US2595078A - Dielectric wave guide - Google Patents

Description

RR 2: 5%, ma S ARCH ROOM SUBSTITUTE FOR MISSING XR April 29, 1952 H. IAMS 2,595,078

DIELECTRIC WAVE GUIDE Filed May 28, 1948 FIG. 2 I 3 i T I FIG. 4

I H INVENTOR HARLEY JAMS BY 7 Z v flam i ATTORNEY Patented Apr. 29, 1952 DIELECTRIC WAVE GUIDE Harley Iams, Venice, Calif., assignor to Radio Corporation of America, a corporation of Delaware Application May 28, 1948, Serial No. 29,853

Claims.

This invention relates to improvements in the art of guided transmission of radio waves, and more particularly to guiding wave energy of wavelengths of the order of one centimeter. The preferred prior art method of transferring centimeter wave power over short distances is by means of a metal pipe, generally referred to as a wave guide. Power losses in such guides are so large that transmission over distances of more than a few meters is out of the question; for example. the attenuationobserved in the best silver wave guideat 1 /4 centimeter wavelength is about db per meter.

Dielectric rods have been used experimentally to guide centimeter waves. They have been found to exhibit greater attenuations than hollow conductive wave guides, because with previously known methods the best known dielectric materials introduced dielectric losses in excess of the resistance losses of the corresponding metal wave guide. For example, bulk polystyrene excited in the simplest transverse electric mode will exhibit losses of about 3.5 db per meter at a wavelength of 1 ,4; cm.

It is the principal object of the present invention to provide improved methods and means for guiding waves of length of the order of one centimeter.

More specifically, it is an object of the invention to guide ultra high frequency radio wave energy with substantially less losses than are incurred in prior art practice.

A further object is to provide methods and means for exciting a dielectric rod as a wave guide in such manner that the loss characteristics of the dielectric material have a minimum eiiect upon the attenuation of the guided energy.

Wave guides as ordinarily used, both hollow pipes and dielectric rods, exhibit low frequency cutoff; for example, when the maximum transverse dimension of a polystyrene rod is less than about 0.6 wavelengths of the applied energy, no transmission takes place in the TEo mode According to the present invention, a dielectric rod or ribbon is excited in a mode not generally used heretofore, with which no low frequency cutoff appears. Preferably the dimensions of the dielectric guide are substantially less, with respect to the wavelength, than those required for propagation in accordance with prior art practice.

In accordance with the invention, the low-loss mode of a dielectric waveguide having no out-- of! frequenc is employed. In order to protect against radiation and losses from obstruction, a surrounding metal shell is spaced from the di- 2 electric waveguide at such a distance that th low-loss characteristic is maintained. The dielectric waveguide is supported within the shell by low-loss means as will be more fully apparent hereinafter.

The invention will be described with reference to the accompanying drawing, wherein:

Figure 1 is a perspective view of a dielectric rod used as a wave guide in the manner contemplated by the instant invention,

Figure 2 is a longitudinal section of a portion of the rod of Figure 1, showing approximately the distribution of the lines of electric force in the operation of the rod as a wave guide as in Figure 1,

Figure 3 is an end view of the rod of Figure 1 showing some of the lines of electric force represented in Figure 2,

Figure 4 is an end view like that of Figure 3, showing others of the lines of electric force,

Figure 5 is a transverse section of a rectangular metal wave guide suitably excited to couple to a dielectric guide to induce propagation therein in a manner characteristic of the present invention,

Figure 6 is a transverse section of a circular metal wave guide excited in a mode analogous to that of the rectangular guide of Figure 5,

Figure 7 is a transverse section of a modified form of-dielectric wave guide,

Figure 8 is a transverse section of a further modification, and

Figure 9 is a perspective view of a preferred type of wave guide structure of. the invention including a spaced metallic shield and supporting means.

The term dielectric wave guide" as used herein is intended to mean a rod or column of material having a different and generally a greater dielectric constant than its immediate surroundings. In addition, the term is intended to exclude dielectric rods or columns which are closely surrounded by conductive sheaths or shields, such as the well known hollow pipe or tubular wave guides which may be regarded as shields surrounding a column of dielectric material which is usually air. However, it is to be understood that I do not intend to exclude structures which are provided with conductive shields sufficient distant from the boundary between the high dielectric and low dielectric media to have little or no eiiect on the propagation along the dielectric guide.

Referring to Figure 1, a cylindrical rod I of low-loss dielectric material, such as polystyrene,

conductive wave guide 3.

3 extends into the mouth of a rectangular hollow The diameter of the rod I is preferably, though not necessarily, considerably less than one-half the wavelength of the energy to be guided. The metal guide 3 is coupled in well known manner to a source, not

shown, for excitation in the simplest transverse electric mode which is usually denoted in the wave guide art as the 'I'Eo,1 mode.

Figure shows approximately the electric field in a transverse section of the guide 3, with the lines 5 of electric force perpendicular to the wider walls of the guide. The width b must be greater than one-half the wavelength in free space of the applied energy in order for propagation to occur.

As an alternative, the rectangular guide 3 may be replaced by a circular guide 1, excited as shown in Figure 6 in the so-called TE1,1 mode. This is the circular counterpart of the TEo,1 mode in the rectangular guide, and also requires, for a given wavelength, a certain minimum diameter for propagation. In the case of the circular metal guide enclosing air or other material of unity dielectric constant, excited in the TIE-1,1 mode, the diameter d must be at least 0.578 wavelength. I

Either of the above-described metal walled wave guides may be filled with dielectric material having a dielectric constant greater than unity. In such case, the cutoff dimensions depend upon the wavelength of the energyin the dielectric material, and will be correspondingly smaller than those for air-filled guides.

The dielectric rod 2 may extend from the guide 3 to a load or other utilization device, not shown, and may be coupled thereto by means of a short section of metal-walled guide like the wave guide 3, or by other known means such as a radiator or horn.

In prior art practice it has been assumed, and perhaps even demonstrated mathematically, that dielectric rods having transverse dimensions less than a certain fraction of a wavelength cannot propagate energy of said wavelength. However, I have found that propagation can and does take place along a dielectric rod which is smaller than the critical size. in what is believed to be in a hybrid mode which is similar to the TEi,1

mode. (Fig. 6) but with the major part of the field outside the guide. Typical electric lines of force in this mode are substantially as shown at 9, II, I3, I5, I'I, I9, 2I and 23 in Figure 1.

Figure 2 shows a plan view of a portion of the rod, I, indicating the electric field distribution and instantaneous polarities. It will be observed that some of the lines, such as 25 and 21, extend across the rod I, while others such as 29 and 3| run axially along the rod. Figure 3 shows how the lines 25 and 21 cross within the rod; Figure 4 indicates that the lines 29 and 3I are also partially within the rod, but do not cross it. The lines of magnetic force are similar to those illustrated, but displaced 90 degrees around the rod.

To clarify the notation whichhas been used hereinbefore and by way of explanation, the modes of propagation of electrical energy in a dielectric rod waveguide may be characterized as TE" modes. The number n may take any integral value. Corresponding to each such value there is a root of a complicated mathematical function. Stated another way, the subscript n is associated with or numbers the various roots of 5 a. certain mathematical function or equation. 7

I i l best available metal wave guide.

The satisfaction of the-function or equation is essential to satisfy the boundary conditions associated with the propagation of electrical energy along the dielectric waveguide. With each such solution there exists a mode of propagation which is discrete and distinct from the other modes of transmission.

The TE1 mode illustrated in Figures 2, 3, and 4 is known as the dominant mode in dielectric rod waveguides because it is the mode of transmission having the lowest cut-off frequency. It has been found that the dominant mode in the case of dielectric rod waveguides has a zero cut-01f frequency and will therefore propagate waves of any alternating frequency however low, although it may be that such a dielectric rod waveguide will not support a direct current flow or energy. In common with the language used in waveguide terminology for hollow pipe waveguides, the mode of transmission of the dielectric rod waveguide having next lowest cut-off frequency to that of the dominant mode will be termed the next dominant mode.

As mentioned above, it is preferable to make the rod I of considerably less than the critical "cutoff" diameter. This effectively prevents propagation of the type generally contemplated in a dielectric guide, with concomitant high dielectric losses. I have determined experimentally that the losses in propagation along a small rod (e. g. one quarter wavelength diameter) are about 3 percent of those in a dielectric rod large enough to support propagation in the usual manner, and approximately 20 percent of those in the Experimental evidence indicates that the attenuation decreases indefinitely as the diameter of the rod is decreased. It is believed that this results from the fact that most of the energy travels outside the guide.

At present it is not known how small the dielectric rod can be made and still be eifective as a guide. Little or no radiation from the rod appears to exist as long as the rod is perfect ly straight. A small bend or irregularity causes considerable radiation. The smaller the rod, the greater is the effect of bends and minor discontinuities. Evidently when the most of the energy travels outside the rod, the waves are not very firmly rooted in the guide. However, the energy may be guided along curved lines or around corners by thin rods crossing at a critical angle, as described and claimed in my copending application Serial No. 11,402 entitled RF Coupling Device filed February 27, 1948.

Although a'rod of circular section is shown in Figure 1 oblong sections may be used, such as the rectangular section shown in Figure 7 or the elliptical section of Figure 8. Such shapes have the advantage of maintaining the wave polarized in the desired direction. When the electric vector is in the direction of the greater dimension, it tends to remain so even if the guide is twisted. These shapes of Fig. '7 or 8 may be described generally as oblong.

Since the field spreads out a substantial distance from the guide, it is necessary for optimum results to keep reflecting objects out of the immediate vicinity of the rod. To support the rod where it will be unaffected by such objects, pillars of hard sponge rubber, or other material having a dielectric constant near unity, may be used. Also, the rod may be supported on small wires at right angles to the electric field.

Figure 9 shows a presently preferred embodiment of the invention, comprising a tape or ribbon 33 of dielectric material and of oblong cross-section supported on wires 35 within a metal shield 37. The shield 37 must be large enough to intercept a negligible amount of the electric field around the guide 33, and its principal functions are to provide a rigid support for the wires 35 and to prevent reflecting objects from getting too close to the guide. Because the losses in transmission through dielectric rod waveguide with the dominant mode of propagation are very small, it would be highly desirable to utilize this mode of transmission. However, in the past, such use has been limited because of the fact that objects at a distance from the dielectric rod create serious disturbances and losses due to the fact that most of the energy of the field associated with this dominant mode of propagation is carried outside of the rod waveguide. On the other hand, if the dielectric rod is closely enclosed with a metal pipe, most of the energy is carried within the rod. In fact, in the latter case one has merely a hollow pipe waveguide which is filled partiall or entirely with solid dielectric material. and the modes of propagation of energy therethrough assume the forms characteristic of hollow pipe waveguide. However, by utilizing a structure as illustrated in Fig. 9, the benefits of low attenuation of the dielectric rod waveguide transmission are readily obtained, without the losses associated with hollow pipe waveguide. In fact, the losses in such a structure as that of Fig. 9 are demonstrably and appreciably less than losses associated with hollow pipe waveguide or with dielectric rod waveguide of a size to carry only the dominant mode. In the latter case, the losses due to radiation and extraneous interferring bodies are too great. If, however, a dielectric rod waveguide of the size to carry the next dominant mode at the operating frequency is used, dielectric losses become too great for practical employment.

I claim as my invention:

1. In a system for transferring wave energy at a given operating frequency from a source to a load, a rod of dielectric material of oblong cross section and having a maximum dimension in cross section substantially less than the minimum which will support propagation in the TEnzl) mode at said frequency, means applying energy from said source to said rod with its electric vec- 5 tor transverse to the longitudinal axis of said rod and parallel to the maximum dimension in cross section to excite said rod in a TEn= 1 mode, where a is an integer greater than zero, a conductive shield at least partly surrounding said rod and spaced therefrom by a distance substantially greater than the cross sectional dimensions of said rod, means supporting said rod within said shield comprising thin wires perpendicular to the electric field therein.

2. In a system for transferring wave energy of a given frequency from a source to aload, the combination comprising a dielectric rod of oblong cross-section having a maximum dimension substantially less than the minimum which supports propagation in the next dominant mode at the given frequency, means applying energy from said source to said rod to propagate said energy along the rod in the dominant mode, a conductive shield at least partly surrounding said rod and spaced therefrom by a distance substantially greater than the cross-sectional dimensions of said rod, and means supporting said rod within said shield.

3. The combination claimed in claim 2, said supporting means comprising thin wire-like metallic members oriented substantially perpendicular to the electric field throughout the member length.

4. The combination claimed in claim 2, said supporting means comprising dielectric material. 5. The combination claimed in claim 2, said supporting means comprising sponge rubber.

- HARLEY IAMS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,142,138 Llewellyn Jan. 3, 1939 2,197,123 King Apr. 16, 1940 2,407,257 Glnzton Sept. 10, 1946 2,422,058 Whinnery June 10, 1947 2,422,191 Fox June 17, 1947 2,425,345 Ring Aug. 12, 1947 2,433,368 Johnson Dec. 30, 1947 2,479,673 Devore Aug. 23, 1949 OTHER REFERENCES Bell System Technical Journal," vol. 15, 1936. (Copy in Library (using page 290).)

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