--- title: "Space wave and sky wave characteristics" subject: "Propagation and Antenna" chapter: 4 key: "semester_6/propagation_and_antenna:space-wave-sky-wave" source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/space-wave-sky-wave site: BCT Engineering --- # Space wave and sky wave characteristics Propagation and Antenna · Chapter 4: Propagation and Radio Frequency Spectrum - Asked in: 2073 Bhadra, 8 marks - Video: [RADIO WAVE PROPAGATION - Ground wave, Sky wave and Space wave](https://www.youtube.com/watch?v=wCS2O08ObQg) — Deepa T - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/space-wave-sky-wave - Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-4 ## Question Briefly discuss the propagation characteristics of space wave and sky wave. ## Short answer Space wave travels via direct and ground-reflected paths, limited to radio horizon d≈4.12(√ht+√hr) km, field falls as 1/d², used for VHF and above. Sky wave refracts from the ionosphere, enabling long-range HF communication up to 4000 km per hop; MUF=fc secθi, strongly dependent on solar activity. ## Answer #### Space wave A **space wave** (tropospheric wave) consists of the **direct wave** from the transmitting to the receiving antenna and the **ground-reflected wave**. It is the main mode at VHF, UHF and microwaves. ```text Tx |\---------- direct ------------>| Rx ht | \ / | hr | \____ ground reflected _/ | ===|================================|=== ``` **Characteristics** 1. **Line-of-sight**: range limited by the radio horizon, d ≈ 4.12(√ht + √hr) km (h in m), typically 40–100 km; higher antennas give longer range. 2. Field strength at large distance: E = 88√P·ht·hr/(λd²), so it falls as 1/d² and grows with antenna heights. 3. Interference between direct and reflected waves gives lobes, multipath and fading. 4. Affected by **tropospheric refraction** (4/3 earth, super-refraction, ducts), terrain, buildings and earth curvature (shadow zone). 5. Above ≈ 10 GHz, rain and atmospheric gases absorb the signal. 6. Not affected by the ionosphere; same performance day and night. 7. Wide bandwidth, low noise; small high-gain antennas. 8. Uses: FM, TV, mobile, microwave links, radar, satellite. #### Sky wave A **sky wave** goes upward and is refracted back to earth by the **ionosphere**; it is the main mode for HF (3–30 MHz). ```text ~~~~~~~~~~~~~~/\~~~~~~~~~~~~~ ionosphere / \ / \ ========Tx=/======\=Rx======== earth |<-skip->| ``` **Characteristics** 1. Long range: one hop up to ≈ 4000 km (F2), worldwide by multi-hop, with low power. 2. Works only below the **MUF** (= fc sec θi); frequencies above it pass through. Critical frequency fc = 9√Nmax. 3. **Skip distance** and **skip zone**: no reception between the end of ground wave and the first return. 4. **Fading** due to multipath and changing ionisation; diversity reception needed. 5. Depends strongly on time of day, season, latitude and sunspot cycle; frequency must be changed (OWF ≈ 0.85 MUF). 6. D-layer absorption in daytime affects low frequencies. 7. Disturbed by ionospheric storms and sudden ionospheric disturbances. 8. Uses: SW broadcasting, maritime, military and amateur long-distance links. | Point | Space wave | Sky wave | |---|---|---| | Medium | Troposphere | Ionosphere | | Band | VHF and above | HF (3–30 MHz) | | Range | LOS (tens of km) | Thousands of km | | Stability | Stable | Varies with sun, time | | Bandwidth | Large | Small | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. A space wave consists of: A. Surface wave and sky wave B. Direct wave and ground-reflected wave C. Ionospheric wave and ground wave D. Only the sky wave Answer: B. The space (tropospheric) wave is the direct wave plus the ground-reflected wave. (Key point: Space wave components) 2. What limits the range of a space wave? A. The radio horizon (line of sight), so higher antennas give longer range B. The MUF of the ionosphere C. The D-layer absorption D. Skip distance Answer: A. Space waves are line-of-sight, d = 4.12(sqrt(ht)+sqrt(hr)) km, so taller antennas extend range. (Key point: Space wave range) 3. Why do frequencies above the MUF fail to give sky wave communication? A. They are absorbed fully by the D layer B. They pass through the ionosphere instead of being returned C. They are reflected back too strongly D. They turn into surface waves Answer: B. Sky wave works only below MUF = fc sec(theta); higher frequencies penetrate the layer. (Key point: Sky wave and MUF) 4. What is the skip zone? A. The area where sky wave is strongest B. The region between the end of ground wave and the first sky wave return with no reception C. The region around the receiver with interference D. The shadow zone of a hill Answer: B. Ground wave dies out and the sky wave has not yet returned, leaving a silent zone. 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