Practical_Antenna_Handbook_0071639586
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Practical_Antenna_Handbook_0071639586

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240 P a r t I I I : H i g h - F r e q u e n c y B u i l d i n g - B l o c k A n t e n n a s Figure 9.7 Impedance matching through broadband transformer. A little math will show that designing your radial system such that radial tips are no more than 0.05 l apart leads to the following relationship: < 4 N (number of radials) = L (in wavelengths) × 128/l Broadband RF matching transformer Coax to transmitter Thus, for quarter-wavelength radials, N is about 32. There is nothing magic or abrupt, of course, about the 0.05 l figure; increasing the spacing from 0.05 l to 0.10 l, for instance, simply means a slight increase in ground system losses and a corresponding slight decrease in signal strength at the distant receiving station in return for needing to install half as many radials. There is also nothing magic about making all the radials equal in length. If you are boxed in by property boundaries, a city lot, or structures that get in the way, simply do the best you can. Radials are important to the efficient operation of verticals, but many different compromise geometries can lead to equally excellent results. Over the past two decades, one of the authors has had a commanding signal on 160 m from two different homes with highly lopsided radial fields of between 16 and 24 radials. It is also true that the taller the vertical, the longer the radials should be for optimum antenna system efficiency. That is because a taller vertical tends to put return currents into the earth at a greater distance from its base than a shorter vertical does. Again, as the radials are lengthened, the number of them should increase. A “safe” rule of thumb is that your radials should be approximately the same length as your vertical’s physical height. Probably the most important thing to keep in mind is that transmitter output power headed for the antenna actually gets split, or “used up”, in three ways: feedline losses, antenna and ground system losses, and antenna radiation resistance. Of those, only power dissipated in the antenna radiation resistance contributes to the received signal strength far away. How much power is lost in either of the other two categories is determined with Ohm’s law calculations on a simple resistive divider. The lower the radiation resistance of the antenna, the more attention that should be paid to feedline losses and losses in the ground system. Thus, a radial field used in conjunction with a short transmitting vertical (less than l/4 tall) requires much more wire than one for a full-size l/4 or taller monopole if the radiation efficiencies of the two antennas are to be comparable. Prior to World War II, scientists and engineers performed a series of tests on radial fields for AM broadcast stations. They concluded that anything more extensive than a

C h a p t e r 9 : V e r t i c a l l y P o l a r i z e d A n t e n n a s 241 2-Meter screen Figure 9.8 Comprehensive ground system for vertical antenna. radial field consisting of 120 quarter-wave radials provided insignificant improvements to radiation efficiency and signal strength. Figure 9.9 is typical of the kind of information that these experiments provided. These test results have formed the basis of much folklore over the years and only recently have new experiments and new reports clarified matters. Today we know that a system of 50 to 60 radials is more than plenty for a quarter-wave ground-mounted vertical in virtually any application, and that the exact length of radials on or in the ground is immaterial. One of the least important characteristics of radials is wire size. Antenna return currents are split among the many radials, and the conductivity of even tiny wire sizes is far superior to that of all known soils. Bigger issues are survivability, mechanical strength, and visibility. Wires laid on the ground or on grass are ultimately subjected to

C h a p t e r 9 : V e r t i c a l l y P o l a r i z e d A n t e n n a s 241<br />

2-Meter<br />

screen<br />

Figure 9.8 Comprehensive ground system for vertical antenna.<br />

radial field consisting of 120 quarter-wave radials provided insignificant improvements<br />

to radiation efficiency and signal strength. Figure 9.9 is typical of the kind of information<br />

that these experiments provided. These test results have formed the basis of much<br />

folklore over the years and only recently have new experiments and new reports clarified<br />

matters. Today we know that a system of 50 to 60 radials is more than plenty for a<br />

quarter-wave ground-mounted vertical in virtually any application, and that the exact<br />

length of radials on or in the ground is immaterial.<br />

One of the least important characteristics of radials is wire size. <strong>Antenna</strong> return currents<br />

are split among the many radials, and the conductivity of even tiny wire sizes is<br />

far superior to that of all known soils. Bigger issues are survivability, mechanical<br />

strength, and visibility. Wires laid on the ground or on grass are ultimately subjected to

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