Why separate TX/RX antenna in Doppler sensor gives more sensitivity?

[Terminator3]

If sensor or transceiver have separate transmit and receive antennas. Where does that additional sensitivity comes from?

Assume we have single antenna. If we want to do it without additional microstrip structures, most simple way to do mixing is to put 3-port tee power divider. First port - LO input, second port - single ended diode mixer, third port - antenna. Then single ended mixer start to work. As it does not need much LO power, another possibility is to use "through" series fed antenna. Such antenna will have two ports, one of them is LO, other single ended diode mixer. I read again this topic: https://www.edaboard.com/showthread.php?t=315554 and suspect that sensitivity loss is due to DC component? I mean LO mixing with itself, and that DC maybe will bias diode, so it mixing sensitivity goes down?

If we have separate antennas TX/RX, then we can easily use at least single balanced mixer. But it is known, that balanced mixers need more LO power. So the only difference i see is ease of use of balanced mixer in separate antenna design. Although balanced mixer can be realized with single antenna too. Also it is unclear, why all sensors i have seen use equal wilkinson power divider for TX/RX path? Do we really need half of LO power to the mixer, is not it better to give some more to TX antenna?

 

[jiripolivka]

If sensor or transceiver have separate transmit and receive antennas. Where does that additional sensitivity comes from?

Assume we have single antenna. If we want to do it without additional microstrip structures, most simple way to do mixing is to put 3-port tee power divider. First port - LO input, second port - single ended diode mixer, third port - antenna. Then single ended mixer start to work. As it does not need much LO power, another possibility is to use "through" series fed antenna. Such antenna will have two ports, one of them is LO, other single ended diode mixer. I read again this topic: https://www.edaboard.com/showthread.php?t=315554 and suspect that sensitivity loss is due to DC component? I mean LO mixing with itself, and that DC maybe will bias diode, so it mixing sensitivity goes down?

If we have separate antennas TX/RX, then we can easily use at least single balanced mixer. But it is known, that balanced mixers need more LO power. So the only difference i see is ease of use of balanced mixer in separate antenna design. Although balanced mixer can be realized with single antenna too. Also it is unclear, why all sensors i have seen use equal wilkinson power divider for TX/RX path? Do we really need half of LO power to the mixer, is not it better to give some more to TX antenna?

Doppler radar directs an "illuminating" beam to a target, and the received return is correlated with the original signal. If one antenna is used, a circulator is recommended to separate the return from the transmit signal. Circulators typically isolate by 20-30 dB while separate antennas isolate by more than 60 dB.
The correlator best functions if the transmit and receive signals have specified levels. Mixer as correlator needs typically +10 dBm as local oscillator input while much weaker signal comes as a return. If the mixer RF port receives too much LO power, the result is mixer saturation and a limited sensitivity.