What exactly is the return loss?

what's return loss?

what's return loss? thanks!

Re: what's return loss?

Very easy question.
The reflected power !
more specifically, the reflected power compared to the incident one, expressed in dB.

|RL|=10log(Power Reflected/Power Incident)

A little incongruence about the slang exist about an eventual sign.
As the nomenclature suggest for a passive device the loss is positive signed. But very often the RL is furnished with negative sign.
The user shoud have enough intuition to consider that are the same.

Also another little incongruence may be found in some pubblications that say
RL=20log |s11|.
I don't agree totally, i think is more correct write:
RL= 20 log |Γ|
where Γ is the complex reflection coefficient. Γ=s11 only when all other ports of the Device are closed to a non reflecting load.

Re: what's return loss?

sergio mariotti said:

Very easy question.
Also another little incongruence may be found in some pubblications that say
RL=20log |s11|.
I don't agree totally, i think is more correct write:
RL= 20 log |Γ|
where Γ is the complex reflection coefficient. Γ=s11 only when all other ports of the Device are closed to a non reflecting load.

Click to expand...

I agree with sergio, RL=20log |s11| is a wrong definition !

Re: what's return loss?

Yes, RL= 20 log |Γ| is absolutely the original definition!

Re: what's return loss?

What is the relationship between return loss and the reflection coeficient as it is defined for transmission lines (Z2-Z1)/(Z2+Z1) ?

Re: what's return loss?

The reflection coeficient as (Z2-Z1)/(Z2+Z1) and the return loss RL=-20 log |(Z2-Z1)/(Z2+Z1)|

If Z2 is load impedance and Z1 is characteristic impedance of transmission lines

what's return loss?

reflection coefficient is ratio of incident field to reflected field introduced by mismatch impedance.

reflection coefficient = Γ = Eincident/Ereflected
= (Z2-Z1)/(Z2+Z1)

Return loss is ratio of incident signal's power to reflected signal's power

Return loss = reflection coefficient is ratio of incident field to reflected field introduced by mismatch impedance.

reflection coefficient = Γ^2

what's return loss?

thank you all!
return loss is defined as the ratio of the incident power to the reflected power at any point on the transmission line,expressed in dB and is equal to
Return Loss=10log(Pi/Po)=-200log|Γ|.

Added after 1 minutes:

thank you all!
return loss is defined as the ratio of the incident power to the reflected power at any point on the transmission line,expressed in dB and is equal to
Return Loss=10log(Pi/Po)=-20log|Γ|.

what's return loss?

Return loss (RL) is a very important noise measurement recently defined for local area networks (LANs) and LAN components. This statement raises an obvious question: Since LANs have been around for years and RL is very important, then why is RL relatively new and just now being defined?


The answer is simple but not obvious. Engineers designing LAN systems and components have known that RL is present, but up to now it did not matter in their designs, so they did not bother defining or quantifying it. Now, with faster and bidirectional signaling being implemented, RL matters.


Return Loss did not previously matter because most LANs have operated using Token Ring or 10 Base T protocols. Both of these signaling schemes use two pairs of conductors—one pair to send data and the other to receive. In each scheme, signal energy travels unidirectionally from a transmitter at one end of each pair to a receiver at the other end. Thus, only two pairs of the commonly-used four pair cables are actually energized. Before we get into detailed discussions on RL, we should briefly explain the differences between unidirectional and bidirectional signaling.

Re: what's return loss?

All right that's a good point. What can you say about this?

Re: what's return loss?

I believe that you are making a mistake!
RL was and is, until now, a relatevely important parameter in unidirectional cable systems.
It is defiened in EIA/TIA 568 and is incuded in all serious cabling testing measurement parameters
in order to certify a cabling system.
Return Loss (RL) is a measure of all reflections that are caused by the impedance mismatches at all locations along the link and is expressed in decibel (dB). Return Loss is of particular concern in the implementation of Gigabit Ethernet.
(see **broken link removed**)

Concerning the bidirectional systems the RL parameter is not either a relatevely big problem to have in mind! Alien cross-talk (or cable-to-cable cross-talk) is the main electrical parameter limiting the performance of the structured cabling system when applied to 10 Gigabit transmission rates.

I read carefully the article at **broken link removed** a part of which is joelo post (!!!!) and it is obvious that they agree with that!