All antennas should be DC shorted to ground at the receiver input because of
another reason: a sometimes much lower level of receiving back-ground noise.
de Hans
----------
>Från: "Carl Clawson" <cclawson@transport.com>
>Till: "Amps Reflector" <amps@contesting.com>
>Ämne: [AMPS] Re: Static discharge caused arcs
>Datum: lör 11 mar 2000 06.43
>
>
>> 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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