This page is part of the N0NJY self-study course for the USA Amateur Radio Technician License.
Radio wave propagation describes how radio signals travel from a transmitting antenna to a receiving antenna. Understanding propagation helps explain why some contacts are easy while others are impossible, and why VHF/UHF signals behave differently from HF signals.
The Technician exam focuses on the basic ways VHF and UHF signals (the primary bands for Technician operators) travel, and the factors that affect range and reliability.
This lesson covers:
VHF (2 meters and above) and UHF (70 cm and above) signals travel primarily by line-of-sight. This means the transmitting and receiving antennas must have a relatively unobstructed path between them.
The usable range is limited by the radio horizon, which is farther than the visual horizon due to the slight bending of radio waves around the Earth’s curvature (refraction in the lower atmosphere).
Typical handheld-to-handheld range on 2 meters is about 5–10 miles in flat terrain, depending on antenna height and power. Raising antennas (mobile on car roof, base on mast or tower) significantly increases range.
The distance to the radio horizon can be estimated with the formula:
Distance (miles) ≈ 1.415 × √(height of transmitting antenna in feet) + 1.415 × √(height of receiving antenna in feet)
Example: A 50-foot tower to a 6-foot mobile antenna gives roughly 30–35 miles of reliable range under normal conditions.
Under certain weather conditions, a temperature inversion in the troposphere (lower atmosphere) can create a “duct” that traps VHF/UHF signals and allows them to travel hundreds of miles—far beyond normal line-of-sight.
Ducting is most common during warm, stable weather, especially near large bodies of water or during temperature inversions. It can make distant repeaters suddenly accessible or allow long-distance simplex contacts.
When a radio signal encounters a sharp obstacle (ridge, mountain, building edge), some energy bends around it through diffraction. This allows communication even when there is no direct line-of-sight path.
Knife-edge diffraction is weak but can be enough for reliable contacts over hills or between valleys, especially on VHF/UHF with sufficient power and good antennas.
The wavelength of a radio signal is calculated as:
Wavelength (meters) = speed of light (300,000,000 m/s) / frequency (Hz)
or approximately: Wavelength (meters) ≈ 300 / frequency (MHz)
Examples:
Higher frequency = shorter wavelength = more line-of-sight behavior, less ability to bend around obstacles, and generally shorter reliable range in typical terrain.
These questions are representative of those found in the Technician license exam question pool.
Understanding how VHF/UHF signals travel helps you choose the right times, locations, and antennas for reliable communication.