[Elecraft] Balun at input or output of tuner

[Joe Subich, W4TV]

> 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
>
>
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