Practical_Antenna_Handbook_0071639586
C h a p t e r 5 : a n t e n n a A r r a y s a n d A r r a y G a i n 157 pattern breaks up into multiple smaller lobes. Thus, the broadside gain of an 80-m l/2 dipole operated on 40 or 30 m is greater than the 80-m gain; on 40 m the increase is on the order of 1.5 dB. The increased gain is accomplished through a narrowing (or “sharpening”) of the main radiation lobe. On 30 m, the gain of the main lobe is still about 0.5 dB greater than on 80, but it is accompanied by two additional lobes exhibiting gain equal to the main lobe but somewhat broader at their 3-dB points. A horizontal wire antenna 5/4 l long on any band is known as an extended double Zepp (EDZ) (or, alternatively, double extended Zepp) for that band. See Chap. 6 (“Dipoles and Doublets”) for further discussion of the EDZ. Of course, if one side of the dipole antenna is thrown away and the remaining side oriented vertically and fed against a ground plane as a vertical monopole, the length for maximum broadside gain is one half of 5/4 l, or 5/8 l; hence, the popularity of VHF and UHF whips of that length! An entire class of antenna—the loop, which we’ll discuss in later chapters—is actually a form of array. To see this, consider the bent dipole. This is nothing more than an ordinary l/2 dipole with the outer half of each side bent straight down. If a second such bent dipole is turned upside down, attached at its ends to the corresponding ends of the first dipole, and one of the two feedpoints shorted out, we have what everyone calls a loop antenna. (See Fig. 5.4.) Because of the “automatic” phase reversal in adjacent l/2 sections of a longwire, the currents at the centers of the two dipoles are always in phase and we have a two-element broadside array, with the elements spaced l/2 apart. The loop can be suspended so that it lies in a horizontal plane, a vertical plane, or some plane partway in between. If suspended vertically and only a single high support is available, the loop takes a triangular form and is called a delta loop. If multiple loops are attached to a common boom, the resulting “array of arrays” is called a quad. Regardless of its name, however, each loop is essentially two half-waves in phase, and the direction of maximum radiation for that array is broadside to the plane formed by its component dipoles. Further, there is nothing magic about feeding the loop at the center of the bottom dipole. As long as the entire wire path around the loop is one wavelength, the loop can tie together 8 8 8 8 feedpoint 8 8 tie together tie together 8 8 Figure 5.4 The full-wave loop array of two bent dipoles. Arrows show direction of current everywhere at a given instant.
) ) ) ) ) ) 158 p a r t I I : F u n d a m e n t a l s Anywhere to left of element 1: I 1 = I 2 , so I 1 lags I 2 by 90˚. Anywhere to left of element 2: I 1 = -I 2 , so I total = 2I I 1 ) ) ) I 2 ) ) ) I total = I 1 = -I 2 = 0. 4 Figure 5.5A Two-element quadrature or cardioid array. be broken and fed anywhere on that path. If a vertically suspended loop is fed in the middle of a side, it becomes predominantly vertically polarized and may well exhibit better ability to work or hear long-haul DX. If a horizontally suspended loop is fed from the “next” side, the azimuthal orientation of maximum performance will rotate 90 degrees. Unfortunately for the HF loop, low dipoles and arrays of low dipoles are (usually adversely) affected by the ground beneath them. The effect of the ground, in fact, is to create yet another array with its own array factor, so proper analysis of the operation of a loop antenna or array must take that into account. (Array factors multiply each other.) Ground reflection effects are discussed later in this chapter. Chapter 13 goes into loops and quads in much more detail. Thus far we have limited the discussion of arrays to those with feedpoint currents that are either in phase or out of phase. Actually, any phase relationship between feedpoint currents can be used, but as a rule the math gets more complicated, the patterns less intuitive, and the physical implementations more difficult when the phase difference between fed elements is other than 0 or 180 degrees. One popular array that employs quadrature phasing of element currents is the two-element cardioid array (Fig. 5.5A). Here two identical omnidirectional radiators (vertical monopoles, for instance) spaced l/4 apart are fed Outer circle = 3.1 dB greater than single element. Figure 5.5B Cardioid pattern using omnidirectional radiating elements. with currents of identical magnitude phased 90 degrees apart. Visualizing how the array pattern is formed is not that much more difficult than for the broadside and in-line arrays: Suppose the current in element 1 lags the current in element 2 by 90 degrees. By the
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C h a p t e r 5 : a n t e n n a A r r a y s a n d A r r a y G a i n 157<br />
pattern breaks up into multiple smaller lobes. Thus, the broadside gain of an 80-m l/2<br />
dipole operated on 40 or 30 m is greater than the 80-m gain; on 40 m the increase is on<br />
the order of 1.5 dB. The increased gain is accomplished through a narrowing (or “sharpening”)<br />
of the main radiation lobe. On 30 m, the gain of the main lobe is still about 0.5<br />
dB greater than on 80, but it is accompanied by two additional lobes exhibiting gain<br />
equal to the main lobe but somewhat broader at their 3-dB points. A horizontal wire<br />
antenna 5/4 l long on any band is known as an extended double Zepp (EDZ) (or, alternatively,<br />
double extended Zepp) for that band. See Chap. 6 (“Dipoles and Doublets”) for<br />
further discussion of the EDZ.<br />
Of course, if one side of the dipole antenna is thrown away and the remaining side<br />
oriented vertically and fed against a ground plane as a vertical monopole, the length for<br />
maximum broadside gain is one half of 5/4 l, or 5/8 l; hence, the popularity of VHF<br />
and UHF whips of that length!<br />
An entire class of antenna—the loop, which we’ll discuss in later chapters—is actually<br />
a form of array. To see this, consider the bent dipole. This is nothing more than an<br />
ordinary l/2 dipole with the outer half of each side bent straight down. If a second such<br />
bent dipole is turned upside down, attached at its ends to the corresponding ends of the<br />
first dipole, and one of the two feedpoints shorted out, we have what everyone calls a<br />
loop antenna. (See Fig. 5.4.) Because of the “automatic” phase reversal in adjacent l/2<br />
sections of a longwire, the currents at the centers of the two dipoles are always in phase<br />
and we have a two-element broadside array, with the elements spaced l/2 apart. The<br />
loop can be suspended so that it lies in a horizontal plane, a vertical plane, or some<br />
plane partway in between. If suspended vertically and only a single high support is<br />
available, the loop takes a triangular form and is called a delta loop. If multiple loops are<br />
attached to a common boom, the resulting “array of arrays” is called a quad. Regardless<br />
of its name, however, each loop is essentially two half-waves in phase, and the direction<br />
of maximum radiation for that array is broadside to the plane formed by its component<br />
dipoles.<br />
Further, there is nothing magic about feeding the loop at the center of the bottom<br />
dipole. As long as the entire wire path around the loop is one wavelength, the loop can<br />
tie together<br />
8<br />
8<br />
8<br />
8<br />
feedpoint<br />
8<br />
8<br />
tie together<br />
tie together<br />
8<br />
8<br />
Figure 5.4 The full-wave loop array of two bent dipoles. Arrows show direction of current everywhere at a<br />
given instant.