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Message-ID: <B65469FEB5B8408487388114379FCE1E@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <9afca2641003090337t4062bb3avc9b952612014c0ba@mail.gmail.com> <4B9678C3.7070907@iup.uni-heidelberg.de>
In-Reply-To: <4B9678C3.7070907@iup.uni-heidelberg.de>
Date: Tue, 9 Mar 2010 19:06:11 -0000
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Subject: LF: Re: Possible VLF HV problems, or not?
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Dear Stefan, LF Group,

Text-book discussions of professional VLF antennas suggest operating=
=20
voltages limited by corona at the order of 250kV - but of course these=
 are=20
using much larger conductor sizes, and separation between antenna wire=
s and=20
other objects, than is normal with amateur antennas.

My experiences on 136k and 73k suggest that about 20kV can be sustaine=
d on a=20
small-scale wire antenna, provide care is taken to reduce corona - I=
 found=20
it neccessary to fit corona rings about 150mm in diameter to the ends=
 and=20
sharp bends on the antenna wires, and where they attach to insulators,=
=20
otherwise corona would occur at these points.

The effect of corona discharge was to produce apparent rapid, erratic=
=20
changes in antenna impedance, which made tuning to resonance and match=
ing=20
resistance impossible. Sometimes, the antenna would work OK for some=
 time=20
before breakdown occured. It is often quite hard to see corona, just=
 a faint=20
blue glow until it sets fire to something! Sometimes I could only loca=
te it=20
through the hissing sound it makes. I found quite a good "corona detec=
tor"=20
was a broadcast FM receiver, on which crackling noises would appear wh=
en=20
breakdown was occuring. Corona can cause a lot of damage, melting or=
 setting=20
fire to plastic insulators, ropes, wooden objects near the antenna wir=
e,=20
etc., antennas suddenly falling down and so on. Several of us have=20
experienced fire damage due to corona, so beware!

Cheers, Jim Moritz
73 de M0BMU


----- Original Message -----=20
From: "Stefan Sch=E4fer" <Stefan.Schaefer@iup.uni-heidelberg.de>
To: <rsgb_lf_group@blacksheep.org>
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=
=20
> antenna wire at some 10kV.
>
>=20