Reg Edwards wrote:
No arcing problem that I ever detected. Transmitter was 100
watts. I
think the relay was rated at 750 volts and it may have been a
vacuum
type.

That's a pretty husky reed relay but you are lucky
it didn't arc. Peak-to-peak across that relay was
higher than 750 volts if it was installed in a 1/2WL
dipole.
--
73, Cecil
========================================

The voltage between the relay contacts, when open, can be considerably
less that the voltage at the end of the live wire relative to ground.

The capacitance between the open-circuit contacts of a reed relay is
very small. And there is a larger capacitance between the two antenna
wires on either side of the relay contacts. There is also capacitance
between each of the two wires and ground.

If the input impedance of the wire on the remote side of the relay is
not low then we have a voltage divider.

So the voltage which appears across the relay contacts can be
considerably less than the volts between the live wire and ground.

If I remember correctly, the relay was located more than
1/4-wavelength along an end-fed wire on the 160m band. There was a
random but not very long length of wire on the far side of the relay.
The antenna was only about 20 feet above ground, ie., quite lossy.
The details of what experiments took place I can't remember. Perhaps
something to do with input impedance measurements and treating antenna
wires as transmission lines.

If the open-circuit relay contacts did not arc over with 100 watts
then it was more by design than good luck. ;o)

I've just had a search round my junk boxes to see if I stll have the
reed relay. It was built into a small plastic box with a few
decoupling capacitors and 3 binding posts. But, unhappily, no signs
of it.

And my suggestion to operate a relay in an antenna wire via Radio
Shack speaker wire stll holds good.
----
Regards, Reg, G4FGQ.

Reg Edwards wrote:
If I remember correctly, the relay was located more than
1/4-wavelength along an end-fed wire on the 160m band.

Thanks Reg, that makes sense now. That's quite different
from breaking a 1/2WL loop in the middle using a relay.
The voltages at each end of 1/2WL are 180 degrees out of
phase so the relay has to handle double the voltage existing
at the ends of the wires. A quick estimate of the voltages
at the ends of a 1/2WL dipole being driven by 100 watts is
1000 volts RMS. The relay would have to stand off almost
3kV PTP.
--
73, Cecil http://www.qsl.net/w5dxp