[AMRadio] FCC's AM power


Donald Chester k4kyv at charter.net
Thu Sep 29 13:04:10 EDT 2016


> John Coleman wrote:

> Another way to look at it is to imagine a XMTR whose power is varying by a
> knob and some one is turning the knob back and forth.  The output power is
> varing along with the knob. on a slow sweep scope it would look like an AM
> signal with very low freq modulation depending on the speed the knob is
> turned up and down.  The peak envelope power is the power (Erms *
> Erms/50ohms) at the time the knob is at its max.

It depends whether you are measuring in the time domain or  the frequency
domain.  An example of time domain  would be an oscilloscope or a crystal
detector with no front-end selectivity between the sample and the indicator.
An example of a frequency domain instrument would be a broad-band spectrum
analyser or panadaptor.

We all know that an AM signal does not  consist of a carrier varying up and
down in step with the audio, but a steady carrier and two sidebands, all
existing independently of each other.  This  can be easily viewed on a
spectrum analyser; the steady unvarying carrier is there all the time, and
the sidebands vary according to the audio used to modulate the carrier.  The
amplitude of the carrier stands well above that of each sideband, 6 dB  in
the case of 100% modulation with a steady tone. This is observable with a
receiver switched to narrow selectivity; we can tune USB, LSB and carrier
independently.  With a narrow-selectivity CW filter, we can tune in the
steady carrier and hear nothing of the modulation.

> "The Power of a single sine wave (360 deg) RF  at the peak of the
modulation envelope"  
> The mesurment and calculations above should satisfy this statement.

That statement is dependent on the false assumption that the AM signal is
indeed a single carrier frequency varying up and down in step with the
modulation,  a concept that was disproved back in the 1920s. Numerous
articles devoted to this topic appeared in early radio magazines of that
era; a debate raged for several years whether sidebands existed physically
or only in the mathematics describing the modulation process. Sidebands were
proved to physically exist when receiver selectivity was developed
sufficiently well to actually tune through an AM signal and tune in USB,
carrier and LSB as distinct signals on separate frequencies. There is no
single RF sine wave that varies up and down, peaking at the crest of the
modulation envelope.  Instead,  we see a steady carrier, and broad spectrums
of frequencies to each side representing the modulation, each a mirror image
of the other, and each with a peak amplitude many dB down from the steady
amplitude of the carrier.

Clear as mud, eh?  They need to come up with a better definition of  p.e.p.
than that.

Don k4kyv



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