Re: antenna
I don't know anything about vehicular radar system design, but generally one wants transmit all the availabe power, no matter what the system.
If the return loss is high, power is reflected back into the system and so interfere with other signals.
If the same antenna(s) is used both for transmitting and receiving at the same time then there could be a problem with the phase noise in the system. If the transmitter is leaking (phase noise), then, during receive the transmitted signal and the received signal mix and so cause even more noise which then appears in the baseband. Also, the smaller the losses the smaller is the noise in the system.
There is usually a circulator in the tranceiver to separate the received and the transmitted signal. Circulators are not perfect, the amount of leakage from the TX to the RX depends strongly on the isolation in the circulator.
I don't know if the above is the correct answer for your question.