[AMPS] Re: Static discharge caused arcs

[Carl Clawson]

> Colin K7FM
> So, the question is, when the amplifier is actuated and relay closed, is
it
> possible that a static discharge returns down the antenna - and while not
of
> sufficient voltage to blow a hole in your roof, might add to the existing
> voltage to create an arc which otherwise would not occur.  Perhaps the rf
> choke that all good amps have installed from antenna to ground would
address
> this and short all to ground.  This might apply to dc - but the rf choke
> might appear as a high resistance to the static.
>
> Could this be one of the causes of the arcs?

Interesting, Colin!

What's happening isn't a sudden discharge from the sky to the antenna, but
rather a slow buildup of charge that occurs when the antenna terminals are
open I've always believed that the final RF choke had two purposes. First,
to blow the HV fuse if the plate block cap fails, thus saving the life of my
cat who is out back sniffing around my antenna. Second, to act as a static
drain and keep the sparks from happening. (Note that many antennas have a
built-in DC short and won't allow static buildup. And a spark gap or
lightning arrestor will keep the voltage down.)

But here's the rub - in the half dozen amp schematics I just looked at, the
choke is on the *tank* side of the antenna relay. You're now depending on
the exciter's receiver to provide the DC path to ground while in receive. If
the exciter isn't connected (which it won't be if you are using a different
receive antenna, for instance) an arc across the output antenna relay is
likely. This creates a bunch of frequencies, some of which won't be shunted
by the final choke.

Should we move our chokes to the other side of the relay, or add another one
there? Is there a good reason not to?

Here I am unable to continue much further. Not knowing the frequency content
of a typical arc I can't estimate how much stimulus would be applied to the
tank and thence to other components. However, we can get an idea of the
energy involved. The capacitance of the antenna is probably negligible, but
coax has 25 to 30 pF/ft. Assuming 100 ft we have 2500 pF. Let's say it takes
10 kV to initiate an arc. The stored energy is then 0.5 * 2.5 * 10^-9 *
(10^4)^2 = 0.125 J. Probably not enough energy to do serious damage to a
robust contact. But could it be the trigger for an exciting "event?" I can't
say.

At least my cat is still safe.

Respectfully submitted,
Carl WS7L



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