Test For The Complex Permittivity And The Attenuation Distribution Of The Lumped Attenuator Along Its Axial Line

Traveling wave tube is a microwave power device, which can amplify microwave signal through the exchange of energy between electromagnetic wave and the electron beam. Because of the advantages of high power gain and a wide band, it can be widely applied in many fields, such as varieties of radars, electronic warfare and satellite communications equipment. In order to make traveling wave tube have the function of amplification, it requires that the electron beam and the electromagnetic wave have the same speed, which requires the phase velocity of the electromagnetic wave to slow down, but slow-wave circuit cannot be self-sustaining, which need be clamped with the dielectric rod in the traveling wave tube, the dielectric rod also played a role inhibiting traveling wave tube self-oscillation action, The performance of traveling wave tube is determined by attenuation distribution of the attenuation coating and complex permittivity of the dielectric rod.It is built on the model insert the dielectric rod in the center of the rectangular waveguide width edge that tests the attenuation distribution of the dielectric rod. In waveguides, the electric field is along the axis of the dielectric rod, it is same when traveling wave tube is in work. Although the electric field strength is different in the waveguide and traveling wave tube, the attenuation distribution of the concentrated attenuator is same in different electric field. By analysis of microwave network before and after inserting the dielectric rod, we can deduce the formula that calculate the value of attenuation coating, and then obtain the rule of the attenuation distribution.In order to measure the attenuation distribution of the dielectric rod accurately, we design X-band, Ku-band and Ka-band test sensors to test the attenuation distribution of attenuation coating at different frequencies. The accuracy of the test is closely related to the reflection coefficient of the test sensor, the reflection coefficient is smaller, the results more accurate. The resolution of the dielectric rod’s attenuation distribution is determined by the narrow edge of waveguide. In this paper, we test the dielectric rod in the center of flat waveguide; we design three band test sensors, which are based on Klopfenstein taper model. When the dielectric rod with attenuation coating inserted into the hole, the field will leak out along the rod, so we need to set device for preventing this field leak out. In the simulation result, the reflection coefficient of the test sensors in three bands designed in this paper are lower than-40 dB, and field shield means is to achieve-100 dB insertion loss or more, these are very good to ensure the accuracy of the test.The test of complex permittivity of the dielectric rod is based on TM0n0 mode cylindrical cavity perturbation method. In the TM0n0 mode cylindrical cavity, the electric field is along the axial direction of the dielectric rod. In this paper, we propose optimization program for the control software of the automatic test system, and two additional conditions are judged in automatic search mode identification to solve the problem that may not be found correct resonance point during the test.