On Fri, 2006-05-05 at 22:25 -0400, Larry DiGioia wrote:
> I personally believe not in disconnection, but moving antenna connectors
> to a good ground system. I use the multiple grounding blocks (#CQ857)
> and quick disconnects from The Wireman.
>
> That is not to ignore the fact that there is a spark gap at the base of
> my Hy-Tower, Polyphaser protectors at the building entrances for all
> coax, and spark plug arrestors (#CQ878) at the entrances on all my
> ladderlines... However, I think that, contrary to popular belief, a
> grounded antenna is LESS likely to take a direct hit, as it allows it to
> "bleed off" charges in the air before they become huge.
>
> Stuart Rohre wrote:
>
> >Lynn makes a good point. Our club station helper grounded every rig
> >negative and every chassis to earth bus. Of course there was coax to the
> >VHF/UHF rig. The lightning surge came down the shield of the coax from the
> >tower, into the radio on coax connector and back to AC ground thru the DC
> >negative copper of the circuit board, (until that vaporized). We had
> >created a ground loop. In this particular hit, the AC ground looked lower
> >impedance than the bronze strap bus,and its connection to cold water pipe
> >just outside the wall. (2 foot of braid to it).
> >
> >How you treat grounds can be a damaging situation.
> >
> >Total disconnection of the coaxes at entry to building when not in use would
> >have make a bigger spark gap for the current to cross than the circuit board
> >did.
> >
> >We had survived a direct hit on the power pole 100 feet across the parking
> >lot some years before. The 65 foot tower usually discharges the
> >surroundings to its earth rods.
> >
> >Stuart
> >K5KVH
> >
The tower sometimes may discharge the surroundings, but its not an
absolute guarantee of lightning prevention.
Back in the early 60s my dad (K0CPN) and I put up a rotating mast. It
stood until about a year ago. We never knew it to be hit, though a
neighbor's house 4 or 5 tower heights away and lower than the top of the
tower was hit once.
I've lived on this spot in the country for over 35 years. My tower is
grounded with double rods at the base (8' rods drive 10' into the
ground) with no insulators in the guy wires, utility sized 8" screw in
anchors and with added ground rods where the coaxes get to the basement
window and one added at one of the guy wire anchors. Its been hit at
least three times, direct hits. It doesn't discharge the sky all the
time even though there are probably at least 50 elements on VHF/UHF
antennas on the top giving lots of tips for corona.
WA0OBN over at Jamaica, Iowa, told me about finding his tower and ground
rods were insulated from the nearby earth. He dug some up and found them
coated with glass, apparently melted silica sand melted to quartz from
lightning hits. Jamaica is on a sandy location in the Raccoon River
bottom.
There used to be a company, as I recall called Lightning Elimination
Associates, that claimed their towers would indeed prevent lightning in
their vicinity. NASA bought their scheme for Cape Canaveral. What they
did was to put a triangular screen on top of a tall tower (a gaggle of
towers were required to protect an area). The triangular screen was made
of wire with barbs much like that used to hold critters in fields except
they put more points per foot than needed for critters. Which put
thousands of points in their "air terminal" to create lots of corona
discharges and to discharge the atmosphere. Apparently the scheme did
work occasionally, but NASA lost faith in them when those towers took
photo documented direct lightning strikes. I think the company quit
advertising after that.
Back in the early 60s one night at W0QEV (Washington University RC, St.
Louis) while studying in the hamshack on a gloomy night I was
interrupted by loud snaps from the antenna patch panel. It was the 80
meter dipole developing so much voltage from the charged atmosphere that
the SO-239 connectors were flashing over. As I recall, that 80 meter
dipole was hung between prominences on the south face of the building,
nearly at the central roof level. On that central roof there was a tower
sticking up another 40 or 50 feet, one the towers used to hold up a flat
top for KSD radio in the 1920s. I know that tower was grounded, I helped
check the ground and add more. It did not discharge the cloud as sampled
by the 80 meter (and probably 40 meter dipole).
So my overall experience of a bit over 50 years of being a ham is that
the best laid schemes to discharge a cloud don't work absolutely
perfectly and so things sticking up will be hit if there are any
thunderstorms within 15 or 20 miles of the location sometime. Not
always, but sometimes.
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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