[AMRadio] Re: Modulation transformer ID and repair

[Donald Chester]

Brett Gazdzinski <brett.gazdzinski at mci.com> said:

>If I was to run the rig at 2000 watts pep, would anyone know or care
>that I was exceeding the power limit (by a little bit)???
>
>Positive peaks are good, but I don't think they are effective
>on the air, if you have to limit the negative mod and the pep.
>better to make sure you go 90% negative at least, and whatever positive,
>... although I doubt the FCC
>truck will pull up outside the house if you go over 1500 watts pep.
>If you ran 10,000 watts pep, someone might notice eventually, but they
>really
>should pay more attention to the intentional interference on the bands.
>
>I personally don't think they pay any attention to anything other than
>blatant
>abuse  over a long time, and even then, they seem to play games, warnings,
>going to
>court, etc.

>I did track down Ron boot (anyone remember him?), go over his house, and
>confront him face to face.
>he turned REAL nice and gave me a tour of his shack (junk).
>I told him, I know where you live, and can come back anytime day or night,
>and although I am mellow and tolerant, I can pound your face if I get
>pissed enough.
>
>The FCC had gone after him somewhat, with zero results  ((Kinda like the 
>current farce on 3890 kc/s?   - Don)), so I personally
>don't worry about going slightly over 1500 watts pep.

>
> > 	My voice is so lopsided; it has peaks 2 squares pos on the scope
> > for one square down.  I've tried it on many scopes direct
> > from many good
> > microphones.  When connecting the same mic to the rig, a 600 watt
> > carrier output yields 5400 watts PEP output just as the carrier is
> > pinched on the 100% negative cylce.


I agree wholeheartedly.  Also, careful examination of the wording of Part 97 
reveals that the regs define the pep figure as "under normal conditions".  
Most of the time the voice remains at a fairly low value of voice peaks 
(about 30% modulation), with a few reaching 100% while an occasional peak 
attempts to go sky high.  These exceptional peaks add next to nothing to the 
total sideband power or the interference potential of the signal.  It's 
where the majority of peaks go, along with the modulation density of the 
audio that determines how loud a signal is.  That is what could be 
interpreted as "under normal conditions".

One advantage of modulating down to just below 100% negative and letting the 
positive peaks go where they may is a cleaner signal.  With limited positive 
peaks (most transmitters are not capable of modulating much more than 100% 
if even that much), the temptation is to modulate to 100% in the negative 
direction, and let the occasional high positive peaks clip off at the 
maximum positive modulation capability of the transmitter.  It is very 
difficult to even see the positive peak clipping on the envelope pattern of 
a scope unless you drive the audio so high that a large percentage of the 
dominant peaks are  flat-topped.  If a transmitter is capable of 150 or 200% 
positive peaks to accomodate the occasional wayward peak (comprising less 
that 1% of the total number of positive peaks), there will be no splatter 
resulting from the hard clipping of these miniscule peaks.  When these peaks 
are clipped, as in the case of 99% of the AM transmitters on the air, both 
amateur and broadcast, they add to the spurious sideband components of the 
signal, even though the clipping effect may otherwise be inaudible and 
invisible on the envelope pattern.

A disadvantage of high positive peak capability is low modulator efficiency. 
  A class-B amplifier's efficiency is directly proportional to signal 
amplitude, reaching a maximum at peak output capability.  If the modulator 
is capable of delivering peaks at 200% modulation, but you run it at about 
100% most of the time, the average efficiency will be much less than that of 
an amplifier capable of delivering only 100% at its peak output.  However, 
the duty cycle of human speech is so low that this will make very little 
difference in the ratio of overall 60~ a.c. power input to r.f. output 
delivered to the antenna.

One of the early double-sideband schemes just after WWII described in QST by 
Villard employed the "upside-down tube" circuit.  Although this was 
exploited later to make the signal louder by transmitting DSB reduced 
carrier, keeping the DC input within the 1 kw limit while generating several 
kilowatts of sideband power, the original intent of the scheme was to take 
care of occasional negative overmodulation peaks without generating suprious 
sideband products.  The article was entitiled "overmodulation without 
splatter".  The sole purpose of the original proposal was to clean up the 
signal back in the days when few hams had monitor scopes or fully understood 
the principles of modulation, and the phone bands were full of trash from 
overmodulated signals.

I think some AM'ers are overly paranoid about p.e.p.  I wonder if these same 
folks are as cautious about going even 1 mph over the posted speed limit 
while driving their automobiles; certainly driving a 4000-lb. machine beyond 
the legal speed limit on a public highway causes greater endangerment to the 
  public than hitting a few positive peaks beyond 1500 watts on a ham rig .

The fault really lies with the FCC (Johnny Johnston and his cronies) for 
coming up with such a foolish power limit rule for amateur radio in the 
first place.  Common sense would have dictated AVERAGE (mean) power output.

Don K4KYV

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