Design of FM Broadcast Systems

Frequency Modulation

Design of FM Broadcast Systems

A system of modulation where the instantaneous frequency of the carrier and its rate of change varies in proportion to the instantaneous amplitude of the modulating signal.

ECE 211L Broadcast Engineering Laboratory

The FM Broadcast Band That portion of the radio frequency spectrum from 88 MHz to 108 MHz. The band is divided into 100 channels, starting at 88.1 MHz up to 107.9 MHz. Each channel is 200 kHz wide, designated by its carrier frequency. Co-located stations should be spaced at least 800 kHz apart to avoid possible interference.

Classes of FM Broadcast Stations Class-A Stations „

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A station authorized to have a transmitter power no more than 25 kW and an effective radiated power (ERP) not exceeding 125 kW. The minimum transmitter power shall be 10 kW. The antenna height is limited to 2,000 ft. above the average terrain. Class-A stations shall only be allowed in Metro Manila and Metro Cebu.

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Classes of FM Broadcast Stations Class-B Stations „

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Classes of FM Broadcast Stations Class-D Stations

A Class-B station shall have an authorized transmitter power not exceeding 10 kW and an effective radiated power (ERP) not exceeding 30 kW. The minimum transmitter power shall be 1 kW. The antenna height is limited to 500 ft. above the average terrain.

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A Class-D station shall have an authorized transmitter power not exceeding 10 W Educational stations shall be allowed to operate with a Class-D license.

Class-C Stations „

A non-commercial, community station having an authorized radiated power not exceeding 1 kW.

Radio-Frequency Protection Ratios All classes of FM stations shall be protected to the 1 mV/m or 60 dBu contour. The following ratios provide for the minimum separation between stations and protection of stations from interference: Frequency Spacing, kHz

R.F. Signal Ratio

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60 dBu : 15 dBu

200

60 dBu : 33 dBu

400

60 dBu : 80 dBu

List of Requirements When Applying for a New FM Station I. Congressional Franchise II. File a petition for a Certificate of Public Convenience (CPC) with the NTC. 1. Articles of Incorporation and By-Laws duly approved by the Securities and Exchange Commission (SEC). 2. List of present Officers and Board of Directors and the corporate secretary's affidavit attesting to its present corporate structure. 3. Duly accomplished information sheet of each and every member of the Board of Directors. 4. Audited financial statement of the corporation for the last three (3) years and copy of income tax returns for the same year. 5. Economic viability study (for commercial stations), source of funds (for non-commercial stations).

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List of Requirements When Applying for a New FM Station 6. Technical feasibility study and engineering plans and diagrams (signed and sealed by an Electronics and Communications Engineer duly registered with the Philippine Regulation Commission). 7. Duly accomplished application for: a. Permit to Purchase Transmitter b. Construction Permit (Forms are available at NTC)

Pre-planning Considerations in the Design of FM Stations Studio and Transmitter Site Selection Studio and Control Room Requirements Remote Studio Facilities (STL)

III. A Public Hearing shall be conducted. IV. The Commission shall render a decision on the petition.

Site Selection Transmitter Site „

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The transmitter site is primarily considered due to its effect to existing stations located in the same area. Also, site selection is important because of the fact it will determine your coverage area (service area). The location should be chosen so that line-of-sight can be obtained from the antenna over the general service area.

Remote Studio Facilities In cases where the studio and the transmitter cannot be located in the same facility, or it is better for the transmitter to be located elsewhere (economically or technically), then a studio-totransmitter link (STL) may be employed.

Studio Site „

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The studio usually contains equipment for program origination. It is a usual practice to co-locate the studio and transmitter in a single facility, mostly for economic purposes.

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Technical Considerations in the Design of FM Stations Terrain Elevation Site Elevation and Antenna Height Above Average Terrain (HAAT) Transmitter System Antenna System Transmission Line Tower Structure Miscellaneous Concerns

Terrain Elevation Terrain elevation is a primary concern since there must be a LOS to the antenna over the principal area of coverage. However, the variations in terrain will not give 100 percent LOS over the entire area. In such case, the average terrain elevation (ATE) around the transmitter site is considered to represent the elevation of the service area. ATE is measured starting from 3 km to 16 km from the transmitter site.

Site Elevation and HAAT To provide LOS within the principal area, the antenna must be conveniently above the average terrain. A tower can be used to elevate the antenna Elevated structures (buildings) may also be used. Hills and mountains may also act as natural towers. The height of the transmitting antenna above the average terrain is referred to as HAAT.

Transmitter System The following criteria are to be considered in choosing a transmitter: „

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Shall operate satisfactorily in the operating range with a maximum carrier swing of ±75 kHz, defined as 100% modulation. Shall be capable of transmitting a band of frequencies from 50 to 15 k Hz Output noise level (frequency modulation) shall be at least 60 dB below 100% modulation by 400 Hz using 75μs de-emphasis Automatic means shall be provided in the transmitter to maintain the assigned center frequency within the allowable tolerance.

Source: KBP Manual

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Antenna System The antenna used in FM broadcasting is a half-wave dipole. The selection of the antenna depends on the following criteria: „

Polarization. The antenna shall employ a standard horizontal polarization. However, circular or elliptical polarization may be used if desired.

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Power handling capability. Directivity (Polar pattern) Array gain. Location of the feed point. Null fill and beam tilt adjustment. Dead load and wind load.

Tower Structure The tower the structure to which the antenna system is anchored to. It should be able to support the antenna system attached to it. The tower, along with the consideration of the transmitter site location, should provide sufficient elevation to provide adequate service throughout the area. It must follow the standards set by the KBP and the National Structure Code of the Philippines. „ „

Transmission Line A transmission line delivers power from the transmitter to the antenna system. Several factors are to be considered in choosing the right transmission line „ „ „ „

Attenuation Voltage standing wave ratio (VSWR) Power handling Price per unit length

Miscellaneous Concerns Antenna mounting Cable mounting Flanges and connectors

Obstruction painting and lighting Lighting protection

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