Protected Contour for FMs: Technical Guarantee or Coverage Limitation?

What does the protected contour of an FM radio station actually represent: is it a limitation on coverage or a guaranteed right for the licensed broadcaster?

According to ANATEL (FCC BRAZIL) regulations, the most accurate answer is that the protected contour represents a zone of technical exclusivity in which the licensed station has priority in the use of the frequency. This means that, within this radius, it has the right to operate without suffering harmful interference from other stations—whether due to spurious emissions, use of the same frequency, or inadequately filtered adjacent frequencies.

If interference occurs within this contour, the affected station may report it to ANATEL, which must notify the source of the interference and may impose penalties or require technical adjustments.

However, it is important to note:

• The protected contour is not a physical limit of signal reception. Often, due to topographical conditions, equipment quality, and propagation, the signal may be received well beyond this official limit without constituting a violation.

• What is prohibited is causing interference outside the limits defined for your operation, even if the station is operating within the permitted power and technical specifications.

Therefore, we can understand that the protected contour is a right, not a limitation. It defines how far the station has priority and regulatory protection, but it does not prevent its signal from exceeding that limit, as long as this happens without affecting other licensed stations.

Class	(ERP)	(HAAT)	R
E1	100 kW	600 m	78,5 km
E2	75 kW	450 m	67,5 km
E3	60 kW	300 m	54,5 km
A1	50 kW	150 m	38,5 km
A2	30 kW	150 m	35,0 km
A3	15 kW	150 m	30,0 km
A4	5 kW	150 m	24,0 km
B1	3 kW	90 m	16,5 km
B2	1 kW	90 m	12,5 km
C	0,3 kW	60 m	7,5 km

Source of the table: https://informacoes.anatel.gov.br/legislacao/atos-de-requisitos-tecnicos-de-gestao-do-espectro/2022/1687-ato-8104#tabela3

Protected Contour: Refers to the area in which the station has priority in the use of the frequency, with protection against interference.

ERP (Effective Radiated Power): The power transmitted by the antenna, taking into account system gains and losses.

HAAT (Height Above Average Terrain): The average height of the antenna in relation to the surrounding terrain, directly influencing signal reach.

In Brazil, according to ANATEL regulations:

The protected contour is related to the field strength level (in dBμV/m) within which a station’s signal must be protected against interference.

For FM radio stations of classes A, B, C, etc., there are different minimum field values considered to define this area.

Purpose of the protected contour:

• Prevent interference between stations operating on the same frequency or on adjacent frequencies.

• Frequency planning to ensure orderly coexistence between broadcasters.

• Provide a basis for technical feasibility analyses for new stations or changes in power/height.

How is it determined? The protected contour is calculated based on:

• Effective Radiated Power (ERP).

• Antenna height.

• Local topography.

• Signal propagation (ANATEL’s standard-curve model, based on ITU-R P.1546 or similar methods).

The protected contour for FM in Brazil was defined based on international agreements and the influence of U.S. FCC (Federal Communications Commission) models. Below are the main elements that influenced the definition of this concept in Brazil:

Origin and Influences:

Influence of the FCC (USA):

• The FCC has defined protected contours for FM since the 1940s, using propagation curves (F(50,50)) to define service areas based on power and antenna height.

• The station classes (A, B, C, etc.) and protection levels (in dBμV/m) in the U.S. directly influenced the Brazilian model.

• In Brazil, ANATEL adapted this system but adjusted it to local geographic and regulatory characteristics.

Geneva 1984 Agreement (GE84):

• This international agreement, promoted by the ITU (International Telecommunication Union), standardized FM frequency allocation and coordination in Region 1 (Europe, Africa, part of Asia) but also influenced technical standards in other regions.

• Brazil is not a direct signatory of GE84, but the agreement served as an international technical reference.

MERCOSUR regulations:

• There are harmonized guidelines among MERCOSUR countries (such as Argentina, Brazil, Paraguay, and Uruguay), but Brazil adopts a model more aligned with the American than the European system.

ANATEL’s technical criteria:ANATEL defines protected contours based on:

• F(50,50) propagation curves – derived from FCC and ITU studies.

• Minimum protected field, which varies according to the station’s class:

  • Class A1: 66 dBμV/m

  • Class A3: 60 dBμV/m

  • Class A4 (most common): 54 dBμV/m

• Minimum spacing between co-channel and adjacent channels, following U.S. models.

These criteria are specified in ANATEL’s FM Technical Regulation (RTF-FM), which is periodically updated.

In practice:The protected contour is the basis for frequency planning in Brazil.

It determines:

• Where a station has the right to protection against interference.

• Where it can cause interference, and up to what level it must not exceed.

• Whether it is feasible to modify ERP or tower height without affecting other stations.

Protected contour for community radio (RadCom)

In Brazil, it is very specific and more restrictive than for commercial radio, precisely because it is a low-power, local service aimed at specific communities. The technical definition comes from ANATEL, based on the Community Broadcasting Service Regulation (Resolution 10/2022) and the RadCom Technical Regulation.

Technical specifications of the protected contour for community radio:

• Maximum power: 25 watts ERP (Effective Radiated Power in the direction of maximum gain).

• Maximum antenna height (HAAT – effective height relative to terrain): 30 meters.

• Protected coverage radius: 1 km from the transmitting antenna.

This 1 km radius defines the protected contour for the purposes of protection against interference from other stations. In other words, no other FM station (commercial, educational, or community) may cause interference within this 1 km area around the community station, according to ANATEL’s technical criteria.

What does “protected contour” mean in this case?

• It is the minimum guaranteed area where the community radio station has priority and protection against interference.

• If another nearby station is causing interference within this radius, ANATEL can be called upon to investigate and, if necessary, require changes to the interfering station.

• This protection is limited: outside the 1 km radius, the RadCom has no guarantee of protection and cannot demand improved reception or absence of interference.

And the service contour (or useful area)?

In practice, even with only 25 W, many community radio stations have a greater reach; under good technical and topographical conditions, the signal can reach 3 to 5 km or more, but this extra area is not officially protected.

Important:

• ANATEL considers only the 1 km radius for technical feasibility analysis and legal protection.

• Projects with very tall antennas or aimed at covering areas larger than 1 km may be rejected or require adjustments.

LPFM (Brazil)

Maximum ERP: 25 watts

Maximum antenna height: 30 meters

Protected contour: Radius of 1 km around the antenna

Actual usable area (not protected): Up to 3–5 km (depending on terrain)

Interference protection: Only within the 1 km radius

Tools for viewing protected contour maps:

Radio Mobile (free software):

• Technical tool that calculates signal propagation based on terrain and station parameters.

• Allows generating maps with contours in dBμV/m, such as the 54 dBμV/m contour, which corresponds to the protected contour of Class A4 FM stations and, by analogy, is also informally used as a reference for community stations.

• Enables precise plotting of the 1 km protected radius for RadCom.

QGIS with propagation plug-ins:

• For advanced users, it is possible to create a 1 km buffer around the antenna coordinates.

• This is the method used by engineers for technical studies.

Radio Locator (more common in the U.S.):

• Allows viewing estimated coverage maps of U.S. FM stations.

• Not directly applicable to Brazilian RadComs but helps visualize the concept of a protected contour.

Google Earth + manual projection:

• Using the circle (ruler) tool, you can draw a 1 km radius around a point (antenna), simulating the protected contour of a RadCom.

Practical example (explained):Suppose a community radio station is installed in the center of a neighborhood, with a 20-meter-high antenna and 25 watts ERP.

• The protected contour will be a circle with a 1 km radius around the antenna location.

• Beyond this radius, reception may still be good, but the station has no legal right to protection against interference.

• Within the radius, no other FM station (community, educational, or commercial) may harm the signal of this RadCom.

Example of a protected contour map for a station in Quincy, Illinois, under FCC (Federal Communications Commission) supervision

Example of a station in the Boston area: coincidentally, there is another nearby city also called Quincy. In the United States, all are under FCC supervision.