---
title: "RF spectrum; VHF and UHF characteristics"
subject: "Propagation and Antenna"
chapter: 4
key: "semester_6/propagation_and_antenna:rf-spectrum-vhf-uhf"
source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/rf-spectrum-vhf-uhf
site: BCT Engineering
---

# RF spectrum; VHF and UHF characteristics

Propagation and Antenna · Chapter 4: Propagation and Radio Frequency Spectrum

- Asked in: 2074 Bhadra, 2+6 marks
- Video: [Basic VHF and UHF Fundamentals](https://www.youtube.com/watch?v=g331FDP8_cA) — Dan Vanevenhoven
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/rf-spectrum-vhf-uhf
- Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-4

## Question

What is a radio frequency spectrum? Give major propagation characteristics of VHF and UHF bands.

## 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.

## Answer

#### 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)
1. **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.
2. **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.
3. **Ground wave is negligible:** surface-wave attenuation is very high at these frequencies.
4. **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.
5. **Tropospheric scatter** at UHF gives beyond-horizon links of 100–800 km with high power and large antennas.
6. **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.
7. **Diffraction and shadowing:** waves bend slightly over hills and buildings, less as frequency rises; UHF suffers more shadowing and building penetration loss than VHF.
8. **Small, high-gain antennas:** short wavelength allows Yagi and panel antennas, giving directivity and frequency reuse.
9. **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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. The main propagation mode for VHF/UHF is:
   A. Sky wave via F2
   B. Space wave (line of sight)
   C. Surface wave
   D. Earth-ionosphere waveguide
   Answer: B. Ground wave is negligible and the ionosphere does not return them. (Key point: VHF/UHF mode)

2. Tropospheric scatter at UHF provides:
   A. Beyond-horizon links of 100-800 km with high power and large antennas
   B. Worldwide links
   C. Links only under 10 km
   D. Satellite links
   Answer: A. It relies on scattering in the troposphere. (Key point: Troposcatter)

3. Multipath in VHF/UHF causes:
   A. Longer range
   B. Zero noise
   C. Ghosting in TV and fast fading in mobile systems
   D. Skip zones
   Answer: C. Reflections from ground, buildings and hills interfere. (Key point: Multipath)

4. Compared with VHF, UHF suffers:
   A. Less shadowing
   B. Longer ground wave
   C. More ionospheric reflection
   D. More shadowing and building penetration loss
   Answer: D. Diffraction around obstacles decreases with frequency. (Key point: UHF vs VHF)

## 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
- [Short note: Radio frequency spectrum](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Arf-spectrum-short-note) — Propagation and Antenna, chapter 4, asked 3×
- [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×
- [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×
- [Duct propagation; critical frequency and MUF](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aduct-critical-frequency-muf) — Propagation and Antenna, chapter 4
- [UHF band and log-periodic antenna](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Auhf-band-log-periodic) — Propagation and Antenna, chapter 3

## Related in other subjects

- [Frequency bands of waves and rays](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Afrequency-bands-waves-rays) — RF and Microwave Engineering, chapter 1
- [Spread spectrum frequency hopping in Bluetooth](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Afreq-hopping-bt) — Instrumentation II, chapter 5, asked 2×
- [Major characteristics of Bluetooth](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Abt-characteristics) — Instrumentation II, chapter 5
- [Transmission line behavior at low and microwave bands](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Atransmission-line-low-vs-microwave) — RF and Microwave Engineering, chapter 1