[AMRadio] The Physical Reality of Sidebands in the AM signal


Donald Chester k4kyv at charter.net
Wed Oct 5 11:50:56 EDT 2016


> Here is where the uncertainty thing comes in.  It is in the act of
receiving
> that causes the signal to appear to be canceled.   It has been proven that
> any electromagnetic waves, radio or light, does not interfere with each
other
> except in the detection
 
> John, WA5BXO

If the phase of the carrier is rotated 90 degrees relative to the sidebands,
the amplitude modulation is nulled out completely, and the signal becomes
phase modulated.  Recalling from memory, I believe that's what Armstrong did
during his 1930s experiments with FM.  He rotated the phase of an AM
carrier, then applied de-emphasis to the audio that modulated the signal, to
convert phase modulation to frequency modulation.  This was done at some low
frequency below the broadcast band, and then multiplied up to around 40 MHz
where it appeared as a wide-band FM signal.

The problem with generating sidebands and carrier with separate
transmitters, you would have to use the same antenna to  radiate both, or
two separate antennas with negligible spacing between them. Otherwise, with
any significant physical distance between the antennas, the signals would
add up properly to form an AM signal only in certain directions, but in
other directions the carrier and sidebands would be out of phase with each
other.  At 90 degree spacing between the antennas, some receiving locations
would receive pure PM, not AM.  If outputs from both transmitters could be
fed into the same antenna without  the transmitters intermodulating each
other's' signals or the power output of one transmitter appearing in the
transmission line of the other, then  the total power output would be the
simple algebraic sum of the outputs from both  transmitters.  Build a  high
level balanced modulator to generate a double-sideband suppressed carrier
signal, and use a regular class-C amplifier to generate a carrier derived
from the same master oscillator that feeds the balanced modulator.  Adjust
the intermediate amplifier tank circuit settings so that the carrier and
sidebands come out in proper phase, then the output power would be the
measured carrier power from one transmitter + the measured sideband power
from the other.  Adjust the output level of the DSB generator to just barely
modulate the carrier 100%. The legal maximum output would be 1000 watts
carrier, modulated by the 500-watt p.e.p. double-sideband generator, to make
1500 watts total.  The vector summation that causes the peak power to appear
as 4 times the carrier power would occur at the detector in the receiver,
not at the modulated stage in the transmitter. Transmitter power would be
read by adding  together the  readings of two Bird 43s, one in the
transmission  line of the DSB transmitter, and the other in the
transmission line of the carrier transmitter.

Don k4kyv



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