[AMRadio] AM Transmitter Advice??


Donald Chester k4kyv at hotmail.com
Sat Feb 18 22:33:08 EST 2006


>Bacon, not trying to be contrite, but where in the old rules did the .25 
>second time constant appear?  I have a copy of the old rules and don't see 
>it.  It says only that "have the means".

It was not stated in the rules, but the FCC declared in a public notice that 
they would use that measurement standard for enforcement purposes.  I recall 
seeing it somewhere in a 1950's era QST or CQ.  I believe it is also stated 
in the ARRL SSB handbook that was published in the late 50's to early 60's. 
It wasn't an issue until SSB came along.  The equipment manufacturers began 
using p.e.p. to rate their linear amplifiers because p.e.p. inflated the 
power ratings by a factor of two.  It looked more impressive in the ads to 
claim that an amplifer was rated at 2 kw (p.e.p. input) than one kw (average 
dc input).  Kinda like the stereo amplifiers that were (are?) rated at some 
absurd power rating called "peak music power" which inflates the actual 
power  by a factor of several times.  I  recall that the FTC went after some 
manufacturers sometime in the 1960's because they were overly inflating the 
power ratings of their stereo amps.

Strictly speaking, the p.e.p. rule hurt SSB almost as much as AM, if the SSB 
amplifier is operated below the saturation (flat-topping) point.  With most 
human voices, the average power is about 10 dB below the peak power, so that 
with 100% modulation, the average modulation is about 30%.  That's why the 
average level using a V-U meter is set tor about 30%.  So a clean SSB signal 
at 1500 watts p.e.p. should be putting out only about 150 honest-to-God 
watts to the antenna.  Under the old rules it was legal to run a SSB 
amplifier at 1 kw average DC input, and let the peaks go where they may, as 
long as the signal was clean.  That allowed maybe 600 watts r.m.s. output, 
if the amplifier had enough peak power capability to reach that power level 
without flat-topping.  Of course, most slopbucket rigs fall far short, and 
flat-top long before they reach that level, and you hear the garbage 10-15 
kHz on both sides of the signal.  With ALC and speech processing, SSB can 
legally increase its average power to maybe 300-400 watts before splattering 
if everything is adjusted properly.

k4kyv

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