[Phys-L] physics in the real world : WAN in a cage

[John Denker <jsd@av8n.com>]

Hi --

Here's a fun example of physics in the real world: What's wrong with
this picture?

Overview:
  https://av8n.com/physics/img48/wan-printer-cage.png

Close-up:
  https://av8n.com/physics/img48/wan-cage.png

When I saw that setup, I sent an email to the guy in charge, saying:
Fastening the wide-area-network (WAN) base station inside the cage is
clever in terms of convenience, but it's asking for trouble. Physics
says the cage will interfere with radio wave propagation.

The email was tacitly ignored. I surmise that the thinking was, why
should we trust the physics guy? Why should we not believe our own
eyes? We tested this setup, and it works fine.

Here's the rest of the story: They tested it at headquarters, which is
literally in the shadow of a cell tower. When they schlepped the cages
out to the 200+ sites where they are actually needed, they found that
some of them worked OK, some of them didn't work at all, and some of
them depended on which way the open door of the cage was facing.

Suddenly the setup seemed a lot less convenient.

In more detail: Here's how I instantly knew it would be trouble: You
can figure out the wavelength of the WAN system by looking at the
antenna. You can guess that it's a 1/4 wave antenna. And in fact
that's a very good guess; I checked that the device is primarily using
the 700 MHz bands. Occasionally 600, and occasionally 850, but that
doesn't change the conclusions.

Now look at the grid spacing on the cage. It's tight compared to the
wavelength. Then apply the Huygens construction. Conclude that as seen
from the far field, a tight grid is indistinguishable from a solid
slab of metal.

You can *see* through the grid because your eyes are using a shorter
wavelength. Shorter by a factor of a million.