James Robinson writes:

I understand it is because of a characteristic of FM called "capture
effect" that blanks out weaker transmissions when two radios transmit at
the same time. The listener would have no idea that a second, weaker
transmission was being made.

It might be possible to use digital FM and employ anticollision
methods such as those used in other media (networks and so on).
Digital FM would be completely noise free. GSM cellphone technology
already works this way. Also, spread frequency methods such as those
used by GPS can help resolve collision issues, although in aviation
voice communications you really want only one channel speaking at a
time (but I'm sure this could easily be worked out).

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