In article , Pdigmking
wrote:

I'm looking at these GMRS motorolas, one is a t6510xmrs and another is a
t5520. Here's the thing: both have a power output of one watt, but one
claims to have a distance range of 8 miles, and the other 5. Is this BS or
what? Why would one 1 -watt radio have more range than another 1 watt
radio? They both have the same size antenna.

Paul-

Both are theoretical values for person-to-person communications over level
ground with no obstructions or reflecting surfaces. Since the
communications path is essentially line-of-sight, slight efficiency
differences in power or antenna gain make little difference. Therefore,
the differences must be based on some assumption that was different when
calculations were made.

For line-of-sight, you can approximate the distance to the radio horizon
for each, and add the two for total maximum range. Suppose a person holds
the handheld radio up to their face, and it is five feet above ground.
The distance to their radio horizon in miles, is approximately the square
root of twice the antenna height in feet, or 3.2 miles. Range to another
similar person would be 6.4 miles under these ideal conditions.

You should see that estimating distance based on different heights can
result in differeing ranges. A height of 3.125 feet (sitting down?) has
range of 5 miles, and a height of 8 feet (basketball player?) has a range
of 8 miles to a similar person.

This approach works fairly well from mountain to mountain or airplane.
Based on power and receiver sensitivity, the limiting range might easily
be greater than a hundred miles. In the real world there are reflections
that cancel signals (Multipath), tall objects that block signals and
vegetation that absorbs them. Person-to-person on typical terrain might
easily be limited to less than one mile.

Fred