Hi, this is my first post to this great site. I'm a tinkerer with RF hardware, licenced some years ago as G0RUE but the licence has lapsed in the intervening years.

I have been trying to understand field strength calculations to get an idea of transmitter range, in the 70cm region. If there's anyone who can confirm or correct my maths so far, I would welcome it.

If a given transmitter with a known power output can reach a receiver that is, say, 30 miles away, then the power is reduced by 1dB, or 3db or 5db, how much less will the range be?

Please understand there's no particular receiver that I want to work. I'm not licenced to do that anyway, and I know that range depends on far more factors than distance alone. I just want to check the relationship between power changes and range, according to calculation.

So, I figure out that a 3dB drop in power is antilog(-3/10) which is 0.5. So the field strength is half what it was. To get the same field strength at the receiver I would need to be closer. I have to reduce the distance to the receiver by 0.5 squared, as field strength reduces with distance in proportion to the square of the distance. My 30 miles would reduce to 7.5 miles.

Is that correct way to calculate the reduction in transmitter range for a given drop in transmitter power?