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Gamma matching

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Subject: Gamma matching
From: (Chris R. Burger)
Date: Tue, 10 Dec 1996 10:58:46 +200
Response to my gamma matching rules of thumb has been interesting.  
Several individuals (including one extremely prolific station 
builder) responded individually, and confirmed most of my empirical 
observations.  They also added that an omega match is useful in 
achieving a better match without having to tweak the dimensions of 
the gamma rod.

However, there was one very critical response.  It is someone whose 
technical credentials are beyond reproach, so there must be something 
to his criticisms.  He contended that:

1.  The gamma attachment point must be as low as possible, failing 
which the gamma itself becomes the radiator.  
My response, although once again empirical, is twofold.  Firstly:  My 
discussion was specifically confined to radiators with an electrical 
length of less than a quarter wavelength.  In these circumstances, I 
maintain that longer is better.  Secondly:  I am not in a position to 
express an opinion on what is radiating; I can only say that it 
works.  That being the case, I'm not particularly concerned whether 
the tower or the gamma rod is doing the radiating.

2.  The gamma diameter is immaterial, as only the transformation 
ratio changes.
Quite so.  However, the stated objective with my rules of thumb was 
to provide a convenient impedance.  That being the case, a diameter 
ratio of as close as possible to 1:1 is preferable.  The intention is 
not to optimise one particular situation, but to maximise the 
probability of having a convenient impedance when a random tower is 
being loaded.

3.  My suggested matching technique was dismissed as "not a match", 
due to the 2:1 (or higher) SWR.
Guilty as charged.  However, for those of us who struggle to find a 
few hours a year to operate (normally the last weekend of several 
months in the third quarter), a 1:1 SWR is a luxury one reads about in 
books.  I feel that, especially considering the low line losses on 
LF, the idea of bringing the antenna impedance into a manageable 
ballpark by preliminary matching and then using a suitably beefy 
tuner in the shack is forgiveable.  Should you harbour resentment 
against the likes of me, I hope you'll find it in your heart to 
forgive me.

Chris R. Burger

PS:  Anyone who simply cannot find this forgiveness in his/her heart, 
please shred your 3DA0/ZS6BCR, ZS0Z, ZS3Z, ZS6EZ, ZS6BCR, ZS9Z, 
ZS9Z/ZS1, V51Z, 3DA0Z, A25/ZS6BCR, ZS6BCR/A22, 7P8EN and W6O QSLs and 
return the remains to me by registered mail.  

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