[Elecraft] [K3-KPA500] SWR jumps up with 5 second key down

[Fred Jensen]

On 6/4/2012 10:00 AM, ai6ii wrote:
> I have installed a new antenna and have strange changing swr. When testing
> with RigExperts meter I find good resonance at say 15m (21.200). With the K3
> at 100 watts reading is 1 bar on K3 meter and 1.0 on KPA500 (STBY). When I
> bring the amp online and drive it at 200 watts, all is still good even with
> a 15 second key down period. Driving the KPA500 to 275 watts, still okay
> with key down, although the KPA500 meter now shows 1.1. BUT when I increase
> the drive a little more (300 watts) the swr jumps up to 2.5+ and the KPA500
> shows REFL HI and drops into STBY.
>
> And here is the kicker...NOT ALL THE TIME!
>
> So I think it has to be in the coax. But the 75feet of RG213 is brand new
> premade from Cable Xperts with the connectors.

I strongly doubt that.  Loose connector maybe, but you've probably 
already checked that.  Some suggestions:

1.  Does it do it with dummy load on output of KPA500?

2.  Does it do it with dummy load on far end of coax in place of antenna 
[not always easy to do]?

3.  Does it do it with another antenna [if you have one for 15]?

4.  Does it do it on any other bands?

5.  You said it is one of Tom's multiband yagi's.  Did you tighten 
everything down when you assembled it?  If I were to bet on it, I'd bet 
this is your problem.

6.  Is there a balun?  If so, what happens to your TV when this high SWR 
occurs?   All transformer cores will saturate at some power level, the 
flux in them just cannot increase anymore, all the little magnet-lets 
are already aligned.  The power at which this happens is sensitive to 
the temperature, and when it does, the balun become non-linear and you 
radiate all sorts of trash.  However, if I were to bet, I'd bet this is 
*not* your problem.

I haven't read the rest of the email list yet and I'll bet there are a 
lot of suggestions for you there by this time.  I'd have typically taken 
mine off-list to you and let you publish the resolution once, however 
apparently, that is poor list etiquette.  I'd like to know what the 
resolution is, I occasionally have a sort of similar problem on an old 
antenna that's been up for maybe 15 years.

73,

Fred K6DGW
- Northern California Contest Club
- CU in the 2012 Cal QSO Party 6-7 Oct 2012
- www.cqp.org