Practical_Antenna_Handbook_0071639586

432 P a r t V I : A n t e n n a s f o r O t h e r F r e q u e n c i e s
38 in
19 in
19 in
38 in
B
19.5 in
B
A
19.5 in
C
C
1 in.
VHF
balun
4:1
1 in.
Figure 19.5 Vertical collinear antenna.
A
Coax to
XMTR
in an omnidirectional pattern. Each
array consists of a quarter-wavelength
section A and a half-wavelength section
C separated by a phase reversing
stub B. (The total length of the stub—
out on one side and back on the
other—is a half-wavelength, but if
made from open-wire line, only a l/4
line is required. The phase reversal
stub adds a 180-degree phase shift between
the inner l/4 section and the
outer l/2 section. Because either outboard
half-wavelength section is l/2
or 180 degrees above or below the center
half-wave section, the in-phase
currents in the sections cancel in the
vertical direction. In any horizontal
direction, however, the current in each
outer section is in phase with that of
its respective inner section, and the
antenna exhibits about 3 dB gain at
low elevation angles over a conventional
l/2 vertical dipole or l/4
ground-plane vertical. Note that the
phase reversal stub, even if made out
of open-wire line, is not being operated
as a transmission line and the
usual length correction for velocity
factor is not applied.
As is the case with an ordinary dipole,
the feedpoint is at the midpoint of
the array (i.e., between the A sections).
Unlike an ordinary dipole, the freespace
feedpoint impedance is around
280 Ω resistive at 146 MHz—a reasonably
good match to a 4:1 balun transformer
(see Fig. 19.6), especially if fed
with 72-Ω coaxial cable. Alternatively,
300-Ω twin-lead can be connected directly
to the two sides of the dipole
center. If the transmitter lacks the balanced
output needed to feed twin-lead,
use an appropriate balun at the input
end of the twin-lead (i.e., right at the
transmitter). For proper performance,
make sure the feedline is kept at right
angles to the dipole axis near the dipole.