US2848689A - Matching device for microwave shunt tee - Google Patents

Description

1958 J. F. ZALESKI 2,348,689

MATCHING DEVICE FOR MICROWAVE SHUNT TEE Filed Feb. 28. 1.955

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, ji 4 Jo A 244 5 /0 MATQHING DEVICE FOR MKCROWAVE SHUNT TEE John F. Zaleski, Valhalla, N. Y., assignor to General Precision Laboratory Incorporated, a corporation of New York Application February 28, 1955, Serial No. 490,802

3 (Ilaims. (Cl. 333-9) This invention relates to microwave matched shunt tee guide components and particularly to such components having wide band characteristics.

In the use of microwave guides employing the dominant T35 mode of transmission such as rectangular and square hollow guides, dielectric-filled guides, round hollow and dielectric-filled guides, one useful component is the shunt tee having a rectangular or square shunt or side arm. This type of tee is also termed the l-I-plane tee since the plane of the tee is parallel to the plane of the field magnetic component in all arms.

Shunt tees normally have three arms, one being the side arm and the other two being in a straight line or collinear and perpendicular to the side arm. Normally microwave energy is applied to the side arm and taken out of the other two arms in equal amounts. However, other forms of so-called shunt tees exist such as the folded shunt tee having all three arms parallel in the same plane, and under special conditions in all shunt tees the side arm may be the sole output arm.

In the design of shunt tees one of the desirable properties is that of transmitting a wide band of microwave frequencies with small loss. To accomplish this the three tee arms are impedance matched to their connecting components and the tee is internally matched to reduce the several kinds of internal losses. These several kinds of loss include the loss due to impedance mismatch between the two output arm impedances seen in parallel by the input arm, and the characteristic impedance of the input arm. When all three arms are made of the same type or" guide, for example, this impedance mismatch ratio is 2: 1. Another kind of loss is due to the geometrical discontinuity presented by the tee to shunt arm input energy. Both kinds of loss result in energy reflections toward the source which are ordinarily detected by the standing waves which they generate, and which are measured by measuring the voltage standing wave ratio (VSWR) of these waves.

It is no problem to match out practically all reflection at any single selected frequency. For example, a thin post may be so positioned at the junction of the collinear arms, joining their broad faces and intersecting the shunt arm axis as to produce such a match. It is well recognized that the design problem is to produce a tolerable match over a band of frequencies. Heretofore, representative solutions of this problem have produced shunt rectan ular hollow guide tees matched well enough to reduce the VSWR to 1.10 over a band of 10% of the bands central frequency.

The present invention discloses matching components which match a rectangular hollow guide shunt tee over a 17% hand to a VSWR of 1.025, and over to a V SWR of 1.1. This is so different from previous practice as to make the tee losses and bandwidth more comparable to those of a straight run of rectangular hollow guide rather than to the losses and bandwidth of other shunt tees. For example, a straight run of rectangular guide is limited, by cutoff proximity and by generation of higher modes, to a bandwidth of 40%, comparable to the 25% bandwidth of the tee of the invention. Also, the absorption attenuation of a straight run of a representative rectangular hollow guide is 0.036 db per ft., which is even higher than the reflection loss of the instant tee over the greater part of its 25% bandwidth. in this sense the improvement provided by the instant invention in shunt tees over existing forms may be said to be qualitative.

The invention consists essentially of provision in the collinear arms of a shunt tee of a transverse septum and of provision at the junction of the shunt arm of an inductive iris. This combination is shown by theory and experiment to be wide band.

The general purpose of this invention is to provide a microwave guide tee having a TE mode shunt arm with improved efficiency over a frequency band of improved width.

More specifically, a purpose of this invention is to provide improved internal matching components for microwave rectangular guide shunt tees operating in the TB mode.

A further understanding of this invention may be secured from the detailed description and associated drawings, in which:

Figure 1 is a longitudinally sectioned view of a rectangular hollow guide microwave shunt tee of conventional external form matched in accordance with the invention.

Figure 2 is a cross section view taken on the line 22 of Fig. 1.

Figure 3 is a cross section view taken on the line 33 of Fig. 1.

Figure 4 is a frequency vs. VSWR characteristic curve of a shunt tee embodying the invention.

Figure 5 is a longitudinally sectioned view of a folded shunt tee embodying the matching devices of the invention.

Figure 6 is a cross section view taken on the line 66 of Fig. 5.

Figure 7 is a cross section view taken on the line 7-7 of Fig. 5.

Referring now to Fig. 1, a shunt tee has its two collinear arms 11 and 12 and its shunt or side arm 13 all made of the widely used hollow rectangular guide. All three arms are made of the same size of conductive guide having an interior cross-sectional larger dimension a, interior cross-sectional smaller dimension b, and wall thickness t, as shown in Figs. 2 and 3. The shunt arm 13 joins the collinear arms 11 and 12 at right angles and the lower and upper broad sides of all these arms are in two respective continuous parallel planes.

One of the two matching devices of this invention consists of a conductive septum 14 conductively secured at right angles to the continuous narrow side 1l6 of the collinear arms 11 and 12. This septum is positioned 50 as to comprehend the longitudinal axial plane of the shunt arm intermediate of its cross-sectional greater dimension and perpendicular to both broad sides. The septum 14 is conductively secured along its length 0 to the broad faces 17 and 18, Fig. 3, of the collinear arms 11 and 12 at their juncture. The septum thickness d and the dimension e, which is equal to the difference a minus c, are significant matching dimensions.

The other matching device consists of an aperture or window placed in the side l91' of the collinear Wave guide arms. This aperture is at the junction of these arms, so that it forms the mouth or opening of the shunt arm 13. The aperture is smaller than the internal cross section of the guide, so that it restricts the mouth or opening of shunt arm 13 through the collinear side 1919'. The restriction is effected by two concomponent sometimes used consisting of an inductive iris placed further back in the shunt am: away from the junction, usually one-quarter'wavelength in guide from the junction of the shunt arm with wall 119'. Such an iris is a completely conventional matching expedient and produces a highly frequency-sensitive match. That is, the tee becomes very sharply tuned and narrow-band. In fact, such an iris, if used,would completely destroy the value of this invention and prevent its principal objective, broadband operation.

Since energy is fed into each ofthe collinear arms through the oblique window formed by the edge of a vane and the end of the septum, the dimensions of these windows may be considered more significant than the dimensions d, e and g from which they are derived.

In order to understand how the septum and iris match the tee let it be assumed that microwave energy in the X-band, having a frequency f and transmitted in the TE mode, is applied to the shunt arm, entering its end 24. in the absence of any internal matching devices this energy will enter the collinear arms perpendicular to the common narrow wall 16-16 thereof, from which it will be reflected back into the shunt arm and in general will be reflected back to the source, setting up a standing wave in the shunt arm which can be taken as a measure of the loss. This type of loss'is occasioned by what may be termed the geometrical discontinuity 'of the tee. It produces a reflected wave which at the junction of the shunt arm with the collinear arms, is in general out of phase with the applied. energy. If the distance from the junction of the shunt arm to the wall 16-16 and return be equal to in which A is the wavelength in guide of the microwave energy having the frequency 1, then, adding the 180- phase reversal at the metallic surface, the reflected energy meets the incoming energy in phase and if the amplitudes are equal, no standing wave is set up inthe shunt arm. This situation calls for a distance from the shunt arm junction to the reflecting surface of 7 If the distance should have any other value, standing waves will be generated, maximum standing waves being generated when the distance to the reflecting surface is When the septum 14 is inserted, reflection no longer occurs at the wall 16-16, but occurs at a plane parallel to wall 1616 at the end 26 of the septum. Therefore to eliminate all effects of the geometrical discontinuity it is simply necessary to make the distance e, Fig. 1, equal to 4 This theoretical dimension is, however, somewhat modified by other considerations, as will be discussed subsequently.

Reflection from the edge 26 of septum 14 may in one aspect be considered to occur, rather than from the wall 1616', because in a narrow sense the two spaces on either side of the septum behave as if they were guides significant dimension.

4 dimensioned below cutoff. for the reflected energy in the TE mode.

The septum 14 behaves in matching in a different manner from the behavior of a thin post connecting the guide broad sides. Since such a thin post at optimum position will not include the point at which the guide axes intersect, violent local waves will be set up in the vicinity of the post. When the septum 14 is employed its plane includes the axial intersection point and the field discontinuity which would cause the. violent local waves is thereby compensated at its source without producing surrounding field disturbance.

The three arms of the tee, being alike, all have the same characteristic impedance and, since the .side arm is of the shunt type, the incoming energy sees the two collinear arm output impedances in shunt. It is therefore necessary, in order to match impedances, either to increase the collinear arm impedances or to reduce the shunt arm impedance. The latter method is chosen and the shunt arm impedance is reduced by the employment of an iris in the shunt arm where it joins the collinear arms. This iris is composed of the two similar vanes 21 and 21, Figs. 1 and 2, havingthe window opening g and having the vane surfaces 23 and 23 coplanar with walls 19 and 19. The impedance of such an iris is almost pure shunt inductance, the value being :2 21 X Mtan 2a This value for the inductance is somewhat modified by the thickness h of the iris vanes, which reduces the inductance as a direct function of h.

All of the four significant matching dimensions g, e, h and d, Fig. l are interdependent, the dimensions g and e being perhaps somewhat more critical than dimensions h and d. The presence of septum 14 thus has a constricting effect on the collinear arm cross sections as all energy to the collinear arms must pass between vane 21 and the septum end 26, or between vane 21 and end 26. Therefore the presence of the septum requires the iris dimensions to be modified somewhat to match the modified collinear arm input impedances. By changing the thickness d of the septum this influence of the septum on the iris is changed without notably affecting the primary geometrical discontinuity matching function of the septum, and thus the septum dimension d becomes a The presence of the iris has an effect upon the operation of the septum because the iris modifies the phase of the input energy at the junction of the shunt arm, thereby changing the phase which the energy reflected from the septum is required to match.

As is well understood in the art, a microwave component such as a shunt tee is reversible in a special sense. If in the described shunt tee energy inputs be applied to both collinear arms 11 and 12 and the output taken from the shunt arm 13, and if the input energies be exactly equal in amplitude and have exact phase equality at the junction, then the described matching devices will operate satisfactorily for this direction of energy flow.

As a numerical example of the shunt tee embodiment described in connection with Figs. 1, 2 and 3, using a present standard size of hollow rectangular guide having the dimensions a=0.900 inch, 12:0.400 inch and t=.050 inch, the following dimensions of the matching devices were found suitable:

0:0.466 inch d=0.015 inch e=0.434 inch g=0.8l5 inch h=0.050 inch A shunt tee having these dimensions was found to have the frequency vs. standing wave ratio characteristic of Fig. 4, extraordinary in both smallness of VSWR and in arm the wideness of the band of frequencies over which the VSWR is low.

The extremely wide frequency band pass characteristic of the shunt tee is explained by an analysis of its operation. The wavelength in a selected guide, k is inversely proportional to the frequency, or

Therefore, if the shunt tee is well matched at a selected frequency, F so that the distance e is closely related to then at a second selected higher frequency, f the new wavelength in guide N will be less than A That is, the distance e is now larger than it should be for perfect matching, disregarding other factors, introducing an error similar to that introduced by inductive reactance. At the same time, in accordance with Equation 1 the inductive reactance of the iris is increased. These phase changes of the septum and of the iris are equal and remain equal over a very wide band of frequencies, so that over this entire band the match remains excellent.

A second embodiment depicted in Figs. 5, 6 and 7 is radically difierent from the shunt tee of Figs. 1, 2 and 3 in physical appearance, but is similar in its principles of operation and in its characteristics. In this case the two output arms 27 and 28 are laid side by side and are parallel to each other and to the input arm, with one narrow side 29 common to both output arms. This narrow side 29 is terminated at the distance e from the junction of the input arm 31, so that the end 32 of the common Wall together with the wall behind it performs the function of the septum 14 of Fig. 1. Furthermore energy is coupled into each output arm through an oblique aperture such as i, Fig. 5, which cuts across the corner of the output arm 27 and is like the aperture between vane 21 and septum end 26, Fig. 1, which cuts across the corner of collinear arm 11. In both cases energy is applied through the oblique aperture to excite the output arm. The input arm 31 is provided with an iris having two identical vanes 33 and 34 at the junction of this arm with the two output arms, similar to the iris described inconnection with Fig. l. The iris window and thickness, together with the dimension 2 and the thickness of wall 29, are the significant matching dimensions of the folded shunt tee of Fig. 5, and the same rules of design are employed as were described in connection with the embodiment of Fig. 1.

What is claimed is:

l. A broadband microwave guide shunt tee comprising, a microwave rectangular guide shunt arm, a pair of similar collinear guide arms joined to said shunt arm at right angles, a conductive septum parallel to the electric field across said pair of arms comprehending the longitudinal axial electric plane of said shunt arm, and an inductive iris in said shunt arm at its junction with said collinear arms.

2. A broadband efficient microwave guide shunt tee comprising, a microwave rectangular guide shunt arm, a pair of microwave rectangular guide collinear arms joined to said shunt arm at right angles, a conductive septum in the electric field extending transversely part way across said pair of collinear arms and comprehending the longitudinal axial electric field plane of said shunt arm, the septum being joinedat one end to one common narrow side of said collinear arms and terminating at its other end distant from said shunt arm by one quarter of the wavelength in guide of the applied microwave energy, and an inductive iris in said shunt arm at its junction with said collinear arms.

3. A tee in accordance with claim 2 in which said iris consists of two similar vanes secured at right angles to the two short sides of said shunt arm with the edges of the vanes parallel to said short sides.

References Cited in the file of this patent UNITED STATES PATENTS 2,432,093 Fox Dec. 9, 1947 2,704,351 Dicke Nov. 15, 1955 2,759,154 Smith Aug. 14, 1956 FOREIGN PATENTS Marcuvitz, Waveguide Handbook, vol. 10, M. I. T. Radiation Lab. Series, McGraw-Hill, 1951, pages 360-62.

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