DK3 screwdriver antenna and artificial ground
 

Hi to all.

I choose to install a DK3 screwdriver on a second floor balcony. No
good grounding is possible fo the base of the antenna. So I decided to
use the MFJ-931 artificial ground as a substitute. My question is
this.. Would it be equivalent to a RF ground to connect the MFJ right
at the ground post of the transceiver (Icom 706) and to disconnect the
coax shield from the base of the antenna?

Or, more generally, what is the best way to use an artificial ground
with an antenna like that one, when a proper metallic ground is not
available?

Thanks de Pierre

Pierre wrote:
Or, more generally, what is the best way to use an artificial ground
with an antenna like that one, when a proper metallic ground is not
available?

An artificial ground tunes the ground wire such that it appears as a very
low impedance point at the rig. That ground wire should be tied to the rig
chassis. Another way to solve the problem is to put resonant radials on the
base of the vertical. In fact, your DK3 configuration will be a dog
without radials.
--
73, Cecil http://www.qsl.net/w5dxp



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No good grounding is possible fo the base of the antenna. So I decided to
use the MFJ-931 artificial ground as a substitute.
The artificial ground is designed to tune a 1/4 wave counterpoise to present
a low impedance rf ground to the chassis of the rig. This is usually done by
cutting the 1/4 wave ground wire to something less than 1/4 wave length. Then
the artificial ground (series resonant circuit) is tuned so it looks like a 1/4
wave radial, or counterpoise. The tuning is done by looking for maximun
current in the counterpoise.
If you are only interested in a single band, just attach a single wire radial
1/4 wave long and forget about the artificial ground. If you want maximum
performance from a screwdriver or any type of vertical, a good ground system is
necessary, but you already knew that.

73 Gary N4AST

A more common use of an artificial ground is to "tune" a dc ground lead so that
it is also at RF ground. This helps to keep the solid state rigs contented but
in no way has an effect on radiation properties of the antenna in most cases.

Unfortunately, the "artificial ground" is just as much an antenna as the
"antenna" is. And it's usually in a lousy place for an antenna -- often,
low and parallel to the ground. If it is, the radiation from the
"artificial ground" (representing a sizeable fraction of the RF power
your rig is producing) will be largely absorbed by the ground. If it's
in a more favorable spot, its radiation will end up modifying your
overall radiated pattern.

Just because we connect one wire to one terminal of our rig's output and
call it an "antenna" and connect the other to the other terminal and
call it "ground" doesn't impart any magical properties to them. They
behave just the same.

Roy Lewallen, W7EL

K9SQG wrote:
A more common use of an artificial ground is to "tune" a dc ground lead so that
it is also at RF ground. This helps to keep the solid state rigs contented but
in no way has an effect on radiation properties of the antenna in most cases.

On Sun, 12 Oct 2003 22:49:35 -0400, Pierre
wrote:

Hi to all.

I choose to install a DK3 screwdriver on a second floor balcony. No
good grounding is possible fo the base of the antenna. So I decided to
use the MFJ-931 artificial ground as a substitute. My question is
this.. Would it be equivalent to a RF ground to connect the MFJ right
at the ground post of the transceiver (Icom 706) and to disconnect the
coax shield from the base of the antenna?

Or, more generally, what is the best way to use an artificial ground
with an antenna like that one, when a proper metallic ground is not
available?

Thanks de Pierre

I used to run a similar set-up from a 3rd floor apartment. Just attach
a 1/4 wave radial (for each band) to the ground side of your antenna.
Run the radial(s) out, across your floor. Or hang them from your
balcony.

Forget the artificial ground. You don't really need it.

You'll have a considerable amount of RF indoors, but that's about what
you get with this kind of set-up.

Bob
k5qwg