Practical_Antenna_Handbook_0071639586

410 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
Figure 18.3 Gamma-matching a grounded
vertical tower antenna.
h
element 20-m monobander with a
32-ft boom and full-size elements approximately
36 ft long. Above that, at
the 98-ft level, is a two-element 40-m
“shorty 40” monobander with a 20-ft
boom and shortened (linear-loaded)
elements about 42 ft long. All elements
of both beams are directly
Gamma rod
grounded to their respective booms.
The tower is well grounded at its
base, and approximately 16 radials of
varying length extend outward in all
s
compass directions. Of course, all
guy wires have their uppermost insulators
located 3 ft or less from their
C
Coax to XMTR
respective attachment points to the
tower. Since a 98-ft tower is a substantial
percentage (70 percent) of a
quarter-wave on 160 m, the capacitive
top loading provided by the two
beams should easily bring the total electrical height to more than 90 degrees—that is, to
an equivalent height of more than 135 ft at 1.8 MHz.
Initial experiments with attempting to find the “right” gamma rod tap point, using
a fixed spacing of 2 ft from the tower, were frustrating; dozens of trips up the tower by
the owner to heights ranging between 25 and 65 ft to adjust the gamma rod tap point
had resulted only in establishing a very sharp resonance (narrow bandwidth) that was
very difficult to reliably tune with the transmitting air variables on hand and that could
never quite be tuned to a perfect 1.0:1 VSWR with either a simple gamma capacitor or
the more complicated omega match.
The owner then turned to EZNEC antenna modeling software from W7EL to model
the combined effect of his tower, gamma rod, and HF monoband beams on 160 m. The
most time-consuming part of this was in creating accurate models of the two beams,
although simplified models would probably have been quite adequate for determining
their capacitive loading effect on 160 m.
The results were eye-opening! Instead of using a gamma rod spaced 2 ft from the
nearest edge of the triangular tower, the model suggested that a spacing of between 6
and 8 ft in combination with a tap point of about 57 ft would optimize bandwidth at the
desired center frequency while developing a feedpoint impedance very close to 50 Ω
resistive with a small amount of inductive reactance deliberately introduced to allow
fine-tuning of the VSWR on the feedline with a single air-variable transmitting capacitor
at the base.
Here is what the owner finally installed:
• An 8-ft length of sturdy aluminum angle stock from a local hardware store
supports the gamma rod (which is made from lengths of copper plumbing