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Range of a radio transmitter

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How would I determine the range of a radio transmitter? What if I wanted to approximate the range of a radio transmitter depending on frequency and etc? Does not pertain to SDR exactly, but I thought somebody here might know the answer. What variables influence the range? So apparently voltage probably does and the antennae length. Wavelength also probably does. What I need is basically a formula I can solve for myself.

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Wow. From the last paragraph of that first link:

[...] about 10 to 30% of the energy in the tank circuit reaches the antenna [...] you may find that only about 1 or 2% of the energy in the coil gets transmitted [...]

I didn't expect things to realistically be that bad...

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Path prediction is based on the sum of all gains and losses in dB. This assumes you have access or measurements of all the gains and losses of the system. First add up the gain of the transmitter output and receiving amplification and then subtract out all the attenuation and losses in the desired path. The trick in calculating is not mess up the terms when converting dBmW to dBi where mW is based on the milliwatt and the I is based on the gain of an isotropic antenna when compared with a dipole at the resonant frequency. Having said all that, I use tool based on a web site called "radiomobile" where you plug in all those variables mentioned above and the output is a map with the predicted results. And then there is nothing better than to develop your own empirical results, hams do this everyday. KD6W

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