[TowerTalk] TH3MK4 15M SWR fluctuation

Gary Smith Gary at ka1j.com
Wed Apr 2 07:21:02 EDT 2014

I rehabbed my father's old TH3MK4, renewing the electrical contacts, 
replacing the traps and installing a new DXE-BAL050-H10-A balun. It 
worked well on all three bands for the last week and its really a 
pleasure to have access to a beam again as I've only been able to use 
vertical wires since 1995.

But last night I noticed the SWR was wildly swinging up to 5:1 on 15M 
when using the amp and when running barefoot it dropped back to 1.2 
as it had been at before. More testing finds that anything over 
70-80W raises the SWR and it remains stable at 1.2 below that power 
level. 10 & 20 are unaffected and have a 1.2 SWR. I ran the amp into 
a separate 15M antenna and there was no issue with SWR so it appears 
isolated to the TH3.

I'm suspecting a 15M trap has failed and I either need to replace it 
or rehab it. I don't know how to tell if it is the driven element 
trap or one of the parasitic traps that has failed. Deciding which 
trap is involved is the first problem, my guess is it would be the 
driven element trap. 

Once I have that sorted out, are these traps easily reparable & if 
so, any caveats? I've done a google search but haven't found that 
answer yet.

Suggestions on how to determine which trap & how to best rehab it?



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