--- title: "RF spectrum; MW and SW propagation" subject: "Propagation and Antenna" chapter: 4 key: "semester_6/propagation_and_antenna:rf-spectrum-mw-sw" source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/rf-spectrum-mw-sw site: BCT Engineering --- # RF spectrum; MW and SW propagation Propagation and Antenna · Chapter 4: Propagation and Radio Frequency Spectrum - Asked in: 2080 Chaitra, 2+4+4 marks - Video: [Understanding the Radio Frequency Spectrum (#715)](https://www.youtube.com/watch?v=E0ePePS1Iys) — David Casler Ask Dave - 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-mw-sw - Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-4 ## Question What is a radio frequency spectrum? Explain characteristics of MW and SW radio 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. ## Answer #### 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. The radio frequency spectrum used for radio communication is roughly: A. 3 kHz to 300 GHz B. 30 Hz to 3 kHz C. 300 GHz to 3 THz D. 3 MHz to 30 MHz Answer: A. The ITU divides this range into decade bands from VLF to EHF. (Key point: RF spectrum range) 2. Why is MW (AM) reception in daytime mainly by ground wave? A. The E layer reflects it too strongly B. The F2 layer blocks it C. The D layer absorbs the sky wave D. MW is above the MUF Answer: C. Daytime range is only about 100-300 km. (Key point: MW by day) 3. Why does MW travel far at night? A. The ground conductivity rises B. The D layer disappears and the E layer reflects the sky wave C. The F1 layer absorbs less D. Noise decreases to zero Answer: B. Night range reaches 1000-2000 km, with fading and co-channel interference. (Key point: MW at night) 4. A key feature of SW (HF) propagation is: A. Ground wave over thousands of km B. Fixed frequency for all seasons C. Line-of-sight only D. Sky wave via F2 with multi-hop, a skip zone, and frequency changes by time of day Answer: D. Frequency is chosen with OWF about 0.85 MUF, higher by day and lower at night. 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