[Elecraft] multiband counterpoise with K1

Thu Dec 5 12:17:01 2002

The wires being connected to the coax braid where they are will drive
that point on the braid to near zero r-f potential. What effect that
has on the rest of the system depends upon the details of the rest of
the system.

If it works for  you, don't worry - have fun!  <:}

Any conductor will radiate r-f provided that r-f can be delivered to
it. And all the r-f that is delivered to the conductor will either be
radiated as r-f or as heat - it must go somewhere!

The reason you don't see much about compromise antenna analysis and
design is that there is an unending array of such systems to be
analyzed. Usually you have to start with an "ideal" or perfect
situation and then try to see what effects are produced when
real-world changes are made. Not easy to do since, for example, the
antenna you are using is probably unlike any other in the world,
believe it or not!   <:}

A couple of K1/K2 pals and I have done a lot of modeling of QRP field
antennas, feedlines, etc. to try to understand what sorts of portable
antennas work "best" under given conditions, how well the K1 and K2
tuners can match them, what sort of lines to use, etc. We have learned
a lot, but the outcome is that each situation is unique and has to be
evaluated by itself. The EZNEC program is quite good for giving you an
estimate of the probable radiation pattern and of the driving point
impedance of the antenna. The TLDetails program can then estimate the
performance of whatever feedline you want to use and give you the
input impedance of that line for a particular frequency. Then there
are programs to design exact L-network tuners for that input
impedance. Once you know the exact L and C values required, you can
then compare these with what is available in the KAT1 and KAT2 tuners
and see if either can tune the impedance.

Running through this litany of programs can be tedious and
time-consuming, but to the extent that EZNEC can model a given antenna
configuration, we have found that we can pretty well predict whether a
given setup can be tuned by either radio, the loss in the line, the
probable antenna pattern, etc. Interesting stuff!

73/72, George
Amateur Radio W5YR -  the Yellow Rose of Texas
In the 57th year and it just keeps getting better!
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe
K2 #489      Icom IC-765 #2349     Icom IC-756 PRO  #2121

----- Original Message -----
From: "Wallace, Andy" <[email protected]>
To: <[email protected]>
Sent: Thursday, December 05, 2002 10:31 AM
Subject: RE: [Elecraft] multiband counterpoise with K1

> > -----Original Message-----
> > From: George, W5YR [mailto:[email protected]]
> >
> > Andy, I have posted on this topic at least a dozen times over the
past
> > two years.
>
> OK, guess I have not spotted any recent ones! Will check the
archives. ;-)
>
> > This "bundle of wires" is not a true counterpoise in any sense of
the
> > word. It is, rather, a "driven ground" in which the wires being in
the
> > near-field of the antenna develop a current and associated
voltage. By
> > being 1/4 wavelength long and open at the "far end" the end
attached
> > to the station equipment is driven by the induced r-f to a near
zero
> > r-f potential, effectively "grounding" the equipment.
>
> OK, here's a divergence -- I have these attached to the coax braid
> about 25' from the rig. What will this do?
>
> > Good luck with your new discovery of a very old application of the
> > "driven ground."    <:}
>
> Part of my problem is that most, if not all, antenna theory books
are
> written about "perfect" antennas. My situation is such that I have
> no trees on my property (thus the clothesline pole for support)
> and am not near earth ground, AND my antenna exits a window down the
hall
> from the room I use for the shack. Unfortunately, I can't change
that
> anytime soon. So if I can make the rig happier (and possibly reduce
> radiation
> of the coax) with this bundle, I will be happy.
>
> There's a market, I am sure, for a book about exactly how to make
compromise
> antennas and what parts of the system can have the most effect.
>
> Back to Elecraft topics: my luck with DX in this situation is proof
that
> the little K1 and its autotuner KAT1 is doing a nice job to get my
signal
> out into the ether. This discussion is not entirely off-topic,
because
> people considering portable K1/K2 ops may need to worry about not
having
> a perfect, resonant antenna to pack with them.
>
> Direct emails to me are welcome if people don't think it's fodder
for the
> List.
>
> Thanks for the help!
>
> -Andy
>
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