List,
Here is another perspective on grounding, used from large PABX (Telephone)
systems where grounding must be uniform across multiple cabinets, even under
a lightning induced impulse situation. One trick was to make sure each
ground lead from the star earth point was exactly the same length, to make
sure that any impulse traveling along the ground system got to each cabinet
more or less at the same time, ensuring that all equipment was more or less
at the same potential.
Nowdays the equipment fits inside far less cabinets, and interconnects are
fibre optic.
Roger
----- Original Message -----
From: "Richard" <2@mail.vcnet.com>
To: "Gary Schafer" <garyschafer@attbi.com>; "Richard" <2@mail.vcnet.com>
Cc: "Rob Atkinson, K5UJ" <k5uj@hotmail.com>; "AMPS" <amps@contesting.com>
Sent: Wednesday, April 17, 2002 8:24 AM
Subject: Re: [Amps] RF ground
>
>
>Richard wrote:
>
>>
>> ? I use a Hertz antenna (end-fed multi half-wave; high feed-Z). Equal
>> currents flow into the antenna and the antenna tuner ground. The lead to
>> the ground system is 5' of two-inch wide copper strap (c. 200nH of
>> inductance). Thus, there is an RF voltage drop across the tuner ground
>> lead. If the tuner ground is connected to the radio ground, the radio
>> becomes hot with RF. I installed a coaxial choke balun to prevent RF
>> from going back to the radio.
>
>If you got an 18 kA lightning strike the voltage on the ground at your
tuner
>would rise to around 18 kV due to the impedance of the 5 foot ground lead.
>That is assuming you had a perfect ground at the other end of the strap.
>
? 18kV is a semi-reasonable assumption,Gary . However, an 18kVpulse of
c.100kHz RF on the tuner case should not be a problem since the antenna
tuner is isolated from the amplifier with a 30uH, air-core coaxial
choke-unun. Also. the Hertz antenna has a 300k-ohm shunt resistor to
ground to bleed off static-charge build-up during thunderstorms.
cheers
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
end
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