US2493569A - Composite loop antenna system - Google Patents

Description

Jan. 3, 1950 B. P. BROWN, JR

COMPOSITE LOOP ANTENNA SYSTEM 2 Sheets-Sheet 1 Filed May 15, 1946 Fig.

PE5/S7'4/VCE FREQUI/VCY Invemtor: Burtorw F. BPCSWrmJr;

Zz zirub am His Attorrwe Jan. 3, 1950 B. P. BROWN, JR 2,493,569

COMPOSITE LOOP ANTENNA SYSTEM Filed May 13, 1946 v 2 Sheets-Sheet 2 37 Inventor:

Buhcorw F. BTOWTXJYJ by% 2% His Attorney.

Patented Jan. 3, i950 COMPOSITE LOOP ANTENNA SYSTEM Burton P. Brown, J r., Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application May 13, 1946, Serial No. 669,259

3 Claims. (01. 250-33) My invention relates to antennas for operation over a wide band of radio frequencies and it has for its object to provide a new and improved antenna system suitable for use in television and other purposes requiring wide band operation.

In an antenna system for broadcasting high frequency waves over a broad band of frequencies, it is desirable that the structure radiate horizontally polarized waves omnidirectionally in a horizontal plane which passes through the antenna. It is further desirable that the radiating structure present a constant purely resistive impedance to the transmission line which supplies the energy to be radiated'and that this impedance be maintained constant and substantially purely resistive over a relatively wide range of frequencies. A substantially circular radiation pattern in the horizontal plane may be maintained by constructing the antenna inthe form of a continuous circular loop which is open at one point for the purpose of being energized, the loop having a circumference of such length that it is considerably less than a half wavelength at the mid frequency of the band of frequencies to be radiated. Such a structure produces an approximately uniform ring of current from which results an omnidirectional radiation characteristic measured in the horizontal plane of the loop. However, when the impedance at the point of energization of such a structure is examined, it is found that it exhibits a narrow band width characteristic. Thus, the impedance does not remain constant and substantially purely resistive over a relatively wide range of frequencies. This limitation is brought about by the fact that the currents on opposite sides of the loop are in time phase opposition and the relatively short phase distance across the diameter of the loop produces a small resultant radiated field per unit current relative to the radiated field per unit current which may be produced if the segments of the loop are located in free space.

Furthermore, this condition causes a relatively high amount of energy to be stored in the circuit so that the resultant structure exhibits imped-- ance characteristics at the point of energization which havelarge variations in both resistance and reactance components as the exciting frequency is varied.

It is an object of my invention to provide an- It is still another object of my invention to 2 provide a new and improved antenna system in which the impedance at the input terminals is substantially constant and purely resistive over a wide range of frequencies.

It is still a further object of my invention to provide a new and improved wide band antenna which presents a relatively low resistance to wind so that the forces on a supporting structure are relatively small.

One of the features of my invention is the use in a broadcast antenna system of a plurality of capacitively coupled radiating segments which are arranged in a loop and in which the lengths of the individual segments and the spacing between the segments are adjusted so that each segment carries substantially the same current both in amplitude and phase. Each of the individual segments is furthermore so constructed that it has a large current carrying surface. Another feature is the provision at the input terminals of the antenna structure of a reactance in series with the antenna impedance and adjusted so that the resultant impedance of the system remains substantially purely resistive over a wide band of frequencies.

The features of my invention which I believe to be novel are set forth with 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 understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 is a perspective view of a wide band antenna suitably embodying my invention; Fig. 2 is a graph illustratin certain features of the system of Fig. l; and Figs. 3 and 4 are, respectively, perspective and plan views of a modification of the antenna of Fig. 1.

Referring to Fig. 1, I have there illustrated a broad band antenna which comprises a vertical metallic supporting mast I having a first pair of transverse supporting arms 2, 3 rigidly attached thereto and a second pair of transverse arms 4, 5 arranged at right angles to the arms 2, 3. The arms 2, 3 may be solid metallic rods, While the arms 4, 5 may be metallic tubes. Supported at the outer ends of the arms 2, 3 are circular segments 6, 1. Circular metallic members 8, 9 are rigidly attached, respectively, to the segments 6, I and extend outwardly from the mast l in a horizontal plane.

A pair of transmission lines of the coaxial type and comprising, respectively, inner and outer conductors I0, II and l2, l3 are supported in a mission lines are bent toward each other. The

ends of the arm 4 and of the outer conductor IL are spaced apart by a small gap I6; The inner this arm, the distance of which depends upon 4 impedance to the supply transmission lines over a relatively wide range of frequencies.

The manner in which the inductive reactance due to the short-circuited portion of the inner conductor compensates the impedance to provide a broad band characteristic to the antenna is illustrated in Fig. 2 which frequency is plotted as abscissa and impedance as ordinate. The upper curve denotes the resistance component of the composite antenna structure, while the curve marked X; denotes a typical reactance curve of the antenna of Fig. 1. The curve XA illustrates that the impedance, oi the composite antenna has considerations which are pointed out later, the

inner conductor I0 is conductively connected by 'ductors I2, I 3 is similar to that described inc'on nection with the arm 4*and the conductors I0, I I. The conductor I2 is similarly short-circuited at a point within the tubular arm'5.

Supported "from and conductively connected to thearms, 4," 5 'areshortjcircular sagments or loops I8, I9; Similarly supported from and conductively connected to the outer ends of the conductors l1; l3 arershert ci cular se ments 28, 2|. Each of the "segments I8J++2l at its end remote from its point of support is provided with 'a me:- tallic plate 22 which is spaced from "the adjacent endoi one of the segments 6; l; The segments |B-+=2i are-further provided with a pair of vertically extending loop-like members 23, 24 which form a radiatin fin secured to the associated one of the segments lB-2l.

In my composite antenna structure, the circular segments 6, 1', Hi -2t are equally spaced from the mast I so that they are arranged as a loop, each of the segments being capacitively coupled to the others. Thus, for example, the segment 6 is capacitively coupled to the segments l8, 19 by means of the plates 22; When the capacitance between plate "and the segment 6 is adjusted correctly, each of the segments 5, I, I842! car-.- ries substantially the same current both in amplitude and phase; which is the conditiondesired 'to provide an omnidirectional radiating characteristic. The loops 8, 9, attached to the segments 5, 1 and the members 23, 24,1attached to the segments IBTZI, all provide an increased current carrying surface for the radiating segments so that the resultant structure is provided with a broad band width characteristic. a

It is found that, in the composite antenna structure comprising the circular segments and their complementary radiating surfaces, the int ermi ls oit aan ehh presen a erta ed nce h ch usu notnumly es stiv ut has a apacitive reactance com e t- Accordingly, the point ofconnection ll of the inner conductor I0 within thearm 4 is arranged so that the portion of-theconductor l0 within the arm 4 provides aninductive reactance in series'with the antenna impedanceyso that the resultant overall impedance presented to the input transmission line at the gap 16 is substantially purely resistive. A similar inductive reactance is provided at the upper end oi-the transmission line 2 3- As a r sult'thei om o ite an e na s r ture, to et er w th the seri s indu ti e r actance. p esent-s a-substantiall constant resistive a capacitive reactance component throughout the entire frequency band. The curve marked X17 illustrates thecompensating reactance which is provided by the inductive reactance in series with the antenna impedance. This inductive reactance is serially added to" the capacitive reactance Xi to Pr -ids ecmhehsationcrar uhstanha h en re n a'ei i cgu hcies to he r diat d As a su t, he composi e antenna strhcturepre sehts a ubst nt ally const ntre s ive-" aw ance' o the input l n H); I -3.

n i s. 3 --and 1; haveshow a me fi i f h tenna a rangem nt a F g ni the m st I q ded with ear trans se a m 2 hic are spaced apar by ang es o h rm .25 11 su port; at their ter e s, c la e men s lflfil rl-uopliha-fih it 32-. ich extend. out ardly respec velyirom he se men sZh-ifl. ha e the ends clr'rie aed he the associa d-s gment, iihs L 2; re ke ise u ed wit -a vertical axi nsion-toihc sa the c ent carr i g-surface of the sse iated radiaars to ide. ah ua hv wid h aracter stic- S pp te fromthe- 6 zi rec rs lar see 3 -3'6 Earh oi hese segmen s ke is is provided Witha radi ting f l l-wh chhoth xtends outwa dly iramthe a s ated se ment an lik wise a a vertical extens he antenna s supplied at. twopuint with ehs gytu e rad d- Thus, e segments i3 3 are. cchnec o e onduc orshi an inhutcuaxial tra missi n n 38 th ough a i .a balance con erter #9.- Sim: arly t i 3.5, i are energ ed. by co nection t a ma ltransin ssi n ne 40 hro h a. n balance e ter 3 i st uctu e an m ri e auxiliary conductur 2 iawh h e inn condueter ci t cocoa al-line .38 or mp e is Q9hhe d. l e mh1234i directly nne te 1 th. outcre ndu r o the transmissio i e .8 and ihcs sment onnec d o t liar ,cunduc r 'l.;. r .7 V In he stru ture Qiri a 3 and a serie apa i nc v a coupl ng adjac n or. the c rcular segme tsi cehs i ited y ia ent hd o the segme ts. Depcnsiiusiupen t val eide ir d, th has am v mar-ha adjusted n the co re line ba anc 7 polarized waves omnidirectionally in a horizontal plane. In addition, by controlling the capacitance between adjacent segments and the value of a series inductive impedance, the composite antenna may be arranged to present a substantially constant resistive impedance to an input transmission line over a relatively wide range of frequencies.

Another important advantage of my composite antenna system is that currents to be radiated are applied thereto at two diametrically spaced points so that a truly circular radiation pattern is obtained. By this method of supplying the current to be radiated to the antenna, less variation in horizontal pattern is obtained as the frequency is varied over a broad range of frequencies.

Another important advantage of my composite antenna system is that it presents a low value of projected area for wind loading, the structure being relatively compact and providing no extensive surfaces. Furthermore, the spacing between the adjacent ends of the circular radiating elements, for example the plates 22 and the end of segment 6 of Fig. 1, is sufficiently rw great that even under adverse and inclement weather conditions ice does not build up on the antenna to a sufficient extent to injure the electrical characteristics of the antenna. Thus, in a typical antenna, the spacing between the plate 22 and the circular segment 6 is eight inches or greater even for operating at a low frequency. Such a spacing is sufficiently large that all troubles from build up of ice on the antenna are avoided.

Still another advantage of my composite antenna system is that all of the radiating elements are directly grounded to the supporting mast so that improved lightning protection of the radiating segments is provided.

While I have shown and described particular embodiments of my invention, it will of course be understood that various changes and modifications may be made without departing from my invention and I, therefore, aim in the appended claims to cover all such changes and 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. A broad band, high frequency, antenna system comprising an even number of radiators symmetrically arranged to form a discontinuous planar loop, each radiator comprising a linear conductor having its ends spaced from and capacitively coupled to the ends of adjacent conductors, a plurality of transmission lines connected respectively to center feed points of the linear conductors of even-numbered radiators around said loop, odd-numbered radiators being coupled to said lines through said capacitive couplings, each even-numbered radiator further comprising two auxiliary radiating surfaces conductively bridging the halves of the adjacent linear conductor on either side of the center feed point, each odd-numbered radiator further comprising an auxiliary radiating surface conductively bridging substantially the full length of the adjacent linear conductor, and means to adjust said transmission lines and capacitive couplings to cause substantially equal, in-phase currents to flow in each of said radiators when said antenna system is energized.

2. A broad band, high frequency, antenna system comprising an even number of radiators symmetrically arranged to form a discontinuous planar loop, each radiator comprising an arcuate linear conductor having its ends spaced from and capacitively coupled to the ends of adjacent conductors, a plurality of transmission lines connected respectively to center feed points of the linear conductors of even-numbered radiators around said loop, odd-numbered radiators being coupled to said lines through said capacitive couplings, each even-numbered radiator further comprising two flat, arcuate, metal bands conductively bridging the halves of the adjacent linear conductor on either side of the center feed point, each odd-numbered radiator further comprising a flat, arcuate, metal band conductively bridging substantially the full length of the adjacent linear conductor, each of said bands having greater curvature than the conductor to which it is connected and lying outside said loop in the plane thereof with its Widest cross-sectional dimension transverse of said plane, and means to adjust said transmission lines and capacitive couplings to cause substantially equal, in-phase currents to flow in each of said radiators when said antenna system is energized.

3. A broad band, high frequency, antenna system comprising a vertical conductive mast,

an even number of horizontal antenna elements symmetrically positioned around said mast and each conductively supported thereby, each of said elements comprising a linear conductor in the form of a circular segment with its ends spaced from and capacitively coupled to the ends of adjacent conductors thereby to :form a discontinuous, horizontal, circular loop surrounding said mast, even-numbered conductors around said loop having center feed points, a separate transmission line connected to each feed point, odd-numbered conductors being coupled to said lines through said capacitive couplings, each even-numbered antenna element further comprising means providing two auxiliary radiating sheets lying outside said loop and conductively bridging the respective halves of the adjacent conductor on either side of the center feed point, each odd-numbered antenna element further comprising means providing a single auxiliary radiating sheet lying outside said loop and conductively bridging substantially the full length of the adjacent conductor, said sheets having substantially greater current-carrying areas and greater curvatures than said conductors, and means to adjust said transmission lines and capacitive couplings to cause substantially equal, in-phase currents to flow in each of said radiators when said antenna system is energized.

BURTON P. BROWN, JR.

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

UNITED STATES PATENTS Number Name Date 2,188,389 Cork Jan. 30, 1940 2,239,724 Lindenblad Apr. 29, 1941 2,267,889 Aubert Dec. 30, 1941 2,289,856 Alford July 14, 1942 2,293,136 Hampshire Aug. 18, 1942 2,324,462 Leeds July 13, 1943 2,391,026 McGuian Dec. 18, 1945 2,393,981 Fuchs Feb. 5, 1946 2,405,123 Fyler Aug. 6, 1946 2,447,879 Scheldorfl Aug. 24, 1948

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