--- title: "Media classes and hybrid transformer" subject: "Telecommunications" chapter: 2 key: "semester_8/telecommunications:media-classes-hybrid-transformer" source: https://www.bctengineer.com/q/semester_8/telecommunications/media-classes-hybrid-transformer site: BCT Engineering --- # Media classes and hybrid transformer Telecommunications · Chapter 2: Transmission Media - Asked in: 2081 Chaitra, 6+4 marks - Video: [Topic 7 Transmission Media Guided)](https://www.youtube.com/watch?v=g3S4xtxB4zg) — JPSays.com - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/media-classes-hybrid-transformer - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-2 ## Question List out different types of transmission media with their classes and basic characteristics. Describe the working principle of a Hybrid Transformer. ## Short answer Guided media include twisted pair, coaxial, optical fibre, and open wire; unguided media include radio, microwave, satellite, and infrared. A hybrid transformer converts two-wire to four-wire and back using a centre-tapped transformer and balance network; when balance impedance equals line impedance, receive signal cancels in the transmit path. ## Answer #### Transmission media: classes and characteristics | Class | Medium | Basic characteristics | |---|---|---| | Guided | Twisted pair (UTP/STP) | Cheap; up to a few MHz (loop) or 100s of MHz (LAN, short); high attenuation; crosstalk | | Guided | Coaxial cable | Shielded; bandwidth up to ~1 GHz; less noise; used in CATV, feeders | | Guided | Optical fibre (SMF/MMF) | Light by total internal reflection; loss ~0.2–0.5 dB/km; Tbps capacity; EMI immune | | Guided | Open wire lines | Old rural telephone lines; weather affected | | Unguided | Radio (HF/VHF/UHF) | 3 MHz–3 GHz; omnidirectional; HF sky wave long range; fading | | Unguided | Terrestrial microwave | 1–40 GHz; line of sight; repeaters 40–60 km; rain fade | | Unguided | Satellite | GEO at 36,000 km; wide coverage; ~250 ms delay | | Unguided | Infrared / free-space optics | Short range; line of sight; no licence | **General characteristics to compare:** bandwidth/data rate, attenuation per km, repeater spacing, noise immunity, security, cost and installation, mobility support. #### Working principle of a hybrid transformer A **hybrid transformer** converts a **two-wire** circuit (both directions on one pair) into a **four-wire** circuit (separate transmit and receive pairs) and back, while preventing the received signal from passing into the transmit path. ```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 is guided? A. Terrestrial microwave B. Open wire lines C. Satellite D. Infrared free-space optics Answer: B. Open wire lines, twisted pair, coaxial and fibre are guided; radio, microwave, satellite and infrared are unguided. (Key point: Classes of media) 2. What does a hybrid transformer do? A. Converts between 2-wire and 4-wire circuits while preventing the received signal passing into the transmit path B. Amplifies speech C. Generates DTMF D. Multiplexes channels Answer: A. It separates directions of the 2-wire line into separate transmit and receive paths. (Key point: Hybrid transformer function) 3. What does poor balance (Z_B not equal to Z_L) cause? A. Echo, and singing if loop gain exceeds unity B. Increased bandwidth C. No effect D. Better balance return loss Answer: A. Leakage across the hybrid causes echo, and oscillation (singing) if loop gain round the four-wire circuit exceeds unity. (Key point: Poor balance consequences) ## Related in Telecommunications - [Guided media comparison and hybrid operation](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aguided-media-hybrid-operation) — 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× - [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 transmission media](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atransmission-media-explain) — Telecommunications, chapter 2, asked 2× - [Crosstalk causes and hybrid transformer](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Acrosstalk-hybrid-transformer) — Telecommunications, chapter 2 - [Role of hybrid transformer](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ahybrid-transformer-role) — Telecommunications, chapter 2 ## Related in other subjects - [Transmission characteristics of guided media](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aguided-media-transmission-characteristics) — 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× - [Define transmission media](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Adefine-transmission-media) — Computer Networks and Security, chapter 2, asked 2× - [Guided vs unguided transmission media](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aguided-vs-unguided-media) — Data Communication, chapter 4