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comments. There seems to be some confusion about the need for baluns depending on antenna SWR. The two have nothing to do with each other. The need for a balun comes about because of current flowing on the outside of the unbalanced line (coax shield) due to antenna and feed line asymmetry. The antenna and feedline system could have very high SWR and still be symmetrical and have no need for and benefit from a balun (other than if it is an impedance transformation type). This is usually not the case since it is difficult to have a perfectly symmetrical system in practice. Likewise, the antenna and feedline system could have no standing waves (SWR=1) and could benefit from a balun because of system asymmetry. Slight asymmetries can be achieved in practice and to my understanding, not cause much harm to the antenna pattern. I know Jim Lindsay (W7ZQ), a former EE professor at University of Colorado and avid experimenter and developer of the quad antenna ran some experiments for the us government many years ago with scale models of 20m quads at UHF. He and his colleagues found no detectable pattern degradation without a balun and no inherent need for a for one due to the antenna and symmetric feedline alone. To this day, I believe that he doesn't use baluns on his giant quads up on 180' towers in Wyoming. On the other hand, most hams don't have antennas that are so high and in the clear. A problem that can manifest itself even more easily than pattern degradation is that of RFI caused by minute amounts of current flowing on the coax braid. I had a brief discussion with LB Cebik about this regarding the need for a balun on my new homebrewed 6m beam. I hope he doesn't mind me rehashing this. Here's part of what he said: (LB) "The level of common mode currents for a reasonably balanced antenna assembly is rarely sufficient to make a pattern difference. However, it only takes some microvolts at the station end to create problems with rigs, SWR indications, etc. Blocking them somewhere along the line--as close as feasible to the antenna and perhaps at the shack entry to catch any developed by direct radiation interception by the line--is thus the main point of the exercise. You certainly can install beads on the parallel 1/4 wl section if you wish, but that tends to make it bulky. The existence of the outer braid path does not mean that current will take it. It has to "rival" the impedance of the antenna element in parallel to be an effective path for the standing antenna currents. However, even ineffective paths bring wenough current to the shack end of the line for trouble. I also have a plate attached near the top of the tower with a bulkhead connector. The coax to the shack and the coax to the antenna meet there, with the tower leg as a ground for the braid exterior. This also helps, since the tower is reasonably well grounded." (end LB) 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 |