I have such a tuner that I built years ago because my favorite antennas are doublets (center fed wires of various lengths) or, when necessary, an end fed wire. In the latter case I prefer a 1/2 or multiple of 1/2 wave long wire because the very high impedance at the feed point means very low "ground" currents and so very low "ground" losses.
Mine has a center link that is tuned with a variable cap in series and a split rotor variable cap across the main coil. Taps set the number of active turns in the link and the main coil. The feeders are tapped onto the main coil as well. With all those taps (accomplished by alligator clips) it looks like something Igor would have in Dr. Frankenstein's lab, hence my name for it: Frankentuner. It takes a little fiddling to find the tap positions but once noted one can switch bands quite easily. If feeding a doublet, the taps are set equidistant from the center. The further toward the end, the higher the impedance being presented by the feeders. At the other extreme, the main coil can be opened at the center to put the feeder in series with the main coil for very low impedances. For an end-fed wire, only one tap is used where it best matches the impedance of the radiator. The right positions are found with two criteria: 1) low SWR on the link between the Frankentuner and the rig and 2) lowest over all "Q" in the tuner (high Q results in unnecessarily high circulating currents and losses and requires more frequent adjustment while moving around a band). It's a design that was very popular in the days before the demand to hop across the Ham bands in milliseconds. It does take 15 or 30 seconds to configure it for a new band and a few more to do a "tune" for adjusting the caps. It does not fit in a tiny enclosure with nice lighted buttons that look "modern". All I care about is that it works. 73, Ron AC7AC -----Original Message----- From: Elecraft [mailto:elecraft-boun...@mailman.qth.net] On Behalf Of Don Wilhelm Sent: Thursday, September 29, 2016 5:41 PM To: elecraft@mailman.qth.net Subject: Re: [Elecraft] how to optimize end-fed? Mel, Your points are well taken when you are feeding an end-fed half wave or a radiator of a multiple of a half wave. The ideal parallel tank circuit should be fed with a coupling coil that is isolated from the larger secondary coil - that reduces or eliminates common mode currents in the shack. That solution is practical for open coil configurations where the user can tap the antenna to any point on the coil of the tuned circuit. However, the tapping choices are limited in a compact, portable implementation for use in portable operation which use toroids for the inductor. We used to use those isolated link coupled ATUs constructed of open coil inductors where it was practical to tap the coil at any point, but today's world of toroid wound inductors, that is not as easy. If one has a resonant parallel tuned circuit, it will match very high impedance, and a series tuned circuit will match very low impedance. The link coupling will provide isolation from common mode currents. Those type of ATU's work very well with a wide range of antennas with varying feedpoint impedance. However, the physical implementation of the ability to tap the antenna to any turn of the high impedance tuned resonant inductor requires a physically large coil. While it will work *very* well, it is not consistent with small ATUs used for portable operation. If you have a fixed length radiator and work only a single band, you can devise a link coupled tuner that will do a great job, but if you are multiband, and do not want to fiddle with coil taps, the auto-transformer is a good compromise. Yes, I still have my link coupled ATUs with plug-in coils for each band and also have a Johnson Matchbox which is also link coupled. They do the job well, but the convenience of toroid wound ATUs is an advantage in simplicity and convenience. 73, Don W3FPR ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
