On 9/28/2015 11:47 AM, Ian Jackson wrote:
In message , rickman writes



Definition of Return Loss

In technical terms, RL is the ratio of the light reflected back from a
device under test, Pout, to the light launched into that device, Pin,
usually expressed as a negative number in dB.

RL = 10 log10(Pout/Pin)

Here is a link for a table of return loss and VSWR....

http://www.jampro.com/uploads/tech_d.../VSWRChart.pdf

It shows a higher return loss (assuming you mean magnitude since the
values are all negative) for lower VSWR.

I'm surprised to see negative quantities. For 50 years, I've always
understood the Return Loss Ratio (RLR) to be exactly what it says on the
tin, ie the ratio (in dB) of the LOSS (the attenuation) of the reflected
signal wrt the incident signal. This is a +ve quantity. Things are
already sufficiently confusing without having to start thinking in
unnecessary -ve figures!



Ian,

Yes, you are correct. But the return signal can never be greater than
the incident signal, so the actual ratio of the reflected signal loss to
the incident signal must always be 1. And the log of a value 1 is a
negative number.

And yes, it is specified as a negative quantity because the result of
the calculation is a negative quantity. Physicists and engineers
specify loss as a negative number rather than confuse issues by changing
signs to suit them. Changing signs only leads to problems in later
calculations.

Non-professionals do change signs because it's often easier to
understand that a 10db loss is greater than a 5 db loss. Not so easy to
grasp quickly is that a -10db gain is more lossy (less gain) than a -5db
gain.

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