In this book, an important principle in mobile phone antennas is described. An equivalent circuit representation of the antenna element, the coupled PCB "chassis" and the coupling between them were analyzed. The investigation of the coupling was by 2 port measurements of the two radiators in the antenna-chassis combination. The chassis was tested as a dipole antenna by cutting two arms from the middle at different angles. Several types of balun transformers were used to measure the (chassis-) dipole experimentally. The antenna element was mounted on the (chassis-)dipole to investigate the coupling. The coupling was measured for orientation, and different positions of the antenna element. The relation between the coupling of antenna and (chassis-) dipole and the bandwidth of the combined antenna-chassis system has been recognized. The accuracy of the measurement was also tested by comparison with simulations using an electromagnetic simulation tool. The modeling of all the structure is performed by using the measured data and the model is improved by tuning the center frequency of the chassis.