--- title: "Multiple hop and atmospheric ducts" subject: "Propagation and Antenna" chapter: 4 key: "semester_6/propagation_and_antenna:multiple-hop-atmospheric-ducts" source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/multiple-hop-atmospheric-ducts site: BCT Engineering --- # Multiple hop and atmospheric ducts Propagation and Antenna · Chapter 4: Propagation and Radio Frequency Spectrum - Asked in: 2079 Chaitra, 2+6 marks - Video: [Duct Propagation in Antenna and Wave Propagation | Simplified | Super Refraction | AWP Module 6](https://www.youtube.com/watch?v=4XGi7HdfjBA) — THE BACKBENCH ENGINEERING COMMUNITY - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/multiple-hop-atmospheric-ducts - Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-4 ## Question Describe multiple HOP and explain how atmospheric ducts can be used for microwave propagation. ## Short answer Multiple hop uses successive ionospheric and ground reflections for long-range sky wave; single-hop max ~4000 km. Ducts are atmospheric layers with rapid refractive index decrease that trap microwaves, guiding them beyond normal horizon for hundreds of km, but are not reliable for fixed links. ## Answer #### Multiple hop **Multiple-hop (multi-hop) propagation** is sky-wave transmission in which the wave is reflected by the ionosphere, comes down, is reflected by the earth's surface, goes up again, and so on, several times before reaching a distant receiver. ```text ~~~~~~/\~~~~~~~~/\~~~~~~~~/\~~~~~~ F2 layer / \ / \ / \ / \ / \ / \ ===Tx=====\==/======\==/======Rx=== earth earth hop 1 hop 2 hop 3 ``` - Single-hop range is limited by earth curvature: about 2000 km via E layer and about 4000 km via F2. - Longer distances (round the world) need multiple hops. - Each hop adds loss (ground reflection loss, D-layer absorption twice per hop), so the signal weakens and fades more; sea surface is a better reflector than land. - Multiple paths with different hop counts can arrive together, causing multipath fading and echo. #### Use of atmospheric ducts for microwave propagation An **atmospheric duct** is a layer of the lower atmosphere in which the refractive index decreases so rapidly with height (dM/dh < 0, where M = (n − 1 + h/R) × 10⁶) that rays bend down more strongly than the earth curves. The duct traps the wave between its boundaries (or between the upper boundary and the ground), guiding it like a waveguide. ```text ~~~~~~~~~~~ top of duct ~~~~~~~~~~~~ /\ /\ /\ /\ /\ / \ / \ / \ / \ / \ Tx===\=/====\=/====\=/====\=/====Rx ground / sea surface ``` **Causes:** temperature inversion (warm, dry air above cool, moist air), especially over sea and on calm evenings; sharp fall of humidity with height. **How it helps microwaves:** 1. Microwave antennas placed inside (or near) the duct and aimed at low angles launch rays that are repeatedly bent back, so they follow the earth's curvature. 2. The duct acts like a waveguide; only wavelengths small compared with duct thickness are trapped (λ_max ≈ 2.5 d √(ΔM × 10⁻⁶)), which suits VHF high end, UHF and microwaves (thickness from a few m to some hundred m). 3. Ranges of several hundred to over 1000 km beyond the normal radio horizon become possible with low loss. **Limitations:** ducts depend on weather and are not permanent, so they cannot be relied on for regular links. They often cause unwanted long-range interference and radar false echoes ("anomalous propagation"). Planned microwave links use them only opportunistically, mostly over sea. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Multiple-hop propagation is: A. Sky-wave transmission repeatedly reflected between ionosphere and earth B. A wave travelling along a duct C. Several transmitters in sequence D. Ground wave reflected by hills Answer: A. It gives distances beyond the single-hop limit (about 2000 km via E, 4000 km via F2). (Key point: Multi-hop) 2. Why does the signal weaken with more hops? A. Each hop increases the power B. The frequency drops each hop C. Each hop adds ground reflection loss and D-layer absorption (twice per hop) D. The F2 layer absorbs more at each hop Answer: C. It also creates multipath fading and echoes. (Key point: Multi-hop drawbacks) 3. An atmospheric duct forms when: A. The refractive index increases with height B. The refractive index decreases so rapidly with height (dM/dh < 0) that rays bend more than the earth curves C. Temperature falls steadily with height D. The ionosphere gets denser Answer: B. The duct traps the wave like a waveguide. (Key point: Duct condition) 4. Which gives ducts and what is their limitation? A. Heavy rain; permanent and reliable B. Sunspots; works only for HF C. Strong winds; works for LF only D. Temperature inversion / humidity drop; weather dependent so unreliable for regular links Answer: D. They are used only opportunistically, mostly over sea, and also cause interference. 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