1. Coverage of FM transmitter and TV transmitter. The main parameter which decides the coverage is height of tower, so people install the antenna on very tall tower or build the tower on top of mountain. The signal has attenuation in air transmission, so people use large power of transmitter to supply the power loss. In a word, the main reason of coverage is relative height of tower to ground, secondly is power of transmitter, thirdly is antenna gain.
2. How can I know my coverage. You can tell us the power of transmitter you have and height of tower plus coordinates if possible, then we can calculate the coverage for you.
3. Can I change the frequency. For FM transmitter, the frequency is agile, you can select any frequency within 88~108Mhz. For TV transmitter, the bandwidth is narrow, you can just select adjacent 2 or 3 channels.