As I understand it, each base station is tracking the
cell phones it can hear and is alert for a hand-off.
So the more base stations that a cell phone can be
heard by, the more load on the network. Thus a cell
phone in the air may put more load on the network than
one on the ground. I don't know how significant this
load is.

As per the example of the mountain top repeater, I
think that different base stations use different types
of antennas depending on the surrounding terrain they
are trying to cover. In flat areas the antennas have
very horizontal propagation characteristics. You can
see that if you watch your signal strength as you take
off as a passenger from a flat terrain airport. It
disappears after one or two thousand feet. But if
you are flying over rougher terrain, say Telluride,
the signal strength will be fine at 13,000 feet directly
over the town or many miles from it, for example.

Forest

At 15:12 28 April 2007, Colin Lamb wrote:
Eric hit it right on the button - about focusing.
Gain is attained at the
expense of directivity. The more gain, the further
the signal transmits and
receives and the less power required. Assuming the
cell tower is at the
same terrain height (or slightly higher), it will try
to concentrate all of
the power along the horizon. Any signal going upwards
will mean less signal
where people need it.

If the antenna is designed correctly, your signal will
decrease as your
angle above the horizon increases. Directly above
the cell tower would
likely have a total null. That means that at altitude
you will likely have
the strongest signal from the cell tower furthest away
- because it is the
lower angle.

Vhf omni-directional antennas have much less gain and
directivity, so it is
much less of a problem on the aircraft frequencies
- but still can be an
issue if you are directly over the station on the ground.
I have seen
repeater antennas on mountain tops near the town they
want to cover actually
invert the ground plane to provide better penetration
into the town below.

Colin