Practical_Antenna_Handbook_0071639586

C h a p t e r 9 : V e r t i c a l l y P o l a r i z e d A n t e n n a s 245
Figure 9.11A Delta-fed grounded vertical.
Coax
to
XMTR
Tuner
The gamma feed system is shown in Fig.
9.11B. This method is commonly used by amateurs
to feed Yagi beam antennas, so it is quite
familiar to many in the amateur radio world. In
effect, the gamma match and its cousins use
distributed components to accomplish the same
impedance-matching functions a coupler or
L-network does in the series-feed case. The two
critical dimensions for a gamma match are the
distance from the base or centerline of the antenna
to the tap point on the antenna (i.e., the
active length of the gamma rod) and the spacing
of the gamma rod away from the antenna element.
The lower end of the gamma rod is not
grounded; rather, it is connected to the hot side
of the transmission line. It is important that the
rod not be anywhere near a quarter-wavelength,
or it would become a vertical antenna in
its own right, and in fact would resemble the
J-pole antenna.
One advantage of the gamma match and
other shunt-feed techniques is that the vertical
antenna element retains a direct connection to
ground, providing superior static discharge
during and ahead of approaching thunderstorms. This is especially useful on the lower
amateur frequencies, where verticals are often built using triangular guyed tower sections
and it is less expensive to construct the tower with a direct electrical connection
(through the tower legs) to ground.
The biggest disadvantage of the gamma matched vertical is that it is much more
difficult to initially adjust because the gamma rod tap point may be well beyond the
user’s reach—especially on the lower bands. A secondary disadvantage is that some
methods of feed for multielement phased arrays of verticals use current-forcing techniques
that are easier to control with the series feed.
Unless the user has the ability to adjust not only the gamma rod spacing and length
but also the overall length of the vertical element, the gamma match may require a
simple matching network between the base of the antenna and the transmission line. In
the easiest cases, a single series capacitor or inductor between the base of the gamma
rod and the hot side of the feedline may be all that is needed to obtain a satisfactory
match and SWR. More generally, an omega match (Fig. 9.11C) can be inserted, although
the user may need to experiment with which side of the series capacitor (C S ) the hot end
of the gamma capacitor (C G ) should attach to.
Chapter 18 (“Antennas for 160 Meters”) includes details of a gamma match for a
top-loaded tower on 160 m. In some difficult cases, when the tower length or available
loading is not ideal, an omega match feed system (shown in Fig. 9.11C) may be necessary.
The omega match adds a shunt capacitor to the basic gamma match.