[Elecraft] Strange SWR change on 20 at

[Brian D]

I have had similar problems with swr trips on my KPA500, only after several
transmissions on some bands. By a process of elimination it was traced to a
toroidal voltage heating up in an external coupler. It came on suddenly
after about 5 minutes of transmissions, and came good after a few minutes to
cool. Replacing the toroid with a larger one cured the problem.



"Ron D'Eau Claire" <ron at cobi.biz> wrote:

> Hi Dave: I thought you said it happened after you sent CQ+Call a few
> times. When a core heats to its Curie temp, the inductance change happens
> "instantly". You are right - if it happens instantly when you hit the key
> it is not heating. Something is breaking down.
> 
> 73, Ron AC7AC
> 
> -----Original Message----- From: Elecraft
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Dave Cole Sent:
> Saturday, May 13, 2017 10:39 AM To: elecraft at mailman.qth.net Subject: Re:
> [Elecraft] Strange SWR change on 20 at
> 
> I added the SWR meter...  My actual SWR jumps instantly up about1 SWR
> unit...  I believe it is a bad connection on the Challenger where the main
> antenna connects to the 20 meter tuning stub...
> 
> Just sort of surprised me to see a second person with the exact same issue
> is all...  SWR change is instant, so it is not heating...  Happens at 30
> watts, or 1 KW...  Time for a rebuild.
> 
> 73s and thanks, Dave NK7Z http://www.nk7z.net
> 
> On 05/13/2017 10:07 AM, Dave Cole wrote:
> > Hmmm...  I am having the exact same issue...  My SWR changes after a few
> > transmissions, then returns...  Up until now, I have been suspecting my
> > antenna...  Time to add an external SWR bridge between the tuner and
> > antenna as a permanent thing...
> >
> > 73s and thanks, Dave NK7Z http://www.nk7z.net
> >
> > On 05/13/2017 09:07 AM, Terry Brown wrote:
> > > I recently started using my horizontal loop that is about a full
> > > wavelength on 80, for 20 meters.  I took down my yagi as we are
> > > moving; drat!  My Horizontal Loop is at 40 ft.  height and fed with
> > > 450 window line into a 4:1 voltage balun with about 15 ft. of coax to
> > > rig.
> >>
> >>
> >>
> > > I am running 100 watts through my KXPA100.  I tune a frequency in the
> > > CW portion of 20 to about 1.2 :1 and start sending CQ.  After about 3
> > > to 4  3x3 CQ's,, the SWR goes off the charts and power drops to zero.
> > > I can retune the antenna and the process repeats itself.  20 meters
> > > is the only band affected.
> >>
> >>
> >>
> > > I am wondering if I am having RF problems in that part of the 20 meter
> > > bands This only occurs on 20 meters.  The Amp is grounded, but the KX3
> > > is not bonded to the Amp.
> >>
> >>
> >>
> > > I would appreciate any advice.
> >>
> >>
> >>
> > > Thanks and 73's,
> >>
> >>
> >>
> > > Terry de N7TB
> >>
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> >>
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-- 
Brian Duffell   Yarm    England