US3657668A - Hybrid t-junction constructed in waveguide having a cut-off frequency above the operating frequency - Google Patents

Abstract

A hybrid T-junction constructed in waveguide for operation below its cut-off frequency. The series arm of the hybrid provides a parallel resonant circuit in series with the symmetrical arms of the hybrid. The shunt arm provides a series resonant circuit at the midpoint of the series arm. The equivalent circuit is a lumped circuit bridged-T, which exhibits the properties of balance and isolation associated with bridge networks.

Description

United States Patent Craven et al.

1 1 3,657,668 [4 1 Apr. 18, 1972 HYBRID T-JUNCTION CONSTRUCTED IN WAVEGUIDE HAVING A CUT-OFF FREQUENCY ABOVE THE OPERATING FREQUENCY Inventors: George Frederick Craven; Raymond Richard Thomas, both of Harlow, England Assignee: International Standard Electric Corporation, New York, N.Y.

Filed: May 25, 1970 Appl. No.: 40,074

Foreign Application Priority Data June 6, 1969 Great Britain..... ..28,698/69 U.S. Cl. ..333/1l, 333/73 R, 333/28 R Int. Cl. ..HOIp 5/12 Field of Search ..333/l1, 28 R, 98 BE ADJUS MBL E CA PA 6/ 7/ V5 TUNING SC/QE W References Cited UNITED STATES PATENTS 2,685,065 7/1954 Zaleski... 333/11 X Primary Examiner-Paul L. Gensler Attorney-C. Cornell Remsen, Jr., Walter J. Baum, Paul W. Hemminger, Charles L. Johnson, .lr., Philip M. Bolton, lsidore Togut, Edward Goldberg and Menotti J. Lombardi, J r.

[5 7] ABSTRACT A hybrid T-junction constructed in waveguide for operation below its cut-off frequency. The series arm of the hybrid provides a parallel resonant circuit in series with the symmetrical arms of the hybrid. The shunt arm provides a series resonant circuit at the midpoint of the series arm. The equivalent circuit is a lumped circuit bridged-T, which exhibits the properties of balance and isolation associated with bridge networks.

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A Home y HYBRID T-JUNCTION CONSTRUCTED IN WAVEGUIDE HAVING A CUT-OFF FREQUENCY ABOVE THE OPERATING FREQUENCY BACKGROUND OF THE INVENTION This invention relates to electrical waveguide arrangements, and particularly to an evanescent mode waveguide hybrid T- junction.

SUMMARY OF THE INVENTION According to the broadest aspectof the invention there is provided a waveguide hybrid T-junction constructed in waveguide having a cut-off frequency above the operating frequency, comprising a rectangular main waveguide forming first and second symmetrical arms of the junction dimensioned to have a cut-off frequency above the operating frequency, a first branch rectangular waveguide, dimensioned to have a cut-off frequency above the operating frequency, directly coupled into a first wall of said main waveguide forming a series arm to provide a parallel resonant circuit at the operating frequency and a second branch retangular waveguide dimensioned to have a cut-off frequency above the operating frequency, directly coupled to a second wall of said main waveguide forming a shunt arm to provide a series resonant circuit at the operating frequency at the midpoint of said series arm.

The invention will be described with reference to the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of an evanescent mode waveguide hybrid T-junction,

FIG. 2 is a basic schematic diagram of a known form of hybrid junction,

FIGS. 3 and 4 are equivalent circuits of the junction of FIG. 1

FIGS. 5 to 12 are equivalent circuits of alternative forms of evanescent mode resonators,

FIG. 13 is an equivalent circuit of the junction of FIG. 1,

FIGS. 14 and 15 are all-pass filter networks,

FIGS. 16 and 17 are equivalent circuits of single-section and multi-section phase equalizers respectively using an evanescent mode hybrid junction and junctions,

FIG. 18 is a perspective view of a multi-section phase equalizer,

FIG. 19 shows part of a known form of phase equalizer,

FIG. 20 is a perspective view of an evanescent mode phase equalizer,

FIGS. 21 and 22 are perspective views of alternative forms of an evanescent mode waveguide series (E) T-junction,

FIGS. 23 and 24 are perspective views of alternative forms of an evanescent mode waveguide shunt (H) T-junction,

FIGS. 25 and 26 are perspective views of alternative forms of an evanescent mode waveguide right angle (E) bend, and

FIGS. 27 and 28 are perspective views of alternative forms of an evanescent mode waveguide right angle (H) bend.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, a rectangular main waveguide 1 has a first branch rectangular waveguide 2 directly connected into one side wall, and a second branch rectangular waveguide 3 directly connected into one broad wall, the branch 3 being located transversely to the main guide 1 symmetrically about the broad wall center line of the branch 2.

Located on the broad wall longitudinal center line of the guide 1 are adjustable capacitive screws4 and 5, each at the mid-point of the length l of the respective portion of the guide 1 on either side of the branch junction and each extending into the respective portion of the guide.

There is an adjustable capacitive screw 6 located at the junction plane of the branch 2 with the guide 1 and extending into the branch 2.

Located in the. plane of the junction between the branch 3 and the guide 1 and completely filling the junction aperture is a thin dielectric plate 7 which constitutes a capacitive obstacle. An adjustable dielectric screw 8 moveable in or out of a corresponding threaded transverse aperture 9 in the plate 7 may be provided for tuning purposes.

Located on the broad wall longitudinal center line of the branch 3, at a distance 1 from the junction plane and the dielectric plate 7 is an adjustable capacitive screw 10 extending into the branch 3.

Located at the center of the junction, i.e. at the intersection point of the guide land the branch 2 center lines, is an adjustable capacitive screw 11.

The waveguide arrangement shown in FIG. 1 is designed to function as an evanescent mode hybrid T-junction, and accordingly all the guide is dimensioned to have a cut-off frequency above the required operating frequency.

As is well known, dominant mode waveguide ceases to propagate progressive waves below its cut-off frequency, and the mode is said to be evanescent. Waveguide in which the dominant mode is evanescent has a positive imaginary (inductive) characteristic impedance 2,) to an incident TE mode and a real propagation constant (7), and therefore behaves es sentially as a pure reactance. If a short section of this guide is terminated in an obstacle which presents a conjugate (capacitive) reactance at a frequency below the cut-off frequency, the incident power at that frequency will be completely transmitted through the section.

This full transfer of energy through evanescent waveguides is more fully described in Waveguide Bandpass Filters Using Evanescent Modes", G.F. Craven, Electronics Letters, Vol. 2 No. 7 July 1966, pp. 25-26, and in British Patent Specification No. 1,129,185.

It will be apparent, therefore, that in "the waveguide arrange ment shown in FIG; 1, full energy transfer through any one of the four arms of the junction is basically achieved, in the evanescent mode, by suitably adjusting the respective capacitive obstacle or obstacles associated with thatparticular arm to obtain the required conjugate match condition.

However, for the arrangement to function as a hybrid T- junction, the necessary requirements of balance and matching must also be met.

How this is achieved in the present embodiment is most readily described by first considering certain basic aspects of a known form of hybrid junction constructed in propagating guide.

The basic requirement for a four-port matched hybrid junction is a type of lattice (bridge) network which in its more restricted form consists of a bridged-T network. One example of a network with these properties, realized in propagating guide, is a resonant slot hybrid junction which has been fully described in Resonant-slot hybrid junctions and channel dropping filters G.F. Craven, D.W. St-opp and RR. Thomas, Proc. I.E.E., Vol. 112, No. 4, Apr. 1965, pp. 669-680, and British Patent specifications 987,593 and 1,053,071.

In this resonant slot hybrid junction two of the conjugate arms are provided by a length of main rectangular waveguide, and each of the other two arms is provided by a branch waveguide joined to a broad wall of the main waveguide. There is a transverse resonant slot in the main waveguide providing coupling between the first mentioned two arms and one of the other arms, and a longitudinal resonant slot in the main waveguide providing coupling to the remaining other arm.

This can be represented as shown in FIG. 2, with the main rectangular propagating waveguide length 20 having a transverse (series) resonant slot 21 and'a longitudinal (shunt) resonant slot 22. The two branch guides, one to each slot, are not shown.

The equivalent circuit of this configuration is shown in FIG. 3, with its reduction to the bridged-T of FIG. 4.

I The shunt and series slots are in the same reference plane, and the transverse (series) slot 21 couples only to the transverse component of the magnetic field; the longitudinal (shunt) slot 22 coupled only to the longitudinal component of the magnetic field. As a result of the symmetry associated with the configuration, the two slots do not couple to each other. Although the two slots are identical parallel resonant slots, the longitudinal slot 22 appears at its reference plane in the main guide as a series resonant circuit.

The explanation of this is that the phase of the longitudinal component of the field lags the transverse component by 90 Thus the equivalent circuit of FIG. 3 shows an equivalent quarter-wave line (M4) which inverts the parallel resonant circuit to a series resonant circuit in the main guide.

The necessary conditions for matching the junction are derived from inspection of FIG. 3. The impedance connected to the series transformer terminals is 22 so that for a match.

Similarly the impedance connected across the shunt transformer terminals is Z /2 and for a match 01/ o1 n3' ip' 2n For operation of the arrangement shown in FIG. 1 as an evanescent mode hybrid T-junction, it will later be shown that the branch guide 2 functions as the shunt arm and that the guide 3 functions as the series arm.

There is symmetry between the shunt arm 2 and the series arm 3 of the junction and therefore isolation exists between these arms. However a propagating mode does not exist in the guide (by definition) and as a result the two fields exciting the series and shunt arms are in phase. Thus no impedance inversion of the shunt network occurs. The problem of realizing the necessary dual elements for the respective series and shunt arms can be resolved in the following way.

Two types of evanescent mode resonator are possible 11' section and T section. The classification follows from either a capacitance at each end of a length I of evanescent guide (11 section), or one capacitance in the center of a length l of evanescent guide (T section). These two types are shown in FIGS. 5 and 6 respectively, and their equivalent circuits in FIGS. 7 and 8.

These equivalent circuits can be shown in a different way involving the concept of impedance inverters. This concept is in itself well known, being described for example in Transmission Networks and Wave Filters" T.E. Shea, p. 329, and involves ideal transformers using negative elements (which in practice are absorbed in a positive element) to provide networks which are essentially broadband impedance inverting networks.

FIGS. 9 and show the circuits of FIGS. 7 and 8 respectively redrawn to include such a network, the impedance inverting networks being enclosed in solid rectangles.

In FIGS. 9 and 10 the resonant elements are effectively connected together by equivalent quarter-wave lines. Thus the 11' section resonator appears as shown in FIG. 11, and this is the type of circuit (parallel resonant) required for the series arm 3 It can be demonstrated that the image impedance, Z,, of the network enclosed in the rectangle of FIG. 9 is given by Z, Z sinh yl and the phase constant of the network is given by It will be seen that the network is equivalent to a transmission line with a resistive characteristic impedance (which varies with frequency) but which is a quarter wave long at all frequencies.

The T section resonator appears as shown in FIG. 12, and seen from its input terminals behaves as a series resonator. This is the type of circuit required for the shunt arm 2. A second characteristic of the series resonator of FIG. 12 is the shunt element (shown within the dashed outline rectangle) which is the remnant of the original section of FIG. 10 when it is modified to include the impedance inverter at the input terminals.

With a load resistance of R, the equivalent source impedance consists of the load resistance and the remnant, which is an inductive reactance Z tanh in parallel.

The equivalent source impedance, 2,, expressed in series form is then I This shows that the equivalent series source impedance is reduced by the inductive reactance of the shunt element. If this reactance is controlled by a shunt capacitance, the source resistance can be varied and matched to the load impedance.

Returning now to FIG. I, it has been demonstrated that for the arrangement to function as a hybrid T-junction, the required series resonant circuit at the mid-point of the series arm 3 is realized by the shunt arm 2, and the required parallel resonant circuit is realized by the series arm 3, in series with the symmetrical arms 1.

FIG. 13 shows the equivalent circuit of the evanescent mode hybrid T-junction as a bridged-T network, analogous to the bridged-T network shown in FIG. 4, but representing the impedance of the main waveguide 1 as functions of the inductive characteristic impedance (2 of evanescent waveguide, y, and the length I, of each portion of the guide 1 on each side of the junction.

The matching parameters of the series arm 3 are the tuning of the parallel resonant circuit (by screw 8 and/or screw 10) at the input terminals to control the reactive component, and the suitable selection of the length 1 to control the impedance transformation.

The matching parameters of the shunt arm 2 are similar. The capacitive screw 6 is tuned to control the reactive component of the series resonant circuit, and the length 1 is selected to control the impedance transformation. Minor fine adjustment can be made by the screw 11 to provide a variable source impedance to the shunt arm as already described.

The screws 4 and 5 are tuned to give the earlier mentioned conjugate match condition for full energy transfer through the respective portions.

It will be appreciated that each of the main waveguide portions I, instead of containing a single centrally located capacitive screw, may each contain two spaced capacitive obstacles, one at each end of the length 1,.

Although capacitive screws have been described for obtaining the necessary capacitive reactances, it is to be understood that any suitable means of obtaining the required capacitive reactances may be used.

The characteristics of an all-pass filter or all-pass network are illustrated in the lattice or bridge network of FIG. 14 and FIG. 15. Such a network transmits energy from zero frequency to infinite frequency, and with correct choice of component values no amplitude change occurs. The phase of the input to output voltage is, however, a function of frequency, as is evident from the extreme conditions; at zero frequency cur rent takes the ADCB path, whereas at infinite frequency current takes the ACDB path. Thus the network has a maximum phase change of All-pass filters of this type are used to correct the phase characteristics of conventional low-pass filters. By taking a further step, FIG. 16, to introduce series and parallel resonant circuits, the network can be used as a correction network for bandpass filters. This general lattice structure can be reduced to a bridged-T network, and therefore the structure of FIG. 4, and FIG. 13, is applicable to all-pass network applications.

In this application a single evanescent mode waveguide hybrid T-junction as an all-pass phase equalizer network could appear as shown in FIG. 16. The basic conditions that must be satisfied by the network for it to be reflectionless have been derived in the paper Resonant slot hybrid junctions and channel dropping filters already detailed. The series and parallel resonant circuits must have equal loaded Q-factors. Thus if Qp Q:

Q, 2wL,/R where R source impedance and The realization of the equivalent resonant circuits with evanescent mode resonators has already been described. The basic design variable is the cavity length I. Z, Z sinh -yl) is used as a variable and the L/C ratio treated as a constant.

A multi-section phase-equalizer is shown in FIG. 17, and is realized either by directly coupling together in cascade the main waveguide portions of a corresponding number of hybrid junctions each as shown in FIG. 1, or by an integral structure as shown in FIG. 18, in which there is an effective merging of output-input main arms of successive hybrid junctions into a single common arm, and like references to FIG. 1 have been used.

In either the directly coupled arrangement of individual junctions or the integral structure, the input for phase-correction is applied to one end main guide 1A and the corrected output derived from the other end ID of the main guide, and each series and shunt arm 2 and 3 is terminated by a short circuit (not shown).

The phase equalization of bandpass filters using hybrid junctions or their equivalents has been described for example in Development of group-delay equalizers for 4 Gc/s, D. Merlo, Proc. I.E.E., Feb., 1965.

In propagating guide the equalizer is realized with two identical bandpass filters 30 each in the two shortcircuited (31) arms in the way shown in FIG. 19. In evanescent guide the equivalent quarter wave shift can be realized as shown in FIG. 20 by having one main arm 1 of the hybrid junction comprising a series resonant ('l" section) cavity containing a single central capacitive screw 5, and the other main arm 1 comprising a parallel resonant (11' section) cavity containing a capacitive screw 4 located at the end of the section remote from the junction and a dielectric plate 40, which may have an adjustable dielectric screw 41, filling the guide section at the junction plane between the end of the section and the broad wall of the series arm 3.

Each of the main arms then couple into identical multi-section bandpass filters (not shown) terminated by a short circuit. Input is the shunt arm 2, and output the series arm 3.

Thus in practical realization the bandpass filters integrate into the hybrid junction assembly.

Tjunctions both series (E) and shunt (H) are necessary components in many systems. Either type of junction can readily be realized in evanescent guide because either junction is merely a special case of an evanescent mode hybrid junction in which one of the isolated arms (shunt or series) is short circuited at its junction with the other arms.

In the same way, a right angled bend can be considered as a special case of a T-junction with one of its arms short cir-' cuited.

FIGS. 21 and 22 show alternative forms of an evanescent mode series (E) waveguide T-junction, with, in FIG. 21, three T-section evanescent mode resonators, each of length 1, between capacitive screw 4 and dielectric plate 7, between capacitive screw 5 and dielectric plate 7, and in the series arm 3 between dielectric plate 7 and capacitive screw 10.

In FIG. 22 there are three 1r section evanescent mode resonators, each of half length 1/2, provided respectively between dielectric plates 40A, 40B and 7.

FIGS. 23 and 24 show alternative forms of an evanescent mode shunt (H) waveguide T-junction. In FIG. 23 there are three T-section evanescent mode resonators, each of length 1, between capacitive screws 4 and 11, between capacitive screws 5 and 11, and in the shunt arm between capacitive screws 6 and 11.

In FIG. 24 there are three 11' section evanescent mode resonators, each of half length 1/2, formed respectively between dielectric plates 40A, 40B and 6.

FIGS. 25 and 2 show alternative forms of an evanescent mode waveguide right angle (H) band. In FIG. 25 there are two 71' section evanescent mode resonators each of half length l/2 formed respectively between dielectric plate 40A and 7. In FIG. 26 there are two T-section evanescent mode resonators, each of length 1, formed between capacitive screws l0, l1 and 4 FIGS. 27 and 28 show alternative forms of an evanescent mode right angle (E) band. In FIG. 27 there are two 11' section evanescent mode resonators, each of half length 1/2, formed respectively between dielectric plates 40 and 6.

In FIG. 28 there are two T-section evanescent mode resonators, each of length 1, formed between capacitive screws 4, 6 and l l.

I claim:

1. A waveguide hybrid T-junction constructed in waveguide having a cut-off frequency above the operating frequency, comprising:

a rectangular main waveguide forming first and second symmetrical arms of the junction dimensioned to have a cutoff frequency above the operating frequency;

a first branch rectangular waveguide dimensioned to have a cut-off frequency above the operating frequency, directly coupled into a first wall of said main waveguide forming a series arm to provide a parallel resonant circuit at the operating frequency;

a second branch rectangular waveguide, dimensioned to have a cutoff frequency above the operating frequency, directly coupled to a second wall of said main waveguide forming a shunt arm to provide a series resonant circuit at the operating frequency at the midpoint of said series arm;

a first capacitive obstacle located at the center of the length of said first symmetrical arm;

a second capacitive obstacle located at the center of the length of said second symmetrical arm;

two capacitive obstacles each located at opposite ends of said series arm; and

a third capacitive obstacle located at the junction of said first and second symmetrical arms.

Claims (1)

1. A waveguide hybrid T-junction constructed in waveguide having a cut-off frequency above the operating frequency, comprising: a rectangular main waveguide forming first and second symmetrical arms of the junction dimensioned to have a cut-off frequency above the operating frequency; a first branch rectangular waveguide dimensioned to have a cutoff frequency above the operating frequency, directly coupled into a first wall of said main waveguide forming a series arm to provide a parallel resonant circuit at the operating frequency; a second branch rectangular waveguide, dimensioned to have a cut-off frequency above the operating frequency, directly coupled to a second wall of said main waveguide forming a shunt arm to provide a series resonant circuit at the operating frequency at the midpoint of said series arm; a first capacitive obstacle located at the center of the length of said first symmetrical arm; a second capacitive obstacle located at the center of the length of said second symmetrical arm; two capacitive obstacles each located at opposite ends of said series arm; and a third capacitive obstacle located at the junction of said shunt arms and said first and second symmetrical arms.

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