Revised September 14, 2013.
You may wonder why may notice that there are "open spaces" on the FM dial but you can not place an LPFM station there. This is due to the FCC's rules regarding interference protection. Let's discuss the different elements involved in determining how interference is predicted.
The FCC uses certain criteria to determine an FM radio station's service areas and interference potential by using measurements distances at certain field strengths. Field Strength can be measured in either microvolts per-meter (µV/M) or in decibels unterminated (dBu) (commonly just decibels). While µV/M is used in FM broadcasting, many of the broadcast rules are written in decibels so this is what we will use. The higher the decibel number, the stronger the signal.
Service and Interference Contours
Each broadcast station has two basic contour measurements:
Also sometimes referred to as a protected contour indicates the primary service area of the station. This is the primary area that the FCC protects for FM stations. Most stations including LPFM are protected to a 60 dBu (1 µV/M) contour. For 100-watt LPFM stations (LP-100), this creates a service contour of 5.6 kilometers (km). Certain full power FM stations in certain areas of the country have a larger protected area by extending the service contours to 54 or 57 dBu. Full power stations in Puerto Rico and the Virgin Islands have much larger service contours due to the area's geography. Service contours are measured using a method called F(50,50) meaning that estimated field strength exceeded 50 percent of the potential receiver locations 50 percent of the time at a receiving antenna height of 9 meters. Non-commercial FM rules (not LPFM) require that the community of license be within the primary service contour of the radio station.
The Interference Contour is a signficantly larger contour. This is a contour that a station has the potential to radiate to but is less likely but there is still a potential for interference. There are different interference contours for co-channel (same channel), first adjacent, second adjacent and third adjacent channels with the co-channel being the largest of the interference contours. Unlike the service contour, the interference contour is measured using F(50,10) or estimated field strength exceeded 50 percent of the potential receiver locations 10 percent of the time at a receiving height of 9 meters.
In addition to these contours, commercial FM stations also have a City Grade Contour. This contour is intended to show the area where a superior quality signal is available. The City Grade contour for all commercial stations is 70 dBu, which makes it smaller than the service contour. In full power commercial FM rules, the entire city of license must be in the city grade contour of the FM station.
How Interference is Determined
The formula for determining interference between two stations involves determining the size of both station's service and interference contours. There is a formula based on the station's power and antenna height above average terrain (HAAT). The distance between the two stations is measured. If the sum of the sizes of the service and interference contour exceed the distance between the two stations, then you have overlap and the potential for interference. In normal full power rules, this measurement must be made both ways. There are additional considerations that are used when directional antennas are used which we will not get into here. Another interference model, referred to as Longley-Rice also uses actual terrain data to determine a station's coverage areas. This model is currently accepted in the digital television rules.
Several groups in the LPFM grassroots are asking the FCC to look at a prohibited overlap model that does take into consideration the actual powers of the stations, directional antennas for either station and other factors. The FCC has interpreted provisions of the Local Community Radio Act to prohibit contour models as it may be seen as "reducing the spacing between LPFM and full service stations". There are provisions in the LCRA to allow the FCC to use overlap models for waiving second adjacent channel protections.
Depiction of two stations fully spaced. In this drawing, you will see two stations. The black circles represent the service and interference contour of one station. The blue circles represent the contours of a higher powered station. Notice how that even though the interference contours of the two stations are overlapping that neither interference contour is actually crossing into a service contour. This means that these two stations are fully spaced and therefore legal as neither station will place interference into the other station's primary service contour.
In the commercial rules (as well as the LPFM rules), all broadcast stations as having full facilities with non-directional antennas over flat terrain. Therefore, distance separation tables are used. To determine the distance, we look at the distance from "Station A" to its 60dBu (or 57 or 54 dBu) service contour and the distance from "Station B" to its interference contour and then we add those two distances together. Then we go the other way and look at the distance to Station B's service contour and the distance to Station A's interference contour. The greater of the two distances is then the minimum distance required.
How terrain can affect the contour size
When the service and interference contours are measured, they are based on several factors. If the station has a directional antenna, that is first taken into consideration. For example, if a directional antenna for a 1kW station only directs 50% (0.500) of power in a particular direction, then the Effective Radiated Power (ERP) will be considered as 0.5kW in that direction. Then height above average terrain (HAAT) is considered. The FCC makes elevation measurements at various points along a radial in the specific direction being checked and then uses the average measurement to determine HAAT (which we call "effective HAAT"). Then based on the adjusted power (if any) and the HAAT in a particular direction, the distance to the contour is measured. This is repeated up to 360 times for each possible direction.
If you have a station that is located in the foothills with a large mountain behind you and a valley in front of you, you may get a low overall HAAT because the high elevation of the mountains and the lower elevation of the valley average out (usually in favor of the valley). As a result, an LPFM station will have a very small contour towards the mountain but towards the valley, the service contour size can extend well past twice the size of the normal 5.6 km service contour size. HAAT that controls what power the LPFM station can actually operate from takes the average HAAT in 12 different directions and then averages that result. As long as your overall HAAT is 30 meters or less, you can run a full 100 watts.
The "Buffer Zone"
Now in the LPFM rules, the FCC did something a little different. In order to maximize the number of LPFM stations possible, the FCC will allow a full power FM station's interference contour to overlap partially into a LPFM station's service contour. In relation to FM translators, an LPFM station can cross well within a the translator's interference contour. In no case shall the service contours of the LPFM and the other station overlap.
For full power stations, a 20 km "buffer zone" is given. This was designed to permit a full power station to be allowed to change to a different location without causing the two service contours to overlap. Therefore, a service contour of an LPFM station can not cross into the buffer zone.
When you search for an available LPFM Channel on REC, you will sometimes see the channel referred to as an "interference channel". This means that the other station's interference contour crosses into the proposed LPFM station's service contour. Those on interference channels may or may not receive actual interference or you may receive interference in a portion of the LPFM service contour. This can be caused by the actual power of the other station (remember, for the rules, all LPFM stations are considered at full facilities), the other station's location and the terrain in between (are there mountains the block the signal from the other station reaching your area?).
Not all stations are created equal
In order to simplify the LPFM rules, the FCC used minimum distance seperation requirements based on a full-power station's maximum allowed facility. You may find that not all full power stations (especially Class-C stations) do not operate at the full facilities for their class. Clicking the [Interference] button next to a channel is a good pre-step to look at the location of the LPFM station in relation to the estimated interference contour based on the actual facility including power, terrain and directional antenna pattern (if there is one). Any reputable consultant or engineer can also look up the interference contours to give you an idea of the field strength of other stations. Also, you can take a drive around your service contour while monitoring the desired channel for the LPFM station. You can determine the noise floor. Just know for very weak stations, the LPFM station may be able to overcome any interference but in fringe areas towards the other station, coverage may be limited.
Encroachment is a term that REC had coined in the LPFM community to indicate application activity being made by a full power station that can result in either increased interference to or complete displacement of a LPFM station. LPFM is a secondary service, meaning that if the LPFM station causes interference to a full power station, the LPFM is responsible for resolving the interference. FCC rules indicate when full legal interference takes place. Encroachment will not be covered here, instead you can read our section on Encorachment.
Intermediate Frequency (IF)
In addition to the interference restrictions above, the FCC also has a restriction called intermediate frequency. All radio receivers also generate radio frequency. FM receivers normally generate a signal at 10.7 Mhz. Some FM receivers may mix that 10.7 frequency with the signal of a very strong FM station and create interference approximately 10.7 MHz up or down from the strong station. As a result, the FCC has a requirement that full power FM stations, LPFM and translators over 100 watts also protect the IF channels. Since FM channels are spaced every 200 kHz, there is no true FM channel exactly 10.7 MHz above or below the station's frequency. Instead, the FCC protects both the channel 10.6 MHz (53 FM channels) and 10.8 MHz (54 FM channels) above and below the station's frequency. The IF is calculated by taking the sum of the 91 dBu contour (F 50,50) of both stations.
LPFM stations are only required to protect Intermediate Frequency in respect to foreign FM stations.
TV Channel 6 Protection
Full Power and LPFM stations operating in what is known as the Reserved Band from 88.1 to 91.9 MHz must also protect TV Channel 6 stations. Channel 6 protections are measured by adding the sum of the Grade-B contour (which is TV's version of a Service Contour) and a certain interference contour of the radio station. The size of the interference contour is different for each of the 20 channels in the reserved FM band and as you go up in channel, the interference contour gets smaller. In the LPFM rules, all TV Channel 6 stations (both full power and low power) are considered having full facilities (even though only a dozen low power TV stations have anywhere near full facilities).
Now that you have an understanding of how interference protection is calculated. You may refer to §73.807 of the FCC rules for LPFM stations, §73.207 for full power commercial FM stations, §73.507 for full power non-commercial stations and §74.1204 for FM translators to determine the required distances or field strengths needed to protect other stations.