Both transmitting and receiving equipment operate on specific frequencies; the higher the construction quality of a set, the more accurate is the matching between the real operating frequency and the indication on the frequency dial. Highly accurate frequency stability can be achieved by using crystals. Since crystals are expensive, specific crystals have been provided for only a few frequencies in a set, or there have been ideas to find ways to achieve high frequency accuracy on many frequencies with a small number of crystals.
Frequencies are measured in Hz (oscillations / second), or in multiples of Hz, so kHz (1 kilohertz = 1000 Hz), MHz (1 megahertz = 1000,000 Hz) and GHz (1 gigahertz = 1000 MHz). In the early days of wireless telegraphy, frequencies were also indicated as wavelengths in metres; the formula 300'000/wavelength in metres can be used to calculate to the frequency in kHz.
The whole range of electromagnetic frequencies was divided into bands which were assigned to specific activities:
- VLF (Very Low Frequency): 3 - 30 kHz; used, among other applications, for communication with submerged submarines, not used in the Switzerland.
- Long waves: 30 - 300 kHz; used for time signal transmitters, radio navigation (aeronautical long wave beacons) and on 150 - 300 kHz for Long Wave Broadcasting (not used in Switzerland).
- Medium waves: 300 kHz to 3 MHz; used on 500 - 1600 kHz for medium wave broadcasting, for amateur radio (160 m band) and as a for military communications and coastal radio, the so called Maritime communications band.
- Shortwaves: 3 - 30 MHz; used for shortwave broadcasting and amateur radio on the defined frequency bands allocated to these services, the whole range is used for military communications.
- VHF: 30 - 300 MHz; used mainly for military communications in the range 30 - 88 MHz, for FM broadcasting in the range 87.5 - 108 MHz, for aeronautical communications in the range 108 - 132 MHz (here, amplitude modulation is used as a single exception in the VHF band), for amateur radio (2m band), professional communication services (police, firearms) and television (old VHF television channels 5 - 12, today DAB / DAB+).
- UHF (decimetre waves): 300 MHz - 3 GHz; used, among other applications, for military aeronautical communicaitons from 225 - 400 MHz, for amateur radio and for television (old UHF television channels 22 - 70), for mobile telephony and WLAN, Bluetooth.
- SHF (centimetre waves): 3 - 30 GHz; used, among other applications, for (military) microwave communications, radar, but also for mobile telephony and WLAN.
In historical sets, frequencies above 30 MHz (in the VHF range used today by military forces) were sometimes calles ultra-short waves (UKW), which explains why the „UKW receivers“ of the German Wehrmacht, for example, operate on frequencies around 30 MHz and not as you would expect from the name in the FM broadcast band.