greetngs
After I thought about your project for a while I remembered something an
engineer amateur told a group of amateurs about 20 plus years ago.
This group of engineers was building a tall, very tall, structure. This
included a number of antennas for both tv and radio transmission, plus some
two way radio antennas.
To see how good, or bad, the antenna system was the engineer, who was giving
this talk, told of an experiment he did to prove a point that you did not
need power at high altitudes. He took his handheld and dialed up the direct
channel. 146.52 he connected it to one of the tv channel antenna. The
resultant swr was considerable. The cable from the radio to the antenna
consisted of a long length, perhaps 500 feet, of cable, decoupling networks
for the various antennas, etc. He switch over to low power, and called cq.
He got a contact in a town about 75 miles away. full quieting.
The point is that once you have gotten away from the earths curvature power
on vhf and uhf has almost little meaning.
I sent an email to the jet propulsion labs a few years ago asking what power
the space crafts like the jupiter series that are going beyond the solar
system are transmitting. Now granted they are using dishes at both ends and
rather suffisticated data decoding etc. But the power level used is
somewhat in the order of 1 (one) watt from the spacecraft.

Personally I find that incredible and I find myself humbling, and somewhat
embarassed, when I used high power on my handheld to transmit 1 mile away.

Larry ve3fxq




"Art Harris" wrote in message
m...
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