Part 73, Subpart G, Section 73.816 is a regulation that addresses antennas that can be used by LPFM stations.
Main article: Antennas for LPFM stations
Non-directional antennas
Most LPFM stations operate nondirectional antennas. This can include horizontally polarized antennas, vertically polarized antennas such as the Norwalk Dominator and the Comet CFM-95 and circular polarized antennas such as the Nicom BKG-77, Nicom BKG-88 and Shively Labs 6812.
Directional antennas
Overview
In some rare cases, LPFM stations may operate directional antennas. For most LPFM stations, a directional antenna does not provide any benefit, if anything, using a directional antenna would actually decrease the station's overall coverage. There is a common misunderstanding that directional antennas will provide "more range". Much of this misconception comes from those who are familiar with directional antennas used in the Amateur Radio Service and does not take into consideration the limitations of Effective Radiated Power (ERP) in the broadcast services.
When using a directional antenna in the LPFM service, the ERP along the peak lobe (the most powerful direction) cannot exceed the maximum ERP authorized to the station.
Proof of Performance, computer modeling and verification requirements
LPFM stations that use directional antennas for reasons other than those specifically exempted by rule are required to provide either a proof of performance to a diagram based on computer modeling that shows the radiation pattern of the antenna as well as the same values in tabular form. The license application that is filed after construction must also include a statement from a licensed surveyor that the antenna was placed at the correct location and a statement from the tower installer or engineer that the antenna has been pointed in the appropriate direction.
Directional antennas being installed by LPFM stations on 88.1~91.9 to demonstrate protection to a TV broadcast facility on Channel 6 is not exempt from these requirements.
Exempt applications of directional antennas
Despite all of those restrictions shown above, most needs for directional antennas in the LPFM service will be for exempted activities. These activities include:
Second adjacent channel waivers
Most second adjacent channel waivers are based around the design of the antenna in order to prevent a certain interference contour from reaching populated areas. While in most cases, these showings are based on the vertical pattern of the antenna, there are some cases where the horizontal pattern can be used, such as using a directional antenna to protect only one building. A directional antenna can also be used in cases where the 100 dBu interfering contour of the proposed LPFM facility does not overlap the 60 dBu contour of short spaced second adjacent channel station (those field strengths are 97 dBu and 57 dBu respectively for Class B1 FM stations on 92.1~107.9 as well as 54 dBu and 94 dBu respectively for Class B stations on 92.1~107.9). A technical study that clearly shows no overlap between the directional LPFM station's interfering contour and the existing station's protected contour is all that is needed to satisfy a second-adjacent channel short spacing.
Travelers Information Stations
LPFM stations operated by state and local government agencies to serve a public safety purpose may use a directional antenna in lieu of a nondirectional antenna.
International agreements
LPFM stations within 320 kilometers (198.8 miles) of the Canadian and Mexican border may have additional restrictions on contour size and on LPFM stations within 125 kilometers (77.7 miles) of Mexico, additional restrictions on effective radiated power would apply.
The directional antenna is exempt from the proof of performance or computer modeling and the verification requirements if the sole reason for using a directional antenna is to satisfy a requirement of the international agreements between the United States with Canada or Mexico.
Filing applications
When filing an application for a directional antenna use that is not exempt, the FCC Form 2100, Schedule 318 must indicate that the antenna is directional. The application must include the tabulated field values of the antenna in 36 different directions. These field values are available from the antenna manufacturer. LPFM stations using a directional antenna for a non-exempt activity will be protected by FM translators based on their directional pattern.
When filing an application for a directional antenna for an exempted activity, the FCC Form 2100, Schedule 318 must indicate that the antenna is nondirectional. Using the directional functionality will indicate to the FCC that the directional antenna is not for an exempted activity and they may impose the computer modeling, proof of performance and verification requirements. Even though the directional antenna information is not filled out on the form, the application should include an exhibit that shows the field values of the antenna in 36 equally spaced directions. LPFM stations using directional antennas for an exempted purpose will be protected by FM translators as if they were a nondirectional station.
Regulatory history
This rule was originally enacted in the 2000 LPFM Report and Order with an across-the-board prohibition on directional antennas. In the Order on Reconsideration enacted in September, 2000 at the request of the New York State Thruway Authority, the rule was amended to allow for directional antennas operating LPFM stations for a traveler's information station. In the 2012 Sixth Report and Order, the FCC allowed for the use of second adjacent channel waivers. With those new rules came the ability for LPFM stations to use directional antennas solely to meet a second-adjacent channel short-spacing issue in addition to the existing exempt use for Travelers Information Service. In 2020, the LPFM Tech Order expanded the scope of directional antennas where they could be used for any purpose. At that time, the FCC required that LPFM stations using a directional antenna would need to get a proof of performance (which would be expanded in a subsequent non-LPFM proceeding to allow for computer modeling), as well as the verification process. The 2020 order did exempt specific activities from the proof and verification requirements including second-adjacent waiver, travelers information service and for the first time, due to international agreement.
Federal Register citation
65 FR 67303, Nov. 9, 2000, as amended at 78 FR 2106, Jan. 9, 2013; 85 FR 35573, June 11, 2020
Text of regulation 47 CFR §73.816
§ 73.816 Antennas.
(a) Permittees and licensees may employ nondirectional antennas with horizontal only polarization, vertical only polarization, circular polarization or elliptical polarization.
(b) Permittees and licensees may employ directional antennas in the LPFM service, in accordance with paragraph (d) of this section.
(c) [Reserved]
(d)
(1) Composite antennas and antenna arrays may be used where the total ERP does not exceed the maximum determined in accordance with § 73.811(a).
(2) Either horizontal, vertical, circular, or elliptical polarization may be used provided that the supplemental vertically polarized ERP required for circular or elliptical polarization does not exceed the ERP otherwise authorized. Either clockwise or counterclockwise rotation may be used. Separate transmitting antennas are permitted if both horizontal and vertical polarization is to be provided.
(3) An application that specifies the use of a directional antenna must provide the information identified in § 73.316(c) except that such information shall not be required of:
(i) Public safety and transportation permittees and licensees eligible pursuant to § 73.853(a)(2) using directional antennas in connection with operation of Travelers' Information Service stations;
(ii) LPFM permittees and licensees proposing a waiver of the second-adjacent channel spacing requirements of § 73.807 for the sole purpose of justifying such a waiver; and
(iii) LPFM permittees and licensees using directional antennas solely for the purpose of meeting the international border zone distance requirements of § 73.807(g).