---
title: "Radio signal in twisted pair and coaxial cable"
subject: "Telecommunications"
chapter: 2
key: "semester_8/telecommunications:radio-signal-twisted-coaxial"
source: https://www.bctengineer.com/q/semester_8/telecommunications/radio-signal-twisted-coaxial
site: BCT Engineering
---

# Radio signal in twisted pair and coaxial cable

Telecommunications · Chapter 2: Transmission Media

- Asked in: 2069 Chaitra, 2 marks
- Video: [communication medium twisted coaxial fiber optics wireless radio  infra computer network lecture 8](https://www.youtube.com/watch?v=SaehKwB47Xs) — History Science and Fact
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/telecommunications/radio-signal-twisted-coaxial
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-2

## Question

Explain the transmission of radio signal in twisted pair and coaxial cable in case of telecommunications.

## Short answer

Twisted pair carries the signal as a voltage difference between two balanced wires; twisting makes induced noise equal in both and cancels it, limiting use to about 100 MHz over short distances. Coaxial cable confines the field between centre conductor and shield, giving little radiation or pick-up, fixed 50/75 ohm impedance, and RF operation up to several GHz.

## Answer

Radio/electrical signals travel along guided copper media as an electromagnetic wave between two conductors.

- **Twisted pair:** two insulated wires are twisted together. The signal travels as a voltage difference between the wires (balanced line). Twisting makes noise picked up by both wires nearly equal, so it cancels at the receiver, and it reduces crosstalk. Loss and radiation rise quickly with frequency, so it suits voice, DSL and LAN signals up to about 100 MHz over short distances.
- **Coaxial cable:** a centre conductor surrounded by a dielectric and an outer braided shield. The field is confined between the conductors (unbalanced line), so there is very little radiation or pick-up. It has a fixed characteristic impedance (50 Ω or 75 Ω) and carries RF up to several GHz, used for antenna feeders, cable TV and old carrier trunks.

## Check yourself

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

1. Why does twisting the two wires of a twisted pair reduce noise?
   A. Noise is absorbed by the insulation
   B. Noise picked up is nearly equal on both wires, so it cancels at the receiver (balanced line)
   C. Twisting increases the characteristic impedance to infinity
   D. Twisting shields the wires with a metal braid
   Answer: B. The signal is a voltage difference between wires, so equal noise on both is cancelled in the difference. (Key point: Noise cancellation in twisted pair)

2. Why does coaxial cable have little radiation and pick-up?
   A. It is a balanced line
   B. It uses twisting
   C. Its signal is carried in the glass
   D. The field is confined between the centre conductor and the outer shield
   Answer: D. The outer braided shield confines the field between the conductors (unbalanced line). (Key point: Coax field confinement)

3. Which statement about frequency use is correct?
   A. Twisted pair suits short distances up to about 100 MHz; coax carries RF up to several GHz
   B. Twisted pair is used for antenna feeders up to GHz
   C. Coax loss rises faster than twisted pair so it is limited to voice
   D. Both carry the same maximum frequency
   Answer: A. Twisted pair loss and radiation rise quickly with frequency; coax with fixed 50/75 ohm impedance handles several GHz. (Key point: Frequency suitability)

## Related in Telecommunications

- [Full duplex on a single twisted pair](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atwisted-pair-full-duplex-converter) — 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

## Related in other subjects

- [Compare twisted pair, coaxial, fiber](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Acompare-twisted-coax-fiber) — Computer Networks and Security, chapter 2
- [Optical fiber advantages over coax and twisted pair](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Afiber-advantages) — Data Communication, chapter 4, asked 3×
- [Why wires are twisted in twisted pair](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Atwisted-pair-why-twist) — Data Communication, chapter 4
- [Coaxial cable vs optical fiber](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Acoax-vs-fiber) — Data Communication, chapter 4