US3522561A

US3522561A - Pyrolytic graphite waveguide utilizing the anisotropic electrical conductivity properties of pyrolytic graphite

Info

Publication number: US3522561A
Application number: US788531A
Authority: US
Inventors: David J Liu
Original assignee: Individual
Current assignee: Individual
Priority date: 1969-01-02
Filing date: 1969-01-02
Publication date: 1970-08-04
Anticipated expiration: 1987-08-04

Description

Aug. 4, 1970 I DAVID J. uuv 3,

'PYRQLYTIC GRAPHITE WAVEGUIDE -UTILJIZING THE ANISOTROPIC ELECTRICAL CONDUCTlVlTY PROPERTIES OF PYROLYTIC GRAPHITE Filed Jan. 2, 1969 2 Sheets-Sheet 1 33 I ll Mom-z /l/NPUT FILTER OUTPLT 3. Illll 32 INVENTOR. DAVID J. LIU

Fig. 3 WM ATTORNEY v, g- 4, 1970 DAVED -J. LIU 3,522,561

PYROLYTIC GRAPHITE WAVEGUIDE UTILIZING THE ANISOTHOPIC ELECTRICAL CONDUCTIVITY PROPERTIES OF PYHOLYTIC GRAPHITE Filed Jan. 2, 1969 TRANSMISION INVENTOR.

DAVID J. LIU BY 2 Sheets-Sheet 2 v United States Patent US. Cl. 333-95 6 Claims ABSTRACT OF THE DISCLOSURE A section of waveguide fabricated from pyrolytic graphite material suppresses all modes except the transverse H mode thereby permitting large distant transmission of millimeter Waves by eliminating the parasitic effects of unwanted modes.

Communications systems of ever increasing channel capacity are in greater demand; accordingly, efiicient transmission at millimeter waves is very attractive. The present invention provides a system for eflicient transmis sion at these frequencies.

Prior art dielectric filled waveguides have not been very satisfactory at millimeter wave lengths due to the required dielectric materials being dissipative. At millimeter wave lengths overmoded waveguides are not easily avoided and overmoded waveguides produce additional attenuation from dielectric materials which is an order of magnitude larger than the attenuation due to the metal walls.

Guides with anisotropic surface impedance provide better performance than other types of waveguides. Among these are disc, corrugated, and helical waveguides. Disc and corrugated waveguides provide the ease with which the designer can choose, and realize, the required surface impedance. However, some technological difiiculties are encountered in manufacturing tolerances and sharp edges on the disc, and so forth.

Helical Waveguides overcome ditficulties associated with the production of disc waveguides yet retain the same benefits. The pyrolytic graphite hollow circular Waveguide contemplated by the present invention provides a more efficient anisotropic surface impedance than any of the prior art waveguide mentioned above.

Pyrolytic graphite is formed by passing hydrocarbon gas over a hot surface held at about 4000 F. The carbon atoms are removed from the gas by a thermo-decomposition process and are deposited in a manner similar to vacuum plating operations. The material deposits such that it is always a poor conductor perpendicular to the deposition surface and a good conductor parallel to the deposition surface. A sheet of pyrolytic graphite thus formed consists of many a layer of carbon atoms. In the direction perpendicular to the deposition surface very weak bonding forces hold these atomic layers together. This results in a dielectric constant of five times that of vacuum and slightly larger than the dielectric constant of structural ceramic. A companion result is that the mechanical strength of the material is weak in a direction perpendicular to the deposition layer, tensile strength of about 500 to 1000 p.s.i. being normal. This means that the material is mechanically easy to work in this particular direction. While in the direction parallel to the deposition surface the electric conductivity is at least 40% greater than copper and the dielectric constant is about the same order of magnitude as that of a good conductor.

A circular waveguide of pyrolytic graphite made in such a way that the transverse cross sectional plane being the deposition surface and a hole of proper size drilled through perpendicular to the deposition surface can cer- 3,522,561 Patented Aug. 4, 1970 tainly support the circular electric modes better than other modes. The attenuation suffered by the-lowest circular electric mode in a pyrolytic graphite circular hollow waveguide is only about 71.5% of that of a uniform copper pipe. On the other hand, the attenuation suffered by the circular magnetic mode and other hybrid modes (including electric modes of higher order) in a pyrolytic graphite waveguide is about 26 db greater than a uniform copper pipe. The realization of 26 db additional attenuation for higher modes is not possible to achieve by means of prior art waveguide including the iris, dielectric, disc loaded and helical waveguides mentioned earlier. The inventor has discovered that this characteristic feature of pyrolytic graphite waveguides is of importance for millimeter wave transmission over very long distances. It is contemplating in practicing the present invention that a small section of waveguide will be made of pyrolytic graphite material according to the principles of the present invention. Thereafter the Waveguides can be constructed of any suitable material, perhaps of a metal such as copper and so forth. The idea being that the small section of waveguide fabricated in accordance with the present invention will act as a mode filter suppressing all modes except the one desired, namely the H (or the TE mode.

It is well known that there is a continuous interaction of the various modes; accordingly, if a wave is launched at one end of a waveguide and is multi-moded, as will be the case in all prior art devices, these other modes will constantly inter-react with the wanted mode and will increase dissipation for every mode and will cause separate eddy currents to be circulated resulting in energy loss. If all modes but the desired mode are suppressed right from the beginning by the mode filter action provided by the present invention, the millimeter wave can travel great distances retaining much of its initial power.

Therefore an object of the present invention is to provide a wave device for operation at millimeter wave frequencies.

Another object of the present invention is to provide a millimeter waveguide fabricated from pyrolytic graphite.

Another object of the present invention is to provide an efficient mode filter.

Another object of the present invention is to provide a section of waveguide which suppresses all modes but the TEm mode.

Another object of the present invention is to providean antenna for launching millimeter waves suppressing all modes except the TE mode.

Other objects, features and advantages of the present invention will be better understood from the following specifications when read in conjunction with the attached drawings of which:

FIG. 1 is a section of pyrolytic graphite waveguide.

FIG. 2 is a mode filter.

FIG. 3 is a millimeter wave antenna.

FIG. 4 shows one end of a waveguide with coaxial connection.

FIG. 5 is an end view of a waveguide.

FIG. 6 is a side view of the waveguide shown in FIG. 5.

Referring to FIG. 1 we see a waveguide which has been fabricated in accordance with the principles of the present invention. The waveguide consists of several sections. Between sections joint 24 is seen. Each section is /2" thick and is cast separately. A waveguide is made up of several separate sections. Each section being about /2" thick, to make a waveguide 4 long requires 8 sections bonded together. The bonding material needed can be any of a number of strong adhesives for example Armstrong C7 epoxy which are well known in the prior art. Each section is cast in the desired shape by passing a hydrocarbon gas over a surface of the die-shape that is held at approximately 4000 F. The carbon atoms are removed from the gas by thermal-decomposition and deposited in a manner similar to vacuum plating. The material is deposited such that it becomes an insulator perpendicular to the deposition surface and a good conductor parallel to the deposition surface. Accordingly a mandrile can be inserted on the central portion of the required surface to provide the desired wave guide opening, with regard to the ends, openings for bolts 22 four separate smaller mandriles can be used. The outside shape is provided by the shape of the die-like shape of the required surface wherein the gas is passed over it to form the waveguide. Each section of waveguide is shown with lines running through it indicating the direction of the layers. 35 indicates these layers run in the required direction.

Once an appropriate number of sections are bonded together with the flanges at each end making up a waveguide, the internal surface is to be sanded and polished until smooth and clean. 21 shows the Waveguide opening.

In FIG. 2 a section of waveguide is shown ideally with layers 35 making up mode filter 33. Input 31 is launched into the waveguide and emerging therefrom is output 32. The input will be a multimoded wave having a substantial number of modes. However only TE mode will emerge from waveguidethe parasitic modes are suppressed.

The internal dimensions of a waveguide fabricated for use at 100 gHz. has a dimension of 1 inside diameter and a 2" outside dimension. The flange dimensions are approximately 3 /2" square and providing satisfactory bolt holes in its four corners. Obviously at 1" inside diameter a 100 gHz. signal would be considerably overmoded in this waveguide.

If it is desired to launch this wave into the air, the ends can be flared as shown in FIG. 3 with an appropriate angle 29 of approximately 45 from the outside edges. Each section is cast in a similar manner to the preceding sections but with mandrile to provide the desired angular shape. The individual sections will then be bonded together to make the antenna as shown in FIG. 3.

If the wave is to travel a great distance the section made of pyrolytic graphite need only be small and probably only occupy the first portion of waveguide. Thereafter the waveguide would be made of metal which is well known in the present state of the art. In any event the waveguide will have an input applied as shown in FIG. 4, coaxial cable 41 having a flange 42 mates with the flange section of waveguide 35. Bolts 38 which extend through both flange elements providing the means for launching a wave into the waveguide which provides mode filtering. Probe 43 is any of the well known configurations which are state of the art.

Referring to FIG. 5 we see the end view of a section of waveguide fabricated in accordance with the present invention. We also see the transverse H mode. This mode will be discussed further as we proceed with the details of the mathematical relationship consistent with the description of the present invention. We see that the magnetic field of the H mode is radial While the electric field is normal thereto. Referring to FIG. 6 we see the side view of that section of transmission waveguide shown in FIG. 5. Transmission is in accordance with the direction indicated in 44. Walls 21 are polished walls within the waveguide. We note layers 16 are normal to the direction of transmission. These layers are the same as that which the direction of deposition is accomplished.

In order to fully understand the relationship of the pyrolytic graphite in its interaction with the various modes to suppress some while permitting certain others to pass without significant attenuation, the pyrolytic graphite region is examined mathematically in the following paragraphs.

In the pyrolytic graphite region, Maxwells equations are:

typical frequency of gHz. in the millimeter waves range, one has:

and

(G) a. 2.03(10s =7D32 103 In other words a permittivity tensor of the pyrolytic graphite may be formed as follows:

E; 0' 0 0 al Therefore, in the millimeter waves range, the following permittivity tensor is valid.

a, 0 0 6 (l-j Q) One can solve Equations 1 through 6 for E E H and H in terms of E and H as follows:

Introducing the notation A =A one derives from Equation 12:

To solve Equations 13 through 17, let

and one obtains, in general and so on. To solve Equations 18 and 19, let

cr ='l 'l' z z and one obtains, in general and for H mode one obtains:

6.4(' f n vl- (mm nep ers/ meter where a is the radius of the waveguide in meters, and f the operating frequency, and

For E-modes one obtains the attenuation factor aEv also by perturbation method resulting at a typical frequency in the millimeter waves range of gHz.:

(26) db/ kilometer Thus we see in Equation 28 that the attenuation factor of the H mode is significantly small in comparison with the attenuation factor of the E modes. Accordingly the TE mode as represented by the attenuation factor of the H mode will result in a wave guide which permits the TE mode to pass with minimal attenuation while all other modes are completely dissipated within a few Wave lengths of waveguide. Accordingly unlike prior art waveguides where the interaction of the different order modes produces a significant attenuation due to the interaction of these modes with one another such that higher order modes become parasitic. Each have dissipation associated with their own mode which is eliminated by the present invention.

Accordingly with the present invention millimeter waves can be launched at one end of a mode filter according to the invention and emerge to continue any distance through conventional waveguide having only one desired mode which experiences minimal attenuation thereafter. It is apparent then with the present invention Wave guide systems for millimeter waves can be increased in efliciency by merely the insertion of a section of wave guide in accordance with the principles of the present invention which acts as a mode filter. Furthermore the waveguide can be shaped as to permit mode filtering and launching of a wave simultaneously in the form of an antenna. My invention has been described with reference to specific apparatus and many of those skilled in the art will be able to make many substitutions and variations in the above described apparatus without departing from the true scope and spirit of the present invention. Accordingly, the inventor wishes only to be limited in his invention by the appended claims.

I claim:

1. A pyrolytic graphite wave device having,

a plurality of layers of pyrolytic graphite,

a smooth polished circular opening perpendicular said layers of preselected dimentions, and

flange means for attaching a probe to launch a wave therein.

2. A pyrolytic graphite Wave device according to claim 1 wherein said opening flares out at a predetermined angle at the end opposite said flange.

3. A pyrolytic graphite wave device according to claim 2 wherein said predetermined angle is 45.

4. A pyrolytic graphite wave device according to claim 1 which further includes second flange means for connecting said wave device to sections of metallic wave guide thereby acting as a mode filter.

5. A pyrolytic wave device according to claim 4 where in said opening is one inch in diameter.

6. A pyrolytic wave device according to claim 4 where in said plurality of layers make up a number of segments, said segments bonded together to form a continuous wave guide.

References Cited UNITED STATES PATENTS 3,016,502 l/l962 Unger. 3,158,824 11/1964 Larsen et -al 333-98 X (Other references on following page) 7 FOREIGN PATENTS 8/1958 Germany. 5/1962 Germany.

OTHER REFERENCES 5 General Electric, Pyrolytic Graphite Eng. Hndbk.

8 its and Boron Alloys of Pyrolytic Graphite, Chem. and Physics of Carbon, vol. 2, pp. 225-256, Marcel Dekker Inc., New York, 1966.

HERMAN KARL SAALBACH, Primary Examiner W. H. PUNTER, Assistant Examiner US. Cl. X.R.