These questions do not have a simple or a single answer. The reason is, there are so many factors that will determine how far your transmitter will travel. In theory, the range of a transmitted radio signal is infinite regardless of power used, basically it goes on and on forever until it encounters an obstruction. More transmitter power helps to penetrate any obstructions. Given this, when engineering and planning a station it is always better to consider these factors that limit the range.
Factors that will determine the range of an FM transmitter or how far it can reach include the amount of output power of the transmitter (TPO), the type of antenna used, the type of coaxial cable used, the length of coaxial cable used, the height of the antenna above average terrain (HAAT), the amount and height of trees in the area around the antenna, the amount and height of buildings around the antenna, the type of terrain surrounding the antenna site whether flat or hilly.
FM A and FM B using a 100 watts FM transmitter with 2 bays antenna system, FM A will be getting 24 kilometers (15 miles) while FM B only gets 1.6 kilometers (1 mile). The reason is simple. FM A getting 15 miles (24km) has its 2 bays antenna mounted on a 150 foot tower/mast or on a 15 storey building located on top of a hill with gradually downward sloping terrain in all directions while FM B has a cheap home made 2 bays antenna or same as FM A mounted on a 1 storey building or on a 10 foot pole that is at ground level and the terrain slopes upwards in all directions around it or in a city where there are very tall buildings.
Three tests were done by our CEO, Schofield (Nakadif), in 2014, 2016 and 2019. Test one (2014) was done in the then Brong Ahafo with 150 watts transmitter and 3 bays antenna mounted on a 70 foot tower with 7/8'' feeder cable and output power was set at 140 watts with 0 watt reflected. This area was flat with few dispersed two to six storey buildings. This setup was heard at 27km away.
Test two (2016) was done in the western north region with 1000 watts (1kW) transmitter and 4 bays antenna mounted on a 120 foot mast with 7/8'' feeder cable and output power was set at 950 watts with 0 watt reflected. The setup was surrounded by forest. Signal for this setup was good at 55km away and was heard at 85km away. On favorable conditions, this setup was heard at far as 121km.
Test three (2019) was done in the same western north region with 100 watts transmitter and 4 bays antenna mounted on a 120 foot mast with 7/8'' feeder cable and with output power set at 100 watts with 0 watt reflected. The setup was surrounded by forest same as the test two. Signal for this setup was was heard at 25km away.
Take into account the cable losses, terrain, interference and the type of FM receiver being used. On flat terrain, your transmission will cover farther, as there will be less obstructions. Mountainous terrain can severely limit your station's range. If your FM radio station is in or near a big city, interference from more powerful FM transmitters could limit how far your station will travel. An FM receiver that uses an outside antenna as opposed to an indoor antenna will have an easier time picking up your radio signal.
The general rule of thumb is, it will take four times the power to double the transmission distance. Let's take the 150 watts transmitter in the test one for example. 140 watts out of 150 watts maximum power was used to achieve 24km range. To achieve 48km, you will need four times of the power used to achieve 24km, in this case, 140 watts times four (140 x 4), therefore, 560 watts is needed.
In conclusion, the best way to know for sure what the range of your transmitter or station will be is to set it up and try it out. Making sure coverage radius of your transmitter or station does not exceed the coverage radius authourised to cover. If you are in Ghana, click here to read more about coverage radius limitation for all types of radio broadcasting.