On the design of the null probe by open-end circular waveguide

According to the needs of the very-low sidelobes antenna measured in a near field measurement system, a new type of null probe is designed by a open-end circular waveguide, using the property of high modes in a circular waveguide. The null probe is a probe which has the ability to spacially filter the test antenna's spectrum by reducing the sensitivity of the main beam but making little change at the sidelobes. Thus it can improve the dynamic range of the tested antenna, and the low sidelobes of the tested antenna will be accurately measured using such a probe. The null probe that has been designed has the characteristic that the the pattern fits more adequately the theoretic requirement than the the two element array of a rectangular waveguide. Also the null-probe property is exhibited in the two main planes of the antenna. At the same time, the designed probe can be used as an ordinary probe as well as a null probe. The measurement result shows that the null probe has the ability to improve the sensitivity of the sidelobes of an antenna, and. can be used in a near field measurement system.