Why should the PA draw MORE current on low voltage? If its being driven
for all the impedance matching transformer can do, then the load line
resistance is constant with supply voltage and the current peaks will
necessarily be smaller when the supply voltage is smaller. Unless the
ALC cranks up the drive to try to make up for the low supply voltage and
thus low power (power of a solid state PA varies with the square of the
possible voltage swing which is limited by the supply at the midpoint
and the saturation voltage of the transistor at the low point). Vsat
might be 1.5 volts, so the maximum swing down is 13.8 - 1.5 = 12.3
volts. When the supply voltage is 12 volts, the maximum swing down is 12
-1.5 = 10.5 volts. With the constant R load, the power at 12 volts is
10.5 * 10.5 / (12.3 * 12.3) of that at 13.8 volts. The maximum power
possible at 12 volt supply compared to 13.8 volts is 72.8%. Since the
transformer turns ratios are in integer steps there might be some
mismatch at 13.8 to allow an amplifier to hold up the power at a lower
voltage for a bit.
As for voltage regulators, it seems that some if not many scouts do have
the LM-2830 or LM-2840 family of low drop regulators and they still are
quite voltage sensitive at 12 volts. There just might be some series
resistors between regulator stages (the VFO, as I recall has two
regulator in its supply line, maybe 8 and 10 volts) to cause excess drop
that can't be made up by a .5 volt regulator drop. For a mobile/portable
likely to be run on battery radio, one would wish that frequency
stability from low voltage would not be a problem down to 11 volts
anyway.
73, Jerry, K0CQ
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Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.
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