[Elecraft] K3/KPA500/KAT500 SWR problem

Mon Dec 30 09:05:44 EST 2013

Hi all,
While it may not solve the problem that the original rp posted, here are some things I've learned while putting together the K-Line book (out soon).

It is best to operate in MAN mode after you have trained your tuner to your antennas.  The reason is the KAT is continually measuring VSWR as you transmit and it takes a little time to do that.  If it measures the forward voltage at one point in a SSB waveform and the reverse in another, you can see that the VSWR calculation will be off.  If  you are in AUTO mode, it will then try to go into a tune.  There isn't much of a downside to operating in MAN mode.  It basically ignores the transient wrong VSWR and if there is a really bad antenna (it fell down or something) the tuner will fault and take the amp off line and you will then be able to correct the problem.  

You can also set the Key Interrupt VSWR threshold to something other than 99.  This kind of acts as a back up to the KPA's innate ability to protect itself.  I have mine set at 4:1 (I'm kind of a belt and braces kind of guy),

The other thing to do is to make sure you have the Amplifier Key Interrupt Power optimized for the KPA500.  The factory default is about 30 watts which means the tuner won't interrupt the amp keying when it sees > 30 watts.  

Cheers and Happy New Year all,
Fred KE7X

-----Original Message-----
From: elecraft-bounces at mailman.qth.net [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Brian Linn
Sent: Monday, December 30, 2013 5:36 AM
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] K3/KPA500/KAT500 SWR problem

All:

This issue sounds familiar. I'll be operating along in the middle of an SSB QSO and suddenly the KAT500 goes wild. The SWR displayed on the meter is HIGHER than the antenna WITHOUT a tuner. The sad part is that it will go on for maybe 10 seconds and no match is found (odd when considering the nearly resonant antenna). The amp then goes into fault and so on. It seems that if I stop the QSO and retune manually all can again be well, but a serious problem indeed.

Because it's so inconsistent and unexpected, I've been unable to try it against a dummy load; however, I did rebuild both 10 meter antennas (just before the recent contest) thinking they might be at fault. No joy ...
problem continued and is showing up on other bands too.

Brian KD0HII

-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Ray Coles
Sent: Sunday, December 29, 2013 2:46 PM
To: elecraft at mailman.qth.net
Subject: [Elecraft] K3/KPA500/KAT500 SWR problem

I do hope someone out there can give me a hand in solving a 2 month old problem with my Elecraft station, even though the problem may not be due to my Elecraft rig at all. I had been happily operating my K3/KPA500/KAT500 rig for about 10 months with my simple wire doublet antenna, about 25 metres long and 5m high. I work mainly digital modes, especially JT65 so I rarely switch the KPA500 to Operate, using it mainly as a Wattmeter. I have used it on SSB on my local Inter-G  club net on 40m however, and everything used to be fine and dandy at all power settings from 40W to 400W with the KAT500 set to Auto.

THEN one evening operating at about 60W JT65 on 20m the Kat500 started hunting and the SWR shot up into the red. Since that event the problem has dogged me continuously. Of course, I immediately suspected a short (or open) in my antenna system. My antenna feed used RG58 out to a 4:1 balun outside in a convenient hawthorn bush. 300 ohm ladder line went from the balun to the doublet T piece. Which was offset 1/3-2/3. This antenna matched, and had bagged QSOs on all bands from 80m to 6m. The RG58 was suspect number one. I replaced it with UR8 and used a new route to the balun which obviated the need for any burial. THE PROBLEM REMAINED. The 300W rated balun was the next suspect, and I already had a replacement 1kW unit to hand. I changed it. THE PROBLEM REMAINED, but by now I had learned that if I ran the KAT500 on Manual and kept power below 40W I could operate, and so I continued to add DX QSOs to my log. All that was left were the aerial components, so I dropped the antenna and changed the ladder line, the T piece the antenna wire and the end insulators. THE PROBLEM REMAINED and it seemed that I would be forever limited to 40W. I had an alternative antenna, an MFJ Magloop which could be happily driven to 100+W connected either to K3 Ant2 or KAT500
Ant3 and I worked surprising DX with it, but I really wanted my lovely Doublet back. (Forgot to mention that I also changed rig connecting leads).
Since then I have played with my rig to try and understand things better and I am beginning to wonder whether there COULD be a rig problem. I also note that when I start transmitting, power is always less than I have selected and it climbs steadily to the demanded value over time. I have set up ALC as documented. If I set power greater than 40W (this varies a little band to
band) the power climbs until....BAM!.... The SWR of 1 to 1.4 suddenly shoots up into the Red with the KAT500 utility recording values of 5-25!

This is a weird set of observations I know, but I would be grateful for any further suggestions from the trusty Elecraft Reflector Brains Trust.

HNY and 73,  

Ray Coles, C.Eng. M0XDL

10 Littlemoor Road,

Weymouth DT3 6AA

Tel: +44 (0) 1305 833699

Mob: 07831 516517

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