Everything in this subject travels over one thin slice of radio spectrum. This lesson places that slice: where VHF sits among the radio bands, which part of it aviation speaks on, how the band was cut into channels and then cut again when Europe ran out of them, what limits how far a transmission carries, and the Morse code still ticking underneath every navigation aid.
The radio-frequency spectrum
The radio-frequency spectrum is divided into eight named bands, treated fully in the radio navigation course's propagation lesson; the ladder itself is exam material here too:
very low frequency (VLF): 3-30 kHz;
low frequency (LF): 30-300 kHz;
medium frequency (MF): 300-3,000 kHz;
high frequency (HF): 3-30 MHz;
very high frequency (VHF): 30-300 MHz;
ultra high frequency (UHF): 300-3,000 MHz;
super high frequency (SHF): 3-30 GHz;
extremely high frequency (EHF): 30-300 GHz.
The ranges cycle 3-30, then 30-300, then 300-3,000 through kHz, MHz and GHz in turn, each band ten times higher than the one before; reconstruct the ladder from that cycle and only the names need remembering.
The VHF band and the aeronautical slices
The rung that matters here is VHF: 30 to 300 MHz. Within it, aviation holds two neighbouring slices. From 108 to 117.975 MHz sits the navigation band of the VOR and the ILS localizer, which belongs to the radio navigation course. Immediately above it, 117.975 to 137 MHz is the band allocated to aeronautical mobile service voice communication: every tower, approach, area and information frequency a pilot speaks on lives inside those 19 MHz.
Channel separation
The band was originally divided into channels 25 kHz apart, which yields 760 channels. European congestion outgrew that, so each 25 kHz block was cut into three: the separation allocated between consecutive VHF frequencies is now 8.33 kHz, raising the count to 2,280 channels, with 25 kHz spacing still in use outside the converted airspace.
The subtlety the exam probes is that an 8.33 channel is selected by a channel name, not by its frequency. Names are six-digit designators stepping in increments of 0.005, and the name is deliberately not the tuned frequency: within the block starting at 132.0000 MHz, the three channels are named 132.005, 132.010 and 132.015, tuning 132.0000, 132.0083 and 132.0167 MHz. The figure draws the whole chain from band to block to channel.