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A New Class of Asymmetrical Directional

Couplers for Power/Antenna Control

Applications

A new class of asymmetrical directional couplers, convenient for power and antenna control

applications, is proposed. The couplers can be fully or partially integrated into a printed circuit board

(PCB). The couplers operate over -20 to -40 dB coupling...

Andrzej Sawicki, Ericsson AB

monitoring the transmitted and reflected powers. The requirements for such a coupler are of high importance:

very low insertion loss, very good matching of the main line (carrying the power), sufficient power handling and a

directivity better than 20 to 26 dB in both directions. Because the two coupled ports of the coupler are used

simultaneously, there is no possibility to “improve” the directivity by tuning one of the coupled line ports. A

modular approach is often implemented. This dictates that a high quality coupler would be placed separately,

outside the integrated units, and connected utilizing cables or special transitions. This, of course, adds to the

manufacturing cost as well as increases the number of steps in the process.

There are few known solutions allowing the partial integration of the coupler into the PCB. Four of them are

patented1-4 and another one is offered by Teppati and Ferrero.5 Most of them are of the coaxial-to-stripline or

coaxial-to-microstrip configurations, where the printed line is placed on top of the coaxial line. Persson4 uses a

inductive coupling coefficient

capacitive coupling coefficient

characteristic impedances of terminating lines

characteristic impedance of line i in the presence of line j (j = 1,2, j ? i)

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

i), respectively

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

The scope of this work was to identify a directional coupler structure having high power handling in the main line,

being fully or partially embedded into the PCB, achieving a coupling coefficient in the range –20 to –40 dB, and

being always theoretically compensated. Three examples of possible solutions are presented in Figure 1.8 The

tuning ground plane(s), towards the center conductor of the coaxial line at a distance s2.

Fig. 1 Three cross-sections of novel asymmetrical directional coupler structures.

There are many possible modifications of the coupler topology using the proposed concept. Other cross-section

shapes and locations of the main line, as well as modifications of the coupled line are possible. Generally, the

structure should contain the low loss main line (embedded into the PCB, or not), the coupled line (embedded

into the PCB), tuning ground plane(s) and the dielectric material of the PCB protruding past the edge of the

tuning plane(s), towards the main line. These last two features are necessary to assure theoretical compensation

of the coupler.

Areas of achievable coupling coefficients under the compensation conditions, dependence of the coupler

directivity on the deviation from these conditions and structural parameters for realizing a –30 dB coupler in a

Most of the calculations were carried out for the FR-4 Matsushita, halogen-free, substrate material. The results

of the calculations for the fully embedded structure are presented in Figure 2. It is clear that this structure can be

compensated in the range of coupling coefficients from –40 to –20 dB by changing the distance s1 between the

main line and the coupled line along with adjusting distance s2. The widths of the main (w2) and the coupled

(w1) lines vary from 5.5 to 5.4 mm, and from 1.5 to 0.5 mm, respectively.

Fig. 2 Inductive and capacitive coupling coefficients versus s2 for the fully embedded structure.

The results shown in Figure 3 were obtained for the structure using a coaxial main line and a stripline coupled line

with a modified outer ground enclosure. It is shown that even “pure air” and “pure 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. This feature distinguishes the asymmetrical coupled lines from