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: [email protected] [mailto:[email protected]] On Behalf Of John Shadle Sent: Friday, June 17, 2011 7:20 AM To: [email protected] 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 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
