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Old 30th Sep 2017, 9:21 pm   #14
trh01uk
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Join Date: Nov 2010
Location: Ross-on-Wye, Herefordshire, UK.
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Default Re: Why is aeronautical VHF AM?

Yes, I can well believe the commercial world was looking forward to 500MHz operation. At the same time, some people were already doing it - when money was effectively no object. The Germans started a radar development on 380MHz around 1938. And they had the Wurzburg ground radar operating on 500MHz and in service by 1940.

That rather proves my point that the military were usually in advance technically of anything the commercial world could do - and usually because the commercial world always has its eye on the pounds and pence, whereas the military world just says "We need it" (and they get it).

What the question of why AM for airborne comms really bears on is a more tricky historical matter of who influenced the final decision? Clearly various people were working on this. But who decided to set a world standard, that has been set in stone ever since (regardless of how antiquated it now is)?

A further clue as to who really influenced this decision is the choice of frequency band. Sandretto says the frequency range of interest was 140 to 144MHz. The band chosen by the RAF in the UK was 100 to 124Mc/s. The airband now is 108 to 137MHz. So that suggests the commercial airline developments didn't get to choose the frequency band.

Interestingly, when you examine the early British developments, with the TR1133 transceiver, that rapidly progressed to become the TR1143. Both of them become eclipsed though when the Americans took a TR1143 and turned into the much better known (and now far more common) SCR-522 transceiver. That set operated over a much wider frequency band of 100 - 156Mc/s, (another words the Americans modified the original British design) suggesting that the question of where the airband would end up was an open question at that time during WWII (1942 I think).


Richard
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