---
title: "Guided vs unguided media; 2-wire to 4-wire"
subject: "Telecommunications"
chapter: 2
key: "semester_8/telecommunications:guided-unguided-two-four-wire"
source: https://www.bctengineer.com/q/semester_8/telecommunications/guided-unguided-two-four-wire
site: BCT Engineering
---

# Guided vs unguided media; 2-wire to 4-wire

Telecommunications · Chapter 2: Transmission Media

- Asked in: 2078 Bhadra, 3+4 marks; 2075 Chaitra, 2+6 marks (asked 2 times)
- Video: [Guided Vs Unguided Transmission Media | Differences & Comparison | Types of Transmission Media](https://www.youtube.com/watch?v=vUDtFjiNib4) — About Technology
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/telecommunications/guided-unguided-two-four-wire
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-2

## Question

Compare and contrast between guided and unguided media with examples. Describe the need and operating mechanism of the two-wire to four-wire hybrid converter with the help of neatly labeled diagram.

## Short answer

Guided media (twisted pair, fibre) confine signals along a physical path; unguided media (radio, microwave) radiate through space. The hybrid is needed because the 2-wire local loop must connect to 4-wire long-distance circuits using one-way amplifiers. It uses a centre-tapped transformer and balance network to separate directions, cancelling leakage when impedances match.

## Answer

#### Guided vs unguided media
| Point | Guided media | Unguided media |
|---|---|---|
| Definition | Signal travels along a solid physical path | Signal radiates through free space |
| Examples | Twisted pair, coaxial cable, optical fibre | Radio waves, microwave, satellite, infrared |
| Direction | Confined to the cable | Broadcast or beamed by antennas |
| Capacity | Very high in fibre (Tbps) | Limited by spectrum |
| Impairments | Attenuation, crosstalk | Fading, multipath, rain, interference |
| Cost | Cable laying costly | Towers/antennas; cheap for remote areas |
| Mobility | No | Yes |

#### Need for a two-wire to four-wire hybrid converter
- The local loop uses **two wires** for both directions to save copper.
- Long-distance transmission needs **four-wire** operation because **amplifiers, repeaters, multiplexers and PCM codecs are one-directional**.
- Hence at the junction a **hybrid** separates the two directions of the 2-wire line into separate "go" and "return" paths, without letting the received signal leak back into the transmit path (which would cause echo and singing).

#### Operating mechanism
```text
                    4-wire receive (from far end)
                         │  (amplifier)
                         v
              ┌──────────────────────┐
  2-wire  ────┤ T1             T2    │
  line   ─────┤  (hybrid transformer)│
  (Z_L)       │   centre-tapped      │
              │   windings           │
  Balance ────┤                      ├───> 4-wire transmit
  network Z_B └──────────────────────┘     (to far end)
```

**Operation of the hybrid (bridge principle):**
1. **Receive → 2-wire:** a signal arriving on the 4-wire receive pair is fed to the centre-tapped winding. It divides equally between the **2-wire line ($Z_L$)** and the **balance network ($Z_B$)**. Half the power reaches the line (3 dB loss).
2. **No leakage to transmit pair:** the two halves flow in opposite senses through the windings coupled to the transmit pair, so their magnetic effects cancel when $Z_B = Z_L$. Ideally no receive signal appears on the transmit side.
3. **2-wire → transmit:** speech from the 2-wire line divides between the transmit and receive ports; the transmit port gets half (3 dB loss), and the part going to the receive port is absorbed by the amplifier output and does no harm.
4. **Balance:** in practice $Z_B$ only approximates $Z_L$ over the band, so a small signal leaks across. This is measured by the **balance return loss**
$$
B = 20\log_{10}\left|\frac{Z_L + Z_B}{Z_L - Z_B}\right|\ \text{dB}
$$
Poor balance causes **echo**, and if the loop gain round the four-wire circuit exceeds unity, **singing** (oscillation).

Typical loss through the hybrid is about 3.5 dB in each direction, and good balance gives a transhybrid loss of 20 dB or more.

## Check yourself

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

1. Which pair correctly lists examples?
   A. Guided: satellite; unguided: optical fibre
   B. Guided: twisted pair, coaxial, fibre; unguided: radio, microwave, satellite
   C. Both: coaxial and microwave
   D. Guided: infrared; unguided: twisted pair
   Answer: B. Guided media use a physical path; unguided radiate through free space. (Key point: Guided vs unguided)

2. Why is a hybrid needed at the junction of the loop and the long-distance circuit?
   A. Loop is two-wire but amplifiers and codecs are one-directional, so directions must be separated without leakage
   B. To raise the voltage
   C. To add ringing
   D. To increase bandwidth
   Answer: A. The hybrid separates go and return paths while preventing the received signal reaching the transmit path. (Key point: Need for hybrid)

3. How much power of the 4-wire receive signal reaches the 2-wire line?
   A. All of it
   B. Half, a 3 dB loss, since it divides between the line and balance network
   C. One tenth
   D. None
   Answer: B. The signal divides equally between Z_L and Z_B, so half reaches the line. (Key point: Hybrid loss)

## Related in Telecommunications

- [Wired vs wireless media; four-wire and hybrid](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Awired-wireless-four-wire-hybrid) — Telecommunications, chapter 2, asked 2×
- [Noise and two-wire to four-wire converter](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Anoise-two-four-wire-converter) — Telecommunications, chapter 2
- [Media types, four-wire system and hybrid](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amedia-types-four-wire-hybrid) — 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
- [Guided and unguided transmission media](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atransmission-media-explain) — Telecommunications, chapter 2, asked 2×
- [Guided media comparison and hybrid operation](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aguided-media-hybrid-operation) — Telecommunications, chapter 2

## Related in other subjects

- [Guided vs unguided transmission media](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aguided-vs-unguided-media) — Data Communication, chapter 4
- [Guided and other transmission media](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Atransmission-media-explain) — Computer Networks and Security, chapter 2, asked 3×
- [Compare and contrast pollution in a city](https://www.bctengineer.com/question.md?key=semester_5%2Fcommunication_english%3Acompare-contrast-pollution-city-nepal) — Communication English, chapter 1
- [Physical description of guided media](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aguided-media-description) — Data Communication, chapter 4, asked 2×