[TowerTalk] TH7-DX 15M problem

[Radio K0HB]

Yes, I installed a full set of 6 new 15M traps (4 on the DE's, and 2 on the 
reflector).

The symptom did not change.

The BN86 bit the dust years ago, now have a 4KW version up there.

Before installing, I gave each of the screws on the traps a snug-up quarter 
turn (even through they were new-in-the-plastic).

Closely inspected all the phasing hardware and insulators.

I've swapped the feedline with another antenna on the same tower.  The 
problem does not follow the feedline.  The other antenna (a 40 meter 
inverted V) loads just fine on 15 meters)

The antenna performs flawlessly at all power levels on 10 and 20 meters, and 
also on 15 meters (at powers below 500W).

On 15 meters (cw end of the band)  at power < 500W the antenna shows about 
1.2:1 SWR, identical to the reading I recorded when I installed the antenna 
(about 20 years ago).  When I put power to it, the SWR stays "good" for 
20-30 seconds, then ramps up to 2.5:1, the transition is a "ramp" (not a 
sudden jump) lasting about 2 seconds.  If I kill the amp, SWR is normal at 
the next keydown (in just seconds) at 200W.

It seems to point to a 10 meter trap issue (just because that's the only 
piece left in the food chain), but a faulty 10 meter trap should cause a 
problem on 10 meters when it is seeing the highest voltage!

Frustrating!



-----Original Message----- 
From: Don W7WLL
Sent: Wednesday, August 26, 2015 11:35 PM
To: Radio K0HB ; Towertalk
Subject: Re: [TowerTalk] TH7-DX 15M problem

Some questions, some probably obvious!

1. Full set - you mean all 6 15M traps??  If a one band issue and the SWR is
high (5-10)  I'd be looking for the problem in the DE's. If lower (assuming
it was working below 3:1 before) in the 3-5 range and the DE traps check out
OK, I'd be looking at the reflector traps. Examine the screw between the
inner and outer trap tube inside a trap (none are SS and none have any
factory applied non-corrosive compound, at least that I've found).

2. Are you using the original BN-86 excuse for a balun?  I have burned out 2
of these using an honest amp.

3. You might also check the two crossover straps used on the beta match
phasing tubes. Being aluminum they are subject under vibration to cracking.
I just experienced this, one totally cracked and clearly disconnected and
the other cracked but not totally and not obvious without close examination.
I've had two of these TH7DX's up and now keep one for spare parts for the
one in the air.

Don W7WLL

-----Original Message----- 
From: Radio K0HB
Sent: Wednesday, August 26, 2015 12:21 PM
To: towertalk at contesting.com
Subject: [TowerTalk] TH7-DX 15M problem

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
--
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