Why are/were some FM frequencies used more than others?

[101tm]

This isn’t an area of expertise for me, but how is it that some frequencies are “used” a lot more than others across the country? For example - 93.9, 101.1, 100.3, 103.5, 104.3, etc are heavily used frequencies, especially in large markets but other mid-sized to smaller ones as well. Meanwhile, a channel like 106.3, 101.3, 100.1, 95.9, etc are a lot less commonly used. Is there anything behind this phenomenon (if it even is one)?

A lot of the large markets like Detroit, NYC, LA, and Chicago seem to have gotten the exact same allocations for FM, while they didn’t for AM.

 

[kevtronics]

The high-power FM signals in a market had to be 800 kHz apart in order to prevent bleed-over on early, not-very-selective, drift-prone tube-based FM receivers. With that required spacing, you're bound to see the same patterns of frequencies repeated in many markets around the country. The gaps were then eventually filled in by lower-power (Class A) suburban stations.

 

[Kent]

This isn’t an area of expertise for me, but how is it that some frequencies are “used” a lot more than others across the country? For example - 93.9, 101.1, 100.3, 103.5, 104.3, etc are heavily used frequencies, especially in large markets but other mid-sized to smaller ones as well. Meanwhile, a channel like 106.3, 101.3, 100.1, 95.9, etc are a lot less commonly used. Is there anything behind this phenomenon (if it even is one)?

The likely reason for this is that we originally had only three classes of FM stations, A, B, and C, and, much like with AM's, certain frequencies were reserved for certain classes. The classes changed by 1980, which allowed B1, B2, C1, C2, and C3 stations.

Originally, channels 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, and 107.1 were reserved for Class A signals, at least on the US mainland. Around 1990, that changed, and we saw a realignment of stations as Class A channels on those frequencies wanted to upgrade, and other stations nearby were forced to move.

Class A signals were generally not desirable in large cities as they didn't cover the entire area. A handful of large cities had some stations on those channels in their suburbs, but you usually didn't see them in the city proper. Most of the stations on those Class A channels that you see in larger cities today were either moved in from the suburbs and upgraded or were forced onto those channels so other stations nearby could upgrade (see KILO 94.3 in Colorado Springs, which was originally on 93.9, for an example of the latter).

So, in larger cities, you had Class B and C stations that couldn't be on those reserved channels. I haven't observed fewer stations on those graveyard FM channels. Smaller towns had fewer radio stations, which meant fewer FM's, but those stations on graveyard channels could be squeezed closer together. Docket 80-90 meant, starting in 1980, more stations got authorized on those channels. Not many markets saw prime real estate handed out thanks to Docket 80-90, though a few did. Most of the Docket 80-90 stations were Class A pea shooters that couldn't do much.

 

[davideduardo]

So, in larger cities, you had Class B and C stations that couldn't be on those reserved channels. I haven't observed fewer stations on those graveyard FM channels. Smaller towns had fewer radio stations, which meant fewer FM's, but those stations on graveyard channels could be squeezed closer together. Docket 80-90 meant, starting in 1980, more stations got authorized on those channels. Not many markets saw prime real estate handed out thanks to Docket 80-90, though a few did. Most of the Docket 80-90 stations were Class A pea shooters that couldn't do much.

The biggest effects of Docket 80-90 were the allowing stations to upgrade in class and/or to move to a different city. A's could become B's or C's without going into a competitive hearing.

Docket 80-90 went into effect towards the end of the 1980s and was the result of the changes brought on after the FCC analyzed the Bonita Springs case where an "A" tried to upgrade, was filed against and lost its license.

 

[101tm]

Thanks! This explains a lot. Here in the southeast, at least SC/GA/NC, it’s common to have stations that were - in a lot of cases but not all - class A’s originally that wiggled their way in to bigger markets via moves, upgrades and sometimes frequency changes thanks to docket 80-90. In most southeastern markets it’s rare to see more than a few FM’s licensed to the principal city of the market since most are move-ins/upgrades that are licensed to suburbs and bedroom communities.

Many southeastern markets were not initially very big and market growth massively outpaced their northeastern/midwestern counterparts resulting in markets now having the same number of stations, but the southeastern markets having more later move-ins. Compare Atlanta to a market like Detroit, for example. Greenville and Columbia, SC, for example only have a few commercial FM’s licensed to the principal city, each…compared to a market like Buffalo where most of the FM’s are licensed TO Buffalo.

What got me thinking was here in Greenville-Spartanburg we have a 106.3 - a frequency I haven’t seen used in many larger markets if any. The 106.3 in question was originally a small town class A that made its way in to the market and was upgraded to a C3 in 2003. We also had a 96.7 which was a class A licensed to a small town - moved in to the market as an A in 2002, upgraded to a C3 in 2014 after moving to 104.9.

 

[Mark Roberts]

Class A signals were generally not desirable in large cities as they didn't cover the entire area. A handful of large cities had some stations on those channels in their suburbs, but you usually didn't see them in the city proper. Most of the stations on those Class A channels that you see in larger cities today were either moved in from the suburbs and upgraded or were forced onto those channels so other stations nearby could upgrade (see KILO 94.3 in Colorado Springs, which was originally on 93.9, for an example of the latter).

Actually, KILO was originally on 94.3 as a class A station, then moved to 93.9 in 1974 and upgraded to class C. Later, it moved back to 94.3, where it is today.

 

[PTBoardOp94]

I did lookups on a few frequencies you listed via FCC.gov, to see if your hunch was in fact correct.

95.9 has 114 full-service licenses
100.1 has 97 full-service licenses
101.1 has 100 full-service licenses
104.3 has 97 full-service licenses

 

[Greg Strickland]

This is an approximate count. Licensed on-air and licensed silent.

Frequency​	Full power licensed​
88.1​	282​
88.3​	187​
88.5​	183​
88.7​	167​
88.9​	170​
89.1​	154​
89.3​	170​
89.5​	153​
89.7​	153​
89.9​	168​
90.1​	155​
90.3​	158​
90.5​	174​
90.7​	158​
90.9​	151​
91.1​	172​
91.3​	178​
91.5​	179​
91.7​	208​
91.9​	208​
92.1​	142​
92.3​	88​
92.5​	94​
92.7​	120​
92.9​	92​
93.1​	73​
93.3​	93​
93.5​	102​
93.7​	101​
93.9​	79​
94.1​	87​
94.3​	102​
94.5​	95​
94.7​	74​
94.9​	77​
95.1​	85​
95.3​	110​
95.5​	82​
95.7​	91​
95.9​	106​
96.1​	95​
96.3​	69​
96.5​	84​
96.7​	106​
96.9​	81​
97.1​	69​
97.3​	74​
97.5​	85​
97.7​	106​
97.9​	88​
98.1​	81​
98.3​	106​
98.5​	80​
98.7​	82​
98.9​	73​
99.1​	89​
99.3​	99​
99.5​	103​
99.7​	64​
99.9​	101​
100.1​	90​
100.3​	91​
100.5​	70​
100.7​	99​
100.9​	91​
101.1​	90​
101.3​	71​
101.5​	96​
101.7​	102​
101.9​	83​
102.1​	87​
102.3​	118​
102.5​	90​
102.7​	78​
102.9​	85​
103.1​	105​
103.3​	80​
103.5​	71​
103.7​	83​
103.9​	108​
104.1​	90​
104.3​	84​
104.5​	84​
104.7​	94​
104.9​	123​
105.1​	96​
105.3​	91​
105.5​	136​
105.7​	90​
105.9​	90​
106.1​	96​
106.3​	113​
106.5​	84​
106.7​	83​
106.9​	90​
107.1​	114​
107.3​	80​
107.5​	81​
107.7​	76​
107.9​	97​

 

[RadioDaze]

Looks like 88.1 mHz leads the pack by far with 282. Wonder how many of those stations came online after the 2009 Digital TV conversion, as analog VHF channel 6's across the country went silent?

 

[kevtronics]

Looks like 88.1 mHz leads the pack by far with 282. Wonder how many of those stations came online after the 2009 Digital TV conversion, as analog VHF channel 6's across the country went silent?

There are only a few dozen "FrankenFMs" (channel 6 LPTV stations acting as radio stations), and the FCC has now allowed them to continue transmitting an analog audio carrier on 87.75 MHz.

I think 88.1 simply became the default "stick 'em at the bottom of the band" channel for low-power non-commercial stations in small markets.

 

[101tm]

Interesting list! I notice that the very frequencies that don’t seem very “occupied” to me - such as 106.3 and 107.1 - actually are! I guess so many are class A’s that they are in smaller markets generally or perhaps communities, while the seemingly more common frequencies like 101.1 actually aren’t, just more powerful signals and somehow the top markets mostly ended up with a 101.1 (San Francisco got a 101.3!).

 

[davideduardo]

There are only a few dozen "FrankenFMs" (channel 6 LPTV stations acting as radio stations), and the FCC has now allowed them to continue transmitting an analog audio carrier on 87.75 MHz.

I think 88.1 simply became the default "stick 'em at the bottom of the band" channel for low-power non-commercial stations in small markets.

And... there are few signals to protect on the "south side" of 88.1. So it nearly cuts in half the protection requirements with low band VHF vastly reduced.

 

[101tm]

And... there are few signals to protect on the "south side" of 88.1. So it nearly cuts in half the protection requirements with low band VHF vastly reduced.

I’m surprised there aren’t more 107.9s for this reason.

 

[kevtronics]

I’m surprised there aren’t more 107.9s for this reason.

The 108-136 MHz aircraft band is a pretty important thing to protect.

 

[Greg Strickland]

I will try to revisit this at some point one weekend. Do not over analyze the totals because they may not be exactly correct.
Totals were not hand counted, but there could have been an error in the Excel based process I used.

 

[TexasTom]

Interesting list! I notice that the very frequencies that don’t seem very “occupied” to me - such as 106.3 and 107.1 - actually are! I guess so many are class A’s that they are in smaller markets generally or perhaps communities, while the seemingly more common frequencies like 101.1 actually aren’t, just more powerful signals and somehow the top markets mostly ended up with a 101.1 (San Francisco got a 101.3!).

Keep in mind that it could be more complex than just counting the number of stations on each frequency. It also matters where those stations are and what class they are. So just doing a count won't tell you whether certain frequencies are predominantly class A stations in smaller cities.

 

[trusty]

You also have to factor in power/HAAT/population/etc to get the most for your coverage area. For example, NYC stations cover a lot of population with lower powered stations than most of the rest of the country. The spacing of NYC stations seems to be the shortest allowed at the time, also.
Makes sense; take a map of the U.S. How many 100kw stations would it take to cover it? How many 3kw stations would it take to cover it? Mix all that up with growth patterns and predictions, and you really begin to have fun.
It's almost like the "solution" to all this would be to sandwich in a bunch of translators. Wow - what a concept.

 

[101tm]

I’m surprised all of Florida is allowed to have class C FM’s. Especially considering states like IN, OH, PA, IL and CA have such limitations yet they aren’t as “cramped” as FL. Indianapolis is limited to class B’s but Orlando gets to have C’s.

 

[ftballfan]

Speaking of graveyard FMs, northern Michigan has a lot of stations that were formerly 3kW Class A's:
99.3C2 WATZ Alpena was originally on 93.5
96.3C1 WLXT Petoskey was originally on 96.7
101.5C2 WMJZ Gaylord was originally on 95.3
100.3C1 WQON Grayling was originally on 100.1
107.5C2 WCCW Traverse City was originally on 92.1
97.5C2 WKLT Kalkaska was originally on 97.7

 

[PTBoardOp94]

I’m surprised all of Florida is allowed to have class C FM’s. Especially considering states like IN, OH, PA, IL and CA have such limitations yet they aren’t as “cramped” as FL. Indianapolis is limited to class B’s but Orlando gets to have C’s.

The population distribution of the USA was way different back then. Florida had roughly 1/3 the population of Illinois, or Ohio and 1/4 that of Pennsylvania in 1950.