US3680147A

US3680147A - Colinear antenna apparatus

Info

Publication number: US3680147A
Application number: US60236A
Authority: US
Inventors: Robert W Redlich
Original assignee: Individual
Current assignee: Individual
Priority date: 1970-08-30
Filing date: 1970-08-30
Publication date: 1972-07-25
Anticipated expiration: 1989-07-25

Abstract

A colinear antenna array having three half wave dipoles equally spaced and driven by currents which are equal in magnitude and phase and are properly polarized for a three element broadside array. The equal and in-phase currents are achieved by connecting the outboard dipoles to the common feed point through 360 electrical degree lengths of coaxial cable. The 360 electrical degree length coaxial cables pass through the interior of tubing which forms the dipole arms. A half wave balun transformer changes the input impedance of the dipole elements to match the characteristic impedance of the readily available coaxial cables. A predetermined separation between the dipoles determines the angular separation between the central maximum and the first null of the radiation pattern transmitted by the antenna array.

Description

July 25, 1972 COLINEAR ANTENNA APPARATUS [72] Inventor: Robert W. Redlich, 9 Grand Park Boulevard, Athens, Ohio 45701 [22] Filed: Aug. 30, 1970 [21] Appl. No.: 60,236

[52] U.S. Cl .Q ..343/792, 343/801, 343/817, 343/840 [51] Int. Cl. ..H01q 21/00 [58] Field of Search ..343/722, 792, 801, 827, 817, 343/840 [56] References Cited UNITED STATES PATENTS 1,874,983 8/l932 Hansell ..343/827 2,148,906 12/1938 Cork et al ..343/82l 3,246,333 4/1966 Hacking et al ..343/872 P- A -+F- -1 Primary Examiner-Eli Lieberman Attorney-Irving M. Weiner [57] ABSTRACT through the interior of tubing which forms the dipole arms. A

half wave balun transformer changes the input impedance of the dipole elements to match the characteristic impedance of the readily available coaxial cables. A predetermined separation between the dipoles determines the angular separation between the central maximum and the fust null of the radiation pattern transmitted by the antenna array.

10 Claims, 6 Drawing Figures TENTH- I97? 3.680.147

sum 1 or 2 i Q j) T Mr T 9 gw LL T INVENTOR ROBERT W. REDLICH- PATENTED 3.680.147

9b 85 711405041) 3oa'o I0 612; 2'0 3'040506'0 70509'0 INVENTOR ROBERT W. REDLICH ATTORNEY COLINEAR ANTENNA APPARATUS The present invention relates to a colinear antenna apparatus. In particular, the invention relates to a three element colinear antenna array intended for use in the transmission of 5 Instrument Landing System Glide Slope signals for aircraft guidance during landing approaches.

BACKGROUND OF THE INVENTION slope beam which is narrow in azimuth is not novel, nor is the concept of a colinear antenna array novel. However, the present invention and the particular antenna configuration made in accordance with the present invention as described hereinbelow will provide important advantages for Instrument Landing System Glide Slope service and systems. In particular, the colinear antenna apparatus according to the present invention provides: a simple and inexpensive antenna construction with a minimum of parts; relative insensitivity to ice, rain, and other weather conditions because of its low driving point impedances; an input impedance which is close to 50 ohms to match the conventional and readily-available coaxial cables; little if any change in the radiation pattern or the input impedance over the entire frequency band employed for the glide slope transmissions; soldered joints, rather than coaxial connectors, for reliability over long periods under adverse weather conditions; and finally a precisely determined radiation pattern transmitted by the colinear antenna apparatus.

SUMMARY OF THE INVENTION The present invention provides a colinear antenna apparatus which includes a plurality of dipole elements which are arranged in a predetermined array, and first means for supporting the plurality of dipole elements. The colinear antenna apparatus also includes second means for driving the plurality of dipole elements by substantially equal and substantially in-phase currents from a common feed junction at or adjacent to the center of the predetermined array. The antenna apparatus includes at least one conductor which is substantially 360 electrical degrees in length for connecting the common feed junction to at least one of the dipole elements which is remote from the common feed junction.

In one possible embodiment of the present invention which might be used for glide path service, a three element colinear antenna array would be mounted in a horizontal fashion along the center plane of a comer reflector screen.

In another possible embodiment of the present invention, the colinear antenna array may be mounted on a substantially vertical mast with its dipole elements arranged in a horizontal fashion in the manner in which it could be used as an element of an Instrument Landing System Glide Slope transmitting array. The dipole elements can desirably be enclosed by a nonmetallic structure, such as a Plexiglas greenhouse, for weather protection. In addition, the dipole elements can be backed up by a reflecting screen. The reflecting screen could be substantially flat in shape, be a parabolic reflector, or be a comer reflector.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a schematic of first embodiment of the colinear antenna apparatus according to the present invention.

FIG. 2 depicts an isometric view of the first embodiment of the present invention which is shown in FIG. 1, including a mast for supporting the colinear antenna array and a substantially flat back-up reflector screen.

FIG. 3 illustrates the dipole current relationships for the center dipole of the first embodiment of the present invention which is shown schematically in FIG. 1.

FIG. 4 illustrates the dipole current relationships for the left dipole element of the first embodiment of the present invention which is illustrated in FIG. 1.

FIG. 5 shows a curve of the measured azimuth radiation pattern attained by employing a colinear antenna apparatus in accordance with the present invention.

FIG. 6 illustrates schematically the relationship between the colinear antenna array and the half beamwidth.

DETAILED DESCRIPTION OF SOME EMBODIMENTS OF THE PRESENT INVENTION Among the most common radiators in the antenna field is the dipole antenna, which may consist of a straight conductor, such as a thin wire or a circular cylinder of larger diameter, broken at some point where it is excited by a voltage derived from a transmission line, a wave guide, or directly from some other type of generator. In most situations, the exciting source is at the center, yielding a symmetrical dipole, although asymmetrical dipoles have been used as well. If the arms on the dipole are very short as compared with the wavelength, it is known as an infinitesimal dipole, or a I-Iertzian dipole. Resonant dipoles, and especially the half-wave dipole, are more common. In a half-wave dipole, the dimension from one end of the dipole to the opposite end of the dipole is approximately equal to a half-wavelength.

With reference to FIG. 1, there is shown a schematic of a three element colinear antenna array or apparatus according to a first embodiment of the present invention. The colinear antenna apparatus includes a center dipole element 1, a left dipole element 2, and a right dipole element 3. If desired, the rods or radiating arms 4 of the dipole elements may be formed from metallic tubing, such as thin walled brass tubing.

With reference to FIG. 2, the colinear antenna apparatus according to the present invention includes first means, such as a vertical mast 5 for supporting the plurality of dipole elements 1, 2 and 3. The colinear array of dipole elements may be backed up by a reflector element, such as the flat reflecting screen 6. If desired, extensions 7 of the back-up reflecting screen 6 may be employed.

In order to make sure that the antenna apparatus is insensitive to adverse weather conditions, such as icing and rain, the dipoles or dipole elements 1, 2 and 3 may be enclosed in a non-metallic structure, such as the Plexiglas enclosure 8 shown in FIG. 2.

Referring again to FIG. 1, the colinear antenna apparatus according to the present invention includes second means for driving the plurality of dipole elements 1, 2 and 3 by substantially equal and substantially in-phase currents from a common feed junction or point generally indicated by the reference numeral 9 disposed at or adjacent to the center of the predetermined array of dipole elements 1, 2 and 3. Such second means may include a coaxial input feeder cable 10 which is connected to a one-half wave balun transformer 11 disposed at the common feed junction or point 9.

The balun or balun transformer 11 is sometimes referred to as a line balance converter, and it is used as a transition between an unbalanced coaxial system and a balanced twowire line or dipole antenna system. At lower frequencies, the balance-to-unbalanced transition might possibly be made with a conventional transformer. However, insofar as high frequency alternating currents are concerned, the outer conductor of a coaxial cable is generally at or near the potential of any large nearby conducting surfaces, either because of an intentional connection between the two, or simply because of the capacitance existing between them. A two-wire line, on the otherhand, should be balanced with respect to ground so that the conductors may carry equal and opposite current. Unbalance will result in the induction of currents on the nearby conducting surfaces.

In addition, if a coaxial cable is connected to an unbalanced two-wire line, a wave will be transmitted along the outside of the coaxial cable. The currents so produced generally serve no useful purpose and lead to unnecessarily high losses and undesired radiation.

The simple balun or balun transformer II is sometimes called a decoupling sleeve or a bazooka. It may consist of a quarter-wave or a half-wave sleeve at the end of a coaxial cable. The end at the point of transition is open, and the other end is closed. The balun transformer 11 shown in FIG. 1 may be used for changing the input impedance of the plurality of dipole elements 1, 2 and 3 to match the characteristic impedance of the conventional and readily available coaxial cables, such as the coaxial input feeder cable 30. However, impedance transformation is not the primary purpose of the balun transformer 11; its essential purpose is coaxial to balanced conversion. Impedance transformation with a )t/2 balun transformer is inherently 4:1, which in this case results in a favorable input impedance of about 50 ohms.

Thecolinear antenna array includes at least one conductor or transmission line which is substantially 360 electrical degrees in length for connecting the common feed junction or point 9 to at least one of the dipole elements 2 or 3 which is remote from the common feed junction or point 9. In the embodiment of the invention which is illustrated in the drawings, such conductor or transmission line takes the form of 360 electrical degree lengths of coaxial cable 12 in the particular manner shown in the figures. It should be noted that the coaxial cable portions 12 shown in FIG. I take the form of substantially U" shaped members having their major portions disposed substantially transverse to the longitudinal axis of the predetermined colinear array of dipole elements ll, 2 and 3. Alternatively, the cable portions 12 could be bent into a coil shape.

FIGS. 1, 2, 3, 4 and 6 illustrate a three colinear half-wave dipole antenna apparatus which is driven by equal and inphase currents from a common feed point or junction 9 at the center of the array. The dipole elements 1, 2 and 3 are spaced apart by substantially equal distances. The equal and in-phse currents, which are illustrated in FIGS. 3 and 4, are achieved by connecting the outboard or remote dipole elements 2 and 3 to the common feed point or junction 9 through the 360 electrical degree lengths of coaxial cables 12 in the particular manner shown. This current drive feature of the present invention is important because the dipole currents will determine the radiation pattern, such as that illustrated in FIG. 5. Furthermore, with this current drive feature, the radiation pattern according to the present invention is virtually independent of any mutual coupling between the dipole elements 1, 2 and 3. However, mutual coupling does change the impedance at the common feed point or junction 9.

As best seen in FIG. 1, the balun transformer 11 is connected to the conductors 13 and M which form the central inner conductor of the coaxial cables 12. Conductor 13 is con nected to the remote arm of dipole element 2, and the conductor 14 is connected to the remote arm of dipole element 3. The central interior conductor 13 is coaxially associated with an outer conductor 15 which feeds adjacent portions of dipole elements 1 and 2. The central inner conductor 14 is coaxially associated with an outer conductor 16 which feeds adjacent portions of dipole elements 1 and 3. It has been found satisfactory in practice if the coaxial cables 12 have a solid copper outer conductors l and 16 as described.

The 360 electrical degree length coaxial cables 12 pass through the interior of the metallic tubing which forms the dipole arms 4 of the dipole elements 1, 2 and 3. In the space between the dipole elements 1, 2 and 3, the coaxial cables 12 are bent into a substantially U" shape or coil shape to take up slack in the coaxial cables 12 and to serve as an inductor to choke off current induced on the outside of the cables 12.

The impedance seen at the balanced terminals is approximately that of three half-wave dipole elements connected in series, or about 200 ohms. If the balun transformer 11 shown in FIG. 1 is a one-half wave balun transformer, it will change the input impedance to approximately 50 ohms, which is the characteristic impedance of many readily available coaxial cables. In practice, mutual coupling and gap capacities will cause the input impedance to be reactive and less than 50 ohms. Consequently, an external match may be necessitated. Nevertheless, the impedance ratio required to be transformed by the external match will be very close to unity, so that the matching network can be a single stub (not shown) and the match will be broadband.

I FIG. 3 illustrates the dipole current relationships for the central dipole element 1. FIG. d illustrates the dipole current relationships for the dipole element 2 shown in the left portion of FIG. 1 and 2. The dipole current I is equal to the dipole current I and the dipole current 1 is equal to the dipole current I because the currents in the coaxial cable 12 are in the TEM mode. The TEM mode refers to a condition wherein the waves contain neither electric nor magnetic field in the direction of propagation. Since the electric and magnetic field lines both lie entirely in the transverse plane, these waves may be called transverse electromagnetic waves (TEM), and are also known as principal waves. With regard to the present invention, the important property of the TEM waves is that the currents in the, inner and outer conductors are necessarily equal in magnitude and opposite in direction.

With further reference to FIGS. 3 and 4, the dipole current I is equal to the dipole current of I and the dipole current I is equal to the dipole current 1,, and the dipole current I is equal to the dipole current I in accordance with the continuity of current relationship. In addition, the dipole current I is equal to the dipole current 1 in view of the passage through the 360 electrical degree length of coaxial cable 12. Therefore, all the dipole currents are equal in magnitude and phase, and are properly polarized for a three element broadside array.

With reference to FIGS. 1, 5 and 6, a working embodiment of the .present invention will now be described to illustrate a three element colinear antenna array for use in the range of 329 megacycles per second to 335 megacycles per second. In this particular operable embodiment of the invention, the dimensions A and B of the arms 4 made of thin walled brass tubing of the dipole element 2 are both dimensioned at 7 inches. The dimension C of the metallic tubing of the dipole arm 4 of the central dipole element 1 was made equal to 6 inches. The dimension D, which represents the distance between the common feed point or junction 9 and the midpoint of one of the outboard or remote dipole elements 2 or 3, was made equal to 19 /8 inches.

With the dimensions of the three half-wave dipole antenna apparatus as indicated above and with the mentioned separation between the dipoles, a radiation pattern will result having an angular separation of 37 between the central maximum and the first null of the radiation, as illustrated in the curve of the measured azimuth radiation pattern shown in FIG. 5. In FIG. 5, the abscissa axis represents the angle 0 in degrees, whereas the ordinate axis represents the magnitude of the measured azimuth radiation pattern on a linear scale. The significance of the angle 0 is depicted in FIG. 6 which shows the colinear antenna apparatus and the angle 0 demarking the half angular beamwidth. It should be noted that the 37 figure for half beamwidth is quite appropriate for glide path service.

With reference to the particular embodiment described above and illustrated in FIG. 1, it should also be noted that another separation between the dipoles could be used with equal facility to obtain a beamwidth other than the 37 degree figure if required.

With reference to FIG. 2, the reflector element, shown in the form of a flat reflecting screen 6 with optional extensions 7, may take many other forms or shapes depending upon the particular application or use for the antenna apparatus. For example, the reflector element may be a parabolic reflector, or a comer reflector, etc.

Shaped reflectors may be used to reflect in desired directions the waves from primary sources such as a dipole. One of the simplest examples is the parabolic reflector with primary feed at the focus of the parabolic shape. Geometrical optics will predict an exactly parallel beam from such a reflector if an infinitesimal source is placed at the focus. Physical optics or an electromangnetic wave analysis shows that there is always some spreading of the beam, and this decreases as aperture size is made greater. To produce a beam width of 1, an aperture of about 140 wave lengths is required.

For glide path service, the three element colinear antenna apparatus as described here and above would probably be mounted in a corner reflector along the central axial plane thereof.

As indicated here and above with respect to FIGS. 3 and 4, all the dipolecurrents are equal in magnitude and phase, and are properly polarized for a three element broadside array. It should be noted that if all the currents in a colinear antenna array are equal in magnitude and in phase, it is evident from physical reasoning that the contributions to radiation will add in phase in the plane which is perpendicular to the axis on the array. For this reason, the array is termed a broadside array. Furthermore, it is evident that, if the total length of the colinear array is long as compared with the wave length, then the phase of contribution from the various antenna elements will change rapidly as the angle is changed slightly from the maximum, so that maximum in this case would be expected to be sharp.

In accordance with the Patent Office Disclosure Document program, the present invention was disclosed in Disclosure Document No. 00l90l filed on Apr. 20, 1970 as evidence of the conception of the present invention, but this Document is not the earliest evidence of the conception of the invention.

Whereas the present invention is herein illustrated and described with respect to certain preferred forms thereof, it should be understood that the inventive concept defined by the accompanying claims may be embodied in other forms by those skilled in the art without departing from the generic teaching or scope of the invention.

1 claim:

1. A colinear antenna apparatus comprising, in combination:

' at least three dipole elements arranged in a predetermined array; I

each said dipole element including dipole rods as radiating arms thereof;

first means for supporting said dipole elements;

second means for driving said dipole elements by substantially equal and substantially in phase currents from a common feed junction at or adjacent to the center of said predetermined array;

at least one coaxial cable which is substantially 360 electrical degrees in length for connecting said common feed junction to at least one of said dipole elements which is remote from said common feed junction; and

said coaxial cable being disposed substantially concentrically with said dipole rods, passing through the interior of said dipole rods, and connected between said common feed junction and the driving point of the outer dipoles, with a U-shaped bend in the space between the center dipole and the outer dipole.

2. A colinear antenna apparatus substantially characterized in accordance with claim 1, wherein each said coaxial cable which is substantially 360 electrical degrees in length for connecting said common feed junction to at least one of said dipole elements which is remote from said common feed junction has a substantially U' shape with its major portions disposed transverse to the predetermined colinear array of said dipole elements.

3. A colinear antenna apparatus characterized substantially in accordance with claim 11, wherein each said coaxial cable which is substantially 360 electrical degrees in length for connecting said common feed junction to at least one of said dipole elements which is remote from said common feed junction has the shape of a coil which is disposed transverse to the predetermined colinear array of said dipole elements.

4. A colinear antenna apparatus characterized substantially in accordance with claim 1, wherein said dipole elements are spaced apart by predetermined distances which determine the angular separation between the central maximum and the first null of the radiation pattern transmitted by said colinear antenna apparatus.

5. A colinear antenna apparatus characterized substantially in accordance with claim 1, wherein said dipole elements include three colinear half wave dipoles which are equally spaced apart and which have a non-vertical longitudinal axis, and wherein all dipole currents are equal in magnitude and phase and are properly polarized for a three element broadside array.

6. A colinear antenna apparatus characterized substantially in accordance with claim 1, including a reflector element disposed in back of said dipole elements, and wherein said reflector element may be a substantially flat reflecting screen.

7. A colinear antenna apparatus characterized substantially in accordance with claim 1, including a reflector element disposed in back of said dipole element, and wherein said reflector element is a parabolic reflector.

8. A colinear antenna apparatus characterized substantially in accordance with claim 1, including a reflector element disposed in back of said dipole elements, and wherein said reflector element is a corner reflector.

9. A colinear antenna apparatus comprising, in combination: a plurality of dipole elements arranged in a predetermined array, first means for supporting said plurality of dipole elements, second means for driving said plurality of dipole elements by substantially equal and substantially in-phase current from a common feed junction at or adjacent to the center of said predetermined array, at least one conductor which is substantially 360 electrical degrees in length for connecting said common feed junction to at least one of said dipole elements which is remote from said common feed junction, said plurality of dipole elements comprises three colinear half wave dipoles which are equally spaced apart and which are arranged in a substantially horizontal predetermined array, said dipole elements being separated by a predetermined distance which determines the angular separation between the central maximum and the first null of the radiation pattern transmitted by said colinear antenna apparatus, said dipole element having dipole arms which are formed of thin walled metallic tubing, said first means for supporting said plurality of dipole elements comprising a substantially vertical mast, said plurality of dipole elements having an input impedance which is substantially equal to that of three half-wave dipoles connected in series, said second means for driving said plurality of dipole elements by substantially equal and substantially in-phase currents from a common feed junction at or adjacent to said center of said predetermined array includes a coaxial input feeder cable which is connected to a one-half wave balun transformer which changes said input impedance of said dipole elements to substantially the characteristic impedance of said coaxial cable, each said conductor which is substantially 360 electrical degrees in length for connecting said common feed junction to at least one of said dipole elements which is remote from said common feed junction passes through the interior of the metallic tubing which forms the dipole arms, said at least one conductor which is substantially 360 electrical degrees in length being formed from coaxial cable, and the substantially 360 electrical degree in length coaxial cable is disposed in the space between said dipole elements and is bent into a U shape to take up slack in the cables and to serve as an inductor to choke off current induced on th outside of said cables.

10. A colinear antenna apparatus comprising, in combination: a plurality of dipole elements arranged in a predetermined array, first means for supporting said plurality of dipole elements, second means for driving said plurality of dipole ele ments by substantially equal and substantially in-phase currents from a common feed junction at or adjacent to the center of said predetermined array, at least one coaxial line which is substantially'360 electrical degrees in length for connecting said common feed junction to at least one of said dipole elements which is remote from said common feed junction, said plurality of dipole elements comprising three halfwave dipoles which are equally spaced apart and which are arranged in a colinear predetermined array, said dipole elements a common feed junction at or adjacent to said center of said predetermined array includes a coaxial input feeder cable which is connected to a one-half wave balun transformer which changes said input impedance of said dipole elements to substantially the characteristic impedance of said coaxial cable, each said coaxial line which is substantially 360 electrical degrees in length for connecting said common feed junction to at least one of said dipole elements which is remote from said common feed juncfion passes through the interior of the metallic tubing which forms the dipole arms, said at least one coaxial line which is substantially 360 electrical degrees in length being formed from coaxial cable, and the substantially 360 electrical degree in length coaxial line being disposed in the space between said dipole elements and being bent into a U shape to take up slack in the cables and to serve as an in ductor to choke off current induced on the outside of said cable.

i i t I t my mm 0mm;

'- esriricirt or (IQREQ'HN Patent No. 3,680 ,llfl Dated Julv 25 9 1972 Invent fl Robert W. Redlich It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

r v "1 in the title page column 1, line it, below inventor information insert:

{73] ASSIGNEE: The President and Boardof Trustees of Ohio University.

Signed and sealed this 3rd day of April 1973 (SEAL) Attest:

EDWARD M.FLETCHER,Z1R. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents

Claims

1. A colinear antenna apparatus comprising, in combination: at least three dipole elements arranged in a predetermined array; each said dipole element including dipole rods as radiating arms thereof; first means for supporting said dipole elements; second means for driving said dipole elements by substantially equal and substantially in phase currents from a common feed junction at or adjacent to the center of said predetermined array; at least one coaxial cable which is substantially 360 electrical degrees in length for connecting said common feed junction to at least one of said dipole elements which is remote from said common feed junction; and said coaxial cable being disposed substantially concentrically with said dipole rods, passing through the interior of said dipole rods, and connected between said common feed junction and the driving point of the outer dipoles, with a U-shaped bend in the space between the center dipole and the outer dipole.

2. A colinear antenna apparatus substantially characterized in accordance with claim 1, wherein each said coaxial cable which is substantially 360 electrical degrees in length for connecting said common feed junction to at least one of said dipole elements which is remote from said common feed junction has a substantially ''''U'''' shape with its major portions disposed transverse to the predetermined colinear array of said dipole elements.

3. A colinear antenna apparatus characterized substantially in accordance with claim 1, wherein each said coaxial cable which is substantially 360 electrical degrees in length for connecting said common feed junction to at least one of said dipole elements which is remote from said common feed junction has the shape of a coil which is disposed transverse to the predetermined colinear array of said dipole elements.

9. A colinear antenna apparatus comprising, in combination: a plurality of dipole elements arranged in a predetermined array, first means for supporting said plurality of dipole elements, second means for driving said plurality of dipole elements by sUbstantially equal and substantially in-phase current from a common feed junction at or adjacent to the center of said predetermined array, at least one conductor which is substantially 360 electrical degrees in length for connecting said common feed junction to at least one of said dipole elements which is remote from said common feed junction, said plurality of dipole elements comprises three colinear half wave dipoles which are equally spaced apart and which are arranged in a substantially horizontal predetermined array, said dipole elements being separated by a predetermined distance which determines the angular separation between the central maximum and the first null of the radiation pattern transmitted by said colinear antenna apparatus, said dipole element having dipole arms which are formed of thin walled metallic tubing, said first means for supporting said plurality of dipole elements comprising a substantially vertical mast, said plurality of dipole elements having an input impedance which is substantially equal to that of three half-wave dipoles connected in series, said second means for driving said plurality of dipole elements by substantially equal and substantially in-phase currents from a common feed junction at or adjacent to said center of said predetermined array includes a coaxial input feeder cable which is connected to a one-half wave balun transformer which changes said input impedance of said dipole elements to substantially the characteristic impedance of said coaxial cable, each said conductor which is substantially 360 electrical degrees in length for connecting said common feed junction to at least one of said dipole elements which is remote from said common feed junction passes through the interior of the metallic tubing which forms the dipole arms, said at least one conductor which is substantially 360 electrical degrees in length being formed from coaxial cable, and the substantially 360 electrical degree in length coaxial cable is disposed in the space between said dipole elements and is bent into a ''''U'''' shape to take up slack in the cables and to serve as an inductor to choke off current induced on the outside of said cables.

10. A colinear antenna apparatus comprising, in combination: a plurality of dipole elements arranged in a predetermined array, first means for supporting said plurality of dipole elements, second means for driving said plurality of dipole elements by substantially equal and substantially in-phase currents from a common feed junction at or adjacent to the center of said predetermined array, at least one coaxial line which is substantially 360 electrical degrees in length for connecting said common feed junction to at least one of said dipole elements which is remote from said common feed junction, said plurality of dipole elements comprising three half-wave dipoles which are equally spaced apart and which are arranged in a colinear predetermined array, said dipole elements being separated by a predetermined distance which determines the angular separation between the central maximum and the first null of the radiation pattern transmitted by said colinear antenna apparatus, said dipole elements having dipole arms which are formed of metallic tubing, said first means for supporting said plurality of dipole elements comprising a substantially vertical mast, said plurality of dipole elements having an input impedance which is substantially equal to that of three half-wave dipoles connected in series, said second means for driving said plurality of dipole elements by substantially equal and substantially in-phase currents from a common feed junction at or adjacent to said center of said predetermined array includes a coaxial input feeder cable which is connected to a one-half wave balun transformer which changes said input impedance of said dipole elements to substantially the characteristic impedance of said coaxial cable, each said coaxial line which is substantially 360 electrical degrees in length foR connecting said common feed junction to at least one of said dipole elements which is remote from said common feed junction passes through the interior of the metallic tubing which forms the dipole arms, said at least one coaxial line which is substantially 360 electrical degrees in length being formed from coaxial cable, and the substantially 360 electrical degree in length coaxial line being disposed in the space between said dipole elements and being bent into a ''''U'''' shape to take up slack in the cables and to serve as an inductor to choke off current induced on the outside of said cable.