Comparison of theoretical and experimental data for the near field of an open-ended rectangular waveguide

A comparison of theoretical and experimental data on the radiating near field of an open-ended waveguide (OEG) is presented. Two theoretical methods are examined. The first is an approximation based on simple plane-wave equations with the electric field expressed in terms of the gain of the OEG. The gain equation is an empirical equation obtained from scaled measured data. The second approach is based on far-field-to-near-field transformations. Its purpose is to provide an alternate method for computing the fields as well as a means of assessing the accuracy of the first approach. Theoretical data computed using both methods are presented along with measured data obtained in the anechoic chamber. The discrepancy between the two theoretical approaches is typically less than 0.5 dB and increases with the distance between the OEG and the field point. For the measured data, ripples attributed to internal reflections in the chamber are observed. Also observed was a trend whereby the measured data fell consistently below the expected value for r⩾3λ. Although the cause of this trend is difficult to isolate without additional experimental work, it is believed to be due to the measurement setup and not by the near-field effect of the OEG