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Way to go, Biff44!! I like that...
Suvitha, there is no compulsion that your antenna should have a return loss of better than 10dB. Its just that its better for your design from the point of view of the maximum power transfer/losses etc..
The return loss measures the reflected wave to the incident wave, that is RL = -20 log(Γ). So, a return loss of -10 dB means that the reflected wave is 10 dB lower than the incident wave. This is approximately equal to a reflection coefficient of 0.3, so 30% of the incident wave is wasted. Observe that this is for a voltage or a current wave, not power.
thanks for your reply..my requirement for the antenna is to deisign one for a vehicular collision avoidance radar at 24Ghz.i have learned in so many papers which all designed to get returnloss better than 10dB..can you please tell me why it is like that for vehicular collision avoidance radar 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.
There is no magic with the -10 dB number. It is often used as a criteria for very broad band antennas. Power is reflected at an antenna feed and is thus "lost". The lower the reflection the better. -10 dB is what typically be achieved over very wide bands.
When pressed I use -15 dB as a guideline but what is acceptable really depends on the antenna and application. I recently worked on a corrugated horn that had a requirement for a -23 dB s11.