On Thu, 13 Jul 2006 09:08:11 -0700, Bill Turner wrote:
>The underlying problem is you have an antenna which is not putting ALL
>the RF up in the air where it belongs. Fix that and you will fix the
>RF in the shack problem.
It isn't quite that simple, Bill. Many of us don't have ideal situations
for antennas, and we MUST load something as a long wire. At my new QTH in
CA, I have nice, nearly ideal, wire dipoles. But at my old QTH in Chicago,
I had to load a dipole as a long wire to get on 160, and to work stations
off the end of the dipole on 80.
Tom, W7WHY said:
>OK, maybe you are right. But, if I don't have the ground hooked up to the
>rig and I touch my tuner, the SWR varies. When I hook up the ground, it
>doesn't do that. And I know for a fact, I have run long wire antennas and
>if I don't have a good ground, I get bit bad by RF on the mike. When I
>hook up the ground, it quits. It must be doing something.
Yes, it is doing something. Remember in my earlier post when I talked about
a counterpoise? A counterpoise is PART OF YOUR ANTENNA -- think of it as
the other half of a dipole (the long wire is one half). You can also think
of it as the return path for antennna current. When you have nothing
connected to the chassis of your antenna tuner but the green wire in your
power system, that green wire is the counterpoise. But it is not EARTH that
you're connecting to, it is the wiring in your house!
Now, when you're loading a long wire and hook another wire to the chassis
of the antenna tuner, that wire also becomes part of the counterpoise -- in
parallel with the green wire in your house wiring. (One of my friends in
Chicago, KK9H, uses the HVAC ducts in his house as his counterpoise). You
can tune that wire just like you can tune the long wire antenna that you
stuck up in the air. That's what the MFJ "artificial ground" is doing. And
you can tune it whether it is connected to earth or not -- all that the
earth connection does is change the tuning!
Here's something else to help you understand what's happening. Go to the
ARRL Handbook or Antenna Book, and study the drawings of voltage
distribution and current distribution on an antenna. The peaks and nulls in
those distributions are established by the boundary conditions of the wire
- that is, the current MUST be zero at the end of a wire, and the voltage
will be at a maximum. One quarter wave along the wire will be a current
maximum and voltage minimum. Another quarter wave, and you'll be back to a
voltage max and current min. When you tune the "artifical ground box," you
are moving that voltage minimum to your shack by adding XL or XC in series
with the "ground" wire. When you tune your antenna tuner, you are adding XL
or XC (and changing the impedance ratio) to get the rig to see an impedance
close to the 50 ohms it wants to work into.
Hope this helps.
Jim Brown K9YC
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