FAQ: I have heard that if I have a single bay antenna (such as a Nicom BKG-77) being fed by 150 feet of LMR-600 cable with a lightning arrestor, I should set my transmitter's power to well over 100 watts. Is that legal?

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When your station is authorized, you are assigned an effective radiated power (ERP). ERP is the power from your antenna, not the power from your transmitter. Your feed line (coax) and any equipment on the line between your transmitter and antenna such as lightning arrestors will create a loss. In addition, the Nicom BKG-77 in a single bay configuration has a gain of -3 dB. In other words, a loss. In this specific case, let's say that the feedline loss is 0.8 dB per 100 feet and the lightning arrestor loss is 0.1 dB. 150' feet of LMR-600 will create a 1.2 dB loss and the lightning arrestor will increase the loss to 1.3 dB. Taking into consideration that the antenna is also creating a 3 dB loss, running 100 watts transmitter power output (TPO) will actually create an ERP of 38 watts, which is below the legal minimum. In this case, setting the TPO to 263 watts will create an ERP of 99.98981 watts. Keep this in mind when you are shopping for transmitters. Now, if you were to add an additional bay to the Nicom, your antenna would become unity gain, thus no loss/no gain. You will still have feedline and insertion loss but setting the transmitter at 132 watts TPO would create an ERP of 100.1322 watts. REC can assist you with your TPO calculation and filing your 319 when you are ready.

LPFM station planning and construction
Answer Date: 
Thursday, May 14, 2015