>
>> Anyone using a PA on an antenna without DC grounded driven element is
>> certifiable.
>
>Then I am in that class. I can count the following here:
>
>160 dipole at 310 feet above ground,
An impressive erection, Tom.
> 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?
>
>Guess again.
My guess is that the "safety" choke becomes copper vapour within mS.
>
>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.
Amen. I can see UHF TV interference from lightning storms that are over
the San Bernardino Mountains, 90 miles to the East.
>
>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.
Indeed, toss 'em out the window lest they burn your house down after a
direct hit. .
> 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.
>
>When lightning hits, even miles away, EVERYTHING arcs in my
>yard. The guy wires arc across the insulators, the gaps on the
>tower bases arc (grounded towers or not), the capacitors in a
>matching network WITH a grounded center-tap inductor feeding a
>long open wire line and curtain array arc.
>
>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.
>
>I stupidly did not have the feedline unhooked, and depended on a
>"Blitz Bug" (an old days coax lightning arrestor) to stop feedline
>differential voltages.
>
The Blitz Bug is, without doubt, a delightsome laugher.
>> 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.
>
My 0.3 wavelength 80m vertical builds up several kV within 15 sec. when
the East wind from the desert blows, providing that the bleeder resistor
is disconnected.
>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.
Which is why I bleed things down with a c. 10Meg ohm HV resistor from the
antenna to gnd.
>........
-
- Rich...
R. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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