[AMRadio] AM power

Gary Schafer garyschafer at comcast.net
Sat Jun 18 22:39:49 EDT 2011

Well here we are again. While it is good to have discussions of any type and
especially this one, it still amazes me how things get so twisted when this
375 watt power issue comes up.

I have copied some notes that I posted on this subject about 2 months ago. 

But I have to tell you that I am a little disappointed when I see some guys
that strongly profess to be "knowledgeable and upstanding amateurs" be
selective in what they want to adhere to in the FCC rules. Where is that old
"honor system of yesteryear?"

The 1500 watts PEP power limit is not a "jackass rule". If you take the time
to investigate and understand, it makes perfect sense with today's
technology. Nothing mythical or complex about it. Continuing to try and
belittle the rule does nothing but confuse many that are trying to further
their education of AM radio.

It is not hard to measure PEP to reasonable accuracy. A scope will do
wonders as will some of the new PEP wattmeters (LP100) available. Some with
more accuracy than the old faithful Bird. 
Since the FCC doesn't specify to what accuracy we need to measure to we
don't need to be worried about 3 or 4 decimal places in our power
But being that most AM operators profess to be "highly technical in order to
be capable of setting up and operating an AM station" it would seem that
those same operators would also be interested in doing their best to at
least make an effort to measure power of their transmitter to the best of
their ability.

End of rant. Now on to the why.

375 watts of carrier allows you to run the most audio power in your side
bands, the thing that counts!


PEP is found by the sum of the carrier voltage and the audio voltage. 

If you want to maintain legal limit and not over modulate, you have a brick
wall at the top (PEP limit) and a brick wall at the bottom (over modulation
on negative peaks).

My point is that with 375 watts of carrier and maintaining legal limit on
the positive side and not over modulating on the negative side, you will
come out with the same amount of audio if you have higher negative peaks or
higher positive peaks. At legal limit you gain nothing by increasing or
decreasing the carrier. Your best bet is to run 375 watts of carrier.
With the magic 375 watts of carrier you don't gain a thing by inverting the
audio phase.

If you have unsymmetrical audio with positive peaks higher than negative
peaks here's what happens. 

With 375 watts carrier, with positive peaks hitting 100% positive
modulation, this will yield 1500 watts PEP, the legal limit. You will not be
hitting 100% modulation on negative peaks.

If you invert the phase with the same unsymmetrical audio and same 375 watts
of carrier you will hit 100% modulation on negative peaks, the most that you
can go, before you will hit 1500 watts PEP with the positive peaks.

Either way you have the same amount of modulation density (same audio

100% modulation occurs when the modulation voltage is equal to the modulated
stage plate voltage. This is true for both positive and negative modulation

You could lower the carrier power and increase the modulation to over 100%
on the positive peaks (while still staying below the 1500 watt PEP limit)
provided the negative audio peaks were lower and didn't cause over
modulation on negative peaks.

If you increase the amount of carrier you could increase the modulation on
negative peaks and not over modulate on negative peaks, but you would have
to decrease the positive peaks in order to stay under the 1500 watt PEP

Increasing or decreasing the amount of carrier (from 375 watts) will only
lessen the amount of power in the side bands, provided that you still stay
under the 1500 watt PEP limit and do not over modulate on the negative

It would seem that aiming for symmetrical audio would have the most benefit.
This would give the highest percentage of modulation in both positive and
negative direction giving the most power in the side bands.

For those that wonder how PEP is found, you do so by adding voltages in the
carrier with the voltage in the applied audio. Since 100% modulation is the
doubling of the plate supply voltage which causes the plate current to also
double (2x2=4) this is where the 4 times PEP comes from.

Gary  K4FMX

> -----Original Message-----
> From: amradio-bounces at mailman.qth.net [mailto:amradio-
> bounces at mailman.qth.net] On Behalf Of Rob Atkinson
> Sent: Saturday, June 18, 2011 8:51 PM
> To: Discussion of AM Radio in the Amateur Service
> Subject: [AMRadio] AM power
> I asked about the "375 watt power limit" but I confess it was a loaded
> question, with an agenda, for which I apologize because I hate being
> on the receiving end of such questions and try to stick to saying what
> I think instead of fooling around with manipulative questions.
> So my agenda was to make a point, the point being that the mythical
> 375 watt power limit is 100% b.s. on two or three levels, the first
> being it's having a peak envelope power limit as its basis, which is a
> jackass rule since, for one thing, how is this measured with precision
> by an average ham.
> Now we get to AM where measuring a carrier to meet an exact numerical
> requirement is basically impossible unless you are at the NIST, and
> measuring peak positive modulation with any accuracy is impossible,
> made more complex with asymmetric voices, which most males have.  We
> can estimate; that's about it.   If the FCC were realistic, the rule
> would provide a fudge factor like a figure +- 5 or 10 % on carrier
> power with a positive modulation limit of 150% and forget the p.e.p.
> crap.  Most hams could stay under that without a lot of fancy
> measuring equipment.    But they don't so I give it all the attention
> it deserves, scaling to its grip on reality.   If there is any tone of
> irritation here it is because it is irritating to me when hams prattle
> this 375 watt stuff off as if it is just this trivial little thing
> like getting inside out of the rain.   I am accustomed to getting the
> 375 watt baloney from slopbucket operators but I am surprised to see
> it here.   Even if you try to do something reasonably accurate like
> measure your RF current into a measured 50 ohm load, your current for
> 375 watts is 2.7386 amps or something and try getting that nailed down
> with a 3 or 5 amp RF amp meter.   So it is all ludicrous to me and I
> therefore give it all the attention it deserves.
> 73
> Rob
> K5UJ
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