[Boatanchors] CAA modulation indicator question...

[Richard Knoppow]

    FWIW, the CAA became the FAA in 1958, which gives a hint about the 
date. From about the late 1920s to the 1950s medium frequency radio 
range stations were used for air navigation. These stations operated in 
the band roughly from 200 to 400 Khz and used directional antennas to 
provide four "beams" for navigation. They could be followed with very 
simple equipment, essentially an AM radio. Each beam produced a steady 
tone when on course (on the beam) composed of combined Morse characters 
A and N. When the aircraft deviated to one side the A signal would 
predominate, the N signal on the other side. Maps were provided showing 
the layout of the beams. The system worked pretty well but was subject 
to problems from reflections from geographic features (hills, etc.) and 
ionospheric propagation conditions. Its possible this monitor was 
intended to be a service tool for these stations.  Many stations also 
had a voice capability superimposed on the navigation beams. Sometimes 
to provide weather information and sometimes to allow check ins as 
flights progressed
     Range station receivers had an audio filter for 1020 Hz. the filter 
could be set as a band pass filter, for the A and N signals, or a band 
elimination filter where the voice signals were wanted. If this unit has 
such a filter its a sure clue as to its purpose.
    About the early 1950s Visual Omni Range (VOR) began to supplant the 
MF range stations. They provided 360 degree directional information and 
had other features such as providing a compass like indicator rather 
than audio to indicate direction.
     Only a guess and a chance to wrench this history out of my memory.

On 6/19/2024 8:50 AM, Gary Peterson wrote:
> I have an interesting piece of test equipment in a very nice, green, felt-lined, wood case.
> 
> The label on the hinged lid contains the following information:
> 
> MODULATION INDICATOR
> made for
> DEPARTMENT OF COMMERCE
> CIVIL AERONAUTICS ADMINISTRATION
> TYPE CA-1433  CONTRACT Cca-28705
> SERIAL NO. 43
> NATIONAL ELECTRONICS LABS., INC.
> WASHINGTON 9, D.C.
> 
> The unit has a percent modulation meter with two ranges (35% & 100%) with switch for positive and negative peaks.
> 
> There are tuning and sensitivity controls and a headphone jack.
> 
> There is a laminated schematic with operating instructions on the inside of the wood box’s lid.  There is no indication as the the frequency range covered by this unit.
> 
> There is an attachable whip antenna with N male connector.
> 
> I have not been able to find any information regarding this unit on the Internet.
> 
> I am guessing that this was used to measure the modulation on transmitters on the aircraft band (108 to 137 MHz?).
> 
> I would appreciate hearing from anyone who has familiarity with this cool device.  Thanks in advance.
> 
> Gary, KØCX


-- 
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998