Generalised correction to the friis formula: quick determination of the coupling in the fresnel region

A simple yet effective correction term to the Friis formula is presented to improve its accuracy in the Fresnel region. Based on the concept of gain reduction factor, the proposed asymptotic term with an empirical coefficient can be widely applied to a variety of antennas without lengthy calculations. The information required in the computation is only the antenna gain, separation and wavelength for the boresight scenarios. Case studies of several representative antennas using full-wave simulations show that the accuracy of the estimated power transmission levels between two antennas in the Fresnel region can be improved by up to 2-5 dB, for both boresight and off-axis cases. Measurement results using two horn antennas also confirm the validity of the proposed correction term.