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I thought it prudent to comment about the issue of balun use on our products considering some of the recent comments seen on the reflector. 1/ We use a balun or RF choke to effectively decouple the outside of the shield of the coaxial cable from the antenna circuit. The antennas are designed to be used with a RF choke or balun. If the choke or balun is omitted, you will have issues of distorted azimuth pattern, reduced front to back and possible VSWR issues depending on the length of your feedline. The radiation of the feedline is another undesirable afterproduct of omitting a balun. Use a balun or RF choke if you expect the array to perform as specified. 2/ A B1 balun, or similar will indeed lower the frequency of an array since the wire/pigtails coming out of the balun become part of the driven element circuit. In each manual, this "phenomenon" is discussed and trimming instructions of those pigtails are given. On most antennas, we trim the pigtails to a finite length and that length is considered when determining the length of the tip of the driver. If you need to make small driver length adjustments, you can shorten or lengthen the balun pigtail instead of changing element length. 3/ Most power related failures in a balun are attributable to water excursions within the balun or RF saturation of the ferrites used. The design and materials used in the B1 balun were selected to prevent this failure mode. 4/ Measurement of a standing wave on a transmission line using a field strength meter may show the presence of waves even if the balun or RF choke is doing their job. A sensitive field strength device may well be "seeing" the energy on the center conductor if the shield of the cable is not perfectly shielding the center conductor. 73 Natan W6XR Force 12 East Ithaca, NY FN12sk force12@qth.com -------------------------------------------- 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 |
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