Hmm, yes. RJ Mitchell's "Schoolboy to Spitfire" notes the replacement of the "old and inefficient TR9 radio" with VHF in 1940-41.
From the RAF Signals Museum:
My memory was clearly faulty on some items!
From the RAF Signals Museum:
So, yes, you're clearly right about most of those factors. I'm not saying that you're wrong about the others, only that I haven't dug up anything conclusive one way or the other.There was a need for increased frequency stability for the TR9. Frequency drift resulted from vibration and temperature changes during flight. In April 1937, crystal control was applied to the TR9; that was then designated the TR9C.
Later, to facilitate transmissions for Direction Finding (DF) purposes within a flight of aircraft while still allowing R/T contact to be maintained, a second frequency channel was provided on the TR9; that became the TR9D.
The proposed VHF set was to be built such that it would be physically interchangeable with the TR9, so that the aircraft fit could be changed from VHF to HF, and vice versa, at short notice!
My memory was clearly faulty on some items!