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Waveguide power dividers are employed for applications where signal or energy needs to be distributed or combined in rectangular waveguide media [1]. Typical applications include the combining of microwave high-power sources [2]-[4], the antenna array feed network [5][6], and the waveguide multiplexer [7][8]. A waveguide power divider is implemented in the form of T-junction [9], Y-junction [10][11] or septum/bifurcation junction [12]. The T-junction offers the advantage of structural simplicity and compactness. Depending on the requirement, the T-junction divider can be realized either in the electric-field plane or in the magnetic-field plane.

Various approaches to the design of the E-plane T-junction power divider have been proposed. A half height (b/2) waveguide has been employed with a branching mound and stepped impedance transformers at the output branches [13][14], where the output ports have twice the input port's characteristic impedance. The output waveguides may have the same height (b) as the input waveguide [14][15]. In this case, stepped impedance transformers are employed either at the output ports or at the input port, and a mound or a similar structure is used to compensate for the junction effect [16][17]. A continuously tapered impedance matching structure can be used in place of a stepped transformer [18].

Another approach to the design of the divider is to use a metal post in the junction region and an impedance matching iris at the input waveguide [19]. The metal post can be replaced with a mound compensating for the junction effect [20].

This paper explains a new power divider in the electric-field plane that has a simply-shaped mound for junction-effect compensation and a dielectric insert for impedance matching. Compared with existing structures, the proposed design offers the advantage of structural simplicity and compactness while providing reasonably wideband performance. Optimum design of the proposed power divider is obtained from iterative parameter sweeps using the CST Studio Suite. Finally, fabrication methods of the proposed power divider are presented.

The structure of a rectangular waveguide E-plane T-junction is shown Fig. 1 where only the inside surfaces of the waveguide are shown for simplicity. In Fig. 1 the background or exterior material is a perfect conductor and the interior material is air. A standard rectangular waveguide has the narrow wall width b that is 50 percent of the broad wall width a. The simple T-junction shown in Fig. 1 has poor input matching due to discontinuity effect. Fig. 2 is the coefficient of reflection (S1,1) and the coefficient of transmission (S3,1) of the structure of Fig. 1. In Fig. 2, one observes that input matching is poor.

Fig. 1. 
Structure of a waveguide E-plane T-junction

Fig. 2. 
Reflection (S1,1) and transmission (S3,1) coefficients of the uncompensated waveguide power divider in the electric-field plane

To improve input matching, various forms of the compensating structure can be employed. Fig. 3 shows the power divider proposed by Zou and co-workers [15] and its reflection and transmission coefficients. It has a mound and three -step impedance transformers at the output waveguides. The coefficient of reflection in the Zou's power divider is less than -20 dB at 23-33 GHz (42.9% bandwidth). It has a large bandwidth but with the structural complexity.

Fig. 3. 
Structure and coefficients of reflection/transmission of the Zou's divider [15]

Fig. 4 shows the structure proposed in this paper, which consists of a mound for junction-effect compensation and a dielectric insert for impedance matching. Compared with the Zou's design, the proposed structure is simpler and thus easier to fabricate. The cost of structural simplicity is a reduced bandwidth of about 20%.

Fig. 4. 
Structure of the authors' power divider

Fig. 5 shows the dimensional parameters of the divider. Design parameters of the divider are as follows. L₂: mound length, L₃: mound height, R₁: mound edge radius, R₂: mound corner radius, L₁: dielectric insert length, ε_r: relative permittivity of the dielectric insert.

Fig. 5. 
Design parameters of the divider

The power divider design will be illustrated for a power divider operating in W-band where a and b are 2.54 mm and 1.27 mm respectively. The cyclic olefin copolymer (COC) is chosen as the material of the dielectric insert for low dielectric constant (ε_r = 2.30 and low loss (tanδ = 0.0002 at W-band), good dimensional stability, excellent precise moulding property, low moisture absorption, high rigidity/strength/hardness and good machinability [21][22]. Other dielectric materials can be used as far as its dielectric constant is not too high.

Starting from initial values obtained by guess work, design parameters R₁, R₂, L₂, and L₁ are iteratively adjusted for low reflection over a wide frequency range.

Table 1 shows the result where reflection coefficient of less than -20 dB is obtained at 75.5-95.7 GHz (23.6% bandwidth). Fig. 6 shows the H-plane cross-sectional view of the divider. Fig. 7 shows the reflection and transmission coefficients of the power divider. The coefficient of reflection is less than -20 dB and the coefficient of transmission is greater than -3.04 dB at 75.5-95.7 GHz.

Fig. 6. 
H-plane cross-sectional view of the designed power divider

Fig. 7. 
Reflection (S1,1) and transmission (S3,1) coefficients of the divider

For power dividers to be operated at a lower or higher frequency range, L₁/b is increased or decrease respectively and other parameters are optimized. The bandwidth of the optimally designed power dividers is about 20% (a little smaller and larger than 20%) throughout W-band.

In the following we illustrate the dependence of the divider's input reflection coefficient on each design parameter. Fig. 8 to 12 shows the reflection coefficient S11 at the input port versus each design parameter (one of L₁, L₂, L₃, R₁ and ε_r) with all the other parameters remaining at the optimum value in Table 1.

Fig. 8 shows the reflection versus the dielectric insert length L₁ with the case S1_1(L₁) corresponding to the optimum value of L₁.

Fig. 8. 
Reflection versus the length L₁ of the dielectric insert

Similarly, Fig. 9 and 10 are the coefficient of reflection with various values of the mound length L₂ and height L₃ respectively. In Fig. 8 to 10, one can observe that the coefficient of reflection is sensitive to these parameters.

Fig. 9. 
Reflection versus the length L₂ of the mound

Fig. 10. 
Reflection versus the height L₃ of the mound

Fig. 11 is the variation in the reflection coefficient with different values of the mound's edge radius R₁. The reflection coefficient at middle frequencies can be further reduced by using smaller values of R₁. This, however, results in reduced 20-dB bandwidths. The divider's reflection is not sensitive to the mound's corner radius R₂ so that the result is not presented. Finally Fig. 12 shows the reflection versus the relative permittivity ε_r of the dielectric insert. Again, the reflection is sensitive to ε_r.

Fig. 11. 
Reflection versus the edge radius R₁ of the mound

Fig. 12. 
Reflection versus the relative permittivity of the dielectric insert

The proposed power divider can be fabricated using the methods shown in Fig. 13. In the first split-block method, the structure is halved in the E-plane central surface. Each half block is fabricated using a suitable method such as CNC machining [23].

Fig. 13. 
Fabrication methods for the divider

The dielectric insert is separately fabricated and installed between two blocks. Two half blocks are then joined together using screws.

In the second top-face split method, the structure is decomposed into the top surface of the branched waveguides and the remaining structure. The rectangular hole for the input waveguide is fabricated, for example, using the electrical discharge machining [24][25]. The remaining processes are the same as in the first method.

Authors of this paper have proposed a new waveguide divider in the electric-field plane and of the T-junction type. The divider design method has been presented. An uncompensated power divider shows reflection coefficient from -9 dB to -2 dB. In the proposed structure, the T-junction discontinuity is compensated using a simply-shaped mound and input-output impedance matching is obtained using a dielectric insert. To illustrate the divider's structure, a design of the proposed power divider has been presented that works in the W-band.

The proposed power divider shows reflection lower than -20 dB at 75.7-95.7 GHz (23.6% bandwidth). Using the proposed structure one can design a divider of T-junction type in the electric-field plane operating anywhere in the standard waveguide band with more than 20% bandwidth. The power divider proposed in this paper is simple and easy to fabricate offering a significant advantage over existing design for applications at high millimeter-wave and terahertz frequencies where machining tolerances of 0.02 mm or less are routinely required.