[AMRadio] Monitoring Modulation Accurately

Donald Chester k4kyv at charter.net
Sat Nov 16 21:22:23 EST 2013

From: rbethman <rbethman at comcast.net>

>>I run the BC-610.  I have to rein it in every time I light it off. It
allows 300cps through 4000cps through the audio chain, and if set by the
manual, it will far exceed the 1500 PEP.  I was stunned to see it break
2500W PEP when I got the Bird.  The Heathkit dual meter with peak reading
was absolutely useless.  Sold it after getting the bird.

So, as one other pointed out, I feed the 455KC if out from either an R-390A
or SP-600 to my SM-220 with BP-8.  I see what is coming in on receive, and
see what my TX signal looks like.  That's AFTER throttling the Speech
Amplifier to stay close to 1500W PEP.

Where am I actually at?  I haven't any better clue than anyone else does!
I know that I'm somewhere in the ballpark.  I haven't had the FCC holler
nor any OO slips.>>

Monitoring the signal on a scope by picking it off the receiver i.f. will
inevitably give a false picture. The highly  selective i.f. amplifier
stages and receiver selectivity filters will alter the envelope pattern;
furthermore, viewing another ham's over-the-air signal  through your
receiver degrades the envelope picture even more, when you add the
selective fading and other effects of ionospheric perturbations. The only
way to accurately view the waveform of a transmitter is to sample the
signal right at the transmitter's  output stage and feed that sample
directly to the vertical input of the scope. In mine, I by-passed the built-
in vertical amplifier altogether and feed the sample directly to the
deflection plates of the scope tube, because I don't trust the linearity of
the amplifier stages of a cheap Hammy Hambone monitor scope like my HO-10.
You MIGHT get a worthwhile approximation of your waveform through the R-
390A if you use the 16 kc/s bandwidth position, but even then, mechanical
filters are known to muddle up the phase relationships of the signals that
lie within the passband. 

Are you sure the Bird is calibrated accurately? Are you running everything
in the BC-610 within its normal ratings at the proper settings? The BC-610
is a classic example of a transmitter that SHOULD fully comply with the pep
nonsense, no questions asked. 

The 250TH in the BC-610 is supposed to run at 2000 volts on the plate @ 250
MA in AM mode. That's 500 watts input. According to Eimac
(http://tubedata.milbert.com/sheets/088/2/250TH.pdf ),  the carrier output
should be 335 watts.  Now, that's the hypothetical plate power output,
under ideal conditions, as measured right at the plate cap of the tube.
Considering plate  tank circuit losses, transmission line losses, and
antenna tuner losses (if a tuner is used), you would be lucky to even get
300 watts of carrier to the antenna, probably substantially less. Those
tube chart figures are kind of like  the EPA mpg fuel efficiency ratings of

The BC-610  modulation transformer total primary to secondary turns ratio
is about 1.65:1. With that much step-down, using the BC-610's common plate
supply for the modulator and final amplifier, it is impossible to modulate
the transmitter significantly beyond 100% positive . It probably won't even
modulate that high before flat-topping begins to occur. Therefore, with
that transmitter, unless it has been extensively modified including
changing out the modulation transformer,  the p.e.p. will actually be close
to 4 times the carrier power, most likely no more than 1200 watts, or if
the tank circuit, tuner and feedline were 100% efficient (impossible),
theoretically 1340 watts.  

If you are loading that transmitter heavily enough to generate 2500W PEP
(625w carrier output), I'd suggest to be in the market for some spare
250THs, because that one isn't going to last very  long...  but I  suspect
everything in the BC-610 is working normally; the problem is more likely
that the Bird is not giving an accurate reading, possibly because it isn't
working into a load close enough to the prescribed 50 Ω, non-reactive.

If an AM transmitter is running 500 watts DC input or less and you know the
carrier power is somewhere around 300-375 watts, I suggest relax, give
yourself the benefit of a doubt and don't worry about it or agonise over
it. When was the last time one of those periodic FCC enforcement releases
listed anything about an amateur station, particularly an AMer, being cited
for "too much p.e.p."?

"Throttling the speech amplifier" of an AM transmitter to limit the p.e.p.,
has to be the worst possible way to do it,  inevitably  resulting in an
anaemic, undermodulated signal. The total sideband power of a full carrier
AM signal, fully modulated,  is  low enough already; the last thing we need
is for hams to start making it a practice to run big carriers and
deliberately modulate less than 100%.

Don k4kyv

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