The TenTec design is OK but it would requrie an extra switch and if really
getting down to the nitty gritty it is no longer a pure L-match.
There was an article in the ARRL Antenna compendium vol 3 that stated that for
an ATU used for a coax match it should be able to match up to an SWR of approx
4.5. There were some valid reasons for that, cannot remember exactly what those
were. The implication was that if it is higher then the antenna needs first to
be matched closer before attaching it to the coax.
One option could be not to use 450 Ohm but something like a 300 which would
make matching on the highest HF bands easier. I saw some two toroid design for
this but did not investigate this further for two reasons: an extra toroid (
more stray capacitance impacting matching and also an extra toroid would
introduce more losses) and secondly I am not interested in the bands above 21
Mhz.
Alternatively it would be quite feasible to use a tap somewhere on the third
winding and I think it would work FB. I'll make a note of this should I ever
get the urge to open the ATU up again and make adjustments enabling higher band
functionality. I suspect that more capacitance will be required on the low
bands.
73, Marinus, ZL2ML
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