Practical_Antenna_Handbook_0071639586

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 239
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Radiation resistance (R RAD )
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Antenna length (degrees)
Figure 9.6B Radiation resistance as a function of antenna length.
plane antennas shortly.) There we see (by looking down from directly above) 16 radials
equally spaced to cover the full circle around the antenna. At one time it was commonly
understood that each radial was to be a quarter-wavelength, where L (feet) = 246/F
(MHz). In truth, radials in direct contact with the ground are sufficiently detuned from
their free-space characteristics that an exact length is not at all important. Instead, what
is important is that enough radials are installed that their far ends are typically no more
than 0.05 l apart. A little thought will lead to the counterintuitive realization that the
shorter your radials are, the fewer of them you need!
And this is, in fact, true. However, it is also true that the shorter your radials are, the
less efficient your ground system will be and the higher your ground resistance losses
will be. The basic point being made is this: If you have a fixed amount of wire to apply
to your system of radials, a few long radials are not as helpful as a greater number of
shorter ones. Stated yet another way, it’s better to have longer radials than shorter ones,
but if you can only have shorter ones, you don’t need as many before you will have
maxed out the performance gains you can accomplish by adding more radials.