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How important is S11 in a receiver antenna

denizduran

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I am currently in the process of designing a dual band horn antenna for the frequencies 24.5 GHz and 37.5 GHz. This antenna will only be used as a receiver. My co-workers are proposing that a S11 is not that important when since the antenna is only used for receiving purposes, and not for transmitting. Doesn't having a poor return loss also mean poor sensitivity? Can someone explain how a poor S11 would effect the performance of my device in all aspects?
 

tyassin

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Hi
It will be the same whether used as transmit or receive antenna.
Also be aware that antenna loss can hide in the return loss. Meaning that loss will improve the return loss.
 

denizduran

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But why would it be the same for the receiver antenna? What does S11 represent for a receiver antenna? How can I distinguish antenna loss with the return loss?
 

FvM

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S11 is actually a combination of mismatch, antenna losses and radiation resistance. Mismatch can be eliminated by matching network or virtually in processing of S11 data, but antenna losses and radiation resistance can't be distinguished in an impedance measurement. The radiation effeciency can be only determined in anechoic chamber. See also https://en.wikipedia.org/wiki/Antenna_efficiency
 

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I'd say, S11 is important if your receiver sensitivity is noise limited and want to recover small signals. I'm not sure how difficult it is to design dual band horn with good S11, for single band it's rather easy.
 

volker@muehlhaus

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how S11 is important for a receiving antenna?

You can do the math yourself, assuming a 50 Ohm receiver input. If S11 is -3dB, half of the received signal is reflected and missing.

However if we go into the details, input impedance of LNA in the receiver front end might be optimized for noise figure and not matched to 50 Ohm exactly.
 

denizduran

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So what you're saying essentially is, if the LNA input impedance is optimized for noise figure then the S11 of the receiver antenna is less important?
 

denizduran

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Can you explain what you meant by "However if we go into the details, input impedance of LNA in the receiver front end might be optimized for noise figure and not matched to 50 Ohm exactly"
 

volker@muehlhaus

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This means the LNA is designed for a 50 Ohm antenna.

You try to find excuses for bad S11, but truth is that antenna S11 matters for both transmit and receive case.
 

denizduran

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I am not trying to find excuses, I am just an intern trying to learn RF, my boss says that he wants to go with an antenna that has a worse S11 (-7 dB) and as a student who took Microwave Characterization and Design class I opposed to that idea. People on this platform are hear to learn from people's experiences and knowledge, not to find excuses?! Please change your attitude or do not even answer to peoples questions at all.
 

volker@muehlhaus

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Please accept my apologies. I tried to emphasize that signal loss from mismatch in the antenna is no different from mismatch loss in other parts of the circuit. You don't damage the receiver, but you loose signal.

Your S11 of -7dB means 1dB of loss from mismatch. Depending on your link margin, that might be acceptable or not.
 

BigBoss

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Can you explain what you meant by "However if we go into the details, input impedance of LNA in the receiver front end might be optimized for noise figure and not matched to 50 Ohm exactly"
Since the Optimum Noise Figure Match is desired, the Impedance which is seen by active device will absolutely be Optimum Noise Impedance and this impedance should be converted to conj(S(1,1)). So, the active device will see Optimum Noise Impedance, Input Impedance will be conj(S(1,1)).
Finally, max. Power Transfer would be realized AND Minimum Noise Figure would be obtained.
Therefore S(1,1) of an Antenna is important. It specifies Resonant Frequency and Source Impedance ( consider an antenna as a source )
 

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