Design and fabrication of an ultra compact Gysel power divider with harmonic suppression by using U shaped resonators

1 Introduction

The power dividers are important three ports elements for working in microwave devices and antenna systems. As their name suggests, they are responsible for power splitting, when their input port is triggered. Furthermore, they are used for mixing the power in the opposite direction [1]. So, a power divider is a critical component in microwave communication systems. Power dividers are used as basic parts in microwave systems. They can be used as feeding networks for an antenna array, power amplifier and mixer [2]. For example, Figure 1 shows a phased array antenna that uses power divider.

Figure 1:

Phased array antenna with 4 elements including three power dividers.

Many types of power dividers/combinators were invented in the MIT Radiation Laboratory in 1940, including the E-plane, H-plane and T-junction waveguides. After that, by inventing strip line and Microstrip lines, they were used in the redesign of power dividers because of their cheap properties and surface structure [1].

Two typical categories of power dividers are Wilkinson and Gysel power dividers. Gysel power dividers have large size and some merits compared with Wilkinson types [1–3]. So, due to have some merits, efforts for reducing the size of Gysel power divider are important. Both of them suffer from harmonics. Therefore, getting a compact power divider with harmonic suppression and high performance simultaneously is still an ongoing challenge. Many studies about miniaturization and harmonic suppression on the power dividers have been done [4], [5], [6], [7], [8], [9]. In this article, a very compact Gysel power divider that reduces occupied area to approximately 85% with suppression of 10 harmonics is presented.

2 Low pass filter design

Filters can be used to optimize the Gysel power divider [1], [2], [3], [4], [5]. In this paper, instead of the 70.7Ω lines in the Gysel power divider, equal low pass filters with proper features are used, Which will eliminate the harmonics and will minimize it. The proposed filter is shown in Figure 4 with w17 = 5.2 mm, w18 = 2.55, w19 = 5.1 mm, w20 = 4.6 mm, w21 = 4.1 mm [12], [13], [14], [15], [16]. For determining the design parameters of filter, first the LC equivalent circuit related to the resonators and the values related to the inductors and capacitors of the LC equivalent circuit are obtained by the method described in references 12 and 14. Finally, by obtaining the values of inductors and equivalent circuit capacitors, the dimensions of each resonator are calculated [12, 14].

Figure 2:

(a) Resonator 1 structure (b) frequency response.

The values of capacitances (Cx) and inductances (Lx) are computed using open ends and high-low impedance lossless lines formulas, [14]. Equations (1) and (2) express these formulas, where Zs is the characteristic impedance of the lines and λg in Equations (1) and (2) is calculated by the width of lines [14] (λg is the calculated guided wavelength at operating frequency).

The design procedure is such that a simple resonator is first introduced and reduced by Minimization methods. The final filter will be designed by combining the resonators together. Finally, these resonators are combined and a power divider is designed.

In the first step, resonator 1 was designed. This resonator consists of placing a step impedance resonator on a straight high impedance line. This resonator produces a transmission zero at a frequency of 1.513 GHz. This transmission zero results in a cut off frequency at 1.14 GHz (Figure 2). It was observed that this resonator does not have a suitable bandwidth and acceptable sharpness. For this reason, other resonators were used to improve response and increase cut off bandwidth and sharpness. Spiral or U shaped resonators have been used to reduce the filter size and so the power divider size.

Figure 3:

(a) Resonator 2 structure (b) frequency response.

In the second step, suitable attenuation units were considered to produce transmission zero in the cut-off band. Rectangular resonators were selected for this purpose (Figure 3).

Figure 4:

(a) proposed filter (b) frequency response.

It is observed that the added rectangular structures produce a strong transmission zero at a frequency of 4.19 GHz. Due to the power of this transmission zero, the cut-off bandwidth of the filter increases up to 4 GHz. These resonators also shift the first transmission zero to lower frequencies, resulting in increased sharpness. As the first transmission zero, which was in the first step at 1.513 GHz, has shifted to 1.413 GHz. It was observed that at this stage, the cut-off bandwidth of the filter is 4 GHz. But to remove the harmonics in the final power divider, a suitable bandwidth (at least up to 5 GHz) is required in the filter structure. So, in the third step another structure was designed to improve this factor (Figure 4). In this step, two resonators consisting of microstrip lines of different lengths are added to the structure. So, the cut-off bandwidth has been upgraded.

In this filter, the cutoff band is up to 5.66 GHZ with an attenuation level of more than 20 dB. Meanwhile, the bandwidth of the filter transition (sharpness) is 240 MHz, which is suitable for replacement in a divider (Figure 4). So, it was observed that the desired specifications of a filter were improved step by step.

3 Gysel power divider

The conventional Gysel power divider is depicted in Figure 5 [10]. This structure consists of 70.7 Ω lines and 90° angle. Also, there are two 100 Ω resistors for isolation between the output ports. In the power divider structure, the λ 4 transmission lines have an impedance equal to 2 Z 0 , which is equivalent to 70.7 Ω and an electrical length of 90°. So, designed low-pass filter is used instead of λ 4 transmission lines. This structure will improve the efficiency of this power divider in terms of size reduction, elimination of unwanted harmonics, increase of bandwidth and wide stop band. The filter removes the harmonics.

Figure 5:

The conventional Gysel power divider [10].

Figure 6 shows proposed Gysel power divider by using designed low pass filter with w22 = 6.46, w23 = 1, w24 = 1.84, w25 = 3.8, w26 = 4.4, w27 = 7 [12], [13], [14], [15], [16].

Figure 6:

proposed Gysel power divider.

The structure is simulated with ADS simulator and for verification, this power divider is fabricated on Rogers 4003 PCB with a relative permittivity of 3.38 and a 31 mm-thick substrate. In Figure 7, fabricated power divider is shown. Results of simulation and fabrication for proposed power divider are shown in Figures 8 and 9.

Figure 7:

Fabricated Gysel power divider.

Figure 8:

Frequency response of s11 and s21.

Figure 9:

Frequency response of s22 and s23.

The considered operating frequency is 580 MHz. In addition, the bandwidth of proposed power divider is equal to 22% (470–680 MHz). In operating frequency, losses of port 1 and port 2, isolation, and transmission losses are S11 = 17 dB, s22 = 37 dB, s23 = 34 dB, s21 = 3.09 dB. The physical size of the circuit is 34.5 × 63 mm, which is equal to the electrical size 0.200λ _g  × 0.109 λ _g , where λg is the guided wavelength at the operating frequency. By regard to the size of a typical Gysel power divider at 580 MHz, the obtained size of proposed power divider is equal to 15.83% of the size of a typical Gysel power divider at this frequency. The proposed power divider operates in the same way as the conventional one [10], but effectively rejects the second to tenth (2–10) spurious harmonics by 16, 27, 24, 61, 60, 70, 48, 35 and 19 dB, respectively. As shown in Figures 8 and 9. It is evident from the Table 1 that the proposed power divider has good features among the other power dividers.

4 Conclusions

In this research, a novel design for compact Gysel power divider with higher order harmonic rejection has been described using a new low pass filter. The proposed power divider operates at 580 MHz and it has been simulated, fabricated and measured. Moreover, the results show that the proposed structure is more compact than other published power dividers and it can suppress more harmonics than others that these are good results for this power divider. So, the proposed power divider is suitable to be applied in small size circuits with higher order harmonic suppression.