There is a diffrence between FM bandwidth and channel spacing, though they can
be related.

The original two meter FM gear used in the 60's were converted surplus land
mobile radio service equipment. Due to the FCC imposed reduced channel spacing
of 30 kHz in the VHF LMR band, the two meter band inherited the same 30 kHz
spacing. The actual IF bandwidth of the receivers was 15 to 20 kHz, since the
combination of crytal frequency drift over temperature, the poor IF filter
quality available then and the signal bandwidth of a 5 kHz deviation transmitter
would stay within that passband.

Later improvements in IF filters (ceramic compound filter blocks) and crystal
oscillator temp stability allowed using 14 to 16 kHz bandwidth and that became
the norm for 5 kHz deviation in the two meter band too.

At UHF, the situation was similar, converted LMR UHF gear was adequate for 25
kHz channel spacing since that was what they used. Same 5 kHz deviation and
typically 20 kHz IF bandwidth since temp stability was less than VHF. Again,
improved fitlers and xtal osc design allowed narrowing to the same 14 - 16 kHz
IF bandwidth.


Now when you say narrow bandwidth, I wonder if you mean sliver bandwidth FM?
That was the latest change to hit LMR. VHF channel spacing went to 15 kHz (or
12.5 Khz at the top end) in the VHF LMR band and deviation dropped to 2.5 kHz,
plus 12.5 kHz spacing at UHF and 2.5 Khz deviation too. When I worked for
Midland years ago, I came up with a dual mode modification for their LMR gear
that used a compromise IF bandwidth of 9 kHz (four pole xtal filter in the first
IF) and a set of audio switches to go between 5 kHz and 2.5 kHz deviation,
programable by channel.



On Thu, 31 May 2007 01:52:03 -0400, "Bob D." wrote:

What is the bandwidth of standard narrow band FM as we use it on 2 meters?
Is there a rule of thumb based on deviation? (I don't want to get into the
heavy math, I just know it's more complex than AM or SSB.)

As an old timer, I remember 2 meter amateur FM started out with 30 KHz
channel spacing. Then, it went to 15 KHz. Then some deviants went to 12.5
KHz. New rigs have 5 KHz tuning. Is there an official channel spacing?