Full Waveguide-band E-plane Power Divider with High Output Power Ratio

I. Introduction

The power divider is one of the building blocks of waveguide components such as multiplexers, transmitters and receiver modules and array feed networks. Wideband power dividers with high output power ratio are often required in the feed network of the array of waveguide radiators [1][2]. Full waveguide-band power dividers have been investigated for H-plane [3] and E-plane [4] geometries. Literature on wideband power dividers with high output ratio is quite limited. Gómez and co-workers presented an E-plane gamma-junction power divider with output ratios of up to 10 [5], while Huang and co-workers investigated an E-plane T-junction power divider with output ratios of up to 4 [6].

In this paper, we present a new E-plane power divider that provides the output ratio of up to 100. The proposed power divider is structurally simple and can easily be fabricated using split-block technique or metal casting. The design and performance of the proposed power divider are presented below.

Ⅱ. Power Divider Structure

The E-plane junction power divider in a rectangular waveguide shows high reflection since it has large junction discontinuity reactance and no measure for input impedance matching. Fig. 1 shows a waveguide E-plane junction divider whose reflection coefficient is shown in Fig. 2 [7].

Fig. 1.

Waveguide E-plane junction power divider [7]

Fig. 2.

Reflection and transmission (S3,1) coefficients of a waveguide E-plane junction divider [7]

In Fig. 2, we see that the reflection coefficient of an uncompensated E-plane junction in WR-10 waveguide ranges from -8 dB at 75 GHz and -3 dB at 110 GHz. Clearly the E-plane junction requires matching structures if it is to used as a power divider. The E-plane junction can be compensated over narrow or wide frequency bandwidths. In this paper we present an E-plane power divider structure that provides reflection of less than -20 dB and high output power ratio over the full bandwidth of the WR-10 waveguide.

Fig. 3 shows the structure of the proposed power divider. The power divider consists of a T-junction with rounded corners and a smoothly tapered septum. The output waveguides are denoted by Port 2 and Port 3, whose output power is P₂ and P₃, respectively. Output power ratios P₂/P₃ of up to 10 are achieved by offsetting the septum as shown in Fig 3. The maximum offset is dictated by the smallest possible gap between the septum tip and the rounded corner that can accurately be fabricated.

Fig. 3.

Structure of the proposed power divider

For power ratios higher than 10, the broad-wall width of the Port 3 waveguide is linearly tapered and directly connected to the output standard waveguide for as shown in Fig. 4. High output power ratios are in effect achieved by reducing the broad-wall width of the Port 3 waveguide below the cut off value.

Fig. 4.

Power divider with a reduced-width output waveguide for greater output ratios

Fig. 5 shows design parameters of the proposed power divider. Rounded corners of the junction have radius R₁ while the septum has a circular taper of radius R₂. The sharp tip of the septum is removed such that the septum has length L₂. The septum offset L₀ from the center line controls the output power ratio.

Fig. 5.

Dimensional parameters of the proposed divider. Plan view (top) and side view (bottom)

For output power ratios of greater than 10, the broad-wall width of the Port 3 waveguide is linearly reduced by 2S over a length of L₃. For split-block fabrication using an end mill with a cut along the center line of the broad wall, corners in the step are rounded with radius R₃ (0.25 mm) [8]. Parameters R₁, R₂ , and L₂ are kept constant for all values of output power ratios.

Ⅲ. Power Divider Design

Design of the proposed power divider is carried out using a commercial electromagnetic-wave simulation tool (CST Studio Suite). With R₁ of about 0.7R₂ and L₀ equal to 0, R₂ and L₂ are adjusted for reflection coefficient of less than –20 dB over the operating frequency range 75-110 GHz of the WR-10 waveguide.

Fig. 6 shows the divider with no septum offset for determining the values of R₁, R₂, and L₂. First the effect of R₁ on the input reflection coefficient is investigated with R₂ and L₂ set at their optimum values 1.78b and 1.29b respectively.

Fig. 6.

Divider with no septum offset

Fig. 7 shows the input reflection coefficient (S₁₁) versus R₁. The optimum value of R₁ is 0.91b giving the input reflection coefficient less than -20 dB at 74-120 GHz. Notice that larger values of R₁ does not result in smaller reflection since the optimum value of R₁ is also related the values of R₂ and L₂.

Fig. 7.

Input reflection versus R1

Next the input reflection is calculated for various values of R₂. Fig. 8 shows the result. The optimum value of R₂ is 1.78b. Larger values of R₂ does not result in smaller reflection since in this case the septum reduces the narrow-wall width of the branching output waveguides. Rather sharp reduction to -42 dB of the reflection at 85 GHz is caused by a mutual resonant cancellation of discontinuity reactances of the divider junction.

Fig. 8.

Input reflection versus R2

Fig. 9 shows the input reflection versus L₂. Smaller values of L₂ make the septum center edge flat and wide while larger values make the septum center edge sharper. The value of L₂ is same as R₂, the septum edge will be sharp as a razor's edge, which is not preferable in manufacturing the power divider. The optimum value of L₂ is 1.29b

Fig. 9.

Input reflection versus L2

For output power ratios P₂/P₃ of up to 10, only the septum offset L₀ is adjusted. For P₂/P₃ greater than 10, L₀ is set at the value for P₂/P₃ = 10 and the taper step S is adjusted with the taper length L₃ fixed at about 1.2a (a: the broad-wall width). The input reflection coefficient at Port 1 remains less than –20 dB for all output power ratios.

The proposed concept is applied to the design of a power divider for the WR-10 waveguide operating at 75–110 GHz. Table 1 shows the dimensions of the designed power divider normalized by the narrow wall width b.

Table 1.

Dimensions of the proposed divider

Optimum parameters for R₁, R₂, and L₂ have been obtained using the divider with a symmetric septum shown in Fig. 6. The power divider with unequal output power ratios is obtained by offsetting the septum of the structure shown in Fig. 3. The reflection coefficient and the output power ratio are calculated for various values of the septum offset L₀ and plotted in Figs. 10 and 11.

Fig. 10.

Reflection coefficient of divider shown in Fig. 3

Fig. 11.

Output power ratio of divider shown in Fig. 3

Figs. 10 and 11 show the reflection coefficient and the output power ratio of the structure of Fig. 3 while Figs. 12 and 13 show those of Fig. 4. The input reflection coefficient in all cases is less than –20 dB at 75-110 GHz. The output power ratio is maximum at 75 GHz and gradually decreases as the frequency increases. The maximum output power ratio is about 100 at 75 GHz and 4 at 110 GHz.

Fig. 12.

Reflection coefficient of divider shown in Fig. 4

Fig. 13.

Output power ratio of divider shown in Fig. 4

The output power ratio can be increased if the input reflection is allowed to increase up to –15 dB. With additional impedance matching structures such as irises, posts and steps in waveguide width and/or height, one can achieve higher output power ratios while keeping the input reflection below –20 dB. With proper impedance matching, power ratios can arbitrarily be increased by increasing the output waveguide taper while keeping reflection coefficient less than a specified value.

Ⅳ. Conclusion

In this paper, we presented a new E-plane power divider operating over the full rectangular waveguide band with maximum output power ratios of up to 100. Full-band operation is realized with a smooth tapered septum while output power ratio of greater than 10 is obtained by reducing the width of the broad wall in one of the output waveguides. A design has been presented for the proposed power divider with reflection coefficient of less than -20 dB and power ratio from 4 to 100 over the full waveguide band (75-110 GHz) of WR-10. The proposed power divider offers high output power ratio with simple structure. The proposed power divider concept can be applied to the design of waveguide components where the power needs to be divided over wideband with high ratios.

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