Anyone using a PA on an antenna without DC grounded driven element is
certifiable. The antenna ground MUST be in parallel with the shack's
mains earth.
This lets one off the hook regarding the protective choke, provided the
PA is connected to the antenna at the time.
If the antenna is not grounded, or the PA works into a switch feeding a
mix of antennas, a safety choke is nearly mandatory.
There are reasons other than the obvious HV voltage appearing on the
antenna. For example, a lightning flash ("near strike") could build up a
EMP voltage on an ungrounded antenna that would make the voltage
generated by a hi-pot tester seem like child's play. Guess where that
voltage goes to: the components on the output circuit of the linear.
(Point to check: is the coax connector on the exciter DC grounded? Some
are not).
Once, during a distant thunder storm, about 30 km distant, I watched
discharges arcing across a PL-259 connector on some RG213 coax (voltage
rating over kV) which was feeding an ungrounded 80m dipole. The cable
was disconnected and lying on the ground at the time.
As the antenna gets physically bigger, it's exposure area builds up
higher EMP voltages, high impedance antennas such as long wires and
helicals are particularly prone.
73,
Ian ZS6BTE
>
> >
> >the rf safety choke is there for a definite safety reason---to blow
> from the
> >dc supply gitting thru the plate blocking cap--check it--bet it
> shorted.
> >new call w5set.
>
> When the plate blocking cap fails, this puts the HV across the antenna
> terminal.
> The purpose of the safety RF choke is to trip the primary CB's or
> fuses to
> remove the HV from there WITHOUT destroying itself. If it is not
> robust enough
> to take the "hit" and fails open, the lethal HV is STILL on your
> antenna!
> (Not good!)
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