[AMRadio] AM Power Level

[Mike Cowart]

Hi Joe,

you have asked a somewhat controversial question about an often
misunderstood concept in radio. Unfortunately the answer is NOT simple.

Yes, the FCC limits us to 1500 watts PEP. And yes if you modulate an RF
carrier with a sinusoidal waveform to 100% - meaning the peak composite
envelope is twice the amplitude as the unmodulated carrier - then the PEP
(and that is in RMS watts by definition) of the composite envelope is four
times that of the unmodulated carrier. Because:

Power = E^2/R

when the envelope voltage doubles, the power increases by four. Therefore,
with the above conditions a 375-watt carrier modulated to 100% with a single
sine wave will produce a peak envelope power of 1500 watts RMS.

Having said that, we do not modulate with a single sine wave (there is no
information in it). Normal audio waveforms are very complex and asymmetrical
in nature. That means that the positive peaks are not equal to the negative
peaks on average. If we modulate with an asymmetrical waveform so that the
larger excursions of the modulating waveform are INCREASING the composite AM
waveform, then it is possible to get a composite waveform that is greater
than twice the unmodulated carrier. Assuming an unmodulated carrier power of
375 watts, the PEP would be greater than 1500 watts. You would have to lower
the power of unmodulated carrier power to stay within 1500 watts.

On the flip side if the larger excursions are DECREASING the composite AM
waveform - as would be the case if you inverted the waveform somewhere in
the audio chain - then when 100% modulation occurs (just pinching off the
composite AM waveform) , the peak composite envelope would not double but be
something less than double. And a 375-watt carrier would NOT produce 1500
watts PEP. In this case, one could increase the carrier power and still stay
within 1500 watts PEP. This is not, however, the optimum situation, because
the amount of power in the sidebands would be much less compared to the
first case. And audio power is the key as the carrier is "along for the
ride."

Increasing the positive excursions of the composite AM waveform to greater
than 100% while maintaining no greater than 100% on negative excursions will
yield stronger audio simply because the average power in the sidebands is
greater. Broadcast stations do this as a matter of standard operating
procedure. They run the positive excursions to 125% or greater to get that
increased audio presence. Does deliberately forcing asymmetry cause
distortion? Yes, but not so much as to be unpleasing to the ear. In fact
this little bit of distortion helps make the audio sound louder.

I hope this helps. See I told you the answer was not simple. I'm sure others
will contribute to this discussion. It seems that it goes around on an
annual basis -hihi.

Get that AM rig on 75 soon! We would love to have you join us on 3880.

73
Mike
WA5CMI


-----Original Message-----
From: amradio-admin at mailman.qth.net
[mailto:amradio-admin at mailman.qth.net]On Behalf Of N6DGY at kirtland.com
Sent: Sunday, May 18, 2003 12:15 PM
To: amradio at mailman.qth.net
Subject: [AMRadio] AM Power Level


I'm a new subscriber to the list, and I have not discovered the way to do a
search
through the old archives.  Forgive me if this has already been covered.

I'm currently in the process of collecting the parts to build a Class-E Am
transmitter
and Class-H modulator for 75 meters.  (I was quite surprised at the relative
simplicity of the circuit!)  While I have been a boatanchor collector for
years, and
also an on-and-off-again Electric radio subscriber (though I have every
issue) I have
never yet been on the air in AM mode.  Listening on 75 off and on over the
years
has me itching to finally change that.

I'm interested to know if I understand the newer FCC power guidelines
correctly and
how peak power on AM is measured.  The FCC now stipulates 1500 watts peak
output.  Am I correct in understanding that in AM, the peak output at 100%
modulation is four times the carrier level?  Does this mean that if one is
to be strictly
within the FCC rules, you can have a carrier of only 375 watts in the AM
mode?

My proposed transmitter will be capable of a unmodulated 600 watt carrier.
Fortunately, it appears to be a simple matter to reduce power in a Class-E
amp just
by lowering the supply / modulation voltage to the MOSFET drains.

Any help in furthering my understanding is appreciated.

Thanks!

73,
Joe
N6DGY


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