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


Donald Chester k4kyv at charter.net
Wed Oct 5 16:18:39 EDT 2016


> From: John Coleman [mailto:jc at pctechref.com]

> Correct as usual Don.  I used the separte XMTR and ANT senario for
> imagination only.  But, seams that I remember a high efficency XMTR being
> built using balanced modulator and a limiter and then into class C.  and a
> separate carrier XMTR and something abouot a 90 deg transmision line mix
at
> the antenna.  As I recall the thing was old even back when I was reading
all of
> this.

The Doherty and Terman-Woodward high-efficiency linear amplifier and grid
modulation systems use the 90 degree transmission lines (usually lumped L
and Components, not actual full-length transmission lines).  No balanced
modulators; a "carrier tube" that saturates essentially at full unmodulated
carrier level, allowing only negative modulation peaks, and a "peak" tube
that remains cut off until the modulation peaks go positive, taking on where
the carrier tube left off to transmit the positive half of the modulation
waveform.  The outputs of the carrier and peak tubes are combined to
generate the full AM signal.

Another system that claims not to require the quarter-wave transmission
lines is so-called "Taylor Modulation".  I have seen articles; some
construction articles ran in CQ magazine back in the 50s and 60s.  I'm not
sure anyone has ever  successfully got it to work; I have never heard one on
the air.  But many broadcast transmitters have used the Doherty system for
many years, including Continental Electronics.

Don k4kyv



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