---
title: "Short note: Radio frequency spectrum"
subject: "Propagation and Antenna"
chapter: 4
key: "semester_6/propagation_and_antenna:rf-spectrum-short-note"
source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/rf-spectrum-short-note
site: BCT Engineering
---

# Short note: Radio frequency spectrum

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

- Asked in: 2078 Chaitra, 3 marks; 2075 Bhadra, 4 marks; 2072 Asoj, 3 marks (asked 3 times)
- 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-short-note
- Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-4

## Question

Write a short note on radio frequency spectrum.

## Short answer

The radio spectrum 3 kHz–300 GHz is divided by ITU into VLf to EHF bands. Each band has characteristic propagation: VLF/LF ground wave, MF ground/sky wave, HF sky wave, VHF/UHF space wave. Higher frequencies allow smaller antennas and wider bandwidth but suffer greater atmospheric absorption.

## Answer

The **radio frequency (RF) spectrum** is the part of the electromagnetic spectrum used for radio communication, roughly 3 kHz to 300 GHz. ITU divides it into bands, each a decade in frequency, and each band has its own propagation mode and uses.

| Band | Frequency | Wavelength | Main propagation | Typical use |
|---|---|---|---|---|
| VLF | 3–30 kHz | 100–10 km | Ground wave, earth–ionosphere waveguide | Submarine, navigation |
| LF | 30–300 kHz | 10–1 km | Ground wave | LW broadcast, beacons |
| MF | 0.3–3 MHz | 1000–100 m | Ground wave (day), sky wave (night) | AM broadcast |
| HF | 3–30 MHz | 100–10 m | Sky wave (ionosphere) | SW broadcast, long-distance links |
| VHF | 30–300 MHz | 10–1 m | Space wave (LOS) | FM, TV, aircraft radio |
| UHF | 0.3–3 GHz | 1 m–10 cm | Space wave, troposcatter | TV, mobile, GPS, Wi-Fi |
| SHF | 3–30 GHz | 10–1 cm | LOS, satellite | Microwave links, radar, satellite |
| EHF | 30–300 GHz | 10–1 mm | LOS (rain/gas absorption) | Radar, 5G mmWave, radio astronomy |

**Key trends:** as frequency increases, ground-wave range falls, the ionosphere stops returning the wave (above ≈ 30 MHz), antennas become smaller and more directive, available bandwidth increases, and absorption by rain and gases rises. The spectrum is a limited natural resource, so its use is regulated by the ITU internationally and by the Nepal Telecommunications Authority (NTA) in Nepal.

## Check yourself

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

1. Which band mainly uses ionospheric sky wave for long-distance links?
   A. VLF
   B. HF (3-30 MHz)
   C. UHF
   D. EHF
   Answer: B. HF is returned by the ionosphere, giving long-distance short-wave links. (Key point: HF band propagation)

2. How do VHF and higher bands propagate?
   A. Mostly by space wave (line of sight), since the ionosphere stops returning waves above about 30 MHz
   B. By ground wave over thousands of km
   C. By D-layer reflection
   D. By earth-ionosphere waveguide
   Answer: A. Above about 30 MHz waves pass through the ionosphere, so VHF/UHF and above use line-of-sight. (Key point: VHF and above)

3. As frequency increases, which trend holds?
   A. Antennas get larger and ground wave range increases
   B. Antennas get smaller and more directive, bandwidth increases, and rain/gas absorption increases
   C. Bandwidth decreases and absorption decreases
   D. Ionosphere reflects more strongly
   Answer: B. Higher frequency means smaller, more directive antennas, more bandwidth, shorter ground-wave range and more absorption. (Key point: Trends with frequency)

## 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
- [RF spectrum; VHF and UHF characteristics](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Arf-spectrum-vhf-uhf) — Propagation and Antenna, chapter 4
- [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
- [Refractive index, critical frequency and MUF](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Arefractive-index-fc-muf) — Propagation and Antenna, chapter 4
- [Ionosphere structure and ground wave](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aionosphere-structure-ground-wave) — Propagation and Antenna, chapter 4

## Related in other subjects

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- [Spread spectrum: FHSS and DSSS](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aspread-spectrum-fhss-dsss) — Data Communication, chapter 6, asked 4×
- [Spread spectrum and FHSS](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspread-spectrum-fhss) — Wireless Communication, chapter 4
- [OFDM principle and spread spectrum types](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aofdm-spread-spectrum-types) — Wireless Communication, chapter 4