Practical_Antenna_Handbook_0071639586

304 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
The first-time constructor of a gamma-matched driven element would be wise to
make the gamma rod itself substantially longer than L/10, since these dimensions are
only approximate, and both the position of the shorting bar and the setting of the capacitor
will need to be varied by trial and error during the tune-up procedure described
subsequently.
The capacitor in series with the center conductor of the coaxial cable has a value of
approximately 8 pF per meter of wavelength at the lowest frequency in the band of interest
or approximately
2400
C (pF) =
F (MHz)
(12.2)
For any transmitter power levels more than a few watts, the capacitor must be a
high-voltage transmitting variable type. In general, gamma match capacitors are either
air or vacuum variables although those found on the older Cushcraft family of four-element
monobanders for 20, 15, and 10 m consist of a fixed length of RG-8 coaxial cable
with the outer jacket and shield braid removed so that just the center conductor and
dielectric remain. This insulated wire is inserted into a portion of the hollow aluminum
gamma rod, where the center conductor of the stripped coaxial cable forms one plate of
the capacitor and the hollow gamma tube is the other.
Adjustment of the gamma match is accomplished in two steps:
• Adjust the length of the driven element (or dipole) for resonance, using a noise
bridge, wattmeter, dip meter, impedance bridge, or other means.
• Alternately adjust the capacitor and the shorting bar/clamp for best impedance
match between the antenna and the transmission line. Results of this tuning
operation are best monitored at the antenna with a small portable analyzer (see
Chap. 27) capable of measuring and displaying SWR or impedance curves over
the frequency range of interest. The author uses a battery-powered AEA HF
Analyst hauled to the top of his tower in a water bucket or hanging from his
belt, but AEA and others have newer analyzers that are more compact and/or
have more “bells and whistles” on them.
If a good match (as indicated by very low SWR) cannot be obtained with the initial
spacing of the gamma rod from the element, write down the best SWR obtained at the
design frequency and then increase the distance of the gamma rod from the element
and go through the adjustment process again. If the SWR obtained this time is lower
than the value written down, continue to increase the gamma rod spacing until no further
improvement is possible. If, on the other hand, the SWR increased, reduce the spacing
between the rod and the element and perform the adjustment procedure again. In
some Yagi designs, it may also be necessary to shorten the driven element to get a
match.
T Match
Not much needs to be said about the T match since it is nothing more than a balanced
(or two-sided) gamma match. All the design considerations are identical. Some antenna
experimenters prefer the T match because it presumably provides a more symmetrical