US2509196A - Electric wave guide - Google Patents

Description

May 23, 1950 E. c. CORK El'AL ELECTRIC WAVE GUIDE Filed April 18, 1945 [N TOR Edwand/ dork, l lidhael/Bgglman-fl fold mm,

ATTORNEY Patented May 23, 1950 UNITED STATES PATENT OFFICE ELECTRIC WAVE GUIDE Application April 18, 1945, Serial No. 589,068 In Great Britain September 1, 1942 Section 1, Public Law 690, August 8, 1946 Patent expires September 1, 1962 Claims. (01. 250-3353) This invention relates to electric wave guides for the transmission of TM or transverse magnetic type electromagnetic waves,

The invention is concerned with wave guides consisting of hollow conductors, the electromagnetic energy being transmitted through a dielectric medium within the conductors.

For many purposes it may be required to employ a guide which at positions along its length contains different dielectrics. Where such a guide is required it is found that the presence of difierent dielectrics can give rise to undesirable reflections of the electromagnetic energy at the dielectric interfaces.

It is the object of the present invention to provide an improved construction of wave guide having portions filled with different dielectrics in which reflection of energy at the boundary between the different dielectrics is substantially reduced or eliminated.

According to one feature of the invention a wave guide for the transmission of electromagnetic waves having an electric component of the field therewithin along the axis of the guide and hereafter generally referred to as waves of the E or TM type is provided having portions filled with different dielectrics wherein the transverse dimensions of the guide at the boundary between said different dielectrics are so related to the dielectric constants of the said different dielectrics and to the frequency of the E or TM type waves that reflection of said waves at said boundary is substantially reduced or eliminated.

The invention is of particular use where it is desired to employ two sections of waveguide of different size, one of the sections being filled, for example, with a solid dielectric and the other section having an air dielectric. It is known that two guides of different transverse dimensions employing the same dielectric can be connected together and matched to avoid reflection by joining the two sections of guide by an intermediate section in which the transverse dimensions thereof slowly change from the section of larger size to the section of smaller size. If, for example, the two sections of the guide are of circular form in cross-section then the two sections may be connected together by means of a conical section having an angle of taper not exceeding about Where such a construction is employed and one of the sections of the guide is filled with a different dielectric from the other section of the guide, then it is possible in accordance with the present invention to arrange the boundary between the different dielectrics at a suitable position along the section having the slowly changing diameter so that reflection is substantially eliminated.

In order that the said invention may be clearly understood and readily carried into effect, it will now be more fully described with reference to the accompanying drawings, in which:

Figure 1 illustrates a wave guide filled with difierent dielectrics, and

Figure 2 of the drawings illustrates the invention as applied by way of example to a wave guide aerial employing sections of wave guide of difierent diameter.

As shown in Figure l, the reference numeral 3 indicates a wave guide which is of circular form in cross-section and is designed for the transmission of electromagnetic waves of the so-called E01 mode. The first subscript indicates the order, that is, the number of full period variations of radial component of the field along angular coordinates within the guide and the second subscript the mode; that is, the number of half period variations of angular component of field along radial coordinates within the guide. Thus a TM wave of the mth order and nth mode is rep resented by Emn. In the form shown in Figure 1 the guide 3 has a constant cross-sectional area and one portion of the guide is filled with a dielectric K1 which dielectric may b air and another portion of the guide is filled with a solid dielectric K2 such as that known by the trade, name Polystyrene. The boundary between the different dielectrics is arranged in the plane S-S', which is located at right-angles to the axis of the guide. The arrangement is identical with the system formed by joining two wave. guides of the same diameter but having difier-v ent dielectric materials. To insure a match between the two wave guides, it is necessary that the radius of the guide 3, the dielectric constants and the frequency of the waves transmitted should be related so that a minimum of reflection occurs at the surface SS.

It is known that wave propagation in a guide may be regarded as a case of reflection, there being a certain system of sources inside the guide and the total field inside the guide being the sum of the impressed and reflected fields. For example in Figure 1, the wave reaching the boundary SS is the sum of the fields of the waves refiected from the surface of the guide enclosing the medium K1. Such reflected waves reach the boundary SS at an angle to the normal to the SS determined by the dimensions of the guide.

This angle is important in that it determines the amounts of transmission and reflection that occur at SS. In particular, in accordance with this invention, when the wave is of transverse magnetic or TM type and the angle is equal to the Brewster or pseudo-Brewster angle, then reflection is wholly or substantially eliminated and the wave is effectively totally transmitted into the medium K2. This is then equivalent to having the guide containing K1 matched to the guide containing K2.

The above explanation is based upon well known optical theory. A more detailed discussion where the optical theory is identified with a general electromagnetic theory is to be found in Schelkunoifs Electromagnetic Waves (published in 1943) pages 251-254 while the use of the Brewster angle to obtain impedance matching at a surface of discontinuity is referredto on page 253.

If instead of the wave guide 3 having a constant diameter as shown in Figure 1, two sections oftthe wave guide are employed having different diameters, then as stated above it is known that these two sections can be matched without introducing appreciable reflection providing the two sections of different diameter are joined by an intermediate section having a slowly changing diameter. Consequently, in the case where the two sections of difierent diameter are filled with different dielectrics a minimum of reflection will occur at the boundary between the two dielectrics providing the boundary is located at a position where the radius of the intermediate section has a value in accordance with the above considerations.

Figural? of the drawings illustrates the invention as applied to a wave guide aerial having two sections of different diameter joined by an intermediate section. As shown in Figure 2 the section of guideof larger diameter is indicated bythe reference numeral 4 and the section of smaller diameter is indicated by the reference numeral 5 and constitutes the aerial section of the guide. Energy can be radiated from the aerial or picked up by the aerial via a slot 6 provided on the circumference of the guide and, in theconstruction shown, in order that the section S'can have a minimum diameter and at the same time the slot 6 can be protected against the ingross of foreign matter, the section 5 is filled'with a solid dielectric indicated at 1, such as the material known by the trade name Polystyrene. In: order to provide a match between the sections and 5 of diiierent diameter an intermediate section 8 -is' provided of conical form having a small angle of taper. Providing the angle of taper is sufliciently small, substantially no reflection will occur at the intermediate section. In accordance with the invention the boundary between the solid dielectric l and the air dielectric of the section 4 is so disposed that it is located at a position where the radius of the section 8 has a value as defined by the above theory. In this manner a minimum of reflection will occur at said boundary. The end of the section 5 is closed by a metal disc 5 to prevent the escape of energy at the end of the section 5, and in order to prevent currents fiowing on the outside of the section 5, the latter is eifectively earthed at a plurality of spaced points by providing two pairs of segmental fins In of conducting material secured to bands II, in turn attached to the outer surface of the guide 5, the members Ill extending around the periphery of the guide for about 90". These members H] are proportioned so that, in

4 effect, they have a radial length equivalent to a quarter of a wavelength of the transmitted waves. The fins of each pair are also spaced apart by a distance of approximately a quarter of a wavelength.

The invention is not limited to the above mentioned examples but may, on the contrary, be applied to any transverse magnetic type of wave and any crosssection of wave guide provided that the dimensions of the wave guide are such as to cause the component waves to fall on the boundary at the Brewster angle. In other words, the dimensions of the guide must be such that the impedance of' the guide on one side of a transverse boundary surface is equal to the impedance on the other side of the transverse boundary surface.

What we claim is:

l. A wave guide aerial, for the radiation or reception of electromagnetic waves having a compcnent of electric field along. the axis of said guide, said guide having portions filled with dif ferent dielectrics, said wave guide'having an aperture on one side thereof: whereby the interior of said guide is coupled to the exterior thereof, a discontinuity in the dielectric within said guide at the position of said aperture, a portion of said exterior serving as radiating means, said wave guide being of reduced transverse dimensions in the portion which serves as a radiating means, the transverse dimensions of said guide slowly decreasing from the section-of larger size to the section of reduced size, a plane boundary between said different dielectrics being disposed within the portion ofsaid guide which is or slowly decreasing size, transverse dimensions of the guide at the boundary between said difierent dielectrics being so related: to the dielectric constantsof the difierent dielectrics and the fre quency of the waves that reflection of said waves at said boundary is substantiall reduced.

2. A wave guide aerialfor the radiation or re ception ofelectromagnetic waves having a component of electric field along the axis of said guide, said guide having portions filled with different dielectrics, said waveguide having an aperture on: one side thereof wherebythe interior of said guide is coupled to the. exterior thereof, a discontinuity in the dielectric withinsaid guide at the position of said aperture, aportion of said exterior serving asradiating means, said wave guide being of reduced transverse dimensions in the portion which serves as a radiating means, the transverse dimensions of said guide slowly decreasing from the section of larger size to the section of reduced size, a plane boundary between said different dielectrics being disposed within the portion of-said guide which is of slowly decreasing size and means ateach end of said radi-- ating portion for inhibiting the flow of current along the exterior of saidguide-beyond said radiating portion, transverse dimensions of the guide at the-boundary between said different dielectrics being so related to the dielectric constants of the different dielectrics andthe frequency of the waves that reflection ofsaid waves at said boundary is substantially reduced.

3. A wave guide aerial for the radiation or reception of electromagnetic waves having a component of electric field. along the axis of said guide. said guide: having portions filled with different dielectrics, said wave guide having an aperture on one side-thereof whereby the interior of said guide is coupled-to the-exterior thereof,

- a discontinuity:inthe dielectricwithinsaid guide at the position of said aperture. a portion of said exterior serving as radiating means, said wave guide being of reduced transverse dimensions in the portion which serves as a radiating means, the transverse dimensions of said guide slowly decreasing from the section of larger size to the section of reduced size, a plane boundary between said difierent dielectrics being disposed within the portion of said guide which is of slowly decreasing size and means at each end of said radiating portion for inhibiting the flow of current along the exterior of said guide beyond said radiating portion, each of said means including a pair of conductiv fins on said guide spaced apart a distance equal to a quarter of the operating wavelength and extending away from the surface of said guide a distance equal to a quarter of the operating wavelength, transverse dimensions of the guide at the boundary between said different dielectrics being so related to the dielectric constants of the different dielectrics and the frequency of the waves that reflection of said waves at said boundary is substantially reduced.

4. A wave guide in the form of a hollow conductive tube for the transmission of electromagnetic waves of the transverse magnetic type and comprising two sections having different transverse dimensions, an intermediate section joining said first two sections, the transverse dimensions of the latter section slowly decreasing from the section of larger size to the section of smaller size, and different dielectrics disposed within said first two sections and extending in to said intermediate section with a plane boundary between said different dielectrics disposed within said intermediate section.

5. The wave guide claimed in claim 4 having a circular internal cross section.

EDWARD CECIL CORK. MICHAEL BOWMAN-MANIFOLD.

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

UNITED STATES PATENTS Number Name 7 Date 2,129,712 Southworth Sept. 13, 1938 2,142,138 Llewellyn Jan. 3, 1939 2,147,717 Schelkunofl Feb. 21, 1939 2,197,123 King Apr. 16, 1940 2,202,380 Hollmann May 28, 1940 2,304,540 Cassen Dec. 8, 1942 2,407,690 Southworth Sept. 17, 1946

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