---
title: "Sources of attenuation and distortion"
subject: "Telecommunications"
chapter: 2
key: "semester_8/telecommunications:attenuation-distortion-sources"
source: https://www.bctengineer.com/q/semester_8/telecommunications/attenuation-distortion-sources
site: BCT Engineering
---

# Sources of attenuation and distortion

Telecommunications · Chapter 2: Transmission Media

- Asked in: 2073 Shrawan, 8 marks
- Video: [Types of Signal Impairment: Attenuation, Distortion & Noise | Computer Network Course | Lecture 17](https://www.youtube.com/watch?v=4O6yyHxJDDI) — Mind Your Exam Channel
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/telecommunications/attenuation-distortion-sources
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-2

## Question

What are the sources of attenuation and distortion? Explain with neat diagram.

## Short answer

Attenuation sources include conductor loss (skin effect increases resistance with frequency), dielectric leakage, radiation, free-space spreading, and poor joints. Distortion sources are amplitude distortion (attenuation varies with frequency), delay distortion (velocity varies), non-linear distortion from amplifiers, noise/crosstalk, and echo.

## Answer

**Attenuation** is the loss of signal power as it travels along a transmission medium; **distortion** is the change in the *shape* of the signal because different frequency components are treated differently by the medium. Both limit how far a signal can go before it must be amplified or regenerated.

#### Sources of attenuation
A telephone line or cable is a distributed network of series resistance $R$, series inductance $L$, shunt capacitance $C$ and shunt conductance $G$ per unit length.

```text
   R dx     L dx
 o--/\/\/--mmmm--+----+--o
                 |    |
              C dx  G dx
                 |    |
 o---------------+----+--o
   one small section dx of a line
```

1. **Conductor (copper) loss** – $I^2R$ heating in the wire resistance. Resistance rises with frequency due to the **skin effect**, so loss rises with frequency.
2. **Dielectric (leakage) loss** – current leaking through the insulation (conductance $G$), which also rises with frequency.
3. **Radiation loss** – part of the energy is radiated away, mainly from open-wire lines and unshielded cables at high frequency.
4. **Spreading / free-space loss** – in radio links power spreads over a larger area ($\propto d^2$); rain, fog and atmospheric gases add absorption.
5. **Coupling and joint losses** – poor joints, connectors, splices and impedance mismatch (reflection) waste power.

Attenuation is expressed in dB: $\alpha = 10\log_{10}(P_{in}/P_{out})$ dB, and per km of cable. On a matched line the attenuation constant is the real part of $\gamma = \sqrt{(R+j\omega L)(G+j\omega C)}$.

#### Sources of distortion
1. **Attenuation (amplitude) distortion** – $\alpha$ varies with frequency, so high voice frequencies are lost more than low ones and the received waveform changes shape.
2. **Delay (phase) distortion** – the velocity varies with frequency, so components arrive at different times. It is serious for data and pulses (causes inter-symbol interference).
3. **Non-linear distortion** – amplifiers and saturating devices create harmonics and intermodulation products.
4. **Noise and crosstalk** – thermal noise, impulse noise and coupling from neighbouring pairs add unwanted signals.
5. **Echo** – reflections at hybrids or mismatched points return delayed copies of the signal.

```text
 sent pulse        received pulse
   +---+               __
   |   |             _/  \__
 --+   +--   ==>  __/       \____
                 (attenuated, spread out)
```

#### Remedies
- **Loading coils** (Heaviside condition $R/L = G/C$) reduce attenuation distortion on voice cables.
- **Equalisers** flatten amplitude and delay response.
- **Repeaters/regenerators** restore level (analogue) or reshape pulses (digital).
- **Impedance matching** removes reflections and echo.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why does conductor loss rise with frequency?
   A. Dielectric becomes a conductor
   B. Skin effect increases wire resistance at higher frequency
   C. Capacitance falls with frequency
   D. Inductance cancels resistance
   Answer: B. Skin effect raises R with frequency, so I^2R loss increases. (Key point: Skin effect)

2. How does distortion differ from attenuation?
   A. Distortion changes the shape of the signal because frequency components are treated differently; attenuation is loss of power
   B. They are the same thing
   C. Distortion is loss of power; attenuation changes shape
   D. Distortion happens only in radio
   Answer: A. Attenuation reduces power; distortion alters waveform shape. (Key point: Attenuation vs distortion)

3. What does delay (phase) distortion cause in data pulses?
   A. A uniform loss of power
   B. Harmonics and intermodulation products
   C. Frequency components arrive at different times, causing inter-symbol interference
   D. Reflections at joints
   Answer: C. Phase velocity varies with frequency, spreading pulses and causing ISI. (Key point: Delay distortion)

4. What does the Heaviside condition for loading coils require?
   A. R = G
   B. L = C
   C. RC = LG
   D. R/L = G/C
   Answer: D. R/L = G/C gives a distortionless line, reducing attenuation distortion on voice cables. (Key point: Loading coils)

## Related in Telecommunications

- [Impairments to voice channel transmission](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Avoice-channel-impairments) — Telecommunications, chapter 2

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