This answer does not reflect rules currently in effect but instead, the potential outcome of rulemaking, if enacted.
The answer to this question may vary based on what your height above average terrain (HAAT) is.
For LPFM stations that are at HAAT 30 meters or below (currently 100 watts and upgradable to 250 watts): The transmitter and antenna combination must be able to put out 250 watts ERP at the antenna. For LPFM stations with single bay circular polarized antennas (like Nicom BKG-77, BKG-88 or Shively 6812), you currently need about 250 watts transmitter power output based on your feedline type and length. This is due to the 3 dB loss that these single bay antennas exhibit. Most LPFM transmitters (like the BW TX-300 and the Nautel VS-300) only put out a maximum of 300 watts output. If you want to keep the antenna, you will likely need about 650 watts power output. The selection of FCC certified transmitters that put out that much power is very small. Instead, we suggest that you switch out the single bay antnena for a two bay circular polarized antenna and preferably a configuration that exhibits gain (such as the Nicom BKG-77 or BKG-88 at 0.85 wavelength spacing). The gain on the antenna will permit you to keep the transmitter power output to less than 300 watts. You can also consider a gain vertical antenna such as the Norwalk Dominator, however, vertical only antennas have their own share of performance issues.
For LPFMs at HAATs greater than 30 meters, you may need to consider a two bay anenna or gain vertical if the calculation based on antenna gain/loss and feedline loss would require a transmitter power output more than your transmitter can handle.
For LPFMs at HAATs 49 meters or above, if you have a 300 watt transmitter, then using the existing antenna and transmitter may work. You will only need to increase the transmitter power output if authorized for LP-250 operation.
It is also important to remember that under the FCC's radio frequency radiation guidelines for LP100, an antenna needs to be at least 20 feet away from any "human head". At LP250 (and you are running a full 250 watts), that guideline increases to about 27 1/2 feet.
For more in-depth technical information on this subject, please see REC Advisory Letter # 11.
