US2640915A - Circularly polarized antenna arrangement for radar - Google Patents

Description

June 2, 1953 I w. SICHAK 2,540,915

CIRCULARLY POLARIZED ANTENNA ARRANGEMENT FOR RADAR Filed Dec. 28. 1950 TEAIZ5-CIYER ZR. '1] TUBE O day? 1 l0 2 72R.. 25 7a TmNs-rE/vER/ TUBE V A a 7a ANTENNA EMENT6 #7 z 'z A C 9 6 0 i B /a- A [4 F 1.6 /5 L E ZR 4 was a a 2' Z 7'0 ANTENNA ELEMENT6 7a ANTENNA ELEMENT6 n rmNs ceNsnl 708530 v INVENTOR WILL IAM SICHAK through the arrangement.

Patented June 2, 1953 UNITE STATES PATENT OFFICE 9 cl ims. (01. capo-.13)

-The present invention relates to circularly polarized antenna arrangements for use especially with radar and articularly .to'such an arrangement which transmits circularly polarized waves of onescrew sense and isadaptedto receive circularly polarized wave of the opposite .SGrew sense.

Ithas heretofore been-proposed. to use circular- 1y polarized antennas for radar. See article in the Proceedings of the I. R. E., August 19% page .997 etc. Antennas for Circular Polarization. 'When however a circularly polarized wave of one screw sense is transmitted and strikes a highly conducting smooth surface the reflected wave will have the opposite screw sense. leretofore circularly polarized antennas have had the property that they will not receive a wave with a screw sense opposite to that which they trans.- mit. Since in radar practice it is desirable and customary to usethesame antenna arrangement for transmitting and receiving, the problem of screw sense has proven anobstacleto the use of circularly polarized-antennas for radar.

.An object of the present invention is the provision of a circularly polarized antenna arrange,-

' lment adapted to transmit circularly :polarized waves incne screwsense and to receive circular lypolarized waves according to the-opposite-screw sense.

A circularly polarized wave may he produced poles, is a quarter wave longer-at the operating frequency of said arr,angez-nentv than the line feeding the other set. It is theoretically possibleto reverse the screw sense-of this arrangement byrotating one of said elements-through 180 degrees. As a practical matter this is "not feasible for high speed-operation such as would berequired for radar operation.

In accordance-with a feature of the present iiventio .it is proposed tcpmvide. a c rcu ar y polarized antenna arrangementadaptedto transmit a Wave having a given screw sense and to re iv za Wav .havin th op osit sc ew sense. the particular screw sense being controlled according to the .smplitud lo the en y .he ns fed Accord n t on a p of th -inv nt on the eversa o the cr w sense is accomplished @by .a ffe iv y sh ft n the ener y. in; n of the trans i ssicnlin s tor :WBNB

enesis-adios s .hns o the two ortho onally usingtwo orthogonally polarized elements such-as .polarized element, such-.asone set of these dia ed a te na e e ent b 18 9 an dd multiple thereofio q et n t er aspecte th en invention the reversal of the screw sense is accomplished by .e iec ve addin r su r ng an extra half wave length transmission section or .oddmul pl thereo i .Qhe ,Qf-t in or r uide ad n tonne o sai a t na nt he a ov ent oned and th r ea an o c s f thi nventio t e man of a. them il hsse hem a pa n n h vinvent'on itself willbe best understood, by referenc t th v .l. 9. v s si ss int on of a em dime i o t h stieh cqniu t on wi h th eq qmhehvihs drawi g w e in:

Fig. l is a schematic and block diagram of an antennaarlfangementin conjunction-with a radar ran m te and r ceiver in ich h a tenn ngement is provided with screw sense .reh s contro eans- Fig. 2 is aschematicdiagram of a coaxial form of screw sense control means.

Fig. 3 is a Coaxial form of ring circuit pro.- i ihs uch c n ol me ns and i 4 1 ma T Wave 18 1 ar an ment crow in fill h swi e m ans Referring now to Fi g. 1 a trans-ceiver I for ada is. c u d to an tenna an e en- .eraflsr desi n te by th nu e a 2, y eans ai line s-w i h ha wo b n h 4 an connected respectively toantenna elements Band 1 which antenna elements are orthogonally w a zed w thr spect t each Other and-wh c a e us d vto o id dll r pq a izafi q by h i ne of th branch ih sl o am it ,qdarter wave lengthlonger thanthe other branch 1. T usz h sxam l if ranch l n 4 a l n .1. the le th of br nch l 5 i lea:-

e rah -Qsi e vI sda icdto hammi uhes of e at ve high ampli ude r y onte t th ant nna arra gement 2, 1311B antenn a rang men he s se vincth re a vely w em :nl tud o energy co te ech u e a d ee .kee the ransmitte pulse out or the receiver and if desired to keep the received pulses outof the t nsm tt h sw l ns me ns o co pl n t ransjmitt er s e t ma n li e m y whom e .t Qa -L While w hav .rcierredt e eme ts transmission media in which the transmitted energy follows a limited predetermined path and includes wave guides, dual transmission lines, G lines and coaxial line-s, etc. While the plane polarized antenna elements 6 and 1 are illustrated as single dipoles, each'of these may consist of a number of antenna elements and may be of any conventional form for producing circularly polarized radiant action.

The screw sense of the circularly polarized wave transmitted by the antenna arrangement will be determined by the length Z in branch lines 4 and and by the position of the antenna elements. Thus for example if one of the dipoles is rotated 180 it will change the screw sense of the Wave. branch lines 4 or 5 is increased or decreased a half wavelength or an odd multiple thereof, the

screw sense will likewise be reversed. This increase or decrease of the length of either of the branch lines produces an 180 phase shift, or odd multiple thereof in the energy transmitted through the line whose length has been altered and it is also therefore possible to change the screw sense by altering the phase shift of the energy in one of the branch lines by 180 or odd multiple thereof. In accordance with the present invention a control means ii is provided in one of the branch lines, such as for example in branch line 4, to effectuate this 180 or odd multiple thereof additional phase shift of the energy in said line. Such control means may take a variety of forms such as those indicated in Figs. 2, 3 and 4.

- The control means illustrated in Fig. 2 physically causes the energy therethrough to traverse an extra half wave length when transmitting, which half wave length is removed from the path when receiving. The network or circuit of Fig. 2 is illustrated as being a coaxial line and consists of a direct or main section 9 of coaxial line which may form part of branch line 4.

"One end A of said section being towards the transmitter-receiver side and the other end B being towards antenna element 6. At an intermediate point between these ends or points A and B there is provided a stub line in which is half a wave length long and is terminated by a TR box or tube I I (see Microwave Duplexers Radiation Lab. series 14 McGraw-Hill page 5 etc.) This tube is adapted to short circuit the stub line II] when a pulse of the amplitude of the trans mitted pulse passes along the section 9. At a quarter wave length from stud line in, two additional stub lines [2 and I3 are provided each a quarter wavelength long, the points at which they meet the main section being designated by the letters C and D. fIhese two stub lines 12 and I3 are open ended and are connected together at their outer ends E and F by a half wave length line 14, lines I2, l3 and [4 providing a shunt path for the energy when transmitting pulses, in contrast to the direct path between points C and D when receiving. To aid in controlling this shunt path a quarter wave length stub line [5 terminated in a T. R. tube 16 is connected at point E with lines I3 and It.

The operation of this arrangement is as follows. When a pulse is being transmitted, the leading edge of this pulse (the initial energy thereof) causes TR tube I l to fire and short the end of line [0. Since line I0 is a half wave length line, the short appears acrosssectiori- 9 at point Likewise if the length of one of the I G in parallel with said section and since the distance from G to C is a quarter wave length this short at G will appear as an open circuit at C and the transmitted pulse cannot go from point A into the portion of section 9 between C and D but instead follows the path through stub line l2 and half wave length line Hi to point F. The initial portion of the energy at point F enters stub line It causes the TR tube it to fire shorting the outer end of stub line [5 and producin an open circuit at point F so that the rest of the energy of the transmitted pulse travels up through stub line l3 until it arrives at point D. Since at point G there is effectively a short and D and F are a quarter wave length apart, an open circuit is produced looking to the left into the portion of section 9 between D and G and therefore the energy arriving at D proceeds towards the endB of the network and towards the antenna element 6.

'When energy is being received from antenna element 6 it travels from point B directly along section 9 to point A and cannot follow the indirect path through stub lines 43, i4 and [2. The reason for this is as follows. When pulses are being received their amplitude is insufiicient to fire tubes H and 56. Since the distance from TR tube it to point D is a half wave length and the stub line It is open-ended at I6, an open circuit appears at point D looking into stub. line It. Therefore the received pulse travelling from B to D will not enter stub line I3. On the other hand looking into the section 9 from D towards G a closed circuit will appear in parallel with the section 9 at point D since the distance from tube It to point D is three quarters of a wave length and section It is open at tube II. This open circuit is inverted in the quarter wave length of the line between G and D to produce a closed circuit in parallel with the portion between D and G. The received pulse travels from D to G and continues towards C. It cannot enter stub line It] because looking into the stub line from point G an open circuit is presented. At point C the received pulse continues on to A for it cannot enter stub line l2 since from the outer end of stub line it: to point V via half wave section 14 is a full Wave length and the outer end of stub line i5 is open circuited (tube It unfirecD, so that an open circuit is presented looking into stub section 12 from line C. Thus the energy of the received pulse goes on towards point A and towards the trans-ceiver I.

It will thus be .seen that the received pulse travels between points C and D a distance equal to half a wave length. 011 the other hand the transmitted pulse travels between points C and .1) via stub lines (2, it and 3 a distance of a full wave length. Thus the transmitted pulse travels ahalf wave length more than the received pulse and is shifted in phase by more than the received pulse by virtue of its passage through the network. Accordingly, the antenna arrangement has its circularly polarized screw sense during -transmission, reversed with respect to its screw points H, I, J and K,the distance between points H andLI and J, J and J and. K, being one quarter'wave lengthuand between Kand H being three quarters of a wave length. Lines l9 and 21' may bereferred to herein as the side arms and'are'terminated by TR tubes 23 and 24respectively. One of the arms l9 and 2|, for example line. I 9, has a length land the other arm 21 has a length Of the other two arms 20 and 22 one is connectedto the trans-ceiver and the other is connected to antenna element 6. Arms 20 and 22 being inserted in a break ina branch line 4. Arms 20 and 22may be considered as the main arm and I9 and 2| as the side arms.

This arrangement operates as follows. During' transmission a pulse is fed into line 20 and uponreaching point I divides equally and goes into-"lines l9 and 2|. The TR. tubes 23 and 24 fire thus making lines It and 2| appear as shortcircuited lines. The energy is reflected from the outer ends of these lines back to points H and J where the energy appears 180 out of phase due to the extra quarter wave length in line 2|. Since the reflected energy at H and J is now 180 out of phase it will not feed back to line 20 but will be fed to point K and into line 22 towards theantenna element 6.

Upon reception of the echo pulse on antenna element 6 the energy thereof is fed along line 22 to point K where it divides and is fed to point H and J where it appears 180 out ofphase. The energy then goes into side arms l9 and 2| which now are open ended at both ends since the energy of the received pulse is insufiicient to operate TR. tubes 23 and 24. Since there is an extra quarter wave length in line 2| the reflected energy at J and at H will be in phase. This energy will then not go back to line 22 but will be fed out of line'20to trans-ceiver I. It is to be noted that when a line is :open as-are lines 2| and 23 during reception of a pulse it is the same as if this open end were an extra quarter wave-length shorted stub away from the open end. In other words the efiect of the open endis to produce 180 phase shift in the energy reflected from that open end as contrasted with what would occur if the end were shorted. Thus during reception the transfer of energy between lines 22 and 20 is increased by 180 as contrasted with the phase shift produced during transmittal when the TR tubes 23 and 25 short circuit the side arms |9 and 2|. Accordingly the requisite 180 phase shift for changing the screw sense of the circularly polarized wave is produced in the ring circuit of Fig. 3. While the ring of circuit of Fig. 3 has been disclosed as a coaxial arrangement a ring circuit composed of wave guide sections is obviously substitutable in place thereof especially for higher frequencies. Likewise a wave guide type magic T arrangement could be used to produce the required 180 phase shift for changing the screw sense of the circularly polarized wave emitted by the antenna arrangement as indicated in Fig. 4.

The magic T wave guide network or arrangement 25 provided in Fig. 4 consists of the usual two side arms 23 and 21 lying in the same phase as the H section 28, the E section 29 extending from between the arms in a plane at right angles to the aforementioned plane. Details of such structures may be found in the book referred to hereinbefore entitled Microwave Marker Wave Duplexers on page 361 etc. For the purposes of the presentinvention, one side arm, for example 21', is made a quarter wave length longer than the other arm and both arms 26 and 21 are provided with TB boxes 30 and 3| respectively at their'outer ends.

The trans-ceiver may be connected to either arm Her 29 and the antenna element 6 maybe connected to the other of said two arms. In Fig. 4 arm 28 is shown as connectedto the transceiver whereas arm 29 is connected to the antenna element 6. The arrangement operates in a manner similar to the arrangement of Fig. 3 and will .be but briefly described herein. A pulse from the trans-ceiver passes from arm 28 into lines 26 and 21 causing the 'T'. R. boxes to operate and reflect the energy into the arm 29. When an echo pulse is received it is fed via arm 29 into the side arms 26 and 21. Since the received echo pulses are of insufiicient amplitude to ignite the T. R. tubes the energy reflected back from said side arms25 and 2'! into arm 28 in effect travels a half wave length longer in each of said side arms, or is shifted in phase in each of the side arms, more than in the case of the high amplitude pulse from the transmitter. This additional phase shift is due to the fact that the side arms 26 and 21 when open-ended produce a 180 shift in phase more than the same length of side-arm when closed by firing of the T. R. boxes. It will thus be seen that there is provided the required 180 extra phase shift for the received pulse than for the transmitted pulse, so that consequently the screw sense of the antenna arrangement is reversed in changing from transmission toreception.

While I have hereinbefore described several devices for effectuating this change of screw sense it will be seen that other arrangements such as transmission line bridges or their wave guide equivalents may be employed for this same purpose.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only byway of example and not as a limitation to the scope of my invention as set forth in the objects thereof and in theaccompanying claims.

I claim:

1. A circularly polarized antenna arrangement for a radar transmitter and receiver comprising a first antenna element providing radiant action in a given plane of polarization, a second antenna element providing radiant action in a plane of polarization substantially at right angles to said given plane, a transmitter producing pulses of relatively high amplitude, a receiver for receiving pulses which are of relatively lower amplitude, means coupling said transmitter and receiver to said antenna elements, said means providing two transmission paths, one of said paths being an odd multiple of quarter wave lengths longer than the other of said paths and means coupled in one of said paths for varying the eflective length by an odd multiple of half wave lengths thereof, said varying means including a switching device operative in response to the higher amplitude of the transmitted pulses and unresponsive to the lower amplitude of the received pulses.

2. A circularly polarized antenna arrangement according to claim 1 wherein said switching device is a gaseous discharge gap adapted to fire in response to the transmitted pulses but not in response to the lower amplitude received pulses,

length which is a multiple, including one, of a quarter wave section.

5. A circularly polarized antenna arrangement according to claim 1 wherein said coupling means is a magic T having two side arms and two main arms,- one of said main arms being coupled to said transmitter and receiver and the other to one of said antenna elements, one of said side arms being an odd multiple including one of a quarter wave length longer than the other of said side arms, said switching device comprising two TR tubes each coupled to the outer end of a separate one of said side arms and adapted upon firing to electrically close its outer end.

6. A circularly polarized antenna arranegment according to claim 5 wherein said magic T is in the form of a ring circuit with a center transmission ring to which said arms are each connected at their inner ends, the inner ends of said arms being spaced, as measured around the ring going in one direction as follows: beginning from one side arm one quarter wave length to the one main arm, from the latter arm one quarter Wave arm one quarter wave lengthto the other main arm, from the last mentioned main arm three quarters of a wave length back to the beginning.

'7. A circularly polarized antenna arrangement according to claim 5 wherein said magic T' is constructed of wave guide sections, one of the main arms and the two side arms lying in one plane, the other main arm extending in a plane at right angle to said one plane.

3. A circularly polarized antenna arrangement according to claim 1 wherein one of said paths comprises a main path and a shunt path coulength to the other side arm from the latter side 8 pling twop'oints of said main path, the shunt path being an odd multiple, including one, of a, half wave length 'longerthan the distance between said two points, said switching device comprising an open ended transmission section associated with saidmain path, said switching device being in the form of a TR tube positioned at the outer end of said section and adapted to be fired by a transmitted pulse to close said outer. end and electrically block the main path between said two points thereby causing the transmitted pulse from the transmitter to travel around the shunt path. 9. A circularly polarized antenna arrangement for use with a radar transmitter and receiver comprising a transmitter for transmitting pulses of relatively high amplitude; a receiver for receiving pulses which are of relatively lower amplitude; an antenna arrangement coupled to said transmitter receiver and providin circular polarization of a given screw sense comprising a first antenna element radiantly acting with a given plane of polarization, a second antenna ele ment radiantly acting with a plane of polarization substantially at right angles to said given plane, and means coupling said transmitter and receiver to said antenna elements, said coupling means including two transmission paths each to a separate one of said elements, one of said paths being an odd multiple including one of a quarter wave length longer than the other of said paths, and means for reversing the screw sense of said circularly polarized antenna arrangement including a resonant transmission section coupled to one of said paths, means responsive to the transmitted pulses for shorting said section and means responsive to the shorting of said section for varying the efiective length of said path by an odd multiple including one, of a half wave length,

WILLIAM SICHAK.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,217,911 Lindenblad Oct. 15, 1940 2,412,315 Brown Dec. 10, 1946 2,412,320 Carter Dec. 10, 1946

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