[RFI] Ground Question
blackburn at qnet.com
Mon Feb 3 18:58:29 EST 2003
It never ceases to amaze me how many old wives tales and utter nonsense there is
floating around with regard to "grounding" a ham station. Part of the problem is
that the term "ground" is used for multiple requirements, some of which have very
little to do with the other.
For a ham station, there are typically three concerns relating to "grounding".
The first is providing a fault protection ground. This is the 3rd wire in your
AC outlet (the green or bare wire), which provides an independent conductive path
back to the power company's reference point in the event of a fault. Maintaining
the fault protection ground in accordance with the applicable codes is important
from a safety standpoint, but typically does nothing for RF compatibility.
The second "ground" is for lightning protection, where you want to provide a low
impedance path to physical earth for lightning currents in order to minimize the
amount of energy from a lighting strike that ends up flowing through your
equipment. In some instances, the lightning ground and RF reference "ground" can
be combined into a single reference system, but this may be a little difficult to
do with the station on the second floor. Lightning grounding is a subject unto
itself and is kind of beyond the scope of a short e-mail.
The third "ground" is an RF reference. In this case, you want to accomplish two
things. The first goal is to maintain your station equipment at a single RF
potential so you minimize RF potential differences (and the resulting fields)
created by circulating currents between the station components. This is
accomplished by providing a low impedance RF bond between all the components.
Note that providing this equipotential reference has absolutely nothing to do
with a connection to physical earth. The earth is not a sink into which we can
dump unwanted RF!
The second goal is to prevent RF currents from flowing through your station
equipment via the various conductors such as antenna coax, power leads and other
conductors. One approach, which doesn't seem to be particularly practical in
your instance, is to build an equipotential plane with a good solid connection to
earth (for lightning) and reference everything else to this plane. What's
probably more practical is to ensure that your antenna has a good lightning
ground, provide some means of disconnecting feedlines (including the shield) when
you are not operating, bond all your equipment together, and use common mode
chokes on the feedlines to minimize RF currents circulating through the station
equipment. Note again, that with the exception of the lightning ground, this has
very little to do with a connection to physical earth. You may also require
common mode chokes or filters on power cables as well, depending on how close
your antennas are to your station and how much power you run.
If you wish, you can use the commercial "line isolators" sold by various sources,
however, I've had success cleaning up RF intereference problems with nothing more
than simple common-mode chokes formed by winding the coax into a coil of about
8-10 turns about 6" or so in diameter right where it comes through the wall into
the shack. If you're interested, theres a bunch of info on coaxial RF chokes in
the towertalk archives.
Counterpoises or "artificial grounds" may help in some instances, but in my
estimation are a band-aid fix to a problem that shouldn't exist in the first
place. These work by moving voltage or current nodes away from the equipment,
but don't solve the root problem of having common-mode currents that don't need
to be there circulating through the station.
Hope this helps a little.
STRutledge at aol.com wrote:
> Hi folks. My shack is on the second floor of my house. No other options. I
> had the same situation years ago in another house, another city. I was just
> getting started in the hobby and several "old timers" told me about a method
> of getting a good RF ground on the second floor. It involved using
> capacitors on a piece of RG-8U. You installed one at the shack and at the
> ground across the shield and center conductor. The ones I have, bought at
> Dayton years ago where they were sold as part of this "lash-up," are marked,
> "RMC, .01HD, 1KV, Z5U. They are discs, tan in color.
> Maybe some of the more technically inclined can comment on this arrangement.
> I would appreciate it.
> Steve, N4JQQ
> RFI mailing list
> RFI at contesting.com
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