hand effect handset antenna

The main idea is to keep the same impedance of the antenna for free space test, and for human presence test. Especially the imaginary part because usually the real part remain constant.
Changing the antenna impedance in the presence of human hand is related mostly to antenna type. Different types of antennas can make huge difference for this kind of test.
PIFA that is widely used in mobiles have a nice behavior, if is well designed. It has the advantage that the currents can be better distributed.
Also a proper antenna match can help to pass this issue. Sometimes when is no way out, is made a compromise in tuning, somewhere between free space and human presence, because the mobile shall pass both tests.