LPFM Second Adjacent Channel
Revised July 13, 2013
The Local Community Radio Act of 2010 has given LPFM stations the ability from locations and on channels that are short spaced to second adjacent channel broadcast stations under the conditions that no interference will result.
The Current Policy
Under the new second adjacent waiver policy, there is a small overlap area where the LPFM station is predicted to interfere with the full power station operating on a second adjacent channel. The LPFM applicant must make a showing that there iare no "potential listeners" within that overlap area.
This can be done through three different methods:
Lack of contour overlap
For second-adjacent stations that are either not operating at the maximum power for their service class or are operating directional antennas. The first check should be is to determine if the interfering contour of the proposed LPFM facility does not overlap the protected service contour of the short-spaced second adjacent channel station as shown:
- For class B stations operating on 92.1~107.9 (Channels 221~300), the 94 dBu LPFM interfering contour cannot overlap the 54 dBu protected service contour of the short-spaced second adjacent channel station.
- For class B1 stations operating on 92.1~107.9 (Channels 221~300), the 97 dBu LPFM interfering contour cannot overlap the 57 dBu protected service contour of the short-spaced second adjacent channel station.
- For all other service classes (A, C3, C2, C1, C0 and C) on all channels, class B & B1 stations operating on 88.1~91.9 (Channels 201~220) and FM translator stations on all channels, the 100 dBu LPFM interfering contour cannot overlap the 60 dBu protected service contour of the short-spaced second adjacent channel station.
The waiver request will require a contour map showing this lack of overlap.
The "Living Way" method
You need to determine the predicted field strength in F(50,50) curves of your second adjacent channel short spaced station(s) at the proposed LPFM site. If there are two stations, you must use the weaker station (lower dBu). Add 40 dBu to that number and then determine the free space distance based on that calculated field strength and the proposed LPFM station's power in ERP. That distance is your overlap zone. If you measure from the radiation center of your proposed antenna downward the distance of the overlap zone, if it does not reach the ground or any occupied floor on a building, then your station is predicted to not cause interference. If your antenna is on a self-supporting tower and the overlap zone would reach the ground, draw a circle on a map around your tower at the distance of the predicted overlap zone. If there are no occupied buildings or major highways within that overlap zone, then your station is predicted to not cause interference.
The "Downward Radiation" method
If your antenna is on a rooftop or otherwise in a situation where using the "Living Way" method will cause your overlap zone to reach occupied structures or highways, you may be able to use the vertical elevation data for your antenna available from your antenna manufacturer to make a case that the design of the antenna limits the radiation downward within the overlap zone and as a result, the overlap zone would not reach any occupied structures or major 4-lane highways.
REC's myLPFM can determine if there's a possibility of a second adjacent channel waiver using either method. For filing assistance on second adjacent channel waivers, please contact REC Networks at 1-844-REC-LPFM.
In some rare cases, a directional antenna can be used by an LPFM station in connection with the three methods above to demonstrate a lack of interference to a second-adjacent channel station. LPFM stations operating directional antnennas must operate their authorized effective radiated power in the peak direction(s) of the antenna. It is very important to remember that in broadcasting, unlike in ham radio, a directional antenna cannot be used to "improve" coverage. In fact, a directional antenna will diminish coverage in the directions where the antenna does not radiate out towards. LPFM stations constructing directional antennas for the sole purpose of achieving a second adjacent waiver are not required to have a proof of performance and verification by a surveyor. If using a directional antenna solely for a second adjacent channel, you will still need to show on the LMS application, that the facility is "non-directional" and you will need to include the antenna's directional pattern in an exhibit. Selecting non-directional in LMS assures that (1) it does not trigger the proof of performance requirement on the construction permit and (2) assures that the LPFM station will still be protected by FM translators as if they are a nondirectional facility. Directional antennas can be very tricky and should not be achieved by those who do not have the appropriate software to engineer such an antenna. For assistance with this, please contact REC for application handling.
For more information about directional antennas for LPFM, see REC's LPFM Directional Antenna information.
Things to remember about second adjacent waivers
- In most cases, it is better for the LPFM station to be closer to the short-spaced second adjacent channel station(s) than further away. The closer you are, the smaller the overlap zone will be.
- The short spaced FM stations can, at any time, file a complaint against the LPFM station claiming interference. In these rare cases, the LPFM will need to shut down until the interference issue can be resolved or proven that the LPFM station was not the cause of the interference.
- If a second adjacent waiver is based on a downward radiation study, that study was based on a specific antenna model and configuration (number and spacing of bays). To prevent future complaints, it is best to construct the station using the antenna model and configuration detailed in the downward radiation study.
The Old Policy from prior to 2012
(Not current policy) The new LCRA version of second adjacent channel waivers differ from the FCC's previous second adjacent channel waiver process as it requires that no interference takes place. In the previous policy, the FCC permitted population to exist in the interference area. The FCC's justification was that the retention of an LPFM voice is more in the public interest than a small amount of predicted interference. The FCC's old policy only applied to existing LPFM stations who were being displaced from their current channel because of a full power station making a move that would "encorach" on the LPFM station. See also: REC advice for LPFM stations currently on Second Adjacent Channel waiver STAs.
For more information
See the FCC decision in Living Way Ministries, Inc.