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

[Francesco Ledda]

I remember my father describing an early 50s ILS transmitter.  The AM modulation was achieved by motorized antennas. 


-----Original Message-----
From: milsurplus-bounces at mailman.qth.net
[mailto:milsurplus-bounces at mailman.qth.net]On Behalf Of Mike Morrow
Sent: Sunday, September 14, 2008 12:37 PM
To: milsurplus at mailman.qth.net
Subject: [Milsurplus] Re: mechanical modulator ... definitely not a
wobulator


>The 2/42 Radio News defense edition says the localizer has 90 & 150 hz fm
>a mechanical modulator

It's very, very safe to say that NO wobulator was ever used in any localizer transmitter design.  That would create frequency modulation, not amplitude modulation.  ILS localizer receivers, regardless of frequency, have always been AM.

As an aside, wobulators might not even produce significant FM if the localizer was NOT operating above the VHF band.  The USN tried before WWII (and dropped before end of WWII) the "Air Track" ILS (ZA, ZA-1, ZAX, AN/ARN-9) that used an MF (not VHF) localizer receiver (hence those R-23 and R-24/ARC-5 receivers with the somewhat rare MX-19/ARC-5 ILS adapter).  If the localizer that Marty mentioned was MF (say, 350 kHz), it would *not* be easy to mechanically FM-modulate it with a "wobulator" device.  For the USAAF's SCS-51 ILS with a VHF localizer, then perhaps, with difficulty.  But there is no point to FM modulation in a AM system.

The AN/APN-1 altimeter transmitter is a 440 MHz oscillator that is frequency modulated by a wobulator.  It's a very different application from that of a localizer transmitter.  

As far as "mechanical" modulation goes, another candidate would be the system used by the SCR-178/179 modulator BC-188.  It uses a vibrating element to generate a MCW tone, which is applied to the input of a couple of 27 tubes to modulate the 865 PA tube.  In that sense, mechanical means are only used to generate the tone frequency, not to do the actual modulation.

The term "wobulator" still has *current* use today for large steam turbine control systems.  A speed wobulator takes effect during turbine start-up to accelerate the RPMs through regions of rotor resonance (critical speeds).

Mike / KK5F
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