[Elecraft] KXPA100 Intermittent VSWR Issues

[Cady, Fred]

The SWR circuitry in the KXAT100 (and KAT500) measure the forward and reverse voltage values at different times.  They cannot be measured simultaneously because they don't have dual A/D converters or sample-and-hold circuits.  To be sure, there is only a short time between sampling the forward and reverse directions but if your output power is changing with time as it will with a high speed CW signals and more especially and more troublesomely with a SSB signal, the SWR metering may indicate an incorrect value.  If you have the tuner in AUTO mode, this may trigger a retune. For this reason it is best to operate either of these tuners in MAN mode and train the tuner(s) to store good L-C values for different frequencies in each of your operational bands.  The tuners store this information in "frequency buckets" and when they detect the frequency within that bucker they recall the L-C values and if they need to be changed they will be.

Summary:
Train the tuner so that it can remember the L-C values to use for a specific range of frequencies.
Don't use AUTO mode;  use MAN mode.

Cheers and 73,
Fred KE7X
"The Elecraft K3: Design, Configuration and Operation 2nd ed"
"The Elecraft KX3 - Going for the summit"
"The Elecraft KPA500 and KAT500 - the K-Line Dream Station"
Printed, coil bound copies of these are all available at www.elecraft.com
KPA500 and KAT500 Quick Set Up Guide
http://www.ke7x.com/home/k-line-introduction-and-set-up-guide

> -----Original Message-----
> From: Elecraft [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of
> Jim Kutsch, KY2D
> Sent: Monday, December 08, 2014 9:18 AM
> To: Elecraft at mailman.qth.net
> Subject: Re: [Elecraft] KXPA100 Intermittent VSWR Issues
> 
> I'm experiencing the same thing but mostly on 40 meters where the best
> match with the ATU isn't as good as on other bands. It *SEEMS* to have
> started Saturday when I installed the latest KX3 beta and set
> compression to 20. The antenna is a Carolina Windham.
> 
> 73, Jim, KY2D
> .
> 
> 
> -----Original Message-----
> From: Elecraft [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of
> Jim Stahl via Elecraft
> Sent: Sunday, December 07, 2014 10:08 PM
> To: elecraft at mailman.qth.net
> Subject: [Elecraft] KXPA100 Intermittent VSWR Issues
> 
> I've run into several occasions where in mid-CW transmission my KXPA100
> (with internal tuner) SWR takes a momentary jump from matched to maybe
> 3:1.
> I see the SWR jump on both the KXPA front panel LEDs and on  the KX3
> VSWR display.  It lasts for maybe half a second, I hear  several clicks
> of the relays in the tuner, and them it's back to normal. When  normal
> the SWR is pretty much 1:1 on the KX3, and the second green LED lit on
> the KX3. I've not seen it happen at the start of a transmission, but
> rather several characters into a CQ or exchange..
> 
> In the ARRL 160 contest Saturday night this was happening perhaps once
> every
> 2-3 minutes. I'm using a fairly well matched antenna - not random
> wires.
> I  also encountered this problem on 15 meters when operating from
> W1AW/8 in WV  in October. In both cases I was running a pretty high CW
> duty cycle while  running at a pretty good clip. The amp was warm, but
> the heat sink air flow is not blocked.
> 
> Any thoughts?
> 
> 
> 73  -  Jim  K8MR
> 
> 
> 
> 
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