Conical Spiral Loop Antenna

Abstract

The invention relates to an antenna having a plurality of pairs of spirally wound radiating arms. The radiating arms are wound in the shape of a cone and terminate at one end in a truncated portion. Impedance matching means are provided between each of the pairs of radiating arms at the truncated end. A ground plane is provided for each frequency of operation.

Description

The invention relates in general to conical spiral loop antennas, and, more particularly to a compact antenna for transmitting and receiving circularly polarized waves.

BACKGROUND OF THE INVENTION

With the advent of navigation systems utilizing satellites, it has been found necessary to provide antennas which are compact, while simultaneously having good impedance matching characteristics at the frequencies of interest. Moreover, such antennas must have low back lobe suppression and when circularly polarized waves are utilized, they must have high selectivity in the selected polarized sense.

Conventional antennas such as conical logarithmic spiral antennas which normally can be used for receiving circularly polarized navigational frequencies become unusually large for such frequencies, and thus, shipboard mounting of such antennas becomes a problem. For example, the United States Navy Navigational Satellite System transmits signals at 150 MHz. and 400 MHz. At such frequencies, a conventional conical logarithmic spiral antenna would have to be approximately 8 feet high with a base diameter of 31/2 feet to provide good patterns of efficiency. Further, since signals are to be received at two frequencies, the use of a single antenna for both frequencies requires that undesirable nulls and peaks in the antenna radiation pattern caused by undesirable phasing be eliminated.

In order to overcome the attendant disadvantages of prior art antennas, the antenna of the present invention provides a compact structure which may be used on receiving stations such as ships where a minimum size antenna is necessary. Moreover, the antenna provides high selectivity in a selected sense for circularly polarized waves. Further, excellent impedance match is provided at the frequencies of interest while, simultaneously, back lobes are adequately suppressed. Moreover, for the lower frequency of operation an improved ellipticity ratio is obtained for signals emanating from the horizon.

SUMMARY OF THE INVENTION

More particularly, the invention comprises an antenna having at least one pair of spirally wound radiating arms. The radiating arms are wound in the shape of a cone and terminated at one end in a truncated portion. Impedance matching means are provided between each of the pairs of radiating arms at the truncated end. Should it be necessary to operate the antenna at more than one frequency, separate ground planes for each frequency may be provided.

The advantages of this invention, both as to its construction and mode of operation, will be readily appreciated as the same becomes better understood by references to the following detailed description when considered in connection with the accompanying drawings in which like reference numerals designate like parts throughout the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a perspective view of a preferred embodiment of the antenna in accordance with the invention.

FIG. 2 is an exploded side view, partly in section, of one of the arms of the antenna which contains a coaxial feed line at its junction with the base of the antenna taken along the lines 2-2 of FIG. 1.

FIG. 3 is a top view, partially broken away, of the antenna of FIG. 1 showing only the top portion of the antenna.

FIG. 4 is an exploded view of the top portion of the antenna within line 4 of FIG. 3.

FIG. 5 is a side view in section of the top portion of the antenna taken along the lines 5-5 of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, there is shown in FIG. 1, a preferred embodiment of the antenna 10 in accordance with the invention. The antenna 10 comprises a first pair of hollow spiral radiating arms 12, 14 and a second pair of hollow spiral radiating arms 16, 18. Each of the arms 12, 14, 16 and 18 are secured at one end to a base member 22 and at the other end to a disc member 24. The base member 22 is generally flat and formed of a top surface area 26, and a sidewall 32.

Referring now to FIG. 2, the base member is shown in greater detail. The top area 26 has discs 34, attached thereto by means of a nut 36 and bolt 38 arrangement at the junction of each of the hollow arms 12, 14, 16 and 18. The arms are normally brazed to the discs 34. The arms 14 and 18 each have a coaxial cable 42, 44, respectively, passing therethrough and through a rubber grommet 46 which is inserted in an opening in the top area 26 and disc 34. The coaxial cables 42 and 44 pass through the base member 22 into a cylindrical casing 48 which contains auxiliary equipment and is mounted on the center of the top area 26 and secured thereto by means of mounting flanges 49, 50.

Referring now to FIGS. 3 and 5, the disc member 24 comprises a top cover member 52 and a bottom cover member 54 which are made of Fiberglas and are secured together by means of bolts 56 to form a cylindrical disc having a hollowed center 58. An O-ring 59 prevent moisture from entering the center 58. Cylindrical metal plate members 60 are secured to the bottom side of the bottom cover member and each forms top termination for the hollow arms 12, 14, 16 and 18. Rubber grommets 61 pass through openings in the plate member 60 and bottom cover member 54 to allow the coaxial cables 42, 44 in arms 14 and 18, respectively, to enter the hollowed center 58.

Mounted within the hollow center 58 are four substantially pie-shaped metal members 62, 64, 66 and 68 which are associated with each of the hollow members. Each of the members 62, 64, 66 and 68 are connected to the plate member 60 associated with its respective hollow member 12, 14, 16, and 18 by means of a nut 72 and bolt 74 arrangement.

The pie-shaped members 64 and 68 each contain an opening through which a grommet 76 is inserted and the coaxial cables 42, 44, respectively, pass therethrough. The braided outer conductor of each of the coaxial cables 42, 44 terminate in receptacle members 78, 82, having lug members 84, 86 secured thereto and to the members 64, 68, respectively. The pie-shaped members 62 and 66 each have lugs 94, 96, respectively secured thereto.

An inner conductor 98 of coaxial line 42 is connected through a pair of parallel capacitors 102, 104 to the lug 94 of pie-shaped member 62. Further, an inner conductor 106 of coaxial cable 44 is connected through a pair of parallel connected capacitors 108, 112 to the lug 96 of pie-shaped member 66. An inductor 114 is connected between the lug 84 of pie-shaped member 64 to the lug 94 of pie-shaped member 62, and an inductor 116 is connected between the lug 86 of pie-shaped member 68 and the lug 96 of pie-shaped member 66.

Referring once again to FIG. 1, a pair of parasitic ground planes for the high frequency signal comprises a first cylindrical shaped plate 122 which is secured to a second cylindrical shaped plate 124 by means of metallic rods 126 which are fastened to the plates 122 and 124 by means of bolts 128. The plates 122 and 124 are mounted in a plane parallel to the plane of the base member 22 and the disc member 24. Further, the plate 124 has secured thereto, a hollow metal cylindrical skirt 132 which extends downwardly from the plate 124 towards the base member 22. A plurality of support rods 133 are fastened at one end by means of bolts 134 to the plate 124 and the cylindrical skirt 132, and at its other end are secured to feet 136 mounted on the base member 22.

The loop antenna thus far described has less than one full turn in its spiral length, the antenna depicted in the drawings having approximately a 270.degree. turn in its length. Normally, a single loop antenna having less than one full turn in its spiral length would have a poor VSWR and polarization sense and a high-back radiation. To overcome these drawbacks while simultaneously providing an antenna of manageable size, the antenna depicted adds a second loop 16, 18 to the first loop formed by the arms 12, 14.

In the transmitting mode, the signal is divided into the two coaxial feed lines 42, 44 by a 90.degree. phase shift hybrid network located in the casing 48. Each of the feed lines is matched to the antenna arms 12, 14, 16 and 18 by means of the matching network located within the disc member 24. This matching network is composed of the inductive and capacitive network within the disc together with the pie-shaped members 62, 64, 66 and 68. The transmitted signals propagate down the arms pairs toward the base 22 in phase quadrature until they reach a region on the arms where the cone diameter is approximately one-third of a wavelength at the frequency of operation at which the radiation into space occurs. This radiating energy propagates back to the disc member 24 with a polarization sense opposite to that conductively propagated down the arms as is conventional of backward wave radiators. The radiated energy which propagates in the direction of the base member 22 is reflected by the ground planes formed by the base member 22 at the low frequency of operation. At the high frequency of operation, the energy is reflected by the ground planes formed by the plates 122 and 124. This reflected energy returns in the proper phase to reinforce the energy traveling in the direction of disc member 24.

Two plate members 122, 124 were chosen instead of one for the high frequency of operation since it was determined that the radiation which occurs at discrete concentric belts on the spiral loops 12, 14, 16 and 18 would produce undesirable phasing should only one plate be used. By utilizing the plate members 122, 124 which are scaled to be resonant at the upper frequency of operation, and nonresonant at the lower frequency of operation, the higher frequency of operation is essentially decoupled from the base member 22. Further, the vertical skirt 132 improves the vertical component of the signal at the higher frequencies thus improving polarization ellipticity near the equatorial plane.

For an antenna as depicted, for transmission and reception at both 150 MHz. and 400 MHz. the following design criteria was utilized: ---------------------------------------------------------------------------

mean distance between base member 22 and disc member 24 37.8 in. diameter of radiating arms 12, 14, 16, 18 2.0 (in.) mean diameter between arms at disc 24 6.0 (in.) mean diameter between arms at base 22 37.2 (in.) diameter of plate 122 14.9 (in.) diameter of plate 124 15.9 (in.) distance between plate 122 and plate 124 8.06 (in.) distance between plate 124 and base 22 11.9 (in.) value of capacitors 102, 104, 108, 112 3 .times. 10.sup..sup.-12 (f.) value of inductors 114, 116 0.1 (.mu.h.) __________________________________________________________________________

Claims

I claim:

1. An antenna comprising a pair of spiral radiating arms, said spiral radiating arms being wound in the shape of a cone and terminating at one end in a truncated portion of said cone, and having a base, a feed line mounted in insulating relationship with one of the arms, and impedance matching means connecting said feed line to said radiating arms at said one end.

2. An antenna in accordance with claim 1 wherein said antenna comprises a plurality of pairs of arms, each of said pairs of arms being diagonally opposed and equally spaced in the planes perpendicular to said cone.

3. An antenna in accordance with claim 2 and comprising feed means associated with each pair of said radiating arms.

4. An antenna in accordance with claim 1 wherein the base of said cone forms a ground plane for said antenna and wherein the other ends of said arms are secured to said base.

5. An antenna in accordance with claim 1 wherein said antenna is designed to operate at either a first predetermined frequency or a second predetermined frequency, the base of the antenna forming the ground plane for the lower of said frequencies, and a first platform extending in a plane parallel to said base intermediate said one end of said arms and said base and connected to said base, said platform forming the ground plane for the higher of said frequencies.

6. An antenna in accordance with claim 5 and further comprising a second platform associated with said first platform, said first and second platform being resonant at said higher frequency and nonresonant at said lower frequency.

7. An antenna in accordance with claim 1 wherein said antenna is designed to transmit and receive circularly polarized waves.

8. An antenna in accordance with claim 1 wherein said impedance matching means comprises a substantially pie-shaped member associated with each radiating arm.