[TowerTalk] Adjusting a Gamma Match on a Yagi with an Antenna Analyzer

[Jim Thomson]

Date: Sun, 14 Nov 2010 19:52:38 -0500
From: hanslg at aol.com
Subject: [TowerTalk] Fwd: Fwd: Adjusting a Gamma Match on a Yagi
with an Antenna Analyzer
 


 

 

-----Original Message-----
From: Jim Hargrave <w5ifp at gvtc.com>
To: hanslg at aol.com
Cc: towertalk at contesting.com
Sent: Wed, Nov 10, 2010 8:54 am
Subject: RE: [TowerTalk] Fwd: Adjusting a Gamma Match on a Yagi withan AntennaAnalyzer




Hans,



Most gamma matches have a capacitor consisting of the feed coax or an

insulated rod inside the outer rod. This is the series capacitor. Its

adjusted by moving the outer rod in and out. This gives you two adjustments

of the Gamma match. Inductance by adjusting the shorting bar and capacitance

by sliding the outer tube.



Is it possible that your yagi has this type of adjustable capacitor?



     73's Jim

      W5IFP

##  back in the 80's  there was a qst article, where a fellow took the infamous 
'PV4'  4-el yagi..and used it on 2m.   The catch was... he tossed the gamma cap
and just shortened the DE  a bit more... and  shortened it on the side that has the 
gamma  rod.    Now this kill's  a bunch of birds with one stone.   That now puts the
center conductor of the coax at DC grnd potential.   No more gamma cap to blow up. 
All the cap is there for..is to introduce XC  to cancel out the XL of the gamma  rod..
and that can be done easily by shorteneing up the DE.... just like you do when a 
hair pin is used.  Since it's plumber's delight config, no need for a split DE, with an insulator
in the middle.  

##  You never see adjustable caps  on any T match yagi..like an old telrex, same deal.
Dump the caps,  and shorten the DE.  

## in the case of the gamma  match, with  or without the cap, the DE has to be a bit shorter anyway,
as a starting point.   With no cap, the DE has to be shortened even more.   IF you shorten it a tiny bit 
more on the side that has the gamma rod,  you will eliminate the slight pattern skewing you normally
get when a  variable cap is used. 

## I have not tried using a clamp on RF ammeter on the coax, so dunno if a bead balun is required or not.
Never seen either gamma  config with a bead balun, so dunno if it buys  you anything or not. 

later...... Jim   VE7RF