Sandy,
You are correct that using a balun (either 4:1 or 1:1) is not
necessarily the most efficient solution. The 'old fashioned' link
coupled tuner will most always be more efficient.
When the feedline input impedance (and that has nothing to do with the
characteristic impedance of the feedline) is close to the output
impedance of the balun, the balun will be just almost as efficient as
the link coupled balanced tuner, but that rarely happens in practice.
Yes, using a balun following an unbalanced transmatch is a compromise.
It lends itself to easy bandswitching and its associated convenience.
If one is searching for the most efficient antenna tuning mechanism,
then either dedicated resonant antennas are required, or one must accept
the inconveniences of changing coils in a simple balanced link coupled
tuner or accept the compromises of an easy bandswitching system. The
old Johnson Matchbox was an effort to provide bandswitching convenience
with a link coupled balanced tuner, but even it has limited matching
range compared to the simple single-band tuner designs.
Bottom line, one must either accept the compromises dictated by the
conveniences of bandswitching or accept the inconveniences of using the
most efficient tuners that can be constructed. There is no 'best of all
worlds'.
73,
Don W3FPR
Sandy wrote:
When a high VSWR exists with a toroid ferrite balun due to a high
inductive or capacitive reactance and the toroid begins to heat, the
losses will rise to quite unacceptable losses and can possibly destroy
the balun itself, even tough the tuner used appears to have reduced
the VSWR on the input side of the circuit to a very low value.
Generally a 4:1 or 1:1 transformer type balun should be preferably
used for just a resistance transformation, not where there is a
highly reactive component on the output side of the balun. Lots of
people "get away" with this situation, but it isn't a very good idea.
I commonly did this for years with very large ferrite cores and a
vanilla high pass "T" network tuner (like the many MFJ and other "T"
network tuners)
For the last 15 years I have used nothing but the common Parallel type
balanced line link coupled tuner configuration and had extremely good
results with the old fashioned and sometimes very cranky to get setup
right circuit.
"choke" type baluns (the ones that traditionally use a large number of
ferrite beads on a length of coaxial cable) are much less troublesome
than the transformer type. If your "balun" setup runs cool, then you
probably have hit upon a length of feeder that is "just right" and you
are "OK". If it is running warm then you are treading on dangerous
ground and things may be getting ready to surprise you one day with a
catastrophic failure, especially when you run the legal limit!
This no matter what the VSWR meter says between the tuner and the
ferrite balun in question.
73,
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