You can have the biggest and the best FM transmitter in the world, but if the power doesn't get from it to the antenna you may as well turn it off. A transmitter is only as good as the antenna it is connected to. A poorly tuned antenna and antenna cable will send the power back to the transmitter and in worse case cause it to over heat resulting in permanent damage. It's a bit like driving a car with flat tyres, it will move slowly, but most of the power will be used in chewing the wheel rims to pieces. A well tuned antenna will take all the power put into it and radiated it all into free space. The antenna is effectively a transformer between the transmitters RF electrical current and free space
The term SWR is used to measure the performance of an antenna. SWR is short for Standing Wave Ratio. An SWR of 1:1 indicates that the antenna is perfectly matched and there's no reflected power. At the other end of the scale an SWR of 1:Infinity indicates that no power is being absorbed by the antenna and all the transmitter power is being reflected back to the transmitter.
A useful piece of test equipment that measures SWR is an SWR meter. All serious radio stations should have one of these. This connects in line between the transmitter and the antenna and antenna cable. SWR meters have a switch on them usually labeled up as FWD and REF. In the FWD position the meter indicates the forward power traveling on the antenna cable, this is the power going from the transmitter to the antenna. In the REF position the meter indicates the reflected power traveling on the antenna cable, this is the power that has not been absorbed by the antenna returning back to the transmitter. To make an SWR reading the transmitter is switched on, the meter is set to FWD position and the CAL control is turned to make the meter needle point to 100%. The SWR meter is then set to REF position, the reading shown on the meter is the amount of reflected power relative to the forward power.
An SWR of 1:1.0 or sometimes just referred to as 1.0 indicates that all power from the transmitter is being absorbed by the antenna.
An SWR of 1.5 indicates that virtually all the power is being absorbed by the antenna. The power being reflected back is only a very small amount is not anything to worry about.
An SWR of 1.8 is starting to get a little bit over the ideal, however most transmitters will run happily at this. Also some broadband type antennas will have SWR of up to 1.8 at the far ends of their tuning range, this is acceptable.
An SWR of 2.0 is starting to get a bit too high. However, even at this level most transmitters will run without a problem. If you are using high power (100W and over) at this SWR it is time to service and adjust your antenna system to get the SWR to 1.5 or less.
An SWR of 2.5 or over is too high and some transmitters will be damaged at this level. It's advisable not to use a transmitter at this level of SWR.