[Elecraft] Baluns and 450 ohm line

[Stuart Rohre]

Don is right that with multiband doublet use, the feeder may, at the
transceiver, present various impedances.  Thus, the use of parallel high
impedance lines, Ie the 300 and 450 and 600 ohm lines is attractive, as they
are lower loss at the mismatch SWRs which would present unacceptable losses
in Coax.  In fact, on the lowest band of the doublet where it may be exactly
a half wave at some frequency, its center would be closer to 72 ohms
feedpoint than to the line impedance, as long as the transmatch can adjust
for the combination of feeder, balun, etc., you are in good shape.  The 1:1
balun may work better for cases where your line ends up being closer to 100
ohms at the transmatch.

As various authors point out, one can test any particular transmatch and
balun combination by measurement of temperature rise in the coil of the
transmatch and in the inductor balun.  With the several low and high power
Dentron Transmatches I have tested with larger all band doublets, I have
never found evidence of heating the balun or inductor.  This Field Day, I
will be trying another small transmatch, as I have extra antennas to match.
The small B&W I will be using is rated at 300 watts and in prior field days
at another site, it was used to multiband an 80m dipole, with coax feeder.
It did this surprising well, after we found that it did have coupling
effects if set upon the Yaesu transceiver being used.  This low profile
transmatch, is so compact that B&W used Mu metal at the end of the short fat
vertical coil to provide magnetic shielding.  However, the entire case top
and bottom are not magnetic shielding material and we found coupling to the
transceiver.  This caused very ambiguous adjustments, until we moved the
transmatch to use an external SWR meter to see if the internal one was
faulty.  Simply setting the transmatch beside, rather than on top of the
radio cured all problems!  I have noted the same construction in some short
models of MFJ transmatches, thus it is something to be aware of, if ever you
get strange results using any transmatch.

Thus, when selecting a transmatch, for demanding applications, you may want
to inspect the internal coil arrangements to be sure not to have undesired
coupling to other circuits.
-Stuart K5KVH