Have a look at QEX this month. W6WO shows an excellent method I have not seen before. He feeds 10% in from the end and uses a simple 4:1 transformer. His capacitor is just a compression trimmer. It is coax fed via a choke and runs to 100W.
Also look at AA5TB website for end feds. Kite flyers use them a lot. The attraction for me is that I can have a single lightweight wire supported by a flimsy fishing pole and the heavy coax is near the transmitter at near-ground level. The 10% tail end does not need to be in-line, it can be bent out of the way for safety and tidiness. I've modelled several configurations and they are mostly good for NVIS unless vertical. I modelled 30ft horizontal at 4ft above ground then 34ft vertical (up the fibreglass pole) and that gives a small f/b of 6dB and useful low angle take off. Great for mini-dxpeditions and back-packers, especially good next to the sea of course. David G3UNA ----- Original Message ----- From: "Stephen W. Kercel" <kerc...@suscom-maine.net> To: <wrco...@flash.net> Cc: "'Elecraft Discussion List'" <elecraft@mailman.qth.net> Sent: Sunday, May 17, 2009 3:37 AM Subject: Re: [Elecraft] OT - SteppIR Vertical and Elecraft Products > Willis and others: > > There is a much cheaper trick than big coils and capacitors that ought > to work, and could easily handle high power. > > Although I have never tried end feeding a half wave antenna, I have had > great success in center feeding full wave antennas. The trick is to use > an open wire feedline as a quarter wave transformer. I used the old > fashioned "ladder line" with plastic spreaders and bare copper wire > rated at 450 Ohms. Since the velocity factor is practically 1, I cut the > length to the theoretical free space quarter wavelength. One end feeds > the center of the antenna, and the other end goes to a 1:1 balun. It > presents a very low SWR to 50 Ohm coax. I have done this on various > bands with great success. > > I have not tried the "end fed Zepp" configuration implied by an end > feeding scheme, but for the sake of this discussion, I have run a quick > simulation. > > Here's what the math says: > > > I ran the "Backyard Dipole" File that Roy provides with with EZNEC. I > Moved the feedpoint to one end, changed the ground conditions to > "Real-Medium" and included the copper losses in the wire. > > The computed end point impedance at 14 MHz (where it is a half > wavelength) 1282-J1791. > > If you use 450 Ohm window ladder line in the TLW program and use a 14 > foot length of line in the traditional single ended Zepp fed > configuration, the impedance at the input of the transmission line is > 43.15-J18.06. Connecting RG-8 to that works out to an SWR of about 1.5 > on the coax. > > In the simulation the transformer length did not turn out to be exactly > a quarter wavelength. I had to tweak it a bit to find a workable > transformation. In practice, the ground losses will not be exactly the > same as the simulation, and one might have to play a bit with the > transformer length when you try it for real. > > However, my experience with the "double Zepp" scheme above is that it is > not that hard to find the right line length and the antenna is quite > forgiving in practice. I see no reason why a single ended Zepp scheme > should be significantly more difficult to do in practice. > > Some posters may object that the antenna's behavior is impractical to > predict because the end point impedance can vary over a wide range. > That is true, but by using a quarter wave transformer with relatively > high characteristic impedance, that wide range at the high-Z end is > transformed to a narrow range at the low-Z end. Thus, in practice, with > perhaps a bit of tweaking, using this scheme will give you acceptable > SWR in the coax, even if your actual impedance numbers are not the same > as my numbers. > > 73, > > Steve Kercel > AA4AK > > ______________________________________________________________ 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
