[Milsurplus] Command Set Receiver Coffee-Grinder

[Mike Morrow]

Breck wrote:

>The "coffee-grinder" tuning was necessary during the enroute navigation and 
>approach phase where two or three different Low Freq Nav Aids were utilized 
>and  the crew would attempt multiple bearings for a fix. Obviously it was 
>advantageous to take the bearings as close together time wise so that 
>further adjustments did not have to be made to the LOP(line of positioning) 
>due to change in the aircraft position during the process.

That decribes more accurately the use of an RDF/ADF connected to a loop. 

This is not applicable to the command sets, using a range receiver like a BC-453 that had no loop antenna.  Radio beacon navigation from station to station using such a receiver was simply done by pilot's ear, by flying into one of the four arms (radials) of the directional A-N Adcock beacon of interest until the A's and N's merged to a solid tone.  This indicated that the aircraft was now on the radial.  The charts of the era gave beacon frequency and the true orientation of the radials coming from each directional beacon.  The four radials from a particular beacon weren't always separated by 90 degrees, either.

I don't know when all the LF/MF directional beacons shut down in the US...I'd bet in the mid- to late 1950s.  But since then, only non-directional beacons (NDBs) have been in service on the LF/MF band.  A loop is necessary to determine direction to an NDB.  But in its heyday, the LF/MF dirctional beacon system made even the smallest, cheapest, and simplest of battery-powered portable LF/MF receivers the one item of electronics to be carried, if any electronics were carried at all.

The reason for the remote tuning capability of command set receivers is pretty simple:

The navigation receivers of the command sets had to be tunable by the pilot to select the frequency of the desired directional beacons, and to select the tower frequencies operating in the LF/MF band (of which 278 kc was the most common).

Before the stabilized R-25, 26, and 27/ARC-5 receivers were available, remote tuning of the communications command set receivers was required to adjust for drift and other disturbances.  When the US Navy finally got their three stabilized communications AN/ARC-5 receivers, they were most often set by a radio tech with the tuning fixed at the rack, and no provision for remote tuning.  Just like the associated transmitter, in-flight adjustment of the stabilized communications receivers was no longer required.  But even in the AN/ARC-5 system, a navigation beacon band receiver like the R-23/ARC-5 was configured for remote tuning, even if no other receiver in the rack was.

Mike / KK5F