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My earlier comment regarding SWR was a red herring. There is an SWR effect, but its minor and it isn't the real issue. Sorry for the mis-lead. The original question was how badly does an antenna deteriorate if there is no balun. So far there's been lots of religous talk, but not much that would help answer the question. I spent some time last night re-reading the N6BV text and examples of pattern skewing due to no balun. I wanted to find a way to calculate the real effect of no-balun use, but couldn't figure out a way to model it. Dean has a couple of explicit examples in the Antenna Book, but he doesn't say how the modeling works. It finally dawned on me that a coax is nothing more than an ideal, balanced transmission line in parallel with a single wire (the outside of the coax). To model the effect, simply use an ideal voltage source at the feed point and hang an extra conductor a couple inches off center of the driven element. Sure enough, using NEC, this matches very well with Deans' examples. Now I have a tool to calculate the real effect of no-balun. Amazingly, the effect on pattern is quite small for a "normal" installation. If you drop a coax down from any antenna, the effect on the main lobe is only a small fraction of a dB. I tried several variations, and the worst was only a 0.5dB loss of main lobe power. This power was re-radiated from the coax and could fill in some of the antenna nulls. Nulls can also be enhanced if the coax happens to radiate out of phase with whats' comming off the antenna. What makes this "problem" so minor is that coax normally comes off the antenna orthogonal to the driven element. If you route the coax in parallel with the driven element and only a few feet away, then all bets are off and that could be a serious problem ... but who would do that anyway? This seems to validate my experiences (and other guys experiences reported here) that the lack of a balun isn't a disaster. Things still work pretty well without one. Would I put up an antenna without a balun? No, but if I'm on an expedition and forgot to bring one I wouldn't let that stop me from getting on the air. If your balun blows up, just bypass it until you can get a replacement. On a related topic, its always good to keep RF out of the shack. A balun certainly helps that, as does sprinkling ferrite beads randomly along the coax as Dave suggests, to knock down RF currents induced by antenna radiation. 73, Al AD6E In a message dated 1/16/2001 7:15:57 AM Pacific Standard Time, DAVED@ctilidar.com writes: << ..... stuff deleted ..... So, I decided to use rf current chokes at several places along the line anyway to keep things tidy. 73 Dave K0QE >> -------------------------------------------- Force12Talk mailing list provided as a service by Force 12 Antennas, Inc. Force 12 Web site: http://www.qth.com/force12 Submissions: send to Force12Talk@qth.com To unsubscribe: send a blank e-mail to Force12Talk-leave@qth.com Force12Talk Message Archive: http://www.qth.com/force12/list/force12talk For problems with the list, contact n4zr@qth.com |