"Pete KE9OA" wrote in message
...
If we are talking about a ferrite loopstick antenna, and if the coil is
wound on a cardboard former, the coil can be slided on the form to peak
the
sensitivity on the lower end of the band. The trimmers peak the high end
of
the band, and if the tuning capacitor has slits in the outer plates,
sections of these plates are bent to peak the sensivity in the middle of
the
band, typically at 1000kHz. This is known as three-point tracking.
With some loopstick antennas, this is not an option because the wire is
wound directly over the ferrite form.

Pete


It's worth mentioning that perfect oscillator - RF tracking won't happen on
typical radios which have the same profile on both the oscillator and RF
sections of the tuning capacitor. It can be correct at two or three points,
but it's going to be off a bit between those points. There's a wider
frequency for the RF stage to cover than the oscillator stage and three
point tracking is the best economical comprimise for most radios. Alot of
the AM radios (not multiband) of the past had different profiles cut on each
of the sections of the tuning cap, but I don't know if that's done with the
common plastic dielectric tuning cap. I imagine anything beside a constant
radius profile would put a radial stress on the dielectric film, which might
be a problem. But I don't know for sure. It's been years since I've taken
one of those things apart, and I don't remember the profile of the plate.

I've aligned a few radios and I haven't noticed much consistant problems
with the factory alignments, but I don't get real fussy about oscillator -
RF tracking. Getting the best dial tracking is more important to me. I
figure getting it right somewhere near the high end and low end is about as
good as it gets. Getting it perfect at the extremes would just knock it off
more somewhere else.

Frank Dresser