Why odd FM frequencies in the USA?

[danikayser84]

Everyone knows that only odd FM frequencies are used in the USA, although some countries are even-only and others use both odd and even... was it to disambiguate from AM where every frequency ended in a 0? Was it completely arbitrary, or is there something technical behind the use of odd frequencies?

I know on some mainly older tuners, trying to seek past a strong station you would land on, for example "105.70, 105.75, 105.80" before moving to the next frequency; however, newer radios (such as in my MP3 player or my Tecsun) don't seem to have that issue

 

[Kent]

Was it completely arbitrary, or is there something technical behind the use of odd frequencies?

Is it arbitrary? Yes and no. No in the sense that FM stations are essentially allotted 200 kHz apiece. Since each station is given 200 kHz, you can't go from 92.1 to 92.2. Yes in the sense that the 200 kHz allotment and whether that .2 mHz difference should be odd or even are indeed arbitrary. In most of Europe, FM's get 100 kHz each.

 

[davideduardo]

Everyone knows that only odd FM frequencies are used in the USA, although some countries are even-only and others use both odd and even... was it to disambiguate from AM where every frequency ended in a 0? Was it completely arbitrary, or is there something technical behind the use of odd frequencies?

It is fairly simple. When FM was moved to 88 to 108 mcs in 1946 the first channel had to be 88.1 since a channel was defined as 200 kcs. Were they to start at 88.0 the band would really be 87.9, which was not the assignment.

 

[davideduardo]

Is it arbitrary? Yes and no. No in the sense that FM stations are essentially allotted 200 kHz apiece. Since each station is given 200 kHz, you can't go from 92.1 to 92.2. Yes in the sense that the 200 kHz allotment and whether that .2 mHz difference should be odd or even are indeed arbitrary. In most of Europe, FM's get 100 kHz each.

In Europe and even places in the Western Hemisphere, different allocations and protections are used. But a channel has to be at least 150 kHz wide since 100% modulation is almost universally defined as +/- 75 kHz.

 

[DJKraze]

Not all the world's AM ends in zero either, that's basically the western hemishpere. The rest is mostly 9 kHz channel spacing such as 540, 549, 558, and so on.

I'm not sure if I understand the spacing on Longwave though. At times it seems to be 9 kHz, and then along comes a channel not divisible by 9. I ran into this on the web controlled tuners on occasion, as you can set the channel spacing to 9k, but at some points, the channels are off pattern, anyone know?