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Formula For figuring ERP on FM Transmitters

We are often asked how to figure out what power to set your transmitter at in order to achieve a certain Effective Radiated Power or ERP. Below is an example of an LPFM station that is using 190 feet of Andrew LDF4 Cable and our PCP1 antenna. In this particular example the customer is approved for a 29 Watt ERP. You can plug-in your own figures to determine your ERP.

Here is the formula to figure out what power to set your transmitter at in order to achieve a given ERP:

Transmitter Power Output x Line Loss x Antenna Gain = ERP

For this example we know the PCP1 has an antenna gain of -3dB. We also know that the loss of LDF4 cable is .661dB per 100 feet. Since, in this example, we will have 190 feet of cable we multiply .661 x 1.9 = 1.255. So you will lose 1.255dB in 190 feet of LDF4.

Now we need to convert dB into POWER GAIN figures. According to my chart (below), -3dB = .5 power gain. And -1.255dB = .75 power gain.
Formula For figuring ERP on FM Transmitters - Blog - Progressive Concepts - Power_Gain

Now we have all the information we need in order to figure out what your transmitter needs to be set at:

79 Watts TPO (Transmitter Power Output) x .75 (cable loss) = 59.25 Watts x .5 (antenna gain) = 29.6 Watts ERP.

The same formula can be used for Belden 9913 cable. The loss per 100 feet of Belden 9913 at 100 MHz is 1.4 dB which gives you a power gain of .75 per 100 feet. If we used 9913 in the example above with 190 feet of cable you would multiply 1.4dB x 1.9 (190  feet) = 2.66dB loss. Using the chart above, this would give you a power gain of .54 (in some cases it is necessary to "read between the lines" on the chart.

Therefore, if we used Belden 9913 in the above example instead of Andrew LDF4 cable, we would get the following results:

110 Watts TPO (Transmitter Power Output) x .54 (cable loss) = 59.4 Watts x .5 (antenna gain) = 29.7 Watts ERP.

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