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 two different methods:
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). 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 of 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.
The Old Policy from prior to 2012
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.