First off, when determining the range of a radio transmission, frequency really comes into play. For instance; some high frequency transmissions such as short wave (a radio wave of a wavelength between about 10 and 100 m (and a frequency of about 3 to 30 MHz) have attendancy to bounce off of the ionosphere and therefore travel around the earth- this is how some ham operators can talk to people on the other side of the world using only radio waves.
You also have to think about the frequency of the signal.
Given two signals of equal strength and different frequencies, lower frequencies travel further than higher ones. AM radio signals, for instance, in the range of 520 to 1,710 KHz, can often be pickedup at distances of 100-300 miles, while FM frequencies of 88 MHz to 108 MHz are limited to what’s known as line-of-sight transmission, topping out at around 50-60 miles, at
best. We can expect the same result the higher we get in the frequency spectrum.
To answer your question, let’s assume that we are talking about 5.8ghz which at the time of writing, is the most popular video frequency for FPV drones. And much higher than FM radio (not very good at penetrating objects, or traveling very far due to it’s high frequency)
Conventional logic would make one think that 600MW is 3x the power of 200mw, so using a 600MW video transmitter over a 200mw Video Transmitter must give you 3x the range, but that simply isn’t true due to the nature of radio waves and more specifically, the inverse square law.
An inverse-square law is any physical law stating that a specified physical quantity or intensity is inversely proportional to the square of the distance from the source of that physical quantity. (Wikipedia) This phenomenon is common in many aspects of the natural world and also has a lot to do with the range vs. Power output of our miniquad video transmitters.
So to put it into some simpler terms, to DOUBLE THE RANGE of your Video transmitter (Vtx), you would have to QUADRUPLE THE POWER OUTPUT.
So essentially, moving from 200mw at 5.8ghz to 600mw would only yield about 1.5x the distance in range.
The further you want to go, the more power you will require. For instance, a 25mw vtx has a doubled range at 100mw, 100mw is doubled at 400mw, and 400mw is doubled at 1600mw, so there is a point of diminishing returns especially when it comes to flying machines and the amount of power that they can transmit. The majority of FPV video transmitters on the market range from 25mw to 1.5W.
So you would notice a slight distance increase going from 200mw to 600mw, but it wouldn’t be anything incredible.
WHY NOT JUST USE THE HIGHEST POWER I CAN GET?
Well, too much power can knock others out of the sky, and also can cause issues with multipathing, especially in areas with lots of reflective surfaces like parking garages. Multipathing can cause signal loss, grey bands, and more video problems.
So what is the best solution?
Use a video transmitter with adjustable power output such as the TBS Unify. That way you can dial it to low power in reflective or competitive environments, and then crank the power when you are by yourself and want better range in open spaces.