Dear Stefan, LF Group,
Text-book discussions of professional VLF antennas suggest operating
voltages limited by corona at the order of 250kV - but of course these are
using much larger conductor sizes, and separation between antenna wires and
other objects, than is normal with amateur antennas.
My experiences on 136k and 73k suggest that about 20kV can be sustained on a
small-scale wire antenna, provide care is taken to reduce corona - I found
it neccessary to fit corona rings about 150mm in diameter to the ends and
sharp bends on the antenna wires, and where they attach to insulators,
otherwise corona would occur at these points.
The effect of corona discharge was to produce apparent rapid, erratic
changes in antenna impedance, which made tuning to resonance and matching
resistance impossible. Sometimes, the antenna would work OK for some time
before breakdown occured. It is often quite hard to see corona, just a faint
blue glow until it sets fire to something! Sometimes I could only locate it
through the hissing sound it makes. I found quite a good "corona detector"
was a broadcast FM receiver, on which crackling noises would appear when
breakdown was occuring. Corona can cause a lot of damage, melting or setting
fire to plastic insulators, ropes, wooden objects near the antenna wire,
etc., antennas suddenly falling down and so on. Several of us have
experienced fire damage due to corona, so beware!
Cheers, Jim Moritz
73 de M0BMU
----- Original Message -----
From: "Stefan Schäfer" <[email protected]>
To: <[email protected]>
Sent: Tuesday, March 09, 2010 4:35 PM
Subject: LF: Possible VLF HV problems, or not?
Dear LF,
I have had some thoughts about HV problems when operating a vertical
antenna wire at some 10kV.
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