US2660718A - Microwave protective system - Google Patents

Description

Nov. 24, 1953 H. R. SUMMERHAYES, JR ETAL MICROWAVE PROTECTIVE SYSTEM Filed Dec. 30, 1949 n, J m m y a o IK: u 5 w; 9 n M P 0 n m 0 m w w a t m m r I L n m m 4A p i a v q vw .m I X Y C T 2 .v Al. A rdg B y 3 VJ 3 2 A 1 H b 8 A x W M l N p f m w m A v 7 m w Patented Nov. 24, 1953 j Ellswofthpj cube; Scotia; N.'Y.,;assinors' to GeneraL-Electric Company, a 'corporation of New York a was. (cm-404th) ".hhother characteristic 6f the system which is semetiir'ies desirable is that" the 's'ystem *does not ini'pede necessary 'tfaific yilhe'n the system need not hein'iise. In" sueh' a'easefanbbstiuotionsuoh as a' eneeis impractibame b eafise it is noteasn removed 'anu'iater -rpialdt Piotection of-an airport is an example'in whioh a fenee inight be uhi'iesii'able. '"Proteetion of aspan of water is" another such eiiarnple. 7

T0 prevent an intruder from discovering and using armeans'wherebytheprot eetive uevicemay be 'outwitted, it is desira'ble that theintruderhe unaware of the existence of the protetive'sys tein. "Another desirable characteristic 'is' thatl'the requiredla'pparatus to outwit the "device be too complex or too bulky tohe readily transported without being notice'd.

In accordance With thisinvention, an object entering. a given area is detted although there is*no visible means ofpiotection at'the'pointflin question. This system is unaffected by :normal ehanges in weather conditions and no physical obstructions are required. Very briefiy,this is ac'complishe'd with a microwave signal transmitted by an antenna toward-a pair-fofreeiving antennas which are connectedtoa balanced receiver. If an intruder pa'sses imam-e transmitted wave, his en'try'into the field'either. mbalances theyamplitudes, of signalsw'at the receivenby'abso ptionjof part of. the wave orthe signal eflected from him arrives at the two receiving antennas out of phase and unbalances thereceive'r, either of which produces aisignal. e

therefore, an ohjedt of..this[' przsvide a reliahle proteuve siysti n v et t an p i te ing a peq fie area.

An theI objecjtfof this invention the pi ovi- 9 -P ap ot y m Whih fb'ei 'nf feet'ed by normal changes in weatheneon'ditions asl nbw, '"a eta 4.

'fii i e '05 3 ;b fii l i iwi i h pfl iiis16 qifa proteetiv 't'e m" 'tvhi'eh 'ap'osible 1,? 1 1?? wi t mf fu r A "still further obj 'e'et 'of this invention the time when w believe Ito {be bhai-ae'tei'istihfj thiemyenuon are set forth with pai ticulai'ityin thappnded claims. This invention itseif; however, Both as to 'its orgafiiiation and hie'thzid titoperatien inay testb tndrsteou by rfeieneto the'fig resofj the aecompanying tiraw'irig} in whi hFigfl' is a plan v-iewof the-entire fsystmin 'scheiii'atic form showing the vvave propagation-from'ati ansinitting antenna wh'ih is utiliz'd injeai'ryin'g this invention into effect; Fig. 2 is a '131 aii vieit'r oa rnogiified for'm'of this invention showing he wave propagation from a polarized transmitting antenna; and Fig. '3=is a table (if vectors vthih 'failitates an understanding of the operation of the system "shown iniFig. 2. .k a

RferringttotFiga 1, there is shown. at-high frequencyitrahsmitter 1, which is of a conventional type comprisingl'anlulti'a high frequencyose'illator12, a=mdfiulatorfstage in whiehthe high frequency signaliisamodulated with a low'fred'ueney signal,v and preferably-Lone or more stages of amplifieationxiirwTheihigh lfrequency electromagnetic avvave :fgenerate'd and modulated; with, for. example,raesiiity;:cycle .=wave. in vvthe transmitter:l ,;is fed through a suitable matching .net work, whih isHshown-in thedrawing as a high frequency coaxial-"cabled; to. a transmittingante'nna systemci"butmhich mayalso be a stub matching .system or some other high frequency matching day-ice... H; Y. a v. Althoughatdirectiveantenna systemi comprising a simpleadipolezfiyand .a parabolic reflector 1 isv shown in theidrawing, the functioning of this invention is not dependent upon the employment 011a directive typetransmitting antenna. In: some appliations of this invention; such .1 as. the,.ease in whiema transmitting antenna is positioned in the;.c.enter nf. a largegarea to be jprotectedyand aplurality oizreceivingastations are placed around it a non.-directiona1,. antenna is advantageous; Threfore the;tditctional :antenna.-.syst,e1m 5 .has heentshownionly for thelpurpose ofiacilitatine an understanding 6f the operation of this inven .Transmitting antennasys'tem iradiates .the modulated electrhiagrie'ticwave Whichuwas gennot have directional characteristics, are shown ceived by these antennas I and l l is fed through 2. suitable matching network to a balanced receiver it. High frequency coaxial cables I1 and I8 are shown in the drawing as the matching networks.

The balanced receiver It consists of a pair of conventional detectors l9 and 20 and an amplifier 2! which produces a signal when the diilerence between the signals applied to its input terminals is changed. The outputsof detectors is and 2e are individually variable which permits their output signals to be made equal even though their input signals difier. Because the outputs of the two detectors l9 and 20 may thus be made equal when the signals received from receiving antennas l9 and I! are unequal, it is unnecessary for the receiving antennas in and l i to be equidistant from the transmitting antenna system 5.

in the input signals to the detectors changes, their outputs will not be equal. This difference in output signal is amplified by the amplifier, and

output of the amplifier maybe used to sound alarm or perform some other task.

The direct rays 22 and 23 leave the transmitting antenna system or antenna and go directly to the receiving antennas l0 and H. These particular direct rays 22 and '23 are only two of the many direct rays which travel directly between the transmitting antenna 5 and the receiving antennas Ill and I i. In addition to these direct rays, there may be other rays which reach the receiving antennas which do not travel directly between the transmitting antenna and the receiving antennas. These other rays include those reflected from the ground and those reflected from permanent obstructions within the operative When an intruder 24 enters into the area included within the boundaries 8.9, l2 and I5, the transmitter field will be modified either by ab-" sorption or reflection, a number of the transmitter rays being reflected from his'person and many others being absorbed by his person. One such ray'25 may strike the intruder 24, and many rays will be reflected from him in many directions. Two of these reflected rays 26 and 21 travel to the receiving antennas l0 and II, respectively,

Where they excite voltage signals which are supplied to the balanced receiver l6.

The voltage signals developed atthe receiving antennas by the reflected rays will, in all prob-- ability, be out of phase because the distance from; the-intruder 24 to-receiving antenna lllis shorter The balanced receiver, which is 50 adjusted Once the output signals from these detectors have been made equal, if the'difference than the distance between the intruder and receiving antenna H. If the difference between the distances traveled by the reflected rays from the intruder .to the two receiving antennas is exactly one wavelength, a condition which cannot be maintained by an intruder, the signals arriving at the antennas are in phase and no unbalance of the receiver results. If the transmitter field is modified by absorption, then on entering the protected zone the intruder causes an unbalance of the receiver.

A modification of this system which increases its sensitivity is the use of transmitted polarized waves in place of the unpolarized waves previously described; It is known that an electromagnetic wave which has been reflected from a fiat surface is partially polarized. This is because the components of the wave in some planes of polarization are absorbed more by the reflecting body than are the components of the wave in some other planes of polarization. Hence, the physical shape of the reflecting body determines the degree of absorption of a polarized electromagnetic wave. If the transmitted wave is not completely polarized, which is so in a practical case, this greater absorption of the component of the wave in one plane of polarization than in another causes the plane of. polarization of a reflected polarized wave to be rotated.

Referring to Fig. 2, a polarized electromagnetic wave. transmitting antenna 26, represented conventionally in the drawing, transmits a polarized wave. that is represented by the vector 21'. Because Fig. 2 is a plan View of the system, the direction of thevector 21 indicates that the transmitting antenna radiates a horizontally polarized wave, and .it is conventionally termed a horizontally polarized antenna.

A pair of receiving antennas polarized in thedirections indicated by the arrows in column A of the table in Fig. 3. Antennas 28 and 29 are each polarized at an angle of degrees with respect to the plane of polarization of the transmitting antenna 26 and at an angle of 90 degrees with respect to one another. Hence, they are said to be cross-polarized.

Column B of the table indicates the direction and magnitude of the direct polarized electromagnetic waves arriving at the receiving antennas 28 and 29. The components of the direct ray which are in the planes of polarization of the receiving antennas are equal or very closely thereto. The signal voltages which are developed at the individual receiving antennas are consequently also equal and are supplied through the 'coaxial cables 30 and 3| to the balanced receiver 32. Because the input voltages to the balanced receiver are equal, its output is zero.

Referringagain to Fig. 2, an intruder 33 .is in a position to reflect and to absorb many rays radiated by antenna 26. One such ray 21, striking the intruder, is reflected by him in many directions. Two of these directions are represented by rays 34 and 35 which travel between the intruder and the receiving antennas 28 and 29, respectively.

Having been reflected from the intruder, the planeof polarization of these waves 34 and 35 is, in general, somewhat different from that of the transmitted rays. tation of the plane of polarization of an electromagnetic wave upon reflection was previously explained. A possible set of the directions and. magnitudes of the reflected wave arriving at 23 and 29 are Thereason for. this 1'01 5, 1 receiving antennas 28 and: 25%? is indicatediin: columnrC. Qfl' the. tablein Ei z. 3;;

Th commentertheserefiecteiiiwaveainethe direction of the, planeot. polarization f tenna 28 is greater than the component of the refiected wave in the direction-of the plane of polarization of antenna z-e, There will thushe a larger-- signal voltage developed at antenna 2 8 than at antenna 29. This change in the difference between the voltages applied through coaxial cables 30 and 3| to the balanced receiver 32 will, by means previously discussed, cause an output signal to appear at the output of the receiver. Of course, absorption of one of the direct rays will also unbalance the receiver.

In addition to the unbalance of the receiver which occurs because of the change of the plane of polarization of a wave when it is reflected, there is the condition of phase difference between the waves arriving at the receiving antennas 28 and 29 which occurred in the nonpolarized system. It is the combination of these two effects which makes this system more general than the simple unpolarized system.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. Apparatus for protecting an area by means of a microwave electromagnetic system comprising an oscillator for generating a microwave signal, an amplifier for amplifying said signal, a directive antenna for radiating said signal through an arc in a predetermined direction, a pair of directive receiving antennas, said receiving antennas being so positioned that they are most sensitive in the direction of said directive antenna, and a balanced receiver connected to said receiving antennas for indicating a change in the relationship between the strengths of the signals received from said receiving antennas.

2. Apparatus for protecting an area by means of a microwave system comprising means for producing a microwave signal, a directive trans mitting antenna for radiating said signal a pa r of directive receiving antennas for receiving said radiated signal, said directive antenna radiating said signal toward said directive receiving antennas, said directive receiving antennas comprising arrays positioned to have their maximum sensitivity in the direction of said directive transmitting antenna, said directive receiving antennas being separated from one another by a distance greater than one wavelength, and a balanced receiver connected to said receiving antennas for indicating a change in the relationship between the strengths of the signals received from said receiving antennas.

3. Apparatus for protecting an area by means of a microwave system comprising means for producing a microwave signal, a transmitter for radiating said signal, a pair of directive receiving antennas for receiving said radiated signal, said receiving antennas comprising arrays positioned to have their maximum sensitivity in the direction of said transmitting antenna, said directive receiving antennas being separated from each other by a distance greater than one wavelength, a balanced receiver connected to said receiving antennas, said balanced receiver including means for indicating a change in the relationship between the strengths of the signals received from said receiving antennas.

4. Apparatus for protecting an area by means of a microwave electromagnetic system comprising means for producing a microwave electromagnetic signal, a polarized transmitting enna i n-radiatin aidsland; inaonlaone p a of pplelzi aticn. aepeir' z.croesenolariz d receiving 6 antennas, and a balanced receiver properly--connected to saidreceiving antennas; forindicating a change irr the relations h ip ;between the strengthsotthe signals received by; said" antennas.

53- Apparatus for protecting anarea by; means;- of ax-microwa-ve electromagnetic system com-prising a means for producing a microwave electromagnetic signal, a polarized directive transmitting antenna for radiating said signal in only one plane of polarization in a predetermined direction, a pair of cross polarized receiving antennas for receiving said signal, a balanced receiver connected through a matching device to said polarized receiving antennas for indicating a change in the relationship between the strengths of the signals received from said antennas.

6. Apparatus for protecting an area by means of a microwave electromagnetic system comprising an oscillator for producing a microwave electromagnetic signal, a polarized directive transmitting antenna for radiating said signal through an arc in a predetermined direction, a pair of polarized receiving antennas for receiving said signal, the plane of polarization of one of said receiving antennas being at right angles to the plane of polarization of the other said receiving antenna, said polarized directive transmitting antenna radiating said signal toward said receiving antennas, and a balanced receiver connected through a matching network to said polarized receiving antennas for indicating a change in the relationship between the magnitudes of the signals received from said antennas.

7. Apparatus for protecting an area by means of microwave system comprising means for producing a microwave signal, a polarized directive transmitting antenna for radiating said signal in asingle plane of polarization and through an arc in a predetermined direction, a pair of polarized directive receiving antennas for receiving said signal, said receiving antennas being cross-polarized, said polarized directive transmitting antenna radiating said signal toward said polarized directive receiving antennas, the maximum sensitivity of which are in the direction of the transmitting antenna, and a balanced. receiver connected to said receiving antennas for indicating a change in the relationship between the strengths of the signals received from said receiving antennas.

8. Apparatus for protecting an area by means of a microwave system comprising means for producing a microwave signal, a polarized directive transmitting antenna for radiating said signal in a single plane of polarization and through an arc in a predetermined direction, a pair of cross-polarized receiving antennas, the plane of polarization of each of said receiving-antennas making an angle of 45 degrees with the plane of polarization of the transmitting antenna, said polarized directive transmitting antenna radiating said polarized signal toward said receiving antennas, the maximum sensitivity of which are in the direction of the transmitting antenna, a balanced receiver connected to said receiving antennas, said receiver including means for indieating a change in the relationship between the strengths of the signals received from said receiving antennas.

HARRY R. SUMMERHAYES, JR. ELLSWORTH D. COOK.

(References on following page) References Cited in the file of this patent UNITED STATES PATENTS Number Name Date Chubb May 14, 1934 Wolff Apr. 16, 1940 Southworth July 9, 1940 Lindsay et a1 June 24, 1941 Hammond Feb. 10, 1942 Number 5 Number 8" Name Date Salinger June 6, 1944 Malter Dec. 17, 1946 FOREIGN PATENTS Country Date Great Britain Aug. 8, 1940 Great Britain Oct. 22, 1946

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