US2943325A - Electro-mechanically scannable trough waveguide transmission lines and antennas - Google Patents

Description

m a mm m m ma E on w 7/2 1 8 Z 2 2 w b e Z n a m Z INVENTOR. M47272 07/ /44! BY: w flaw/V8 The invention describedherein may be manufactured and used by or for the United States Government for governmental purposes without payment to me o f any royalty thereon.

This invention relates generally to antennas and transmission lines and more particularly to apparatus for the transmission of high-frequency electrical energy. The invention is characterized by the utilization of a special waveguide structure as a means for transmitting wave energy and radiating energy to the surrounding space and involves the use of elements within the waveguide structure for controlling the guide wavelengthor phase velocity and/or radiation intensity or attenuation rate in the waveguide. Control of these parameters in efiect enables the obtaining of any desired controlled radiation pattern.

A trough waveguide of either rectangular channel shape, U-shape, V-sh'ape or variations thereof having a symmetrically disposed fin therein comprises the transmission line which is modified to produce the controlled radiation. A basic concept on which this invention relies is that anti-symmetrical obstacles in a trough waveguide couple energy from the bound symmetrical trough guide mode, a transverse electric (TE)'mode, into energy in a transverse electromagnetic (TEM) field which radiates into free space from the open side of the waveguide. Discrete symmetrical obstacles, on the other hand, react as tuning elements. In accordance with above principles, a large variety of radiating devices may be constructed.

The trough waveguides, together with their radiating elements, have utility alone or as primary line source arrays for use with secondary reflectors, horns, or lenses in radar and communication antenna systems. In this respect, they are superior to slots or dipole equivalent arrays on rectangular or circular waveguides. When not used for wave propagation into free space the devices have utility as variable delay lines, frequency dispersive lines and as variable phase shifters with multiple pick off points.

Because of the superior electrical and mechanical properties of the antennas andtransmission lines of this invention, many new advantages are realized.

It is contemplated that the subject invention will open new vistas in the fields of radio astronomy, radar, and communications.

It is an object of this invention to produce an antenna comprising a trough waveguide with means for controlling radiation therefrom. a

It is another object of the invention to produce an antenna with means for controlling guide wavelength.

it is still another object of the invention to produce an antenna of extremely broad impedance bandwidth.

It is a further object of the invention to produce an antenna which may be composed of either resonant or non-resonant radiating elements.

It is a still further object of the invention to produce a non'resonant traveling wave array composed of either resonant or non-resonant element's.

It is another object of the invention to produce an atent 2 antenna which is easily and economically produced by conventional, commercial manufacturing techniques.

It is still another object of the invention to produce a novel variable delay transmission line.

or aperture of the guide.

A further object of the invention'involves, the production of a variable phase shifter with multiple pick olf points. I

A still further object of the invention involves the production of a traveling-wave leaky, waveguideiantenna.

Another object of the invention involves the production of a traveling wave array "of non-resonant radiating elements in which there is achieved a phase reversalh'etween successive elements. i

I Still another object of the invention involves the production of a traveling wave leaky waveguide formed by a continuous asymmetry along a trough waveguide.

A further object of the invention involves the production of a novel scannable antenna.

A still further object of the invention involves the production of novel means for varying phase in a transmission line. p

These and other advantagm, features and objects of the invention will becomemore apparent from the following description taken in connection with the illustrative embodiments in the accompanying drawings, wherein:

' Figure l is a cross-sectional view ofa trough waveguide with a representation of the electric field of "the transverse electric (TE) mode therein, 7

Figure 2 is a cross-section of a trough waveguide with a representation of the electric components of th'e'transverse electromagnetic (TEM) field; and I Figures 3-5 are cross-sections of pictorial views of trough waveguides with various means for varying the phase velocity of a traveling wave.

The invention utilizes a symmetrical non-radiating trough waveguide which acts as a transmission line. This type of waveguide in its mechanical construction is left open on one side; however, it still acts as a transmission line. Its characteristics, frequency-wise follow those of a waveguide while it retains mechanical simplicity of a strip transmission line. The general configuration of the waveguide has two side walls and a bottom, thus forming a trough, and a substantially centrally disposed fin member of less height than the side walls. The embodiment of Figure 1 is a channel or rectangular form of trough waveguide but various alternative forms canxbe achieved by deformation of the side walls, .for example, the trough may be of U or V shape as long as symmetry about the central fin is maintained in 'order 'to avoid spurious modes.

Like numerals willdesignate like parts of the waveguide throughout the specification.

Figure l, for illustration purposes, shows a rectangular trough waveguide having side walls 20 and 21, a bottom wall 22 and a substantially centrally disposed fin member 23. Whenthe side walls 20 and 21 are less than one half wavelength apart, a TB (transverse electric) mode may be propagated along the axis of the guide. This modeis bound to the center finfand has an electric held with a general configuration as shown in Figure l. .The intensity of the field lines of the electric vector increases from the bottom [22 of the waveguide to the top of the central vane or fin 23. The transverse currents on the sides of fin 23 vary from a minimum at the free edge to a maximum at its base. 1 1

The electric components of the TEM (transverse electromag'netic) field is depicted in Figure 2 and can be propagated wherever side walls exist. As is shown by the dashed lines at the open end of the trough, radiation into space is achieved whenever the field strikes the open top The creating of an asym:

Patented June 28, .19 0

wavelength of the highest frequency in the range. .TE mode is critically dependent upon the dimensions of conventional means as long as the interior of the trough .is plated or otherwise lined with a highly conductive material.

The characteristics of a trough waveguide are such that the cut-oif wavelength depends upon the electrical height of the center fin 23, i.e., the cut-off wavelength is approximately that at which the center fin is a quarter wavelength. I The sidewall (20, 21) height above the center ,fin 23' and the spacing between side walls act to prevent unwanted, uncontrolled radiation. Less than half wavelength spacing between the side walls allows for opera- .tion of the line over a range of frequencies. The spacing of the side walls, at any rate, should not exceed a half The the center fin while the TEM or radiating mode is independent of the fin 23.

The embodiments of Figures 3-5 utilize various methods of varying the phase velocity and producing radiation from a trough waveguide.

Variation of phase velocity by changing the height of the center fin 23 is achieved in Figure 3. p

In this embodiment, the solid line portion of the figure illustrates the principle of varying the height of the center fin 23 cyclically by utilizing driving means (not shown) connected to a shaft 24 supported by bearing means 25. An eccentric 26 is mounted on shaft 24 for rotation therewith and drives a link 27 which is pivoted to the eccentric. The link 27 is also pivoted to a channel shaped member 29 which is limited to vertical movement by its attachment to fin 23 by means of pin 29. To assure at any particular time a'uniform height of the fin 23 in the trough waveguide, it is necessary to duplicate the eccentric 26 and line 27 in at least one other position along the length of the fin. Variation in the height of the fin 23 caused by rotation of shaft 24 produces a change in phase velocity in the waveguide which will always be greater than the phase velocity of light.

Variations in phase velocity in a trough waveguide which is made to propagate energy into free space produces a scanning of the propagated beam. Accordingly, means for creating an asymmetry, as taught in my copending application titled: Trough Waveguide Antennas, Serial No. 647,454, filed on even date herewith, may be introduced into the trough waveguide to give it antenna characteristics. As illustrated in dotted lines in Figure 3, a discrete horizontal radiating rod 30 emanating from wall 20 creates the asymmetry which tends to cause an energy interchange from the TB mode to the TEM field. An R.F. choke 32 is utilized to prevent radiation through the slot in bottom wall 22. Vertical tuning post 31 for the explanation which follows is considered as mounted on the fin 23 to move therewith and acts to cancel reflections in the waveguide. Repetition of elements 30 and 31 at half wavelength intervals creates an antenna array; however, in order to sweep the propagated beam to or through a broadside position, phase reversal must be introduced by placing elements 30 in a staggered relationship on walls 20 and 21 at half wavelength intervals.

Although post 31 is depicted as extending through channel member 28, it could be mounted on top of fin 23 or made an integral part thereof. As shown, however, post 31 may be described with reference to a different type of operation which follows.

Variations in fin height may introduce a variation in the reflected energy to be cancelled by post 31; therefore, it may be necessary to independently vary the height of posts 31 to create the appropriate cancellation for each of the conditions encountered at all the positions of the tin 23. Post 31, to cover these changing conditions, may be made to run through a clearance hole in channel 28 to a fixed support (not shown) in order. to keep its height with respect to the side walls constant, thereby presenting a mean value for cancellation of reflections. If a fixed support were not used, post 31, as illustrated, could be made to ride with a separate cam 26a on shaft 24 by means of link 27a so that the system would be matched in impedance at each position of fin 23.

Although the embodiments of Figures 3-5 show particular means for producing a radiation of energy into free space, it should be understood that any means which creates an asymmetry, as taught in the aforementioned co-pending application, may be utilized.

Figures 4 and 5 illustrate slow wave structures wherein the phase velocity or guide wavelength are varied.

In Figure 3 discrete vertical posts 31 were used which act as inductive shunt elements while the post 33 of Figure 4, and posts 38 of Figure 5, being of less height than a quarter wavelength, act as capacitive shunt elements to periodically load the trough waveguide, thus producing a slow wave structure. An increase in scan angle over that obtained in Figure 3, which is limited by the guide wavelength and for large angles, requires operation near .cut-ofi, is obtained since the guide wavelength may be either greater or less than the wavelength in free space. Furthermore, the embodiment of Figure 4 may be made to be frequency scannable.

In the embodiment of Figure 4, elements 24, 25, 26, 27, 28 and 32 have the same operation as defined with reference to Figure 3. Fin 23, however, is split in this embodiment to accommodate vertical post elements 33 which are tied together at their lower extremities for conjoint movement. Attachment of the lower portion of posts 33 to channel shaped member 28 is effected by at least one threaded rod 34 inserted into its internally threaded boss 35 in element 33 and secured to channel shaped member 28. Adjustment of element 33 above fin 23 may be effected by the threaded rod 34 and a locknut 36.

For consideration of the operation of the post elements only, the fin would be held stationary by attaching its lower extremity to bottom wall 22 thereby eliminating that part of the fin shown below the bottom wall. Variations in the height of the closely spaced (less than a quarter wavelength) posts 31 then would produce changes in phase velocity.

Frequency dispersive characteristics may be introduced by utilizing pin 29 as a securing means for fin 23 to produce its movement. For this condition, vertical posts 33 are held stationary by allowing a clearance for rod 34 through element 28 and attaching it so some fixed structure (not shown). Movement of shaft 24 creates a vertical motion for split fin 23 which varies the post height to fin height ratio, thus introducing frequency dispersive characteristics into the transmission line.

Propagation of the wave energy into free space may be accomplished, for example, by placing a block 37 (shown in dotted lines) in the waveguide and running case with frequency scannable radar. This innovation gives a greater degree of freedom from jamming sources. Of course, instead of cyclical variation of the post to fin height, a manual adjustment could be provided.

In addition to the production of changes in phase velocity by varying post height (Figure 4) in a slow wave structure, either a change in post pitch or post width would produce a phase velocity change. Figure 5 illustrates a device for changing phase velocity by varying the tooth or post width. A shorting plate 39 terminates the waveguide while a coaxial input 40 is provided to present an input signal to the center fin of the waveguide. The center fin 2-3 comprises three plates, the center one of which is free to slide on bottom wall 22. Identical rectangular posts 1 or teeth 38 are cut on all three plates 23; the ratio of tooth width to gap being close to unity. As the center plate slides between the outer ones, the efiective tooth width varies and a change in phase velocity is obtained. In the extreme case when the spacing is reduced to zero, the effect of a solid center fin 23 is obtained and the phase velocity is greater than that of light. The effective tooth width varies repetitively, i.e., it alternately increases and decreases as the centre plate moves linearly, going through a complete cycle whenever the center plate is shifted by an amount equal to the spacing of the teeth. The width variation produces a corresponding multicyclic change in phase velocity. The center plate of fin 23 may be extended external to the waveguide and joined into a loop, as a blade in a 'bandsaw. Rapid motion of the center. plate may then" be obtained, causing a very rapidly repeating'phase velocity variation.

By combining this technique with the introduction of periodic asymmetric radiating means 41 (in dotted lines) at half wavelength intervals on alternate sides of fin 23,

a radiated beam that swings infspace in accordance with the change in guide wavelength will occur with motion of the loop or center plate of fin '23. w

A rectangular tooth structure 38 has the disadvantage of producing a nonlinear variation of phase velocity as the gap approaches zero and, with small gaps, the power handling capability of the guide may be reduced because of voltage breakdown across the gap. To overcome these difficulties a tapered tooth 38 is preferred.

The tooth depth is made to gradually increase from the input point to present a proper transition from the coaxial line 40 or from any suitable waveguide to trough transition for impedance matching purposes. Such a procedure is, of course, applicable to the embodiments of Figure 4.

Frequency dispersion or frequency scanning may also be introduced in this embodiment by choosing the proper ratio of post to fin height.

Although the invention has been described with reference to particular embodiments, it will be understood to those skilled in the art that the invention is capable of a variety of alternative embodiments within the spirit and scope of the appended claims.

I claim:

l. A slow wave trough waveguide comprising a generally trough-shaped member, a fin symmetrically disposed therein and running longitudinally thereof, said fin comprising a pair of parallel plates, and closely spaced post means mounted for conjoint movement between said parallel plates transverse to the length of said fin, said movement creating a change in the height. of said posts above said fin to produce changes in phase velocity.

2. A device as defined in claim 1 includingmeans for creating a cyclical variation in post height.

3. A slow wave trough waveguide comprising a generally trough-shaped member, a fin symmetrically disposed therein, said fin comprising a pair of parallel plates extending longitudinally of said waveguide, a series of closely spaced posts fixedly mounted between said pair of parallel plates, and means for varying the fin height with respect to the post height above said fin for changing the frequency dispersive characteristics of said waveguide.

4. A device as defined in claim 2 including means for creating a controlled radiation from said waveguide.

5. A device as defined in claim 3 including means for producinga controlled radiation from said waveguide.

6. A slow wave trough waveguide comprising a generally trough-shaped member'and a centrally disposed fin in said member and extending the length thereof, said fin comprising three plates having identical toothed portions therein, the center plate beingmoveable longitudinally to produce variations in tooth width, 7

7. A device as defined in claim 6 including means for producing a controlled radiation from said waveguide. V 8. A trough waveguide suitable for the transmission of microwave signals comprising a generally trough-shaped member, a symmetrically disposed fin extending longitudinally of said member, means for varying the phase 9 velocity of said signal by mechanically varying the height of said fin, means for producing a controlled radiation from said waveguide, and tuning means for cancelling reflections caused by said means for producing a con trolled radiation.

I 9. A device as defined in claim 8 wherein said tuning means is cyclically varied in synchronism with variations in fin height.

References Cited in the file of this patent UNITED STATES PATENTS 2,602,893 Ratlifi July 8, 1952 2,735,958 Brown Feb. 21, 1956 2,799,831 Fubini July 16, 1957

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