Joe: Right on -- certain unnamed baluns had a quite reputation as being RF fuses.
But as the suject title above says, I'm still talking about the pros and cons of placing a CM choke balun at the input or at the output of an unbalancing antena tuner to feed balanced lines. Both positions are valid ones, and like most engineering matters there are tradeoffs to both approaches. Some tradeoffs involve significant smoke and flames... ! 73, Dean, N6BV -----Original Message----- From: Joe Subich, W4TV [mailto:li...@subich.com] Sent: Tuesday, December 13, 2011 11:21 AM To: Dean Straw Cc: j...@audiosystemsgroup.com; elecraft@mailman.qth.net Subject: Re: [Elecraft] Balun at input or output of tuner > 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. Moving this discussion away from the tuner and to the feedpoint of the antenna ... I would never use a bifilar wound CM choke with a high HF antenna. Years ago I tried to use a well known, third party high power "balun" on a triband antenna with a reputation for blowing its OEM (fuse) balun. That attempt was spectacularly unsuccessful on 15 meters where the 90-100 Ohm Zo of the bifilar winding coupled with a "line length" of slightly over 12 feet transformed the normally benign 50 Ohm SWR of the antenna into something that was poor across the entire band. With an antenna supporting more than three bands, it is likely that the transformer effect would impact at least one band! 73, ... Joe, W4TV On 12/13/2011 1:43 PM, Dean Straw 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
