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Re: Antenna identification



Tony Wayne wrote:

The pictures are located at http://physics.k12albemarle.org/tower

That's a pretty standard configuration.

In this figure
http://physics.k12albemarle.org/tower/tower-Images/0.jpg
we have a cell-phone base station. The word "antenna" can
be applied to the widgets about the size of a large spray
can, slightly flattened. We have here an array, or actually
three arrays, one array facing in each of three directions
spaced roughly 120 degrees apart. This station sits at
the corner where three _cells_ meet.

Lower down on the same tower we have
http://physics.k12albemarle.org/tower/tower-Images/1.jpg
the things that look like drums are microwave dish antennas,
used as part of the backhaul network. Backhaul is a major
expense for cable companies; they use fiber optics when
they can, but in this case they needed some microwave links.
(You don't know whether this is backhaul to and/or from
other towers.) Obviously some sort of backhaul is needed;
it doesn't do much good if your cellphone can contact the
tower, unless the tower can contact the rest of the PSTN
(public switched telephone network).

The two different antenna designs reflect two radically
different tasks. The cell-base antennas need to lay
down a broad swath of radiation, covering roughly a 120
sector, with uniform gain within the sector but falling
off very steeply at the edges of the sector. And that's
not all; sometimes they adaptively steer the beam within
the sector, on the fly, electronically, to aim it at a
particular mobile. There's serious wizardry involved;
this barely scratches the surface.

The backhaul antenna has a relatively simple task. It
needs to form a tightly focussed beam. It is carefully
aimed at a specific, stationary peer.