What is frequency?
Frequency is the number of complete cycles of an alternating voltage, current, or wave that
occur every second. It is measured in Hertz, shortened to Hz.
In an AC signal, the voltage rises positive, falls through zero, goes negative, and then
returns back through zero to complete one full cycle. The polarity changes during the cycle,
but the frequency is counted by the number of complete cycles per second.
Simple rule: 1 Hz means 1 complete cycle every second. 1,000 Hz is 1 kHz.
1,000,000 Hz is 1 MHz.
Positive
Negative
Time
One complete cycle
AC power and radio frequency
The electricity in a UK home is normally 50 Hz AC. In some countries it is 60 Hz.
This is still an alternating electromagnetic signal, but it is far below the frequencies
normally used for radio communication.
A mains cable can produce changing electric and magnetic fields, but it is not designed
to work as an efficient transmitting aerial. Radio systems use much higher frequencies
and correctly designed aerials so that energy is radiated into space efficiently.
Frequency and wavelength
Frequency and wavelength are linked. As frequency goes up, wavelength gets shorter.
As frequency goes down, wavelength gets longer.
Lower frequency
Longer wavelength, larger aerials, often better ground-wave or ionospheric behaviour.
Higher frequency
Shorter wavelength, smaller aerials, more line-of-sight behaviour.
Radio example
3.5 MHz needs a much larger aerial than 144 MHz.
The radio spectrum
Radio waves are part of the electromagnetic spectrum. They are the same basic type of
wave as light, but at much lower frequencies. As frequency rises above radio, we move
through infrared, visible light, ultraviolet, X-rays, and gamma rays.
Electromagnetic Spectrum
Frequency increases from left to right.
Radio
Microwave
Infrared
Visible Light
Ultraviolet
X-rays
Gamma
Long wavelength
Short wavelength
Radio frequency bands
The radio spectrum is split into named bands. These bands help describe the range of
frequencies being used. Amateur radio operators commonly use parts of the HF, VHF,
UHF, and microwave spectrum.
| Band name |
Frequency range |
Typical use or meaning |
| LF / Long Wave |
30 kHz to 300 kHz |
Long-range low-frequency services, navigation, and specialist uses. |
| MF / Medium Wave |
300 kHz to 3 MHz |
Medium wave broadcast radio and some lower-frequency communication services. |
| HF / High Frequency |
3 MHz to 30 MHz |
Short-wave radio and many amateur bands including 80 m, 40 m, 20 m, 15 m, and 10 m. |
| VHF / Very High Frequency |
30 MHz to 300 MHz |
Local and regional radio communication, including the amateur 6 m, 4 m, and 2 m bands. |
| UHF / Ultra High Frequency |
300 MHz to 3 GHz |
Shorter-range communication, repeaters, satellites, Wi-Fi, mobile systems, and the 70 cm amateur band. |
| SHF / Super High Frequency |
3 GHz to 30 GHz |
Microwave communication, radar, satellite links, and amateur microwave experimentation. |
| EHF / Extremely High Frequency |
30 GHz to 300 GHz |
Millimetre-wave systems, specialist links, research, and advanced microwave work. |
How the main amateur bands fit in
Amateur radio band names are often based on wavelength rather than frequency. For example,
the 40 metre band is around 7 MHz, and the 2 metre band is around 144 MHz.
Amateur Radio Examples
Lower frequency bands have longer wavelengths. Higher frequency bands have shorter wavelengths.
160 m
1.8 MHz
80 m
3.5 MHz
40 m
7 MHz
20 m
14 MHz
10 m
28 MHz
2 m
144 MHz
70 cm
430 MHz
HF
Longer-distance communication using ionospheric propagation.
VHF / UHF
Local, repeater, satellite, and line-of-sight communication.
Frequency and propagation
Different frequencies behave differently. HF signals can often be refracted by the
ionosphere and return to Earth many hundreds or thousands of miles away. This is why
the HF bands are so important for DX communication.
VHF and UHF signals are generally more line-of-sight, which makes them ideal for local
communication, repeaters, portable work, and satellite contacts.
Frequency and aerial size
Aerial size is linked to wavelength. A half-wave dipole for 40 metres is much longer
than a half-wave dipole for 2 metres. This is why HF aerials often need more space,
while VHF and UHF aerials can be compact and portable.
- 40 m band: around 7 MHz, long aerials.
- 20 m band: around 14 MHz, medium-sized HF aerials.
- 2 m band: around 144 MHz, compact aerials.
- 70 cm band: around 430 MHz, very compact aerials.
In simple terms
Frequency tells us how fast a signal is changing. Low frequencies have long wavelengths,
while high frequencies have short wavelengths. Radio, microwaves, infrared, visible light,
X-rays, and gamma rays are all part of the same electromagnetic family, but they occupy
very different parts of the spectrum.
For amateur radio, understanding frequency helps us choose the right band, the right aerial,
and the right operating method for the contact we want to make.
