0 Replies Latest reply: Sep 29, 2011 11:52 PM by Harald Naumann RSS

Why SAR tests are expensive?

Harald Naumann Master

SAR tests are expensive, because there is an expensive robot arm plus expensive software on a PC in use

Nevertheless, there is some manual work to do as well. The pictures as below will make the described process visible.


specific absorption rate test 150x150 Why SAR tests are expensive?

SAR test with robot moving a probe


The robot has on the end of its arm a probe to measure the radiated  energy. This probe is measuring the absorbed RF energy in a bath tub.  This bath tub has the shape of a human body. The liquid has nearly the  same electrical characteristic like the human body. The liquid has to be  changed during tests with different frequencies, because the human body  has non linear electrical characteristic. The liquid contains sugar and  some kind of other materials. A liquid with sugar cannot be used  endlessly, because the liquid will change the characteristic over the  time. We all know that already – just take grape juice and wait a while.  You get wine, vinegar or something that you cannot use for cooking.  Anyhow, let us go back to SAR tests. The Device under Test (DUT) will be  mounted under the bath tub with liquid. In first step the probe will  move in 2D on the surface mount of the liquid to find the hot spot.  After founding the hot spot, the probe will start to measure in 3D and  dive deeper in the liquid. Remember, that the human body has a non  linear electrical characteristic. This test has to be repeated on  different frequency bands again.

How to save money on SAR tests?

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SAR test 150x150 Why SAR tests are expensive?Probe moving in 3D inside a virtual human heat

The easiest option to save money is to ask for consulting for the SAR  tests and the used antennas in beginning of your design. By listening  to a consultant and antenna specialist before starting the development  of the mobile device a lot of money can be saved.

If your device will contain two RF generators like GSM and WIFI and  the two antennas have a distance of more than 20 cm, then both antenna  systems will be observed separately. If they are closer together, then  the radiated energy will be observed together. Powering on one RF  generator in same time only, then this can save trouble as well.

Selecting the right antenna and enclosure in beginning of the design  can save trouble as well. If the selected antenna radiates directional  instead of omni directional it will help to minimize the absorbed energy  by the human body. Another possibility is to install the antenna in the  near of the microphone. The microphone is far away from ear and head.  Interference of the antenna with the microphone can be limited or  eliminated by using microphones with digital audio interface. Other  option is to use audio codec’s with digital interface and to convert the  analogue audio into digital audio and to wire to the DVI (Digital Voice  Interface) of the GSM module.


Bath tub
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SAR test mobile phone 150x150 Why SAR tests are expensive?Mobile device mounted under the "bath tub"

If you have interest in SAR tests, antennas and DVI, then just drop an email to harald.naumann (at) gsm-modem.de to get further details.