Why do Radio Frequency's always end in odd numbers?

[timf65]

I know alot of people will think im stupid for not knowing this but ive always wanted to know the reason why Frequencies have to end in odd numbers like 101.3, 105.7 etc.

 

[BarryATL]

In other countries they do not. In North America it was decided to odd numbers to achieve channel separation. Just as on the AM band North America separates by 10khz (660khz, 670, 680, etc.). In Europe they go to 1khz (660khz, 661, 662, etc.).

It was just an easy way to map out where stations would go geographically and on what frequency and not interfere with each other.

 

[KeithE4]

In other countries they do not. In North America it was decided to odd numbers to achieve channel separation. Just as on the AM band North America separates by 10khz (660khz, 670, 680, etc.). In Europe they go to 1khz (660khz, 661, 662, etc.).

The rest of the world (outside the Americas) separates their AM channels by 9 kHz, and the frequencies are normally divisible by 9, starting with 531, 540, 549, 558, and so on up to 1602 kHz. I'm not sure if they use the expanded band, but if they do, the highest frequency is 1701.

FM channels are on odd decimals because a channel's width is 200 kHz wide, and the band extends from 88.0 to 108.0 MHz, including the bandwidth of the channel. A station on 88.1 actually transmits in the band 88.0 to 88.2 MHz (analog FM is a bit narrower, but a guard band was built in when the channels were allocated in the 1940s).

I believe that pretty much anything goes on FM outside the Americas. Even-number-decimals (94.0, 107.2) are allowed and, IIRC, there are even some stations on 50 kHz channels (98.15 MHz, for example).

 

[cd637299]

I believe that some African nations are still on the 2-decimal plan (97.15 MHz for example)....maybe Italy too? not sure.

As to the 10 KHz AM separation, that's the plan in all the Americas, except for some "grandfathered" AM stations, ZIZ 555 kHz in St. Kitts being the most prominent. ZIZ might be the last one remaining. With more and more digital receivers out now, ZIZ is truly the oddball.

As to even-decimals, there are exceptions in the Americas. Colombia adopted the even-decimals (as well as the odd-) almost 20 years ago. I am in the skip zone from Colombia (I am in Florida), and have been able to hear Colombian FM on these frequencies. The Francophone islands (including Guadeloupe & Martinique) have evens as well.

cd

 

[stormy01]

The rest of the world (outside the Americas) separates their AM channels by 9 kHz, and the frequencies are normally divisible by 9, starting with 531, 540, 549, 558, and so on up to 1602 kHz. I'm not sure if they use the expanded band, but if they do, the highest frequency is 1701 kHz.

Yes, in Europe, Africa, Asia and Oceania (Australia/New Zealand/etc.), the MW band continues in 9 kHz steps from 1602 until 1701 kHz.

 

[davideduardo]

I know alot of people will think im stupid for not knowing this but ive always wanted to know the reason why Frequencies have to end in odd numbers like 101.3, 105.7 etc.

That's because the band is 88 MHz to 108 MHz. Each station uses a channel 200 kHz wide. So if you are licensed to 92.3, your bandwidth is really 92.2 to 92.4: modulation at 100% occupies 150 kHz bandwidth, and there is a 25 kHz safety band above and below the 100% modulation portion.

So, with the band beginning at 88.0, the first channel has to be 88.1, and the last one has to be 107.9.

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Oops. I posted this and did not realize Keith had already made the same point. Sorry for the duplication.

 

[Ai4i]

Also, although 108.0 is a brick wall with aircraft navigation above it, many countries outside the Americas extend well below 88.0. China and Japan extend two TV channels below our bottom edge.