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Design of FM Broadcast

Systems

ECE 211L

Broadcast Engineering Laboratory

Frequency Modulation

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.

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

„ 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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Class-B 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. Class-C Stations

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

Classes of FM Broadcast Stations

Class-D Stations

„ 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.

Classes of FM Broadcast Stations

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:

60 dBu : 80 dBu 400 60 dBu : 33 dBu 200 60 dBu : 15 dBu 0 R.F. Signal Ratio Frequency Spacing, kHz

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

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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)

III. A Public Hearing shall be conducted.

IV.The Commission shall render a decision on the petition.

List of Requirements When

Applying for a New FM Station

Pre-planning Considerations in the

Design of FM Stations

Studio and Transmitter Site Selection Studio and Control Room Requirements Remote Studio Facilities (STL)

Site Selection

Transmitter Site

„ 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.

Studio Site

„ The studio usually contains equipment for program origination.

It is a usual practice to co-locate the studio and

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

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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:

„ 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.

„ Power handling capability. „ Directivity (Polar pattern) „ Array gain.

„ Location of the feed point. „ Null fill and beam tilt adjustment. „ Dead load and wind load.

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

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.

„ Obstruction painting and lighting „ Lighting protection

Miscellaneous Concerns

Antenna mounting Cable mounting

References

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