Electromagnetic Waves and Communication
Easy Overview
Your phone is talking to a tower right now using invisible waves traveling at the speed of light. That's electromagnetic waves — oscillating electric and magnetic fields that carry energy through empty space. This chapter covers the EM spectrum (from radio to gamma rays) and how we use different parts of it for communication. It's basically how the wireless world works.
What Are Electromagnetic Waves?
EM waves are oscillating electric (E) and magnetic (B) fields perpendicular to each other and to the direction of propagation. They travel at the speed of light c = 3×10⁸ m/s in vacuum. Maxwell figured out that a changing electric field creates a magnetic field, and a changing magnetic field creates an electric field — so they sustain each other. No medium needed: EM waves travel through space. The ratio E/B = c.
The Electromagnetic Spectrum
EM waves range from radio waves (λ meters to km) to gamma rays (λ < 10⁻¹² m). In order of increasing frequency (decreasing wavelength): radio → microwave → infrared → visible → ultraviolet → X-rays → gamma rays. Radio: communication. Microwave: radar, cooking. Infrared: heat, remote controls. Visible: what our eyes see. UV: cause sunburn, sterilize. X-rays: medical imaging. Gamma: cancer treatment, astronomy.
Communication — How Signals Travel
For communication, we encode information onto a carrier wave (high frequency) through modulation. Amplitude Modulation (AM): vary the amplitude of the carrier. Frequency Modulation (FM): vary the frequency. AM is simpler but more noisy; FM gives better quality. Bandwidth is the range of frequencies a signal occupies. The antenna size needed is roughly λ/4 — that's why TV towers are tall (low frequency = large antenna).
Propagation of EM Waves
Ground waves follow Earth's surface (used for AM radio, up to a few hundred km). Sky waves reflect off the ionosphere (used for shortwave radio, can go thousands of km). Space waves travel in straight lines (line of sight) — used for TV, FM, and satellites. For space waves, the range depends on tower height: d = √(2Rh), where R is Earth's radius and h is antenna height. Higher tower = longer range.
Satellite Communication
Geostationary satellites orbit at about 36,000 km above the equator, matching Earth's rotation. They look stationary from the ground — perfect for TV broadcasting, weather monitoring, and communication. The uplink frequency is higher than downlink (to avoid interference). A satellite acts as a repeater in the sky — receives signal, amplifies it, and retransmits it back to Earth. Three geostationary satellites can cover the entire Earth.
Key Points
- •EM waves have E and B fields perpendicular to each other and to direction of travel
- •Speed of EM waves in vacuum: c = 3×10⁸ m/s
- •EM spectrum (increasing f): Radio, Microwave, IR, Visible, UV, X-ray, Gamma
- •AM: amplitude varies; FM: frequency varies (better quality)
- •Ground waves: follow Earth; Sky waves: reflect from ionosphere; Space waves: line of sight
- •Antenna height determines range for space waves: d = √(2Rh)
- •Geostationary satellites at ~36,000 km altitude, T = 24 hours
Practice Questions
- Arrange the following in order of increasing frequency: infrared, gamma rays, radio waves, visible light, X-rays.
- Distinguish between AM and FM radio.
- What is the height of a TV tower if the range is 40 km? (R = 6400 km)
- Explain how a geostationary satellite is used for communication.