In the early days of experimental wireless development, it was common practice to calibrate electromagnetic waves by measuring the distance from one crest to the next crest in much the same way as you might measure the waves of the ocean, from one crest to the next. This distance is called the wavelength, and because many of the prominent early experimenters were European, it came to be measured electrically in metres.
Thus you will find the dials of most of the very old radio receivers marked in metres. In the medium wave broadcast band, one station in particular might be on the air for example on a wavelength of 200 metres, and another on 300 metres. On shortwave, one station could be shown as 30.99 metres, and another on 30.96 metres.
But, there's another way of calibrating a radio signal. Instead of measuring the distance between crests, or length of complete wave cycles, you can count how many of those complete cycles pass a fixed point in one second. This is known as the frequency.
The existence of electromagnetic waves was first demonstrated by German physicist Heinrich Hertz, and in 1930 the International Electrotechnical Commission, or IEC, established the unit of frequency of one cycle per second as being one Hertz, in his honor. This is abbreviated with a capital H and a small z.
Following standard metric usage, a frequency of 1,000 cycles per second, or 1,000 Hz, is thus equal to 1 kilohertz.
As time went by, it was discovered that measuring a radio signal by its frequency in kilohertz was more accurate and easier to express than its wavelength in metres. In addition, as the broadcast bands became more crowded, it was decided by international convention to adopt on mediumwave a 10 kilohertz separation between channels in the western hemisphere, and a 9 kilohertz separation in the rest of the world, including longwave in Europe. On shortwave, where signals have a narrower bandwidth, a 5 kHz separation between channels was adopted globally.
In spite of this, however, radio stations in Europe continued to announce their spot on the MW or LW dial in metres, right up until June 1984. During that month, they switched to using kilohertz instead, like most other regions had already done.
There is an inverse relationship between metres and kilohertz of 1:300,000. You can divide the wavelength in metres, or the frequency in kilohertz, into 300,000, and obtain the reciprocal.
As an example, for a shortwave station such as WWV which broadcasts on 10,000 kilohertz, you can divide 10,000 into 300,000 and see that its wavelength is 30 metres (just above the 31 metre broadcast band). Decades ago when I lived in the London area, the BBC had a 150 kW mediumwave transmitter there on 330 metres carrying Radio 4. Today that same transmitter now carries Radio 5 Live. You can divide 330 into 300,000 and see that the frequency being used is 909 kilohertz.
Sometimes, instead of kilohertz, you might find a radio station channel designated in megahertz. Longwave and mediumwave stations always use kilohertz, and VHF (FM) stations always use megahertz. When you hear that a station is on 89.9 FM, the frequency being used is actually 89.9 megahertz. But on the shortwave bands, between the mediumwave and FM bands, both kilohertz and megahertz are used.
To convert back and forth between kilohertz and megahertz, all you need to do is move the decimal point by three places, remembering that 1,000 kilohertz equals 1 megahertz. Thus for example, the shortwave channel 30.99 metres can be expressed as either 9680 kilohertz, or 9.68 megahertz.
The standard abbreviations for these three radio terms are expressed in this way:
Metres m Small m
Kilohertz kHz Small k, capital H, and small z
Megahertz MHz Capital M, capital H, and small z
(AWR/Wavescan NWS 264)
