[Antennas] MFJ Artificial Ground

[Robert Lay]

> Bob, is it your opinion that a properly installed 1/4 wave counterpoise
> will be as effective as the tuned artificial ground?

Dear Joe,

Not in the configuration that I understand that Bill Marx has.

Under normal circumstances, the two things do not compare - apples and
oranges.

When you need a counterpoise or radials, it's for the benefit of the
antenna -period.

When you need an artificial ground, it's because you are trying to ease
yourself out of a bad situation with respect to choice of antenna type and
shack location.

The artificial ground was never intended as a substitute for a proper
earthing of a Marconi type antenna. It was intended as a means of cooling
down the chassis of shack equipment that should never have been at elevated
RF potentials in the first place.

As I said in my original comments, the artificial ground is nothing more
than a series tuned circuit designed to bring the equipment to which it is
attached down to zero potential with respect to earth. A half wave length of
heavy wire would do as well or better - but only at the one frequency to
which it is cut. The advantage of the artificial ground is that it is
tunable over a large frequency range - 80 m through 10 m, for example.

When you are operating out of a shack on the 2nd floor and you are operating
a Marconi type antenna, then you are stuck. You have no choice but to employ
the artificial ground in the interest of cooling down the hot chassis. As a
result ALL of the antenna current is passing through the artificial ground -
that's not what the designers had in mind, but it works, up to a point. Try
running a KW, and see how long it lasts!

Imagine that the antenna circuit starts at the top, far end of the antenna.
It then comes down into your shack to a tuner and out the ground side of the
tuner. From that point the antenna circuit either goes to ground via a long
ground wire to earth, or it goes through the artificial ground circuit and
again through that long wire to earth. Either way the antenna current gets
to ground, but with the artificial ground, there is now an artificial half
wavelength between your chassis and the earth, so the chassis seems to be at
zero potential. Without the artificial ground that long length of wire to
earth might just be 1/4 wavelength and then your chassis could be a thousand
volts above ground.

Now, back to your counterpoise question. Where does the counterpoise fit in?
It's down there at near ground level trying to create a better earth than
you are getting with your 8' copper rod that you drove into the ground as
your antenna ground. All well and good, but what has that to do with the
artificial ground, which is serving an entirely different purpose?

Faced with such a situation, I would change to a Herzian antenna, so as to
NOT be working against ground. Then, the only grounding problem I would have
would be the stray, common mode RF leakage coming down the outer shield of
my coax. Or, in the preferred design, using a balanced feed line, I would
have virtually no common mode RF anywhere, and my shack grounding system is
there only to provide a path for precipitation static and the occasional
lightning bolt - God forbid!

I should mention that around 40 years ago, when I was younger but old enough
to know better, I had a 40 meter rig (using the old Heathkit DX-40) that was
Pi coupled to a long wire running directly out of a second floor window to
the telephone pole at the back of the lot. I also had a ground wire that
went out the same windows to an earth ground near the air conditioner
unit.(I probably took advantage of someone's existing earth rod at that
point). Needless to say, I could only operate at 2 am, because I had every
kind of BCI, TVI, and other kinds of problems that one could ever have -
including 2nd harmonic Pink Tickets from the Grand Island monitoring post.

Good Luck,
and 73,

Bob Lay (W9DMK), Dahlgren, VA
w9dmk@crosslink.net
http://www.qsl.net/w9dmk