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.
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?
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.
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 or use the form to get further details.