On Tue, 9 Mar 2010, [ISO-8859-1] Stefan SchДfer wrote:
> 
> I have had some thoughts about HV problems when operating a vertical 
> antenna wire at some 10kV.

Dear Stefan,
there was no such a problem (seems to me) in Markus experiment with 10kV.
May by with 20kV... But anyway corona protected rings (rather small i
think) to be usefull on the ends of wire. Remember, high E is only arround
ends of wire, not at the center of wire. Besides such a problems arises in
50 Hz systems at some about 100kV, not at 10 kV. 10 kV is rather small
voltage:-)

> 
> Normally, e.g. in a 50Hz system there will flow a current into the air 
> (capacitive with a restistive component) due to the ionisation of the 
> air when partial discharges occur.
> This is due to exceeding the maximum E field strange on the wire (a 
> function of its radius and geometry and insulation and the gap between 
> wire and insulation and the epsilon r of the insulation and and and ... 
> ;-) ).
> In the special case of a VLF antenna there might be some special 
> conditions that can limit the influence of the partial discharges. For 
> example, the Z  (sqrt (L/C) of such an antenna is very high, so the 
> resonance is quickly lost if C is changing slightly. The presence of 
> partial discharges will apparently increase the C since the surface is 
> virtually increasing. There will be additional resistive losses but 
> since a change the C will bring the circuit out of resonance (what would 
> reduce the voltage at the wire), there might be a limitation of the 
> momentary value of the voltage. This will be anything else than a stable 
> process but there might be the effect, that the voltage will be limited 
> like with a varistor or a spark gap in series with a resistor. So, 
> theoretically, there will be many harmonics and my signal might be 
> better visible at its harmonics than on the ground wave ;-) So, have 
> some of you, perhaps on LF, have experiences with theese effects??
> 
> 73, Stefan/DK7FC
> 


Regards,
Alexander