[Elecraft] K2 fixed, but now what?

[George, W5YR]

I would like to add a coment or two to Tom's excellent suggestion.

The term "counterpoise" has come to mean many things in amateur radio usage
and I will steer clear of trying to give any sort of generally accepted
technical definition for the term. The function of a true counterpoise
differs from that of the wire under discussion, but it matters little what
we decide to call it - the wire works the same regardless.

Historically, the term "counterpoise" originally was used to describe a
system of wires placed near but not on the ground to provide an r-f
connection to ground by means of the capacitance between the counterpoise
and the actual ground. The counterpoise was not necessarily designed to be
resonant but was more intended to provide as much capacitance as could
reaonably be obtained. A similar effect accounts for the success of our
vehicular HF installations where the capacitance of the vehicle frame and
body to ground provides our r-f connection to ground and thus allows our
vertical whips to operate with a degree of efficiency. At 10 meters or so,
the vehicle body may serve as a groundplane of sorts but clearly at 80 and
40 meters something else is needed for any degree of efficiency.

The purpose of the wire that Tom is recommending is not primarily to
provide a capacitive connection to the Earth (which it does to a degree)
but rather is to place the cabinet, chassis, etc. of the K2 - specifically
the GND post on the rear panel - at as near zero r-f potential as possible.
Thus, the term "driven ground" is usually applied to a wire connected as
described.

If the wire is 1/4-wavelength long at the operating frequency, has its far
end insulated and is not too close to the ground or metallic structures -
either of which would serve to detune it - then in the near-field of the
radiating portion of the antenna system, the wire has current induced in
it. This current sets up a voltage gradient along the wire, with the "far
end" having a voltage maximum and the end connected to the radio a near
zero voltage level, due to its 1/4-wavelength dimension.

Thus, the "driven ground" serves to collect r-f from the "antenna" for the
purpose of *driving* the radio "ground" to near zero r-f potential.

Because of the current in the wire, it is actually a radiating part of the
overall antenna system and should be treated as such. Even with our 5 watt
power levels, substantial voltages can be developed at the open end.
Radiation from the driven ground wire is normally not a great problem in
most field setups - it probably helps! -  but in home stations, some care
should be taken in how the wire is placed to minimize r-f pickup in nearby
devices.

From a theoretical viewpoint, the actual impedance to "ground" - whatever
and wherever that mystical reference point may be - from the KAT2 ground
post is probably much much lower with the driven ground than one could
obtain by using a ground rod with a wire between it and the K2. An
exception might be if one uses a series-tuned resonant circuit in such a
lead to minimize the overall impedance. 

Before rushing out to pound several 8-10 ft ground rods into Mother Earth,
be aware that most ground rods are fine for 60-Hz and possibly for
lightning abatement, but are essentially worthless for r-f "grounding"
purposes. Even if they did manage to maintain a low-impedance connection to
"ground", connecting the station equipment to the rods involves conductors
with finite and frequently large impedances. Hence the use of the "ground
line tuners" such as those sold by MFJ. Incidently, if you want to build
one, the circuit diagram of the MFJ-931 is in ON4UN's "Low-Band DXing" on
page 11-22.

In over 57 years of amateur radio I have never used a "station ground"
other than the safety ground for 60 Hz provided by the power line system.
R-f ground requirements for antennas are best resolved at the antenna, not
in the shack.

<steps down from pulpit>

73/72, George    
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe   
Amateur Radio W5YR -  the Yellow Rose of Texas
In the 57th year and it just keeps getting better!


Tom Hammond N�SS wrote:
> 
> Daniel:
> 
> Add a COUNTERPOISE to the antenna configuration... this can consist of
> nothing more than a 15'-30' (depending upon what bands you plan to operate)
> length of wire, connected to the GND terminal of the KAT2 and laid on the
> floor, but try to stretch it out as much as possible... bends are probably
> OK, but try to use up as much area as possible.
> 
> If you can droop the wire out the window and let it hang DOWN, so much the
> better.
> 
> Generally, I counterpoise will make the RF situation look much less 'hot'
> to the K2/KAT2.
> 
> 73,
> 
> Tom Hammond   N0SS