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I have been following the balun discussion with great interest, but at a distance. I can see how easily misunderstandings and frustration can evolve if people don't communicate especially carefully. Basically, everyone is correct from the perspective of their frame of reference. Natan took the correct position that most antennas are designed to function with an equal impedance balance to ground at both feedpoints of the driven dipole. This equal impedance balance is insured by using a modern high impedance balun at the feedpoint. Natan is incorrect to imply that not using a balun guarentees a deterioration in antenna function. Any deviation from the balun specification DOES open the big "maybe" of improper antenna function. Barry's frame of reference is that of an antenna manufacturer who has the responsibility to deliver product that functions as designed. Roy is correct, for some installations, by a miracle of a combination of feedline dress and feedline length, it is possible to get a fine, equal, impedance balance to ground at both sides of the feed point without a balun, at least over a limited bandwidth. Roy is also correct that amateurs should be encouraged to experiment. A big problem with his argument is that an "experiment" must be conducted and the data evaluated. Most hams don't actually "experiment". They just do something based on a belief system and leave it at that. They view their antenna's function through the unreliable lens of their expectations. "Anything works". A proper "experiment" in some people's eyes is a subjective assessment of some antenna performance parameter. For other's, a proper "experiment" is an objective collection of comparative data. It's your choice. Roy's frame of referrence is that of an independent individual who's experience and individual circumstance has been that baluns aren't necessary for his antenna expectations to be met. The question of common mode currents and balun function has no relation to feedline SWR per se. The proper relationship to consider in designing for a balun application is the ratio of the feedpoint impedance to the common mode impedance of the balun (or the miraculous feedline installation). A high SWR may indicate a high feedpoint impedance, (or a low feedpoint impedance) but consideration of SWR is irrelevant to the theoretical discussion. High SWR can cause failure in real world balun from high voltages and currents but failure modes is not what this discussion is about. The point is: BALUNS ARE NOT NECESSARY. EQUAL IMPEDANCE TO GROUND AT THE FEEDPOINT OF ANTENNAS DESIGNED FOR EQUAL IMPEDANCE TO GROUND AT THE FEEDPOINT, IS NECESSARY. If a balun helps me get there in the quickest, safest, most consistent manner, I'll take the balun, everytime. Barry brings up the other point that people don't talk enough about: "antenna currents"-their origin and impact on antenna function. I believe that we sometimes make the mistake of thinking and talking only about feed point baluns (and the occasional miracles of feedline dress and length) as the sole determiner of whether a system will demonstrate common mode currents on the feedline or not. This is a narrow view. I agree with Barry, antenna currents can be a source of significant common mode currents on the feedline AND on the rotator and antenna switch control lines, as well. Antenna currents (common mode currents) on feed and control lines are almost universal when ever the feed and control lines make a significant horizontal run anywhere in the vicinity of the near field of the antenna. "Everything couples to everything". I treat feed and control lines the same. I attempt to decouple them from the antenna by careful line dress and by RF chokes. I have installed one set of ferrite RF chokes (baluns) at the top of the tower on both the feed and control lines, another set of RF chokes (baluns) at the tower base where both lines begin a horizontal run, and another set of RF chokes (baluns) at the point where both lines enter the shack. I'm certainly interested in further amplification on the topic and criticism of the theories. Regards, John Petrich, W7HQJ -------------------------------------------- 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 |