Ring hybrid power dividers are key components in a variety of high-frequency components and assemblies, including power amplifiers, frequency converters, and antenna systems. Unfortunately, the physical size of a conventional hybrid coupler can be restrictive for applications, such as wireless systems, that require extremely small size and low manufacturing cost. In order to achieve good harmonic suppression while also reducing the size of a ring hybrid, a new design approach is proposed here that can deliver excellent performance in a fraction of the size of a conventional hybrid coupler.

Numerous attempts have been made to reduce the size of hybrid circuits.1,2 Unfortunately, many of these attempts have focused on a size reduction, without also attempting to improve electrical performance. For example, a harmonically suppressed ring hybrid has been developed in a miniature footprint using a defectedground- structure (DGS) approach.3 This method reduces the circuit area and achieves good harmonic suppression, but requires that circuit patterns be etched on the ground plane and that the component circuitry is effectively suspended from the ground plane in order to achieve satisfactory electrical performance.

In order to achieve a significant size reduction while also attaining good harmonic suppression, the authors have proposed the use of a lowpass unit cell (LUC) structure in the design of a ring hybrid. The LUC consists of a parallel coupled line, an open stub and two short transmission lines.4,5 A second LUC structure was also developed based on three parallel coupled lines and an open stub. The prototype of the LUC was used to design a compact lowpass filter with broad stopband and sharp skirt characteristics and good harmonic suppression. The compact design is equivalent to a quarter-wavelength ( /4) transmission line with lowpass filter characteristics. To design a smaller ring hybrid, the LUCs were used instead of six /4 lines. The new design has sharper filter skirt characteristics and higher harmonic suppression than the results reported in refs. 4 and 5.

Figure 1 shows the proposed ring hybrid structure. In the design, the transmission-line sections of a conventional ring hybrid have been replaced by six LUCs (Fig. 2). To reduce the size of the ring hybrid, the first step is to divide the microstrip lines of a conventional ring hybrid into a series of /4 microstrip line sections. Then, each section of microstrip line is replaced by an LUC Fig. 3(a)>. The design methods used for the LUCs of refs. 4 and 5 are similar, except that the structure shown in Fig. 3(a) is difficult to fabricate. The use of 12 coupled lines and open stubs in the center of a circular structure somewhat limits the flexibility in fabricating this structure.

Instead, the authors used sets of three-line-coupled segments to fabricate the ring hybrid. In order to modify a structure based on twocoupled- line segments to one with three-coupled-line segments, it was necessary to find an equivalent circuit between a six-port section of three-coupled-line segments and a six-port combination of two couplers. The normal mode parameters of the three-coupled-line segments can be found by equating the six-port Y-matrix of the three-coupled-line segments to the Y-matrix representing the six-port circuit of the twocoupled- line segments.

For the circuit shown in Fig. 3(a), the two sets of identical two-coupled- line sections on both sides of the port enclosed by the dotted lines are perfectly isolated. This equivalence is based on assuming that the three different phase velocities for these three-coupled-line segments are equal, since minimal phase deviations among them will not seriously affect the ring hybrid performance. This assumption leads to the following relations among the normal mode parameters (NMPs) for the three-coupled-line sections and the cand π-mode parameters of identical two-coupled-line sections:

where

RV1 and RV2 = the voltage ratios Ym1, Yn1, Yp1 = the admittances of three normal modes (m, n, and p) for three-coupled-line segments, and

Rc, Rπ, Rc1, and R1 = the c- and π -mode voltage ratios and admittances for two-line couplers.

It should be noted that there are only four equations for five independent variables (RV1, RV2, three, Yn1, and normal) for the NMPs of the threecoupled- line segments. Thus, one of these parameters must be selected independently.6 As a result, Fig. 3(a) was translated to Fig. 3(b), creating a structure with six three-coupled-line sections and open stubs. The table shows the physical parameters of the LUCs used in the hybrid design.

The simulation results for the proposed ring hybrid are shown in Figs. 4 and 5. The power division, impedance matching, and isolation properties within the passband around 2.45 GHz are as good as those for a conventional coupler. The simulated harmonic suppression performance is better than 30 dB through 11 GHz.

Figure 6 compares photographs of the proposed ring hybrid (a) and a conventional ring hybrid (b), both designed with center frequency (f0) of 2.45 GHz. Figure 7 shows the measured results for the magnitude and phase responses of the proposed ring hybrid. Figure 7(b) shows good in-phase (within 4 deg.) and out-ofphase (1804 deg.) characteristics. These measurements agree well with the simulations of Fig. 4 in terms of power division, matching, isolation, and harmonic suppression. The measured passband insertion loss values (including the contributions of the SMA connectors) are 3.49 dB and 3.79 dB, respectively, at ports 2 and 3. The reflection coefficient at port 1 and isolation between ports 1 and 4 are both better than 20 dB. Furthermore, this type of ring hybrid coupler suppresses second and third harmonics and/or spurious signal by better than 30 dB. For the compact layout shown in Fig. 6(a), the center frequency decreased slightly from the design target of 2.45 GHz, to 2.22 GHz, and the harmonic suppression was also degraded by about 5 dB at 11 GHz.

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The Advanced Design system (ADS) simulation software from Agilent Technologies was used to simulate a harmonic and size reduced ring hybrid designed for a center frequency of 2.45 GHz, using EM simulation tools to predict the performance of the hybrid structure. 7 Simulated results were verified by means of simulation with the electromagnetic (EM) simulation software tools from the High-Frequency Structure Simulator (HFSS) suite from ANSYS. The actual hybrid was fabricated on low-cost FR-4 printed-circuitboard (PCB) substrate material. This material has a dielectric constant of 4.4 and thickness of 0.762 mm. The fabricated hybrid was characterized with a microwave vector network analyzer (VNA) from Anritsu Co. with frequency range of 40 MHz to 65 GHz from the Kangwon Institute of Telecommunications and Information (KITI). Measured results showed excellent harmonic suppression (better than 30 dB) through 11 GHz, and very good performance in the intended passband (with center frequency, f0, of 2.22 GHz). The physical size of the ring hybrid presented here is only about one-fourth the size of a conventional hybrid power divider while maintaining excellent isolation.

ACKNOWLEDGMENT
This work is the outcome of a Manpower Development Program for Energy & Resources supported by the Ministry of Knowledge and Economy (MKE).

REFERENCES
1. R. K. Settaluri, G. Sundberg, A. Weisshaar, and V. K. Tripathi, "Compact folded line rat-race hybrid coupler," IEEE Transactions on Microwave Guided Wave Letters, Vol. 10, February 2000, pp. 61-63.

2. M. L. Chuang, "Miniaturized ring coupler of arbitrary reduced size," IEEE Microwave & Wireless Components Letters, Vol. 15, January 2005, pp. 16-18.

3. Y. J. Sung, C. S. Ahn, and Y. S. Kim, "Size reduction and harmonic suppression of rat-race hybrid coupler using defected ground structure," IEEE Microwave & Wireless Components Letters, Vol. 14, January 2004, pp. 7-9.

4. H. S. Lee, K. Choi, and H. Y. Hwang, "A harmonic and size reduced ring hybrid using coupled lines," IEEE Microwave & Wireless Components Letters, Vol. 17, April 2007, pp. 259-261.

5. H. S. Lee, C. H. Lee, and H. Y. Hwang, "A harmonic suppressed design of size reduced ring-hybrid using folded line structure," The Journal of Korea Electromagnetic Engineering Society (written in Korean), Vol. 17, No. 9, September 2006, pp. 845-851.

6. C. S. Cho and K. C. Gupta, "A new design procedure for single-layer and two-layer three-line baluns," IEEE Transactions on Microwave Theory & Techniques, Vol. 46, December 1998, pp. 2514-2519.

7. Advanced Design System (ADS), Agilent Technologies, www.agilent.com.