- 2074 Bhadra · 2+6 marks
RF spectrum; VHF and UHF characteristics
Short answer
The radio spectrum ranges from 3 kHz to 300 GHz, divided into bands. VHF (30–300 MHz) and UHF (300 MHz–3 GHz) propagate mainly by space wave, limited to the radio horizon d ≈ 4.12(√ht+√hr) km; the ionosphere does not reflect them, ground wave is negligible, multipath fading occurs, small high-gain antennas are possible, and they are used for FM, TV, mobile, and GPS.
What is a radio frequency spectrum? Give major propagation characteristics of VHF and UHF bands.
Radio frequency spectrum
The radio frequency spectrum is the range of electromagnetic frequencies used for radio communication, about 3 kHz to 300 GHz, divided by the ITU into bands:
| 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 |
Propagation characteristics of VHF (30–300 MHz) and UHF (300 MHz – 3 GHz)
- Space-wave (line-of-sight) propagation: direct wave plus ground-reflected wave; range limited to the radio horizon, d ≈ 4.12(√ht + √hr) km. High antenna masts or towers are used.
- Ionosphere does not return them: they pass through, so no sky-wave skip (except rare sporadic-E or F2 openings at low VHF). This makes them suitable for satellite links.
- Ground wave is negligible: surface-wave attenuation is very high at these frequencies.
- Tropospheric refraction bends the waves slightly, extending the horizon by about 15% (4/3 earth radius); abnormal conditions cause super-refraction and duct propagation with long-range interference.
- Tropospheric scatter at UHF gives beyond-horizon links of 100–800 km with high power and large antennas.
- Multipath and fading: reflections from ground, buildings and hills cause interference patterns (field varies as 1/d² far away), ghosting in TV and fast fading in mobile systems.
- Diffraction and shadowing: waves bend slightly over hills and buildings, less as frequency rises; UHF suffers more shadowing and building penetration loss than VHF.
- Small, high-gain antennas: short wavelength allows Yagi and panel antennas, giving directivity and frequency reuse.
- Low noise and wide bandwidth: atmospheric and man-made noise is low; enough bandwidth for FM, TV and data.
Uses: VHF – FM broadcast, TV, aircraft and marine radio; UHF – TV, mobile (GSM/4G), GPS, Wi-Fi, Bluetooth.
✓ Answer checked by Claude Opus 5.5 (high effort)
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