[Milsurplus] Re: mechanical modulator ... definitely not a wobulator

[C Whitaker]

de WB2CPN   The word you're looking for is
"Pedantic".
The USAF ILS systems, MRN-7, MRN-8, and SCS-51
used mechanical modulators which were really
sideband generators.  The hardware in the MRN-7
and MRN-8 were the same except for their size as
the 7 runs about 109 MHz, and the 8 about 390
MHz or something.  The RF transmitter, (localizers
were AM modulated only with Station ID at about 5
percent, and control tower voice at about 5 percent),
fed a complex power dividing and phasing system made
up mostly of two balanced capacitive bridges,
one to generate 90 Hz sidebands and suppressed
carrier, the other for 150 Hz. Each bridge had
two identical variable capacitor rotors. The rotors
in the 90 Hz bridge had three arms about 4 inches
long sticking out from the shaft.  The rotor in
the 150 Hz bridge had five similar arms.
The 50 Hz synchronous AC motor had a long shaft
sticking out of either side, the 90 Hz rotors on
one side, the 150 Hz rotors on the other.  The
AC for the motor was generated by a high-stability
RC oscillator that we checked regularly.  50 Hz
was used to not let the motor speed be affected
by the Commercial AC frequency.  Grief for we
maintenance people was the bearings on that shaft.
They were way too small, and took many hours to
change, and the whole facility had to be realigned.
These sideband bridges, and other combining bridges,
fed signals to the horizontal Localizer antenna
array to produce a complex left/right blue/yellow
90/150 audio signal in the aircraft.  The antenna
arrangement in the Glide Slope was much different.
It was three horizontal antennas mounted on a
vertical pole.  That antenna depended heavily on
ground reflection to produce many signal lobes. The
CAA/FAA method of generating ILS and VAR sidebands
was entirely different from you've seen here. That's
because their sideband generators I've maintained must
have weighed tons.
Wobulators:  Trivia:  There were such things from
the time of the ancient Romans, but we'll save that
for later.  (I have a TRM-3 Sweep Signal Generator
here that uses one.)
73  Clete  (Some of you may know that us old
ACS and AACS types got into a lot of things at one
time or another.)