Testing Cellular connectivity

[98roamer]

I'll be doing some volunteer work deep in the woods of KY this summer and I need to stay connected to the internet. I'm testing a AT&T, T-mobile and Sprint Aircards with a CradlePoint CB250 wireless bridge. We will be pairing it with a Wilson 30db amplifier with a Laird antenna.
I'm trying to simulate poor cell phone coverage to review a CradlePoints CB250 Wireless Bridge. I've built a Faraday cage and can successful block all voice calls on AT&T and Cincinnat Bell networks. I put the AT&T iphone into engineering mode and it returned a -107 RSSI with normal being -65db. I can't get it to block data from the Aircards. It will cut the signal down about 20-30% but I was expecting the cage to kill all data traffic. The cage is on phase 3 of an over kill all expect for grounding it. It killed the voice calls on phase 1.

Do I have to build different Faraday cages for data vs voice?

I’m hoping that someone can teach me on cellular networks.

Thanks
Lance

 

[chrismc]

The specs for the cage will be dependent on the the frequency that you want to block. Most providers use different frequencies for voice and 3G data. Also, some of the standards (GSM, CDMA, etc...) might be more resistant to interruption than others due to the way they allocate channels. You'll probably need to dig up the specifics for each service you're attempting to tamper with, and attack each individually (there's no reason you couldn't do nested cages).

Sounds like a pretty fun project, I'll be interested to hear more about your results (and specifics of how you accomplished it).

 

[iigs]

At a former employer we did wireless device testing in a 1 cubic meter copper walled box (solid, not mesh). You need to have some way to get data (and probably power) in and out of a box of that sort.

If you have pictures of your setup perhaps we can look at it to see what might be wrong. For starters, the things I'd suggest checking out are the seals for whatever door it has and the hole (if any) you're using for getting power or ethernet into the chamber.

A 1cm x 1mm gap positioned just right/wrong is probably enough to let in enough RF to get some signal. I believe all of the protocols in current commercial use these days have logic to store a frame and retry until they get an acknowledgement, meaning you don't have to have much RF at all to get what appears to be a pretty solid connection.

 

[98roamer]

Thanks for the replies. I think the more I look into and from the replies; the door is going to need improvement. Right now the cage is 5 layers of small square holes of fine galvanized garden fencing/ type of chicken wire. It's a continuous length just folded over but the end is just bolted on, so I'm loosing some connectivity property there. The door however is just hinged at the top of the cylinder, doesn't seem all that protective after all. I've wrapped the cylinder with copper wire and a couple layers of aluminum foil.

It kinda looks like a rat trap with the door they way it is.

My plan:
1. Wrap the cage in copper
2. Fix the door so it's more conductive focusing on the door
3. Research the carriers data frequencies to know what I'm dealing with.

How does the plan look? Am I on the right track? Pictures to come.

Thanks
Lance

 

[98roamer]

I built a solid copper box with just a little opening for the power cable to the wireless bridge. It didn't work. I even soldered the seems to hold it together. It works for voice but it really had no effect on data cards.

I'm kinda stumped. I thought a copper box would have done it. I must be missing something.

 

[chrismc]

I haven't seen it done with solid copper before. Usually, a shielding cage is made of a fine wire mesh that draws the RF waves away from the interior. There is a nice place to buy the shielding mesh here: http://www.lessemf.com/fabric.html (the website seems to be a bit targeted towards the tin-foil hat whack-job crowd, but there are some real products in there) They have the specs for each fabric listed as dB for a given frequency range. The ARGENMESH looks like a good option. It introduces >60dB of loss at 10MHz to 40GHz, which should more than cover any wireless frequency band. The price is pretty reasonable as well.

Build a simple wooden box, and attach the mesh to the box (I'd just use a staple gun, but I think hot glue works well also), being sure to overlap all seams completely. This is a good example:

 

[iigs]

I built a solid copper box with just a little opening for the power cable to the wireless bridge. It didn't work. I even soldered the seems to hold it together. It works for voice but it really had no effect on data cards.

I'm kinda stumped. I thought a copper box would have done it. I must be missing something.

Did the box have a door or did you install stuff through a removed panel and then solder it closed?

In the walk-in RF chambers we were using there was a hole the size of a quarter in the bottom corner of the cage and the lead RF engineer said that it was enough leakage to totally invalidate certain tests.

In the end it's a numbers game -- you're seeking decibels of attenuation. It's hard to tell without measuring (at least using the RSSI indicator on the phone if not with a more sophisticated method) where you are and where you need to be.

My suggestion would be to do whatever you have to do to remove remaining holes and cracks in the seams. Maybe instead of a hole for the power, leave the power adapter outside the case (if it's low voltage) and screw a F-connector or BNC connector to the cage, and pass the power through the chassis and the hot pin. You can then use several feet of quad shield RG-6 or the like to connect the power adapter, hopefully reducing leakage through that interface a little bit more.

If you haven't, you might also try grounding the cage. I don't _think_ it needs to be, but it seems like it couldn't hurt.

I'm not clear exactly on what you're testing, but another thing you might consider is breaking the plastic case of the data card and desoldering the antenna and reattaching a piece of superfine coax-cable. Conducted RF testing isn't the right choice for everything but it may be an easier route than trying to get the appropriate gear in a big box.