Practical_Antenna_Handbook_0071639586

320 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
X X
A
C
X
X
X
X
B
Figure 13.3 Feed options for the quad.
A loop located within a few wavelengths of ground or another reflecting surface is
actually an array of four elements: the two original bent dipoles plus their two “image”
antennas. As discussed in Chap. 5, when the bottom of the loop can be lifted to a height
of at least a half-wavelength above the earth or other reflecting surface, it begins to
make sense to utilize horizontal polarization by feeding it at the midpoint of the top or
bottom side. In this manner, maximum utilization of a ground reflection factor that may
be worth up to 6 dB at a distant receiving point is attained. For most amateurs, this suggests
horizontal polarization at 14 MHz and above. At support heights below l/2, however,
the ground reflection factor for horizontally polarized antennas tends to put most
of the radiation at very high elevation angles, so if long-distance communication is the
user’s objective there is merit to switching to vertical polarization by feeding the loop
on one side or the other. Of course, if you wish to hedge your bets, or your quad loop is
at that “difficult” height, you can feed it at a corner to obtain equal parts horizontal and
vertical polarization.