Robert11 wrote:
Hello,

For the scanner frequencies around 118 MHz, e.g., I realize FM is
essentially line of sight.

Are AM transmissions also line of sight ?

Same question, but for the higher freq's around 800 MHz ?

Thanks,
Bob

Andy replies:

While I agree with the other posters, let me add that "line of
sight"
represents the best possible circumstances, and then only in free
space.

To test this, take your 2 meter handi talki and start up a
conversation
and then walk behind a car, or big truck. Nearly every time you will
be able
to maintain your conversation, even tho you are no longer in "line of
sight".

The environment is so full of reflections that it's like standing
in a hall
of mirrors with a flashlight. Many times you can drive thru a tunnel
without
losing the link. Other times you will lose a signal when you drive
by
light poles. (This is especially noticeable with your FM radio and a
weak signal).

While the radio waves travel propagate thru a non-homogenous
medium with
lots of things present that don't represent 377 ohms, each one of them
causes a reflection that will combine with the original vector, and
can
create sharp nulls or slight enhancement. Also, the combined signals
can effectively make the path of the signal "bend"....

So, while "line of sight" is the simplest answer, I have rarely
found
it to be correct. The close you get to "free space", the more
realistic
it is...... but that hardly ever happens when standing on earth...

Andy W4OAH