LF: Re: Arcing relays &c.

["James Moritz" <james.moritz@btopenworld.com>]

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