--- title: "Guided media comparison and hybrid operation" subject: "Telecommunications" chapter: 2 key: "semester_8/telecommunications:guided-media-hybrid-operation" source: https://www.bctengineer.com/q/semester_8/telecommunications/guided-media-hybrid-operation site: BCT Engineering --- # Guided media comparison and hybrid operation Telecommunications · Chapter 2: Transmission Media - Asked in: 2080 Chaitra, 4+4 marks - Video: [Transmission Media In Hindi | Guided and Unguided Media](https://www.youtube.com/watch?v=WuQKlGaa9Bg) — Explore - The Knowledge Tv - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/guided-media-hybrid-operation - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-2 ## Question Compare guided transmission media in telecommunication system. Explain operation of hybrid transformer. ## Short answer Twisted pair is cheap with low bandwidth and high attenuation; coaxial has medium bandwidth and good shielding; optical fibre has very high bandwidth, low loss, and EMI immunity. The hybrid transformer separates two-wire and four-wire directions; received power splits equally between line and balance network, cancelling leakage to the transmit path when balanced. ## Answer #### Comparison of guided transmission media | Property | Twisted pair | Coaxial cable | Optical fibre | |---|---|---|---| | Signal | Electrical | Electrical | Light | | Bandwidth | Low (kHz–100s MHz) | Medium (up to ~1 GHz) | Very high (THz) | | Attenuation | High (several dB/km at kHz–MHz) | Medium | Very low (~0.2 dB/km) | | Repeater spacing | ~2–5 km | ~1–10 km | 50–100+ km | | Noise/EMI | Susceptible; crosstalk | Good shielding | Immune | | Security | Easy to tap | Moderate | Very hard to tap | | Cost | Cheapest | Moderate | Cable cheap, terminal costly | | Weight/size | Light | Bulky | Very light, thin | | Uses | Local loop, LAN | CATV, antenna feeder | Backbone, FTTH, undersea | #### Operation of hybrid transformer ```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). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which statement correctly compares optical fibre with twisted pair? A. Fibre has very low attenuation (~0.2 dB/km) and is immune to EMI, but its terminal equipment is costly B. Fibre has higher attenuation but is cheaper to terminate C. Fibre is easy to tap and susceptible to crosstalk D. Fibre carries electrical signals with kHz bandwidth Answer: A. Fibre carries light with THz bandwidth, very low loss, EMI immunity and high security; only terminals are expensive. (Key point: Fibre vs copper) 2. Which ordering of typical repeater spacing is correct (smallest to largest)? A. Optical fibre, coaxial, twisted pair B. Coaxial, optical fibre, twisted pair C. Twisted pair, coaxial, optical fibre D. Twisted pair, optical fibre, coaxial Answer: C. Twisted pair ~2-5 km, coax ~1-10 km, fibre 50-100+ km because attenuation falls in that order. (Key point: Repeater spacing) 3. Why does a signal from the 4-wire receive pair not appear on the transmit pair of a balanced hybrid? A. The amplifier on the receive pair blocks it B. Its two halves flow in opposite senses through the windings coupled to the transmit pair, so their fluxes cancel when Z_B = Z_L C. It is filtered out because it uses a different frequency band D. The transmit pair is shorted by the balance network Answer: B. The signal splits equally into Z_L and Z_B; with Z_B = Z_L the opposing magnetic effects cancel in the transmit winding. (Key point: Why no leakage) 4. What happens when the balance network Z_B only approximates the line impedance Z_L? A. A small signal leaks across to the transmit side, measured by the balance return loss, causing echo B. The hybrid stops working completely C. The line loses 3 dB more in every direction D. Noise is removed from the line Answer: A. Z_B is only approximately equal to Z_L over the band, so some receive signal leaks into the transmit path; poor balance gives echo and possibly singing. (Key point: Imperfect balance) ## Related in Telecommunications - [Guided and unguided transmission media](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atransmission-media-explain) — Telecommunications, chapter 2, asked 2× - [Media classes and hybrid transformer](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amedia-classes-hybrid-transformer) — 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 vs unguided media; 2-wire to 4-wire](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aguided-unguided-two-four-wire) — Telecommunications, chapter 2, asked 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 - [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× ## 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× - [Transmission characteristics of guided media](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aguided-media-transmission-characteristics) — Data Communication, chapter 4 - [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×