Design of a Q-band waveguide to CPW transition

A design scheme of waveguide to co-planar waveguide probe transition structure is proposed in this paper, and the structure is modeled and optimized by Ansoft’s electromagnetic simulation software HFSS. The simulation results indicate that the structure’s reflection coefficient is below -20dB and its insertion loss is below -0.3dB at 38∼47GHz. The structure is applicable to tile-type T/R modules.

Because of the signal's totally reflected on the short pavement of the waveguide, the electromagnetic field is strongest at the place odd times  4 away from the pavement, where the probe can get the highest coupling efficiency.Just assuming that the electromagnetic wave spreads on the Z-axis, at the same time, both X and Y axes correspond to the directions of the waveguide's two sides a and b respectively, and the field expression of TE10 is In formula ( 1), E 0 is the amplitude of electric field, a is the long side of rectangular waveguide,  is the circular frequency, β is the propagation constant, and μ is the magnetic permeability.
From formula (1), we can draw the conclusion that the electric field is largest at the middle of the waveguide width, and the strongest electromagnetic wave can be coupled by placing the probe here.To sum up, the coupling is strongest when the transmission probe is inserted in the center of the rectangular waveguide's broad side.When at 45GHz, the quarter wavelength is about 1.6 mm.In this case, the transmission line needs to be placed about 1.6mm away from the short road surface of the waveguide, which will occupy a large wiring area, so the structure is not easy to integrate the lines with high density.
In this paper, the medium in the multilayer board is used to replace the air part between the probe and the short road surface in the traditional transition structure waveguide cavity.As shown in figure 2. In this transition structure, the shielding through hole under the waveguide can be equivalent to the extension of the metal waveguide wall to prevent energy leakage; The bottom metal ground is used as a short pavement to replace the air short pavement in the traditional probe coupling structure, and the coplanar waveguide transmission line with better isolation performance is used in the plane transmission line part, which enables the structure to complete high-density integration in EHF frequency band.

3.Proposed transition from Waveguide to CPW
The dielectric substrate of this design is TSM-DS3 material with dielectric coefficient of 2.94, transmission line is about 0.045 millimeters thick, and the loss tangent δ=0.0014.The waveguide is BJ500 standard (4.78mm×2.39mm).The prepreg is FR-28-0040-50, and its dielectric constant is 2.81.
In this structure, the input impedance of the probe is related to its length、width and the distance to the short pavement, and this impedance usually appears capacitive.In order to counteract the capacitance effect caused by the probe, a high-impedance transmission line is connected in series behind the probe: similar to the traditional probe transition structure, the probe obtains the maximum coupling efficiency at a quarter wavelength away from the short road surface, so H2≈1mm is selected, and H1=0.127mm is selected as the partial substrate thickness of the transmission line; Using LineCalc tool in ADS software, the line width of 50Ω CPW at 44.5GHz is calculated to be Ws=0.3mm,so that the gap width of the coplanar waveguide is 0.1 mm.The multilayer board in Figure 2 consists of six copper layers (L01~L06) of metal and five layers of dielectric.L01 is the plane where the coplanar waveguide transmission line is located, square holes need to be opened from L02~L05 corresponding to BJ500 standard rectangular waveguide (4.78mm×2.39mm),as shown in Figure 3, while the sixth layer is a short signal road surface, so no other treatment is needed.H1=0.127mm is selected as the thickness of transmission line substrate; Using LineCalc tool in ADS software, at 45 GHz, the width of 50 Ω CPW transmission line is calculated to be W s = 0.3mm,Gap =0.1mm.
After determining some basic parameter values, as illustrated in Figure 2 and Figure 3, the remaining parameters should be optimized are: Length of probe inserted into waveguide L in , width W h and height L h of waveguide aperture, length L s2 and width W s2 of coupling patch, linewidth W s1 and length L s1 of high impedance microstrip line, width W h1 of inner metal layer aperture and length L h1 .
After these dimensions are optimized by simulation, the parameter values obtained are shown in  4 illustrates that the transmission performance is satisfactory in a frequency range of 38~47GHz, the insertion loss is below 0.16dB , and the reflection coefficient can be controlled below -20dB.

Figure 4 S parameter simulation results
Figure 5 Tolerance simulation of parameter Lin Through tolerance analysis of various parameters, it is found that L in has the greatest influence on the simulation results among all parameters.As can be seen from Figure 5, as Lin changes by only 0.05mm, it still a great influence on the simulation performance, so it is indispensable to put forward strict accuracy requirements for this part of the machining error.
For the purpose of verifying the feasibility of structure, the symmetrical structure is modeled in the research, and it is illustrated in Figure 6. Figure 7 Simulation results of symmetrical structure The simulation result of the symmetrical structure is shown in Figure 7, and the return loss in 38~47GHz is less than -18dB.Considering the loss of transmission line and waveguide, the results are essentially similar to the simulation outcomes shown in Figure 4, demonstrating the practicality of this transition structure in Q-band.

4.Conclusion
In this paper, a Q-band waveguide-coplanar waveguide probe transition structure is designed.The simulation results indicate that the structure's reflection coefficient is below -20dB and its insertion loss is below -0.3dB at 38~47GHz.The probe transition features a wide working bandwidth, small insertion loss, simple structure and easy assembly.Due to the use of coplanar waveguide structure, its excellent isolation performance can be applied to tile-type T/R module.

Figure 1 H
Figure 1 H-plane probe(a) and E-plane probe(b)

Figure 2
Figure 2 structural side view Figure 3 Structural Disassembly and Key Dimension Marking

Figure 6
Figure 6 Symmetrical transition structureFigure7Simulation results of symmetrical structure The simulation result of the symmetrical structure is shown in Figure7, and the return loss in 38~47GHz is less than -18dB.Considering the loss of transmission line and waveguide, the results are essentially similar to the simulation outcomes shown in Figure4, demonstrating the practicality of this transition structure in Q-band.