Based on the analysis of the structure design of SMA coaxial to microstrip connector which is popular in China at present, the reasons of Hangdians poor EMI capability and the reason of disassembling and replacing the reactor are found out, and the improvement measures are put forward to meet the application requirements of hangdiane jamming.

       SMA type RF connector is a small coaxial connector with characteristic impedance of 50 ohm, screw connection and inner diameter of outer conductor of 4.13 mm. SMA series (for stripline or microstrip transmission line) RF coaxial connector. It refers to the RF connector with standard SMA interface and the other end connected with stripline or microstrip transmission line. Because of its wide operating frequency band (generally 0-18ghz, the improved type can reach 26ghz), small size and light weight, it is widely used in radio equipment and electronic instruments, and is usually connected with the microstrip transmission line in the shielding box, which is the main component of the input and output of amplifiers, filters and wave detectors. Its interface structure and size are specified in military standards such as Mil-C-39012 and gjb681. With the development of science and technology and the emergence of miniaturized and high-density electronic equipment and circuits, people pay more and more attention to the anti radio frequency leakage and anti electromagnetic interference of this kind of connector, which is also a problem that must be paid attention to when using a large number of digital circuit electronic equipment in the future.

       At present, the structure of SMA and its coaxial to microstrip connector are mostly designed according to the early structure. Taking the SMA type coaxial to microstrip connector with extended medium as an example, in the early structure, the way to fix the inner conductor and insulating medium was through drilling holes on the metal shell and insulating support, and pouring epoxy resin, i.e. using epoxy pin; the connection with the microstrip in the shielding box was to lengthen the insulating medium and inner conductor together, and weld the inner conductor of the connector and the microstrip line Connecting together, i.e. lengthening medium type; of course, the lengthening part shall meet the impedance requirements as well as the optimal body size requirements for connecting with the microstrip line; the flange connector of the connector and the paper collection of the fifth academic conference of the switch (two or four holes) and the shielding box shall be directly fixed with screws, and no other measures shall be taken.
        When there is no requirement to resist electromagnetic interference and RF leakage, the above structure can be used. When the ability of anti electromagnetic interference is required, the structure can not be adapted. At present, the structure has the following defects:
       (1) Large RF leakage. Due to the adoption of epoxy pin technology, holes are punched in the metal shell and insulation support, and epoxy resin is injected, then the high-frequency signal or its harmonic signal is easy to leak through the through-hole, or the external signal is closed through the through-hole pot, resulting in electromagnetic interference. Moreover, the main component of the epoxy pin is epoxy resin, its dielectric constant is about 4, the insulation support is usually polytetrafluoroethylene, its dielectric constant is about 2, so the characteristic impedance of the cross section of the epoxy pin deviates from the nominal impedance due to the large deviation of the two dielectric constants, which affects the voltage standing wave ratio performance and reduces its transmission performance.
       (2) poor sealing and shielding performance. Using the extended medium type, the extended insulation medium is connected with the microstrip line through the shielding box. Due to the influence of the ambient temperature, the sealing performance between the medium and the shielding shell is easy to reduce; the connector flange and the shielding shell are fixed by screws, and the contact surface is difficult to avoid the gap, which affects the overall shielding performance.
       (3) it is difficult to dismantle and replace, and the performance is unstable. The inner conductor is welded with the microstrip circuit. Once the connector fails, the whole connector must be removed and the inner conductor and the microstrip circuit must be welded. In this way, the insulation medium and microstrip circuit are easy to deform due to the welding heat, which affects the electrical performance. After removal and replacement, the electrical performance must be re debugged, so it is difficult to maintain all kinds of performance of the original shielding box, or even cause the whole shielding box microstrip circuit to be scrapped.

       In a word, the SMA type coaxial to microstrip connector, which is popular in China, is difficult to meet the requirements of anti electromagnetic interference due to the defects of electromagnetic leakage, poor anti electromagnetic interference ability and unstable performance during the disassembly and replacement, so the structure needs to be improved.
       In order to meet the needs of anti-electromagnetic interference, in recent years, some foreign RF connector manufacturers, aiming at the above defects in the structural design of the early SMA type coaxial to microstrip connector, have gradually made improvements, including the following aspects:
       (1) Improvement of fixed mode. The epoxy pin type is no longer used, but the internal conductor is provided with a barbed structure. In order to reduce the adverse effect of barb structure on VSWR performance, compensation measures should be taken in shell structure design. During the structural design, close cooperation shall be taken to prevent falling off. This fixation method is applicable to the whole SMA series.
       (2) Improvement of the overall structure. In order to overcome the difficulty of replacing the structure of glass insulator and to change the structure of the original extended medium due to the influence of connector replacement on the performance of the whole component. From the original one-piece type to the two-piece type. The inner conductor of the connector is connected with the shielding box end and changed into a jack, and then a low reflection glass insulator is specially designed to replace the original extended medium part. The shell of the glass insulator is welded with the shielding box, one end of the inner conductor is inserted into the inner conductor of the connector, and the other end is connected with the microstrip circuit by welding or other ways.

        The kovar alloy whose linear expansion coefficient is close to that of glass is usually selected as the material of outer shell and inner conductor of low reflection glass insulator, and 7070 glass powder with stable high frequency performance is selected as the glass. The structure of outer shell and inner conductor meets the requirements of characteristic impedance. Glass insulator is made by high temperature sintering. At present, there are specifications of glass insulator at home and abroad (outer diameter division of inner conductor).
       (3) shielding measures. In order to prevent leakage due to contact clearance between the flange plate of the connector and the shell of the shielding box, a sealing ring made of conductive rubber is usually added between them.

       Based on the original SMA coaxial to microstrip connector structure, the RF leakage level is greatly reduced by improving the structure and increasing the shielding measures. It meets the requirements of anti electromagnetic interference. According to foreign reports, after the test, the epoxy pin is used for fixing before the improvement, and the RF leakage is about - 75-85db; after the improvement, the RF leakage is - 154db at 6GHz, - 143db at 8GHz, - 138db at 12gh, - 132dB at 15gh, - 128dB at 18gh. This leakage level is beyond the test range of most test equipment.