ATU Flashover
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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 _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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When you switch from CW to SSB are you also changing
frequencies from the CW subband to the SSB subband? If so, repeat your
test on the same frequency.
My simple world view makes this one easy to solve.
Stay on CW :-)
- Keith N1AS -
- K3 711 - From: [hidden email] [mailto:[hidden email]] On Behalf Of G4MKP Sent: Friday, October 17, 2008 10:47 AM To: [hidden email] Subject: [Elecraft] ATU Flashover 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 _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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In reply to this post by G4MKP
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 > > Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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In reply to this post by G4MKP
Define 275W. Is that PEP or is that average. If it is what the meter reads the SSB peaks might be much higher. I assume you actually knew this.
Another possibility is the K3 power spike on first SSB syllable, that might be higher than the one on CW. Whenever you change mode or power setting, the ALC is reset and there is a high power spike. If that initial spike causes a breakdown, the arc will persist. However, on the second key up after a little delay to let everything cool off there should not be another arc. If there is, that was not the problem. Easy to eliminate this one. Another possibility is simple RF feedback, causing the K3 PA and amp combination to oscillate at some other frequency where the VSWR is poor. Are you wearing the headset when operating CW? If so it should happen in CW so it probably is not that either. Mike
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In reply to this post by G4MKP
Are you running your K3 tuner into the amp? If
so, shouldn't' the amp be giving the radio a nice match to start with? Are
you running a tuner after the amp as well? If you are running the K3
tuner, turn it off.
Is this on the exact same frequency or are you switching
between the cw and ssb portions. What freq is the antenna tuned
for? From: [hidden email] [mailto:[hidden email]] On Behalf Of G4MKP Sent: Friday, October 17, 2008 8:47 AM To: [hidden email] Subject: [Elecraft] ATU Flashover 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 _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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In reply to this post by Don Wilhelm-4
Don Spot on! Thank you and also to Mike, W0MU....should have picked this up myself. Haven't done any operating over the last few weeks but I have been tidying up the antennas around the garden. Whilst doing this I thought that I would "improve" my 80m antenna by firstly spreading the radials out a bit and secondly by increasing their number. Did that, got distracted and stupidly didn't check the antenna with the analyzer until just now. Sure enough the antenna resonance had changed to around 3.46Mhz so I lopped off around 5' and she's back to just over 3.7Mhz. Booming all over Europe with fb reports and no flashover. Cheers, Terry G4MKP 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 > > _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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