[TowerTalk] TH7-DX 15M problem

[David Robbins]

No, running the amp into a dummy load won't rule out the amp.  A high swr like that caused by the amp is because the amp starts detuning and generates harmonics, it’s the harmonics that would be reflected causing the high reverse power.  Running into a dummy load would likely not show that unless there were a good lowpass filter in between to reject them.

David Robbins K1TTT
e-mail: mailto:k1ttt at arrl.net
web: http://wiki.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://k1ttt.net

-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of Robert Harmon
Sent: Wednesday, August 26, 2015 21:32
To: Ham - Tower Talk
Subject: Re: [TowerTalk] TH7-DX 15M problem

Hans,

I would have suspected the 15 meter traps too.   
I would start doing the process of elimination to narrow down the culprit.
Disconnect the coax feeding the TH7 in the shack and put a coax jumper to a dummy load and run the amp full bore on 15 and see if the SWR stays ok.
I know you think the amp is OK, but this will rule it out.
Then reconnect the TH7 coax and go up to the TH7 and disconnect the coax from the TH7 connection terminals.  Connect the coax to the dummy load again.
Back to the shack, load it up on 15 again and see what happens.  If you still have the swr anomoly, then you have ruled out the amp and the feedline.
It is in the antenna.  
 
I had a similar problem with a KLM KT34XA one time and went thru this,  then lowered the antenna down to sawhorses in the back yard and cleaned, and reconnected every element connection point and found a connection to one of the linear loading tubes that was squirrely.  Put the antenna back up and all was good.  

Bob
K6UJ




> On Aug 26, 2015, at 1:38 PM, Radio K0HB <kzerohb at gmail.com> wrote:
> 
> Responding to Dave --- the amp works fine on a different antenna on 15M, and good as gold on all other bands on any antenna.  Hard to suspect the amp.
> 
> Responding to Matt --- the SWR meter is external, downstream from the amp.
> 
> The balun is the 4K version, not the BN86 with the bad reputation.  Hard to envision a balun weird on a single band.
> 
> 
> 
> 
> 
> -----Original Message----- From: David Robbins
> Sent: Wednesday, August 26, 2015 7:43 PM
> To: towertalk at contesting.com
> Subject: Re: [TowerTalk] TH7-DX 15M problem
> 
> a problem with the amp?
> 
> 
> Aug 26, 2015 03:28:05 PM, kzerohb at gmail.com wrote:
> 
> I have an odd issue with a TH7-DXS.
> 
> A while back I started to see a higher-than normal SWR, just on 15M, and just when running some smoke.
> 
> While barefoot the problem never arises.
> 
> The problem never happens on 20M or 10M under any circumstances. I can put a brick on the key at full legal limit on those bands, and the SWR is 1:1 forever.
> 
> If I key down full power (actually any power over about 500W) the SWR will be fine for about 20-30 seconds, then will rise (not instantly, but “ramps” up) to about 2.5:1 across the 15M band. If I unkey, kill the amp, then key up barefoot the SWR drops back to 1:1 and stays there forever.
> 
> It is as if something heats up under power, then changes value. BUT ONLY ON 15M.
> 
> I suspected one of the 15 traps, and hunted down a full new set for a very nice price.
> 
> This morning I replaced all of the 15M traps, and freshened up all the slip joints in the antenna. It looks like new!
> 
> The problem persists.
> 
> Suggestions?
> 
> 73, de Hans, K0HB
> --
> "Just a boy and his radio"™
> --
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