Until recently I've used gamma matches on all of my big Yagis. On 20M I
used a 4 foot length of 1/2" diameter x 0.058" wall tubing for the Gamma
rod. The capacitor was made from RG-217 (aka RG-14) coaxial cable. I
stripped the black outer jacket and copper braid from the cable. The
remaining dielectric (with the embedded center conductor) fits nicely into
the 1/2" tubing and makes an adjustable capacitor by varying its length.
If you are using low power, you can use 0.375" diameter x 0.035" wall
tubing and RG-8 (modified as above) will fit nicely into the 0.375"
diameter tubing. Both types of tubing are inexpensive and readily
available via UPS from Dillsburg Aeroplane Works in Dillsburg PA.
You will find that the driven element of a gamma matched Yagi needs to be
about 2% shorter than its resonant length.
By the way, all of my current Yagis are T-match fed (except my 20M Yagis
which are a little older and are still Gamma matched). Oddly, the length
of a T-matched driven element needs to be about 2-3% LONGER than resonance
for a proper match.
73
Frank
W3LPL
donovanf@sgate.com
On Wed, 15 Jan 1997, kcubilo wrote:
> hi all i have been designing a 2 el 20 meter yagi and wonder if anyone can
> give me some pointers on a gamma match. layout shows a radiation resistance of
> approx 35.8 ohms.
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