> I have a relay system that switches a base loading coil in
and out of a
> 60 foot vertical. While on 160, the open contacts across
the 80 meter
> relay arch, interfering (albeit intermittently) with 160
meter operation
> at the kw level. I understand the voltage is quite high
at the top of
> the coil on 160 with an 1/8th wave vertical, but is there
a way to
> prevent this arching from occurring?
This was just on an eHam Elmers thread also.
The current at each coil terminal is essentially equal so
long as the coil capacitance to surroundings is small
compared to antenna load impedance on the coil. The voltage
drop across the coil is the base current times coil
reactance.
If the coil is compensating a base feed impedance of
15 -j300, current through the coil at a kilowatt is sqrt of
1000/15, or 8.16 amps. Voltage across the coil is 8.16 times
300, or 2450 volts. The relay has to be rated by peak
voltage, so voltage would be 1.414 times the 2450 RMS, or
about 3500V. Now you need a safety factor, so a 5kV relay
contact to contact or contact to frame breakdown would be
OK. That voltage would require an air gap of more than .2
inchs if the contacts were reasonably blunt. Sharp points or
edges would make spacing even wider.
That's why most relays won't be reliable.
>I have tried two relays so far,
> one from Magnacraft another from Potter & Brumfield, and I
have the same
> problem with both. They all work fine on 80, by the way.
I am going to
> replace them with Array Solutions 5kw relays, but it seems
to me the
> problem is not necessarily in the power handling
capability of the
> relay, since the arching is across the open contacts of
the 80 meter
Speaking of realays that are not hot-switching loads....
Relays don't have a power rating, unless you define all the
other parameters associated with that power rating. Relays
have a current rating that is a function of time and
frequency. They have a voltage rating that generally is not
affected much by frequency, although air breakdown it is
affected by humidity and air pressure. With 10kw in a
50-ohm system voltage applied to a relay is only 1000 volts
peak. It's not any surprise a "5kw rated" relay would fail
with the loading coil. That's a useless rating for your
application.
What you need is a high voltage relay that still has
contacts capable of handling the expected current. DXE has a
relay that would give a good shot at working, but I'm not
sure they are going to retail them. A high power vacuum
relay would probably work.
One reason people get away with relay switching is they
might run QRP. Another is they may have a lossy coil, a
lossy ground system, or a combination of the two. Perhaps
the required reactance is low. Or it could be a little of
all.
The bottom line is if you have a short (1/8th wave or less)
vertical and it is an efficient system, the voltage across
the loading coil is probably going to be pretty high even at
modest power.
73 Tom
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