[Antennas] MFJ Artificial Ground

[Robert Lay]

Dear Bill,

> What is the groups experience if any, with the MFJ Artificial Ground. I
want to run a wire outside an apartment and for the ground
> inside, use the device they sell. I am interested in hearing how others
have used. it. I have read the instructions.
>
> Is it really necessary?

I built and tested the MFJ artificial ground for a blind ham about 3 years
ago. My only complaint about it was that MFJ had trouble getting me the
correct screws to hold the box together.
It is not at all difficult to build and it does the job, as advertised.

Whether it is necessary or not depends on whether or not you  care about
having everything in your shack hot with RF. The only way to bring the
chassis of all of the shack equipment to ground potential is to use the
artificial ground. The reason that you have the problem otherwise is that
you are feeding an unbalance Marconi type antenna part way up its height.
Whatever your antenna current is multiplied by the impedance from the rig
chassis to the outside earth ground is the amount of RF voltage you will
have on the chassis. Depending on the length of that wire to ground and the
frequency and the number of amperes of current, it could be a minor problem
all the way up to a catastrophe.

The artificial ground tunes that length of wire to become a series resonant
circuit, and as we all know a series resonant circuit has zero impedance.
It's just as if you had physically brought the earth into the shack and
connected it to your rig.

> I can always run a counterpoise wire inside.

Not understood. Inside from what? The only counterpoise that I know about
should be outside and running from the ground side of the feet point
terminals, outwards, radially and in the clear - not into the shack.

> I will be using a manual tuner. I have an AH-4 available but
> think it is overkill for this.

Does not matter what tuner you are using, the tuner will be many RF volts
above ground unless you use the artificial ground.

> I tried a Hustler system but even with a decent ground to the radiator
heating system it > was barely usable.

Not surprised - if you're talking about the same Hustler that I had mounted
on my rear bumper.  Any vertical antenna or unbalanced Marconi type antenna
working against ground needs a GOOD ground - not a connection to a radiator,
not a connection to the green wire in the house wiring, but a true earth
ground or counterpoise.

Now you know why it is that people usually run coax to the feed point, and
even then they usually have similar problems. It should also be obvious why
the Hertzian dipole is better for these situations, because it is balanced
instead of working against ground.

It's one thing to have a small amount of RF current flowing in your "ground
system" when running a balanced antenna and quite another thing to have your
primary antenna current running from your rig and tuner chassis through a
long piece of wire out to the earth. Altogether much bigger problem and to
be avoided at all cost. One of the side effects of having the rig chassis
"hot" is that you can't avoid having the RF sneaking through your computer,
your radios and TV's, stereo systems, etc. The whole bloody place becomes
part of your antenna system.

Bob Lay (W9DMK), Dahlgren, VA

w9dmk@crosslink.net
http://www.qsl.net/w9dmk