LPFM Checklist: Technical Operations

In this section, we look at the technical operations of the transmission system to assure that your station is within tolerances.

Power vs. Height

For LPFM stations, the maximum facilities will be based on 100 watts effective radiated power (ERP) with an antenna radiation center of 30 meters height above average terrain (HAAT).An LPFM station with a HAAT that exceeds 30 meters will not be permitted to operate with an ERP greater than which would result in a 60 dBu contour of 5.6 kilometers.No facility will be authorized in excess of 1 watt at 450 meters HAAT.

The minimum facilities will be based on 50 watts at 30 meters HAAT or the equivalent necessary to produce a 60 dBu contour of at least 4.7 kilometers.

Transmitter Power Output (TPO)

A station's TPO is the power that is actually produced by the transmitter, which taking into consideration the insertion losses created by feedline, lightning arrestors and connectors as well as the gain or loss of the installed antenna will produce the station's authorized effective radiated power (ERP).

The TPO of an LPFM station is determined by procedures outlined in §73.267 of the rules.If the operating power of a LPFM authorized a TPO more than 10 watts must be maintained as near as practable 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 with an authorized power of 10 watts or less may operate with less than authorized power but nor more than 105% of what was authorized.

In the event that it becomes technically impossible to operate, then a station may temporarily discontinue operation for a period of not more than 30 days without specific authority from the FCC.If the operation is terminated for 10 consecutive days, then a notification must be sent to the FCC Media Bureau, Audio Division, Washington DC, 20554 no later than the 10^th day of the terminated operation.If normal power is restored prior to the expiration of the 30 day period, the licensee must notify the FCC upon restoration of normal operation.

Direct vs. Indirect Method

The operating power of LPFM stations may be determined by the direct or indirect method.

Direct method

The direct method of power determination of an LPFM station uses the indications of a calibrated transmission line meter located at the RF output terminals of the transmitter.This meter must be calibrated whenever there is any indication that the calibration is inaccurate or whenever any component of the metering circuit is repaired or replaced.

Indirect method

The indirect method is determined by applying the appropriate factor to the input power to the last radio-frequency power amplifier stage of the transmitter using the following formula:

Transmitter output power = E_p x I_p x F where:
E_p = DC input voltage of final radio stage.
I_p = Total DC input current of final radio stage.
F = Efficiency factor of the transmitter.

The value of the efficiency factor, F, is to be determined and a record of its value is to be maintained and available upon request.

Licensees need to make certain that all duty operators know which method of power determination is being used and how to calculate the output power based on that factor.

It is likely that LPFM stations would use the direct method that involves connecting a watt meter to the RF output of the transmitter.

Frequency

The departure of the carrier or center frequency of an LPFM station may not exceed 3,000 hz (3 kHz) from that authorized.  The transmitter can't drift more than 3 kHz off frequency in any direction.  

Modulation

The percentage of modulation is to be maintained at as high a level that is consistent with good quality of transmission and good broadcast service.Generally, the modulation should not be less than 85% and may not exceed 100 percent on peaks with frequent reoccurrence with reference to 75 kHz deviation.

Transmission System

LPFM stations must maintain the bandwidth occupied by their emissions in accordance with the following:

• Any emission appearing on a frequency removed from the carrier by between 120~240 kHz inclusive must be attenuated at least 25 dB below the level of the unmodulated carrier.
• Any emission appearing on a frequency removed from the carrier by more than 240 kHz up to and including 600 kHz must be attenuated at least 35 dB below the unmodulated carrier.
• Any emission appearing on a frequency removed from the carrier by more than 600 kHz must be attenuated at least [43 + 10log₁₀ (power in watts) dB] below the level of the unmodulated carrier, or 80 dB, whichever is the lesser attenuation.

Certified Transmitters

Only transmitters that have been granted FCC certification shall be used at LPFM stations.  Certified transmitters will have a permanently attached label bearing an FCC identifier.

Blanketing Interference

For one year after the commencement of transmissions with new or modified facilities, all LPFM stations are required to take remedial action to resolve blanketing interference complaints occurring within the immediate vicinity of the antenna site.  The blanketing contour of an LPFM station will extend approximately 125 meters (410) feet from the transmitter site. 

Resolution of complaints shall be at no cost to the complainant.  These requirements do not include interference complaints involving malfunctioning or mistuned receivers, improperly installed antennas, high gain antennas, booster amplifiers, mobile receivers and non-RF devices such as tape recorders, hi-fi amplifiers and hard wired telephone devices.

Self Check

Items with an asterisk * may not apply to all stations.

• Are the station’s operating ERP and antenna HAAT within the limits specified in §73.811 of the rules?
• Is the station operating power between 90% to 105% of what was authorized?
• Is the efficiency factor known for each transmitter and a record is kept to its value, along with the source from which this value was determined?* (Stations using indirect method only)
• Is the station in compliance with the frequency tolerance specified in §73.1545? (no more than +/- 3000 hz)
• Is the station in compliance with the modulation limits specified in §73.1570(b)?
• Is the station maintaining emissions within the limits specified in §73.317?
• Is the transmitter at this station certified by the FCC and have an FCC ID attached?
• Has this licensee resolved all complaints of blanketing interference within the station’s blanketing contour within the specified one year?

References

• §2.907 – Certification
• §2.925 – Identification of equipment
• §73.267 – Determining operating power
• §73.317 – FM transmission system requirements
• §73.318 – FM blanketing interference
• §73.508 – Standards of good engineering practice
• §73.811 – LPFM power and antenna height requirements
• §73.840 – Operating power and mode tolerances
• §73.1545(b) – Carrier frequency departure tolerances
• §73.1570(b) – Modulation levels: AM, FM, TV and Class A TV aural
• §73.1660(a)(2) – Acceptability of broadcast transmitters

Related REC Knowledge Base Questions and Answers

• The construction permit shows only a horizontal power but no vertical power. Does this mean that LPFM is prohibited from using vertical polarized antennas?
• Why was my station only authorized a very low power?
• I have heard that if I have a single bay antenna (such as a Nicom BKG-77) and is being fed by 150-feet to LMR-600 cable with a lightning arrestor, I should set my transmitter’s power to well over 100 watts. Is that legal?
• Where can I find the rule that requires LPFM stations to operate a certified transmitter?

Continue with Emergency Alert System

Information in this document is current as of February 25, 2019.

Information provided by REC Networks is for educational purposes only and should not be construed as legal advice.  For legal advice, please obtain the services of a qualified attorney that specializes in FCC law, especially where it comes to LPFM.