Originally Posted by
sky9
I seem to remember that when an aircraft transmits it's receiver is "earthed". If the relay sticks reception is not possible until the aircraft transmits again and the relay probably opens.
I'm sure a radio engineer can give a more accurate description of the function.
Whilst I'm not specifically an aircraft radio designer, I can speak for many other forms of transmit/receive combination and relays went out in the 70s there, being replaced by PIN diodes - which are entirely solid state and don't stick, nor is there a limit on the number of times they 'switch' (since they don't actually switch).
I've designed and built a number of transceivers, so this is an area where I have working knowledge, just not of the particular requirements for aviation certification. For all I know relays may still be common practice but that would be a source of considerable surprise.
I'd love to see any concrete evidence - particularly repeatable - of the 'sleeping radio' phenomenon. Whenever you get believable reports of that kind of behaviour, you start constructing imaginary scenarios of a combination of circumstances that could cause the equipment to behave in that way. I'm fairly good at fault-finding in that way but there are no obvious failure modes that I can think of to cause the reported behaviour. If imagination doesn't lead to a cause, next you need an actual example to investigate.