On 12/14/2011 4:59 PM, Don Wilhelm wrote: > The balun has the same loss no > matter where it is placed, so lets assume it is placed at the output.
There we go with that nasty word again (balun) -- we're talking about a common mode choke, right? :) The problem is more complex than it looks. Let's say we have a matched 200 ft line with loss of 3dB (to make the arithmetic simple), the choke is wound with the same Z as the rest of the line, and we drive the line with 1kW. If we put the choke at the transmitter, the differential loss in the choke will be half of what it would be at the transmitter end. Now, let's say we have a mismatched line that we're driving with that same 1kW, and the Zo of the choke is still the same as the Zo of the rest of the line, To compute the differential loss in the choke, we must determine the current distribution along the line so that we know the current in the choke. Now, the TOTAL differential loss in the line will be the same no matter where the choke is, but since the coax going through the choke is coiled up, its dissipation is more concentrated than the rest of the line, and it's thermally coupled to the core, and maybe it's also in an enclosure. The real world may be even more complicated -- perhaps the Zo of the choke different from the Zo of the rest of the line. Or perhaps there's some sort of impedance transformer, not a simple common mode choke. Again, we've got to work through the transmission line problem(s) and find the current in the choke. And we haven't even talked about common mode dissipation yet. :) 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
