- 2080 Chaitra · 2+4+4 marks
RF spectrum; MW and SW propagation
Short answer
The radio frequency spectrum ranges from 3 kHz to 300 GHz, divided into bands. MW (300 kHz – 3 MHz) uses ground wave by day and sky wave by night, with vertical polarization; range limited by ground losses. SW (3–30 MHz) relies on sky wave via F2 layer, enabling long-range multi-hop propagation with skip zones, but suffers fading and requires frequency changes with time.
What is a radio frequency spectrum? Explain characteristics of MW and SW radio propagation.
Radio frequency spectrum
The radio frequency spectrum is the range of electromagnetic frequencies used for radio communication, from about 3 kHz to 300 GHz. The ITU divides it into decade bands, each with its own propagation behaviour:
| Band | Frequency | Main propagation | Use |
|---|---|---|---|
| VLF | 3–30 kHz | Ground wave, earth–ionosphere guide | Navigation, submarine |
| LF | 30–300 kHz | Ground wave | LW broadcast, beacons |
| MF | 0.3–3 MHz | Ground wave (day), sky wave (night) | AM broadcast |
| HF | 3–30 MHz | Sky wave | SW broadcast, long-distance |
| VHF | 30–300 MHz | Space wave (LOS) | FM, TV, aircraft |
| UHF | 0.3–3 GHz | Space wave, troposcatter | TV, mobile, GPS |
| SHF | 3–30 GHz | LOS, satellite | Microwave links, radar |
| EHF | 30–300 GHz | LOS (high absorption) | mmWave, radar |
Characteristics of MW and SW 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.
✓ Answer checked by Claude Opus 5.5 (high effort)
Related questions
- RF spectrum; VHF and UHF characteristics · Propagation and Antenna, chapter 4
- Short note: Radio frequency spectrum · Propagation and Antenna, chapter 4 · asked 3×
- Surface wave factors; MW and SW · Propagation and Antenna, chapter 4 · asked 2×
- Space wave factors; MW and SW · Propagation and Antenna, chapter 4 · asked 2×
- Path loss in radio wave propagation · Propagation and Antenna, chapter 5 · asked 2×
- Radio wave types and band characteristics · Propagation and Antenna, chapter 4 · asked 2×
- RF/MW radiation hazards and safety practices · RF and Microwave Engineering, chapter 7 · asked 4×
- Load sharing: 250/500 MW, frequency falls to 49.5 Hz · Power Plant Equipment, chapter 2 · asked 2×
- Spread spectrum frequency hopping in Bluetooth · Instrumentation II, chapter 5 · asked 2×
- Radiation pressure of radio wave · Engineering Physics, chapter 9