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: Elecraft@mailman.qth.net 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
