Jim
Yes , that's a limiting factor,
The Delco open frame relays have quite a considerable, 'coil to
contact' distance and some 4 / 5 mm contact gap,
2 or 3 mounted in line, daisy chained would produce quite a
symmetrical lay out as the wiring could be simply single core
btc linked across .. stray contact 'c' should even out the voltage
? .. you could remove the un-used contacts to increase the
isolation ...
Other than that, its either plug in coils or a remote contactor
arrangement . but I would try the Delco relays first
G..
--------------------------------------------------
From: "James Moritz" <[email protected]>
Sent: Wednesday, October 12, 2011 9:59 AM
To: <[email protected]>
Subject: LF: Re: Arcing relays &c.
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
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