The rapid development of wireless communication systems proposes more and more requirements for functional devices. In order to meet the market demands, different kinds of well-performed functional devices have been widely studied. In the past decade, substrate integrated waveguide (SIW) has become one of the hotspots in researches [1]. Considering its perfect high-pass transmission with low-loss, high-efficient properties, functional devices such as filter [2], power divider [3–6] and coupler [7] have been proposed. Half-mode SIW (HMSIW) consists of only half of SIW structures, while keeping the same performance as SIW [8, 9]. Miniaturization can be realized. Spoof surface plasmon polaritons (SSPPs) are guiding surface modes along periodic metamaterial structures. Such modes have also received wide attention for its unique properties of perfect low-pass transmission and near-field confinement [10, 11]. SSPPs have been studies in field of different high-efficient designs such as bandpass filter [12, 13], power divider [14, 15], coupler [16], radiator [17].

Filters and power dividers are the most commonly-used functional devices in wireless communication systems. In certain applications, both kinds of devices are required to be integrated with each other to reduce the occupied space. And their performances are crucial for systems stability. In recent years, a series of filters and power dividers based SIW and SSPPs have been reported [18, 19]. In order to improve the integration and operating properties, parasitic structures like resonators [20], interdigital structures (ISs) [21], defected ground structures (DGSs) [22, 23] have also been studied. The combination of SIW and SSPP provides a new solution for the miniaturization designs, since the power dividers of SIW are usually narrow-band and those of SSPP have limits of bulky dimensions.

In this letter, we proposed a novel unequal bandpass filtering power divider based on the hybrid HMSIW-SSPP modes, which shows compact size and power division ratio of 1:3. The hybrid unit can be compatible with the functional techniques of both HMSIW and SSPPs. The lower and upper edges of the passband can be controlled independently by changing the dimensions of HMSIW and SSPPs. Defected ground structure is also loaded aiming for improving out-of-band suppression.

The DGS can disturb the current distribution on the metallic ground and introduce an extra transmission zero. In the optimized model, a two-element array of inverted T-shaped DGSs is introduced. The DGSs are loaded on the bottom layer of the hybrid HMSIW-SSPP units to improve the upper stopband rejection. The structure of the DGS unit is shown in the inset of Figure 6A, in which the red line represents the contour of the grooves on the top layer. The DGS is etched on the bottom layer and it is in the middle of two adjacent grooves. The dimensions of the DGS decide the frequency of its transmission zero. And the optimized dimensions are a1 = 1.9 mm, a2 = 4.4 mm, a3 = 3.8 m, t = 0.6 mm, s = 0.45 mm, g = 0.25 mm. The comparison of S 21 of the design with and without the DGSs is shown in Figure 6A. The transmission with two-element array of DGSs provides an extra transmission zero, providing better out-of-band suppression. The cut-off efficiency of transmission zero of DGS at 10 GHz can be improved by increasing the number of DGSs used in the model. A small shift of the cutoff frequency of SSPPs is observed due to the coupling between DGSs and SSPPs. The transmission of the proposed design loaded with DGSs are shown in Figure 6B. An unequal power divider with bandpass filtering effect is observed within band from 6.5 to 9.5 GHz. The simulated S 11 is lower than −20 dB in the passband. S 21 and S 31 are −2.2 dB and −7 dB, respectively. Within the whole operating band, S21 undulates between −2.6 dB and −2.3 dB, and S31 undulates between −7.4 dB and −7 dB. The simulations indicate a stable power division. The power division ratio of port 2 and port 3 is 3:1.

A prototype of the proposed unequal filtering power divider is fabricated and measured. The substrate is F4B board with relative permittivity of 2.65 and thickness of 1 mm. Photographs of the prototype are shown in Figure 7. The total dimensions of the proposed design are 45 mm × 25 mm × 1 mm. Figure 8 shows the compared of the simulated and measured S-parameters. The measured transmissions are about 1 dB lower than the simulated ones, because of the machining accuracy of the sample and the unskilled welding of SMA connectors in the experiments. The operating bandwidth and power division effect keeps steady.

In this paper, an unequal filtering power divider based on hybrid HMSIW-SSPP mode is proposed. The passband can be controlled independently by changing the dimensions of HMSIW and SSPPs. DGSs are introduced on the bottom layer of the device to improve the out-of-band suppression. A prototype working from 6.5 to 9.5 GHz is designed and fabricated. And power division ratio of 1:3 is obtained. Such design provides solutions for miniaturized multi-functional devices, and could be used in wireless communication systems.