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Mobile terminals include classic mobile phones and smartphones in particular. Routers and tablet computers also belong to this group of devices if they are able to establish and use mobile radio connections. The devices generate high-frequency electromagnetic fields in order to communicate with a mobile phone base station. In almost all devices used today, the transmitter and receiver antennas are incorporated into the devices and not visible from the outside.
For the protection of the consumer, the devices have to be designed in a way that internationally recommended limits for the absorption of high-frequency electromagnetic fields by the body are adhered to during operation. This is aimed at excluding health effects. The high-frequency energy absorbed by the body per time and tissue mass, the so-called specific absorption rate (SAR, in watts per kilogram) is used to evaluate the effects.
Mobile phones operated in GSM mode at 900 MHz achieve transmitter powers of up to 2 watts per pulse (peak transmitter power). In GSM mode at 1800 MHz the maximum pulse power is 1 watt. Owing to the time slot technique, the time-averaged transmitter power is lower: In GSM mode at 900 MHz it amounts to a maximum of 0.25 watt and in GSM mode at 1800 MHz to 0.125 watt. In modern UMTS and LTE networks as well, the time-averaged transmitter powers of the terminals are a maximum of between 0.12 and 0.2 watt.
Limits for the specific absorption rate (SAR)
Before the market launch of every mobile phone or smartphone model, a maximum SAR value the device can produce during operation is determined by the producers or importers under standardised laboratory conditions. A distinction is made between the two application cases of "operating next to a user's ear" and "body-worn operation" (see Specific absorption rates (SAR) of mobile phones). Mobile-enabled tablets are generally only tested for the second application case. The limits set for the specific absorption rate have to be adhered to also in the case of maximum transmitter power in both application cases. Under typical everyday conditions, modern mobile devices often operate at lower present transmitter powers owing to an automatic transmitter power control. When the transmitter power drops, the current value of the specific absorption rate decreases as well.
For the metrological assessment if a device complies with the recommended maximum SAR value in the application case "body-worn operation", present-day European product standards allow a distance of up to 5 millimeters between the mobile device and the body model used for testing. Many manufacturers indicate the distance used for measurement together with the maximum SAR value in the accompanying documents of the device or refer to a site where this information is available.
Exposure during phone calls or while surfing the internet
With the "classic" method of making phone calls, the device and the antennas are very close to the user's head (near field). In the process, energy is absorbed unevenly at the head surface and within the head. The figure illustrates a typical distribution of the specific absorption rate (SAR) at the head surface during a mobile phone call when the transmitter antenna of the mobile phone used (sectional drawing) is incorporated into the upper part of the casing. The SAR values averaged over a volume of 10 grams of tissue should not exceed the limit of 2 watts per kilogram.
If a headset is used for making calls, the device might be worn in a pocket of a user's clothing close to the torso. Also when the smartphone or tablet is employed to transmit data (for example while surfing the internet), the antennas can be near to the body.
Extent of energy absorption by the body
The extent to which high-frequency energy is absorbed by the body when using a mobile terminal depends, among other things,
- on the design of the device,
- on the type of antenna, its distance and position to the body,
- on the transmission frequency,
- on the way it is used as well as
- on the actual transmitter output power of the device.
Compared to the field strengths generated by mobile phone base stations, mobile phone users are exposed to significantly higher field strengths caused by their terminals when these are operated close to the body. This affects mainly areas of the head close to the antenna of the mobile phone when a phone call is made while holding the device to the ear. While surfing the internet, other areas of the body are more or less close to the antennas of the mobile device.
The exposure from mobile terminals is limited to the duration of use (this also includes automatic updates of apps or the automatic delivery of messages). In contrast to that, the exposure from a base station is not limited in time and is distributed more evenly over the body.
State of 2018.03.29