[SOLVED] Reflection coefficient & Retun Loss-Beginners questions

[themaccabee]

Hi,

I ve a basic doubt ie
Is reflection coefficient in dB is same as negative of return loss in dB?

The relflection coefficient is Γ=Vreflected/Vincident from wikipedia
Return Loss = -20*log( Γ)

From another webage i ve read that if the incident and reflected powers are expressed in dBm
then Return Loss=RL(dB)=Pi(dBm)-Pr(dBm)



I assume if 0dB reflection coefficient means the incident power is fully reflected
very negative reflection coefficient means very less power being reflected.
Btw high return loss means very low reflected power.
So is it simply

Reflection coefficient (in dB)=-Return loss (in dB)??
or diffferrent??
Thanks

 

[jayce3390]

You wrote yourself :
Return Loss = -20*log( Γ)

And IRL = Pi(dBm)-Pr(dBm) is also true.

 

[sweetchoto]

Hi themaccabee,

The relation bw Reflection coefficient and return loss is that Reflection coefficient is in terms of incident/reflected voltage whereas R.L is simply in terms of Incident/reflected power,that is the reason for R.L we take square of reflection coefficient to go into power and get the relation -20*log(reflection coefficient).

Here is a bit more explanation about Reflection coefficient:

|Reflection coefficient|= zL-Z0/ZL+Zo

There are always three cases:

1- Open circuit: In this case ZL=infinity and when you will put it in above expression and solve it you will get 1 or 0dB which means everything is reflected with no phase shift and nothing is being passed through.

2- Short circuit:In this case ZL= 0 and after putting it in above expression again and solving it you will get -1 which means again everything is reflected but with phase shift of 180 degrees.

3-Perfectly matched case: In this case ZL=Z0 and putting it in above expreassion and solving you will get 0 which means that nothing is reflected back and you will get maximum transmission.

So by negative reflection coefficient you cant say that very less power being reflected.

Hope it clears out your doubt.

/SC

 

[themaccabee]

So by negative reflection coefficient you cant say that very less power being reflected.

I meant negative reflection coefficient in dB...Cant i say for a negative reflection coefficient in dB very less power is being reflected.??

Thanks

 

[sweetchoto]

I meant negative reflection coefficient in dB...Cant i say for a negative reflection coefficient in dB very less power is being reflected.??

Thanks

Yes you are right. For example, R.L= -10dB =0.1 Which means that only 10% of the incident power will be lost due to mismatch and 90% will go through. That is the reason that in 'Antenna Theory' or' Microwave field' an antenna/Transmission line and circuit is said to be well matched if it has a return loss of -10dB or more(It is a standard).

Hope it clears out the misunderstanding now.

/SC

 

[themaccabee]

Thank you i hope its clear now