Andy,
You've answered your own question. If it doesn't happen when the entire
rig system transmits into a dummy load, but has problems while connected
to an antenna; you know where to look.
In that search, high on the list of suspects (right up there with
failure of any antenna elements) is damage from tiny livestock (mice,
rats, birds that may have gotten into the attic or bugs like mud wasps);
chewed wires, cables and insulators.
However with your extended time using a 40M antenna on 80M even at 250
watts, it's likely that some part of the antenna system (feed, chokes,
transformers, wires etc) has broken down (high voltages are possible
causes). Something changed, you know where to look now. Be ready to
replace the antenna with the means to prevent another similar failure.
73,
Rick nk7i
On 11/20/2023 7:16 PM, Victor Rosenthal 4X6GP wrote:
This is really strange.The only thing I can think of is some kind of
feedback to the TS-590s ALC circuitry caused by common mode current on
the feedline. An easy test would be to connect a common mode choke at
the output of the KAT500 and see if the problem goes away. Does the
antenna have some kind of choke or balun at the feedpoint? If not,
probably it should, just to help reduce local RFI.
Why did this suddenly happen? I have no idea. Changes in grounding or
any conductors attached to the system can affect the path that the
common mode current takes.
73,
Victor, 4X6GP
Rehovot, Israel
formerly K2VCO
CWops no. 5
http://www.qsl.net/k2vco/
On 21/11/2023 1:20, Andy Durbin wrote:
I have a new power oscillation problem that is manifest only on 40 m
when using an attic dipole. Basic station configuration is Kenwood
TS-590S, KAT500, KPA500. As drive power is slowly increased and
output power rises above about 100 W the power output falls to zero
then slowly rises again. If drive power is increased the frequency of
the oscillation increases but the peak output power does not change.
At onset the oscillation frequency is 3.5 Hz. While KPA500 output
power is cycling TS-590S exciter power remains constant, KPA500
remains keyed, and neither LP-100A, KPA500, nor KAT500 indicate any
SWR transient. KPA500 and KAT500 do not fault.
This 40 m attic dipole has been in use for several years and this
problem is new. I have set any arbitrary 250 W limit for this
antenna and have, in the past, experienced no significant problems
running that power level.
What could cause KPA500 to stop power output and then resume? The
KPA500 will output 500 W into a dummy load on the same frequency with
no oscillation so that seems to rule out a KPA500 hardware issue.
Next suspect would be RFI but what feedback mechanism would cause
KPA500 to drop to low or zero power while drive remains constant and
key line remains active. Why would I have RFI now if I didn't
before?
Next suspect would seem to be some sort of breakdown in the antenna
system and that breakdown heals when power is removed. But how could
any breakdown pull power to zero and not fault KPA500 or KAT500?
Next step will be visual inspection of the attic antenna. I'm
posting in case anyone has seen a similar problem and has any
suggestions as to the cause.
Early draft notes and some scope traces here -
https://www.dropbox.com/scl/fi/rxidnm4bkv9pissfyln00/Power-Oscillation-40-m-draft-1.pdf?rlkey=446vn8bjoe2bv2xn4un86zpd2&dl=0
Thanks and 73, Andy, k3wyc
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