[Milsurplus] Command Set Receiver/Transmitter Frequency Spotting

[Mike Morrow]

>> A MONITOR-NORMAL switch for these command sets would simply
>> need to open the connection to pin 5 in the cable between the receiver 
>> rack and the modulator.
>> This would make it simple to adjust a receiver to the associated 
>> transmitter.

>Now why would you want to net your receiver to your own
>transmitter, which might be mistuned from the other transmitters
>in the flight by a Kc or 2 or even more? Wouldn't that result in
>the entire flight's receivers being mistuned all over the spectrum?

I don't know...I suppose for the same reason you might want to net the liaison receiver to its transmitter, as the liaison set MONITOR-NORMAL switch is specifically in place to allow?

The command transmitters would have been adjusted using a BC-221 and locked pre-flight.  Each transmitter is probably as close to being on the desired operating frequency as any transmitter in the tower.  There aren't many reports of command transmitters being re-tuned in flight, so presumably the transmitter frequency was sufficiently stable in flight to maintain comms with other aircraft.  It seems like that would be a very good signal source to use to net your receiver if you suspect the frequency has been changed inadvertently or changed by ambient conditions, especially since you have little other recourse once in flight.  But, this capability was not built in to the ARA/ATA, SCR-274-N, and AN/ARC-5 command sets.  Some external source (another aircraft) is required.

At least for these AM sets with broad receiver IF passbands, being off frequency by several kc is not likely to cause a loss of communications.  Then there's the fact that most sources point to VHF command sets like the SCR-522 and the later AN/ARC-3 as having provided the great majority of aircraft command communications in the ETO.

>Isn't it more likely that the pre-flight check required each pilot to
>switch to CW and use his BFO to zero-beat his receiver with
>the flight commander's or control tower's transmitter?

That would be unlikely.  The radio operator did not operate the command sets, the pilots did.  It would surprise me if most pilots even knew about the BFO and zero-beating.  At best, perhaps they tuned in an AM signal from the tower or from another aircraft in the flight for best voice intellegibility.

US Navy WWII AN/ARC-5 installations that used the C-27, C-38, C-125, and similar controls didn't even have the capability of selecting CW mode for any MF/HF receiver.  That philosophy carried on to the post-WWII AN/ARC-25 liaison set, consisting of an R-105/ARR-15 receiver and a T-47/ART-13 transmitter.  MCW or CW mode on the R-105 can only be selected locally, and not at the C-733 pilot's control panel. 

>Was the pre-flight check the entire justification for the receivers'
>coffee-grinder tuning, or was it necessary that the pilots make
>large changes in receiver tuning during missions when they would
>receive orders to do so for tactical reasons?

There would be little point to making significant frequency changes away from the pre-set command transmitter frequencies.  There were generally just two HF transmitters, hence two HF channels, available from the SCR-274-N command set.  There were also just two HF receivers.  The best approach would be to have one receiver assigned to one transmitter, even if that required two of the same receivers being installed for the mission.  Otherwise, the pilot would be required to adjust one command receiver beween two command transmitters set to frequencies covered only by that one receiver.

I doubt there was much HF jamming, but I'll bet there was a lot of potential for interference as a consequence of the crowded airwaves of WWII!  The allies probably did the most...wasn't jamming the German FuG 16 and 17 aircraft VHF command radios the intended main purpose of the RT-45/ARQ-1 "Sandy" countermeasures set?

Mike / KK5F