Wireless transmitting und reception devices work on specific frequencies. The higher the quality of the receiving or transmitting device, the better is the correlation between the dial markings and the real operation frequency. Very high accuracy of the frequency of a receiver or transmitter can be reached, when crystals are used. Because these crystals used in receivers or transmitters are expensive, engineers developed ways to safe the cost of many different crystals and ensure a high frequency stability anyway.
Frequencies are measured in Hz (cycles / second), or in multiples, for example kHz (Kilohertz = 1000 Hz), MHz (Megahertz = 1000'000 Hz) and GHz (Gigahertz = 1000 MHz).
In the very early time of wireless telegraphy, frequencies have been indicated inthe form of a wavelength in meters angegeben, you can use the formula 300'000/ Wavelength in meters to calculate the frequency in kHz from a given wavelength.
In older British sets, you might encounter the dial marked in cps (cycles per second) instead of Hertz.
The frequency spectrum has been divided in different bands which were allocated for certain use:
In historic german sets, frequencies above 30 MHz have been addressed as „Ultrakurzwellen“, that's why the „UKW Empfänger“ of the german Wehrmacht are operating on frequencies around 30 MHz.