On 3/9/2012 5:48 PM, Guy Olinger K2AV wrote: > OCF antennas, under miscellaneous names, have been working fine, just fine, > just about as long as radio.
"Working fine" depends on your definition of "the big picture," and your ability to diagnose problems. The problem with OCF antennas is COMMON MODE FEEDLINE CURRENT, which causes the feedline to receive noise, make the shack hot with RF, and radiate RF to the TV and stereoand computer in our living room (and our neighbor's living room). Fifty years ago, there was relatively little man made noise to be picked up on the feedline of our antennas, the equipment in our living room did not have Pin One Problems that turned all of the wiring into receiving and transmitting antennas, and the equipment in our neighbors' living room was not full of noise generators (other than their TV set's horizontal flyback system). Equally important, we had not LEARNED about common mode current on feedlines, and its contribution to these problems. We lived in blissful ignorance. We called CQ, we got responses, we had fun, but we also had TVI! And when the electronics world changed, introducing Pin One Problems, digital equipment, and switching power supplies to create noise, COMMON MODE CURRENT on feedlines started biting us in the behind, WHETHER WE KNEW IT OR NOT. As Guy and I have both noted, we can get away with unbalanced antennas if we choke them to death, but we're going to fry chokes if we run power unless we use MULTIPLE chokes. And "choking them to death" means multiple turns around ferrite cores (multiple cores and multiple chokes for high power). A FAR better solution, if we can do it, is resonant antennas for each band, well choked. If you're limited on space that can be done with fan dipoles or traps or loading coils. And there is NO MAGIC to parallel wire feedline -- there can be just as much common mode current on parallel wire line as on coax if the antenna itself is unbalanced. Balance is determined by the entire circuit -- the antenna, the feedline, and the transmitter (including the tuner), not ONLY the feedline. That's why I object to the words "balanced feedline" -- they are are pure fiction. 73, Jim K9YC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
