On Tue, 08 Jun 2004 23:39:09 -0000, Larry W4CSC
wrote:

Gary Schafer wrote in
:




A good lightning ground is also a good RF ground.



Welcome home, Gary. Missed your dissertations, really.

Ah, but once again, BZZZT...WRONG.....

I got two real-world examples to show you from my Navy experiences......

1 - Aboard a wooden MSO (minesweeper, ocean), a sailor was nearly burned
alive when he touched a metal handrail just outside the bridge while on
watch! His hands had bad burns, as did his hip, which was touching a pipe
not connected to the handrail.

An electrical inspection found the handrail heavily grounded, per Navy
requirements, to the boat's electrical grounding system, a good
installation with no problems found. However, the burning continued.

I found out about it from the boat's ET gang because I was in MINELANT's
Mine Force Support Group electronics shop, at the time, around 1970? My
EMO asked me my opinion and for me to take a look.

Not far from the handrail was the antenna tuner and 35' whip of the boat's
AN/URC-32 500W HF transmitter. Curious, I got a list of the frequencies
the boat transmitted on the day he got burned. They operated only three
that day, all on RTTY (FSK at full power). I took a tape measure and, as
best we could, measured the length of the ground strap down into the bilge
where it connected to the ship's grounding system. It was around 31 feet,
total length, and didn't really connect to any other points on the way down
into the engine room. One of the frequencies in question was on the 4 Mhz
band just above the 75 meter ham band. At this frequency, the 31' ground
strap was quite close to a 1/4 wavelength resonant ANTENNA with the open
end right under our burned sailor's forearm and coffee cup he threw when
the FSK started.

So, let's test this theory. I took a scope probe with a 6' ground lead on
it and connected the scope between the pipe he was leaning against, itself
some length of antenna at some other frequency, and our burning handrail.
"Key the transmitter.", I called down the passageway. WOW! The trace of
the 4 Mhz RF was TOO BIG TO MEASURE! I could feel the RF in my fingers!

So, moral, this great lightning ground was NOT ANYWHERE NEAR a good RF
ground on 4 Mhz, or any other odd multiple of 1/4 wavelength. It was a
resonant antenna with a handrail capacitor hat waiting to bite someone!

Solution - After months of fighting the electrical engineers at NAVSEA
about SHIPALTs to allow us to install them, we finally won and installed RF
Chokes into all handrail grounds at the handrails themselves to keep them
from becoming antennas resonant at any HF freq the ship used.

2 - Charleston Naval Shipyard, Metrology Laboratory of the Quality
Assurance Office (Code 132.1). I was asked to look at a crazy alarm
problem at the nuclear refueling docks where we pulled out the reactors
from nuclear subs and replaced them with refuelled reactors. (The hulls
are cut open and the core is swapped by a specially-equipped huge crane
that runs on big rail tracks around the docks).

Every time the crane lowered its big hook down into the hull, all the
radiation alarms went crazy, even before the hook got to where it was
supposed to go! Electrical Engineers (not RF engineers by a long shot)
added more and more ground straps between the rails the huge crane sat on
and the hull of the sub to "make sure" we had a "good ground" on
everything. (More grounds were always their answer to everything.)

I made arrangements to get the crane to where it would normally operate,
with the operator at the controls, but with the hook first hanging over the
rail of the crane, then over the hull of the sub in the drydock for
testing. I snatched a portable Tektronix scope from the shop's inventory
that was battery powered so it wouldn't be part of the grounding systems
and met the crane at the appropriate time. I grounded the scope to the
track at a handy pad eye used to hook the sub ground to it and as I
approached this huge steel hook the trace on the scope went off my screen.
My AC voltmeter read over 80VAC between "ground" and that hook. But wait!
What's this MODULATION all about?? I ran back to the shop to retrieve my
portable radio and quickly returned to the test point. Watching the AM
modulation on my scope while tuning around on the AM band, I matched up the
modulation envelope with WNCG AM 910Khz, a 5KW AM radio station some IDIOT
at the FCC allowed them to construct right outside the hospital gate less
than a mile from where I was standing. THE CRANE WAS A GIANT LOOP ANTENNA
and I was standing at the high-impedance FEEDPOINT of that loop!

Identifying the problem was easy. DOING something about the problem was
NOT! NOONE in Rickover's Navy makes any CHANGES to anything without a
fight. This fight I left, gladly, to much higher powers than me, but it
also resulted in a huge strain insulator added to the cable of the crane to
INSULATE the offending signal from the hook lowered into the sub. Wonder
whatever happened to it, now that it's all gone bye-bye....??

Moral....a great lightning or AC line or DC ground is hardly EVER a good RF
ground.....

Ok, as usual, your turn.....
Larry


Hi Larry, Hope you had a good sail.

Well this is an easy one! Your hand rail was not a good lightning
ground even though it may have had a large ground strap connected to
it. The path to ground was too long providing a high impedance.
Same for the crane. Too long a ground lead at the hook.

A good lightning ground has to have a low DC resistance as well as a
low impedance to AC. Remember that lightning has a large AC component
that is very strong. Any impedance in it's path despite how well the
DC ground may be will allow a large voltage to develop on it.

A good antenna ground must also have a low impedance.

Regards
Gary