# A Compact and Broadband Magic Tee with Waveguide Filter for Ka-Band System

Authors: Youna Jang; Yuseong Choi; Jin-Woo Choi; Dal Ahn
Journal: International Journal of RF and Microwave Computer-Aided Engineering
 (2023)

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## Abstract

In this letter, we propose and fabricate a waveguide Magic Tee with waveguide filter for Ka-band system. The proposed Magic Tee has a reducedz-axis arm, which must typically be long for impedance matching at the input port. A bridge iris is inserted at port 4 for signal application. The z-axis arm at port 4 is significantly shortened by the impedance matching tuning through the iris that is normally uses to connect the adaptor. Thus, the dimension of the proposed Magic Tee is reduced to 7.12×10×3.6mm3, and the insertion loss shows good performance of 0.9 dB by applying the impedance matching tuning. The proposed Magic Tee also contains a 7th waveguide filter, which is used to verify the signal division performance. The results of excitation at each port of the Magic Tee show insertion loss less than 4.5 dB and return loss of over 18 dB.


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## Body

## 1. Introduction

The Ka-band is mainly considered in satellite communication systems for deployment of satellite high-speed broadband networks [1]. In such a Ka-band system, a Magic Tee is a major device that is used as a high-power signal divider. Thus, studies on the Magic Tee have been conducted for different planes (E-plane, H-plane, and coplanar plane) and waveguide-microstrip transitions.In previous works [2, 3], two metal rods or cone fins were inserted in the Magic Tee for impedance matching. However, these only showed limited bandwidth or a return loss (RL) of about 9.5 dB. Magic Tees with coplanar arms were proposed in [4, 5]; although [4] has the same arm plane, the microstrip section causes transmission losses with limited power-handling capability [4, 6]. In [5], which did not have a microstrip unit, although high power-handling capability was reported, the size was considered large owing to the ridge waveguide transitions for impedance matching. Broadband Magic Tees using the gap waveguide technology were suggested in [7, 8]. Although such Magic Tees have low device losses, they have problems with manufacturing complexity and cost.Consequently, in this study, we designed and fabricated a waveguide Magic Tee with reducedz-axis length, which is otherwise inevitably long for the purpose of signal control and impedance matching, as reported in [9–11]. The proposed Magic Tee is a combination of a Ka-band Magic Tee and a 28 GHz bandpass filter. The proposed device has a metal rod and bridge iris, which not only results in higher bandwidth and lower insertion loss (IL) but also reduces the volume compared to the conventional structures [9–11].In previous works, an arbitrary physical length of the inductor was inserted within the Magic Tee to induce interactions among the signals to realize the characteristics of a hybrid Magic Tee.In this study, a new Magic Tee is proposed by inserting two aluminum alloy structures, namely, bridge iris and cylinder-shaped structures, as shown in Figure1. The proposed device operates in the Ka-band and has a standard waveguide WR-28 (7.12×3.56mm) with a center frequency of 27.925 GHz.Figure 1
Schematic of the proposed Magic Tee (ρ=3mm, postheight h=3.8mm, and thicknessofbridge‐iris=1mm).


## 2. Design of the Magic Tee

The proposed Magic Tee has a cylinder-shaped platform for impedance matching of port 1; this structure consists of a platform and a post. Figures2(a)–2(c) show the results of impedance matching at ports 1, 2, and 3 when the postheight is varied from 2 to 4 mm. The proposed Magic Tee also has a bridge iris conductor; signals are induced by the bridge iris conductor attached to an adaptor, which is used to connect port 4 for signal application. The bridge iris does not affect the height of the Magic Tee. In particular, by tuning the E-field through the iris used to connect the adaptor, the z-axis arm can be reduced dramatically. Therefore, the bridge iris reduces the arm size of the proposed Magic Tee compared to those of conventional devices. Figure 2(d) shows the simulation results of impedance matching at port 4 when the thickness of the bridge shape is varied from 0.5 to 1.5 mm. Impedance matchings at ports 1, 2, 3, and 4 are observed to be in good agreement, as seen in Figures 2(a)–2(d).Figure 2
(a) Change inS11 with the postheight of the platform. (b) Change in S22 with the postheight of the platform. (c) Change in S33 with the postheight of the platform. (d) Change in S44 according to the thickness of the bridge iris.
(a)(b)(c)(d)Full-wave simulations of the proposed Magic Tee were conducted. The simulated field distributions for excitations at each of the ports are shown in Figures3(a) and 3(b). The simulation results of the proposed structure show that the proposed broadband Magic Tee is maintained well; excellent signal divisions are observed, as seen in Figures 4(a) and 4(b).Figure 3
Simulated field distributions: excitation signals at (a) port 1 and (b) port 4.
(a)(b)Figure 4
Simulated results of excitations at ports 1 and 4: (a) magnitudes and (b) phase differences.
(a)(b)


## 3. Fabrication of the Magic Tee and Bandpass Filter

The signal division performance of the proposed Magic Tee was confirmed by connecting a 7th-order waveguide filter to the output ports. In the 5G mm-wave band, it is necessary to consider signals and interference in the 50 GHz band, 36-37 GHz band, and 23.8 GHz band, which are used for water vapor and temperature analysis in meteorological satellites.In order to design a filter that reduces interference between these different bands, it is designed with a 7th-order filter. The 3D structure and the simulation results of the waveguide filter are shown in Figures 5(a) and 5(b). The waveguide filter is implemented in the frequency band of 27.525–28.85 GHz.Figure 5
(a) 3D structure of the 7th waveguide filter. (b) Simulated results of 7th waveguide filter.
(a)(b)Figures6(a) and 6(b) show photographs of the Magic Tee fabricated with the waveguide filter having dimensions of 99.8×97.6mm2. The height of the Magic Tee was only 3.56 mm, which was achieved by replacing the z-axis arm with the bridge iris. Thus, the total volume was reduced compared to those of conventional structures. Figure 7(a) shows the measured results of the proposed Magic Tee. The return loss is over 12 dB in 25-30 GHz due to the fabrication error. The amplitude balances of the proposed Magic Tee have ±0.15 dB in 25-30 GHz, as shown in Figure 7(b). Figure 7(c) shows the measured results of output phase differences of the proposed Magic Tee according to the port 1 input (sum port) and port 4 input (delta port). The phase imbalances have ±2° in case of both inputs of port 1 and port 4.Figure 6
(a) Front and (b) back sides of photograph of the fabricated Magic Tee with the waveguide filter.
(a)(b)Figure 7
(a) Measured results of proposed Magic Tee. (b) Amplitude imbalance of Magic Tee. (c) Phase differences of Magic Tee.
(a)(b)(c)The amplitude and phase imbalances of the proposed Magic Tee show good results using the reducedz-axis length as the results are less than 0.3 dB and 3.8°, respectively.Figures8(a)–8(d) show the measured results of the Magic Tee filter when different ports are excited. Excitation results at all ports show ILs under 4.5 dB and RLs over 18 dB.Figure 8
Excitation results according to (a) ports 1 and 3, (b) ports 1 and 4, (c) ports 2 and 3, and (d) ports 2 and 4.
(a)(b)(c)(d)To demonstrate the proposed Magic Tee, the Magic Tee and waveguide filter have been designed at the target bandwidth 27.525–28.85 GHz by following the procedure below.(1)
Design and draw the proposed Magic Tee for the Ka-band(2)
The miniaturization and broadband of Magic Tee is simulated and applied(3)
Draw and simulate a 7th-order waveguide filter at a target bandwidth of 27.525–28.85 GHz(4)
Fabricate and assemble the proposed Magic Tee and waveguide filter(5)
Adjust the filter and measure the proposed circuit(6)
Check the specification for desirable resultsTable1 shows the results of performance comparison of the proposed Magic Tee with previously published works [5, 6, 9]. As shown in Table 1, [6] has better bandwidth performance than the proposed Magic Tee, since our target is for a Magic Tee filter with a Ka-band system in target bandwidth of 27.525-28.85 GHz. However, [6] uses microstrip dual-probe transition, which limits the power-handling capability. Thus, it is not affordable for the Ka-band system even if it operates over a wide Ka-band. The proposed work with the bridge iris is easier to fabricate than the other ridge waveguide transitions [5]. Moreover, the tuning point of the iris reduces the z-axis arm significantly compared to the other metallic matching elements used in [9]. Therefore, the proposed design has a broader bandwidth, a lower size, and smaller IL characteristics.Table 1
Comparison results of the proposed Magic Tee for Ka-band.
Ref.[5][6][9]ProposedFrequency (GHz)28-3626-4024-27.525-30Bandwidth25%42%16%17.8%RL (dB)15152012IL (dB)10.70.20.9Dimensions (mm3)34×25×1226×17×1940×40×407.12×10×3.6


## 4. Conclusion

This work presents a novel broadband Magic Tee structure with reduced length of thez-axis arm. In the proposed structure, two aluminum alloy structures are inserted; the bridge iris element controls signal flow and impedance matching. The z-axis arm of the Magic Tee is markedly reduced. The proposed circuit was verified to have 0.9 dB IL and 25–30 GHz bandwidth. Moreover, the proposed circuit was connected to a waveguide filter at 27.525–28.85 GHz, and the measurement results confirmed proper signal division through the Magic Tee with ILs less than 4.5 dB for ports 1 and 4. Therefore, the proposed circuits achieved excellent performance for the Ka-band system using two matching elements.



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*Source: 6248913-2023-02-07.xml*