--- title: "Surface wave factors; MW and SW" subject: "Propagation and Antenna" chapter: 4 key: "semester_6/propagation_and_antenna:surface-wave-factors-mw-sw" source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/surface-wave-factors-mw-sw site: BCT Engineering --- # Surface wave factors; MW and SW Propagation and Antenna · Chapter 4: Propagation and Radio Frequency Spectrum - Asked in: 2073 Bhadra, 2+6 marks - Video: [Radio Wave Propagation (Types, Basics & Definition) Explained | Ground, Sky & Space Wave Propagation](https://www.youtube.com/watch?v=ONxCEfZ2WnA) — Engineering Funda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/surface-wave-factors-mw-sw - Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-4 ## Question Write down the factors which affect the surface wave communication. Explain the major characteristics of MW and SW radio propagation. ## Short answer Surface wave attenuation rises with frequency, depends on ground conductivity and permittivity; only vertical polarisation works. MW uses ground wave by day and E-layer sky wave at night, range 100–2000 km. SW uses F2-layer sky wave with skip zone, fading; frequency changes diurnally and with sunspot cycle. ## Answer #### Factors affecting surface wave communication 1. **Frequency:** attenuation increases rapidly with frequency; surface wave is useful only up to about 2–3 MHz. 2. **Ground conductivity (σ):** higher σ means lower loss; sea water (≈ 4 S/m) is best, dry/rocky soil is worst. 3. **Ground permittivity (εr):** affects wave tilt and attenuation, especially at higher frequencies. 4. **Polarisation:** only vertical polarisation propagates; horizontal is short-circuited by the ground. 5. **Distance and earth curvature:** field falls as A/d (A = attenuation factor) and faster beyond the diffraction region. 6. **Terrain and obstacles:** hills, forests, buildings and mixed land–sea paths add loss. 7. **Transmitter power and antenna** (height, efficiency, ground system). 8. **Atmospheric noise** (high at LF/MF) reduces usable range. #### Characteristics of MW and SW radio propagation **MW (medium wave) propagation, 300 kHz – 3 MHz (AM broadcast 526.5–1606.5 kHz):** - Daytime: mainly **ground (surface) wave**; the D layer absorbs the sky wave. Range ≈ 100–300 km depending on power and soil (longest over sea). - Nighttime: D layer disappears, the **E layer reflects** the sky wave, giving 1000–2000 km range, but also co-channel interference and fading where ground and sky waves mix. - Vertical polarisation and tall λ/4 mast antennas with radial ground systems are used. - Ground-wave attenuation increases with frequency, so the lower end of the band covers more area. - High atmospheric noise; narrow bandwidth (9 kHz channels in ITU Region 3, including Nepal; 10 kHz in the Americas). **SW (short wave) propagation, 3 – 30 MHz (HF):** - Mainly **sky wave** via the F2 (and E) layer; ground wave dies within a few tens of km. - Very long range with modest power: one F2 hop up to ≈ 4000 km, worldwide by **multi-hop**. - Governed by **critical frequency, MUF and skip distance**; a **skip zone** exists between the end of ground wave and the first sky-wave return. - **Fading** due to multipath and changes in the ionosphere; diversity reception is used. - Frequency must change with time of day, season and sunspot cycle: higher frequencies by day, lower at night; OWF ≈ 0.85 MUF. - Affected by ionospheric storms and sudden ionospheric disturbances (radio blackouts). - Horizontal antennas such as dipoles, rhombic and log-periodic arrays are common. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which polarisation is used for surface wave propagation, and why? A. Horizontal, because the ground reflects it B. Vertical, because horizontal is short-circuited by the ground C. Circular, because it avoids fading D. Either, since the ground has no effect Answer: B. Horizontal electric field is shorted by the conducting ground, so only vertical polarisation propagates. (Key point: Polarisation) 2. How do ground conductivity and frequency affect surface wave range? A. Higher conductivity and lower frequency give longer range B. Higher conductivity and higher frequency give longer range C. Lower conductivity gives longer range D. Frequency has no effect Answer: A. Attenuation rises with frequency and falls with conductivity; sea water is best. (Key point: Conductivity and frequency) 3. Why does medium wave reach 1000-2000 km at night? A. The D layer disappears and the E layer reflects the sky wave B. Ground conductivity increases C. Atmospheric noise stops D. The F2 layer absorbs it Answer: A. By day D absorbs MW sky waves; at night D vanishes and the E layer reflects them far. (Key point: MW day/night) 4. What is the main mode of short wave (3-30 MHz) propagation? A. Ground wave B. Sky wave via F2 layer C. Tropospheric scatter D. Duct propagation Answer: B. HF ground wave dies in tens of km; the F2 sky wave gives hops up to 4000 km and multi-hop worldwide. (Key point: SW mode) ## Close variants set in other exams - [Space wave factors; MW and SW](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aspace-wave-factors-mw-sw) — Propagation and Antenna, chapter 4, asked 2× ## Related in Propagation and Antenna - [RF spectrum; MW and SW propagation](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Arf-spectrum-mw-sw) — Propagation and Antenna, chapter 4 - [Path loss in radio wave propagation](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Apath-loss-derivation) — Propagation and Antenna, chapter 5, asked 2× - [Radio wave types and band characteristics](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aradio-wave-designation-bands) — Propagation and Antenna, chapter 4, asked 2× - [Surface wave propagation pros and cons](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Asurface-wave-propagation) — Propagation and Antenna, chapter 4 ## Related in other subjects - [Radiation pressure of radio wave](https://www.bctengineer.com/question.md?key=semester_1%2Fengineering_physics%3Aradiation-pressure-radio-wave) — Engineering Physics, chapter 9