Topband: Oscillation in Narrow band tuned antenna.

[GEORGE WALLNER]

Rune,

"Out of the blue" output transistor failures have been happening in SS PA-s. 
There are lots of discussions of the possible reasons on the various forums, 
ranging form overdrive, thermal issues to transients in the output circuits, 
etc. Nobody knows for sure yet.
One possible source is lightning or other high voltage discharge events, 
which could be made worse by a high Q antenna. Against that, you should have 
gas discharge (or spark-gap) protection devices on your feed-line.
Otherwise, if your antenna is matched to the coax, the voltage on the coax 
should be determined by the power and the impedance of the coax (about 400 
Vpeak at 1500 W on 50 Ohms). If the SWR is not 1:1 the voltage will be 
higher: about 800 Vp at infinite SWR. If your PA has a 12:1 output 
transformer, the transistors will see 67 V, which should be safe. "Narrowly 
resonant" antennas, when matched, just have narrower low SWR ranges. Think 
of the antenna (and matching circuit) as a transformer: It transforms the 
voltages and currents on the feed line into crazy high voltages (and zero 
currents) at the tips of the antenna. It is the PA driving the antenna, and 
not the other way around.
The best protection is to cover everything: the sequencer you have installed 
will help by preventing hot-switching or switching transients. Lightning 
protection protects against much more than just direct hits. You should also 
have something that prevents overdriving your amp.
GL and 73,
George,
AA7JV/C6AGU





On Wed, 13 Jan 2021 12:13:25 +0100
  Rune Øye <rune.e.oye at gmail.com> wrote:
> Hi all and HNY.
>
>
> I have been told that Oscillation in Narrow tuned antennas can occur and
> make damage to your equipment, radio or amplifier. It is really the first
> time I have heard about this potential issue.  On 160m band, I use a
> Titanex HD with a matching unit, in fact it is 27.5 meter vertical with a
> big loading coil at the feed point. 60 ground radials approx. 80 to 100
> feet long and two ground rods. The antenna is perfectly matched at 1826Khz.
> Reason for asking is that I have burned the final transistor in my radio
> and the final FET`s in my PA is broken. It all happens “out of the blue”.
> Since I now get a bit worried about this issue, I have installed a
> sequencer to be sure that my Power Amp, and all my switches and boxes are
> in time / sync so hope I have done something useful. Oscillation in Narrow
> band tuned antenna, is this a common issue, how do I avoid it! tune the
> antenna a bit out of resonance!
>
> This is really the first time I have heard about this issue…… any
> suggestion please.
>
>
> Thanks,
>
>
> 73
> Rune LA7THA
> _________________
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