This question does not have a simple or a single answer. The reason is, there are so many factors that will determine how far your 500 watts transmitter will go.
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.
In this article, the team at MyTuneIn give you four different test results that will help answer your question. The four tests were done with 500 watts transmitting power and up to four (4) dipole antennas. We used our car radio tuner and a Samsung GT-E1207Y mobile phone as receiving devices for all the tests.
The Samsung phone was used to receive a signal from stations at far as 150km away before making it a sister receiving device to our car tape which also receives signals of some stations at over 150km away.
Please note that, kilometres used for our tests are 'straight line' kilometres without curves which is different from kilometres by road since radio signal does not travel like cars (by road). We used google map to achieve the number of kilometres used in this article. Since we achieved our km by drawing a straight line from the transmitter site to our stopping points, our kilometres are less than kilometres by road.
Also, audio peak deviation for all the four setups were set at 68KHz.
Below are details of each test.
Test One: The first test was done with 500 watts output power with 0 watts reflection connected to a single dipole antenna with 1.5dbd gain mounted at about 100 feet (30m) high through 7/8" feeder cable. This test was done to see if we can receive a good signal with clear stereo sound from this configuration at 30km away.
The area for this test was flat with few dispersed storey buildings. The signal was strong and easily picked by our car radio tuner and the Samsung GT-E1207Y mobile phone in an auto search at about 21km away. At about 31km away, the signal was good on both receiving devices with few hiss.
The signal was still good with clear stereo sound on both receiving devices at about 40km but with hiss at some directions on the Samsung phone. We ended the test at 42km away.
Test Two: In this test 500 watts output power with 0 watts reflection was used. The power was fed to two dipole antennas with 3dbd gain mounted at about 100 feet (30m) high through 7/8" feeder cable.
The area was flat with few dispersed storey buildings. The signal was strong with clear stereo sound on our car radio tuner and the Samsung GT-E1207Y mobile phone at about 30km away.
We received this setup with good stereo sound on both devices at about 42km away. This test was ended at about 50km away. This test was done to see if we can receive a good signal with clear stereo sound at 40km away.
Test Three: In this test 500 watts output power with 0 watts reflection was fed to three dipole antennas with 4.5dbd gain mounted at about 100 feet (30m) high through 7/8" feeder cable.
The area used for test one and two was used for this test; flat land with few dispersed storey buildings. We received this setup with good stereo sound on both devices at far as about 52km away where we ended the search the signal at. We heard hiss at some points but was few.
This test was done to see if we can receive a good signal with clear stereo sound at 45km away.
Test Four: This test was done with same transmitter output power and reflection, 500 watts and 0 watts respectively. It was fed to four dipole antennas with 6dbd gain mounted at about 100 feet (30m) high through 7/8" feeder cable.
With clear stereo sound and strong signal on both devices, this setup was received at about 38km away. We ended this test at about 60km away where we still received this setup with clear sound.
This test was done to see if we can receive a good signal with clear stereo sound at 50km away.
Where we ended the 'search for the signal' weren't the end of the signal from the setups. We ended because we achieved the purpose for the tests. The above tests are only for comparison.
For instance 85km can be achieved with a good transmitter site with 500 watts of transmitter power fed to antennas at high as 150 feet or more. The distances shown are for a reasonable signal on a car stereo and mobile phone with built-in FM reciever.
Check out what we got for 1000 watts FM radio trasmitter.