Phantoms for Radiation Measurements of Mobile Phones

Even though mobile handsets have to comply with, e.g., the GSM standard the radio link performance varies significantly among the handsets on the market. The reason is that the testing for standard compliance does not include neither the antenna nor the user of the handset. A group under COST 273 (Subgroup 2.2) is working on a proposal for a test method based on detailed investigations. To understand the variation in the radio link performance the problem can be divided into the following three parts: * Efficiency of handsets * Efficiency of the handset including the users body * Influence of the propagation environment. The efficiency of the handsets themselves describes the ability to transmit or receive the power which is delivered to the antenna from either the power amplifier or the environment. The efficiency of the handset including the users body has previously been investigated at CPK in a measurement campaign involving 200 test users of a handset equipped with three types of antennas. The so-called body loss is defined as the difference in received or transmitted power when the user is present as compared to the free space case. From the measurements it was found that the body loss varies not only fromone type of antenna to another (differences of 3-6 dBin the mean) but is also highly dependent on the user. Differences in the body loss up to 10 dB was found among the users, and hence the user influence is very important for the radio link performance. Four possible contributors to the body loss have been identified [Pedersen2001]: * Absorption of power in the user * Antenna mis-tuning due to user * Change of radiation pattern * Change of polarization state. With the purpose of investigating which of the above are important for the observed bodyloss a measurement campaign was carried out where the spherical distribution of the transmitted power was measured in the anechoic chamber for 43 test persons using the same handset. From these measurements it was concluded that the absorption in the user is dominating the body loss. The radiation efficiency vary as a function of the user, with an average highly dependent on the antenna type. The absorption was investigated further using measurements in the anechoic chamber on a phantom of the human head and hand. The following three items were investigated as reasons to the large variation in absorption found for different users: * The tilt angle of the handset was found to be of minor importance, with variations of only plus/minus 0.6 dB * Similarly, the distance from the handset to the user's head only has a small influence on the total absorption * In these investigations the position of hand on the handset turned out to be the single most important issue. If the phantom hand was located near the bottom of the handset the loss is nearly the same as the case of no hand, but as the hand was moved closer to the top of the handset, and hence the antenna, the loss increased 5-7 dB, depending on the antenna type. Therefore, it was concluded that the large variation in body loss observed with the real test users is mainly due to the different ways of holding the handset. The correlation of the position of the user's hand on the handset and the power absorption has been verified from video recordings of the real test users measured in the anechoic chamber. (Gert Frølund Pedersen)