[Elecraft] Balun at input or output of tuner

[Dean Straw]

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