[AMPS] Re:(AMPS) sb-200 troubles

[Tom Rauch]

> 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, 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.

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.

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. 
     
> 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.


73, Tom W8JI
w8ji@contesting.com

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