[Aironet] Wireless elevators

[Webcats N. <ad...@we...>]

Magnetic interference and Aironet bridging

As professional site surveyors and installers, we exercise a 
heightened awareness of electronic devices, in and around the 
wireless equipment we install, to reduce or remove as much 
interference as possible.

Recently while installing a dish antenna and bridge atop one of 
Lexington Kentucky's bank buildings I noticed a rather pesky problem 
when trying to aim the antenna.  Using a magnetic compass, I 
carefully took my bearing, looking to the horizon about 12 miles out, 
where a client antenna would (or should) be located.

I climbed the short rooftop tower and aimed the antenna.  I pulled 
out my trusty liquid filled, professional compass to check my work. 
My bearing was off by more than 35 degrees to the south with this my 
second reading.  I re-aimed the antenna and pondered how I could have 
made such a large error in my first reading.

One last check from the roof and I'm going home.  The third bearing 
check showed yet a third vector.  It didn't match either of the first 
two readings taken.  In frustration, I called another tech to the 
scene.

After shooting the bearing a number of times we speculated that the 
compass was receiving interference from metals used in the building 
construction and attempted to find a spot on the roof that might get 
us a reliable reading.  So we shot the bearing from a few locations 
around the roof of this 18 story building; looking for a trend in the 
readings that might reveal where our true target bearing lie...

With more than 80 degrees angle of arc horizon to choose from in our 
survey of compass points, the obvious question becomes, "Which 
readings do you trust?". After a little more speculation, discussion 
and binocular searching of the horizon, we picked a target and 
secured the mounts.

As I reached street level and exited the building, I still pondered 
the incredible amount of magnetic interference that we experienced on 
the rooftop, just 18 stories above.

Again I pulled out my trusty liquid filled, professional compass to 
check my work.  I took the bearing to where the client antenna should 
be located and to my surprise, the reading indicated that the antenna 
I just mounted should be pointed even further to the south than any 
of the readings we had collected on the roof. I crossed the block and 
took another reading from the courthouse steps and got the same 
result. More compass readings from around the area revealed that the 
antenna would again have to by re-aimed.

On the elevator ride back up to the roof I watched the compass.  The 
closer I got the the roof, the more northward of the true bearing the 
compass needle would point. As soon as the elevator stopped, the 
needle readjusted itself swinging slightly back toward the true 
target bearing still short of the actual vector.

By now I was very familiar with how the building was attached to the 
ground in relation to magnetic north. That coupled with the 
inaccurate readings and then the sudden readjustment on the compass 
just as the elevator stopped made me realize that the roof mounted 
motors driving the elevators were the source of the magnetic anomaly 
(even when they were not running).

I've installed on roofs similar to this one, but I was always able to 
see the target antenna I was aiming toward (no compass needed).  In 
this case, I had to rely on a compass because of the 12 mile distance 
to the target antenna.  This information (now realized) adds one more 
element to watch for when mounting or implementing a wireless system.

How clean the area is of electromagnetic (EM) interference is an 
important element to consider.  This situation now begs me to further 
ponder the influence of the magnetic interference generated by 
elevators and how it may be effecting the throughput performance of 
Aironet products.  Typically placed in these notorious areas of high 
EM, what if any effect can I expect from elevators, and if there is 
an effect, how do I eliminate it?  Raise the tower?  So far I have no 
"proof" that there is a problem, only a suspicion.  Here's why.

We have 5 antennas mounted at this location.  One of the bridges, the 
closest client, is only  300 meters from our location.  We are using 
an antenna/bridge combination that will work for up to 12 miles, yet 
the signal strength and quality from this client are "dirty".  There 
are a couple of things we want to try before we write it off as being 
the elevators fault, but it is a consideration we'd like to hear more 
about.

Robert Boak
Webcats Wireless Network
http://Webcats.net/