[AMRadio] Monitoring Modulation Accurately

Donald Chester k4kyv at charter.net
Fri Nov 15 22:33:44 EST 2013

From: Rob Atkinson <ranchorobbo at gmail.com>

>>I agree with some of this but not all of it.  Peak power may be everything
if you are a slopbucketeer, but in ham AM, it's a bogus power
indicator--it's difficult to measure accurately, and gives a false idea of
your transmitting effectiveness because AM has a complex waveform over time
and frequency spectrum.>>

The FCC admitted as much when they passed their bogus power limit rule back
in 1983. Under the original 1000 w DC input  rule, amateurs were required to
possess the necessary instruments to  accurately measure input power, if the
level exceeded 900 watts. Of course, that meant only a DC volt meter and DC
ammeter.  With the  new rules, they deleted altogether the  requirement to
have instruments on hand capable of measuring transmitter power, because
they knew accurate output power measurement was something that many hammy
hambone types wouldn't have a clue how to perform, nor would many hams have
the required instruments, which can be very expensive. They even included
some nonsense in the R & O to the effect that "amateurs may use means other
than accurate measurement to  determine their output power".

The so-called "wattmeters" used by hams are not really wattmeters at all,
but rf voltmeters, with a dial calibrated to indicate power in watts in the
special case of when the meter is working into a perfectly nonreactive
50-ohm resistive load. I have tried out a few of these meters using the same
transmitter power input and the same resistive dummy load and, like
snowflakes, no two of them read the same. Even the venerated Bird 43 is
guaranteed to only 5% accuracy, and that's only if it has been recently
calibrated. The method of subtracting reflected power from forward power for
load impedances other than 50 ohms may work to a certain extent if and only
if the SWR is not too high and the load is not too reactive. With a typical
amateur radio antenna, the resistive and reactive  components of the load
that the transmitter is looking into may vary widely as one tunes to
frequencies from one end of the band to the other.

Another problem is that nearly every one of those  meters are designed to
work with 50-ohm unbalanced coaxial transmission line. Some of us don't use
that type of transmission line. For example, I don't have a sprig of coax
anywhere in my entire antenna/transmission line system between the plates of
the final tubes and the  radiating element of the antenna. Everything is
linked together using balanced open wire transmission line, part of which
operates as a flat, untuned feed line working into a matched load at the
final stage of the output network at the base of the tower, and from there
as a tuned feed line operating with standing waves up to the radiating
element of the antenna. Now, where would I insert a Bird 43?
>>The one useful thing about the positive and negative peak flashers on a
monitor is that if you see the positive flashing a lot more that the
negative, you know you need to flip the phase on the audio.>>

I'm not so sure about that. If the positive peak indicator is flashing, that
means the positive peaks are exceeding 100% or whatever the positive peak
threshold is set at. If the negative peak is flashing, that means that the
transmitter is reaching the verge of overmodulation. Most natural human
voices have a certain amount of inherent asymmetry in the waveform, and it
is more desirable to have the extended peaks to go in  the positive
direction than in the negative. If the negative is flashing more than the
positive, that means the audio is out of phase, and you are hitting 100%
negative before reaching 100% positive. That's when you would need to flip
the phase. If  you see the positive flashing a lot more that the negative,
you merely need to check the waveform using the envelope pattern on an
oscilloscope just to make sure your extended positive peaks are not hitting
the limits of headroom of the modulator or of the modulated rf stage (aka
the modulation capability of the transmitter), resulting in flat-topping,
distortion and splatter.

Don k4kyv

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