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 arch...@mail-archive.com
