Practical_Antenna_Handbook_0071639586

328 P a r t I V : D i r e c t i o n a l H i g h - F r e q u e n c y A n t e n n a A r r a y s
Concluding Thoughts
At HF it is very difficult to erect any horizontally polarized antenna high enough to be able
to forget about the effect of the earth below. In principle, the theoretical 6-dB gain that
results from the ground reflection is worthwhile, but in practice the reflection gain from
real earth is either far less than 6 dB or the antenna is so low (in terms of wavelengths) that
the increased signal goes mostly into the ionosphere at very high angles, where it is often
of no use, depending on your operating or listening interests. Similarly, the theoretical
3-dB gain from the second radiating element (the other half of the loop) is seldom achieved
in the real world. Part of the reason for this is that in the case of 1l loops and quad elements,
the spacing of the opposing sides is only l/4—leading to quadrature phasing and
partial, rather than full, cancellation of the signal in unused directions. Equally important,
the radiated field from the lower leg of a square loop is subject to the ground reflection
pattern of a much lower height, so in general a loop’s maximum gain is at a somewhat
higher elevation angle than it is for the top leg alone.
In the author’s opinion, unless one has some other reason for choosing the quad—
such as the high-power corona discharge problem at HCJB—the mechanical complexity
of the quad may outweigh its benefits. Often, more can be realized by analyzing the
ground reflection factor for different antenna heights and then applying the same (or
less) effort to putting a simple two- or three-element Yagi up at a height that yields the
desired elevation pattern.