On Fri, 19 Sep 2008 10:21:44 -0700, Ron D'Eau Claire wrote: >The balun doesn't have any effect on the efficiency of the antenna unless it >involves lots of coax at high SWR such as the big coil of coax some Hams use >for a "choke balun". (They're FB as long as the SWR is low but, like any >coax, the losses go up with the SWR.)
WRONG! There are three fundamental cases of ferrite "baluns" or chokes. 1) A choke wound with coax 2) a choke wound with parallel wires (bifilar) 3) a transformer (voltage balun). Case #1: Coax contains 100% of the differential power (transmitter feeding antenna) within the dielectric. The ferrite core of a COAX CHOKE sees ONLY the common mode voltage and current. If the impedance of the choke is high enough, the current is very small, so the dissipation in the choke is very small. Case #2: A choke with a bifilar winding (that is, two parallel wires, NOT coax) puts a significant fraction of the transmitted power in the core. This is "leakage" flux from the bifilar winding (really a short length of parallel wire transmission line). This leakage flux is typically 30% of the transmitted power, and is NOT related to common mode current. Case #3: A VOLTAGE balun is VERY different -- it puts 100% of the transmitted power in the ferrite. All of this is discussed in the tutorials, previously referenced. BTW -- earlier work and publications by W7EL, W2DU, and W1JR are all very good, but they were done 30 years ago. My work builds on theirs, and is considerably more advanced. Also, the #31 material that is so useful for HF chokes was developed only a few years ago. 73, Jim K9YC _______________________________________________ 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
