Off-axis gain and field in the finite range of an open-ended rectangular waveguide

Expressions are presented for the approximate evaluation of the off-axis gain and field in the finite-range of an open-ended rectangular waveguide (OEG). These formulas in turn present a method for predicting the off-axis error when an OEG is used as the source antenna. For an OEG operating at 250 MHz, the off-axis error estimated by using this method favorably compares with that experimentally estimated for the 1 m off-axis position.