Consider the following ……

Desired Coverage?

Attached is a predicted coverage for an FM station. Here it can be seen that the range of a station is dependent primarily on the magnitude of the radiated power as well as the height above average terrain. Actually, a change in height has a more dramatic improvement than an increase in the ERP. (Effective radiated power)

Selection of Antenna Location

The highest practical location should be selected for the antenna location. This can be the top of a tall building, a short tower on top of a high elevation such as a hill or mountain or a new independent tower of 100 to 250 meters.

All sites must have a reliable electrical power source. This can be either single or three phase. They must also have a location which assures access for transportation of equipment and personnel.

Selection of Antenna and Transmitter Power

Referring to the “Predicted Coverage map” and recognizing the height above average terrain from the antenna location selected, the minimum required “ERP” can be determined. “ERP” is defined as the “effective radiated power” and is the product of the antenna gain and the transmitter power.

The supporting strength and height of the tower can effect the antenna selection. It should be noted that elements are spread 3 meters apart. Thus, a 4 bay antenna would occupy 9 meters of tower space. The bottom element should be at least 6 meters above the base. Thus, the tower would have to be at least 15 meters high.

The antenna gain of a vertically polarized antenna is equal approximately to 10% greater than the number of elements. Example: A 4 bay vertical antenna has a gain of 4.4. A circularly polarized antenna has a gain of 50% of a vertical antenna. Example: A 4 bay CP antenna has a gain of 2.2.

A CP antenna is desirable when coverage is desired in a hilly area and in a city with many tall buildings.

A low gain antenna (2 to 4 elements) should be used if the antenna location is in the middle of a population area. If the desired coverage begins 5 to 10 miles from the antenna location, a high gain (10 to 12 bay) antenna may be used.

The transmitter output power should be 80% or less of the desired antenna input power to achieve the required ERP. Example: if the desired ERP were 50 KW and the antenna gain were 4, the antenna input power would have to be 12.5KW. Thus, a 15KW transmitter should be selected.

Transmission Line Selection

The line selected must be able to handle the transmitter output power and must have a maximum loss of 10%.

In general, continuous flexible coaxial lines of 50 ohm impedance are used at the following power levels:

1KW or lower --- ½” Foam Line

2KW to 3KW --- 7/8” Foam Line

4KW to 10KW --- 1-5/8” Air Line

11KW to 25KW --- 3” Air Line

26KW to 50KW --- 4-1/4” Air Line

Accessory Items

The following items are normally included in a professional broadcast transmitter site:

FM Modulation Monitor – To verify audio level of transmitter modulation.

AVR - Automatic line voltage regulator with isolation transformer and line surge protector for sites with “non-reliable” power sources.

FM Processor - This equipment is used to achieve audio quality, protect against over modulation and provide stereo performance.

Optional Items

These items are not essential, but depending on the professional level desired as well as the particular situation, they may be considered.

Stand-by Transmitter – For those stations that cannot tolerate any “off air time”. This can be a transmitter with as low as 25% of normal transmitter power. Its selection is determined by the minimum coverage required under emergency conditions.

Motorized Coaxial Switch – For switching in stand-by transmitter within 3 seconds.

Reject Load – For terminating transmitter for engineering purposes instead of feeding the antenna.

Remote Pick-Up Equipment – Permits program origination from location other than normal studio site. Example: sports stadium, shopping mall, etc.

STL (Studio Transmitter Link) – When the studio is not co-located with the transmitter and it is not economically feasible or technically possible to feed the programs to the transmitter site, wireless microwave may be used. If both studio and transmitter sites have reliable, wide band Internet connections, an Internet STL may be considered.