[Milsurplus] Setting and Netting Aircraft Transmitters and Receivers

[Mike Morrow]

Ed asked:

> Was the LM-10 Freq Meter used more for the receiver or  the transmitter?

BOTH...but especially the transmitter.

(Note:  CFI is Crystal (or Calibration) Frequency Indicator.)

The transmitters required an external CFI like the LM-* to have any chance of being put on the correct frequency.  This did not change until the internal CFI-equiped ATC (Collins 17H-2), AN/ART-13, and AN/ART-13A.  The T-47A has a cal book at 1 kHz intervals throughout its entire tuning range from 0.2 to 18.1 MHz.  The earlier models had cal books at 2, 5, or 10 kHz intervals depending on frequency.

Likewise, most aircraft receivers had dials that at best suggested the frequency to which they were tuned, and thus required an external CFI.  That did not change until the USN's R-105/ARR-15 (Collins 51H-3) whose internal CFI system allowed easy setting to 1 kHz intervals.  (The USN AN/ARC-25 is the combo of AN/ART-13 and AN/ARR-15.  Either of these units is capable of accurate tuning without external CFI, yet pictures of most such installations still show the presence of an LM-* CFI.)

A receiver could be set by tuning to its accurately set (by CFI) transmitter if the circuitry on key-down did not mute the receiver and substitute transmitter sidetone AF for the receiver's AF.

The USAAF's SCR-287-A (BC-375-* and BC-348-*) and AN/ARC-8 (AN/ART-13A and AN/ARR-11 a.k.a. BC-348-*) mute the receiver and substitute transmitter sidetone AF for receiver AF on key-down.  These systems have a MONITOR-NORMAL switch near the receiver.  On key-down when MONITOR is selected, the receiver is un-muted and transmitter sidetone AF is disconnected.  This allows the operator to tune the BC-348-* to the transmitter, or (less frequently) the transmitter to the receiver.

Most USN systems do not mute the receiver on key-down except to ground its antenna input.  The transmitter will be heard on key-down.  A notable exception is the ARA/ATA command set and its decendants.  On key-down, the AF from all receivers is disconnected and receiver rack AF busses are connected to modulator AF sidetone.  As designed and wired, these sets have no provision for netting a receiver to its transmitter frequency without LM-* or similar external signal.

But the AN/ARC-5:designers tried.  A special crystal-controlled oscillator O-4/ARC-5 powered from a receiver could be used to set the frequency of a tunable AN/ARC-5 communications receiver at one of the O-4's two crystal frequencies.  The O-4 was of no value to setting transmitter frequency...that required an LM-* at pre-flight. The O-4 is designed to be remote-controlled by the pilot to adjust in-flight the remote tuning cranks of his two command communications receivers.  The O-4 was soon obsolete when frequency-stabilized (yellow circle around S stamped on front) R-25, R-26, and R-27 receivers became available.  That stability allowed these communications receivers to be set pre-flight and locked, with no in-flight pilot tuning controls available except for the R-23* beacon-band receiver (if one was installed in place of the typical R-4*/ARR-2 VHF 'ZB' homing receiver).

Mike / KK5F