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In earlier works, a dual-disk probe was proposed to provide broadband impedance matching from an input coaxial line to a radial line.8 But this impedance-matching structure is rather complicated, requiring a sophisticated mechanical process to form an additional closed cavity on the top side of the substrate. In the current report, a simple one-disk probe was used to avoid any complex manufacturing requirements. A stepped coaxial line transformer together with a tapered SIW was used for broadband input matching for the power divider/combiner, as shown in Fig. 1.

A tapered SIW was formed by using guiding posts, as illustrated in ref. 9. The inner conductor diameter of the central probe is also stepped, from d1 to ds, for broadband matching. Such a stepped structure can be easily realized by adding a metallic central viahole with diameter of ds in the substrate.

Thus, the return loss can be optimized by changing the total lengths of the guiding posts (ls) and the physical parameters of the stepped coaxial line transformer (D2, D4, H, and ds). Compared with those parameters described in ref. 8, the feeding structure for the power divider/combiner in this report is much simpler while also achieving broadband input impedance matching. Four isolation resistors of 100 Ω were soldered between each open end of the HMSIW Y-junction to improve the port-to-port isolation performance.

For many microwave power-combining networks, a microstrip port is preferred for its ease of connection with planar active components, such as power amplifiers. A microstrip port makes it possible to make direct connections with planar active components. In contrast to the designs described in ref. 8, in the current report, the central feedline is on the same side as the microstrip output port, so that two eight-way HMSIW power dividers/combiners can be connected back-to-back by using vertical microstrip transmission lines to form a broadband power combiner, as shown in Figs. 2 and 7.

HMSIW Methods Make Broadband Dividers, Fig. 2

For a power-combined amplifier structure, the microstrip transmission lines can be replaced by planar amplifier modules. Because there is no coaxial-to-microstip transition, insertion loss can be minimized when this approach is applied to a power-combining system. To illustrate the performance of back-to-back connected power divider/combiner circuits, the input port of the power divider was driven by an incident voltage vin, as shown in Fig. 2. If V1 denotes the voltage vectors between two power dividers—i.e., V1 = [vin…vin]T—it can be derived that:

V1 = ([I] + [S]2)vinT   (1)

where T = [tt…tt] and t represents the transfer coefficient of the power divider from the input port to either output branch; [S] is the scattering matrix of the power divider with the first row and column of the matrix excluded. Obviously, the off-diagonal elements of matrix [S] indicate the isolation between the output ports of the power divider. In Eq. 1, [I] is the identity matrix. The output voltage of the power divider can be written by means of Eq. 2:

vout = TTV1 = vin TTT + vin TT[S]2T = vout1 + vout2   (2)

The first term, vout1 of Eq. 2 is the ideal output, while the second term, vout2, is the frequency-dependent interference signal caused by multiple reflections and nonideal isolation. This term should be minimized to improve the wideband response of the power-combining system. Obviously, when power amplifiers are inserted between power divider/combiners, system performance can be improved by optimizing the input/output return losses of the individual power amplifiers.

The design procedure for realizing an eight-way HMSIW power divider is straightforward. The SIW and HMSIW parameters are first calculated according to the desired operating frequency range so that HMSIW Wilkinson power dividers can be designed.10 Next, the parameters required for a transition from coaxial lines to the four SIW arms are determined. Finally, these two circuit segments are integrated and full-wave optimization is applied.

HMSIW Methods Make Broadband Dividers, Fig. 3

To demonstrate the design procedure, an HMSIW eight-way power divider was fabricated on TLX-8 printed-circuit-board (PCB) material from Taconic’s Advanced Dielectric Division. The substrate material has a thickness of 60 mil and relative dielectric constant of 2.55 in the z-direction. Full-wave computer simulation and optimization was performed by means of CST Microwave Studio 2011 computer-aided-engineering (CAE) software from Computer Simulation Technology. As shown in the photograph of Fig. 3, SMA connectors were added to the circuit for testing a single power divider. The dimensions of the various parameters for the power divider are listed in the table.

HMSIW Methods Make Broadband Dividers, Table

The simulated and measured S-parameters of the single eight-way power divider are shown in Fig. 4. Good agreement can be observed in wideband. The discrepancy between the two results is mainly attributed to unexpected tolerance of fabrication and assembling. Over a wide frequency range from 4.5 to 11.2 GHz, insertion loss lower than 1 dB (including SMA connectors) and return loss better than 12 dB are achieved.

HMSIW Methods Make Broadband Dividers, Fig. 4

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