US2553167A - Microwave radio transmission system - Google Patents

Description

May 15, 1951 I D. s. BOND MICROWAVE RADIO TRANSMISSION SYSTEM Original Filed June 25, 1947 lNVENTOR .DON LD 3. BOND sY Q:

ATTORNEY Patented May 1951 MICROWAVE RADI SYS o TRANSMISSION TEM Donald S. Bond, Philadelphia, Pa., assignor to Radio Corporation of America, a corporation of Delaware Original application me 25, 1947, Serial No.

756,955. Divided and this 1948, Serial No. 28,127 V 7 Claims. (Cl. 343100) The present invention relates to radio transmission systems and more particularly to ultra high frequency radio relaying systems employing microwave frequencies.

This application is a division of my copending application Serial No. 755,955, filed June '25, 1947, which contains claims directed to the device for converting circularly polarized radiant energy waves to plane polarized waves, whereas this divisional application contains claims directed to a radio system for reducing fading.

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 reduce the magnitude of reflected rays in micro- Wave 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 polar ized 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 followin 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:

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

Fig. 2 is a vector diagram illustrating the dimetion of polarization of the plane polarized wave application May 20,

radiated from the transmitting antenna of Figure 1 as viewed from the receiving location, while Fig. 3 is a vector diagram illustrating the relationship between the direct and reflected waves at the receiving location;

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

Referring now to Fig. 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 Iand after reflection by reference character A. The wave propagation in Fig. 1 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 is plane polarized .at an angle of with respect to the vertical.

When viewed from the receiving location RS, this is illustrated in Fig. 2 as vector D forming the angle 0. 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 Fig. 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 particular angle of incidence is known as the Brewster angle. By analogy with the foregoing condition for the reflection of light, there is an angle of incidence 4min, for the radio wave case knOWnas 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 350 and the reflected components A and A of vertical and horizontal direction are equal to the yerticaland horizontal impingin components :I and :I -at the point of reflection. However, the direction of the reflected vertical component, A coincides with the impinging vertical component, Iv, 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 Fig. 3.

A consideration of this figure shows that the direct ray D having components D and D as shown in Fig. 2 received at the receiving location RS is polarized at an angle 5 with respect to the reflected component, A. The difference in directionof 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 6 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 Fig. 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 7 is in electromagnetic units and the frequency f is in megacycles per second. The complex reflection coefiicients K and K for vertical and horizontal polarization respectiyely are given by the following expressions:

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 reversed with respect to the similar component A illustrated in'Fig. 3 for the case of infiinte 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 Fig. 3. This presupposes that the pseudo-Brewster angle and the angle 5 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 dmi as a function of dielectric constant, 5, conductivity, andfrequency, one may refer to typical As an example for the case of finite conduc tivity, assume the conditions where the vertical component of the reflected wave A =0.51 and the angle 0 assume the vector A =I and 03:0 as is illustrated in Figure 4. The resultant reflected wave is elliptically polarized with the major .axis at an angle '7 with respect to the vector A (Fig. 5). The construction of the ellipse follows from the relationship:

s n (aw-0H) =3-Z where 0 and 0 represent the phase shifts due to the reflection of the incident waves of vertical and horizontal polarization respectively.

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 reflected 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. Thequarter wave plate has the property that the phase velocity of the wave propagated throughout is dependent upon the direction of polarization. One form ofquarter wave plate is illustrated in perspective inFig. 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 (i and 1 Each cell then becomes a wave guide Whose phase velocities are different for the different polarizations H and V respectively. For finite values of ei e v' n' s he gees ac n s. o

greater than the velocity of light. The thickness of the quarter wave plate dimension lie 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 for the other. This dimension is given by the following expression:

where A and A are the wavelengths of vertical The quarter-wave plate is positioned with longitudinal axis MM' along the direction 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 ccincides with the direction of axis SOS of Figure 5 while the dimension H is parallel to the axis ROB, 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 inphase relationship 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 relative magnitude of the vertical and horizontal components at refiection; that is v 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 5 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 (a] and 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 (Ti-w) 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. For example, while Fig. 1 appears to show that the reflecting surface called for in each of the claims in the case lies on a line between the transmitter TR and the receiver RS, it should be obvious that there will be many situations in which the reflecting surface will lie at a substantial distance to either side of the aforesaid line and still produce the desired results.

What is claimed is:

1. In an ultra high frequency radio system including a transmitting antenna and a receiving antenna at spaced locations, there being a reflecting surface located between said locations whereby some of the energy from said transmitting antenna arrives at said receiving antenna by reflection from said surface, the energy radiated from said transmitting antenna impinging on said reflecting surface at an angle at which the energy reflected from said surface is elliptically polarized, means between said reflecting surface and said receiving antenna for converting said elliptically polarized energy into plane polarized energy at an angle different from that angle of polarization of the energy proceeding directly from said transmitting antenna to said receiving antenna, said receiving antenna being so oriented as to have its maximum response to said directly proceeding energy.

2. A radiant energy system including a source of plane polarized radiant energy waves and a receiver for said waves at spaced locations, there being a reflecting surface between said locations whereby some of the energy from said source arrives at said receiver by reflection from said surface, said radiant energy waves being propagated at an angle with respect to said reflecting surface that said reflected energy is circularly polarized, a converting structure in front of said receiver acting to convert said circularly polarized energy into plane polarized energy at an angle different from that angle of polarization of the energ arriving directly from said source, said receiver is so oriented as to be unresponsive to said converted plane polarized energy. I

3. A radiant energy system including a source of plane polarized radiant energy waves and a receiver for said waves at spaced locations, there being a reflecting surface between said locations whereby some of the energy from said source arrives at said receiver by reflection from said surface, said radiant energy waves being propagated at an angle with respect to said reflecting surface that said reflected energy is elliptically polarized, a converting structure in front of said receiver acting to convert said ellipticall polarized energy into plane polarized energy at an angle different from that angle of polarization of the energy arriving directly from said source, said receiver being so orient-ed as to be unresponsive to said converted plane polarized energy, said converting structure being in the form of a multicellular plate, the vertical and horizontal dimensions of the cells of said plate being so related to the operating wavelength of said system as to give different velocities of propagation of vertically and horizontall polarized components of radiation therethroughf the thickness of said plate being such as to convert circularly polarized radiation incident on said plate to emerging plane polarized radiation.

4. A high frequency radio system including a transmitting antenna adapted to radiate plane polarized Waves and a receiving antenna for said waves at spaced locations, there being a reflecting surface between said locations whereby some of the waves from said transmitting antenna ar- :rive atsai'd receivinganaennarby reflection: from :said :surface, :said transmitting :antenna': bein :50 oriented with :respect to .said ;refiecting zsurfai that said reflected waves are .elliptically polarized, a converting structure in front of said 112 .ceiving [antenna acting to convertgsaidelliptically polarized waves -:.to waves plane polarized at an angle different from that anglciofirmlarizatiln of theenergy arriving directly from said transmittingantenna, said receiving antenna being so oriented as :to be substantially unresponsive to saidconverted plane polarized waves.

"5. .A high frequency radio system including a transmitting antenna adapted to radiate plane polarized waves and a receiving antenna for-said waves at spaced locations, there being a reflectin surface between-said locations whereby some of the waves from said transmitting antenna arrive at said receiving antenna by reflection from :said surface, said transmitting antenna being so oriented with respect to said reflecting :surface that said reflected waves are ellipticallypolarized, 'a converting structure in front of said receiving vantenna acting to convert said elliptically polarized Waves intoplane polarized waves at an angle different from that angle of polarization of the energy arriving directly from said transmitting antenna, said receiving antenna being so oriented as to belsubstantiall unresponsive to said converted plane polarized waves, said converting structure being in the vform of a multicellular plate, the vertical and horizontal dimensions of the cells of said plate being so related to the wavelength of said waves as to, give different velocities of propagation of vertically and horizontally polarized waves therethrough, the thickness of said plate being such as to convert circularly polarized waves incident on said plate to emerg- 1 plane polarized waves.

6. A radiant energy system including a source of plane polarized radiant energy waves, a receiver for said waves spaced from said source, said receiver having a response to said waves varying in accordance with the orientation of the plane of polarization thereof with respect to said receiver, there being a reflecting-surface between said-source and said receiver whereby wave energy from said source arrives at said receiver over both a direct path and by way of reflection from said surface, the plane of said polarized energy waves being oriented with respect to said reflecting surface whereby said wave energy arriving at said receiver byxway1of saidgreflectionfis; llip- =tica1ly polarized, and ,meansjlocated tat saidzreceiver to convert said elliptically'polarized-waves into plane polarized waves oriented :t provide minimum response at said receiver tosaid ir flectedwave energy.

' 7. A; radiant 1 energy system including a source of plane polarized radiantenergy :waves,;a :receiver for said waves spaced from said source, said receiver having a response to said waves varying in accordance with the orientation of the plane of polarization thereof, ther being a r ect n uriae e een sai s urc and ai receiver whereby waveenergy from said source arrives at said receiver over-both a gi L I andby way of reflection fromsaidsur in f sa d pol i d ens-tier W v s et e ented with respect to .said reflecting euriace whereby said wave energy arriving at said receiver by way of said reflection is elliptically polarized, and a multicellular plate, the vertical and horizontal dimensions of the cells of said plate having dimensions in ,terrnsof the oper- .ating wavelength to provide difier-ent velocities .of propagation of vertical and horizontal components of said ellipticallypolarized waves .therethrough andthe thickness of said plate having. a dimension at which said elliptically polarized waves are converted into plane polarized waves,

said plate being,orientedtoprovide minimumresponse .at .said receiver ,to ,said reflected wave en y.

DONALD s. Born),

EFEREN ES L I ED The following references are of record in the file ,0 1 this patent:

UNITED STATES PATENTS OTHER REFERENCES Metal Lens Antennaf by E. Kock, Proc. I. R. E.,-vol. 34, pages 828 to 836, November 1916.

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