T3B: Electromagnetic Wave Properties

Every radio transmission is an electromagnetic wave. Before you can understand propagation, antenna design, or frequency selection, you need a clear mental model of what a radio wave actually is: what it is made of, how it moves, and how its fundamental properties relate to each other. This is the foundation that everything else in amateur radio builds on.

T3B covers the physics of electromagnetic waves — the electric and magnetic field components, their orientation to each other, the definition of polarization, the speed at which radio waves travel, and the inverse relationship between wavelength and frequency. It also establishes the frequency boundaries for the three main spectrum regions Technician operators use: HF, VHF, and UHF.

Structure of an Electromagnetic Wave

A radio wave — like all electromagnetic waves — is composed of two distinct fields that exist simultaneously and travel together: an electric field and a magnetic field. These two fields are oriented at right angles to each other. They are also both perpendicular to the direction the wave is traveling. This three-dimensional arrangement — electric field in one plane, magnetic field in a perpendicular plane, propagation direction perpendicular to both — is the defining structure of all electromagnetic radiation, from radio waves through visible light to X-rays.

The two components are inseparable. An oscillating electric field generates an oscillating magnetic field, which in turn generates an oscillating electric field, and so on — this self-sustaining cycle is what allows electromagnetic waves to travel through free space without any medium. The components are not in parallel, they do not travel at different speeds, and they do not revolve in opposite directions. They are at right angles to each other and travel at exactly the same speed.

Polarization: Defined by the Electric Field

The polarization of a radio wave is defined by the orientation of its electric field. If the electric field is oriented vertically — that is, parallel to the Earth's surface in the up-down direction — the wave is vertically polarized. If the electric field is oriented horizontally, the wave is horizontally polarized. The magnetic field, being perpendicular to the electric field, automatically orients itself accordingly.

It is the electric field that defines polarization, not the magnetic field, and not any ratio between the two. This is important to know because antenna design and polarization matching are based on the electric field orientation — a vertical antenna radiates and receives vertically polarized waves because its conductor is aligned with the electric field direction.

Speed of Radio Waves in Free Space

All electromagnetic waves, including radio waves, travel through free space at the speed of light. In free space (vacuum), this speed is approximately 300,000,000 meters per second — that is, 300 million meters per second, or roughly 186,000 miles per second. This value is a fundamental constant of physics.

Radio waves do not travel at the speed of sound, do not travel faster or slower depending on frequency, and do not travel at a speed inversely proportional to wavelength. Every radio frequency — from 3 MHz HF to 300 GHz millimeter-wave — travels at the same velocity in free space. The number to know for the exam is 300,000,000 meters per second.

Wavelength and Frequency: An Inverse Relationship

Frequency and wavelength are two ways of describing the same wave. Frequency measures how many complete cycles occur per second (in hertz). Wavelength measures the physical distance from one cycle's peak to the next peak — the length of one complete wave cycle in space.

Because all radio waves travel at the same speed, frequency and wavelength are inversely related: as frequency increases, wavelength decreases, and as frequency decreases, wavelength increases. A 300 MHz signal has a wavelength of 1 meter. A 3 MHz signal (ten times lower frequency) has a wavelength of 100 meters (ten times longer). This inverse relationship holds consistently across all frequencies.

This is not a situation where wavelength and frequency are unrelated, or where both increase together. Higher frequency always means shorter wavelength; lower frequency always means longer wavelength. This relationship is fixed by the speed of light.

The Wavelength Formula

The standard formula for converting between wavelength and frequency is straightforward to use for ham radio purposes:

The formula uses frequency in megahertz (MHz), not hertz. If you start with a frequency in hertz, convert it to MHz first by dividing by 1,000,000. Wavelength in meters equals 300 divided by frequency in megahertz — not frequency multiplied by 300, not frequency divided by 300, and not megahertz divided by 300. The numerator is always 300.

Amateur Bands Named by Wavelength

Amateur radio bands are commonly identified by their approximate wavelength in meters rather than by their frequency. When an operator refers to "the 2 meter band," they mean the band with frequencies around 144–148 MHz, where the wavelength works out to approximately 2 meters. When they say "70 centimeters," they mean the 420–450 MHz band where wavelength is roughly 70 cm.

This naming convention is practical because it gives a quick physical sense of the antenna sizes involved — a half-wave dipole for the 2 meter band is about 1 meter long, a fact immediately obvious from the band name. Amateur bands are not identified by letter/number designators, channel numbers, or any other scheme — approximate wavelength in meters (or centimeters) is the standard.

HF, VHF, and UHF: Frequency Range Definitions

The radio spectrum is divided into named ranges based on frequency. Three of these ranges are particularly important for Technician operators to know:

Each range spans exactly one decade of frequency. HF runs from 3 to 30 MHz — this is the range where ionospheric propagation regularly enables long-distance contacts. VHF runs from 30 to 300 MHz — the 2 meter band (144–148 MHz) and the 6 meter band (50–54 MHz) are both in VHF. UHF runs from 300 to 3000 MHz — the 70 centimeter band (420–450 MHz) is UHF.

Know these boundaries exactly. Common exam distractors swap the VHF and UHF ranges or use kilohertz instead of megahertz. VHF is 30–300 MHz; UHF is 300–3000 MHz; HF is 3–30 MHz.

T3B Practice Questions