[Elecraft] bonding of grounding systems

[Ron D'Eau Claire]

I'm not sure what 'system' you are talking about, Ken, but I assume you mean
using an independent ground stake to kill the broadcast RFI.

Nothing in the mains safety ground more than a few feet from the ground rod
is at RF ground. DC (or low frequency AC like the mains power) is a
completely different 'animal' from RF. The inductance of wire is enough to
make a conductor that is hooked to a ground stake very hot with RF only a
few feet away from the ground stake. That's why it's normal practice to
minimize the inductance of any connection for an RF ground. The usual means
is to use a very wide conductor - either braid or a wide copper sheet - and
keep it as short as possible. 

More commonly, for Amateur installations, we tend to do one of two things.
Either we use a balanced, low impedance RF transmission line or we used a
tuned counterpoise. (Coax feed is a form of "balanced" feed, in that the
shield provides a low impedance RF ground to the rig even though the feeder
currents aren't strictly "balanced"). 

First of all, a ground stake, even with a very short run of wire to the rig,
is a lousy RF ground. Unless we're at the sea shore or on a swamp, the
resistance of the soil is terrible at RF. That's why broadcast stations lay
out a circle of radials around their antennas, so the vertical is almost
sitting in the middle of a copper disk. So, wherever possible, we either use
an antenna that doesn't require an RF ground at the rig, or we arrange the
best RF ground we can. Usually the best RF ground is a tuned counterpoise.
The simplest version is a 1/4 wavelength long wire, insulated at the far
end. As long as the wire is 1/4 wavelength long and an open circuit at the
far end, that end will be a high-impedance point. That, by definition,
forces the end at the rig to be a low impedance. One 1/4 wavelength wire
will show an impedance of about 35 ohms at resonance. Use four of them and
you have a ground with about 8 ohms impedance. That's why you will often see
verticals with four radials. It's  a nice compromise and they can be used as
support guys. By comparison, it's not unusual for a ground stake to show 200
or 300 ohms - even higher in many locations - and the impedance goes up with
every foot of conductor we have between the ground and the rig. 

Counterpoises (or 'radials') are isolated from the ground, in general.
Sometime they are laid on the ground, although we now know that raising them
only a short distance makes the counterpoise a far better RF ground than one
on the ground unless we use a great many conductors, and one on the ground
is far better than a buried system. 

So, for HF transmitting, it's very common to have an RF ground that is
completely independent of any direct connection to the earth.

If you managed to kill broadcast RFI with a secondary, independent ground
wire, I'd bet it is simply reducing the RF on your equipment by providing a
better RF ground at broadcast band frequencies than your house wiring
provides. Eight feet of wire to a ground stake is totally ineffective at 28
MHz, but can be rather effective at 1 MHz. And you don't really care all
that much about the quality of the ground to stop RFI, as long as it's good
enough to do the job. Also, bonding your ground stake to the building mains
ground stake shouldn't interfere with the effect of the ground for RF. Even
though the building ground wire may be hot with RF all over the place, it
won't be at the ground stake. 

Ron AC7AC

-----Original Message-----
...One of the local A.M. stations had increased transmitter power with the
result of wild RF running all over the place. It seems standard AC service
ground systems stink for handling stray RF. ... I was always plagued by RFI
until I went to this type of system. Has anybody else on the group ran into
this?

Ken W8OB


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