Wireless Woes

by Alan Graham

I recently moved my 802.11b router to the room next to our living room and quickly started having signal trouble. Nothing shocking about that, except I moved it to a location that was the same distance from my laptops as it was before, and line of sight, by the way. So what could cause such a drastic state of affairs?

Microwave oven? No.

Solid Walls? No.

Electronics? No.

Concrete, Metal, Bullet-Proof Glass? Nope.

Since I couldn't find the source of the problem, I was certain it had to be my wireless card or wireless antenna. Nope and nope.

After one week of experimentation, and about $1,000 worth of my billing time, I finally stumbled on the problem. Attached to our living room is a studio. Between the studio and the living room is a doorway. To keep the cats and dogs from turning the studio into a barnyard, we put up a wood framed screen door. While wood has a low level of interference, it turns out that the metal mesh screen in the door was preventing the Airport signal from leaving the room. Now we're talking a distance of less than 10 feet, line of sight, transparent, and yet I couldn't even get one bar.

I'd love to know the physics of why this happens, if anyone out there can explain it.

Got the answer?


2004-03-07 19:04:24
Faraday Cage
Ever heard of a Faraday cage?
2004-03-07 19:32:03
Faraday Cage
2004-03-07 22:47:24
Faraday Cage
I'll take that as a yes.
2004-03-08 06:34:20
Microwave Engineering 101

You've stumbled into the mysterious world of Microwave Engineering, where a physical volume of space is modeled as a circuit. (I mean Microwave energy, not microwave ovens.) Really, the only way to figure out what your signal problems are is to draw a 3D model of your "space" and run it through a tool similar to Ansoft's High Frequency Structure Simulator - this is the same tool that professionals use to verify that a trasmitter or source (satelittes, radar, etc.) is going to function properly. And, I can attest to the fact that designing microwave enclosures is a fine art bordering on divination. A tool like HFSS is like cheating, it solves Maxwell's equations over a mesh inside a volume, and it can spit out some amazing animations that show you an approximation of how waves travel through space.

The previous responses about a "Faraday Cage" are not necessarily correct. While a Faraday Cage can create a noiseless volume of space, from your description, there is simply a rectangle of metal mesh between you and your wireless whatchamacallit. If your house/apartment/teepee was made of steel, or metal - then you might be having problems. Or, if there is paint on the wall that contains a metallic component, you might be creating a faraday cage unwittingly. Any picture frames on the wall?

A so-called "Faraday Cage" can shield a volume of space from energy of any frequency, and there is a great resource at BOLT Lightning Protection that gives you a good equation for calculating the attenuation of a signal depending on the specifications of your metal mesh doorway.

I think most wireless routers operate near the 2.4 GHz or 5 GHz, and you shouldn't have to worry about being line-of-sight to your router. I've had wireless ethernet work through various materials - (my floors are made of wood, and I frequently find myself on my neighbor's wireless router).

T. O'Brien

2004-03-08 06:51:57
Microwave Engineering 101
Well I discovered the solution to the problem by accident. I thought the problem was my wireless antenna, so I replaced it. Same issue. Then I watched my wife open the screen door and my signal jumped. This door has only a small wood frame, and the rest of it is metal mesh.

Open the door, signal. Close it, no signal. In fact, you can be standing right in front of the door, 3 feet from the base station and not get a signal at all.

2004-03-08 11:09:45
Microwave Engineering 101
You also said the door is in your line of sight to the router - you don't need a full faraday cage to cause yourself a problem, just a big ol' conductor in your way.

A while back, New Scientist mentioned one of the major electronics companies patenting pen-like devices (really just waveguides) that builders could insert into walls to relay 802.11, IR, etc through obstacles. Sounds like they've found a customer.