenna. For receiving, a very low radiation resistance can be tolerated. But for transmitting it can not. In a transmitting antenna the efficiency E of an antenna is given, in percent, by the equation

E = 100 (RR / (RR + RL))

where RR is the radiation resistance and RL is the loss resistance. If the radiation resistance is very low, then it's more likely to be low in relation to the loss resistance, which is a function of ground conductivity and antenna conductor resistance. Consider the ex ample where RL = 10 Ohm, a typical value. If RR = 73 Ohm, then

E = 100 (73 / (10 + 73)) = 100 (73 / 83)

But if RR = 1 Ohm—a liberal estimate, at best, for a whip antenna at medium frequencies—then

E = 100 (1 / (10 + 1)) = 100 (1 / 11) = 9%

Very poor. In practice, it might be far less than even this. For a ferrite loopstick antenna, the efficiency might be less than 0.001%. The power not radiated would be dissipated as heat, mostly in the ferrite or powdered iron. It would not take much transmitter power to crack the core in such an instance.

The consideration of radiation resistance is the main difference between a transmitting antenna and a receiving antenna. Transmit ting antennas must be physically large—at least a few percent of the free-space wavelength—if they are to function well. In theory, even a tiny antenna, say a paper clip at 3.5 MHz, can be made efficient for transmitting if the loss resistance can be made small enough, but there is a practical limit to how low the value RL can be.

Transmitting antennas must not only be at least a certain physical size, but they have to be capable of handling the power dissipated in the conductor(s). A length of AWG No. 40 wire might function well with a 1-W transmitter, but it would melt if subjected to an RF signal of 10 kW.

Outside these fundamental constraints, any kind of antenna can be used equally well for either receiving or transmitting. All gain and loss factors, with the above exceptions, apply to transmitting antennas in the same way as to receiving antennas.

ANTENNA DIRECTIVITY and GAIN

A truly omnidirectional antenna—a device that radiates and picks up signals equally well in all directions in three dimensions—is a theoretical ideal not achievable in practice. Any antenna has favored directions, in which the radiation and response are maximum, and weak or null directions. The half-wave dipole responds and radiates best in directions perpendicular to the wire, assuming the antenna is perfectly straight. The response and radiation are minimum (theoretically zero) in a line containing the wire. In intermediate directions the response and radiation are variable, generally getting smaller and smaller in directions nearer and nearer to the line containing the wire. The response-radiation pattern for a half-wave dipole antenna is shown in ill. 24-9.

ill. 24-9. Radiation pattern of horizontal halt-wave dipole, as viewed from directly overhead.

A vertical (quarter-wave) or ground-plane antenna has similar radiation and response characteristics. The maximum radiation or response is in the horizontal direction; theoretically, zero radiation- response occurs in a vertical direction (straight up). This is shown in ill. 24-10.

ill. 24-10. Radiation pattern of vertical quarter-wave antenna, as viewed from the horizontal.

The directional properties of antennas change when they are operated at harmonic frequencies. In the case of a dipole operated at the second harmonic frequency, where it measures 1 wavelength, the lobes are sharper than in the half-wave case, and about 2 dB of gain occurs over the half-wave case (ill. 24-hA). At the third harmonic, where the antenna measures one and one-half wave lengths, multiple lobes occur (ill. 24-11B). A similar thing hap pens with the vertical and ground-plane antennas. If an antenna is operated at a very large harmonic value—say, the 51st harmonic—there are many lobes, and there may also be considerable gain in some of them.

ill. 24-11. Top view radiation pattern for dipole operated at second harmonic (A) and third harmonic (B).

A long-wire antenna provides perhaps the most illustrative ex ample of gain resulting from operation at a harmonic frequency. An end-fed long-wire can be operated as a quarter-wavelength radiator against ground, and the radiation pattern will resemble that of a half-wave dipole. The only difference will be a slight loss with respect to the dipole. A half-wave end-fed radiation behaves just like a half-wave dipole, except that the feed-point impedance is very high. But when the radiator is made longer than one-half wave- length, either by lengthening the wire itself or by raising the operating frequency, gain is generated and multiple lobes occur. Figure 24-12 shows the orientation of the main (strongest) lobe, approximately, for long-wire antennas measuring 1, 2, 4, and 8 wavelengths. Figure 24-13 is a graph showing the approximate gain, in decibles relative to a half-wave dipole (dBd), as a function of long-wire length.

ill. 24-12. Radiation-pattern maxima for long-wire antenna having lengths of one wavelength (A), two wavelengths (B), four wavelengths (C) and eight wavelengths (D). These are top views.

ill. 24-13. Theoretical gain, over a half-wave dipole, of a long-wire antenna as a function of the length.

Variations of the long-wire antenna, employed for obtaining gain, are the V beam and the rhombic. These types of antennas use multiple long-wire elements for bidirectional or unidirectional operation.

Another method of obtaining gain in an antenna is the use of phased elements. Two vertical antennas may be placed one-half wavelength apart, e.g., and driven in phase to obtain bidirectional gain of about 3 dBd (ill. 24-14A). This is called broad side array because the gain occurs in a direction broadside to the plane containing the radiators. Another form of phased-vertical array, known as a collinear antenna, is shown in ill. 24-14B. The elements are aligned along a common line, and gain occurs in all directions perpendicular to that line (horizontally in the case of the collinear vertical). There are many other methods of obtaining gain and directivity by means of phased elements; ill. 24-14 is only in tended as a sample illustration.

ill. 24-14. Two methods of obtaining gain by phasing. At A, a broadside phased array, viewed from the top and employing two vertical antennas. At B, a vertical collinear array.

Yet another way to obtain gain and directivity is by the use of parasitic elements. This is a rather interesting phenomenon that involves proximity coupling between an antenna-driven element and a free element nearby. Electromagnetic coupling takes place between an antenna and surrounding conducting objects, and in some cases this coupling can greatly affect the radiation pattern of the antenna. If the nearby conductor is about the same length as the active (driven) element, is close by, and is parallel to the active element, radical changes occur in the directional pattern. The parasitic element may act either as a “reflector” (ill. 24-15A) or a “director” (ill. 24-15B). With a single reflector or director, gain of up to about 6 dBd can be realized if the parasitic element is just the right length and is placed at just the right distance from the active element.

ill. 24-15. Examples of yagi antennas using a single reflector (A) and a single director (B).

Generally, only one reflector is used in a parasitic array. But two or more directors can be employed, along with one reflector, to obtain considerable gain over a dipole. The physical details of parasitic-array construction are beyond the scope of this guide, but several good texts are available that discuss parasitic-array design.

At very-high and ultrahigh frequencies, various high-gain antennas are used. A dish antenna, in which a large spherical or parabolic reflector is used, can give extreme gain—up to 30 dBd or more—if the reflector is large enough. At short wavelengths a truly large reflector is not that unwieldy. The principle of operation is just like that of a visible-light reflecting device (ill. 24-16). The driven element is placed at the focal point of the reflector, which serves to collimate, or make parallel, the transmitted rays of electromagnetic energy. For receiving, the rays converge at the focal point where they are picked up by the active element.

ill. 24-16. A dish antenna operates by collimating the outgoing waves and by bringing incoming waves to a focal point.

Another often-used high-gain antenna at very-high and ultrahigh frequencies is the helical antenna. As its name implies, it consists of a helically wound conductor, measuring about one- third wavelength in diameter and having at least several turns (ill. 24-17). A reflector is generally used to enhance the gain. The helical antenna has circular polarization; that is, the electric lines of flux rotate as they are emitted during transmission, and the antenna is not sensitive to the orientation of the received lines of flux. Electromagnetic-field polarization is an important consideration especially at very-high and ultrahigh frequencies; it can also have an effect at lower frequencies.

ill. 24-17. Structure of a helical antenna.

POLARIZATION

An electromagnetic field can be thought of, on a small scale, as electric and magnetic lines of flux that continually alternate in polarity (direction) and are always mutually perpendicular. An electromagnetic field propagates outward from the source in spherical wave fronts that travel at about 186,282 mi/sec (299,792 km/sec) in free space. If we could see the electric and magnetic lines of flux in an electromagnetic field at a great distance from the source, they would appear as shown in ill. 24-18.

ill. 24-18. An electromagnetic field consists of electric flux (solid lines) perpendicular to magnetic flux (dotted lines). The direction of propagation is orthogonal to both the electric and magnetic flux lines.

The polarization of an electromagnetic field is generally defined as the orientation of the electric lines of flux. The electric lines of flux are normally parallel to the orientation of the radiating element. A horizontal half-wave dipole antenna, e.g., produces electric lines of flux that are horizontal; a vertical ground-plane antenna produces electric lines of flux that are vertical. Certain types of antennas (e.g., the helical antenna) produce electric lines of flux that turn around and around as they travel through space; this kind of polarization is said to be circular.

At very-low and low frequencies, vertical polarization is preferable to horizontal polarization. This is because the electromagnetic waves at very-low and low frequencies tend to be propagated in contact with the surface of the earth, and vertical electric lines of flux travel better under such conditions than horizontal electric fields. At medium frequencies, vertical polarization is still desirable for surface-wave propagation, but long-distance skywave propagation is possible with either vertical or horizontal polarization.

At high frequencies the polarization is of little consequence. Good long-distance skywave propagation can be had with either vertical or horizontal polarization. At the extreme low end of the high-frequency spectrum—perhaps from about 3 to 6 MHz—some surface-wave propagation occurs with vertical polarization favored over horizontal, but the effect is not too significant.

At very-high frequencies and also at ultrahigh frequencies (30 to 300 MHz and 300 MHz to 3 GHz, respectively), polarization takes on a different significance. Frequency-modulated commercial and amateur signals are usually vertically polarized, whereas continuous-wave and single-sideband signals are horizontally polarized. Broadcast stations use combined vertical and horizontal polarizations. There is no technological reason for this; it's only a matter of convention.

Infrared, visible-light, and even ultraviolet and X-ray energy can be polarized. Normally it's not; the infrared from hot coals, the visible light from an incandescent lamp, and the ultraviolet rays from the sun are randomly polarized. Polarizing filters can be used, however, to eliminate radiation that is polarized in a certain direction at these wavelengths.

Polarization can be varied in a controlled way, such as around and around as the signal is propagated through space. This is called elliptical polarization, lithe signal strength remains constant throughout a full rotation, the polarization is called circular. There is an advantage to using circular polarization at radio frequencies, because, if circular polarization is used at one end of a communications circuit, it does not matter what the polarization is at the other end. This is valuable in satellite communications, e.g., where the orientation of the satellite varies, and consequently the polarization of its antennas changes with time. A circularly polarized ground-station antenna eliminates the fading that would occur if a linearly polarized antenna were used.

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