Terry,
As another responder indicated, you have a high impedance at the
feedline/ATU junction - a high impedance translated into a higher
voltage for any given amount of RF, and when that voltage is high
enough, the tuner capacitors will arc. When they arc over, the
capacitance changes drastically and quickly, so your SWR at the tuner
input goes up a lot.
The best fix is at the antenna feedpoint - add inductance or capacitance
(or both) in a network there to bring the input impedance closer to 50
ohms at all frequencies of interest. You are not likely to find a single
network that will be best for both the CW and the phone end of 80
meters, but the line SWR may be tolerable if you set the network at the
antenna base for a 50 ohm feed at mid-band. Even so, you can expect the
SWR to go up to 3:1 or even more at both ends of the band.
You will have to do some work with an antenna analyzer at the base of
the antenna to determine what type of matching network is needed there.
73,
Don W3FPR
G4MKP wrote:
Problem on 80m SSB.
I am experiencing what I can best describe as ATU Flashover on 80m SSB.
Background – I have a K3 and Challenger 3 linear amplifier. Headgear
is the Heil Proset plus. I can tune up the rig and amp to well in
excess of 500W without any problem. CW can be operated at full power
without any problems. However, when I modulate the radio using SSB on
80m at anything above approx 275W, the SWR goes from 1:1 to infinity
and I can hear the ATU flashing over. It also occurs when modulating
the radio using AFSK for RTTY operation. Opening the ATU reveals
arcing between the capacitor plates. The problem does not occur on any
other band.
Grounding – all equipment is individually wired to a central rf earth
point which is connected to a buried copper earth rod. I have a
further 4 copper rods connected to the previously mentioned rod that
are spread over a few feet.
Antenna – 80m inverted L with six radials spread over about 45 degrees.
I don’t have another rig at present to substitute.
Any idea what’s going on please?
Cheers,
Terry
G4MKP
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