US3508175A - Contiguous slotted and unslotted waveguide portions having substantially the same characteristic impedance - Google Patents

Description

Aprll 21, 1970 3,508,175 CONTIGUOUS SLOTTED AND EGUIDE PORTIONS HAVING SUBSTANTIA HE SAME CHARACT c IMPEDANCE Filed May 5. 1967 A. ALFORD UNSLOTTED WAV INVENTOR ANDREW ALFORD l5 Y' ATTZRNEYS United States Patent O M 3,508,175 CONTIGUOUS SLOTTED AND UNSLOTTED WAVE- GUIDE PORTIONS HAVING SUBSTANTIALLY THE SAME CHARA'CTERISTIC IMPEDANCE Andrew Alford, 120 Cross St., Winchester, Mass. 01890 Filed May 5, 1967, Ser. No. 636,478 Int. Cl. H03h 7/38; G01r 27/06 US. Cl. 333-33 7 Claims ABSTRACT OF THE DISCLOSURE A slotted waveguide portion contiguous with an unslotted waveguide portion are arranged so that both have substantially the same characteristic wave impedance and propagation characteristics by making the slotted portion a little bit wider and/or a little bit shorter than the unslotted portion.

BACKGROUND OF THE INVENTION The present invention relates in general to contiguous slotted and unslotted portions having substantially the same characteristic impedance and wave propagation characteristics especially useful for making slotted waveguide measurements.

Slotted waveguides are widely used for making measurements at microwave and higher frequencies. A probe penetrating through the slot normally senses an indication of the electric field strength at points along the length of the slotted waveguide to provide indications of the VSWR in the slotted waveguide as a result of devices connected to the ends of the slotted waveguide. Although such slotted waveguides are useful for many applications, prior art devices themselves introduce enough ofa mismatch to seriously limit the accuracy of the measurements being made. These problems are especially serious at the extremely short wavelengths when the width of the slot becomes comparable to a dimension of the waveguide.

It is an important object of this invention to provide methods and means for enhancing the accuracy of slotted waveguide measurements.

It is a further object of the invention to achieve the preceding object with a slotted waveguide having a{ slotted portion that has its wave impedance and propagation characteristics matched to that of the unslotted portion.

It is still a further object of the invention to achieve the preceding objects over a relatively wide frequency range for the TE mode in rectangular waveguide.

It is still another object of the invention to achieve the preceding objects with relatively little additional physical apparatus capable of being reproducible when making slotted waveguides in production quantity.

It is still a further object of the invention to achieve the preceding objects with fixed structure that does not require adjustment from slotted waveguide to slotted waveguide.

SUMMARY OF THE INVENTION According to the invention, there is a waveguide having a slotted portion with a longitudinal slot formed in the waveguide wall intercoupled with an unslotted portion with the cross sectional area of the slotted portion slightly changed from that of the unslotted portion so that the impedance and propagation characteristics of both the slotted and unslotted portions are substantially the same.

Numerous other features, objects and advantages of the invention will become apparent from the following specification when read in connection with the accompanying drawing in which:

3,508,175 Patented Apr. 21, 1970 FIG. 1 is a perspective view of one embodiment of the invention in which a slotted portion of the waveguide is formed with its width slightly greater than that of the unslotted portion to effect the desired wide band impedance and propagation characteristics match;

FIG. 2 is a perspective view of another embodiment of the invention in which the slotted portion of the waveguide is slightly shorter than the unslotted portion to effect the desired wideband impedance and propagation characteristics match; and

FIG. 3 is a perspective view of still another embodiment of the invention embodying a stepped transition portion to elfect the desired wide band impedance and propagation characteristics match.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference now to the drawing and more particularly FIG. 1 thereof, there is shown a perspective view of an embodiment of the invention in which the slotted portion of the waveguide 11 is slightly greater than the unslotted portion 12. The width of the unslotted portion is a and that of the slotted portion a plus Aa while the height of both waveguides is b. The slot width is d. The increase in width Aa is selected to be just enough so that the impedance and propagation characteristics in both slotted portion 11 and unslotted portion 12 are substantially the same. The manner of selecting this difference will now be described.

A waveguide slotted line, if made of the same size as the unslotted waveguide in which measurements are to be made, has a higher characteristic impedance than that of the unslotted waveguide. Let the unslotted waveguide impedance be Z and let the waveguide impedance of the slotted waveguide section (provided with a longitudinal central slot through one of the wider walls) be Z -j-AZ In general, the characteristic impedance of a waveguide is given by Equation 1.

where:

a=the internal width of the waveguide,

b=the internal height of the waveguide,

K=a constant, 7

f =the cutoff frequency of the TE mode, and f=operating frequency.

Since Aj=c where:

a=the velocity of light.

We may substitute the value of A in terms of 1 into Equation 2 and thus obtain the relationship between L, and a in which a=c/2j (4a) From the above it follows that we can make up for the increase in f by M by increasing dimension a by no as given by Equation 5 Aa/a=]cAf /2f Dividing Equation 5 by Equation 4a we obtain Aa/a A When we wish to decrease Z by AZ, We may do this by decreasing the waveguide height b. The relationship be tween AZ, and Ab can be obtained from Equation 1 as Referring to FIG. 2, there is shown an alternate embodiment of the invention in which the height of the slotted portion is reduced by Ab as explained above to establish the characteristic impedance and propagation characteristics of the slotted portion 13 and unslotted portion 14 substantially the same.

The dimensions Aa and Ab can be computed with the guidance of the equations set forth above and the dimensions trimmed experimentally to optimize the matching of impedance and propagation characteristics. Alternatively, there exists an essentially experimental way for determining the parameters Aa and Ab. First, measure the cutoff frequency and impedance in an unslotted waveguide. Then cut a slot in this waveguide of the desired width d and again measure the cutoff frequency and impedance. The first cutoff frequency and impedance measurements are the parameters f and Z respectively, and the differences between each first measurement and the corresponding second measurement are AL, and A2 respectively, Aa and Ab may then be determined from Equation a and Equation 6. It may be desirable to effect part of the compensation by changing the height and part of the compensation by changing the width. This would involve an alteration of the cross sectional area of the slotted portion with respect to the unslotted portion so that the impedance and propagaion characteristics of the two contiguous portions remain substantially the same.

Referring to FIG. 3 there is shown still another embodiment of the invention in which the slotted portion 15 is coupled to the unslotted portion 16 by means including a transition slotted portion 17. The specific embodiment illustrated in FIG. 3 includes both height, width and slot width compensation. Thus, the heights of unslotted portion 16, transition portion 17 and slotted portion 15 are respectively b, bAb and bAb The width of unslotted portion 16, transistion portion 17 and slotted portion 15 are respectively a, a+Aa and a+Aa The width of the slot in transition portion 17 is b and that of slotted portion 15 is d and d being greater than al Preferably, the electrical length of transition portion 17 is an odd multiple of quarter guide wavelengths at the center of the frequency at the desired operating band. The principles set forth above are applicable to choosing the different dimensions.

It is evident that those skilled in the art may now make numerous uses and modifications of and departures from the specific embodiments described herein without departing from the inventive concepts. Consequently, the invention is to be construed as embracing each and every novel feature and novel combination of features present in or possessed by the apparatus and techniques herein disclosed and limited solely by the spirit and scope of the appended claims.

4 What is claimed is: 1. High frequency apparatus comprising, means defining a slotted waveguide portion formed with a slot of predetermined dimensions along the direction of energy propagation, means defining an unslotted waveguide portion contiguous with said slotted waveguide portion, said slotted and unslotted waveguide portions being characterized by finite cutoff frequencies and having different cross sectional areas with means for establishing the relationship between said different cross sectional areas so that the impedance characteristics and cutoff frequencies of said slotted and unslotted portions are substantially the same, whereby said waveguide portions of different cross sectional areas coact with said slot of predetermined dimensions to form a waveguide having substantially the same impedance and finite cutoff frequency along its length. 2. High frequency apparatus in accordance with claim 1 wherein said slotted and unslotted waveguide portions are dimensioned to propagate the TE mode.

3. High frequency apparatus in accordance with claim 2 wherein said cross sectional areas are rectangular.

4. High frequency apparatus in accordance with claim 3 wherein the width of said slotted portion is slightly greater than that of said unslotted portion.

5. High frequency apparatus in accordance with claim 3 wherein the height of said slotted portion is slightly less than that of said unslotted portion.

6. High frequency apparatus in accordance with claim 4 wherein the height b of said slotted portion is slightly less than that of said unslotted portions by a difference Ab to establish substantially the same empedance Z in said slotted and unslotted portions and the width a of said slotted portion is Aa greater than that of said unslotted portion to establish substantially the same cutoff frequency i in said slotted and unslotted portions.

7. High frequency apparatus in accordance with claim 1 wherein said slotted and unslotted waveguide portions have a common axis and said slot is along the length and generally parallel to said axis.

References Cited UNITED STATES PATENTS 2,433,368 12/1947 Johnson.

2,454,530 11/1948 Tiley 3333l 3,005,168 10/1961 Fye 333-31 3,050,689 8/1962 DeLoach 33333 HERMAN K. SAALBACH, Primary Examiner C. BARAFF, Assistant Examiner US. 01. X.R.

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