73.840
Part 73, Subpart G, Section 73.840 is a general technical rule related to the power level that the transmitter is set at.
The difference between transmitter power output and effective radiated power
Broadcast stations, including LPFM stations are authorized to operate a specific effective radiated power (ERP). The effective radiated power is the actual power of the station as the signal leaves the antenna. Between the transmitter and antenna, there are several factors that diminish the signal from the transmitter, this includes the type and length of the feedline (wire) from the transmitter to the antenna. The antenna itself may also play a factor. Some antenna configurations, such as the popular circular polarized antennas like the Nicom BKG-77, Nicom BKG-88 and Shively 6812 in a single antenna configuration will cause a loss in the transmitter power as it reaches the antenna while some antennas, such as the vertical-only Norwalk Dominator will exhibit a gain at the antenna.
To compensate for this, the LPFM station actually needs to operate at a power level that exceeds and in some cases, well exceeds the 100 watts (or whatever power level) ERP they are authorized. This is done through a calculation that looks at the "insertion loss" of these items in the transmission system. Based on this calculation, it will determine how much power needs to run from the transmitter in order to be able to generate the authorized ERP at the antenna. This is referred to as the "transmitter power output" or TPO. When the construction is completed, this TPO will be placed on the license application and when granted, will show on the station license in addition to the ERP.
The TPO tolerance for LPFM stations
Not all transmitters may have a setting to exactly match the TPO that was calculated. Some transmitters may only be adjustable in steps of more than 1 watt at a time. To address this, the FCC permits the LPFM station to operate at a variance in TPO that is between 90 and 105 percent of the value shown on the station license. This does not allow to operate at up to 105 percent of your ERP. Stations should not "push the envelope" on the TPO.
LPFM stations licensed for a TPO of less than 10 watts may run at any TPO less than what is authorized, but no more than 105 percent of that authorized TPO.
Regulatory history
Enacted in the 2000 LPFM Report and Order and has not been amended since.
Text of regulation 47 CFR §73.840
§ 73.840 Operating power and mode tolerances.
The transmitter power output (TPO) of an LPFM station must be determined by the procedures set forth in § 73.267 of this part. The operating TPO of an LPFM station with an authorized TPO of more than ten watts must be maintained as near as practicable to its authorized TPO and may not be less than 90% of the minimum TPO nor greater than 105% of the maximum authorized TPO. An LPFM station with an authorized TPO of ten watts or less may operate with less than the authorized power, but not more than 105% of the authorized power.