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 (k_L) and capacitive (k_C) coupling coefficients are made exactly equal, i.e.,

where:

and

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

and

where:

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

L_m and C_m = the mutual inductance and mutual capacitance per unit length, respectively.