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20180804 - Gamma Match Adjustments

posted Aug 3, 2018, 4:41 PM by Onno Benschop
Denis VK6AKR shares a story about Gamma Matching:

My first experience with the gamma match was educational.   My need was for a match to a 2m dipole for satellite working on a crossed yagi.   Desiring to have the 2m elements removable for transport, a folded dipole was unworkable.

Much research into gamma matches revealed several (conflicting) sets of dimensions and configurations for the capacitor and tapping point.   Eventually settled on a compromise design using the inner conductor and insulation of some RG213 coax, sliding inside a 10mm aluminium tube to form the capacitor and the matching line simultaneously.   So far so good.

However the VSWR would not reduce to anywhere below about 4:1 and despite moving the tapping point over its full range the VSWR variation was small.   Then it dawned upon me.   I had started with a very small capacitance indeed - the gamma tube enclosing only a small amount of the RG213 core.

Herein lay the problem.   With too little capacitance I was effectively measuring the VSWR of an open-circuit coax feedline!

So the moral of the story -- start with maximum capacitance, set the tapping point (between the tube and the antenna element) then reduce the capacitance, re-adjust the tap point, etc until the VSWR reduces to as close to 1:1 as possible.

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