On the design of the null probe by open-end circular wave-guide

According to the needs of the very-low sidelobes antenna measured in 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 circular waveguid. The null probe is a probe which has the ability to spacially filter the test antenna's spectrum by reducing sencitivity 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 tested rang antenna will be accurately measured using such probe. The null probe that designed have the charactors that the the pattern is more fit the theoretic requirement than the the two element array of rectangular waveguid. And the null-probe property is exhibited in two main plane of antenna. On the same time, the designed probe can be used as ordinary, probe as well as null probe. The measurement result shows that the null probe has the ability to improve the sensitivity of the sidelobes of antenna, and can be used in near field measurement system.