For those interested in this band, whilst 8.33khz is the correct channel spacing in use in the European area (some users enjoy/ed an exemption, still using 25khz steps), frequency information passed by ATC's and aircrews is truncated to the nearest 5khz and for practical purposes it is worth using this step size - it seems to work pretty well (with the caveat that I am a novice), and using uno's great scanner function with a 5khz step, the 118 - 138 mhz range can be scanned in about 3 seconds, it's PDQ!
In the past, voice communication used a simple AM carrier with 50 kHz separation and a “telephone”
audio bandwidth of 300 Hz to 3.4 kHz. This became insufficient in the VHF band of 118 – 137 MHz
and so the channel separation was reduced to 25 kHz. This has been the standard for many years
now. In order to meet the demand for more channels, the channel separation in many areas has been
reduced by a further factor of three to 8.33 kHz. This has the advantage of offering backwards
compatibility to older radios which can still use every third channel on the original 25 kHz frequencies.
This closer channel separation leaves very little guard band between adjacent carriers and so the
audio bandwidth has been reduced to 350 Hz to 2.5 kHz which still gives acceptable voice quality.
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