I'm studying for my ham test and have a question that isn't clearly answered in my study guide.

On 2m, it seems channels are spaced 15Khz or 20 Khz or 30KHz apart, and (I think) channels have a bandwidth of 16KHz. Wouldn't that means the channels overlap on 15KHz channel spacing?

Next, I read somewhere that there is 6KHz guard band between channels. But mathematically that doesn't fit either 15KHz or 20KHz spacing.

Can someone clarify this for me? Are there supposed to be 6KHz guard bands between channels? And the above assume 16KHz channel bandwidth (but I read somewhere online about narrow and wide band channels, so I don't know if that is a factor)

I'm in Canada in case that affects the answer.

  • 3
    $\begingroup$ Amateur radio in general is not channelized at all. (There are exceptions.) Repeaters look like they are channelized, but it's by convention only, and who says the conventions need to be sane? It does matter that you are in Canada. In the US, it even matters what state you are in, as different repeater coordinators use different spacing sometimes. Some are even going narrower than the 15khz spacing now. $\endgroup$
    – user10489
    Commented Aug 13, 2022 at 23:55
  • $\begingroup$ Back in the Dark Ages (1970's) repeaters were spaced at 30kHz and all of them were at or above 146.01. As 2m became wildly popular, all the channels became occupied and started interfering with each other, so the community opted to use a "split channel" (i.e. 15kHz) with an inverse offset (below 147, repeater input was the higher frequency). But then some of the stronger repeaters got into the inputs of other repeaters in the next 15kHz. Later (1980's) some groups decided to move to a 20kHz spacing to help alleviate that, but not all groups went along. That's some of the history. $\endgroup$
    – Duston
    Commented Aug 16, 2022 at 13:44
  • $\begingroup$ Also, as technology has advanced and bands got more crowded, some coordinators have reduced channel width to fit more of them in. $\endgroup$
    – user10489
    Commented Aug 17, 2022 at 4:31

1 Answer 1


Amateur FM radio standard is/was 5kHz deviation. Mathematically FM has infinite bandwidth, but for practical purposes there's Carson's rule, in which 98% of the energy is contained, given as BT=2(Δf+fm), where Δf is the deviation (5kHz), fm is the maximum modulating frequency (typically less than 3kHz audio bandwidth for speech communications), and BT is the 98% bandwidth - so yes, 16 kHz.

In practice though, many radios, including repeaters, use much less than 5kHz deviation, and the actual 98% bandwidth is lower. I look at my local repeater on scope (QT GUI Frequency Sink, which I see you are probably familiar with), and it only takes up about 5 kHz with any significant signal over the noise and it sounds just fine. So in practice, 15kHz space works, as long as frequencies are pretty close to nominal, which these days, with good crystal control, they are. I once tested the deviation on my Icom IC-7100 and it was under 3kHz, even though it was set to 5. Older radios I've tried go wider. I think 3kHz deviation is quite common, which puts the Carson's rule bandwidth at a comfortable 12kHz.

I have no idea where the 6kHz guard bands come from :-).

73, Chris VE3NRT


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