Depending on the type of antenna you are using, you may have to purchase a 250-watt transmitter anyway. Since the authorized power is based on effective radiated power (ERP) and not transmitter power output (TPO). If you have a one bay antenna such as a Nicom BKG-77 or a Shively Labs 6812b, you are actually getting a 3 dB loss. With an extra 1dB of feedline loss, a 250 watt transmitter at full power will give you about 99.5 watts ERP. If you upgrade to a two-bay antenna that gives unity (0) gain, 250 watts output will give you about 198 watts ERP. For overall cost savings in the long run, it would be in your best interest, if possible to install a 2 bay antenna and purchase a 500-watt transmitter. Operating at 315 watts from the transmitter with 1 dB of feedline loss into a unity gain antenna will give you the full 250 watts ERP (if LPFM stations are ever authorized to go that high). It's better to have a higher power transmitter where you can turn down the power to the authorized level. Just don't get tempted to "crank it up".
LPFM station planning and construction
