What is the difference between an FM transmitter and an FM exciter ?

both FM exciter and FM transmitter require you to have a license to own let alone transmit. a transmitter connects to the antenna. an exciter connects to the transmitter. some transmitters have the exciter section built in. the exciter is what takes the audio and modulates the RF channel. 40 kms is a long distance, you can't get that far with a lpfm license, you will need a full commercial broadcast license for power levels of 100 watts or more. those are hard to obtain, unless you can find a current license holder that is willing to sell.