In article , Airy R. Bean wrote:

I forgot the SPC, but that appears to need a differential
capacitor that I haven't yet obtained (or made).

The SPC circuits I've seen (including the classic design in the late
'90s ARRL Handbook) does not use a differential capacitor. Its output
capacitor is a dual-section non-differential type - the two sections
track in parallel.

I have seen some T-type tuners which use a differential capacitor,
with one section being the input cap and the other being the output
cap. MFJ sells one of these. These are advertised as being more
convenient, since establishing a match requires tuning only two
controls rather than three. However, I doubt that this arrangement
results in the most efficient match.

Also, ISTR reading somewhere (that I cannot find) that
the SPC arrangement is very lossy, and can result in
3/4 of the power being lost in the ATU itself)

I don't think that's a specific characteristic of the SPC - it seems
to be true of T tuners in general. The T configuration does provide a
wide matching range, but in the case of very low-impedance or highly
reactive loads the "match" may result in extremely high circulating
currents through the coil, with the resulting high losses. A "T"
tuner with sufficient reactance in the components may let you
establish a "match" for a short-circuited load... but you'll end up
dissipating almost 100% of your power in the coil. The SPC probably
shares the same characteristic.

--
Dave Platt AE6EO
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