VHF Propagation — Page 139, Lesson 123

I want to walk you through the basics of VHF propagation, starting with the fundamental units we use to measure frequency. Frequencies are expressed in Hertz, abbreviated Hz, and one Hertz equals one cycle per second. Because radio frequencies are very high, we use convenient larger units. One kilohertz, or kHz, is one thousand Hertz, or ten to the power three Hertz. One megahertz, or MHz, is one million Hertz, or ten to the power six Hertz. One gigahertz, or GHz, is ten to the power nine Hertz. And one terahertz, or THz, is ten to the power twelve Hertz. Now let's look at how the entire radio frequency spectrum is divided into bands. The bands that matter most for radio communications are the VHF band and the HF band. Starting at the lowest frequencies, we have VLF, or Very Low Frequency, from 3 to 30 kHz, with wavelengths from 100 kilometres down to 10 kilometres. It's used for very long range navigation. Next is LF, Low Frequency, from 30 to 300 kHz, wavelengths 10 kilometres to 1 kilometre, used for NDB and Decca navigation systems. Then MF, Medium Frequency, from 300 to 3000 kHz, wavelengths 1 kilometre to 100 metres, also used for NDB. Above that comes HF, High Frequency, from 3 to 30 MHz, with wavelengths from 100 metres down to 10 metres. This is the band used for HF radio telephony, or HF R/T. Then we reach VHF, Very High Frequency, from 30 to 300 MHz, wavelengths from 10 metres down to 1 metre. This is the band used for VHF R/T, VDF, VOR, ILS, and marker beacons. Above VHF we have UHF, Ultra High Frequency, from 300 to 3000 MHz, wavelengths 1 metre to 10 centimetres, used for ILS glidepath, DME, and some surveillance radar. Then SHF, Super High Frequency, from 3 to 30 GHz, wavelengths 10 centimetres to 1 centimetre, used for PAR, some surveillance radar, Doppler radar, and the radio altimeter, abbreviated AWR. Finally, EHF, Extremely High Frequency, from 30 to 300 GHz, wavelengths 1 centimetre to 1 millimetre, used for airfield surface movement radar. Now let's zoom in on the part of the VHF band that concerns a pilot. The frequencies from 88 to 107.95 MHz are used for FM broadcasting. From 108 to 117.95 MHz, we have radio navigation, using both AM and FM modulation. And from 118 to 136.975 MHz, we have radio communication, using AM modulation. This last band, 118 to 136.975 MHz, is the one used for aeronautical voice communications.