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To know the range of a transmitter, you also have to consider the receiver and the antennae at the transmitter and the receiver.
If the sensitivity of the receiver is poor, you will need more transmitter power.
If either of the antennae are poor, you will need more transmitter power.
The free space path loss between two antennae can be calculated using the formula
path loss (dB) = 32.45 + 20 log(f) + 20 log(d)
where F is in Mhz and d is in km.
But this wont do much good in this case because the transmitter and receiver are so close, 10metres. When the distance is less than about 10 wavelengths, we are in what is known as the "near field" and we dont have a true electromagnetic field which obeys nice simple equations. (The wavelength of 27Mhz is about 11 metres).
To give a feel for how little power is required for what you want to do, I did a test.
Using a scanning receiver with a 57cm long antenna, and a signal generator with a 80cm long antenna. I put them about 5 metres apart and adjusted the output power of the signal generator and listened. The frequency I used was 27.1Mhz.
With 1 micro watt from the generator, there was a noisy signal at the receiver.
With 0.1 milli watt from the generator, there was a nice quiet signal at the receiver.
A simple 1 transistor oscillator should give you several milli watts. You may also have a few other problems like frequency drift, but it will be an interesting learning adventure for you!