So long as the balun transformer is not considered to be bothered by
core saturation and hasn't stray c and leakage inductance (e.g.
theoretically perfect, not real world) it makes little difference
whether the balun is before or after the tuner. My complaint is that
some time I tried tuning a tuner with a balun on the output side and
when I was matched, no RF got to the antenna, it was all dissipated in
what appeared to be a series resonance of the balun winding.
Disconnecting the antenna feedline had no effect on the matched input Z
of the tuner. This was a center tapped transformer (voltage balun)
winding. Unless a balun is quite oversized, core saturation at high
impedances is a distinct problem. I've tried to wind a broad band
transformer for 600 ohms Z to handle a bit of power and I ran into the
limitations that to get enough turns on my cores to reach a few times
600 ohms inductive reactance the stray C made the transformer resonant
and it didn't work well at the resonant frequency and the bandwidth was
far less than 3 to 30. More like 14 to 15.
I remember a 50 KW ferrite core Balun Collins made for high impedance in
the 60s. It had a low impedance transformer, then yards and yards of
tapered balanced transmission line to transform high impedance down to
where the transformer could work with some bandwidth.
When I look at the impedances a typical dipole can supply to the tuner
through the worst possible lengths of transmission line, I see that if I
used a 300 or 450 ohm line the range of impedances is something like 50
to 1000 ohms. (full wave antenna with quarter or half wave transmission
line and then half wave antenna with those same line lengths). So a
tuner made to run at 200 or 300 ohms is right in the middle of the
impedance range and needs a smaller range of component values to handle
the range of dipole impedances.
If that same dipole is fed through a 50 ohm transmission line, the tuner
Z range is far worse. Its still 1000 ohms for the full wave dipole and
the half wave feed line, but with the full wave dipole and the quarter
wave feed line its 2.5 ohms. And with reactance that 2.5 ohms is a bitch
to match because it takes TEN times more C than 25 ohms and the currents
get to be unreasonably high. Line current 6.3 amps for 100 watts, 50
amps for a KW. Though some of the L circuit tuners do match that well.
None without considerable loss and that loss is made worse by the high
line current.
Still when the balun is at the input of the tuner (whether the tuner
circuit is a T or a balanced lazy H) when tuned the balun sees its
design impedance when means its not overstressed for either voltage or
current, the core is least likely to saturate, and so the balun losses
are least.
Having said all that, I use only link coupled tuners from 160 through 2
meters for balanced lines. And I do have a small Viking Matchbox in the
collection, though its not as versatile as the swinging link with series
capacitor and tappable balanced secondary. I've tuned a 75 meter double
extended zepp with 450 ohm open wire line as easily as with shielded
direct burial telephone drop wire for the feed from BCB frequencies up
through 2m and worked 2m SSB as far without a band opening as I have
with a 20' yagi at about the same height.
73, Jerry, K0CQ
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Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.
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