Bill, I am aware of Jim's innovative way of achieving center feed while seemingly attaching the feeder to the end. It should be useful in some situations. However, the thread creator wanted coverage of 4 bands with one antenna, and my personal interest right now is also solutions that cover serveral bands without adjustments and compromises. With that as the goal, I see no theoretical reason to want center feed. Only by feeding at the end can you get approximately the same impedance for each band, without moving the feedpoint, for so many bands. (That said, if you compare reality side-by-side to a map, reality tends to win. It would be interesting to compare side-by-side to make sure that the high-ratio transformer isn't lossy enough to hurt performance.) If you are looking at a single band, with a 1/2 wave vertical wire. The current distribution should theoretically be the same whether we feed it at the bottom or if we move the feed point to the center at the cost of 1/4 wav elength of additional coax plus the cost of the common mode choke (but there may be another justification to have the choke anyway). If one is willing to compromise the requirement of covering several bands with very good match, there is also the option of Off-Center-Feed, which can allow operation on several bands, but the match on most or all bands will be a compromise that likely forces the use of a tuner, similar to the situation with an end-fed with 9:1 impedance transformer. If you compare the OCF with the end-fed with 9:1 impedance transformer, it is not clear to me which one has the edge in practical use (assuming both have the antenna wire suspended and shaped similarly). For the OCF the results will depend on the selection of feedpoint, and for the end-fed there is room for a wide range of wire lengths that are non-resonant and should produce "medium impedance". One can easily find suggestions online for both designs. However, it seems to me that the 9:1 fans are more prone to use low elevations and shapes that reduce performance.
73, Erik K7TV -----Original Message----- From: Bill Frantz [mailto:fra...@pwpconsult.com] Sent: Sunday, November 19, 2017 1:03 PM To: Erik Basilier <ebasil...@cox.net> Cc: 'JT Croteau' <jt.to...@gmail.com>; elecraft@mailman.qth.net Subject: Re: [Elecraft] KX3 Field Ant. for 80/40/30 A useful post Erik. Also useful is the "pseudo end-fed" design that K9YC shows on <http://k9yc.com/VerticalHeight.pdf> starting at page 70. This design is really a center fed dipole using the outside of the feed line coax as one half of the dipole and an extension of the center conductor as the other. The RF-electrical length of the feed line outside is controlled by a common-mode choke on the feed line. I built one of these for 30 meters and found that the best tuning occured when the coax shield between the common-mode choke and the "center" feed location was slightly shorter than the other half of the dipole. 73 Bill AE6JV ______________________________________________________________ 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
