Just keep repeating... there is no such thing as a ground for RF or
Lightning. Follow that with... There are 3 types of problems to be
addressed by 'Grounding'.
1. AC Safety - fairly simple... all equipment cases, towers, extra ground
rods, etc must connect to the power entrance ground.
2. Lightning - think differently... problem here is differences in ground
potential from one component to another... voltage differences between
ground at a tower or even from one side of a house to the other can be
substantial. This drives connecting everything to a common ground, but also
adds things like MOV's to equalize voltage between ground and power/signal
leads to prevent backflashovers inside equipment.
3. RF - There is no way to get rid of RF by grounding. Period. All you can
do is keep the impedances low enough so that you don't notice it. First
part is the same as AC Safety, everything must be connected together, but
add to this 'by the shortest possible path'. The biggest misconception is
that one where you have RF on a mic or other piece of equipment... the fix
is not to connect it to ground better, but to provide a better path for the
rf to the jack on the back of the radio. The most important thing to
remember when trying to fix rf in the shack problems is that ALL the power
you pump out on the center pin of the coax connector MUST come back to the
INSIDE of the same connector. RF currents at that point must be equal and
opposite. The simplest illustration is to put a piece of bare wire in that
center pin hole and tx... the rf return path is on the outside of the radio
case back to that connector and then over the outside of the connector to
get to the inside to balance the current going out the wire. Obviously
anything connected to the case of the radio, like microphones, power leads,
etc, becomes part of the return path for the rf... Including YOU if you
touch the case or microphone case, but note, it is not RF FROM the radio
that is doing the biting, it is RF TRYING TO GET BACK TO THE RADIO that is
picked up on your body trying to get back to that antenna connector. Adding
a ground wire, or any other wire not even connected to ground, that provides
a more efficient path back to the antenna connector will reduce the RF
picked up on your body or raise the voltage of the case to match your body
so it doesn't provide a path from your body to the antenna connector. It
gets uglier when you connect a long piece of coax to the antenna connector.
You essentially move the point where the currents must balance to the end of
the coax, but you also make the radio case as long as the shield on the coax
is. When that length gets more than a small part of a wavelength you can
end up with all sorts of current/voltage patterns on the outside of the
coax, this is where ferrites and baluns come into play by making the path on
the outside of the shield to the far end of it a higher impedance than all
the other paths where you would prefer the current to be.
David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://wiki.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://k1ttt.net
-----Original Message-----
From: Jim Brown [mailto:jim@audiosystemsgroup.com]
Sent: Saturday, April 27, 2013 22:59
To: rfi@contesting.com
Subject: Re: [RFI] Shielding a ground wire
On 4/27/2013 1:32 PM, Christopher Brown wrote:
> But it is not a magic RF sink, and as far as the station goes.....station
is second floor...strap or not, it is not a "RF ground" or some such for
anything above 80M.
Right. In general, a connection EARTH is NOT part of a solution to RFI
problems, nor does it make antennas work better (although it CAN provide a
current sink for a receiving antenna, the earth is FAR too lossy to be an
efficient part of a TX antenna).
We MUST provide an effective earth connection for any premises, but the
reason for it is LIGHTNING SAFETY, not noise elimination.
The only example I can think of where an earth connection CAN help reduce
noise is in combination with one or more common mode chokes to shunt common
mode current to the earth rather than a cable that can either radiate it or
couple it to an antenna feedpoint.
73, Jim K9YC
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