Cushcraft 147-20T tuning problems

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Larry R Savage:
Yes, I did say a Cushcraft 147-20T; This antenna is around 25 years old.  I picked this antenna up brand new, still in unopened box a while back. I have assembled it, but cannot seem to get the vertical side tuned very well.  It keeps on favoring the 150Mhz range...I've doubled/triple checked all elements and they are fine.  The horz. side seems to be fine. I can get about a 1:2 SWR there where it needs to be (144.5Mhz).  Have any of you hams out there fiddled with the gamma matching system to maybe improve this antennas match?  This antenna is very similiar to the new 148-20T with only subtle differences. One of them is the gamma matching system. I've thought about exchanging the gamma match unit, but why should I have to do that?  ANy help appreciated.

Larry, KG5I

Steve Katz:
Did you *adjust* the gamma match?

It's not factory pre-set, and always requires adjustment.  If you haven't adjusted it, that's the problem.

WB2WIK/6

Larry R Savage:
Yes I did tune the antenna as per Cushcrafts instructions. Still no good.  That's why this is such a strange problem.  I've assembled many cushcraft beams over the years and never had a problem such as this.  I'm thinking now I should by a new 148-20T and give this one away.......Anybody want a brand new 147-20T that won't tune up???? :-)

Winston A Miller:
I'll take it and pay the shipping!

Chris Harris:


 Did you measure the vertical driven element and compare the length against the table in the manual?


 What's the SWR at 147.95 and 146.5 mc?


 The 148-20 manual calls for 39 1/4" for the vertical DE. Does the 147-20 call for the same length?


 If you scale the formula for a resonant 1/2 dipole (wire) using the dimension of the horizontal DE that seems to resonate properly, that would work out to approx. 489.5/F.

 Using 146.5 Mhz as the approx. center frequency of the 2 meter FM band, the vertical DE should be about 40". (@148 mc it's 39 11/16")  Given what you're seeing, this seems to make sense.


 To prove this out, just swap driven element positions.

 
 If this works, instead of giving up the antenna, try cutting a new piece of 1/2" aluminum tubing to ~40" -about 3/4" longer than the original, drill a hole in the center to mount the boom and try it out.  You could even make it a little longer and trim it down.

 
 Good luck,

 Chris

 

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