Chris writes: > I have been infected with the idea that the > ferrite bead baluns have high loss and relatively low impedance, > and that a better design uses coax wound through one or two binocular cores.
Hope you didn't catch it from me :) Other things being equal, sleeve (W2DU) baluns/ununs aren't lossier than multiturn baluns, given that a bead sleeve is just the N=1 case of an N-turn balun. As long as the choice of coax and ferrite compound is reasonable, sleeve baluns don't have much loss. The advantage of a multi-turn balun at QRP is that the common-mode impedance increases as the square of the increase in the number of turns. Doubling the number of turns raises the common mode impedance by a factor of four; tripling by a factor of nine, and so on. With 5 turns of coax on a one-inch stack of toroids, you've got about the same common-mode impedance as 25 inches of bead sleeve, other things being equal. Thus the multi-turn balun can be smaller and lighter by comparison with a W2DU balun having equivalent common-mode impedance. Being smaller, a multi-turn balun or unun has less surface area than an impedance-equivalent W2DU balun, so it doesn't radiate heat as well. That doesn't matter at QRP, but it's easy to cook a physically small multiturn balun or unun at QRO if it's running against a high resistive mismatch or imbalance. The multiturn balun trades away power handling capacity for a decrease in size, which is usually a safe and advantageous thing to do at QRP. Incidentally the W3EDP isn't expected to have a low SWR on multple bands. You just tune it up with the ATU and go. What's nice about the W3EDP is that it's not a half-wavelength on any of the amateur bands, so it doesn't present thousands of ohms of impedance on those bands. As such, it can be tuned by a simple, low-excursion L network without resorting to the sort of tank circuit that you usually need to tune end-fed halfwaves. 73, Ken K3VV _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
