The issues arising with AutoT 6.25 precluded prolonged FT8 operation at full (100 W) power. And performance (in terms of power loss) was a reason for concern. Time to switch to the backup setup.
AutoT 6.25 was replaced by the ATAS-25 style sliding coil. This move generated new (well, they were known) issues. When radiator length is much longer than one quarter wavelength, there is no way to tune it adding a coil. When it is too short, it is inefficient.
For a given radiator length, maximum operation frequency is limited. The coil can be adjusted for operation below that frequency. Until certain limit.
Wire length in the coil (plus its own height) is around 7.5 m. Plus 5 m of radiator, makes 12.5 m. A full wavelength would be 50 m, or 6 MHz. Possibly a bit lower. But not more than 1-1.5 MHz below]. This would be the minimum tuneable frequency. Not low enough to operate in 3.5 MHz band. In practical terms, 7 MHz would be the limit. Operation in 3.5 or 1.8 MHz bands would require additional coil(s).
3, 4 & 5 m radiators were evaluated. 5 m radiator was the one described in section “2024-06-26. Testing at summer QTH garden.”. To obtain a 4 m radiator, the upper section (10 mm diameter) was removed. In 3 m radiator, the two uppermost sections (12 & 10 mm diameter) were removed.
Maximum operating frequency as a function of radiator length (including coil’s length with 0 turns, 0.34 m) is summarized in Table 6. For operation between 7 and 14 MHz, 5 m rod is the most convenient radiator. For frequencies above 14 MHz, 3 m rod is the only practical possibility.
3 m aluminium rod seems a sweet spot combo. Increasing length by 35 cm impeded autotuner operation in 21, 25 and 28 MHz bands. Using a thinner radiator (8 mm diameter) decreased bandwidth by a small amount, but enough to hinder autotuner operation in 28 MHz band.
Indeed, 3 m rod is an all-round solution. It allows operation from 7 to 51 MHz. Although at the expense of reduced efficiency in the lower part of HF spectrum.
Table 7: Summary of operation settings for sliding tuning coil.
VNA graph. 3 m x 20-16-12 mm Aluminium radiator, sliding coil 0 turns.
VNA graph. 3 m x 8 mm brass radiator, sliding coil 0 turns.
VNA graph. Al and brass radiators comparison, sliding coil 0 turns.
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