Antenna is the most important element
There are 4 simple types that one can use for the FM transmitter. A—GP (Ground plane), B-Dipole, C- Reflected Folded dipole, D- Folded Dipole, E- Yagi. They are Omnidirectional, bi-directional, uni-directional & heavily directional. Discussed below a few for the amateurs. Complicated parameters like bandwidth. Gain, SWR, etc are all not covered for simplification except the impedance(Z) of the antenna which is important. The best way to increase the range of your FM transmitter is to use a matched antenna as per the impedance without which one gets very little range coverage. The cable, impedance matching, and height at which it is installed are also important. Any obstructions in the line of sight also affect the range. For example, one can get up to 1 to 2 KMs for a one-watt transmitter, 5 to 10 Kms for a 5 watt, 7 to 14 Kms for a 7 watt as tabulated below. A range of FM transmitters, antennas, cable, some spare power transistors are available at www.projectntraining.com. or here https://www.projectntraining.com/FM_Transmitter/cat395091_4620806.aspx Much Chinese make claim very high range for their transmitters which are all incorrect. Moreover, they provide no warranty and if it does not work it becomes a waste of money. We not only give warranty but also repair any time after the warranty is over. Being manufacturers we even provide the detailed circuit diagram for technicians to attend repairs easily.
Cable is also equally important depending upon the type of antenna.
Twin core twisted cable, twin-core flat cable, Coaxial cable need to be used. Best is the co-axial cable which is available in 50 (for RG58) ohms. Commonly available RG59, in 75ohms, is the standard cable used by cable TV service providers which are not very suitable for FM transmitters.
Depending upon the antenna impedance (Z), 50 ohms to 300 ohms a matching arrangement has to be made for getting the best range. Thus to use a co-axial cable of RG 58 a matching balloon has to be used. For example, if a 50 ohms cable is used because that is the output from a transmitter then a 50 ohms to 300 ohms balloon transformer has to be used at the antenna end of the folded dipole antenna.
Mounting a folded dipole Antenna
It can be used vertically as seen or horizontally depending upon which area one has to cover. For a long-range in either direction, mount horizontal but for short-range all over mount vertical.
Finally, the receiver is also equally important. The range is good as long as complete hissing free audio is received. FM receiver in Android phones gives the least range as the earphone cord acts like an antenna. Some headphones with built-in antenna for FM receiver gives very little rage because of a very small inbuilt antenna. Using a car radio or good highly sensitive FM radio shall give the best result.