David Harper wrote:
I'm sending a IC-V8 (2m) up on a high altitude balloon, and was
debating on what power setting to use. Ideally, I'd go with the
higher power setting (5.5W), but due to the cold temperatures that are
encountered (even with insulation) and limited battery life (even with
a higher capacity battery), going with .5W is more reliable from a
battery life standpoint.

My question is: can I get clear reception using the .5W power setting,
even when the balloon is, say, 75 miles away at an altitude of 20
miles?

Assuming you have a line-of-sight path to the balloon, you can use the
free space path loss equation to determine your path loss.

At an altitude of 20 miles and 75 miles downrange, the distance to the
balloon will be 77.62 miles. At 146 MHz, the path loss will be 117.6
dB. Assuming antennas with "zero" dBi gain at both ends of the
circuit, the signal provided to your ground receiver will be as
follows:

A 5.5 watt transmitter will result in 21.64 uV at the receiver.

A 0.5 watt transmitter will result in 6.52 uV at the receiver.

Art Harris N2AH