"starman" wrote in message
...
craigm wrote:

"starman" wrote in message
...

The traditional single gang 10-365 cap' was used to tune the
oscillator
in a MW radio, not a front-end preselector. The oscillator usually
operated at 455-Khz (I.F.) *above* the desired frequency. This would
be
about 995-Khz for the low end (540-Khz) of the band. That's why a
variable cap' with a maximum of 365-pf is not really low enough
(practical) when you want to use it as a tuning cap' for a MW loop
antenna or preselector. This is because it has to tune down to the
actual lower limit of the MW band (540) instead of the receiver's
oscillator frequency (995) at the low end.



I don't think so.

If you have a single gang cap in an AM radio, then it is tuning an
antenna
coil. If you have an oscillator in a traditional radio, then you have a
superhet and will see a two or three gang capacitor.

The most frequent thing I've seen for the AM broadcast band is a dual
gang
capacitor with the oscillator section having about 75% of the capacity
of
the antenna/RF section.

When you see a dual 365 pF cap used in a superhet, you will also see a
pad
cap in series with the oscillator section so that the oscillator tracks
at
the needed 455 kHz offset.

With a 265 uH coil and a parallel capacitance of 10 pF it resonates at
3093
kHz.
With a 265 uH coil and a parallel capacitance of 365 pF it resonates at
512
kHz.

This is more than enough for the AM broadcast band. However in real life
one
gets some stray capacitance due to wiring.

Adding 20 pF for stray capacitance, we get.

With a 265 uH coil and a parallel capacitance of 30 pF it resonates at
1785
kHz.
With a 265 uH coil and a parallel capacitance of 385 pF it resonates at
498
kHz.

Still, this is more than adequate.

Given the right inductance and keeping stray capacitance low, 365 pF is
enough.

If you need more capacitance, it probably means your stray capacitance
is
very high and you reduced the number of turns in the loop (inductance)
to
offset that problem and then added more variable capacitance to cover
the
low end of the band.

craigm

I was thinking of a superhet with a single gang cap' to tune the
oscillator and no front-end preselection tuning, which would require
another gang on the cap'.


I've never seen a superhet without an antenna tuning stage (at least not
before the new garbage which is PLL tuned and has no tuned antenna stage,
and therefor also has no gain). All the older superhet designs, whether
tube, transistor or IC (analog and almost all portable digital tuners) has a
tuned input.