[Elecraft] [K2] wire antenna lengths for portable operations

[Ron D'Eau Claire]

John, as a general rule I try to get at least 1/4 wavelength of radiator up.
That's not always practical but it's worth striving for since below 1/4
wavelength the feed point impedance and so the efficiency drops very
quickly. 

If you are working against a counterpoise, something close to 1/2 wave is
ideal since it raises the impedance is highest at the length. However, the
impedance can easily rise above that which the tuner can match.

An easy way to adjust the length of the antenna is to loop the wire through
an insulator that is up as high as possible (at times I have used some
monofilament fishing line with a loop through which the loop of lightweight
antenna wire passed). Now, depending upon the amount of sag allowed, I can
adjust the length by simply letting out or pulling in the radiator wire. The
RF sees the folded back-wire as simply a thicker single wire since the RF
field passes over wire, not through it. That adjustment is often enough to
move the feed point impedance to something the tuner can handle. 

A 1/4 wavelength counterpoise is ideal since it minimizes the RF voltage at
the rig. For that reason, it's good to avoid 1/2 wavelength counterpoise
since that puts a voltage maximum at the rig. 

A "random wire" antenna like this with one "counterpoise" wire is really
just an off center fed radiator. The counterpoise and radiator are one RF
circuit. The advantage is you only need one elevated point. The disadvantage
is that the counterpoise also radiates but it only "warms the worms" Hi! So,
whenever possible I try to get both wires up into the air with at least a
90-degree angle between them (so the RF fields won't tend to cancel each
other) even though one wire might be a lot longer than the other.   

Ron AC7AC 



-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of John Shadle
Sent: Friday, June 17, 2011 7:20 AM
To: elecraft at mailman.qth.net
Subject: [Elecraft] [K2] wire antenna lengths for portable operations

I'm headed up to KL7-land next week and am going to take the K2 with
me. I was thinking about using the KAT2 to tune a wire attached to the
BL2 (at the 1:1 setting).

Probably going to cut some teflon-coated wire this evening and pack it
away in the bag.

I'm seeking input on the best lengths of wire to cut for the active
part of the antenna as well as the counterpoise.

If you have any other comments on this setup, I'd love to hear them,
too. In the past, I've used the K2 attached to my homebrew vertical
(SGC SG-239 at the base of a elevated ground plane vertical), so this
is my first adventure with "random wire" antennas.

Thanks!
-john W4PAH
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