Jim Brown said: Mon, 12 Dec 2011 17:10:36 -0800 > I have not attempted to measure the Zo of the bifilar wound chokes I've > built using #12 and #14 THHN, but Jerry Sevick, in the last of his > books, did wind some using exactly that method and that wire, and he > says the Zo of those he wound were about 100 ohms.
This is a useful data point. (I've got to rummage through my library to find the Sevick book.) I used a bifilar wound CM choke at the input of the ARRL high-powered tuner described in late editions of "The ARRL Antenna Book." It had 12 bifilar turns of #10 AWG Formvar wire on a 24-inch diameter OD Type 43 core. (Nowadays I'd probably use a more optimal Type 31 mix.) In testing the input balun (aka CM choke) 1500 W of RF at 29.7 MHz was applied for 60 seconds. The #10 wire in the balun got warm to the touch (after the RF was shut off!) but the core remained cool, as it should when there are no common-mode currents, only differential-mode current in the bifilar-wound transmission line. Now, #10 wire is roughly the same size as the inner conductor used in RG-213. On 10 meters the majority of loss in the bifilar transmission line wound around the torroid will be I-squared-R conductor loss, rather than additional dielectric losses that come into effect in the VHF and UHF regions. So, I then assume that the matched-line loss in the bifilar-wound transmission line is the same as that for RG-213 at HF so that I can do computations using TLW. I then used the "User-Defined Transmission Lines" capability in TLW as follows: Frequency = 28.0 MHz; Matched-Line Attenuation, dB/100 Feet = 1.142, Velocity Factor = 0.95; R0 = 100 ohms; Computed X0 = -0.698 ohms. Again, a total length of three feet is assumed for the bifilar-wound transmission line. For a 3000 + j 0 load, TLW reports additional line loss due to SWR (which is 30:1) of 0.416 dB, a power loss in the balun of 137.0 W for a 1500-W transmitter. This level of dissipation in a physically small package will result in catostrophic destruction when the balun is placed at the output of the tuner. For a 3 + j 0 ohm load, the SWR is 33.33:1, and the total line loss is 0.449 dB, amounting to 147.3 W dissipation in the balun -- again, this amount of power in the CM choke balun would surely destroy it. The use a a bifilar-wound transmission line instead of RG-213 has resulted in a slightly greater susceptibility to catosphrophic destruction at low-impedance loads when the balun is placed at the output of the tuner. For a 5 + j 0 load (a 10:1 SWR), the total line loss is 0.274 dB, which for 1500 W is 91.7 W for 1500 W input, or 30.6 W for 500 W RF input. This would be about the limit of safe operation for a CM choke balun placed at the output terminals of an antenna tuner. 73, Dean, N6BV ______________________________________________________________ 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
