Return loss

Key takeaways

Return loss belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Return loss to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Return loss from memory before moving on to harder problems.

Reference excerpt

In telecommunications, return loss is a measure in relative terms of the power of the signal reflected by a discontinuity in a transmission line or optical fiber. This discontinuity can be caused by a mismatch between the termination or load connected to the line and the characteristic impedance of the line. It is usually expressed as a ratio in decibels (dB):

RL ( dB ) = 10 log 10 ⁡ P i P r , {\displaystyle {\text{RL}}({\text{dB}})=10\log _{10}{\frac {P_{\text{i}}}{P_{\text{r}}}},}

where RL(dB) is the return loss in dB, Pi is the incident power, and Pr is the reflected power. Return loss is related to both standing wave ratio (SWR) and reflection coefficient (Γ). Increasing return loss corresponds to lower SWR. Return loss is a measure of how well devices or lines are matched. A match is good if the return loss is high. A high return loss is desirable and results in a lower insertion loss. From a certain perspective "return loss" is a misnomer. The usual function of a transmission line is to convey power from a source to a load with minimal loss. If a transmission line is correctly matched to a load, the reflected power will be zero, no power will be lost due to reflection, and "return loss" will be infinite. Conversely if the line is terminated in an open circuit, the reflected power will be equal to the incident power; all of the incident power will be lost in the sense that none of it will be transferred to a load, and RL will be zero. Thus the numerical values of RL tend in the opposite sense to that expected of a "loss".

Sign As defined above, RL will always be positive, since Pr can never exceed Pi. However, return loss has historically been expressed as a negative number, and this convention is still widely found in the literature. Strictly speaking, if a negative sign is ascribed to RL, the ratio of reflected to incident power is implied:

RL ′ ( dB ) = 10 log 10 ⁡ P r P i , {\displaystyle {\text{RL}}'({\text{dB}})=10\log _{10}{\frac {P_{\text{r}}}{P_{\text{i}}}},}

where RL′(dB) is the negative of RL(dB). In practice, the sign ascribed to RL is largely immaterial. If a transmission line includes several discontinuities along its length, the total return loss will be the sum of the RLs caused by each discontinuity, and provided all RLs are given the same sign, no error or ambiguity will result. Whichever convention is used, it will always be understood that Pr can never exceed Pi.

Electrical In metallic conductor systems, reflections of a signal traveling down a conductor can occur at a discontinuity or impedance mismatch. The ratio

Γ = V r V i {\displaystyle \Gamma ={\frac {V_{\text{r}}}{V_{\text{i}}}}}

of the amplitude of the reflected wave Vr to the amplitude of the incident wave Vi is known as the reflection coefficient. Return loss is the negative of the magnitude of the reflection coefficient in dB. Since power is proportional to the square of the voltage, return loss is given by

RL ( dB ) = − 20 log 10 ⁡ | Γ | , {\displaystyle {\text{RL}}({\text{dB}})=-20\log _{10}|\Gamma |,}

where the vertical bars indicate magnitude. Thus, a large positive return loss indicates that the reflected power is small relative to the incident power, which indicates good impedance match between transmission line and load. If the incident power and the reflected power are expressed in "absolute" decibel units, (e.g., dBm), then the return loss in dB can be calculated as the difference between the incident power Pi (in absolute dBm units) and the reflected power Pr (also in absolute dBm units):

RL ( dB ) = P i ( dBm ) − P r ( dBm ) . {\displaystyle {\text{RL}}({\text{dB}})=P_{\text{i}}({\text{dBm}})-P_{\text{r}}({\text{dBm}}).}

Optical In optics (particularly in fiber optics) a loss that takes place at discontinuities of refractive index, especially at an air–glass interface such as a fiber endface. At those interfaces, a fraction of the optical signal is reflected back toward the source. This reflection phenomenon is also called "Fresnel reflection loss", or simply "Fresnel loss". Fiber optic transmission systems use lasers to transmit signals over optical fiber, and a low optical return loss

ORL ( dB ) = 10 log 10 ⁡ P i P r {\displaystyle {\text{ORL}}({\text{dB}})=10\log _{10}{\frac {P_{\text{i}}}{P_{\text{r}}}}}

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Return loss

Start with the simplest possible case. Write down what Return loss claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Return loss before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Return loss ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Return loss

In research: Return loss appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Return loss in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Return loss is common in secondary-school and first-year university syllabi. It links to neighbouring topics Electrical parameters, Engineering ratios, Fiber optics, so understanding it makes those chapters shorter.
In everyday life: Look for Return loss outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Return loss in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Return loss means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Return loss out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Return loss in simple terms?

In telecommunications, return loss is a measure in relative terms of the power of the signal reflected by a discontinuity in a transmission line or optical fiber. This discontinuity can be caused by a mismatch between the termination or load connected to the line and the characteristic impedance of…

Why does Return loss matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Return loss?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Return loss.