Owen Duffy wrote:

Has someone got a good definition of coherent.

I thought that it implied "same phase", as in a coherent source is one
where all rays, photons, whatever are in phase. An antenna may well
receive rays from a single source that are not in phase.
. . .

The way I've always seen it used in this context is meaning "exactly the
same frequency". They don't have to be in phase, but the same-frequency
requirement implies that the phase relationship wouldn't change with time.

This is consistent with the definition from _Merriam Webster's
Collegiate Dictionary_: "relating to or composed of waves having a
constant difference in phase ~light".

Roy Lewallen, W7EL