Wavelength 101
Wavelength is fundamental to how radio waves travel, how antennas work, and why certain ham bands behave the way they do. If you're just getting into amateur radio, understanding wavelength will help you grasp why your gear is designed the way it is—and why some bands work better for certain types of communication than others.
What Is Wavelength?
Wavelength is the distance a radio wave travels in one complete cycle.
It’s often measured in meters, which is why we refer to “20 meters,” “2 meters,” etc.
The basic formula is:
Wavelength (λ) = Speed of Light / Frequency
Since the speed of light is about 300,000,000 meters/second, we can simplify this for ham radio:
λ (in meters) ≈ 300 / Frequency (in MHz)
For example:
A 2 meter wave has a frequency of around 150 MHz
A 20 meter wave has a frequency of around 15 MHz
Antennas & Wavelength: The Resonance Principle
Antennas work best when they are sized to “match” the wavelength of the signal. This is called resonance.
The most efficient antennas are usually:
Full-wave (1λ)
Half-wave (½λ) — most common
Quarter-wave (¼λ) — very popular for mobile and handheld radios
Why Resonance Matters:
Think of pushing someone on a swing:
If you push at the right rhythm (resonance), the swing goes higher.
If your timing is off, your energy is wasted.
Same with antennas — if they “resonate” with the wave, the energy is transmitted efficiently. If not, you get power loss or even damage to your radio (high SWR).
Real-World Examples:
Note: A 20-meter band antenna may require a 33-foot-long wire, while a 2-meter antenna can be just a few feet.
|
Band |
Wavelength |
Common Antenna Size | Typical Use |
|---|---|---|---|
| 2 meter |
~ 2 meters
|
1/2 wave = ~1m | Local communication, VHF |
| 20 meter |
~20 meters
|
1/2 wave = ~10 meters | Worldwide communication, HF |
| 70 centimeter | ~0.7 meters | 1/4 wave = ~6-7 inches | Local, indoor, short-range UHF |
Why Certain Bands Are Better for Certain Things
Wavelength influences propagation (how waves travel):
Long Wavelengths (e.g., 80m, 40m, 20m)
Bounce off the ionosphere (skywave)
Travel hundreds or thousands of miles
Good for worldwide communication
Require larger antennas
Short Wavelengths (e.g., 2m, 70cm)
Line-of-sight travel (like light)
Limited to local or regional communication
Great for repeaters, emergency services
Work well indoors or in cities
Require smaller, portable antennas
Analogy: Guitar Strings
Think of wavelength like string length on a guitar:
Long string = low frequency, deeper tone
Short string = high frequency, higher tone
Similarly:
Longer wavelength = lower frequency = bigger antenna
Shorter wavelength = higher frequency = smaller antenna
If your “instrument” (antenna) is out of tune with the wave, it won’t play well (i.e., won’t transmit efficiently).
Why Half-Wave Antennas Are So Popular
Easy to construct
Highly efficient at radiating signal
Symmetrical, so tuning is easier
For many ham bands, building or buying a half-wave dipole is the go-to option.
Why Half-Wave Antennas Are So Popular
Wavelength determines antenna size and signal behavior.
Resonance is key to effective transmission—matching antenna to wave.
Different bands serve different purposes based on wavelength:
Long = global reach
Short = compact, local use