Another source of radiation-causing common-mode currents is an *unbalanced* two-wire feeder, such as when ladder, window or twinlead is used to feed an off-center fed dipole.
A balun at the transmitter will do little or nothing to improve the balance. It's caused by the unbalanced load at the antenna. That radiation is not always bad if the system is designed to make use of it. The version of the Windom that uses open wire or twinlead feeders is a good example (the original Windom used a single wire feeder). In this case the considerable radiation from the feeder is part of the effectiveness of the antenna, adding a fair bit of vertical radiation (if the feed line runs vertically). The challenge is to recognize that the "feeder" in such antenna systems is actually part of the radiating antenna and must be run in the clear. It can be a source of severe RFI if it's run where it can induce heavy RF currents into nearby conductors, such as TV antenna leads, house power wiring, etc. Even though a two-wire line might be used, when it's unbalanced it has to be treated with the same consideration one would give to running a simple end-fed wire into the shack. An that's why coaxial lines are so popular in spite of the need to hold down the SWR to avoid excessive losses. Ron AC7AC -----Original Message----- I agree with Kieth wholeheartedly. SWR on the feedline does NOT cause feedline radiation - ever. The thing that causes feedline radiation is common mode currents - most commonly observed as current flowing on the outside of a coax line. Properly placed current baluns can eliminate common mode current, but should not change the SWR, 73, Don W3FPR _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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
