On 12/12/2011 5:10 PM, Jim Brown wrote: > Yes, chokes like these would be a great choice for use at the feedpoint > of ANY HF antenna, whether fed with coax or parallel conductor line.
One VERY important exception that I forgot to mention is that common mode chokes can be destructively overheated by running high power into badly unbalanced antennas. Off-center fed antennas like Windoms can place VERY high common mode voltages across common mode chokes. The only solution I know of is to use multiple chokes in series on the feedline IMO, this sort of antenna is a poor choice in today's world, where local RF noise is made worse by pickup on the feedline. A few years ago, I investigated this by modeling the common mode voltage, and the resulting heat dissipation, in a common mode choke at the feedpoint of a 40M dipole whose feedpoint was moved off center in increments of 3 ft, at 1.5kW, fed by a half wavelength of coax (67 ft). At this worst case feedline length, you don't have to go very far off center to produce a lot of heat in the choke. The results are summarized in a table in a Power Point for a presentation I've done for several ham clubs. One of these bifilar chokes would be OK on a Windom at the 600W level produced by the KPA500, but could fail at max legal power. See page 43 of http://audiosystemsgroup.com/CoaxChokesPPT.pdf 73, Jim Brown 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
