A field/polarization diversity antenna for portable phones

The design of a two-element field/polarization diversity antenna for portable phones is considered. The antenna consists of a dipole terminated with a parallel loop capacitor combination. The antenna has a single feed at a point on the loop opposite the junction, and does not require external combining circuitry. The capacitor creates a phase shift between the dipole and loop currents, thereby greatly reducing the probability of deep nulls when the antenna is deployed in the vicinity of a fixed reflector, where standing wave patterns occur. Theoretical and simulation studies based on multiple incident plane wave fields are used to quantify antenna diversity gain in the presence of a fixed reflector. Simulation results are presented for three antenna types: a dipole antenna, a loop-dipole antenna without a capacitor, and the loop-dipole antenna with capacitor. The simulation results are verified by field measurements on an automated outdoor test range, where the incidence angle and distance from the reflector are varied. Both the simulation and field test results show that the loop-dipole-capacitor design provides a substantial diversity gain over the standard dipole antenna and over the loop-dipole antenna without the capacitor.