| Index | Thread | Archive | Jan-2001 | Archive | Send |
 |
  |
|
   |
|
  |
|
Glad to hear from you. Have read your regular contributions to the TowerTalk reflector and appreciate all the information you and others have to offer. Yes, I am quite familiar with that coiled air core balun data and the posting about it, last year, on TowerTalk. Yes, I am aware of the phenomenon of the common mode impedance being reduced when the coils are stacked on top of one another, compared to a single layer, solenoid, method of winding. This change in common mode impedance is the result of a reduction of the Q of the coil. The resonant frequency of the coiled air core balun is also lowered, as well. These coiled air core transmission line baluns owe their high common mode impedance characteristics to the parallel resonant circuit equivalent that the coiling results in. The reduction in common mode impedance that results from the layered winding method often doesn't materially reduce the effectiveness of this type of balun, provided the resonant frequency of the balun is still correct for the intended frequency of operation. The common mode impedance, with either method of winding, is often sufficiently high that you can throw away a lot of common mode impedance and still have enough common mode impedance to choke off common mode currents with low impedance loads. The minimum one needs from a balun is for the balun to provide a common mode impedance of about 2 to 3 times the load impedance. This amount of common mode impedance will reduce the common mode currents to a very low, but not vanishing level. So, to feed a antenna with an input impedance of about 80 ohms, one needs a balun with a minimum of about 250 ohms of common mode impedance. At that point, things will work entirely satisfactorily, except in the most demanding of applications. This example illustrates why a lot of people "just" wind a few turns of coax up near their antenna feedpoint and get a good reduction in common mode currents. They no longer see the lights blink n the house and their stereo's don't squak any more. At that level of performance expectation, the layered air core coiled transmission line balun is entirely satisfactory. A potential positive advantage of the layered coiled air core transmission line balun over the solenoid wound balun is the lowering of the Q of the balun. This lowering of the Q of the balun can result in a significant broadening of the balun's common mode impedance bandwidth. This broadening of the common mode impedance bandwidth can substantially improve the multiband capability of the coiled air core transmission line balun. A high Q narrow bandwidth balun is converted into a lower Q wider bandwidth balun when one shifts from the solenoid to the layered winding method. Not bad, in the correct application. Careful examination of George Badger's origional data on his layer coiled air core transmission line balun, "Badger" balun, shows that he measured a very useful common mode impedance bandwidth from below 20 meters to about 10 meters. This led him to state that he had developed a "new" balun type. He is close to the truth: a relatively broadband, high-ish common mode impedance bandwidth, low cost, with high power capability and without the limitations of the vintage "voltage" baluns popular at the time. Sounds great to me! These are my thoughts after reading your e-mail. It is good to hear from you. I'm interested in any further thoughts and experiences on this subject. Regards, John Petrich, W7HQJ Now days, with advances in radiio technology, some expect higher levels of balun performance, i.e multiband yagi systems, single tower antenna farms, multi-multi contest operating, so care and attention to broad band balun technology is very important. -------------------------------------------- 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 |