FM Frequency Bandplans / LAPs (licence Area Plans)

[dxer2_2000]

Hi There ;D

Here in Australia we have our frequencies sorted into band plans. Eg. for the high power stations in Melbourne are on 105.1, 105.9, 106.7, 107.5 etc. Many other locations share the same band plan - so Mildura VIC or Cairns QLD will use the SAME frequencies mentioned above.

My location (Southern NSW) has the same bandplan as Maryborough QLD - ie 99.3, 100.1, 100.9 etc.

Do they have a system like that in the US or are they just allocated wherever there is space?

dxer2_2000

 

[Lawppy]

For the most part, US FM stations are allocated wherever there is space. But the only real organized frequency band plan I can think of is that cities like Detroit, Chicago, New York and Los Angeles all have their commercial stations spaced at .8 megahertz intervals, starting at 92.3, 93.1 and on up the dial to 107.5.

My location is roughly halfway between Chicago and Detroit. There is just enough space (235 miles) between the two cities that smaller FM signals (3kw to 6kw) can be placed on one of the aforementioned frequencies. However, most of our local stations are bigger signals that broadcast on the next channel up (92.5, 93.3) from the band plan described above. Since these stations also have to be protected, there is limited room for any more FM stations in the area even of the small variety. The last such allotment around here was filled in the late 1990's.

At the same time these stations are being protected, they're still far enough away that they don't wreak havoc on my FM dial, so it's a great DX location!

 

[w9wi]

Do they have a system like that in the US or are they just allocated wherever there is space?

These days they go wherever there's space, but the plan evolved over the years...

Firstly, frequencies between 88 and 92MHz are reserved for non-commercial stations. Secondly, a "table of allotments" exists - a list of frequencies assigned for use in specific cities. For example, 92.9/95.5/97.9/103.3/105.9MHz are assigned for use in Nashville, Tenn. An original table was created by government engineering staff, though today almost all of the original channels are in use and any new station uses a frequency added to the table as the result of a private petition.

Thirdly, for many years commercial FM stations were divided into classes. Class A stations were limited to 3,000 watts at a maximum antenna height of 300 feet. Class B stations (assigned in the densely populated Northeast and California) are limited to 50,000 watts/500', and Class C stations (assigned elsewhere) to 100,000 watts/2,000'. Several frequencies - 92.1/ 92.7/ 93.5/ 94.3/ 95.3/ 95.9/ 96.7/ 97.7/ 98.3/ 99.3/ 100.1/ 100.9/ 101.7/ 102.3/ 103.1/ 103.9/ 104.9/ 105.5/ 106.3/ 107.1MHz - were reserved for Class A operation only - no Class B/C stations on these frequencies - and no Class A stations on any other frequency. Finally, it was determined that stations in the same city must be at least 0.8MHz apart.

The 0.8MHz separation allows for two possible plans that allow the maximum number of Class B/C stations in a city:
92.3/93.1/93.9/94.7/...105.9/106.7/107.5 , and
92.5/93.3/94.1/94.9/...106.1/106.9/107.7MHz

You will find the first plan in use in New York, Detroit, Chicago, and Los Angeles. The second plan, with some modifications, exists in Philadelphia, Seattle, and San Francisco.

There are two other possible plans that maximize the number of stations in a city, but these plans would have 8 and 12 respectively Class A assignments. The FCC tried to avoid mixing Class A and Class B/C channels in the same city, so these plans were not used.

Any order fell apart pretty quickly though, as it was necessary to allow for other cities close enough to the very largest to be subject to interference but not close enough to receive service. To add to the chaos, in 1980 the government lifted the distinction between Class A and Class B/C channels, allowing any class on any frequency.

Today, there is no order. If you can fit it in without causing interference, you're in business.