[Elecraft] KAT500 Performance Clarification

[Dick Dievendorff]

Hi, Phil.

Although the KAT500, like most tuners, will attempt to find a match whenever
it can, it protects itself from the very high voltages and currents that can
appear on internal components when the match is poor. We rate the tuner for
500 watts at 10:1 SWR.  When you use an antenna that shows 17:1 SWR (the
"untuned" or "bypass" VSWR), the ATU measures the forward power and computes
the power it can safely handle with the high mismatch.  It's less than 500
watts.  You should be able to operate safely at 250 watts or so with that
high a bypass VSWR.  If you exceed the ATU's safety values, it should
illuminate the red FAULT led, make a fault log entry, and put the ATU into
BYPASS at the next opportunity.  It should also interrupt the amplifier key
line.

Dick, K6KR




-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Phil Hystad
Sent: Wednesday, January 23, 2013 8:48 AM
To: elecraft at mailman.qth.net List
Subject: [Elecraft] KAT500 Performance Clarification

Gang, I was surprised last night when I got a descent low SWR tuning my
80-meter (ladder-line fed into 4:1 balun) dipole on 160 meters.  I have had
the KAT500 for a couple of months and I never bothered trying it on 160
meters because on all previous attempts with other manual tuners I have
failed to get a good match.

But, last night, just as a lark, I switched up to 160 meters and gave the
KAT500 a try.  Sure enough it found a match of 1.05:1 at 1900 KHz with the
Bypass VSWR reading about 17:1.  Given I have most of the transmission line
in lower-loss ladder line helps with the losses to the antenna with that
high bypass SWR.

This was at 100 watts out from the K3.  Now, I decided to give a try with
the KPA500 in-line at about 475 watts and the best I could get is about 4:1
tuned SWR.  Now, I am not quite sure I understand the dynamics of the KAT500
with higher power.  This is what I did.  I tuned at 20 watts to the 1.05:1
SWR (bypass was ~17:1).  At 100 watts, my LP-100A was still reading about
1.1:1 SWR.  But, when I turned on the KPA500 (switched out of standby) the
reading was roughly 4:1 with a fault on the KPA500.  Question, how does the
higher power affect the match of the KAT500.  I understand how higher power
might stress the KAT500 with maybe arcing or something but I don't
understand why the match actually changed unless that higher power caused
some change in the KAT500 capacitor or inductor components.


73, phil, K7PEH

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