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A High Directivity Coupler Design Using an Inductive Compensation Technique
Abstract
This paper presents the technique to enhance a
directivity of a microstrip parallel coupler. The technique
increases the directivity by equalizing phase velocities with
inductive components at coupled and direct ports of the
coupler. Two configurations based on the technique are
proposed and their design equations are derived. The
practicability of the proposed technique is demonstrated with
both singly and doubly inductive compensation at 0.9GHz on
RF60-0600 substrate, whose improved directivity performance of such uncompensated microstrip parallel coupler about 6 and 33dB, respectively.
... Another approach adopts a second dielectric substrate covering a microstrip coupled line [2], resulting in the decreased inhomogeneity of the dielectric material and increased directivity at the cost of a more complex structure and implementation. Implementing phase compensation can also achieve high directivity [3][4][5][6][7][8]. In Ref. [3], two capacitors are implemented between the ends of coupled lines, adding to the odd-mode phase delay without affecting the even-mode phase delay. ...
... In Ref. [5], high directivity is achieved by loading shunt stubs on the centres of the lines but with increased circuit width. Series inductor loading at one or two ports can also introduce high directivity with increased implementation complexity [6]. High directivity can also be achieved by adding spurs at the ends of a parallel coupled line, adding to circuit complexity [7]. ...
... Step 3: The initial values of ε ee1 /ε eo1 and ε ee2 /ε eo2 in Figure 1a are set using Equation (3). A suitable m e is set to be larger than and close to 5, and m o is computed using Equation (6). Each section's even/odd electrical length is computed using Equation (5). ...
Traditional quarter‐wavelength parallel‐line couplers (QWPLC) in microstrip form suffer heavily in low directivity because of unbalanced even‐mode and odd‐mode phase velocities. Related schemes to improve directivity lead to complex structures and large dimensions. This study derives and validates an iterative synthesis method to design a high‐directivity microstrip coupler adopting a symmetric stepped‐impedance microstrip coupled line (SSIMCL). Given that the even‐mode and odd‐mode effective dielectric constants (EDCs) vary with the characteristic impedances, the authors take the EDCs of traditional QWPLC as the initial values for an SSIMCL, and the characteristic impedances and EDCs can be computed and converged in several iterations, resulting in good phase delay balance and high coupler directivity. A 20‐dB microstrip coupler working at 1 GHz with high directivity is designed, fabricated, and measured. The obtained maximum directivity is 47 dB and approximately 1 GHz, while the relative bandwidth with a directivity higher than 20 dB and a return loss better than 20 dB is larger than 70%. The novel microstrip coupler also features a compact, simple structure shorter than λg/4.
Techniques for compensating the unequal even- and odd-mode phase velocities encountered in parallel-coupled micro-strip are discussed. New results on the use of lumped and semi-lumped capacitors are presented, A newer geometry, a cross between suspended-substrate stripline and microstrip, can be used to manufacture quadrature couplers with improved, broadband directivity performance.
Typically loosely coupled backward wave microstrip couplers have directivities which decrease with increasing frequency. High directivity becomes more difficult to obtain as the coupling is loosened. The problem appears to be that the propagating velocities of the odd and even modes are not equal. This paper deals with a technique which equalizes these velocities up to an arbitrarily high frequency.
A construction technique is described that allows microstrip
parallel-coupled filters to have greater passband symmetry while largely
removing the parasitic passband at twice the center frequency. The
filter design extends the odd mode phase length by allowing the coupled
lines to overlap with lines outside the resonator proper, i.e. the
coupling gap of the resonator is extended out onto the 50-Ω lines.
The even-mode length has been set by the distance between the 50-Ω
lines. Thus, the reference plane for the even mode has been moved into
the resonator. This configuration makes the odd-mode length longer than
the even mode and thus compensates for the phase velocity difference
between modes
Terminal characteristic parameters for a uniform coupled-line four-port for the general case of an asymmetric, inhomogeneous system are derived in this paper. The parameters (impedance, admittance, etc.) are derived in terms of two independent modes that propagate in two uniformly coupled propagating systems. The four-port parameters derived are of the same form as those obtained for the symmetric case resulting in similar port equivalent circuits for various circuit configurations considered by Zysman and Johnson. The results obtained should be quite useful in designing asymmetric coupled-line circuits in an inhomogeneous medium for various known applications.
In this paper, two-port networks composed of two identical, coupled transmission lines embedded in an inhomogeneous dielectric (e.g., suspended substrate, microstrip) are investigated. The ABCD parameters of circuit configurations, considered by Jones and Bolljahn, are obtained for the case of inhomogeneous dielectilc. Equivalent circuits of these networks are also given. It is shown that the characteristics of such circuits differ markedly from those embedded in a homogeneous medium. In addition, experimental results are presented for three types of circuits which have been constructed and tested. There is excellent agreement between the experimental results and those predicted theoretically on the basis of the equivalent circuits.
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