How to Calculate Effective Radiated Power

Find the listed power output of the transmitter in the operating manual. The final power output of the transmitter may vary slightly from the listed ,figure so a check of the actual output should be made with a watt meter which has been calibrated.

Check the feed line loss. The loss of transmitter power through the line is determined by the length and size of the coaxial cable. A chart showing line loss from commonly used cables is available on line from the Iowa Repeaters Association.

Check the manufacturers operating manual for the figures on antenna power gain.

Determining the effective radiated power of the antennae is completed by using these figures. Transmitter output power less the loss of power in the feed line plus the power gain of the antenna as stated by the manufacturer. Take the transmitter output, subtract the feed line loss and add in the antenna power gain.

Using a commercial FM radio station we can show an example of effective radiated power. A 100,000-watt FM station would most likely have a transmitter with a power output of 10,000 watts to 20,000 watts. The number of antenna elements and their design would determine the power gain which would bring the output of the station to 100,000 watts effective radiated power.