Wavelength 101
Understanding how radio waves travel is essential in amateur radio. Whether you're bouncing signals off the ionosphere or trying to reach a nearby repeater through buildings, the physics of wave propagation influences everything.
Let’s break it down into manageable pieces:
The Speed of Light
All electromagnetic (EM) waves—including radio waves—travel at the speed of light in a vacuum:
300,000 kilometers per second (or ~186,000 miles per second)
In air, this speed is slightly slower, but close enough that we still approximate it as 300,000,000 meters per second (m/s) in most ham radio calculations.
How Radio Waves Move
Radio waves don’t just shoot in straight lines and stop. They can do all sorts of things depending on their frequency, power, and the environment they pass through.
Here are the main behaviors:
Reflection
What happens: The wave bounces off a surface (like a metal building, water, or mountain).
Why it matters: Useful for urban environments where direct line-of-sight is blocked.
Example: UHF signals bouncing off buildings can still reach receivers that aren’t in direct view.
Refraction
What happens: The wave bends as it passes through different mediums (like air layers with varying temperatures or humidity).
Why it matters: Causes ducting or bending of signals beyond the horizon.
Example: VHF “tropo” propagation allows distant signals during certain weather conditions.
Diffraction
What happens: The wave bends around obstacles like hills or buildings.
Why it matters: Lower frequencies (like HF) diffract better, allowing communication over obstacles.
Example: An HF signal might bend over a ridge that would completely block a UHF signal.
Scattering
What happens: The wave hits small particles or irregular surfaces and gets scattered in many directions.
Why it matters: Weakens signal but sometimes enables it to reach unexpected places.
Example: Snow, rain, or foliage can scatter and degrade a signal.
Absorption
What happens: Some materials soak up radio energy, reducing signal strength.
Why it matters: Dense forests, concrete, or even human bodies can weaken signals.
Example: UHF signals are heavily absorbed by concrete walls or inside buildings.
Line-of-Sight vs. Beyond-Line-of-Sight
VHF/UHF signals mostly rely on line-of-sight: If something blocks your view, it blocks the signal too (with exceptions like reflection and refraction).
HF signals often reflect off the ionosphere, allowing them to travel around the globe even though they don’t follow Earth’s curve.
What Affects Propagation?
Frequency - Lower frequencies bend better (HF), higher ones are more line-of-sight (VHF/UHF)
Terrain - Hills, valleys, buildings can block, reflect, or diffract signals
Atmosphere - Weather layers, temperature inversions can refract or “duct” signals
Ionosphere - At night, HF signals reflect more easily, enabling long-distance communication
Solar Activity - Sunspots and solar flares dramatically impact HF propagation
Obstacles - Trees, walls, and water can absorb or weaken signals, especially on UHF
Propagation Modes You'll Hear About
Ground Wave – Follows Earth's surface (mostly for low HF frequencies)
Skywave – Bounces off the ionosphere (common in HF bands)
Tropospheric Ducting – VHF/UHF can travel hundreds of miles in weather layers
Line-of-Sight – Direct path between antennas, important for VHF/UHF
Satellite – Uses orbiting repeaters to relay VHF/UHF across the globe
Quick Recap
|
Behavior |
Frequency Range |
Key Benefit |
|---|---|---|
| Reflection |
VHF/UHF
|
Reaching order corners/obstacles |
| Refraction |
VHF
|
Extended range during conditions |
| Diffraction | HF | Signals bend over terrain |
| Absorption | All | Explains why some signals vanish |
| Skywave | HF | Long distance ,global comms |
| LOS/Direct | VHF/UHF | Reliable if unobstructed |