I’d forgotten how old most houses are in the northeast, and my house is no exception. The walls are lathe & plaster, or something equally ancient underneath whatever wall covering is there. Electrical wiring is ancient too, with 2-prong outlets and no grounding wires; another chore for the “honey-do” list.
So what does my house construction have to do with EMC? Well, on the good side, my microwave oven does not interfere with my 2.4 GHz wireless surround-sound. All that lovely thick plaster and wire in the walls effectively attenuates any emissions from my microwave.
On the bad side, my cell phone loses signal inside the house, depending on where I’m located at the time. Outside the house, or near a window, the signal is strong and steady. A typical wall has about 3-8 dB of attenuation. When I measured walls (with no furniture or objects in the rooms) in several rooms, they had anywhere between 10-15 dB of loss.
An annoying, but significant, symptom of this loss of signal strength is the life of my cell phone battery. When a cell phone cannot receive full-strength signal from the cell tower, the phone increases output power. This uses up the battery more quickly, for obvious reasons.
I have mentioned this to a number of people, merely to see what they will offer in the way of getting around this problem. So far, the only comment I’ve received is to change providers. But we both know this won’t help, and my provider has excellent coverage in this area.
Cellular Signal Booster for the Home/Office
One answer is to boost the signal coming in to the house, and there are a number of manufacturers out there who provide this type of solution for the home or office. Surprisingly, it is not that expensive.
These systems consist of a small antenna that receives cell tower signals, an amplifier to boost the signal and send it to your phone, and all the installation accessories required.
The quality of signal outside is the determining factor on where you install the receive antenna. Since I have excellent signal reception outside, I placed the receiver inside the house, at the top of the window facing the closest cell tower. If the signal outside was weaker, say, one or two bars, I would probably need to place the receive antenna on the roof. (The good thing is, the system comes with a nice little holder and clamp that can attach directly to a satelite TV pole.) The amplifier/AP is mounted discretely in the corner of my office, near the ceiling. Not only are there less obstructions at this height, but it is mounted line-of-sight to the receive antenna.
The system covers all Cellular and PCS frequencies, and Laptop Data Cards (except iDEN and Nextel), and is easy for anyone to install.
The booster I installed is manufactured by Wilson Electronics, Inc., the model aptly named SignalBoost DT, part number 801247. I purchased it online for about $199.00. A quick Google search will find it for you at a similar price.
Now my phone battery lasts longer, I don’t get dropped calls, and I don’t hear that annoying chit-chat on my computer speakers that occurs whenever the phone and cell tower communicate. All in spite of the ancient building construction of my home.
Distributed Antenna Systems (DAS)
From an EMC perspective, this type of system is extremely beneficial to hospitals, especially from an EMC management perspective. Commonly known as distributed antenna systems (DAS), they operate in the same manner as my simple system here in my office; bringing in the signal and boosting it. More importantly, they help in ways that have nothing to do with just getting more bars on your cell phone.
Remember that annoying chit-chat I spoke of? Because it is my speakers reacting to my phone, I can hear an audible response, and what I’m hearing is my cell phone interfering with my speakers, aka electromagnetic interference (EMI).
However, if I were to place my cell phone on top of a critical medical device for instance, there might be interference that causes an inaudible result. And this is what you want to reduce or prevent.
It is all about proximity and power level. For we all know (or should) that cell phones operate on completely different frequencies than those commonly used in medical devices.
Power level output in a cell phone increases with its inability to communicate with incoming signals from the tower (dead battery, remember?). The greater the output power of the phone, the more likely it is to affect a nearby device (like my speakers).
Proximity is simply how close it is to another “victim” device. The effects of power decreases with distance. Simple math, but proven when I move my phone further away from my speakers; i.e. I don’t hear that chit-chat anymore. However, it’s convenient for me to place it on my desk next to the right speaker, and that’s where I want it.
Consider this scenario:
A doctor does his rounds late at night in the ICU. Most patients are sleeping, and some might have their Telemetry transmitter sitting on a bed-side table next to them, or other life-support devices nearby. As the doctor reads the patient chart, the cell phone in his pocket decides to “ping” the tower (or vice-versa), or maybe it rings through. The signal is weak, so the phone increases it’s output power and interferes with whatever is close by; maybe the Telemetry transmitter, maybe an infusion pump or ventilator.
The next day, a nurse calls support services and reports a malfunction the night before. This starts the ball rolling, an investigation ensues, but the interference continues to occur intermittently every night and the cause remains a mystery.
The bottom line is, the event caused a compromise in patient care. Understanding how this phenomena happens and taking steps to reduce the probability factor is the answer. A distributed antenna booster system (DAS) in this environment would greatly reduce this factor to almost nil.
Some medical device manufacturers recommend a “no cell phone use” policy in critical care areas. They do this because there is never a guarantee when multiple electronic devices are used concurrently in the same environment. Manufacturers simply can’t control human interactive events 100%, so they provide guidelines and recommendations.
So, what if the doctor wants or needs to carry his phone in the ICU anyway? The DAS takes the chance of interference down to a very low percentage.
As with any electrical or electronic device, and the nature of electromagnetic phenomena, the only way to have absolute 0% probability is to not have one or the other in the environment at all, but we both know that won’t happen either. So manage the situation with what is readily available.
I might also add that cell phone manufacturers are well aware of where their products are used. In the last ten years, these little electronic “goodies” so necessary to our daily lives have come a long way in reduced power output. Cell phone design engineers are paying attention. Their business depends on usability in all environments.
I wish I could say the same for Microwave Ovens, but that’s a whole ‘nother future post.
~Dara~
