In article Martin Gregorie writes:
On Fri, 19 Sep 2008 10:25:42 -0500, Michael Ash wrote:


Anyone know more about it? I'd like to know more than my patchwork
memory....

In the UK, anyway, the base station transmission patterns are quite flat
which can stop you getting a signal in the air.

A year or two back I wanted to annoy a friend with the "ring him and hold
phone by the audio vario" trick, but at 3000ft over Huntingdon, i.e.
above a flat bit of Cambridgeshire, there was no signal at all. I was
using a GSM phone, so the radiation pattern was evidently flat enough the
exclude not only Huntingdon masts but also those further away (Cambridge,
Northampton). This makes sense to me. Why should a telco waste
electricity transmitting a hemispherical pattern when a pancake pattern
will give a better signal strength for less radiated power throughout its
service area.


Interesting. Generally, the attenuation possible from an antenna depends
on the angle of elevation, and if you are 5 miles from the tower, at 3000
feet, you would be 6.5 degrees of elevation above horizontal, which should
be well within the pattern of the antenna. (Any reduction would be easily
made up by the very clear path to the tower.)

I have noticed the same effect on top of mountains here -- at 2600 feet
elevation, looking out at the populated areas, there is no signal.
HOWEVER --- If I walk away and hide behind a building, I get nice strong
signal.

There are a limited number of channels where the cell system
transmits control information. When the phone is not on a call, it
listens to one of them. Each cell site (tower) has one (or perhaps
more) channel for this control information. Like cell calls, it is not
re-used until a "safe" distance away.

When on top of a mountain, there are dozens of towers within sight.
Unfortunately, every available channel is in use by several of these
towers. Thus, the phone cannot receive a clear control signal on any
of the control channels -- each is a jumbled mess of several sites
transmitters.

Much the same happens in the glider.

Hiding behind the building, a few feet back from the edge of the
mountaintop blocks many of these signals. The phone found a good one,
and used it.


IIRC this has been noticed and commented on in the USA too.

And I strongly believe that the signal pattern of the antennas is not
the cause of the problem, or stepping a bit behind the building would not
have made the phone work, as the pattern would have been the same.


Alan