About matching between antenna and LNA

[pavel_adameyko]

Dear all
Imagine the folllowing: we have passive antenna which feeds receiver's LNA input without any filter between them. It appears that S21,S12,S22 and NF of LNA are good, but S11 is very poor. I wonder if we would observe any negative effects due to poor LNA matching in such receiver.
Any opinions would be appreciated

 

[BigBoss]

The overall sensitivity of the receiver will drop because the energy coming from antenna will return back to antenna due to poor S11..

 

[pavel_adameyko]

The overall sensitivity of the receiver will drop because the energy coming from antenna will return back to antenna due to poor S11..

Hi BigBoss
As i know the overall sensitivity is determined by NF of receiver. Surely we can improve NF of receiver by matching input of LNA, so S21 would increase, therefore the noise contribution of subsequenr stages to overall NF would decrease. But what if we have acceptable overall NF even with poor matching at input?

One negative effect i think up reading your post is that antenna would re-radiate the signal, so it would disturb another receivers working near the one we discussed. Did anybody face such effects?

 

[tony_lth]

You can calculate the reverse radiation.
In telecomm system, there is same channel interference and adjecent channel interference, and the specification is about -30dB, if I remember correctly.
So you need to judge whether it is a problem or not.

 

[vfone]

I would recomend to do first a conducted system sensitivity (without antenna) and after that do a radiated sensitivity using the antenna in place.
For radiated test you need some knowledge how to calibrate the setup.
After that you can compare the results and decide if the system needs or not NF improvements (matching the LNA input for best NF when antenna is connected).

 

[pavel_adameyko]

Hi, vfone
Could you explain more about conducted system sensitivity and radiated one? Or any links ?
Thanks