IO540

I agree (as an electronics engineer who at the age of 13 used to build pirate radio transmitters comprising of a pair of 807s, followed by a less than exhaustive PI-tank ) but I think if this worked habitually, it would soon be realised that the NDB at any airport which also has a VOR there would be pretty useless.

Very interesting about 243 being 121.5. Never spotted that It must be that they simply generated a dirty 121.5 and then filtered everything above 243, to achieve a simple design. 406 is much more complicated in comparison.

JW411

Airborne DF has existed for over 50 years. We had the facility on the old HS Argosy to home in on a buddy using a UHF frequency. He transmitted (a sprung loaded 1000 kcs tone switch was also supplied for this purpose) and the homing aircraft followed an instrument just like a zero reader which gave a fly left/right and a fly up/down indication.

The equipment was known as "Violet Picture".

Needless to say, we could find an ELT (we called them SARBE) with great accuracy. I can remember doing an exercise at Thorney Island. The ELT was inside a hangar and we were even able to tell them which corner of the hangar it was in!