[ARC5] Interesting documents

[Jim Haynes]

Some guesses on my part.

VHF was needed for communication in England.  The SCR-522 was perhaps the
first ever "plug compatible" item, meaning internally it was completely
different from the British equivalent model, but they were interchangeable
and interoperable.

To set an HF transmitter on frequency accurately you need time and skill
and at least a BC-221 frequency meter.  So if you had pilots trying to
change transmitter frequencies in flight you would most likely wind up
with each aircraft on a different frequency and a total loss of 
communication.