Four-way radial waveguide power divider with high power-handling capacity

The Journal of China Universities of Posts and Telecommunications October 2011, 18(5): 124–128 www.sciencedirect.com/science/journal/10058885

http://jcupt.xsw.bupt.cn

Four-way radial waveguide power divider with high power-handling capacity ZHANG Jian-qiong ( ), LIU Qing-xiang, LI Xiang-qiang, ZHAO Liu School of Physical Science and Technology, Southwest Jiaotong University, Chengdu 610031, China

Abstract

In this paper, a novel four-way radial waveguide power divider with low insertion loss and high power-handling capacity is proposed. This power divider consists of an input coaxial waveguide, a central probe, a radial waveguide, four equispaced identical coupling probes, four equispaced identical adjusting posts and four output coaxial waveguides. The novel coupling probes and the adjusting posts are used to realize favorable uniform power division. A power divider with the center frequency of 4.1 GHz is designed, fabricated and measured. Good agreement between simulation and experiment is noted. The measured 15 dB return-loss bandwidth of the power divider is about 600 MHz. The measured 0.5 dB insertion loss bandwidth is wider than 700 MHz. The differences and isolations between the output ports are also discussed. The power-handling capacity of the power divider is analyzed through simulation, and the result proves its usability in high power applications. Keywords power divider, high power, low loss, multiport, radial waveguide

1

Introduction 

In design of some high-power microwave system and high-power radar transmitter system, power dividers are required to realize desired high power division. In this particular domain, the power dividers should satisfy several critical requirements, which include high power-handling capacity (exceeding hundreds of MW), low insertion loss and usually multiport outputs [1–2]. Conventional microstrip line and stripline power divider [3–4] can not be used due to their poor power-handling capacity. A suitable type is waveguide-type power divider. T-junction is the mostly used in waveguide-type power divider [1,5], however, when multiport outputs are needed, it is necessary to make a cascade, and this increases its volume and destroys its advantage of planar structure [1,5]. An effective way to achieve multiport outputs is using radial waveguide as feeding waveguide and coupling energy out in some certain way, as having been reported in Refs. [6–7]. However, the use of E-coupled probes and absorbing materials in the power divider reduces its Received date: 01-03-2011 Corresponding author: ZHANG Jian-qiong, E-mail: [email protected] DOI: 10.1016/S1005-8885(10)60114-8

power-handling capacity and increases its insertion loss, also the structure of multi-ring outputs affects the balance of outputs [7]. To overcome these drawbacks, a novel radial waveguide power divider is proposed in this letter. This power divider uses new probes and adjusting posts to realize favorable uniform power division. It has the advantages of high power-handling capacity, low insertion loss, compact structure and the capacity of realizing multiport outputs.

2

Basic structure and design

Fig. 1 depicts the basic structure of a four-way radial waveguide power divider. This power divider consists of an input coaxial waveguide, a central probe, a radial waveguide, four equispaced identical coupling probes, four equispaced identical adjusting posts and four output coaxial waveguides. The input TEM coaxial mode is transitioned to a radial mode by the central probe, the inner conductor of each output waveguide is connected with the coupling probe inserted into the radial waveguide, and excited by the radial mode. All of the output ports are arranged at the same circle on the upper plate. The central probe, four coupling probes and four posts could be fixed directly to the radial waveguide, therefore no

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ZHANG Jian-qiong, et al. / Four-way radial waveguide power divider with high power-handling capacity

medium support and absorbing material exists in the power divider, which can improve the power-handling capacity and reduce the insertion loss. As the structures of the output ports are completely identical, it is evident for the power divider to realize four-way equal-amplitude in-phase power division. The proposed structure also permits us to realize the vacuum condition in the power divider, which can greatly improve its power-handling capacity. The finite element method (FEM) code is used to analysis and design.

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The structure of the central probe is similar with that presented in Ref. [8]. However, a conical conductor is introduced to replace the entire cylindrical conductor to improve the conversion efficiency of the junction. The coupling probe is desired to have a high coupling capacity. So that, most of the energy can be coupled out when the radial mode pass through the probe, and the energy reflected by the edge of the radial waveguide can be diminished, which is helpful to improve the matching bandwidth of the power divider. Two methods have been used to realize the high coupling capacity. The proposed new coupling probe, which is called L-shaped probe, is shown in Fig. 2. It consists of a cylindrical base and an L-shaped conductor.

(a) Basic structure of front view

(a) Stereogram

(b) Basic structure of sectional view

Fig. 2

(c) Photograph of top view

Fig. 1

(d) Photograph of bottom view Basic structure and photograph of the power divider

(b) Sectional view Configuration of the new probe

The L-shaped conductor forms a closed loop together with the cylindrical base and the radial waveguide, which can enhance the coupling capacity compared with the probe only with the cylindrical probe. The characteristics of the probes can be simulated by being placed in a parallel-plate waveguide, which have similar field distributing to radial waveguide. Fig. 3 shows the typical coupling characteristics

Fig. 3

Typical coupling characteristics of the probes

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of the three probes. It can be seen that the L-shaped probe has the highest coupled magnitude and lowest reflected magnitude. Fig. 4 shows the electric field distributions of

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three probes when the input power is 0.5 W. It can be seen that the L-shaped probe has the minimum electric field, which means it has the highest power-handling capacity.

(a) E-coupled probe (b) Cylindrical probe (c) L-shaped probe Fig. 4 Structures and field distributions of three probes

The L-shaped probe is therefore suitable to be used in the power divider. Another important method to enhance the coupling capacity is the use of adjusting posts. It is essential because it changes the field distribution in the radial waveguide to enhance the excitation energy at the location of the probes. The comparison of the electric field distributions before and after the insertion of the adjusting posts is shown in Fig. 5, where the field enhancement can clearly be seen.

a) Without posts (b) With posts Fig. 5 Field enhancement by the adjusting posts

Based on the above analysis, a power divider with the central frequency of 4.1 GHz is designed. In order to be used in high-power application, the input and output ports are non-standard big-size waveguides. The inner and outer radii of the ports are 2.5 mm and 9.5 mm respectively. The distance between the two parallel plates of the radial waveguide is 20 mm, and the radius of the location of the output ports, i.e. R4, is 63.6 mm for practical application. The design procedure is as follows: First, the central probe is optimized to obtain a high conversion efficiency from TEM coaxial mode to TEM radial mode; Second, the L-shaped probes as well as the adjusting posts are optimized to ensure an appropriate coupling amplitude and a low reflection coefficient; Third, the radius of the edge of the power divider is optimized to finalize the power divider design. The optimized dimensions of the power divider are listed in Table 1.

Table 1 Dimensions of the structure (Unit: mm) Parameters R1 R2 R3 R4 R5 R6 H1 H2 H3 L Dimensions 13.0 12.0 2.0 63.6 62.0 85.0 5.0 9.0 12.0 14.5

3 Simulated and measured results To validate the design theory, the power divider is simulated and measured. Fig. 1(b) shows a photograph of the power divider. The FEM code is used for the simulation and a two-port vector network analyzer is used for the measurement. As the input and output ports are non-standard coaxial waveguides, impedance converter is needed to convert these ports to the standard ports in the measurement. The impedance converter is designed by the method of Chebyshev, which is well-known and widely used. It is calibrated by combing two converters together, and the calibration result is shown in Fig. 6. This converter shows good matching performance but an additional insertion loss is added to the measurement. The insertion loss for the two conjugated converters is around 0.11–0.18 dB. As these converters are not needed in practical applications, this additional insertion loss should be deducted from the measured result.

Fig. 6

Calibration result of the impedance converter

The simulated and measured results are shown in Fig. 7. This power divider shows an approximate equal power

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ZHANG Jian-qiong, et al. / Four-way radial waveguide power divider with high power-handling capacity

division over the designed bandwidth while maintaining a good return loss. At 4.1 GHz, the measured return loss is about  23 dB and the measured average insertion loss is about 0.19 dB. The measured 15 dB return-loss bandwidth of the power divider is found to be about 600 MHz. The measured 0.5 dB insertion loss bandwidth is wider than 700 MHz, comparing with the simulated results, the measured return loss and insertion loss are higher. The differences between the simulated and measured results are most likely attributed to the errors of fabrication, assemblage and measurement that include the slight change in the dimension, the flatness and smoothness of the surface and the junction insertion losses that have not been considered in the simulation. Nevertheless, a good agreement of the measured results and the simulated ones could be noted.

Fig. 7

Simulated and measured results of the power divider

It is important to note that the measured amplitude and phase at the output ports of the power divider is not exactly uniform (Fig. 7 only shows the average amplitude). Take the center frequency for example, the output amplitude imbalance is about r0.2 dB , and the output phase imbalance is about r5q . These imbalances are most likely related to the symmetry errors of the power divider that include the fabrication errors among coupling probes and adjusting posts, the asymmetry of actual radial waveguide and inevitable differences among impedance converters. It should also be noted that the isolations between the output ports are not quite satisfactory (as low as 10 dB for the neighboring ports and as low as 5 dB for the opposing ports). This places added importance in maintaining matching at all ports, which is the case for most high power division applications. The power-handling capacity of this power divider is also analyzed by FEM code. Fig. 8 shows the simulated electric field distribution. It can be seen that there is no notable field concentration in the power divider due to the use of L-shaped probe as well as the central probe. The maximum electric field is about 2011 V/m when the input power is 0.5 W. If we maintain the vacuum state in the structure, and assume the

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breakdown threshold to be 50 MV/m, the power-handling capacity for this power divider could reach 300 MW, which is suitable for high power application. Moreover, the maximum electric field locates in the input coaxial waveguide. If the dimension of this waveguide is augmented, the power divider will have higher power-handling capacity.

Fig. 8

4

Simulated electric field distribution

Conclusions

A four-way radial waveguide power divider is proposed and investigated. The results demonstrate its advantages of high power-handling capacity and low insertion loss which are suitable for high power applications. Furthermore, the design theory proposed in this letter will remain valid, and the main performances of the power divider will not be significantly destroyed when we further augment the number of the output ports. More details on this point will be developed in the future research. Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities (SWJTU09ZT38, SWJTU09BR241).

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