Message-ID: <4E957112.8090200@iup.uni-heidelberg.de>
Date: Wed, 12 Oct 2011 12:50:58 +0200
From: =?ISO-8859-1?Q?Stefan_Sch=E4fer?= <Stefan.Schaefer@iup.uni-heidelberg.de>
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Subject: Re: LF: Re: Arcing relays &c.
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Joe,

As i was i child i played with some 10 kV transformers which are used as 
ignition transformers in oil ovens. Also some HV TV cascade experiments 
were done. To discharge the loaded C's i took a motor (out of a cassette 
player) and applied a wire on its axis. When spotting some ligh from a 
hand lamp to a photo diode that opens a transistor the motor begun to 
rotate and closes a contact gap of several cm, good for 100 kV if 
necessary ;-)  Maybe something in that way would be an alternative to 
switching usual mains relays in series. Then your 50 kV are no problem. 
The contact forces is another thing of course...

73, Stefan

Am 12.10.2011 10:59, schrieb James Moritz:
> Dear Joe, LF Group,
>
>> Feeding this into 3 mH (X=2500 ohm at 137.8 kHz) gives 11.2 kV (17 kV 
>> peak) so  relays in series as suggested by G8FZK and G3NYK should work
>> well.
>
> It is a bit more complicated than that... if you have a number of 
> relays in series, when the contacts are open there will be one or more 
> isolated "floating" sections of conductor linking the open contacts. 
> To ensure that the voltage is evenly distributed between the relay 
> contacts, you would need to take steps to ensure that the potentials 
> of these isolated sections are equal divisions of the total antenna 
> voltage - this would mean adjusting the distributed capacitance in the 
> relay wiring somehow. Also, the relay at the "hot" end will have the 
> entire antenna voltage between contacts and coil, so coil-contact 
> breakdown voltage would need to be much higher than the voltage rating 
> for the contacts.
>
>> I'm puzzled by the 6pf/metre rule for monoploes.  It seems the 
>> capacitance
>> of the monopole is much higher.
>
> This is a good first-order estimate for wire antennas - it is 
> reasonably accurate for wires up to a few mm in diameter, but if the 
> antenna element is a conducting mast, the diameter will be much 
> larger, and C significantly higher too.
>
> Cheers, Jim Moritz
> 73 de M0BMU