Symmetric Directional Coupler Research Articles

Comment on "Synthesis of Symmetrical Branch-Guide Directional Couplers" [Correspondence

This note considers the approximation problem encountered in the synthesis of branch-guide couplers. An alternate solution to that proposed by R. Levy and L. F. Lind is considered and shown to give generally inferior performance. It is inferred then, that an important design criterion is the requirement that the passband of the coupler contains the maximum number of frequencies of perfect match consistent with elements that are a quarter-wave-length long at midband.

Directional Couplers with Linear Variation of Coupling Coefficient

Coupling characteristics of asymmetrical and symmetrical directional couplers have been studied for linear variation of coupling coefficient. Comparison of the coupling characteristics for the exponential and linear variation of coupling coefficient has also been included.

The Design of Discrete N-Section and Continuously Tapered Symmetrical Microwave TEM Directional Couplers

A method is presented for the computer-aided design of either N-section discrete or continuously tapered symmetrical microwave couplers. The coupling distribution function k(x) is parametrized in the form k(x, p/spl ovbr/), and a special optimization process (of the generalized Remez type) is used to determine the set of parameters p/spl ovbr/ which produce an optimum power coupling response. Standard parametric forms based on an approximate Fourier analysis as well as more general spline parametric forms for k(x, p/spl ovbr/) are developed and illustrated.

Directional Couplers with Hyperbolic Variation of Coupling Coefficient

Symmetrical and asymmetrical directional couplers employing hyperbolic variation of coupling coefficients have been studied and their results compared with coupled uniform transmission line directional couplers.

Synthesis of Asymmetrical Branch-Guide Directional Coupler-Impedance Transformers (Correspondence)

In a recent paper by R. Levy and L.F. Lind [Synthesis of synunetrical branch-guide directional couplers, ibid., vol. MTT-16, pp. 80-89, February 1968] an exact synthesis procedure for the symmetrical branch-guide directional coupler was described. In this design it was assumed that all port terminating resistances were equal. An extension to this work has been found which allows one pair of terminating resistances to have a different value from the other pair. The resulting device is therefore both a branch-guide directional coupler and impedance transformer. This combination of properties may find application in antenna design work and other fields.

Synthesis of Symmetrical Branch-Guide Directional Couplers

A synthesis procedure is described for the design of branch-guide directional couplers which gives results showing a significant improvement over previous approximate methods. The synthesis technique adopted gives exact Butterworth characteristics and almost exact Chebyshev equal-ripple characteristics, the deviations in the latter case being so small that in most practical cases they may be neglected. The design of branch-guide couplers for bandwidths of greater than one octave is demonstrated. The design information for a large number of cases of practical interest is presented in tabular form, and experimental results for several branch-guide couplers constructed in waveguide and in stripline are in good agreement with the theory. The technique could prove valuable in the design of microminiature stripline hybrids and couplers.

Design Theory of Balanced Transistor Amplifiers

This paper discusses the expected characteristics of balanced transistor amplifiers with symmetrical directional couplers. Provided that pairs of transistors with similar characteristics can be selected from a given distribution, the input and output matches obtained with the balanced configuration are satisfactory over a ±10 per cent bandwidth with simple one-section lumped-constant LC directional couplers and over a ±40 per cent bandwidth (1.2 octaves) with one section distributed λ/4 couplers. For single-stage amplifiers, the decrease in gain is less than 0.1 db and the phase nonlinearities introduced by the couplers are about ±0.16° and ±0.6°, respectively, over the same bandwidths. The requirements on the terminations which are connected to the couplers to absorb the transistor reflections are not stringent: VSWR's less than 1.4 should be acceptable. The noise measure of balanced amplifiers is calculated to be a weighted average of the noise measures of the two component amplifiers, plus a small term which vanishes when the couplers have 3-db coupling and the component amplifiers have identical gains. Gain compression takes place at a 3-db higher signal level compared with conventional single-ended designs, and the expected improvement in the third-order intermodulation is 9 db on the average. In the final section, the cascade connection of identical balanced amplifiers is discussed. With typical microwave transistors, the input and output return losses for a multistage amplifier should be about 4.5 db worse than those for the individual single-stage amplifiers of which it is composed. The gain ripple introduced by the interactions between stages is also investigated in detail.

Synthesis of Symmetrical TEM-Mode Directional Couplers

Exact synthesis procedures are derived for symmetrical three-section and five-section TEM-mode directional couplers. These synthesis procedures are based on the equivalence between the theory of directional couplers and stepped quarter-wavelength filters as previously described by Levy and Young. Explicit formulas for the parameters of three-section couplers are presented. A realizable insertion-loss function is derived for the five-section coupler resulting in an equal-ripple response. Although this function has an equal-ripple characteristic, it is not expressible in terms of Chebyshev polynomials. Results obtained for the five-section coupler show considerable improvement in bandwidth over a three-section coupler. For example, a five-section coupler of -3 /spl plus mn/ 0.5 dB has a bandwidth of 9.6:1 as compared with 5.8:1 for a three-section coupler of -3 /spl plus mn/0.5 dB. A five-section coupler of -10 /spl plus mn/ 0.5 dB was designed on this theoretical basis for the 555-4000 Mc band, and the measured shows good agreement with the theoretical coupling sponse, yielding a minimum directivity of 18.0 dB at 3.7 Gc.

Reflection and Isolation of Symmetrical Lossless Directional Couplers

Reflection and Isolation of Symmetrical Lossless Directional Couplers

Recent advances in the use of coupled transmission lines as directional couplers

If two similar coaxial transmission lines are coupled by sharing a common outer conductor for part of their length, so as to form a 3-conductor line, they constitute a symmetrical directional coupler. In work on coaxial directional couplers at microwave frequencies up to 7 Gc/s, new designs of coupled transmission lines have been developed for use as power monitors and variable attenuators.

Symmetrical directional couplers

Symmetrical directional couplers

Coupled transmission lines as symmetrical directional couplers

If two similar unbalanced transmission lines are coupled together by sharing a common outer conductor for part of their length, so as to form a 3-conductor line, they constitute a symmetrical directional coupler. The output ratio of this varies sinusoidally with frequency, being greatest for lengths equal to an odd number of quarter-wavelengths; the characteristic impedance is independent of frequency, being determined by the cross-section of the 3-conductor line. Two forms of the directional coupler are described. One of these, intended for laboratory power measurement at frequencies up to 1 000Mc/s, consists of two parallel strips enclosed in a square-section outer conductor. The other, a very simple device for monitoring transmitter power, consists of two lead-covered coaxial cables grafted together. Among possible uses suggested are all-pass filters and applications to aerial systems consisting of two parts fed in phase quadrature.