# Just Wanna to Counsult for Return Loss - RF Cafe Forums

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ngks
Post subject: Just wanna to counsult for return loss
Unread postPosted: Thu May 19, 2005 4:41 am

Dear all,

May i know what is the meaning of return loss for S11,S12,S21 and S22?

Thanks

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Guest
Post subject:
Unread postPosted: Thu May 19, 2005 9:16 am

Return loss is only applicable to S11 and S22, in fact S11 and S22 are return loss. Return loss is the ratio of reflected power to incident power.

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Jeanalmira
Post subject:
Unread postPosted: Fri May 20, 2005 1:59 am
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Joined: Tue Mar 15, 2005 11:43 pm
Posts: 65
Location: Singapore
Hi :

I agree with the previous reply. S11 is input return loss and S22 is output return loss. That usually calculated in dB.

This input and output return losses are caused by the impedance mismatch. That's why matching circuit comes to the picture.

and it's also related to VSWR (Voltage Standing Wave Ratio).

I hope it helps.

Regards,
Jean

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ngks
Post subject: am i right?
Unread postPosted: Fri May 20, 2005 5:16 am

Thanks to all,

In order to measure the S-parameter, How many source should us provided?

Is it two? if yes, means

input source to measurement S11 and S21

Output source is to measure S22, s12

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Guest
Post subject:
Unread postPosted: Fri May 20, 2005 1:04 pm

Hello,

You need 2 sources to measure S11, S122 and also the 2 other S-Parameters, S12, S21.

Each measurement will give you a set of 2 parameters, with one port terminated (input or output), while a source is connected to the other one.

The S-Parameters are related to each other like that:

b1=S11*a1+S12*a2
b2=S22*a2+S21*a1

Where b1,2 are the reflected waves, a1,2 are the incident waves.

From these 2 equations you can get the ratios:

S11=b1/a1; a2=0 (Output port is terminated)
S12=b1/a2; a1=0 (Input port is terminated)
S21=b2/a1; a2=0 (Output port is terminated)
S22=b2/a2; a1=0 (Input port is terminated)

Hope this helps.

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ngks
Post subject: higher value or lower value for return loss
Unread postPosted: Tue Jun 21, 2005 12:11 am

which return loss value is more better, higher value or lower value?

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Jeanalmira
Post subject:
Unread postPosted: Tue Jun 21, 2005 12:59 am
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Joined: Tue Mar 15, 2005 11:43 pm
Posts: 65
Location: Singapore
Hi!

For return loss values, the more negative is the better.
For example :
-15 dB is better than -10dB.

In other words, the better your matching networks, there will be lesser loss, so the loss values will be more negative in a sense.

I hope it helps. :D

Regards,
Jean

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Guest
Post subject:
Unread postPosted: Mon Jun 27, 2005 8:42 am

The definition of Return Loss is as follows:

-20*log(S11) or S22, whichever way you're looking.

Therefore, you would want return loss to be as high as possible.
If you are talking about just changing S11 to dB (take the minus sign out of the above equation), that is not technically return loss. For that, you would want it to be as negative as possible.
It's just terminology, though, so I'm just nitpicking. Some people get rather upset, however, when you say you have a negative return loss :)

Hope that helps,
--Dan

Posted  11/12/2012

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