US2553166A - Multicellular microwave lens - Google Patents

Description

May 15, 1951 D. s. BOND MULTICELLULAR MICROWAVE LENS Filed June 25, 1947 v INVENTOR. Zonaldfiflorwl M fi r Mvxl Patented May 15, 1951 UNITED MULTICELLULAR MICROWAVE LENS Donald S. Bond, Philadelphia, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application June 25, 1947, Serial No. 756,955

2 Claims. 1

The present invention relates to radio transmission systems and more particularly to ultra high frequency radio relaying systems employing microwave frequencies.

An object of the present invention is to diminish the effects of fading in microwave transmission systems.

Another object of the present invention is to reducethe magnitude of reflected rays in microwave transmission systems and thereby to diminish the effects of fading due to interference between the direct ray and the reflected ray.

Another object of the present invention is to provide a structure which may be so oriented about its axis of symmetry as to convert an incident elliptically polarized wave into a plane polarized wave.

Another object of the present invention is the provision of a quarter wave plate adapted to be used with microwave radio transmission systems.

The foregoing objects and others which may appear from the following detailed description are attained by so orienting a microwave transmitting antenna that the wave emitted therefrom is plane polarized at an angle of approximately 45 with respect to the vertical. At the receiving location a quarter wave plate is placed in front of the receiving antenna. This quarter wave plate is so oriented about its axis of symmetry as to convert an elliptically polarized reflected component of the transmitted wave into a plane polarized wave. The reflected and converted plane polarized wave then will be oriented at an angle approaching 90 with respect to the plane polarized direct wave. The receiving antenna is so arranged as to discriminate strongly against the reflected component.

The present invention will be more fully understood by reference to the following detailed description which is accompanied by a drawing in which:

Figure 1 illustrates in diagrammatic form an arrangement of transmitting and receiving antennas in a microwave radio transmission system showing the direct and reflected signal ray paths between the transmitting and receiving antennas;

Figure 2 is a, vector diagram illustrating the direction of polarization of the plane polarized wave radiated from the transmitting antenna of Figure 1 as viewed from the receiving location, while Figure 3 is a vector diagram illustrating the relationship between the direct and reflected waves at the receiving location;

Figures 4 and 5 are further vector diagrams 2 illustrating features of the present invention, while Figure 6 is a perspective view of a quarter wave plate adapted to be used with the receiving antenna of Figure 1.

Referring now to Figure 1 there is shown a transmitting antenna TR and a receiving antenna RS mounted on towers T at spaced locations on the earths surface E. The figure illustrates a direct ray path D between the transmitting and receiving antennas and also a reflected ray, the incident ray being identified by reference character I and after reflection by reference character A. The wave propagation in Figure l is shown as taking place over water primarily because the fading conditions which the present invention is designed to mitigate are particularly obnoxious under such conditions. However, it should be clearly understood that the present invention is not limited to radio transmission systems operating over water but may as well be used in other microwave transmission systems.

The present invention will be considered under the assumption that the water at E is of infinite conductivity. Then, the modifications necessary to take care of the case of finite conductivity of the reflecting surface will be considered. The antenna TR at the transmitting location is so oriented that the radiated wave i plane polarized at an angle of 45 with respect to the verical. When viewed from the receiving location RS, this is illustrated in Figure 2 as vector D forming the angle a with respect to the vertical. The vertical component of the radiated wave is denoted by vector D and the horizontal component by the vector D The incident wave denoted by reference character I in Figure 1 just before reflection at C has the same polarization. However, upon reflection at point C there is a change in amplitude and phase given by Fresnels laws of reflection. While Fresnels laws of reflection in optics give the ratio of reflected to incident light for reflection from any plane surface, the results apply equally to a radio wave because the derivation thereof is based upon principles that apply correctly to any type of electro-magnetic waves. Furthermore, in optics it is known that when polarized light is incident on a reflecting surface such as glass plate, reflection does not take place for a certain angle of incidence and for a certain plane of polarization. This angle of inci dence is known as the Brewster angle. By analogy with the foregoing condition for the reflection of light, there is an angle of incidence 4mm, for the radio wave case known as the pseudo-Brewster angle. The vertical component of the reflected wave at this angle will be a minimum but not necessarily zero. For infinite conductivity of the reflecting surface, the pseudo-Brewster angle occurs at an angle =0 and the reflected components A and. A of vertical and horizontal direction are equal to the vertical and horizontal impinging components I and I at the point of reflection. However, the direction of the reflected vertical component, A coincides with the impinging vertical component, I or the direct vertical component, D while the reflected horizontal component, A is rotated 180 with reference to the horizontal components, I or D As viewed from the receiving location RS, this condition is illustrated in Figure 3.

A consideration of this figure shows that the direct ray D having components D and D as shown in Figure 2 received at the receiving location RS is polarized at an angle ,8 with respect to the reflected component, A. The difference in direction of arrival may be neglected inasmuch as the angle is usually small. When the angle a is equal to 45 (Fig. 3), it is evident that the angle 5 is 90. The receiving antenna, if oriented to receive only signals polarized in direction OD (Fig. 3). will entirely reject the reflected wave indicated by the vector A0 of Figure 3. Thus, where the reflecting surface is of infinite conductivity, a simple orientation of the planes of polarization of the transmitting and receiving antennas will result in a total discrimination against the reflected wave component.

Now, we will consider the case of finite conductivity of the reflecting surfaces. The complex dielectric constant of the surface E is given by the following expression:

where the conductivity i1 is in electromagnetic units and the frequency f is in megacycles per second. .The complex reflection coeflicients K and K for vertical and horizontal polarization respectively are given by the following expressin /e lisin it For finite conductivity and with the angle of incidence much less than the pseudo-Brewster angle, the direction of the reflected verticallypolarized component will be approximately 180 reversedwith respect to the similar component A illustrated in Figure 3 for the case of infinite conductivity. Theory indicates that at grazing incidence for a surface of finite degree of conductivity, destructive interference of both vertical and horizontal components of an incident plane wave should occur. For this and other related reasons, this case lies outside the scope of my invention.

On" the other hand, for finite conductivity and for the angle of incidence of the order of or greater than the pseudo-Brewster angle, the directions of the reflected components of vertical and horizontal polarizations will be substantially as shown in Figure 3. This presupposes that the pseudo-Brewster angle and the angle are both small. Otherwise, from the geometry of the case, vectors I and A while always lying in the plane including TR, C, and RS, will diverge from each other until at =90 they will be oppositely oriented. However, the present invention contemplates operation where the angle is not too far different from the pseudo-Brewster angle mentioned above. To illustrate the order of magnitude of the pseudo-Brewster angle (brain as a function of dielectric constant, e, conductivity, and frequency, one may refer to typical examples given in the table.

Table Dielectric Degrees 25 1 ll. 5 25 10 ll. 5 25 100 5. 0 25 1000 2. 5

l0 1 l7. 5 l0 l0 l4. 5 10 100 5. 5 10 1000 5 4 1 2c. 5 4 l0 l9. 0 4 100 6. 0

As an example fo the case of finite conductivity, assume the conditions where the vertical component of the reflected wave A =0.5I and the angle 0 assume the vector A =I and 0 =0 as is illustrated in Figure 4. The resultant reflected wave is elliptically polarized with the major axis at an angle 0' with respect to the vector A (Fig. 5). The construction of the ellipse follows from the relationship:

sin (av-"05) 2% wherein 0 and 0 are the phase shifts due to the reflection of the incident waves of vertical and horizontal polarization respectfully.

Now, it will be seen that there is no possible orientation of a receiving antenna of the simple dipole type about an axis of rotation normal to the axis of the dipole, said axis of rotation lying in the direction of the received ray, at which a null of signal from the elliptically polarized reflectcd wave will be received. There will only be a broad minimum when the dipole is at right angle to line ROR of Figure 5.

However, by the interposition of a quarter Wave plate Q (Fig. 1) this difficulty may be overcome. The quarter wave plate has the property that the phase velocity of the Wave propagated throughout is dependent upon the direction of polarization. One form of quarter wave plate is illustrated in perspective in Figure 6 and may consist of two sets of thin parallel metal strips [5 and 20 oriented at right angles with respect to each other to produce a series of open rectangular cells 30 of unequal edge dimensions d and d The axis M-M' is perpendicular to both edges of dimensions d and d respectfully. The directions of said edges will be referred to as the principal directions V and H respectively in Fig. 6. Each cell then becomes a wave guide whose phase velocities are different for the different polarizations H and V respectively. For finite values of the dimensions d and d the phase velocities of propagation 12 and o through the cells are greater than the velocity of light. The thickness of the quarter wave plate dimension iril WJUEW de is chosen so that the path length, measured in wavelengths, is one quarter or an odd number of quarter wavelengths longer for one component, for example, the vertical component V than fo the other. This dimension is given by the following expression:

A 1 1 )\V )\H where and A are the wavelengths of vertical and horizontal wave components, respectively.

For the practical case where the dimension d is made large so that the velocity v approaches the constant c, the velocity of light,

wherein A is the wave length in free space. The quarter wave plate is placed with axis M-H along the directions of the received reflected wave A of Figure 1. It is then rotated about said axis so that its two principal directions H and V of Figure 6 coincide with the major and minor axes of the ellipse of Figure 5, that is, the principal direction V of Figure 6 coincides with the direction of axis SOS of Figure 5 while the dimension H is parallel to the axis ROR' of Figure 5. The components of-the wave along axes OR and OS differ by 90 in phase. As a consequence of passage of these components through the quarter-wave plate the two components again go into an in-phase relation and lie along the direction TOT (Fig. 5). This direction is then the plane of polarization of the reflected wave upon its arrival at the antenna at RS. It is evident that the angle of polarization 6 (Fig. 5) depends only upon the relative magnitude of the vertical and horizontal components at reflection; that is tan 6- If the length dimension of the dipole of the receiving antenna at RS is made to coincide with the line OT of Figure 5, there will be no reception of the reflected wave. It will be noted that the angle 6 of Figure 5 is not in general equal to the angle a of Figure 3. Thus, there will be some reduction of the strength of the directly received signal. However, this rejection depends upon the factor cos ((1-6) that is, in general, small. Furthermore, if neither principal direction of the quarter-wave plate coincides with the direction OI (Fig. 3) the direct ray will be elliptically polarized to some extent. However, when the angle (a'y) is small the eccentricity will remain very high. The longitudinal axis of the quarter- .wave plate Q will not in general coincide with both the reflected and direct rays but in practice may be set at some compromise angle. None of the effects cited immediately acts to reduce the intensity of the direct ray very seriously.

While I have illustrated a particular embodiment of the present invention, it should be clearly understood that it is not limited thereto since many modifications may be made in the several elements employed and in their arrangement and it is therefore contemplated by the appended claims to cover any such modifications as fall within the spirit and scope of the invention.

What is claimed is;

1. A device for converting elliptically polarized radiant energy waves to plane polarized radiant energy waves in the form of a metallic-walled multi-cellular plate with parallel arranged cells equal to each other in dimensions, the transverse dimensions of the cells of said plate having values effective to form wave guides for said radiant energy waves, the vertical dimensions of said cells being difierent from the horiozntal dimensions to provide different velocities of propa-- gation for waves at the operating frequency polarized in the directions of the transverse dimensions of said cells, the thickness of said plate being selected so that the path length through said cells measured in wavelengths is one quarter or an odd number of quarter wavelengths longer for waves polarized in the direction of one of said vertical and horizontal dimensions than for waves polarized in the other, said cells being formed by thin conductive strips crossing one another.

2. A quarter Wavelength receiving plate in the form of a multi-cellular metallic plate with parallel arranged metallic-walled cells of uniform thickness, each cell having dimensions equal to that of each other cell the vertical and horizontal dimensions of the cells of said plate having values effective to form wave guides for radiant energy waves, the vertical dimensions of said cells being different from the horizontal dimensions to provide different velocities of propagation for vertically and horizontally polarized waves of the same operating frequency, the uniform thickness of said plate being selected so that the path length through said cells measured in wavelengths is one quarter or an odd number of quarter wavelengths longer for waves polarized in the direction of one of said vertical and horizontal dimensions than for waves polarized in the other to convert elliptically polarized waves at the operating frequency into plane polarized waves.

DONALD S. BOND.

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

UNITED STATES PATENTS Number Name Date 1,934,924 Heintz Nov. 14, 1933 1,958,886 Chubb May 15, 1934 2,129,669 Bowen Sept. 13, 1938 2,153,209 Scharlau Apr. 4, 1939 2,206,683 Wolff July 2, 1940 2,405,992 Bruce Aug. 20, 1946 2,464,269 Smith Mar. 15, 1949 FOREIGN PATENTS Number Country Date 668,231 Germany Nov. 28, 1938 OTHER REFERENCES Publication: Metal Lens Antenna by W. E. Kock, Proc. IRE, November 1946.

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