US3276020A - Low profile surface wave antenna - Google Patents

Description

Sept. 27, 1966 MA|LLET 3,276,020

Low PROFILE SURFACE WAVE ANTENNA Filed June 7, 1961 5 Sheets-Sheet 1 F/GJ B 1 i I 02 I 2 O; f F/G.2 5!

5 SheecsSheet 2 Filed June 7, 1961 Sept. 27, 1966 H. MAILLET LOW PROFILE SURFACE WAVE ANTENNA Filed June '7, 1961 I3 Sheets-Sheet 5 United States Patent 1 Claim. (51. 343-705 The present invention relates to microwave aerials. Such aerials are generally excited at one of their ends by a primary source, for example, a horn. Y

lhis arrangement, which is generally satisfactory, requires the primary source to be matched to the radiating body. This matching may be difiicult to achieve if the radiating body .is very thin, for example, if it is a dielectric body along the surface of which surface waves propagate. As is known, in this case, the directivity is the greater, as the transversal dimension of the dielectric body is smaller. It is then extremely diflicult to match a very thin piece to a horn.

It is an object of the invention to provide an aerial which avoids this difiiculty.

The invention provides an aerial comprising a first radiating line which is energized by a second line extending parallel thereto and directly connected to the ultrahigh frequency energy source, this second line being in coupled relationship with said first line. The two lines are dimensioned in such a manner that, when there is no coupling, their phase propagation velocities are identical and their length is such that all the energy fed .to the secnd line by the source is absorbed bythe first line, on account of the coupling between these two lines.

The invention will be best understood from the following' description and appended drawing, wherein:

FIG. 1 is an explanatory diagram of the principle of the invention; i I

FIG. 2 shows graphically the field distribution along the two transmission lines illustrated in FIG. 1 when they are coupled to each other;

FIG. 3 shows an embodiment of a transmission line which may be used for providing a distributed excitation;

FIG. 4 is a cross-sectional view of-the line shown in FIG. 3;

51G. 5 is a perspective view of a dielectric plate aerial; an

FIG. 6 shows an aerial according to the invention, @0111? prising the lines illustrated in FIGS. 3 and 5;

In FIG. 1, two transmission lines 1 and 2 are shown along which the energy propagation is along parallel direct-ions. Line 1 is excited by a source 3 and is terminated in a matched load 4. Line 2 terminates at its two ends in respective matched loads Sand 7. The lines are such that, if, in the absence of any coupling'therebetween, travelling waves propagate along them, they propagate with the same phase velocity.

If 2 is the abscissa of two points M and M located in the same plane, perpendicular to the respective axes of lines 1 and 2, x and y being the two other coordinates in this. plane, the waves propagating along these lines may be expressed by'the following relations:

7; and 7 being the propagation coefiicients or constants; as is well known by propagation coeflicient or constant, is meant a transmission characteristic of a line or medium,

3,276,020 Ce Patented Sept. 27, 1966 2 which indicates the effect of the line on the wave being transmitted along the line;

- If lines 1 and 2 are coupled to each other, 6 being the coupling coefiicient, it may be shown that the field in line 1 results from two travelling waves, namely wave A and another wave A the propagation constant of which s: 'Y31='Y1- Similarly, in line 2, wave A has a propagation constant v =v +2a By coupling coefficient or coefficient of coupling is meant a numeral rating between 0 and 1, that specifies the degree of coupling between the two circuits.

- Waves A and A being in phase with wave A the field in line 1 maybe expressed by the following relation:

In line 2, wave A is in phase opposition with A and the field is then;

This is true even if a diflers from a it is also true when one of the lines, for instance line 2, is dissipative, on account of ohmic losses or radiation.

In FIG. 2, there is shown, for a low radiation, the distribution of amplitudes B and B along the axes of propagation of the two lines O z and O z Between 0 and a point of abscissa 1, line 1 gives up energy to line 2, the field amplitude decreasing with z in line 1 and increasing in line 2.

' Formulae 2 and 3 show that amplitudes B and B vary periodically with z, 'th'e'period being 1r/c.' For z=l, with the wholeof the energy is transferred from line 1 to line 2.

If the respective lengths of lines 1 and 2 are made equal to I, no energy is dissipated in load 4, the whole of the energy in line 1 being transferred to line 2. This energy is either radiated by line 2 or absorbed by loads 5 and 6.

Thus, by conveniently selecting the length of lines 1 and '2, line. 2 may absorb and radiate all the energy of line 1.

The invention thus provides a novel method for exciting radiating lines.

.An aerial of particular interest, incorporating the above principles, will now be described, by way of example, in more detail. FIG. 3 shows a flat-shaped line which does not radiate much energy and which may be used as line 1. It comprises a thin metal plate or strip 10, having a width a, which is separated from two practically infinite conductive plates 20 and 30 by gaps having a width b. The line is excited by a coaxial line 5, whose innerconductor 4 is coupled to plate 10, while plates 20 and 30 are connected to its outer conductor. Line 5 is excited by a source 6. In the embodiment shown in FIG. 3, the line assembly is supported on a dielectric plane support.

In FIG. 4, the same line is shown in cross-section. The lines of force of the electric field are in phase opposition in the two gaps. Consequently, the wave propagating in the line is a TEM-wave, the phase velocity of which is equal to the velocity of the light, with where A is the wave-length in the space.

' The line has a very low attenuation coeificicnt m the ohmic losses and the radiation losses being negligible.

FIG. is an embodiment of a transmission line which may be usd as line 2 of FIG. 1. It comprises a dielectric plate or strip 7, of a thickness '11, which is supported on a metal plate 8. Let Oz be the direction of propagation of a surface wave along the plate, 0x and 0y being the two directions perpendicular to Oz. If h is small with respect to the wavelength A i,e. if

21h h=- r. (6)

is very small, it can be shown that only a TM-wave can propagate in the plate, this wave having a propagation constant 0 which is substantially equal to 6 1)- sin 1 8) with v=B6L= The aerial gain is substantially G=2v 1 0 All .these results are well known in the art. But, up

to the present, it was not known to excite aerials of a reduced thickness h by means of horns located at one end of the line. As a matter of fact, the characteristic impedance of such a plate is the higher, as h is smaller. It is therefore difiicult to match a waveguide to sucha line. The present invention makes it possible to achieve this result FIG. 6 illustrates an assembly comprising lines 1 and 2. Line 2 comprises two conductive plates 81 and 82, having, a width largeenough to be considered infinite for all practical purposes. These plates respectively support dielectric plates 72. Plates 81 and 82 also serve as conductive plates 20 and 30 of FIG. 3, conductor 83 corresponding to conductor 10 of the same figure.- The assembly 81-82-83 thus buildsupline 1 of FIG. 1. The whole assembly hasthe length l defined by Formula 4. The coupling coeflicient c depends on the width h and the distance d between the gaps and-the edge of the dieletcric plates 72. This distance d must be small, since the coupling decreases very rapidly as d increases.

Equation v=sin =1 I defines thethickness h of the dielectric since V f ,i e .1- e l u and h=u/fi This thickness is sufficiently small relative to the wavelength for the phase velocity of the surface wave propa- 4 gating along line 2 to be equal to the velocity of light in free space. 7 In one embodiment of the invention, one had A =l0 cm. and h=0.4 cm. The dielectric selected had-a small loss angle (sih'rite) and 6:4.

. Under these conditions, one had and 18 -5 1,610

therefore The gain G is equal to 32, a half-waive dipole.

The above Formulae 6 and 9 show that parameters u and v, which define the radiating pattern of the aerial and the maximum gain, vary as a function of the first power of the frequency of the transmitted wave.

In addition, no device showing a significant Q-factor is made a part of the aerial. Therefore, the aerial can have a wide frequency range, which may attain and even exceed-theoctave, as confirmed by the experience. The experience has shown indeed that, for a frequency range extending from 2000 to 4000 megacycles, -i.e. for an average wave-length )t of 10 cm., the radiating pattern actually presents the characteristics according to Formulae 8, 9 and 10. V a In this range, the width of the radiation beam at 3 db below the maximum is close to 20 according to For.- mula 8. Y

The; maximum gain has been experimentally found to be comprised between 16 and 20 db, which is quite consonant with the theoretical results of Formula 10.

The aerial has a highly fiat structure. and can bereadily applied onto the surface of an aircraft or other flying body without afiecting the aerodynamic qualities thereof.

What is claimed is:

A device for radiating ultra high frequency energy comprising: a first and a second transmission line parallel to each other; said first line comprising a first conductive strip, having a thickness small relatively to the operating wavelength, extending ina predetermined direction, and having two parallel sides, two infinite and conductive first planes, respectively extending along said sides and separated therefrom by respective gaps, means to nonpling an ultra high frequency source to said line for creating electromagneticfields in said gaps; said second transmission line comprising two second strips of adielectric material, respectively supported. on said first planes, at the same distance from said gaps, said first strip and said second strips having the same length equal to 1r/ 2 C, being the coupling coefiicient between said two lines.

or 15.2 db with respect to References Cited by the Examiner HERMAN KARL SAALBACI-l, Primary Examiner.

GEORGE WESTBY, Examiner. i s; D. MILLER, E. LIEBERMAN, Assistant Examiners,

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