--- title: "Space wave factors; MW and SW" subject: "Propagation and Antenna" chapter: 4 key: "semester_6/propagation_and_antenna:space-wave-factors-mw-sw" source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/space-wave-factors-mw-sw site: BCT Engineering --- # Space wave factors; MW and SW Propagation and Antenna · Chapter 4: Propagation and Radio Frequency Spectrum - Asked in: 2071 Bhadra, 5+6 marks - 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-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 space wave communication. Explain the major characteristics of MW and SW radio propagation. ## Short answer Space wave communication is affected by antenna heights, earth curvature, tropospheric refraction, ground reflection, frequency, obstacles, atmospheric absorption, distance, and multipath. MW propagation uses ground wave daytime and sky wave nighttime with vertical polarization; SW propagation uses sky wave via F2 layer for long-range communication with critical frequency, MUF, skip distance, and fading, requiring frequency changes with time and solar activity. ## Answer #### Factors affecting space wave communication **Space wave** = direct wave + ground-reflected wave, used at VHF and above. 1. **Antenna heights:** LOS range d ≈ 4.12(√ht + √hr) km and field E ∝ ht·hr; higher antennas give longer range and stronger signal. 2. **Earth curvature:** limits range to the radio horizon, reduces effective antenna heights, creates a shadow zone, and spreads (diverges) the reflected wave. 3. **Tropospheric refraction:** normal decrease of refractive index gives k = 4/3 (horizon extended about 15%); abnormal profiles cause super-refraction, ducting or sub-refraction (fading or loss of signal). 4. **Ground reflection:** reflection coefficient depends on ground conductivity, permittivity, polarisation and grazing angle; causes interference lobes and multipath fading. 5. **Frequency:** for fixed antenna gains, a higher frequency means more free-space path loss, more shadowing by obstacles and less diffraction. 6. **Obstacles and terrain:** hills, buildings and trees block or diffract the wave; first Fresnel-zone clearance (≥ 0.6) is required. 7. **Atmospheric absorption and precipitation:** rain, fog, O₂ and H₂O absorb above about 10 GHz. 8. **Distance:** beyond the horizon the field falls rapidly; within LOS it falls as 1/d² at large d. 9. **Multipath from buildings and moving objects:** causes fast fading (mobile radio). #### 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 (10 kHz channels). **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. How do antenna heights affect space wave communication? A. Higher antennas increase range and field strength B. Heights have no effect C. Higher antennas reduce the range D. Only the receiver height matters Answer: A. d is about 4.12(sqrt(ht)+sqrt(hr)) km and E is proportional to ht x hr. (Key point: Antenna heights) 2. With normal refraction (k = 4/3) the radio horizon is: A. Reduced by 15% B. Unchanged C. Extended by about 15% D. Infinite Answer: C. Normal refractive index decrease bends rays downward, extending the horizon about 15%. (Key point: Tropospheric refraction) 3. What Fresnel zone clearance is needed for a space wave link? A. At least 0.6 of the first Fresnel zone B. Zero clearance C. Only 10% D. Full second zone Answer: A. About 60% clearance of the first Fresnel zone avoids significant obstruction loss. (Key point: Fresnel clearance) 4. How does medium wave propagate by day? A. Mainly by ground wave because D layer absorbs sky wave B. By F2 layer reflection C. By troposcatter D. By ducting Answer: A. By day MW ranges 100-300 km using the surface wave. (Key point: MW daytime) ## Close variants set in other exams - [Surface wave factors; MW and SW](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Asurface-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× - [Space wave and sky wave characteristics](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aspace-wave-sky-wave) — 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