W5UN wrote:
> Does anyone here have any experience using automotive type relays for
> switching RF? If so, at high power levels? These relays are
> typically manufactured for 12 vdc switching at 30 amps, but breaking
> one open and looking inside, it appears the spacings and contacts
> could handle RF. Not sure the insulation can, though. I don't know
> what would happen at high frequencies.
>
> Dave, W5UN
I have modified my MonstIR as follows:
I cut the jumper between the element halves for the end elements
(reflector and director 2). I then reconnected the element halves
to the normally closed contacts of a very small 30A automotive relay
about the size of a sugar cube. For normal operation of the MonstIR,
the full element current flows through this relay. Could be up to
10A at the legal limit. Anyway, normal MonstIR operation seems to
be unaffected.
The element halves are also connected to a transformer, which is
shorted out during normal MonstIR operation. When I
energize the relay with 12VDC, the short is removed and the transformer
is used to operate the two 70 ft elements (on a 34 foot boom) as a 2
element driven array for receive only. The elements have individual
coax feeds that go to a combining network on the ground. The array has
been tested on 80 meters and has an excellent pattern. I am working on
getting it down to 160 meters.
In this application, the relay never sees any significant RF voltage.
It is useful to note that relay voltage ratings are based on hot
switching. Assuming you are not going to be doing that, you can
probably quadruple the voltage rating.
You can also try the relay modification I published in May 2009
QST Hinks and Kinks to increase the voltage handling. Basically,
what I do is remove the NC contacts and modify the stops on the
relay to allow the armature to move farther away from the NO
contacts. I haven't tried this on an automotive relay. If
you try this, let us know how it works out.
Rick N6RK
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160 meters is a serious band, it should be treated with respect. - TF4M
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