Michael Coslo wrote:
K1TTT wrote:

and
they can provide interesting effects that can make sorting out signals
in pileups easier, or maybe pulling signals out of the noise a bit
easier, but none of them will really prevent the multipath, arrival
angle, or polarization fading. that is where tom was trying to point
out that any way you combine the rf from two antennas into one you
lose the advantages of the diversity of the antennas.

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.

- 73 de Mike N3LI -

For diversity, something which you can take advantage of has to be
different between signals from two antennas. They can be at the same
physical location but, for example, have different polarization
(polarization diversity). Or they can be physically separate (space
diversity). However, a key necessity is that the signals from the two
can't be phase coherent if you're combining them. That means you have to
separately detect the two signals with receivers that aren't phase
coherent -- you can't use a single LO for both -- then combine the
signals after detection. If you do try to connect the antennas together
or convert/detect them with the same LO, you'll simply have a single
phased array antenna system.

If you're not combining them, but listening to one or the other but not
both at the same time based on some sort of voting system, you can
detect/convert them any way you want, including using the same LO.

Roy Lewallen, W7EL