[Antennas] gamma match

[Robert Lay]

At 11:08 PM 1/12/2005 +1300, Alan Hughes wrote:
>I am attempting to adjust a gamma match on a new home brew yagi antenna 
>for 10 metres. I have a variable capacitor which appears to adjust the 
>reactance to approx zero on my home brew noise bridge, however the 
>impedance appears to be about 18 ohms and it doesnt seem to matter what 
>I do it appears not to change. I am obviously missing something (apart 
>from not knowing enough about what I am doing).  Any suggestions please 
>as to what to look at??
>Alan ZL3KR

Dear Alan,

>From what little I have actually done with gamma matches, I believe it would be correct to say that there are TWO adjustments - one is the capacitor that you mention, and the other is the length of the parallel conductor. It must be lengthened or shortened in order to provide a transformation ratio that is appropriate for the antenna's impedance and the line impedance. I can't get it straight in my head, but I'm thinking that you probably have to lengthen it go get a higher impedance, and then, as you say, you tune out the reactance component as best you can with the capacitor. However, you should expect some cross coupling between the two adjustments and the capacitor will have an effect on both the magnitude and angle of the impedance seen.

I have only used the gamma match on verticals, but the principles are the same whether it's a dipole, a beam or a vertical.

It would also be good to remember that your SWR indicator(s) on the coax feed line are equally valuable in indicated when your match is improving or getting worse. Ultimately, the fine tuning should be done that way and not with the noise bridge.


73 de W9DMK, Bob in Dahlgren, VA
w9dmk at crosslink.net
http://www.qsl.net/w9dmk