Return Loss Ratio

I am having a hard time finding a good definition of return loss ratio. They all use the ratio of the incident wave at the antenna to the reflected wave, but some are Pi - Pr and others are Pr - Pi. Reported values for the ratio in dB are all positive... except for the ones that are negative.

WTF!!!

There is even an IEEE paper on this in "IEEE Antennas & Propagation Magazine", but it is subscription and I'm not willing to pay $100 for this little gem of knowledge.

I have one reference which gives an equation for system loss

Ls = 10 log (pt/pa) = Pt - Pa, dB

where Ls is the loss in decibels, pt is the power delivered to the antenna terminals, pa is the power at the receiving antenna.

The equivalent equation for the return loss at the transmitting antenna would be

Lr = 10 log (pt/(pt-pr))

where pr is the power reflected. The equations I found for return loss are

Lr = 10 log (pt/pr)

and

Lr = 10 log (pr/pt)

Anyone know what is going on?

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Rick
Reply to
rickman
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Den mandag den 28. september 2015 kl. 19.55.07 UTC+2 skrev rickman:

if you can give a the full name of the paper, I can check it out, I probably have access

-Lasse

Reply to
Lasse Langwadt Christensen

formatting link

Definition and Misuse of Return Loss [Report of the Transactions Editor-in-Chief]

This will be greatly appreciated.

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Rick
Reply to
rickman

In theory, if the reflected signal is less than the incident signal the value should be positive (because it is called a loss).

The issue is that people often store the incident level as a reference in the power meter and then read the reflected power using the differential mode. That shows a negative value. They then forget to remove the minus sign.

Bob

Reply to
bob.quintal

I sent it, it is only 2 pages

-Lasse

Reply to
Lasse Langwadt Christensen

Return loss is

RL = -20log10|Gamma| dB

Gamma := reflection coefficient

Gamma is a ratio.

Gamma = Vo-/Vo+ = (ZL-Zo)/(ZL+Zo) (relected voltage wave over the inci dent voltage wave.)

Pi = |Vo+|^2/(2Zo) Pr = |Vo-|^2/(2Zo)

As seen, the i & r voltage waves are related to i & r powers, of course.

The reason for negative return loss is usually a mistake. That usually sho uld be called "return gain." To say *loss* is positive is to say there is actual loss. Negative loss is actual gain. Some people get pissy when you point out the mistake in sign. If all other ports are terminated in their characteristic impedance, and you have an actual negative return loss, that means more power is available from the port than what you put in. You'd h ave a way to make an oscillator. It implies an "internal" energy source.

The expression on the far right is obviously wrong.

re

PL is power delivered to the load GammaL is the reflection coefficient of the load

PL = Pi - Pr = Pi(1 - |GammaL|^2)

so

Pr/Pi = |GammaL|^2

|GammaL| = sqrt(Pr/Pi)

RL = -20log|GammaL| = -20log(sqrt(Pr/Pi)) = -10log(Pr/Pi)

Reply to
Simon S Aysdie

Thanks. I've already shared some quotes from it.

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Rick
Reply to
rickman

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