From: Rob Atkinson <ranchorobbo@gmail.com>
To: MU 4CX250B <4cx250b@miamioh.edu>
Cc: "amps@contesting.com" <amps@contesting.com>
Subject: Re: [Amps] Vacuum Relay Question
<A 1/4 w. monopole load is a low Z load; that means high current, low
<voltage, i.e. it is not a voltage fed antenna. Therefore there isn't
<high voltage involved. You can operate without a vacuum relay. They
<are usually employed in h.v. situations such as a 1/2 w. radiator
<(although there are ways of avoiding them there) and mostly in
<transmitter applications such as switching plate chokes or pi network
<tune caps. Of course you need an enclosure that keeps bugs out and
<vents to evaporate condensation.
<Don't worry about designing to survive a direct hit. Broadcast tuning
<houses get destroyed by direct hits, especially positive strikes.
<It's not cost effective to build to handle that. Just disconnect and
<provide a better path to ground. Try using a ball gap across the base
<insulator.
<73
<Rob
<K5UJ
## Rob, did you read his post ?? He has a 1/4 wave tall 80m vertical,
70 ft tall. He uses the same 70 ft tall vertical on 160m, by vac relay
switching in a special network, with a large toroid etc, etc. He already
has the vertical DC grounded with a 100 uh coil, between base of
vertical..and ground. Open frame relays, when used outdoors are typ UN
reliable, esp loss on RX. And you won't find any that are 12 kv @ 50A
rated. I have had more problems than I care for when using open frame
relays in water proof nema boxes, packed with silica gel etc. Zero
issues when using ceramic vac relays,none.
W8ZR might want to consider a bigger knife switch, or 2 in series, or some
kind of quick disconnect switch, or removeable copper strap at one end,
with a resulting huge gap.
The 100 uh coil he is using at the base of the vert may well impede a
lightning event, since lightning is typ around 1 mhz. An adjustable
spark gap might just do the trick. I made several adjustable spark gaps,
by using solid brass bolts, and solid brass locknuts...and 2 x pieces of
aluminum angle. Then the on the inboard side, where the actual gap is, I
used solid brass acorn nuts, which are sorta dome shaped, like wheel lug
nuts, with the 2 x domes facing each other. The gap is easily set. The
hb spark gaps were not for an ant application though. I used one across
the vac tune cap on a hb linear, which protects the vac tune cap...plus the
expensive bandswitch. Another adjustable spark gap was wired across the
expensive vac load cap. I also used yet another adjustable spark gap
across my big 127 lb dahl 4 Henry choke. The choke is for an
experimental C-L-C filter network, with the choke in the B+ line, between
the 2 x caps. Choke is fully insulated from the chassis.
The gaps are easily set, and everything is fully protected, in case
anything goes sideways. If the solid brass acorn nuts are destroyed/
damaged / burnt, they are easily replaced with spares. 6-32, 8-32,
10-32, and 1/4-20 brass hardware was readily available at any big box
store. The mating brass acorn nuts are much bigger diam than the
threaded material. They also had 3/8" brass material. But for an ant
application, bigger is probably better, and steel could probably be used,
like 1/2". Acorn wheel lug nuts are for either 1/2" threads, or 13mm on
newer vehicles.
Some ground rods to supplement the radial field might be the ticket...like
at the base of the vertical. Lightning is trying to find a path to ground.
Jim VE7RF
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