Jeff wrote:

Yup. Here in NoCal, some parts of the 2-meter spectrum use 20 kHz
spacing, and others use 15 kHz. There was a proposal to move things
down to even narrower 12.5 kHz spacings a few years ago, but some
experiments (which I helped perform) demonstrated that a lot of the
then-available mobile and hand-held radios would suffer some pretty
severe adjacent-channel bleed-through - their IF filters aren't
sharp/narrow enough to avoid it. Getting people to cut their peak
deviation down to 2.5 kHz would also have been difficult (older radios
often don't have this available as an option, and those that do are
often easy to mis-adjust).

Hi

Just to point out that on 2m the Region 1 & 3 band plans used to use
25kHz spacing and moved to 12.5kHz about 20 years ago!!

The commercial world in Europe have used 12.5kHz spacing for even longer
on VHF.

Jeff

Adjacent channel interference is sometimes a problem, depending on the
width of your filters, the deviation of the adjacent station, and the
accuracy of the frequencies.
(FM stations often are accurate only to about 1.5 kHz and of course
when the station above you is 1.5 down it makes the interference worse)

While a 12.5 kHz spacing is used for repeaters here, generally the
frequency coordination is done in such a way that adjacent repeaters
are not 12.5 kHz apart.

In general, 12.5 kHz works well. Note that on 10m (and CB), the channel
spacing is only 10 kHz. That is more problematic, it requires a tiny
deviation and low audio cut-off frequency to do it well.