SAR, Cell Phones, and San Francisco

An interesting article came across my desk this week. It talks about a recent vote in by the Board of Supervisors in San Francisco requiring that cell phone retailers post the specific absorption rate (SAR) for all cellular handsets that they sell. The article caught my attention because many cell phone manufactures use our FDTD libraries to model the electromagnetic field strengths, which are used to calculate SAR as a post processing step.

What is SAR?

SAR is a measure of the amount of power that is absorbed by a human body tissue in Watts/kg, averaged over a 1 gram or 10 gram mass of tissue. The FCC and other national government organizations are responsible for defining the safety limits for normal use.

How do you Measure SAR?

Because it is impractical (and inhumane) to insert a probe into someone’s head while they are using a cell phone, we rely on models to determine how much power the human body is absorbing. Since the absorption rate is highly dependent on the position of the cell phone antenna designers will run hundreds, thousands, or even tens of thousands of simulations to determine the SAR value for a given phone.

The other way to measure the SAR value is to create a physical model of the human head and use probes to measure the value. Ideally, simulations match the model results. Major discrepancies need to be resolved before the phone is sent to manufacturing.

Is Lower Better?

Sure. But if the value is too low, your phone will start dropping calls. Typically, antenna designers will work to minimize SAR while maximizing signal strength from the antenna.

As for the long term effects of cellular phone radiation, medical studies are being conducted, but the results are somewhat inconclusive. The good news is researchers and engineers are constantly working on improving antenna efficiency which reduces the power output and SAR value.

Will the Legislation Make a Difference?

I’m speculating this law will affect the decision of very few consumers. Most people make their decision based on feature sets and brand. Phones that are in the same family (e.g., smartphones) should have similar SAR values anyway. Consumers that want an iPhone will buy an iPhone. Speaking of the iPhone…

iPhone 4 and the Antenna Problem

Apple launched the iPhone 4 on June 24th, 2010 and sold 1.7 million phones in a span of 3 days. However, some users have been complaining of dropped calls and poor signal strength when holding the phone.

Does the Human Hand Affect the Antenna?

Absolutely! The human body absorbs the signal and reduces the effective output power of the phone. In the case of the iPhone, the antenna around the chassis could be completely covered by the hand causing the energy to be absorbed into the body. This is not a new problem and engineers have been working around this issue for many years.

How do you solve the problem?

Often times, a phone will have more than one antenna to support different usage models – holding the phone in the hand, placing the phone next to the head, using a Bluetooth wireless headset. Each use case can change the effective impedance of the antenna which in turn impacts the signal strength at the base station.

Modeling is the easiest and most cost effective way to resolve these issues. Check out our partner SPEAG’s website with their section on mobile phones.

You’ll notice they have an entire section devoted to human hand positions and other body models. In a standard design process, antenna engineers will run thousands of simulations to determine antenna performance under a variety of conditions. If the performance is not up to par, the designer will make a change and re-test. Of course, the simulation needs to match the actual performance of the phone. In many companies, matching the actual performance to the simulated performance is an important part of the process.

Our company plays an integral role in the simulation workflow. Since these models need to be run many times, our libraries reduce the time each simulation takes by a factor of 10x or more. This allows designers more time to modify and tweak their designs more frequently as their deadlines approach.

Is this a real problem with the iPhone 4?

As more information becomes available, we’ll have a better understanding of the issue (if any) with this phone. Without having the simulation results, it is impossible to tell what the performance should be.

The iPhone was simulated… wasn’t it? ;-)