US3184747A

US3184747A - Coaxial fed helical antenna with director disk between feed and helix producing endfire radiation towards the disk

Info

Publication number: US3184747A
Application number: US225249A
Authority: US
Inventors: Kach Alfred
Original assignee: Patelhold Patenverwertungs and Elektro-Holding AG
Current assignee: Patelhold Patenverwertungs and Elektro-Holding AG
Priority date: 1961-10-06
Filing date: 1962-09-21
Publication date: 1965-05-18
Anticipated expiration: 1982-05-18
Also published as: DE1166297B; NL283937A; FR1335403A; CH391803A; GB1012738A; SE320421B

Description

May 18, 1965 A. KCH 3,184,747

coAxIAI. FED HELIcAI. ANTENNA wITH DIRECTOR DISK BETWEEN FEED AND HELIX PRODUCING ENDFIEE RADIATION TOWARDS THE DISK Filed sept. 21, 1962 2 sheets-Sheet I 5 HIIIIIYHIYIIIVHIYHND'NIIIINHJVIVA 8 INVENTOR.

wf-RED Kiew BY Wea ATTORNEY May 18, 1965 A. KcH 3,184,747

COAXIAL FED HELICAL ANTENNA WITH DIRECTOR DISK BETWEEN FEED AND HELIX PRODUCING ENDFIRE RADIATION TOWARDS THE DISK Filed Sept. 2l, 1962 .2 Sheets-Sheet 2 INVENTOR. Abr/ED /Ocw ATTORNEY United States Patent O 3,184,747 COAXEAL FED HELlCAL ANTENNA WHH DlREC- TGR DISK BETWEEN FEED AND HELIX PRO- DUCENG ENDFRE RADIATION TWARDS THE DISK Alfred Kch, Nnsshaumen, Aargau, Switzerland, assignor to Patelhold Patentverwertungs- 8: ElektronHolding, A.G., Glarus, Switzerland Filed Sept. 21, 1962, Ser. No. 225,249 Claims priority, application Switzerland, Get. 6, 1961, 11,600/61 12 Claims. (Cl. 343-840) The present invention relates to helical antennas of the type being comprised essential-ly of a loosely wound wire helix and characterized by their effective directional characteristics.

Helical antennas which radiate the electromagnetic energy predominantly along the helical axis (axial mode of operation) have :heretofore become known in the form of numerous designs and constructions. Such antenn-as ordinarily :are comprised of a wire helix having its input end connected to the end of a feeding line, such as the inner conductor of a coaxial feeder, and being tted in the vicinity :of the junction between the -helix and said line with a parasitic antenna in the for-m of a reecting disk or the like extending at right angle to the helix axis, the inner conductor of said feeder being passed through said disk and the latter being connected to the outer coaxial conductor. With the helix ,and reflecting disk being suitably dimensioned, as described in the following, radiation will be :predominantly along the axis of the helix and in a direction from the junction point between said line land helix towards the free or open end of the helix, this direction being referred to in the following fas the Iforward 'helix axis or direction for the purposes -of this specification. As is well known, this forward radiation of the conventional helical antenna is substantially circularly polarized.

The dimensions of a helical antenna of the type referred to are non-critical within rel-atively wide limits on account of the broad Iband width characteristics of the antenna. In order to effect radiation in the so-called axial mode referred to, as distinct from the normal mode involving radiation in the direction laterally of the helix axis, it is necessary for the mean helix diameter to have a value between 3 and M 4, wherein A represents the operating wave length yof the radiated energy in free space. To the same end, the pitch angle of the helix should be within deiinite limits, such for instance between l-15", while the reflecting disk, in the absence of which the radiation from the antenna would be diffuse or strongly elliptieally polarized and directed in both the forward and backward directions of the helix axis, should have a diameter equal to at least one .wave length, 'but by no means should be less than M 2 in order to ensure a sufficiently sharp or concentrated energy radiation in the forward direction of the helix.

The energy concentration or directivity of a forward radiating helical antenna of this type litted with a reecting disk may furthermore be influenced by varying the number of turns of the helix, as well as by varying the distance between adjacent winding turns. -It h-as also become known, in Ian effort to improve the directional characteristics, -to arrange two helices one within the other. In the case of two helices of opposite winding sense and varranged one behind the other, the resultant radiation will be substantially linearly polarized. Furthermore, it has become known to vary the diameter of the helix yalong its axis, such as by the use of a tapering or conical helix, in an `eort to modify the radiation characteristic of the antenna.

lCe

Por certain practical antenna applications, such as for instance in using a directional antenna as a primary radiator in conjunction with a parabolic or the like redector, it is necessary to provide a primary antenna having :a directional field or radiation pattern point-ing towards the opening of the parabolic reector. For this purpose, it has already been proposed to utilize la forward radiating helical antenna of the type referred to -with the coaxial feeder of the antenna being passed from the free end 'of and through the helix to the input end or feeding point adjacent to the reiecting disk, without essentially affecting the directional radiation characteristics of the composite antenna structure.

Antenna and reflector arrangements of the foregoing type possess various defects and disadvantages in practice. IIn :the first pl-ace, the helical antenna and reflector :struct-uros assume considerable dimensions Within the lower frequency range of about -300 mc., whereby to render their use -impractical if n-ot impossible in connection with the lower operating frequencies. Besides, in Iarrangements of this type, the reiiecting disk `acts to m-ask a considerable portion ofthe parabolic reiiector surface, whereby to result in substantially reduced radiation efli-ciency of the composite antenna structures. ln the second place, it is desirable for practical reasons to mechanically :support both the helix yand parabolic reflector by the coaxial feeder disposed in `line with the helix axis. However, inasmuch as for the achievement of maximum radiation leiiiciency the outer diameter of the coaxial line should not exceed :about 20% Iof the helix diameter, the diameter of the coaxial line or feeder, especially in the case of the higher operating frequencies, will assume `such `a small `dimension as to be no lon-ger able to provide the required mechanical strength necessary to serve as a mounting means or support -f-or the antenna and reector structure. This condition will be aggravated rby the presence of un* predictable and uncontrollable influences, such as snow or wind pressure upon the reiiector which, in addition, may have :to support a protective sleeve or housing en- -closing the wire helix of .the antenna. As an example, the mean helix `diameter of a heli-cal antenna designed for an operating frequency in the neighborhood of 7000 mc. is found to be about l2 mm., in which case the most favorable outer diameter of the coaxial feeder from an electrical standpoint will be only 2.4 mm., whil-e the parasitic reflecting disk should 'have a diameter of at least 40 mm., that'is, of the order of or equal to the operating wave length. Such a feeder is practically unsuited .as a supporting or mounting means for the antenna and reflector structures. Eesides, a coaxial line of such small dimension presents considerable diliculty to the achievement of a wave impedance of 50 ohms as required for norm-al operation, for both physical and electrical reasons. Finally, it has ibeen found that the combination of a forward radiating helical antenna and reiiecting disk of the type known in the art in many cases exhibit an inadequate ratio between the forward and backward radiation, it being impossible to materially improve this ratio without a considerable increase of the dimension of the parasitic reflecting disk.

Accordingly, an important :object of the present invention is the provision of a novel and improved helical directional antenna by which the aforementioned and related difficulties and drawbacks are substantially eliminated or minimized.

iA more specific object of the invention is the provision of a composite :structure `comprising a helical antenna and coaxial feeder, wherein the latter may have yan adequate mechanical strength witho-ut deleteriously Iaffecting the electrical properties and characteristics of the antenna, to serve as a suport for said antenna as well as auxiliary devices, in particular a parabolic reilectcr combined with the antenna acting as a primary radiator.

Another object of the invention 4is :the provi-sion of a helical antenna as a primary radiator combined with a parabolic or the like reiiector characterized I'by improved mechanical as Well as electrical characteristics compared with antenna kstructures of similar type known in the prior art.

Yet another object of the :invention is the provision f radiating helical antenna and coaxial feeder constructed in accordance with the principles of the invention;

FIG. 7 is a cross-sectional view illustrating the cornbination of a helical antenna according to the invention as a primary radiator with a parabolic reflector;

FIGS. 8 and 9 are schematic views illustrating combinations of a backward radiating helical antenna according to the present invention with a conventional forward radiating antenna; and

FIGS. l and l1 are similar schematic views showing multiple helical antenna structures according to the invention.

Like reference 4characters denote like parts and elements throughout the different views of the drawings.

With the foregoing objects in view, the invention is predicated principally upon lthe findings and recognition that a reduction of the diameter of the parasitic reflecting disk of ,a -conventional forward radiating helical anytenna of the type referred to below the critical or minimum value at first causes the radiation to become diffuse or non-directional and thereafter to change into an axially Ibackward radiation as the diameter of the reiiecting disk is further decreased, by reason of the fact that said disk ceases to act as reflector and assumes lthe function ofa parasitic director, in such a manner as to change the original directive radiation in the forward helix direction into a radiation opposite thereto or in the backward direction in respect to the forward helix axis. This radiation will be referred to herein as backward radiationl of the composite helical antenna and director structure. For practical purposes, a backward radiation of this type is obtained by the use of a director disk or the like parasitic antenna at the junction between the feed line and 'the input of the helical antenna, said director having a diameter or radial dimension being less than about one-half the operating wave length in free space.

Such a backward radiating helical antenna, forming the subject of the present invention, has many advantages of both a mechanical and electrical nature, compared with a conventional forward radiating helical antenna, as will become further apparent as the following description proceeds in reference to the drawings.

Referring more particularly to FIG. l, the numeral 1 represents a wire helix having its beginning or input end connected with a fee-der, that is, the inner conductor or wire 2 of a coaxial line having an outer tubular conductor 4. Disposed at the junction between the helix 2 and the conductor 2 is a director disk 3 through which is passed said conductor and which, in turn, is connected tothe outer conductor 4 of the coaxial feeder. Item 5 represents an insulating spacer Ibetween the inner and outer conductors of the coaxial line. The diameter or radial extension of the director 3, in accordance with the present invention, is less than about one-ralf of the operating wave length in free space, as pointed out hereinbefore. The radiation of a helical antenna constructed in this manner is concentrated along the helical axis and directed backward, or opposite to the forward helix direction, as seen from the antenna input or feeding point and indicated by the arrow a in the drawing.. lf fthe diameter of the director disk 3 is substantially greater than M2, the backward radiation will change with increasing diameter first into a diffuse or yomni-directional radiation and then into the known forward radiation with the director 3 becoming a reflector as the diameter is further increased to the limit mentioned. In `other words, in the case of thebackward radiating antenna, the disk 3 no longer acts as a parasitic reflector but rather as a director for the electromagnetic energy being radiated. if the diameter of the disk is made smaller than M10, or if no disk or parasitic antenna is provided, the'radiation again becomes diffuse or more or less omni-directional.

it is advantageous, in the interest of attaining a wellbalanced radiation characteristic as well as a favorable ratio between backward and forward radiation, to make lthe diameter of the director disk Sat least approximately equal tothe mean diameter of the helix, preferably between -thelimits of M 3 and M4. The diameters of the disk-and helix need, however, not be exactly equal, but the disk may have adiameter near the upper limit of the range mentioned, while the diameter of the helix may be near the lower limit kof said range, or vice versa. Especially favorable radiation conditions are obtained if both diameters are thesame and equal to 0.3M

The mean distance between the helix winding turns is also non-critical, as. far as the radiation characteristic is concerned, and may be between M3 and M15. With a smaller distance betweenthe turns, the directional beam is somewhat more concentra-ted. Advantageously, the mean winding distance is chosen to be equal to about M 5. There exists, however, a lower as well as an upper limit for the winding distance beyond which the radiation will no longe-r be inthe axial direction. Thus, in the case `of a very small winding distance, the helix assumes the character of a solid metal cylinder. The limit at which a marked directional radiation may still be ascertained is approximately at M 67.". This limit condition can be recognized bythe fact that the free end of the helix is no longer decoupled, in which case the input impedance of the antenna varies substantially asa function of the frequency. On the other hand, with `too greatl a winding distance, the radiation characteristic Y of the helix approaches thatof a stretched or straight wire, resulting again in the disappearance of the backward radiation.

The wire diameterl exerts only asmall influence on the radiation characteristics of the antenna. its most favor- .able value is between M 100 and M 2G, the smaller value being advantageously chosen for the larger wave lengths. ln the case of FIG. l, the helix consists of a metal wire having a cylindrical cross-section. Alternatively, the helix may consist of a strip-like conductor produced for instance by means .of a printed circuit 4technique upon a cylindrical insulating core, or upon the inner surface of a hollow sleeve or. tube serving as a protective enclosure for theantenna. Besides, Ythe helix may Vbe pressed yor cast in an insulating core, in a manner well understood.

In FIG. 1, the radial connecting link 6 Vbetween the inner conductorZ of the coaxial feederV and the input point of helix l is shown to have a-rise or pitch angle a. The latter in case of a forward radiating helical antenna and reflecting disk should have a value `of about l0-l5, in order to achieve optimum radiation in the forward direction. In the case of a backward radiating helical antenna according to the present invention, the shape `of the helix wire in the neighborhood of the antennarinpu-t or feeding point and ofthe director disk is of equal importance, especially in the interest of achieving a favorable rotational symmetry of the radiation field relative to the helix axis. Especially favorable conditions are obtained if the radial connecting or coupling link 6 between the inner conductor 2 and the helix 1 plus at least a part of the iirst winding turn of the helix coincide with a plane being parallel to the disk 3, whereby the pitch angle of at least the initial portion of the helix is equal to zero. Advantageously, the portion of the helix being parallel to the director disk is designed to gradually merge into the ascending or main turns of the helix.

An antenna of the latter construction is shown in FIG. 2, wherein the coupling link 6 connecting the inner conductor 2 of the feeder with the beginning of the helix 1, together with about one quarter to one half of the rst winding turn, are arranged parallel to the director disk 3, and wherein the portion of the helix being parallel to the director disk merges gradually into the main helix turns having a constant rise or pitch angle.

The distance between the wire portion parallel to the director disk from the latter should be of the order of the diameter of the helix wire. The radial link 6 may furthermore serve in a known manner as a transformation line to match the radiation resistance of the antenna with the wave impedance of the feeder.

The embodiments shown in FIGS. 3-5 differ from FIG. 1 and from each other solely by the construction of the director disk 3. In FIG. 3, the director disk or parasitic antenna is formed by the edge surface of the outer reinforced conductor 4 of the coaxial feeder having a sufficiently large wall thickness, to provide adequate mechanical strength when serving as a support or mount of the antenna. In the embodiment according to FIG. 4 the reinforced outer conductor 4 of the feeder is provided upon its edge surface with a recess 7 acting as a choke to prevent energy return ow along the feeder. Finally, in accordance with the embodiment shown in FIG. 5, the reinforced outer diameter of the feeder is provided with an annular recess near the director disk 3 tapering outwardly to a certain distance from the end of said disk. This construction of the feeder in the vicinity of the director exerts a favorable iniiuence on the ratio between the backward and forward radiations, with the configuration shown by FIG. 5 having been found to be especially favorable in this respect. It is furthermore possible to influence the residual forward radiation within certain limits by varying the thickness of the director disk 3, in that the forward radiation increases for disks of both very thin and very thick wall thickness. The disk 3 may be provided with perforations or replaced by a wire net, in a manner well known.

A backward radiating helical antenna according to the present invention is characterized by its broad band width characteristics, being able to effectively encompass a frequency band having frequency ratio of 1.5, without any substantial variation of the radiation characteristic or radiation resistance of the antenna. Besides, practically the full amount of the energy is radiated by the rst Winding turn of the helix being closest to the director disk, while the next following turns predominantly act to reduce the residual forward radiation. More than tive turns are required only in case of an extremely small winding distance. If the mean winding distance is greater than M 8, the remaining turns no longer contribute to the radiation process. Furthermore, since the current distribution along the helix decreases approximately exponentially, all the turns following the third turn are practically decoupled and merely exert a slight influence on the'iield in the forward direction, which influence disappears completely from the sixth turn onward. As a consequence, a backward radiating helical antenna according to the present invention has the further advantage of achieving an optimum radiation characteristic with a minimum of turns of the helix, thus, in turn, reducing the size and bulk of the antenna structure, compared with forward radiating helical antennas according to the prior art.

Despite the unsymmetrical feeding ofthe antenna, it is possible to achieve a complete rotational symmetry of the radiation eld in respect to the helix axis by a gradual transition of the portion of the lirst winding turn being parallel to the director disk 3, FIG. 2, into the remaining turns of the helix, in the manner described hereinbefore, In such a case, the iield symmetry is maintained for any azimuth angle within the aforementioned frequency ratio of 1.5. On the other hand, the ratio between backward and forward radiation is dependent upon the frequency, inasmuch as the amount of residual forward radiation depends upon the winding distance, the helix diameter and to a slight extent upon the distance between the input end or portion of the helix from the director disk. For the aforementioned frequency ratio of 1.5, it is possible to achieve a ratio between backward and forward radiation of from l5 to 20 db near the limits of the frequency range, and, in contrast to the forward radiating antenna, a ratio of more than 30 db in the center of said range. The antenna gain is from 6 to 9 db in reference to an isotropic radiator, also being dependent Iupon the winding distance. The radiation resistance of the antenna is between and 150 ohms, the same as in the case of a forward radiating antenna.

As an example, a backward radiating helical antenna according to the invention having the aforementioned characteristics and being designed for a frequency range of 5000 to 9000 mc. may have the following dimensions:

Mm: Outer helix diameter 13.5 Winding distance 8 Helical wire diameter 1.5 Number of winding turns 5 Diameter of the director disk l2 Thickness of the director disk 1.7

In the embodiments described hereinbefore, feeding is from the inner end of the helix, that is, with the coaxial feeder and helix being separated by or disposed on opposite sides of the director disk. Alternatively, feeding may be through the helix in the manner shown in FIG. 6. In such a case, the outer diameter of the feeder may have a limit up to one half of the helix diameter without appreciably affecting the radiation characteristic of the antenna, inasmuch as radiation is in the direction away from the helix and disk 3. In FIG. 6 numeral 1 again represents the wire helix having an end connected to the inner conductor 2 of the coaxial feeder which is passed axially through said helix. Item i again represents the tubular outer conductor of the feeder containing a suitable insulating material in which is embedded the inner conductor 2. The outer conductor 4 is again connected vwith the director disk 3 dis-posed close to the feeding or input point, said disk having the same diameter as the helix 1 in the example illustrated and being related to the wave length in the manner described. Both the helix and disk 3 are mounted in an insulating sleeve or enclosure 8 which serves both as a mechanical support of the helix and as a protection against atmospheric inuences. Radiation in the backward direction, or from the helix toward the director disk or antenna, is the same as in the preceding embodiments and indicated by the arrow a in the drawing. The director disk 3 may be provided with a recess 9, to reduce the thickness and, in turn, the residual forward radiation, in a manner pointed out in the foregoing.

A backward radiating helical antenna according to FIG. 6 has the advantage, among others, compared with the known forward radiating antenna, that :the outer diameter of the feeder being passed through the helix may be relatively large, such as 40% of the inner helix diameter, without deleteriously affecting the radiation characteristic of the antenna. As a consequence, the coaxial feeder may serve as a support or mounting means for the antenna with or without any auxiliary devices (parabolic reflector, etc.). Besides, the size and outer dimension of the antenna structure are reduced, in that a large reflector disk is dispensed with and a relatively small number only of helical turns is required for the attainment of an optimum radiation characteristic.

FIG. 7 illustrates the utilization of a backward radiating helical antenna according to the invention as a primary radiator disposed at the focal point of a parabolic reflector. According to this embodiment, the wire helix 1 is supported by means of the outer conductor d of the coaxial feeder at the apex of the parabolic retiector 1d, shown in part only in the drawing. The antenna may be adjusted to a position of exact coincidence with the focal point of said reflector by means of an adjustable mount or clamp 11 of said feeder, as indicated in the drawing. A forward radiating helical antenna is especially suitable as a primary radiator for a parabolic or the like reflector on account of its slight residual forward and lateral radiation. Moreover, the portion of theparabolic reflector surface masked by the antenna with director is considerably less than in the case of a forward radiating antenna with reflector, for the reason pointed out herein and obvious from the foregoing.

According to a further feature of the invention, a backward radiating antenna may be advantageously combined with a forward radiating antenna for the attainment of a special radiation field, provided care is taken to effectively decouple both antennas, or to prevent one radiator from acting as a receiving antennav for the energy emanating from the other radiator of the multiple antenna structure. Inasmuch as the radiation field of both types of antenna is circularly polarized, it is furthermore necessary to take into consideration both the direc- CIL tion of the energy tiux and theV rotational directions of polarization of the separate fields being radiated in determining the characteristics and polarization of the resultant field or radiation. Furthermore, care should be taken, by the proper choice of the directions of rotation of the polarization as well as of the phase positions at the feeding points of the antennas, to prevent ield cancellations in the resultant directional beam or radiation` field.

Referring to FIG. 8, there is shown schematically an arrangement of this type for the production of a linearly polarized directional radiation iield. Disposed at one end of a single helix 12 is a director disk 13 and disposed at the opposite end of said helix is a reflector disk 1d. The inner conductor of the coaxial feeder 15 is connected to both ends of the helix 12, whereby the latter together with the director disk 13 constitutes a backward radiating helical antenna producing an energy flux as indicated by the arrow a, on the one hand, and whereby the helix 12 together with the reliector disk 14 constitutes a forward radiating helical antenna radiating in the same direction, as indicated -by the arrow b in the drawing. Inasmuch as the antenna 12 and director disk 13, on account of its directional characteristic, is insensitive to the radiation emanating from the reliector disk 1d, there exists a practically complete decoupling between both antenna systems. In ease of equal energy distribution upon the feeder 15, the resultant composite radiation will be linearly polarized, its direction of polarization depending upon the existing phase difference between the opposite feeding points of the helix 12, on the one hand, and upon the transit time of the wave emanating from the reflectingV disk 14 along the helix 12, on the other hand. Feeding of the antenna may also be by means of a double line, as will be understood.

A further embodiment of a composite forward and backward radiating helical antenna is shown by FIG. 9. The arrangement which includes a parabolic reflector 16 is designed to produce both a coarsely concentrated and a highly concentrated radiation field or beam in the same direction. For this purpose, there is disposed at the focal point of the reflector 16 a forward radiating helical antenna (arrow c) serving as a primary radiator for said f d reliector and being comprised of a helix 12 connected to the inner conductor of the coaxial feeder 1S .and a reecting disk 14. The resultant radiation field emanating from the parabolic reflector 16 (arrows b) is relatively highly concentrated. Disposed upon the same axis on the side of the disk 14.- opposite to the helix 12 is a-further helix 12' also being connected to the inner conductor of the feeder 15 and a director disk 13, both the helix 12' and director 13 constituting a backward .radiating helical antenna (arrow a) and producing a relatively coarsely concentrated radiation iield or beam of the same direction as the heid emanating from the reflector 16. As is understood, both the reflector 14 and director 13 are connected to the outer conductor of the-feeder 15, in a manner described hereinbefore. The sense ofirotation of polarization of the radiation emanating from the antenna 12', 13 in reference to ythe direction of the energy flux, is opposite to the sense of rotation of the energy emanating from the

antenna

12, 14, assuming both helices. 12 and 12' to have the same winding sense. However, inasmuch as the direction of the energy liux emanating from the

antenna

12, 14 is reversed by the parabolic reflector 16, the two radiated fields (arrows a and b) will have the same rotational direction of polarization. Moreover, both antennas are substantially decoupled from one another, inasmuch as the antenna 12', 13 producing the relatively coarsely concentrated beam or radiation field is located in y the shadow of the reflecting disk14 and (besides, is insensitive to the radiation emanating from` the parabolic reflector 16.

A combined coarsely and highly concentrated radiation field in connection with a parabolic reliector may furthermore be produced by means of a pair of backward radiating antennas in the manner shown by FIG. l0. In the latter, the

antenna

12, 13 being positioned in the focal point of the reliector 16 .ac-ts as a primary radiator (arrows a) for the'latter, whereby to produce a highly concentrated directional beam emanating from said redector (arrows b), while the antenna 12', 13 operates as a direct radiator (arrows a') producing a coarsely concentrated beam or radiation in the direciton of said first beam. The sense of winding of the helices 12 and 12' in this case should be opposite to one another in order to cause the partial radiation beams to have the same direction of rotation of polarization and to cause the antennas to be decoupled from one another.

Composite or multiple antenna structures of greater etiiciency may furthermore be produced by arranging a plurality of backward radiating antennas either one behind the other or in juxtaposed relation, to form multiple antennas or groups. An example of such an arrangement is shown by FIG. l1. In the latter, two

antennas

12, 13 and 12', 13` are disposed side by side with their helices 12 and 12' being wound in the same sense and being spaced by a distance A from one another, to cause both antennas to radiate in the same (backward) direction, as indicated by the arrows a and a' in the drawing, respectively. In order to obtain an equiphased polarizaiton, the feeding points of the helices 12 and 12' are displaced by 120 as shown in the drawing. The distance A may be between A and M2. Both antennas are substantially decoupled on account ofthe relatively weak forward and lateral radiations. The feeding conductors extend only partly along the helix axes and lead to the common double feeder being disposed between both antenna structures.

In the'foregoing the invention has been described in reference to a few specific illustrative antenna structures or systems. It will bev evident, however, ,that variations and modifications, as well as the substitution of equivalent parts and elements for those shown herein for illustration, may be made without departing from the broader scope and spirit of the invention as set forth in the appended claims. The specification and drawings are accordingly to be regarded in an illustrative rather than in a restrictive sense.

I claim:

1. A directional antenna comprising in combination:

(l) a cylindrical wire helix,

(2) a coaxial feeder having an inner conductor and an outer conductor,

(3) a radial coupling link connecting one end of said helix inner conductor,

(4) a parasitic antenna disk disposed at the junction between said helix and said inner conductor, said disk being connected to said outer conductor and extending radially from the helix axis to an outer diameter substantially equal to he mean diameter of said helix, and

(5) said coupling link and at least a portion of the adjoining first turn of said helix being substantially parallel to and spaced from said disk by a distance of the order of the diameter of the helix wire, whereby said disk acts as a director producing a directional radiation characteristic of said antenna in the direction from the free end of said helix towards said disk.

2. A directional antenna system comprising in combination:

(1) a parabolic redector,

(2) a coaxial feeder in line with the axis of said reector having an inner conductor and an outer conductor,

(3) means to support said refiector by said outer conductor,

(4) a main antenna in the form of a cylindrical wire helix disposed at the focal point of said reiiector co- Q axially with and extending outwardly from said feeder,

(5) a radial conductor connecting an end of said helix tothe end of said inner conductor,

(6) a parasitic antenna disk disposed at the junction between said helix and said inner conductor, said disk being connected to said outer conductor and extending radially from the helix axis to an outer diameter substantially equal to` the mean diameter of said helix, and

(7) said radial conductor and at least a portion of the adjoining iirst turn of said helix being substantially parallel to and spaced from said disk by a distance of the order ofthe diameter of the helix wire, whereby said disk acts as a director producing a directional radiation characteristic of said antenna in the direction from said helix towards said reector.

3. A directional antenna as claimed in claim 1, said disk and helix having a diameter of between one third and one fourth of said Wave length.

4. A directional antenna as claimed in claim 1, the distance between adjacent winding turns of said helix being between one third and one iifteenth of said wave length.

5. A directional antenna as claimed in claim 1, said feeder and helix being loca-ted on and extending from opposite sides of said disk.

6. A directional antenna as claimed in claim l, said feeder and helix being located on the same side of said disk with said feeder passing through said helix.

7. A directional antenna as claimed in claim 1, the portion of said helix being parallel to said disk merging gradually into the` adjoining ascending turns of said helix.

8. A directional antenna as claimed in claim 1, including means to support said helix by the outer conductor of said feeder.

9. A directional antenna as claimed in claim 1, said parasitic antenna being formed by the edge of said outer conductor having a diameter substantially equal to the mean diameter of said helix.

10. A directional antenna as claimed in claim 9, including a recess near the edge of said outer condnctor acting as a choke, to prevent high-frequency energy return iiow along said feeder.

l'l. A directional antenna as claimed in claim 9, said outer conductor having a circular recess adjacent to its edge to form a director disk, said recess tapering gradually in the direction away from said disk.

l2. In a directional antenna structure as ciaimed in claim 2, including a protective cylindrical insulating housing hermetically enclosing said helix and the adjacent end portion of said feeder.

iieierenees @Cited by the Examiner UNITED STATES PATENTS 2,885,675 5/59 Simon et al. 343-895 2,919,442 12/59 Nussbaum 343-895 2,982,964 5/61 Bresk 343-895 FGREGN PATENTS 562,302 8/58 Canada.

HERMAN KARL SAALBACH, Primary Examiner.

Claims

1. A DIRECTIONAL ANTENNA COMPRISING IN COMBINATION: (1) A CYLINDRICAL WIRE HELIX, (2) A COAXIAL FEEDER HAVING AN INNER CONDUCTOR AND AN OUTER CONDUCTOR, (3) A RADIAL COUPLING LINK CONNECTING ONE END OF SAID HELIX INNER CONDUCTOR, (4) A PARASITIC ANTENNA DISK DISPOSED AT THE JUNCTION BETWEEN SAID HELIX AND SAID OUTER CONDUCTOR, SAID DISK BEING CONNECTED TO SAID OUTER CONDUCTOR AND EXTENDING RADIALLY FROM THE HELIX AXIS TO AN OUTER DIAMETER SUBSTANTIALLY EQUAL TO THE MAIN DIAMETER OF SAID HELIX, AND (5) SAID COUPLING LINK AND AT LEAST A PORTION OF THE ADJOINING FIRST TURN OF SAID HELIX BEING SUBSTANTIALLY PARALLEL TO AND SPACED FROM SAID DISK BY A DISTANCE OF THE ORDER OF THE DIAMETER OF THE HELIX WIRE, WHEREBY SAID DISK ACTS AS A DIRECTOR PRODUCING A DIRECTIONAL RADIATION CHARACTERISTIC OF SAID ANTENNA IN THE DIRECTION FROM THE FREE END OF SAID HELIX TOWARDS SAID DISK.