---
title: "Short note: Medium wave propagation"
subject: "Propagation and Antenna"
chapter: 4
key: "semester_6/propagation_and_antenna:mw-propagation-short-note"
source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/mw-propagation-short-note
site: BCT Engineering
---

# Short note: Medium wave propagation

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

- Asked in: 2080 Asoj, 3 marks; 2076 Bhadra, 3 marks; 2073 Magh, 4 marks (asked 3 times)
- Video: [Mediumwave propagation or Propagation of the AM broadcast band](https://www.youtube.com/watch?v=y3D0xOq2Kw0) — OfficialSWLchannel
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/mw-propagation-short-note
- Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-4

## Question

Write a short note on MW (medium wave) propagation.

## Short answer

Medium wave (300 kHz–3 MHz) propagates by ground wave during daytime, with D-layer absorption suppressing sky wave, giving range ~100–300 km. At night, the D layer vanishes, E-layer sky wave travels up to 2000 km causing interference and fading. Vertical polarization and Marconi antennas with radial ground systems are used.

## Answer

**Medium wave (MW)** propagation covers the MF band, 300 kHz – 3 MHz; AM broadcasting uses 535–1605 kHz (526.5–1606.5 kHz in Nepal/ITU Region 3).

**Daytime**
- Propagation is mainly by **ground (surface) wave**. Vertical polarisation is used (vertical mast radiators) because horizontal polarisation is quickly short-circuited by the earth.
- The D layer strongly absorbs MW sky waves, so there is almost no sky wave.
- Service range is about 100–300 km, depending on transmitter power and ground conductivity (larger over sea water, smaller over dry or rocky land).

**Nighttime**
- The D layer disappears, so the sky wave is reflected by the E layer with little loss and reaches 1000–2000 km.
- This gives long-distance reception but also **interference** between distant stations on the same channel and **fading** where ground wave and sky wave of similar strength meet (near-fading zone).

**Other points**
- Attenuation of ground wave rises with frequency, so the lower end of the band covers more area.
- Antennas are tall λ/4 Marconi masts with good radial ground systems; anti-fading antennas (≈ 0.53λ high) reduce high-angle radiation.
- Signal is stable in daytime, so MW is used for local and regional AM broadcast, maritime and aeronautical beacons.

## Check yourself

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

1. How do MW signals travel in the daytime?
   A. By E-layer sky wave over 1000-2000 km
   B. Mainly by ground wave, since the D layer absorbs sky wave; range 100-300 km
   C. By ducting
   D. By troposcatter
   Answer: B. The D layer strongly absorbs MW sky waves in the day, so only the vertically polarised ground wave is useful. (Key point: Daytime MW propagation)

2. Why does MW reach much farther at night?
   A. The D layer disappears so the E layer reflects the sky wave with little loss
   B. Ground conductivity rises
   C. Transmitter power increases
   D. The F2 layer vanishes
   Answer: A. Without the D layer, the sky wave is reflected by the E layer and reaches 1000-2000 km, but causes interference and fading. (Key point: Night propagation)

3. Why are vertical mast radiators used for MW broadcasting?
   A. Horizontal polarisation is quickly short-circuited by the earth, so vertical polarisation suits ground wave
   B. Vertical masts are cheaper only
   C. The ionosphere needs vertical polarisation
   D. To get a narrow beam
   Answer: A. Ground wave needs vertical polarisation since earth shorts out the horizontal electric field. (Key point: Polarisation and antenna)

## Related in Propagation and Antenna

- [Surface wave factors; MW and SW](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Asurface-wave-factors-mw-sw) — Propagation and Antenna, chapter 4, asked 2×
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