Why are the VHF and UHF airbands directly adjacent to frequency bands used by vital radionavigation aids? Of the three main frequency ranges used by aircraft for radio communications (HF, VHF, and UHF), two (VHF and UHF) have their aircraft-radio-communications frequencies clumped together into a contiguous (or almost contiguous) airband1 (117.975-137 MHz for VHF, and 225-328.6 MHz and 335.4-399.9 MHz for UHF2).

Interestingly, and somewhat worryingly, each of the VHF and UHF airbands is immediately adjacent to a band of frequencies used by vital radionavigation aids:


The VHF airband (117.975-137 MHz) lies immediately above the 108-117.975-MHz band used by VOR and localiser transmissions.3
The two UHF airbands (225-328.6 MHz and 335.4-399.9 MHz) are separated by the glideslope-transmitter band (328.6-335.4 MHz).


This raises the possibility that a malfunctioning (or even just poorly-tuned) VHF radio could cause interference for VOR or localiser signals if it went even slightly out-of-band, or (much worse) that a mistuned or malfunctioning UHF radio could end up stepping on somebody’s glideslope.

Given the navaid-interference potential thereby caused by this adjacency of radiocommunications and radionavigation bands, why do both the VHF and UHF airbands directly border frequency bands used by vital navaids, and how are out-of-band VHF and UHF radio transmissions prevented from thereby causing interference with these navaids?



1: The HF frequencies used for aeronautical radio communications, in contrast, are split into multiple discrete bands spread out throughout the entire HF range.  This is because the main attraction of HF is its ability to transmit and receive signals far beyond line-of-sight range (potentially even enabling an aircraft to talk to someone at the aircraft’s antipode, on the opposite side of the planet); which HF frequencies work best for this depends on the current state of the earth’s ionosphere, which varies considerably due to things such as solar activity, variations in the Earth’s magnetic field, recent high-altitude nuclear explosions, supernovae, witchcraft, etc., so the aeronautical HF bands are scattered throughout all HF in order to ensure that there should be at least one useable band no matter what (the frequency bands used for shortwave radio broadcasting are likewise scattered throughout most of the HF range [although, surprisingly, not the lower end], for the same reason).  In any case, the HF range has no radionavigation bands of any sort whatsoever, making it not a concern for the purposes of this question.

2: Which, if you want to be pedantic, places a good portion of the UHF airband in what is actually the top end of the VHF range.

3: And the 108-117.975-MHz radionavigation band is, in turn, immediately underlain by the FM broadcast band (88-108 MHz), but, as there are fewer FM radio stations than there are VHF-radio-equipped aircraft, and they are bigger and easier to spank if they get out-of-band, this is less of a concern from the interference-with-navaids perspective.
The most obvious reason is that the ITU allocated the VHF and UHF bands for aviation purposes generally, and then aviation authorities had to split those limited bands into adjacent comm and nav sub-bands.

If aviation had gotten separate ranges for nav and comm, then that would double the risk of interference from adjacent non-aviation users, plus it would have made designing radios and sharing antennae more difficult.

So, we got the lesser of two evils.