P-39C-D & 400

[Elmas]

Exactly. There were sistems, as you mentioned, late in the war, to switch frequency remotely, only for short range (VHF), but even that did not mean that radios coud be placed everywhere in the aircraft.
For HF it was more difficult as, changing frequency, say from 15 Mhz (vawelenght 20 m) to 30 Mhz (vawelenght 10 m), required theoretically different lenghts of the antenna, so transmitter and antenna system had to be closely matched, otherways a too high SWR
Standing wave ratio - Wikipedia .... (for simplicity...)
could cause serious problems, and even destroy the final stage of the transmitter.
Zerstörers had at the beginning of the war a crew of three for reasons.
To change frequency was absolutely fundamental, see the bottom of the blackboard:

 

[MIflyer]

Fighters using the SCR-274-N or ARC-5 systems had the one or two transmitters installed tuned to the required frequency and could not change them after takeoff. This was also true of the earlier SCR-283 system (which the P-400's were equipped with) and the similar system used by the USN. They could change the received frequency used by the two or three receivers by means of a remote dial and "speedometer cable" link to the receivers but this would have been both unwise and probably unnecessary for anything but the LF receiver because the two HF receivers (3 to 6 and 6 to 9 MHZ) were no doubt tuned to the same frequency as the transmitters. Tuning the HF receivers to other frequencies risked not listening on the same frequency you were talking on.

In the ETO most USAAF fighters had SCR-522 installed rather than SCR-274-N. It had four crystal controlled channels that were set up on the ground. The pilot could change from channels 1 through 4 but had no ability to go outside that. One channel, D, was reserved for emergencies. In the CBI many USAAF fighters retained the SCR-274-N and that enabled them to talk to the ground troops they were supporting, something the RAF could not do because they were equipped with the SCR-522. The SCR-522 also came to dominate the USAAF comm in the Pacific. In the later part of the war in the Pacific the USN fighters adopted a very interesting ARC-5 system that had 5 VHF channels and one HF channel.

 

[MIflyer]

Now, as for the blackboard, note it says "Operate On Button" "Homing On Button" 'Mayday on Button" That indicates the aircraft were using SCR-522 and Buttons A, B, C, D, were used for various preset frequencies associated with those functions.

And see the attached. It shows the SCR-274-N and SCR-522 installations for the P-51A. Other P-51's and the A-36A would be the same. It seems likely that for Mustangs the SCR-274-N was only used in combat in P-51-1 and A-36A in the Med in early days following Operation Torch and the P-51A and A-36A in the CBI.

And also attached is a shot of an SCR-274-N rack of receivers being installed in a new production P-51A at Inglewood. The receivers were 190-500 KHZ for the radio ranges and control tower communications and the 3 to 6 MHZ and 6 to 9 MHZ receivers for radio communcations. There were two transmitters installed to cover the two comm frequency ranges, and as I said earlier, they were pre-set on the ground.

 

[Elmas]

To summarise what I meant:
Operationally more than one frequency was used (so multiple transmitters and receivers were used on different bands );
the equipment could not be placed anywhere in the plane but had to be placed as close to the pilot (and C.G.) as possible.

 

[manta22]

Fighters using the SCR-274-N or ARC-5 systems had the one or two transmitters installed tuned to the required frequency and could not change them after takeoff. This was also true of the earlier SCR-283 system (which the P-400's were equipped with) and the similar system used by the USN. They could change the received frequency used by the two or three receivers by means of a remote dial and "speedometer cable" link to the receivers but this would have been both unwise and probably unnecessary for anything but the LF receiver because the two HF receivers (3 to 6 and 6 to 9 MHZ) were no doubt tuned to the same frequency as the transmitters. Tuning the HF receivers to other frequencies risked not listening on the same frequency you were talking on.

In the ETO most USAAF fighters had SCR-522 installed rather than SCR-274-N. It had four crystal controlled channels that were set up on the ground. The pilot could change from channels 1 through 4 but had no ability to go outside that. One channel, D, was reserved for emergencies. In the CBI many USAAF fighters retained the SCR-274-N and that enabled them to talk to the ground troops they were supporting, something the RAF could not do because they were equipped with the SCR-522. The SCR-522 also came to dominate the USAAF comm in the Pacific. In the later part of the war in the Pacific the USN fighters adopted a very interesting ARC-5 system that had 5 VHF channels and one HF channel.

View attachment 597133View attachment 597134View attachment 597135

When I was in junior high school around 1950, the surplus market was flooded with electronics such as SCR522s, ARC5s, and SCR274Ns. They were nicely designed and easily converted to amateur radio use; in fact, I used a modified SCR522 (BC624 & BC625) on 2 meters AM when I got my Novice license (WN1ZUO) in 1953. Using a channel 7 TV antenna I could reach Providence from my home in Newport, Rhode Island. Later, I moved to West Virginia where there was no VHF activity so I used a modified SCR274N (BC-459) transmitter on 40 meter CW as W8TKF.
The SCR522 used a set of Ledex rotary solenoids to move a rack which gang-tuned the transmitter and receiver to pre-set frequencies which were determined by plug-in crystals. This radio set could be mounted anywhere since it was remotely tuned. Later on, Collins Radio introduced their "Auto Tune" system that allowed individual remote tuning. As already mentioned, the SCR274N "Command Set" receivers could be tuned by a mechanical cable so installation choices were versatile. A rack with individual pre-tuned transmitters was able to be placed anywhere within a proper CG envelope and where access could be gained.
Regarding antenna matching: The SCR522 used an antenna with decent broad band performance so no re-tuning was necessary. The SCR274N transmitters used a variable inductor to impedance match its long-wire antenna so each transmitter was matched to the antenna at its particular frequency.

 

[Big Jake]

Years ago we had our airplanes equipped with Plessey VHF radios in which the frequencies were controlled by removable crystals. Every so often, a courier would arrive with a new set of crystals and our radio technician would proceed to modify all the radios by removing and replacing the crystals. The pilot could switch between channels and thus between pre-set frequencies only.

 

[chuter]

While moving the radio for CG reasons is a good consideration (it was done for the 109K - a lot was done for the K) a complication is the weight of the SCR-522, ~48 lbs., and the power supply, ~35 lbs. To relocate it you'd need to move the radio rack back while still being able to tie it in to the airframe structure such that it could support at least ~288 lbs. (for six Gs, ~384 lbs. for eight Gs. What's your requirement?) just for the radio. That might not be very easy and could be a notable source of weight gain above the simple solution ballast ... and you still need reasonable access.

 

[MIflyer]

The ARC-3, which came out late in WWII, was basically a later model SCR-522 and used a separate receiver and transmitter. Aside from the great advantages afforded by the ability to just change out a bad transmitter or receiver rather than having to pull the whole combined unit out of the airplane, it also gave more flexibility in terms of locating the separate units for both accessibility and CG purposes.

The ARC-3 became more or less the standard VHF set soon after WWII. When they needed a bunch of F-51's to fight the Korean War they referbed a bunch of Mustangs, removed the SCR-522, and installed ARC-3, right behind the pilot's seat and added a BC-453 range receiver atop the ARC-3's. The F-86, F-84, and F-86 in Korea had the ARC-3, but the SCR-522 served on in many other aircraft. The VHF ARC-5 was a really neat setup, but about the only place the USAF used it was in the Bell X-1A.