---
title: "Role of characteristic impedance"
subject: "Telecommunications"
chapter: 2
key: "semester_8/telecommunications:characteristic-impedance-role"
source: https://www.bctengineer.com/q/semester_8/telecommunications/characteristic-impedance-role
site: BCT Engineering
---

# Role of characteristic impedance

Telecommunications · Chapter 2: Transmission Media

- Asked in: 2069 Chaitra, 2 marks
- Video: [Transmission Line Characteristic Impedance](https://www.youtube.com/watch?v=R8U8JzoQtKI) — Altium Academy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/telecommunications/characteristic-impedance-role
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-2

## Question

Explain the role of characteristic impedance in the flow of radio signal from one equipment to another equipment.

## Short answer

Characteristic impedance is the voltage-to-current ratio of a travelling wave on an infinite line. If source, line and load are matched, all power is delivered and no reflection occurs; mismatch creates reflections, standing waves, power loss, echo and possible transmitter damage. Cables and equipment are designed to a common Z0.

## Answer

**Characteristic impedance** $Z_0$ is the ratio of voltage to current of a wave travelling along an infinitely long line:
$$
Z_0 = \sqrt{\frac{R + j\omega L}{G + j\omega C}}
$$
(≈ 50 Ω or 75 Ω for coax, ≈ 100 Ω for twisted pair, ≈ 600 Ω for telephone lines).

Its role in carrying a signal from one equipment to another:
- If the source, line and load are all **matched** ($Z_S = Z_0 = Z_L$), all the power is delivered to the load and nothing is reflected.
- A mismatch causes **reflection** (reflection coefficient $\Gamma = \frac{Z_L - Z_0}{Z_L + Z_0}$), standing waves (high VSWR), power loss, echo and ghost images, and in transmitters even damage.
- So cables, connectors and equipment ports are all designed for the same $Z_0$.

## Check yourself

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

1. What is characteristic impedance Z0?
   A. The load resistance at the receiver
   B. The ratio of voltage to current of a wave travelling on an infinitely long line
   C. The total DC resistance of the cable
   D. The source impedance only
   Answer: B. Z0 = sqrt((R + jwL)/(G + jwC)) is the V/I ratio of a travelling wave on an infinitely long line. (Key point: Definition of Z0)

2. What happens when Z_S = Z0 = Z_L?
   A. Full reflection occurs
   B. Standing waves are maximum
   C. All power is delivered to the load and nothing is reflected
   D. Only half the power is delivered
   Answer: C. With a matched line there is no reflection, so maximum power reaches the load. (Key point: Matched condition)

3. Which is a result of an impedance mismatch?
   A. Zero reflection coefficient
   B. Reflection, high VSWR, echo and power loss
   C. Lower VSWR
   D. Higher bandwidth
   Answer: B. Gamma = (Z_L - Z0)/(Z_L + Z0) is non-zero, giving reflections, standing waves, echo and loss, even transmitter damage. (Key point: Mismatch effects)

## Related in Telecommunications

- [Radio signal in twisted pair and coaxial cable](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aradio-signal-twisted-coaxial) — Telecommunications, chapter 2
- [Guided and unguided media for radio signals](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aradio-guided-unguided-compare) — Telecommunications, chapter 2

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