This debate has served to illustrate the poor understanding most hams
have regarding the difference between static build-up on an antenna, and
EMP effects caused by lightning strikes or "swipes".
Static on an antenna, as other correspondents have already indicated,
can be largely overcome by DC grounding the antenna (ALL of its
elements). It doen't help to simply ground the driven element only (in
receiving terms).
However, if the antenna is serving as a "bleeder" for the environment,
DC grounding is irrelevant, go and have a beer. This is perfectly normal
(and desirable). The noise, if it is caused by corona discharge from the
antenna, indicates a reduced risk of a direct strike, as the charge is
being bled off.
To say that antennas are "floating" may be nonsense - is the antenna DC
grounded somewhere else? For example, the rig may have a choke across
the input. Filth (electrolytes) across the antenna terminals, degraded
coaxial insulation (leaching caused by continual high volatge static,
and other effects), could mean that the antenna is anything other than
floating.
The grounding choke could be a mini, there is no power (amperage)
involved, only voltage. There is no frequency component other than the
need to define an impedance to the RF voltage.
Pulse effects cause by lightning are readily leaked off to ground by
chokes (or from ground, through the chokes, etc., to the antenna
system), or even active devices such as diodes. The charge is conducted
before it can build up enough power to start destroying things. To talk
of frequency is probably off course, other than discussing the rise in
voltage vs time, across the resistance (impedance?) of the conducting
medium. It's true that a pulse with a fast rise time has a broadband
frequency component, but when talking grounding against EMP, think of a
clamping action at the threshold voltage rather than a RF path.
Also, more stations are destroyed by pulses coming through the miles of
municipal power wiring, through the rig, etc., and out through the
antenna (or else discharge in the shack itself), than any other mode.
These hams would swear they were struck by a "direct hit". INHO anyone
suffering a direct hit, in the typical ham environment, whatever the
state of the grounding system, would be lucky to not go SK.
Ian, ZS6BTE.
> Then I am in that class. I can count the following here:
>
> 160 dipole at 310 feet above ground, 80 meter dipole at 160 feet
> AGL, force 12 ten, fifteen, and twenty meter yagi stacks with
> floating parasitic elements and floating driven elements, four 130
> foot verticals in a 160 meter four square, and a 200 foot vertical on
> an insulator.
>
> > 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.
>
> And I suppose we should think a small choke that has a high
> enough impedance to not affect SWR will change the level of
> energy substantially from a nearby lightning hit?
>
> The majority of energy in a lightning discharge is in the hundreds of
> kilohertz range, and remaining energy extends from light
> frequencies all the way down. You can see it with your eyes, see it
> on the VHF or UHF television set screen when the hit is dozens of
> miles away, hear hits thousands of miles away on 160, 80 and 40
> meters.
>
> If the choke has enough impedance to NOT affect RF it does little
> to nothing for lightning pulses.
>
> The best rule is if it storms, don't be a fool. Unhook the antennas
> and move the feedlines out of the way. Of course the shields,
> which always carry the bulk of the energy, should be well grounded
> with a proper system.
>
> 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.
>
> Different effect. I can stand outside and watch the gap on a 318
> foot insulated base tower arc when lightning flashes on the horizon.
> That tower IS dc grounded through the chokes that feed tower
> lights and VHF antennas, and it still fires across a 1/4 inch gap.
>
> The only major damage I ever had was when a direct hit occurred
> on a grounded shunt fed tower (with 100 number 8 gauge radial
> wires for 160 meters). The vacuum variable in the shunt wire melted
> inside, and the hit went through the amplifier and into a T4XC where
> it welded the loading capacitor plates and torched the relay board.
>
> > 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.
>
> It's the impedance from dc to light, in particular at a few hundred
> kilohertz, that matters.
>
> Now I'm not saying grounding the element is a bad idea, it just
> means precious little except for the slow gradual build up of
> charge. For example, my 160 dipole even on a calm day will knock
> you flat on your butt if you disconnect the feedline and let it hang in
> the air, and touch it after five or ten minutes.
>
> When my antennas are connected to the station through a switch
> box, they are indeed "dc grounded" through various chokes in the
> equipment. But if anyone for a minute thinks that ground does
> anything much more than eliminate the tick tick tick of the slow
> voltage build-up from charge gradient between the earth and sky
> (and that gradient is there all the time, storm or not) they need to
> re-think the problem. That problem can cause a popping noise in
> the receiver or blow out sensitive front-end components.
>
> That is the only effect "dc grounding" eliminates. The rest is a pipe
> dream. My series fed 200 foot tower is just as quiet as any shunt
> fed tower I could install, as long as efficiency is the same. My four
> square, with series fed UNgrounded elements is just as quiet as
> "grounded" Beverages most hours of the night.
>
> If I take the chokes off, all I get is an occasional "pop" when the
> charge exceeds the voltage rating of a relay or other component.
>
--
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