On Sat, 17 Jan 2004, Arthur Harris wrote:
"Daniel J. Morlan" wrote:
I'm assuming that the frequency chosen might have a slight effect on
the range, but with 75 watts screaming from my mobile, what is the
potential range with a clear line of sight? Can anyone approximate at
all? There's repeaters galore here, but I am quite interested in
simplex operations away from the great Dell of loomis.

Your output power is just one of the many variables that determines range.
That gain and height of the antennas at each end of the circuit, as well as
terrain, will play a much bigger role.

Going from 50 Watts to 75 Watts of transmit power will have almost no effect
on range.

For true "line-of-sight" paths, the "Free Space Path Loss Equation" is one
way to approximate range:

Path Loss (in dB) = 36.6 + 20 Log F + 20 Log D

Where F is frequency in MHz and D is Distance in miles. This equation
assumes isotropic antennas at each end.

Are you certain that the distance should be measured in miles and not
kilometers? Using the metric system instead of English units works much better.
Ever notice why most radios have an RX sensitivity of 0.16 microvolts? That's
a field strength equivalent to 1 watt (isotropically radiated) at a distance of
exactly 1 km. 4 watts at 2 km will produce the same field strength.

FS = P / ( 2 * pi * D ^ 2 )

FS is in volts.
P is in watts.
D is in meters.

If you were trying to work the space shuttle at a distance of 250 miles away
on 146 MHz, the path loss (from the above equation) would be 127.8 dB. If
the shuttle's receiver needed -100 dBm of signal from you, then 1 Watt (or
+30 dBm) of transmit power would be sufficient. (+30 dBm transmiited, minus
127.8 dB of path loss, equals -97.8 dBm received)

The problem is that few terrestrial paths are really line of sight. Once you
get 10 or 20 miles away from another land based station, earth curvature and
other obstructions begin to have a big effect.

Mobile-to-mobile communications will have the shortest range since antenna
heights are so low. Mobile-to-base is better, but two base stations with
high gain yagis on tall towers will have the best range of all. Switching
from FM to ssb will also greatly increase range for a given power.

If you are in flat, open terrain in your mobile running 75 watts, and your
buddy has a base station with an omnidirectional antenna at 40 feet, I would
estimate your simplex range on 2M FM would be about 25 miles under normal
propagation conditions.

Art Harris N2AH