After a week of solid testing, and a hundred feet of wasted/cut coax, I’m finally honing in on a good procedure for building+tuning a Tesla Magnifying Transmitter.😙

Short version: Wind your secondary and extra coil to about 1.5x your target frequency.
Measure each coil separately with a VNA or function generator and scope/meter until you have a match, then join them together and measure the series resonant frequencies.
If your frequency is too low, trim an equal amount from both coils until you are close to your desired target. Keep going back+forth tuning each coil separately to ensure they stay roughly the same.

When both have roughly equal length, they will also have equal weight, which corresponds to roughly equal capacitance, which causes both halves to interact more strongly in longitudinal mode.

Or at least this is the assessment I am arriving at.
My latest proper coil has both secondary+extra at 2.35mc, resulting in a concatenated series resonance of 1.487mc
(still need to clip both coils a smidges to tune it up to 160 meters)🙂

At this rate I should have the first TMT up and running Sunday, though I want to wind a second so I can do short-distance experiments and fine-tuning.🤠😉