|[AMRadio] AM Power|
ars.w5omr at gmail.com
Mon Sep 26 03:40:01 EDT 2016
On 09/25/2016 10:11 PM, Donald Chester wrote:
> Let the newbies figure it out for themselves. No point in providing
> comfort to the enemy.
The newbies aren't our enemy. Ignorant bureaucrats, within our own
ranks are the problem.
(which, btw, was co-authored by some cat in Woodlawn, TN - ever heard of
him? ;-) )
The FCC regulations at the time stated that the general class license
holders could run no more than 1000 watts DC input power to the plate
circuit of the RF final amplifier. This was easy to measure with DC
meters. The plate supply voltage was multiplied times the plate supply
current, and the product was the DC input in watts.
RF amplifier efficiency determined how much output power could be
achieved. The efficiency of a screen modulated RF output stage or a
class B RF linear amplifier at carrier level is about 33% to 35%, giving
you about 330 to 350 watts of carrier output on AM for the maximum legal
input power of 1000 watts. On the other hand, a plate modulated class C
amplifier has about 75 percent efficiency, giving you about 750 watts of
AM carrier output for that 1000 watts input. And the output tube in the
final RF amplifier runs a lot cooler in class C than in class B AM
linear operation, so smaller tubes can be used. But the only practical
way for hams to get high RF amplifier efficiency with AM was to apply
the modulating audio voltage to the plate supply of the RF final, and
the audio circuitry required to do this must be capable of at least 500
watts of audio. So, to get the extra power output within the legal
definitions, most of the big-gun operators opted for the high-level
plate modulation method.
The high-level modulation method is the application of the
modulating voltage to the plate circuit of the class C final, causing
the output amplitude to vary in accordance with the applied modulation.
One hundred percent (100%) modulation was generally defined as the point
where the maximum modulating voltage, during its negative half cycle,
opposed the DC supply voltage sufficiently to reduce it to zero. If this
voltage dipped below zero, over-modulation and splatter were the result.
Most people agreed that the peak of the positive half cycle of the
modulating audio voltage, added to the DC supply, could go as high as
necessary to faithfully reproduce the audio as an image of the
microphone output. Even if the positive peak was more than two times the
amplitude of the negative peak, the modulation was not considered
illegal unless it contained distortion products that caused splatter
over an excessive bandwidth. Over-modulation was only considered to
occur at the point where modulation characteristic became non-linear,
producing distortion and splatter.
The audio voltage from a microphone is often not symmetrical,
unlike a sine wave from a signal generator. This asymmetry is a natural
quality of speech and other sounds. This article discusses the use of
voice waveform asymmetry in AM systems.
When an AM transmitter is 100% modulated by a pure sine wave, the
PEP (Peak Envelope Power), is 4 times the un-modulated carrier power.
This is because the Audio Voltage modulating the carrier doubles the RF
voltage at the peak, since the load resistance is constant, the RF
current doubles at the same instant as the RF voltage. Since P = E * I,
then P at the instant of the positive peak must be 4 times greater than
the power of the original carrier...
the rest of the article, with graphical presentations and solutions (and
Don Chester's comments), are located at
As someone who enjoyed the effects of this circuit and design, lets just
say, that watching a pep meter bang the 1000kW scale end on audio peaks
with just a 100w carrier, and the monitor scope not hitting the 100%
negative line is a feeling that was difficult, at best, to describe.
73 = Best Regards,
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