Design of coaxial rectangular waveguide converter with SMA connector at high frequency

　　In microwave systems, a common component is often used, namely, a transition element that transforms one transmission line to another, called a waveform converter, also known as a waveform exciter. The requirements for a waveform converter are: (1) it can excite the required waveform; (2) the standing wave coefficient is as small as possible.

　　In order to achieve good impedance matching in a wide band, the currently widely used wide-band coaxial-rectangular waveguide converter mainly has two forms, the probe type and the ridge waveguide transition type. The probe type is to connect a metal disk or ball to the top of the conductor in the coaxial line inserted into the waveguide cavity, and set a number of tuning screws on the waveguide cavity. The ridge waveguide transition type is to add a ridge piece in the waveguide to form a step impedance converter, so that the output impedance of the ridge waveguide is close to the characteristic impedance of the coaxial line, so as to achieve the purpose of impedance matching.

　　In order to reduce costs, the SMA coaxial connector is generally a standard product, and its dielectric, inner and outer diameters are fixed. This structure brings two problems: (1) SMA connectors can only work in single mode below a certain frequency (18GHz). At higher frequencies, the high-order modes in the SMA connector will seriously affect the working bandwidth of the converter. If other standard connectors with higher working frequencies are used, such as K connectors, their prices are much higher than SMA connectors, which will greatly increase the cost; (2) The converter design parameters are relatively few, and it is not easy to match.

　　2 Probe type coaxial-rectangular waveguide converter

　　Compared with the ridge waveguide transition converter, the probe type converter has the advantages of wide bandwidth and easy processing, so this article only discusses this type of converter. The probe type coaxial-waveguide converter is to make the inner conductor of the coaxial line into a probe and insert it into the waveguide cavity from the wide side of the waveguide. A disk or a small ball is added to the top of the probe. One port of the waveguide is short-circuited and the other port is output. Several tuning screws are added in the waveguide cavity.

　　A better match of the coaxial-to-rectangular waveguide converter within the operating frequency band is achieved by adjusting the following three dimensions: (1) the distance i from the probe to the short-circuit end; (2) the length f of the probe; (3) the thickness h and diameter g of the disk on the top of the probe; and (4) the position of the tuning screw.

　　Figure 1 Probe-type coaxial-rectangular waveguide converter

　　As shown in the figure above, the outer diameter of the standard SMA connector is

mm, inner diameter

mm, and the relative dielectric constant of the medium

. Its cut-off frequency is around 18GHz.

　　This paper designs a probe-type adapter from a standard waveguide port of waveguide model BJ220 to a coaxial line with inner and outer diameters of 1.3mm and 4.1mm. The operating frequency of the standard waveguide BJ220 is 17.6-26.7GHz, which exceeds the operating frequency range of the SMA connector. Through software simulation, the optimal result is shown in Figure 2.

　　Figure 2 SMA-BJ220 converter simulation curve

　　As can be seen from Figure 2, since the operating frequency band of the adapter exceeds the operating frequency of the SMA connector, the high-order modes generated inside the coaxial connector  produce resonance peaks at 25.9 GHz and 28.1 GHz, causing the reflection coefficient of the adapter to increase sharply.

　　3 Improved coaxial-waveguide converter

　　In order to overcome the above problems, this paper proposes some improvements based on the structure of the previous probe type converter. The cutoff frequency of the coaxial line is

When the outer diameter b and the relative dielectric constant of the medium are reduced  , the cutoff frequency will be significantly increased. Therefore, a transition stage is added between the SMA connector and the waveguide cavity, that is, a circular hole is opened on the upper wall of the cavity where the SMA connector is inserted. The diameter of the hole is smaller than the outer diameter of the SMA  , and the inside is filled with air.

　　Figure 3 Improved coaxial-rectangular waveguide converter

　　Table 1 shows the main structural parameters of the optimized coaxial-rectangular waveguide converter.

Table 1 Structural parameters of the converter

　　The simulation curve is shown in the figure

　　Figure 4 Simulation curve of the improved SMA-BJ220 converter

　　It can be seen from the simulation curve in Figure 4 that within the operating frequency band of the standard rectangular waveguide BJ220, 17.6-26.7 GHz, the reflection coefficient of the adapter is below -27 dB, that is, the standing wave coefficient is less than 1.05. And due to the suppression effect of the transition circular hole, the resonance peak generated by the high-order mode is also increased to 35.6 GHz, moving out of the operating frequency band of the adapter. Therefore, through this improvement, the SMA connector is recognized for use in applications above 18 GHz.

　　As shown in Figure 5, the actual performance indicators of the improved SMA-BJ220 converter are: the working frequency band of the converter reflection coefficient below -15dB is extended to 17.6-31.6GHz; within the frequency band range of 17.6-26.7GHz of the waveguide BJ220 single-mode transmission, its reflection coefficient is below -16dB; through the suppression effect of the transition hole, the resonance peak is increased to 32.3GHz. There is a certain difference between the reflection coefficient in the passband, the simulation curve and the actual measurement curve. The main reason is that the volume of the converter is only 24.3*22.4*22.4  , the relative error during processing is large; and in the simulation process, the microwave reflection of the SMA connector itself during connection is not considered.

　　Figure 6 Measured curve of the improved SMA-BJ220 converter

　　4 Conclusion

　　This article describes some of our research on the conversion of coaxial to rectangular waveguide. Coaxial-to-rectangular waveguide converters are now widely used in various microwave systems, and the annual production and demand are very large. The technology described in this article can replace other high-performance and expensive connectors with low-cost SMA connectors, thus effectively reducing production costs. At present, we are further exploring this new technology and the problems it faces in large-scale production. The technologies described in this article have applied for patent protection.