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Diversity antennas
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 |
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