Directional couplers are useful for monitoring and measuring power levels from antennas and at different points in a communications system. Designing a microstrip or stripline directional coupler requires a clear understanding of the physics of coupled lines and the limitations of a printed-circuitboard (PCB) process. What follows is the design of a novel microstrip/coplanar-to-stripline directional coupler that is convenient for simultaneous monitoring of power transmitted to and reflected from an antenna. The coupler can be designed with coupling coefficients from 20 to 40 dB and features unique wideband compensation for optimum performance. The compensation is achieved by proper displacement of a tuning ground plane between the coupled lines. Using the design approach shown here, 30- and 40-dB couplers will be constructed and tested.

Directional couplers are an important part of modern radar and wireless-communications equipment, for monitoring power transmitted to and reflected from an antenna. The requirements for such couplers include low insertion loss; good impedance matching; sufficient main-line power-handling capability; a coupled path that is weakly coupled (20 to 40 dB less than the mainline power level) to allow measurements with sensitive test equipment on coupled high-power signals; directivity of better than 20 to 26 dB in both directions; and the possibility of integrating the coupler with other circuits on a common PCB.

The only coupler structure in the literature found to fulfill these requirements was composed of three conductive layers embedded in a PCB.1 The coplanar line is placed on the top layer and the stripline is in the middle layer. The bottom layer of the structure is backed with a conductor. The author has verified numerically that the directional coupler presented in ref. 1 could achieve good directivity for coupling values of 20 to 40 dB. The one drawback of this structure is the inability to separate the microstrip/coplanar and stripline circuits since the stripline inner conductor occupies the microstrip line ground layer.

This shortcoming is not an issue in the new directional coupler structure proposed in Fig. 1.2 In this new design, the coupled line is shifted down to the third conductive layer and a tuning ground plane is introduced at the second layer. This ground plane plays an essential role in providing compensation for the coupler. The proposed structure is suitable for handling moderate levels of power. For higher-power applications, quasi-air-dielectric main-line-to-stripline/microstrip directional coupler solutions have already been proposed in ref.3.

The proposed coupler structure belongs to the class of asymmetrical coupled transmission lines in an inhomogeneous dielectric media. It is known that, 4,5 assuming the validity of quasistatic approximation, directional couplers with this type of coupled-line structure can be compensated (be perfectly matched and achieving infinite directivity) if the following conditions are fulfilled: 1. the inductive (kL) and capacitive (kC) coupling coefficients are made exactly equal, i.e.,

where:

and

2. the two coupled lines are terminated with the following impedances:

and

where:

Li, Ci, i = 1,2 = the self-inductance and self-capacitance, respectively, per unit length of line i in the presence of the another line, and

Lm and Cm = the mutual inductance and mutual capacitance per unit length, respectively.

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STRUCTURE ANALYSIS
The analysis of the proposed structure has been performed with a static two-dimensional electromagnetic (EM) solver. 6 The transversal geometrical dimensions of the 50 Ω matched coupled lines have been calculated as a function of the tuning ground plane protrusion distance s2, and changes to the inductive kL and capacitive kC coupling coefficients have been registered.

Figure 2 shows the results of the calculations for the microstrip-to-stripline structure. The structure can be tuned to a desired coupling value, while compensated, by changing the distance s1 between the main line and the coupled line along with adjusting the distance s2. The range of achievable coupling coefficients extends from about 18 to 40 dB and more. The widths of the main line (w1) and the coupled line (w2) vary from 126 to 127 mils, and from 20 to 30 mils, respectively, depending on the coupling level. The microstrip-to-stripline coupled transmission lines can be compensated in spite of large difference between the values of effective dielectric permittivity of the two orthogonal modes propagated in the structure (Fig. 2b). This feature distinguishes the asymmetrical coupled lines from symmetrical lines. The latter lines are compensated if values of these permittivities are equal. Similar results as those presented in Fig. 2 have been also obtained for a coplanar-to-stripline structure.

Figure 3 shows the transversal-structural dimensions and spacing of a 30-dB, 50-ohm matched and compensated microstrip-to-stripline coupler based on different substrate dielectric permittivity values. These curves are useful for a practical coupler realization using any chosen PCB dielectric material. For example, a coupler fabricated on a substrate having a dielectric constant of 10 yields a stripline width of 6.5 mils, which can be realized on fairly standard PCB technology.

DESIGN CURVES
To demonstrate these design curves at work, 30-dB and 40-dB directional couplers designed and fabricated with a coplanar-to-stripline configuration using halogen-free FR4 material (relative dielectric constant of 4.8 and loss tangent of 0.013) from Matsushita (www.matsushita.com). The transversal dimensions of the coupled lines are w1 = 102.5 mils, w2 = 19.2 and 16.8 mils, s1 = 68.8 and 87.5 mils, s2 = 84 and 82.4 mils, and s3 = 60 mils for the 30- and 40-dB couplers, respectively.

Figure 4 shows the physical-layout of the first and third conductive layers for the 3-dB coupler. For measurement convenience, external microstrip lines are connected to the stripline using plated viaholes. The ground copper pattern on the second layer follows the ground pattern on the top layer and fills up the areas of the external microstrip lines.

Figure 5 shows the measured results for the 30-dB coupler from 0.5 to 3.5 GHz while Fig. 6 shows the measured performance of the 40-dB coupler from 0.5 to 2.5 GHz. Midband coupling levels are 0.2 and 0.6 dB lower than the computer-projected design levels of the 30- and 40-dB couplers, respectively. The main and coupled lines are well matched: return loss is better than 30 dB. Directivity exceeds 25 dB across the full frequency band presented for the 30-dB coupler and across almost the entire band for the 40-dB coupler. The insertion loss of the 50-mm-long main line is less than 0.25 dB at 2 GHz.

These excellent results would tend to validate the quasi-static approach as a reliable starting point for the design of high-performance directional couplers. Some corrections should be considered, especially for higher-frequency designs, due to discontinuities and the dispersion of the main line. The coupler structure appears promising for sampling forward and reflected RF power simultaneously, and it supports integration of the coupler into multifunction circuitry as well as separation into microstrip and stripline circuits. The required range of coupling coefficients from 20 to 40 dB is easily realizable with good directivity in a wide frequency

REFERENCES

  1. F. Beyer and P. Schucht, "Stripline directional coupler," United States Patent 4,288,760.
  2. J. Dabrowski and A. Sawicki, Patent application, filed April 2003.
  3. A. Sawicki, "A new class of asymmetrical directional couplers for power/antenna control applications," Microwave Journal, November 2005, No. 11, pp. 102-112.
  4. T. Emery, Y. Chin, H. Lee, and V.K. Tripathi, "Analysis and design of ideal non symmetrical coupled microstrip directional coupler," IEEE MTT-S International Microwave Symposium Digest, 1989, pp. 329-332.
  5. K. Sachse, "The scattering parameters and directional coupler analysis of characteristically terminated asymmetric coupled transmission lines in an inhomogeneous medium," IEEE Transactions on Microwave Theory & Techniques, 1990, Vol. 38, No. 4, pp. 417-425.
  6. A.R. Djordjevic, M.B. Bazdar, T.K. Sarkar, and R.F. Harrington, LINPAR for Windows: Matrix Parameters for Multiconductor Transmission Lines, Software and User's Manual, second edition, Artech House, Norwood, MA, 1999.