[AMRadio] P.E.P (was Power Levels)

[Donald Chester]

 >...a 100w rice-box needs to be lowered to somewhere around
 >25 watts of carrier, to allow for full audio output...

That is basically a correct statement, even if the terminology used is 
slightly in error.  If the rice-box's output power is capable of no more 
than 100 watts, you have to lower the carrier power to about 25 watts in 
order to leave the amplifier with enough headroom to accomodate the positive 
modulation peaks, if you expect to modulate 100%.

That's why so many of the signals on 11m. sound so crummy.  Many "good 
buddies" run their carrier levels up to the maximum output capability of the 
amplifier, so there's no more headroom left for modulation peaks, thus the 
positive peaks are shaved off at the carrier level.  The result is downward 
modulation, distortion and lot of splatter.

When amplifying AM, the efficiency of the linear amplifier is about 33% or 
less - at the unmodulated  carrier level (the efficiency of any linear 
amplifier is a direct function of the drive/output level; zero at zero 
signal level and maximum at the peak output level).  A SSB linear is just as 
inefficient, since most of the time the SSB signal level is far below peak 
output.  Consequently, an AM linear is equal in efficiency to a SSB linear 
on modulation peaks if they run the same peak power.

The crux of the problem is the moronic decision of the FCC to redefine the 
power limit in terms of pep.  Most of today's hams haven't a clue what it 
really means, and typical ham radio quality wattmeters are next to useless 
for measuring it when delivering power to typical amateur radio antenna 
installations.  The FCC was aware of this, and they skirted the issue by 
deleting the requirement in Part 97 that amateurs have the capability of 
accurately measuring output power.  They said that amateurs can determine 
what their power level is by means "other than accurate measurement" (now 
try to figure that one out).

Pep does serve a useful purpose, to define the peak power CAPABILITY of a 
system.  It came into popular use in ham radio back in the 50's as SSB was 
becoming popular, largely because the manufacturers of linear amplifiers 
could inflate the power ratings of their commercially-made products in the 
ads.  A 2 kw pep linear sounded more robust than a 1 kw one.

The purpose of the legal power limit is to put a cap on the amount of 
interference a signal can gererate.  It is AVERAGE or MEAN power, based on 
rms values of current and voltage that determines the loudness, interference 
capability, effectiveness, or whatever you want to call it, of a signal, not 
the power level developed on occasional voice peaks.  When the FCC was 
soliciting comments on the power limit docket, many hams basically told them 
that, but the folks at the Private Radio Bureau paid little notice to what 
the comments said.  As a consequence, we would undoubtedly find that some 
amateurs now pay them the same courtesy.

Don K4KYV

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