Steve, Keith, and List Members,
Keith, you have really hit the nail with this one. So did Ian, G3SEK, in
another reply to Steve's posting. I would just like to add a few comments.
First, I was a second floor op for roughly the first half of my ham career.
I violated a LOT of grounding rules and paid the price in some cases. NONE
of my coax cables were ever grounded external to the house in those days,
and I am just very glad now that I never took a near or direct lightning
strike during that entire time. In actuality, my shack ground was the 3rd
wire ground of the AC power feed. The houses in the Chicago area have been
required to use metal conduit for decades, so having a rather solid safety
ground from a 2nd floor shack is not a problem there. However, stray RF
currents will also travel (or try to travel) that path, and that is where
problems arise. I was generally OK until I tried to use a longwire antenna
with a tuner located in the shack, Disaster!
Now, some specific comments on Steve's situation:
1. If you are using Astron (or similar) power supplies that have
3-conductor power cords, you may already be using the house power ground
wires as your ground. The very popular Astron RS-20 power supply has the
NEG (-) output terminal tied to chassis ground, which is also tied to the
green wire AC ground. Therefore, all equipment connected to that power
supply has its negative return power lead (which is usually chassis ground,
for most rigs) connected in a roundabout, uncontrolled impedance manner, to
ground via the AC power wiring in your house. Note: my wife recently
bought a Pyramid 14 amp DC power supply to run her 2m FM rig in the
kitchen. I was amazed to find that the Pyramid has only a 2-conductor
power cord, so the DC output terminals totally float. I do not know if
this is true for other Pyramid brand power supplies, but it is something to
keep in mind if you do not want to use the house power ground wiring as
your station ground. Also note that both the Astron RS-20 and the Pyramid
14 amp supplies are linear power supplies. I do not know if the switching
power supplies tie the output (-) lead to ground or not.
2. Use balanced feed antennas. These would be dipoles (and variants, such
as inverted-V, G5RV, etc.), loops, and yagis. Avoid unbalanced antennas
like slopers and longwires. Be certain to feed all balanced antenans with
a balun or an effective decoupling scheme to eliminate RF currents on the
outside of the coax shield. If you want to (or must) use an unbalanced
antenna, feed it via one of the remotable auto tuners that you could place
outside of the shack and provide the ground for the tuner external to your
shack. That was my problem with the in-shack tuner for the longwire.
Also, you may have to provide multiple length ground leads for that remote
tuner (if it is well above ground and has a path to ground longer than
about 8 feet), as it can not work correctly unless it can see a low
impedance path to ground for each frequency band of interest. Yes, you
could lay counterpoise wires around the shack to establish an RF ground up
there (I tried that), and it will work. It will also create some very high
RF levels within the shack, which can lead to either excessive fields
around you or excessive fields which affect the equipment (which is what
happened with me - RFI to the xmtr itself).
3. MFJ has a had a "Ground Tuner" for several years. It gives you the
means to establish a low impedance path to ground for each HF band of
interest. Yes, that maximizes ground current and current is what radiates.
If you can't stand that, then stay with balanced feed antennas. Note that
my presumption, and I think that of other responders, is that you are
talking HF operating. I can tell you that I also did a lot of VHF and UHF
operating from the 2nd floor shack and had no problems. At those
frequencies, the wavelength is short enough that any physical path to real
earth ground is too long to be effective for that purpose. However, I used
only yagi, corner reflector, or ground plane (with integral radial)
antennas, so the lack of low Z path to earth was not a problem. I would
not expect it to be a problem for you, either, Steve.
4. I can not emphasize enough the importance of adequate lightning
protection. Keith has it covered. You do have to be creative with a 2nd
floor shack, as it is a bit difficult for you to run out and yank the
cables out of the feed-thru hole when you are finished with the station for
the day or night. I can only suggest rigging either a metal access plate
or a number of short grounding leads at the point of entry to the house and
bonding all of the coax shields to ground at that point. It is even better
is you also locate surge protectors there for each coax, as well. Then,
the whole lot has to get to external ground via the lowest impedance method
available. In practical terms, that would be wide copper or brass strap.
Not braid, not aluminum wire, not old coax used like a piece of welding
cable.
Best luck and I hope you'll let us know what you finally do to solve the
problems.
73, Dale
WA9ENA
Keith Blackburn
<blackburn@qnet.c To: STRutledge@aol.com
om> cc: rfi@contesting.com
Sent by: Subject: Re: [RFI] Ground
Question
rfi-bounces@conte
sting.com
02/03/03 08:58 PM
Steve,
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.
Keith
STRutledge@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@contesting.com
> http://lists.contesting.com/mailman/listinfo/rfi
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