Jerry Sevick used a monster T400A-2 toroid, #2 powdered iron, 4 inches in diameter, intended to be the 4:1 up transformation for the back of high power tuners. (Sevick, Understanding Baluns.... 2003, CQ Communications, pp 60-61) I have run these some times with brick-on-key 1500 watts and never managed to get heat. I've never personally managed to construct anything that would stress one of these.
I'm currently using a 17 turn trifilar winding on a T400A-2 as a 4:1 isolation transformer (not a balun, no direct connection between primary and secondary) feeding the 90 ohm base of my 160m 3/8 wave inverted L plus folded counterpoise to 360 ohm "450" window line (Wireman #554). Particularly with the significant capacitive reactance of the counterpoise, I was definitely expecting this would put some serious heat on the core QRO, and maybe invalidate the concept, but I have gone 15 min QRO BOK, immediately walked out to the base, and the core was stone cold. The whole thing seemed cold. There was a little bit of condensation visible inside the teflon tubing beforehand, and the BOK did not cause it to evaporate. I really don't know why it didn't heat up, but I'll take it. Thing is a killer ant. So I'm thinking if you put up Jerry's 20 turn bifilar on a T400A2 as a Ruthroff balun and slap it on the back of a tuner, that you're going to be very hard pressed to warm it up with ordinary stuff. 73, Guy. On Tue, Dec 13, 2011 at 1:43 PM, Dean Straw <n...@arrl.net> wrote: > > 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:Elecraft@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
