US2412160A

US2412160A - Directive radio system

Info

Publication number: US2412160A
Application number: US412452A
Authority: US
Inventors: Richard C Longfellow
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1941-09-26
Filing date: 1941-09-26
Publication date: 1946-12-03
Anticipated expiration: 1963-12-03

Description

Dec. 3, 1946- R. c. LONGFELLOW V 2,412,150

- DIRECTIVE RADIO SYSTEM Filed Sept. 26, 1941 3 Sheets-Sheet 1 g, 31% 1? 1.31 F" I Re cel'ver- Fansm/ter- Inventor: Richard C.L0n Fellow,

byivfl 'fii His btorney.

DIRECTIVE RADIO SYSTEM Filed Sept. 26, 1941 3 Sheets-heet 2 Invehtor": Richard C. Longfellow,

His tborney 1946- R. c. LONGFELLOW DIRECTIVE RADIO SYSTEM Fig.5.

'- b .mV aaM His tcobney f Invent r -..*-1'- rd C. Lon gfellow Patented Dec. 3, 1946 UNITED STATES PATENT oF-rice Richard C. Longfellow, Schenectady, N. Y., assi'gnorto General Electric Company, a corporation of New York Application September 26, 1941, Serial No. i12,452

10'Clairns.

My invention relates; to directive radio systems and more particularly to means for vary-. ing the directivity of such systems.

In directive systems such asare employed for the location of distant objectsmoving craft,- and the like, it is desirable that the directivity ofthe antennae arrays employed be readily and electrically variable;

An object of my invention is to provide an improved system forvarying. the directivity of such systems.

Another object of my invention is to simplify systems in which the directivity is variable in three or more directions by variationof the phase relation of the waves supplied to the different antennae. 7

In copendingapplication of'Lawrence M. Leeds, Serial No. 410,836} filed simultaneously herewith, entitled Directive radio systems, and which is assigned to the same assignee as my present application, is shown a system involving four radiators located at the corners of a rectangle. Each of these radiators is connected through transmissionlines to the respective corners of a transmission line loop. or phasing rectangle.

Transmission lines then extend from interinediate points on all ofthe sides of the loop, or phasing rectangle to the radio apparatus, which may comprise either a transmitter, orja receiver, or both. Means are provided whereby transmission over any three of these latter lines may be simultaneously interrupted thereby to confine transmission to any single one of the lines extending to the radio apparatus. Thus energy transmitted over any line excites one pair of adj acent' antennae in phase,and' it excites the remaining adjacent pair in phase; but the latter air is excited in displaced phase relation with respect to the first pair by an amount equal to the electrical length of the phasing rectangle.

Thus the antenna array has maximum efie'ct with respect to a direction at an angle to the normal to the plane of the array. This effect may be varied through any of four directions about the normal to the array by changing the line through which transmission between the radio apparatus and. the phasing rectangle is confined.

Such a system possesses certain very desirable advantages. An object of my invention is to pro- Vide a system having improved capabilities with respect to the angle through which the direction of maximum effect of the antenna may be shifted. Aiurther object of my present invention isto: p ovid s a im ro e s s mv d o e 2 which,- at the: same time. is somewhat simpler and more economical toconstruct. i

Another object ofmy invention is to provide a system in which interconnections betweenv lines leading to different antennae in which no useful energy is carried are avoided. i

The novel features which I' believe to be characteristic of my invention are set forthwith particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be imderstood by reference to the following description taken in connection with the accompanying drawings in. which Fig. 1 represents a diagrammatical showing of an embodiment of my invention; Fig. 2" representsa modification there of; Fig. 3' represents the antenna array employed; Fig. 4 represents the capacitance switch utilized in the form of the invention shown in Fig. 1; Fig. 5 represents the structure of the diode employed in my invention, and Figure 6 is a diagrammatical showing of a modification. of my invention.

Referring to Fig. l of the drawings, I have shown therein four antennae, I, 2, 3' and 4, arranged at the respective corners of a rectangle. Theseantennae preferably are disposed in an array and mounted for orientation both in elevation and in azimuth, or train. In Fig. 3, I have shown the antennae l, 2, 3 and 4 as mounted in a suitable array and arranged for rotation in the vertical plane about a pivot 5. This pivot 5 is arranged in the support 6 which may be rotated in the horizontal plane-whereby the array of antennae'may be oriented both in the horizontal plane and in the vertical plane.

In accordance with my invention as illus dated in the embodiment of Fig. 1, the direction of maximum effect of the array when oriented in any'direction may be shifted to any one of four directions about the normal ofthe array.

As illustrated in the drawings, each of the antennae: l, 2, 3 and 4- comprises four dipoles, a, b, c and d, the dipoles a and b being positioned end to end and the dipoles c and 1 being positioned end to end parallel with the dipoles a and D but at a distance one half of a wave length therefrom. The dipoles themselves, of course, have a length equal to half of a wave length of the wave at which they operate. The end of dipole b adjacent dipole d is connected to theend of dipole 0 adjacent dipole d and similarly dipole a is connected to dipole d. The antenna l is also positioned in end-toeend. relation with the gle-ended transmission line.

antenna 2, and similarly antenna 3 is positioned in end-to-end relation with antenna 4, and

antennae

3 and 4 are positioned a half wave length from the antennae I and 2, all, of course, in the same plane. While this relative spacing of the different antennae has been departed from in Fig. 1 for the purposeof clearness of that drawing, the antennae are shown so relatively spaced in Fig. 3.

While I have shown and described a particular form of directive antenna, it will of course be understood that any desired form of antenna may be used at the corners of the rectangle, as, for example, simple dipoles.

The antennae positioned at diagonally opposite corners of the rectangle are connected together through a transmission line; that is, the antenna I is connected to the antenna 4 through a transmission line I, and antenna 2 is connected to antenna 3 through a transmission line 8. These transmission lines may be of the concentric conductor type in which the outer conductor may be -maintained at ground potential. The inner conductor I, or 8, is connected to one side of the feed point of each of the antennae to which it is connected. Conductor I, for example, is connected to the left-hand terminal of the feed point A of antenna I and to the left-hand terminal of the feed point D of antenna 4. Since the impedance of the dipoles at the feed points-A, B, C and D is balanced with respect to ground, means must be provided to effect'the connection between the balance'impedance of the antennae and the sin- This is effected by means of a sleeve I9 positioned about the end of each transmission line, this sleeve having a length equal to a quarter of a wave length of the wave at which the system operates. The sleeve is conductively connected to the outer sheath of the transmission line at a point'one quarter of a wave length from the end of that sheath, and forms with the end quarter of awave length of the sheath a' transmission linehaving a length of a quarter of a wave length and which is'short circuited at the end remote'from the antenna. These sleeves I0 are grounded and insulated from the end quarter of a wave length of the sheath. Since they form a quarter .wave length transmis- Sion line with the end of the sheath, those ends oscillate with respect to ground in opposed phase relation to the inner conductor and .thus the inner conductor and the end of the sheath have an impedance between them which is balanced .with respect to ground and which may be connected directly across the respective feed points A, B, C or D. -These antennae I, 2, 3 and 4 are connected through transmission lines to radio apparatus -comprising a transmitter I1 and a receiver I9, both shown near the bottom of the drawing. These connections comprise transmission lines I I, I2, I3 and I4, as shown on the drawing, all of which are connected together at the point I5 and thence through transmission line It to the transmitter I'I andthrough transmission line I8 to the receiver I9.. Transmission lines II-and I2 are connected to points on the line] removed from the midpoint of that line so that each line II. or I2 is connected to the line I at a point nearer to one of the antennae I and 4 than it is to the other. Thus if energy be suppliedv over linev II to the lines! itexcites antennae I and 4 with a phase fdifierence equal to the electrical difference in the lengths of the path over which thewaves are supplied to the'respective antennae. v

Lines I3 and I4 are similarly connected to line 3.

Means are provided whereby only two of these II, I2, I3 and I4 are employed at a time, one being connected to the line 'I and the other to the line 8, This means comprises the capacitance switch 20. e y

This capacitance devicev comprises four stationary capacitance electrodes 2|, 22, 23 and 24, each of which is connected through the inner conductor of

concentric transmission lines

25, 26, 21, 28 to a point on-the respective transmission lines II, I3, I2 and I4 respectively. This capacitance device also comprises a rotating capacitance electrode 29, which is grounded as at 30 and which may be in the form of half of a disk whereby it simultaneously cooperates with any two of the

electrodes

2I, 22, 23 and 24 depending upon its position about pivot point 3I. Thus in the position shown, this electrode 29 cooperates with electrodes 2I and 22 producing high capacitance between it and electrodes 2I and 22. Similarly,

low capacitance exists between

electrodes

23 and 29, and between

electrodes

24 and 29.

Electrode 2| is connected through the line 28, which has a length equal to an odd number' of quarter Wave lengths, to the point 32 on line I4. Since line 28' is an odd number of quarter wave lengths in length, the low impedance between electrodes 2| and 29 produces a high impedance at the point 32 whereby the line 28 has little effect upon the transfer of energy through the line H past point 32. Electrode 22 is similarly connected through the line 2'! to the point 33 on line I2 and for the same reason has little efiect upon the transfer of energy through the line I2 past point 33.

Electrodes

23 and 24, being similarly connected to points 34 and 35 respectively on transmission lines II and I3, have very great efiect upon the transfer of energy through the lines I I and I3 since high impedance exists between these elec trodes and the electrode 29. That is, these

various lines

25, 26, 21 and 28 operate as impedance inversion networks producing high impedance at the

respective points

32, 33, 34 and 35 when low impedance exists between the corresponding fixed electrode and the rotor 29,'and producing low impedance at the

points

32, 33, 34, 35 when high impedance exists between the respective fixed capacitance electrode and the electrode 29'.

Thus it will be seen that with, electrode 29 in the position shown, conductorsII' and I3 are effectively short circuited in the

points

34 and 35. A short circuit at the'p'oint 34 does not affect transmission of energy through line, 7, however, .because the length or" lin'e'I I between point, 34

and the point of connection with line 7 is also an odd number of quarter wave lengths in length.

other circuits because the length of 'line II from point 34 to point I5 is also an odd number of quarter wave lengths in length. This same relationship between the lengths of'the different sections of the transmission line I I is true of each of the other transmission lines I2, I3 and.I4.

Thus, with electrode 29 inthe positionshown,

"lines I2 and I4 and lines I and 8 to all of the different radiators I, 2,- 3 and-4. This energy is supplied to the radiators 2 and t in phase since the lengths of the paths leading to these radiators are equal. It is also supplied to the radiators I" and 3 in phase since the lengths of the paths leading to these radiators are equal. The energy supplied to the radiators I and 3 is delayed in phase with respect to the energy supplied to radiators 2 and 4, and accordingly the direction of maximum efiect of the array is shift edfrom the normal of the array in the horizontal plane by an angle corresponding to the difference in the length of the paths leading to the antennae 2 andfrom those leading to the antennae I and 3.

Nowletus suppose thatr the capacitance electrode 29 is rotated clockwise through 90. Then low impedance exists between electrodes 22 and 29-, and between

electrodes

24 and 29. This renders lines I 2 and i3 free for transmission of energy between the radio apparatus and the antennae, and produces short circuits at the

p ints

32 and 34 thereby interrupting transfer of energythrough conductors I I and I4. Now for the same reason as that previously described, antennae I and 2 are energized with currents in delayed phase relation with respect to the current supplied to

antennae

3 and 4, and the direction of maximum effect of the array is shifted from the normal in the vertical plane by an amount corresponding to the electrical difference in the length of the various paths.

If condenser 29 be shifted from the position shown through 189, the beam will be shifted from the normal in the horizontal plane in the opposite direction andby an equal amount from that in which it is shifted with the condenser 28in the position shown.

If it be shifted through 2'70- in clockwise direction, then the beam will be shifted downward in the vertical plane.

The receiver i9 is connected to the transmission line at the point 35 distant from the transmitter by an amount such that the impedance ofthe transmitter as viewed from the point 35 is high.

For example; if the transmitter impedance be low the connection to the receiver is made at a point 36 distant from the transmitter by a quarter wave length or odd multiple thereof, whereby, because of the impedance inversion effect of the section of line between point 35 and the transmitter, the transmitter impedance, as viewed from point 36 is high.

System such as that described may be employed in'connection with echo systems in which impulses are transmitted at a very rapid rate and echoes thereof are received in the intervals between the transmitted impulses. It is desirable, therefore, that the receiver be protected from the'intense impulses produced by the transmitter. Unless such protection is provided the intense transmitted impulses may impair the sensitivity of' the receiver and the receiver will not have sufficient time in which to recover before reception byit is required. The result is that it may be insensitive during the time when the impulses tobe received arrive. For this reason the diode .3115 is employed, the anode thereof being connected to the endof the inner conductor of a stubtransmission line 33 extending from the pointx39 on transmission line IS. The cathode of .thediodeB'I is connected to ground and th o h albattery 40 to the outer conductor of the con- .centric. transmission line 38. During the transmission of'the intense impulses produced by'the transmitter, high voltageis produced upon the anode of the diode 31 and this diode becomes conducting, producing'low impedance across. the end of the stub transmission line 38 during. the transmitted impulses. This low impedance, however, does not exist during reception because the diode is. not then conducting. 'This low impedance of the diode 31 doesnot impair transmission over the line I6 because the stub line 38 has a length equal to a half of a wave length so that the diode 3'I- produces low impedance at the point 39. The distance between the points 39' and 36 is an odd number of quarter wave lengths so that the diode has the effect of producing high impedance at the point 36.

Fig. 2 shows a form of my invention involving only three antennae I, 2 and 3-, these antennae being positioned at the vertices of a triangle rather than at the corners of a rectangle as in Fig. 1. Each of these antennae is connected to the transmitter through respective transmission lines 59, El and 52, these lines beinginterconnected by lines 53, 54 and 55', which may be of equal lengths.

Capacitance device 56 is also employed but is of slightly different character from that shown in Fig. 1 in that its rotor is in the form of an arc subtending approximately rather than This condenser com-prises three fixed electrodes 51, 58 and 59, each connected to the

respective transmission line

50, 5|, 52 through transmission lines 80, SI and 62 respectively. These latter lines 69, BI and (52 have a length equal to an odd number of quarter wave lengths, as explained before whereby, with the" capacitance electrode 63 ofswi'tchingdevice 56 in the position shown, the electrodes 5-? and 59 produce low impedance at the points. 64 and I56 on thelines 5i] and 52' and thus impair transmission over these lines. These points, however, arepositioned an odd number of quarter wave lengths from the point I5. so that the short circuit at

the'points

64 and 66 does not impair transmission over line 51' past the point l5. Electrode 58', having low impedance to electrode 63, produces a high impedance at the point 55. and thus line El is in transmitting condition.

Energy is transmitted up line 5| to the point 68 Where it divides one portion passing to antenna 2 and another portion passing, to antennae I and 3 through line 55-, and line '53 andiil in one case, and 5 5 and 52- in the other. The 'sec tion of line between'points 61 and 65 is an odd number of quarter wave lengths in length so that the short at point fi'does not impair transmission-from line 53 to antenna I past point 61. The same is true with respect to the section of line between points so and s9 and also-between points. as and 63.

The-points 61, 68' and'fid are so positioned and the

connections

53, 54 and 55 are of such length that the array, when operating through the line 51, has a maximum effect in the vertical plane at an. angle to the normal.

If electrode 63 be rotated through 120" in clockwise direction, then transmission takes place over line 52 and transmission over lines 50' and 5| is interrupted. The beam is now shifted through an arc of approximately 120 about the normal to the plane-of the array.

If the-electrode 56 be rotated to its third position, the direction of maximum effect of the array willbe shifted through a further 120.

Fig. 4 shows in greater detail'the condenser structure of the form of theinventionshown in Fig. 1, the electrodes 2|, 22', 23,' 24and. 29 all being housed within a housing 65to which the outer conductors of the various transmission lines are securely connected electrically as indicated at 66. These electrodes are equally spaced about a circumference to cooperate in succession with rotating electrode 29.

.Fig. represents the diode employed in connection with my invention. It comprises an anode 10 and a cathode H, the latter of which may be indirectly heated by heater I2 energized from a transformer 13'. The cathode is mounted upon a conducting plate 14 which closes the end of the transmission line 15 but which is insulated therefrom by means of insulating material 16.

The anode 10 of the diode is provided with a screw-threaded projection 11 at the top of which screws into the end of the inner conductor 18 of the transmission line. This anode is provided with a flange portion 19, the outer periphery of which is sealed in the edge of a glass cylinder 80, the lower edge of which is securely sealed to the plate 14 whereby the interior space between the electrodes may be evacuated.

If desired, a source of potential 40 may be connected between the plate 14 and the outer conductor 15 of the transmission line thereby to render the anode of the diode somewhat negative with respect to the cathode normally in order to prevent the flow of electron to the anode by reason of thermal agitation and the like. The connection to the anode for direct current is, of course, completed through any apparatus that may be connected between the outer conductor and the inner conductor of the transmission line.

I have described certain transmission line sections as having a length equal to a odd multiple of a quarter of a wave length. It will, of course, be understood that I include the multiple one since frequently any of these lines may be of but a quarter of a wave length in length.

I have mentioned

lines

25, 26, 21 and 28 as being of an odd number of quarter wave lengths in length; if desired, however, these lines may be of different lengths. For example, the line itself may, if desired, have a length equal to an eighth of a wave length, or to an eighth of a wave length plus a half wave length, or multiple thereof. If line 25, for example, has such a length, the capacitance between electrode 23 and ground, assuming electrode 29 in the position shown, may be adjusted, as by means of variable trimmer electrodes, not shown, within the capaci tance device to cooperate with the line 25 to cause it to have an electrical length equal to a quarter ofa wave length and thus to produce an effective short circuit at the point 34. The same may be true with respect to the line 26. The

lines

21 and 28 being connected respectively to electrodes 22 and 2| having higher capacity to ground, assuming electrode 29 in the position shown, may then have a wave length equal to a half a wave length such that they produce high impedance at the

points

33 and 32. That is, the

lines

25, 26, 21 and 29 operate in either of two modes, the open circuit quarter wave mode or the open circuit half and 28 when so adjusted.

This proportioning and tuning of the

lines

25, 3

26, 21 and 28 in association with the capacitance device 20 is more particularly described and is claimed in the copending application of Franklin G, Patterson, Serial No. 421,126, filed December 1, 1941, entitled Transmission systems and which is assigned to the same assignee as my present application.

I have found, when operating. at certain frequencies, that difliculty may be encountered in obtaining the impedances within the capacitance switching device necessary for operation of the lines connected thereto in the two modes described. No difficulty is encountered in producing the quarter wave length mode of operation because this merely requires adding capacity between the stationary electrodes and ground. This may easily be done by adding trimmer electrodes arranged in adjustable spaced relation with respect to the stationary electrodes. In the half Wave mode of operation larger capacity between the rotor electrode and stationary electrode is present. The circuit then extends from the stationary electrode to the rotor, which is in mesh therewith, and then through the shaft of the r0- tor to ground. This path not only has capacity but also inductance, which must be adjusted for the half wave mode. This may be done by proper proportioning of the parts.

In accordance with my invention, however, the arrangement of Fig. 6 may be employed. In this Fig. 6 I have shown conductors H and 25 of Fig. l joined at point 34. Conductor 25 leads to the electrode 23 of the capacitance switch 20. In this switch the capacitance between the electrode 23 and ground, when the rotor is removed from electrode 23, is represented by the dotted linesat 80. This capacity may be adjusted, as by trimmer electrodes, to produce the quarter wave mode of operation.

A further component of impedance between electrode 23 and ground, when the rotor meshes with this electrode is represented as comprising a series combination of capacitance 8| and an inductance 82, in parallel with capacitance 80. These impedances may be adjusted for the half wave mode of operation. I have found it convenient, however, to provide the stub line 83 at point 34, this stub line being of adjustable length, as by telescoping inner and outer conductors, as conventionally represented on the drawings. The length of this stub line may be adjusted to cause it to resonate with any reactance presentedby line 25 at point 34 to produce very high shunt impedance between point 34 and ground. That is, capacitance 88 may now be adjusted toproduce a short circuit between the inner and outer conductors at point 34 when the rotor is in its most remote position from electrode 23, and line 83 may be adjusted in length to produce very high impedance between the inner and outer conductors at point 34 when the rotor meshes with electrode 23. 7

It will be observed that my invention offers a very satisfactory system for shifting, in two or more planes at an angle to each other, the direction of maximum effect of the antenna array, whether it be a radiator, a receiver, or both, the shift being brought about by simple rotation of the rotor Of the capacitance switch. Any interconnections between lines leading to different antennae, and which, at least at times, carry no useful energy, are avoided. The avoidance of such interconnections increases the capabilities of the system with respect to the angle through which the direction of maximum effect of the systern may be shifted. This results from theavoidance of reactance, or impedance effects, of such interconnections upon the system. Moreover, the avoidance of such interconnections very greatly simplifies the problem of impedance matching throughout the length of the lines between the radio apparatus and the different antennae. In fact, in the systems Of my invention the problem of impedance matching throughout these lines is no more difiicult than it is in the usual transmission systems between a radio apparatus and one or more antennae.

While I have shown particular embodiments of my invention, it will, of course, be understood that I do not wish to be limited thereto since different modifications both in circuit arrangements and in the instrumentalities employed may be made, and I contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

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

1. In combination, a plurality of antennae positioned in an array to produce maximum eifect of said array with respect to certain directions, radio apparatus. a plurality of permanent transmission lines, each line extending from said apparatus to a corresponding one of said antennae, a connection between different of said lines near the antenna ends thereof, the path through any line to its corresponding one antenna being shorter than the path through the same line to any other of said antennae, means to select'any of said lines in accordance with the direction in which said array is to have maximum effect, the lengths of the paths from said apparatus to all the antennae being so proportioned as to produce maximum effect in the direction corresponding to the line selection made by said means, said means comprising an impedance device having a moving member, and means whereby said lines are alternately shorted at respective points thereof in response to movement of said moving member, said points being so positioned on the respective lines as to prevent impairment of transmission through another lin to an antenna corresponding to a line shorted by said impedance device.

2. In combination, a plurality of antennae positioned in an array to produce maximum effect of said array with respect to certain directions, a radio apparatus, a plurality of transmission lines, each line corresponding to one of said antennae and extending from said apparatus to its corresponding antennae, a connection from a point near the antenna end of one line to a point near the antenna end of another line, the path from said apparatus through said line to the antenna corresponding thereto being shorter than the path through said line to another antenna, means to short first one and then another of said lines alternately while maintaining unshorte'd lines in energy transferring condition, the distance of any short on any line from said connec tion being such that transmission through said connection is not impaired by said short and the lengths of the said connection being such that the direction with respect to which said arra has maximum efiect is dependent upon the line over which energy is transferred between said apparatus and said array.

3. In combination, a plurality of antennae positioned at the vertices of a polygon, a plurality of interconnections, each interconnection extending between antennae positioned at different vertices of said polygon, radio apparatus, a pair of transmission lines extending from said apparanections, said points being so positioned that the path from any antenna to said apparatus is longer over one of said lines extending from the respective interconnection than the other, and means to select one line in each pair to transfer energy between said apparatus and each corresponding pair of antennae thereby to control the direction in which said array has maximum effect.

4. In combination, a plurality of antennae positionedv at the vertices of a polygon, a plurality of interconnections, each interconnection'extending between antennae positioned at opposite corners of said polygonyradio apparatus, a pair of transmission l-inesextending from said apparatus to respective points on each of said interconnections, said points being so positioned that the path from any antenna to said apparatus is longer over one of said lines extending from the respective interconnection than the'other, and means to interrupt transmission through either line in each pair while maintaining transmission through the other line of each pair thereby to Vary the directivity of the array comprising said antennae.

5. In combination, a plurality of antennae positioned at the vertices of a polygon, a, plurality of interconnections, each interconnection extending between antennae positioned at opposite corners of said polygon, radio apparatus, a pair of transmission lines extending from said apparatus to respective points on each of said interconnections, said points being so positioned that the path from any antenna to said apparatus is longer over one of said lines extending from the respective interconnections than over the other, and means to produce low impedance across either line in each pair to impair transmission therethrough, said low impedance being produced at a point such that high impedance is presented by the respective line to the other lines over which transmission is not impaired.

6. In combination, a plurality of directive antennae arranged at the corners of a rectangle, the antennae at diagonally opposite corners of said rectangle being connected together through respective transmission lines, radio apparatus, two pairs of transmission lines, the lines of each pair of extending from respective points spaced apart on one of said first mentioned lines corresponding to the respective pair, all of said two pairs of lines extending to said apparatus, and means to interrupt transmission through any two of said last-mentioned lines while maintaining transmission through the other two of said lines.

7. In combination, a plurality of directive antennae arranged at the corners of a rectangle, the antennae at diagonally opposite corners of said rectangle being connected together through respective transmission lines, radio apparatus, two pairs of transmission lines, the lines of each pair extending from respective points spaced apart on one of said first-mentioned lines correspending to the respective pair, all of said two pairs of lines extending to a common point and thence to said apparatus, and means selectively to produce low impedance across any two of said two pairs of lines at points thereon such that transmission over the other lines of said two pairs of said antennae and said apparatus is not impaired by such low impedance.

8. In combination, a plurality of antennae elements arranged at the vertices of a triangle, an individual connection from each antenna element to a common point, a radio apparatus, an individual transmission line extending from each of said connections to said apparatus, and means to disable all but one of said lines while maintaining said one line in condition to transmit energy between all of said antennae and said apparatus.

9. In combination, a plurality of antenna elements arranged at the vertices of a triangle, an individual connection from each antenna vto a common point, radio apparatus, an individual connection extending from a point on each of said connections to said apparatus, means to confine transmission of energy between said apparatus and the different antenna elements .to one of said last-mentioned connections at a time, said points being so positioned that said antenna elements have a direction of maximum effect dependent upon that one of said last individual connection to which transmission isconfined.

10. In combination, a transmission line, a second line connected to a. point on said first line and having a length different from any'integral multiple of a quarter of a wave length, a capacitance connected across the end of said second line proportioned to produce low impedance at said point substantially equal to that of an open line having a length equal to a quarter of the operating wave length and variable to increase said impedance, and a third transmission line connected to said point .to resonate with reactance produced by said second line when said impedance is increased by variation of said first-mentioned capacitance thereby further to increase the impedance at said point toa value approaching that of an open transmission line having a. length equal to half of the operating wave length.

RICHARD C. LONGFEILOW.