[Radio Wrangler]

It's almost designed to be confusing.

The channel plan was designed around radios with a display resolution of 5kHz yet the channels are on multiples of 8.3333333..kHz

When the radio displays 118.000MHz, its transmit frequency is 118.000MHz, the receiver is centred on 118.000MHz and the wide receiver filter is selected (the bandwidth previously used with 525kHz channel spacing) this channel is one of the '25kHz' ones. The receiver is thus wide enough to receive multiple signals on carriers offset +/- several kHz... there is room enough for the centre and four offsets to be used. So ground stations

When the radio displays 118.005MHz, its transmit frequency is still 118.000MHz and the receiver is still centred on 118.000 MHz but now the narrow receiver filter is selected. This is an '8.33kHz' channel! There is just enough receiver width for one offset ground transmitter on each side of the channel centre one.

When the radio displays 118.010MHz it's transmitter is on 118.008333..MHz and the receiver is centred there and is narrow.

118.015 gives you 118.016666..MHz, narrow.

You won't get 118.020 it should just skip it

118.025 is a return to sanity. You get 118.025 and wide receive.

118.030 gives you 118.025 narrow

118.035 gives you 118.033333..MHz narrow
118.040 gives you 118.041666..MHz narrow

118.050 is a return to WYSIWIG and is wide.

And so on and so on

So the indicated frequencies aren't real frequencies at all, they've become just channel names. As you wind upwards through the channels, the radio switches between wide and narrow receiver filters. If you're lucky, the radio has an 8.33/25 switch and in the 25 mode, it just gives you the wide channels on their 25kHz spaced centres and you need only bother with the 8.33kHz channels when you need to use one.

It's no problem for pilots. When told to dial up a channel, they just do it and the radios take care of getting the right frequency and IF filter.

If you're listening to aviation comms using a non-aviation radio, they you're tuning in real megahertz and the discrepancies between channel names and actual frequencies will show up.

Where the ground stations use multiple offset transmitters to increase their coverage, they plan which is on what offset so that if offsets are re-used, at least the transmitters are so far apart that an aircraft at an altitude which will need that ground station will not be getting strong signals from more than one at a time. On '25kHz' channels you may find offsets up to +/-7.5kHz in use. Radios are required to have a lot of audio attenuation above 3.2kHz.

David