Practical_Antenna_Handbook_0071639586

C h a p t e r 1 1 : V e r t i c a l A r r a y s 269
A
B
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/4
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3/4
Coax
to
XMTR
Figure 11.2B Minimum length feed for a phased array antenna.
transmitter’s coaxial cable (L 3 ), while the other is fed from a phasing switch. The phasing
switch is used to either bypass or insert a phase-shifting length of coaxial cable (L 4 ).
In principle this technique allows the selection of varying amounts of phase shift
from 45 degrees to 270 degrees. However, at other than 0 degrees and 180 degrees, the
currents no longer split equally between the two elements because the element feedpoint
impedances are not the same. In general, for angles other than 0 degrees or 180 degrees,
more complicated feed systems, including current-forcing techniques, are employed.
When unequal lengths of transmission line go to the elements of an array, unequal
amplitudes are a likely by-product. The most noticeable effect for small differences,
such as would occur when adding a l/2 section of cable to implement the end-fire
mode, will be to cause the nulls to fill in and lose some of their depth.
With the addition of a phasing transformer, a two-element vertical array can be remotely
switched between broadside and end-fire modes while preserving balanced
drive currents at the two elements. Figure 11.4 shows how a two-element array is fed
through such a two-port phasing transformer. The phase-reversing switch S 1 can, of
course, consist of coaxial relays controlled from the transmitter end of the transmission
line. The transformer itself is made from a 1:1 toroidal balun kit such as those available
from Amidon Associates and others.
Wind the three coils in trifilar style, according to the kit instructions. The dots in Fig.
11.4A show the “sense” of the coils, and they are important for correct phasing; call one
end the “dot end” and the other end the “plain end”—and mark them differently—to
keep track of them. If the dot end of the first coil is connected to J 3 (where the feedline
from the transmitter or receiver attaches), connect the dot end of the second coil to the
0-degree output (J 1 , which goes to array element A). The third coil is connected to J 2
through a DPDT low-loss RF relay or switch rated for the power levels involved. Al-