Sorry, guys, but I must disagree. While a shack ground is NOT a sink for
noise, it IS far more than lightning protection. It is supposed to provide
a low-impedance path for stray RF that inadvertently runs around in the
shack. In an ideal world, perhaps, there should be no stray RF, antennas
should be perfectly balanced, etc, etc, but as anyone who has ever
experienced microphone bite can attest, things ain't always perfect. Mic
bite may be a minor annoyance, but stray RF can let all sorts of gremlins
loose in the shack. Also, as anyone who enjoys vintage equipment knows,
most old rigs did not have three-wire cords so in addition to lightning
protection the shack ground also provided an electrical safety ground for
the chassis of these older rigs, but this feature has nothing to do with its
length.
I did, however, use the term "resonant ground lengths" incorrectly. It is
ground lengths that are odd multiples of 1/4 wavelength for your operating
frequencies that you want to avoid. Sorry about that... should have engaged
the brain before putting the mouth (or keyboard) in gear.
I must admit that I learned this stuff 45 years ago (with a refresher about
20 years ago) so maybe things have changed.
73 - JC, K0HPS
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com]On Behalf Of Jim Brown
Sent: Wednesday, July 12, 2006 11:41 AM
To: TOWERTALK
Subject: Re: [TowerTalk] SECOND FLOOR SHACK
>You probably already know the basics (as short as possible
Yes
>but avoid
>resonant ground lengths for the frequencies you operate). Other than
that,
>consider a ground "tuner" like the MFJ Artificial Ground.
Horsepucky. An earth connection is NOT a sink for noise, and it is a part
of an antenna system ONLY when you're not putting the transmitter power
into the transmission line of a properly balanced antenna. An earth
connection is important for one reason only -- LIGHTNING SAFETY. If the
earth connection is affecting how the antenna works, you've got antenna
problems, not "earth" or "grounding" problems.
The wire going to an earth connection CAN be part of your antenna when
that antenna is some form of long wire, or when using coax with an
inadequate balun. See W8JI's website for a discussion of the balun part of
this.
As to the "long wire," consider everything connected to the chassis of
your rig (and antenna tuner) as the counterpoise for the antenna. That is,
it forms the "other half" of the antenna. As an example, the link below
shows how, at my old QTH, I loaded a dipole as a top-loaded long wire on
80 and 160, using the combination of some ground rods and a big wrought
iron fence as a counterpoise. It worked pretty well. The ground rods were
there ONLY for lightning protection.
http://audiosystemsgroup.com/K9YC/k9yc160TopLoad.htm
As you can see, it was a second floor shack, and, unfortunately, nowhere
near the power service entrance. All the antennas entered the shack and
landed on Polyphasers bonded to a copper bar below the operating desk. The
bar had a #8 insulated copper going straight down to a 10 ft ground rod. I
added another four ground rods alongside the house and tied them all
together. The power system ground is tied into the one nearest the power
service entrance. There were two #8's going to the wrought iron fence on
each side. The rigs, of course, were also bonded to the green wire in the
shack power outlets.
>From a lightning safety point of view, all the ground rods and the fence
offered parallel paths to "ground." From an antenna point of view, the
earth ground and the fence were in parallel to lower the loss component of
the impedance of that long wire; the reactance is whatever it was, the
tuner tuned it out, and both the ground wires and the long wire radiated.
Jim Brown K9YC
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