Thanks for all the comments and suggestions. Some great info to mull over.
N1ESE On Sun, Nov 19, 2017 at 6:36 AM, Igor Sokolov <ua9...@gmail.com> wrote: > Very useful post Erik, > > Thank you > > 73, Igor UA9CDC > > > 19.11.2017 13:22, Erik Basilier пишет: >> >> End-fed antennas have gotten popular lately. When I look closer I see two >> different popular approaches. >> >> The first uses a 9:1 impedance transformer in combination with a wire >> length >> that is not resonant on any band. The idea is that (assuming there is no >> significant feedline length) you have a medium impedance (450 ohms) on the >> antenna side of the transformer, and because the wire is not resonant so >> you >> might have an impedance into the wire that is also "medium". By adjusting >> the wire length, you might get pretty close to 450 ohms on one or two >> bands, >> and with a wide range tuner you can probably get below swr 2:1 for the >> radio >> PA to see. >> >> The second approach, used by MyAntennas and others seems to use a >> transformer with much higher impedance ratio. One way to construct such a >> transformer would be to cascade two 9:1 units for an effective ratio of >> 81:1. This would mean the wire should present an impedance of 4000 ohms or >> so. Another way would be using a single tranformer with a higher ratio. >> The >> impedance ratio is the square of the turns ratio. With a turns ratio of >> 9:1 >> you should again get to about 4000 ohms. Somewhere I saw somebody using an >> 8:1 turns ratio for an ideal antenna wire impedance of about 3200 ohms. >> These impedance levels are achieved by using a resonant wire. >> >> I don't know how you arrived at your parameters, but your wire length is >> too >> close to resonance on 80 and 40. Your transformer ratio wants a >> non-resonant >> wire, so you might see better results if you shorten the wire >> significantly >> and keep using the KX3 ATU. Alternatively, you could replace the >> transformer >> for a much higher impedance ratio, in which case you can probably operate >> with the tuner bypassed at least on 80 and 40 with a well adjusted wire >> length. With this approach you want the wire resonant on each band. It >> should be easy to achieve resonance on 80, 40, 20 and 10. As you double >> the >> frequency, you are changing the number of half wavelengths covered by the >> wire; the end feedpoint is always at the end of one of these half >> wavelengths, and thus you get the very high impedance that you seek. 30 >> meters does not fit as clearly into this scheme. The commercial versions >> use >> a small coil in the wire located close to the transformer end, and seem to >> be able to achieve a reasoable match for all the bands 80 and up without >> using a tuner. Now if you had placed the feedpoint in the center you would >> not have been able to get this consistency of feedpoint impedance from >> band >> to band. As I see it, this is a major reason for the popularity of the >> end-fed approach as contrasted to the conventional center-fed approach. >> Note >> that the 30m coverage of the 80 meter and up design is not replicated if >> you >> try the same approach with half the wire length. In this case you will >> need >> a tuner to get reasonable swr on 30. >> >> An important consideration is antenna height. We all know that antennas >> usually work better when placed higher. Looking a bit closer, we can look >> beyond the general installation height and consider the height(s) of the >> antenna part(s) that carry the most current. Antenna modelling may >> calculate >> the field as resulting from current levels in different individual pieces >> of >> the wire, and then it makes sense to elevate those portions more than >> other >> parts of the wire that carry less current. Another reason for this is the >> effect of ground losses. Jim Brown, in his article that he just linked to, >> shows that ground losses get worse the closer a vertical antenna is to >> ground. This makes sense as currents in the lossy soil are caused by >> induction from currents in the antenna. When we look at the current >> distribution within the vertical antenna wire it again makes sense to >> place >> the part(s) of the wire with high current higher rather than lower. One of >> the simplest portable antennas is a short wire or whip of a quarter wave >> or >> less. It will have a low feedpoint impedance that can probably be matched >> reasonably without a tuner or with a limited-range tuner. However, with a >> low impedance comes a current maximum at the feedpoint. This often means >> close to the ground, so even with a good set of elevated radials, >> considerable ground losses could be expected. (An actual connection to the >> soil would generally be much worse, unless you bury a lot of wires.) With >> a >> longer wire (1/2 wavelengh at the lowest band) we can have a very high >> feedpoint impedance, very low feedpoint current, and more elevated >> location(s) of high curent portion(s) of the antenna, for lower ground >> losses. >> >> Any antenna feed point needs to provide two terminals for the feed current >> to flow through a complete circuit. A end-fed designed for medium to high >> feed impedance has small feed current. In practice this means that >> whatever >> is used as the counterpoise side can be small. The applies with a "medium >> impedance" design as discussed earlier, but it applies even more for a >> resonant end-fed with its tiny feed current. A short piece of wire may be >> used, but often not even that is needed, as the feed return current may >> flow >> on the transformer and feedline (if used), and even on the radio box. This >> is fine for QRP and maybe even medium power, as the current is small >> relative to the higher current higher up on the wire. However, at medium >> to >> high power, if matching problems are encountered, or bothersome RF around >> the rig, I would consider a small counterpoise wire to the high impedance >> side of the transformer, or experimenting with the length of the feedline, >> as the length of it matters when "counterpoise current" flows on the >> outside >> of it. I would not count on the impedance transformer to function as a >> common mode choke for blocking such RF current. You could place a separate >> common mode choke somewhere on the feedline, and move its position as a >> way >> to adjust the length of the effective counterpoise. Look up Jim's article >> on >> how to build an effective common mode choke. >> >> 73, >> Erik K7TV >> >> -----Original Message----- >> From: elecraft-boun...@mailman.qth.net >> [mailto:elecraft-boun...@mailman.qth.net] On Behalf Of JT Croteau >> Sent: Saturday, November 18, 2017 2:34 PM >> To: elecraft@mailman.qth.net >> Subject: [Elecraft] KX3 Field Ant. for 80/40/30 >> >> Friends, what would be a good end fed wire setup that will match well with >> the internal KX3 ATU and cover, hopefully, 80, 40, and 30 meters? >> >> I went out to my winter camp site, with two really tall pine trees, and >> tried to experiment with a 9:1 UNUN, 135' radiator, and 35' of RG8X. It >> was >> a total disaster. Best matches were on 20, 17, and 15 meters but only >> with >> 6.4:1 SWR. No match at all on the lower bands. >> With my two trees, the radiator made for a perfect inverted-L shape. >> >> What should I try next? I did try some pruning of the antenna but quickly >> gave up due to the WX conditions. >> >> Thanks >> N1ESE >> ______________________________________________________________ >> 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 ebasil...@cox.net >> >> ______________________________________________________________ >> 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 ua9...@gmail.com >> > > ______________________________________________________________ > 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 jt.to...@gmail.com ______________________________________________________________ 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
