FREQUENCY V METRES

© RN Communications Branch Museum/Library

  1. Frequency = 'f' versus Metres {Wavelength} = 'λ'. Take the speed of light 'V for velocity' as being 186,000 miles per second = 300,000 metres per second. If we were to reduce this down further to 300 metres per second, resultant 'f' would be in kHz and resultant 'λ' would be in kM. From the formula λ = v ÷ f, we can calculate either Frequency in kHz or Wavelength in kM. For example, if one were to listen to Cricket Test Match Special on Long Wave (LW) on a frequency of 198kHz, the wavelength would be λ = 300 ÷ 198 = 1.52kM = 1520 Metres. Also, a Short Wave (SW) transmission on Band 4 on 20 Metres (0.020 kM) would radiate on a Frequency of f = v ÷ λ where f = 300 ÷ 0.02 = 15000 kHz = 15 MHz.

  Other examples could be taken from the chart below. To work out the length of an aerial, say an AJE, transmitting on 300 MHz, first look at Band 5 to find 300 MHz. Its equivalent in Wavelength is 1 Metre. The AJE is a dipole (centre-fed) so its wavelength needs to be λ ÷ 2, or half wavelength. Thus the wavelength should be half a metre in length. If using a whip (a monopole) aerial which is end-fed and requires λ ÷ 4 (quarter wavelength) say, a type AWF, for a transmission on 8MHz, then the electrical length of the aerial when tuned (in resonance with the associated transmitter) would be, from Band 4, 37.5 metres ÷ 4 = 9.38 metres. The Type AWF was a 10.7 metre physically long whip aerial sitting on a base tuner. At higher than 8MHz frequencies the electrical length was shorter and at lower frequencies, longer. At lower frequencies, the transmitter was routed to a base tuner employing a horizontal wire aerial.

2. dB's

3. dB's Explained

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