Michael Coslo wrote:

I agree. One of the important factors as far as I know is that the
antennas have to be in two different spots, and although I haven't
measured, (I will now that I'm really interested) I'll bet that the best
performance comes at a distance that is well related to the wavelength.
Which is to say the picket fencing I hear on a mobile two meter signal
might allow me to determine his velocity by knowing the frequency of the
picket, the frequency of the transmission, with a likely but small error
via Doppler shift.

But aside from that little foray, I have no doubt that the effects that
call for diversity antennas/receivers also call for some physical
separation of separate antennas.

Actually, the antennas can be co-located, they just have to have
different patterns. There are some researchers in France who did a lot
of work using an active whip next to an active loop.

A pair of crossed dipoles would probably also work, which would have the
advantage that you could use one of them to transmit with.