Broadband Spiral Antenna

Abstract

A frequency independent spiral antenna having a plurality of pairs of arms energized with currents equal to the sine of the respective terminal position angles of the arms for substantially removing bandwidth limitations.

Description

BACKGROUND OF INVENTION

Spiral antennas commonly denominated as frequency independent antennas are actually only relatively insensitive to frequency variations above some minimum frequency. In actual practice it is found that mounting of such antennas relatively close to a ground plane or the like increases frequency dependency which is highly undesirable for many applications. This problem is particularly noted in aircraft installations of antennas directly over the aircraft skin which are often employed with communications or direction-finding equipment.

There has been developed an antenna described and claimed in U.S. Pat. No. 3,624,658 which provides a major improvement in the field of frequency independent antennas capable of producing a stable radiation pattern over an appreciable bandwidth such as a bandwidth of 4:1 or greater.

The present invention provides a further improvement in spiral antennas to substantially eliminate bandwidth limitations.

SUMMARY OF INVENTION

The antenna of the present invention is comprised as a plurality of pairs of spiral arms particularly energized to maximize frequency independence of the antenna. The curvature of the arms of the antenna hereof may be properly delineated as either an Archimedean spiral or as a logarithmic spiral; however, in the following description of the present invention the term "spiral" is taken to denominate an antenna having curved arms of these general types.

The antenna of this invention has a generally flat configuration which may be closely spaced from a ground plane or the like with the outer ends of the spiral arms being terminated as by resistive coupling to the ground plane or the like. The individual arms of the antenna hereof have the inner terminations thereof spaced equally about a circle surrounding the center of the antenna. Considering a center line through two oppositely spaced points on such circle the terminal position angle of each antenna arm is taken as the angle between such center line and the inner terminus of the arm. In accordance with the present invention, the spiral arms of the antenna are energized by excitation currents, I.sub.n, of an N arm spiral equal to the sine of the terminal position angle. With the terminal position angle being identified as .phi..sub.n, excitation is then I.sub.n = Sin .phi..sub.n, where n = 1, 2, 3, . . . N. With this energization the only possible modal regions on the spiral will be 1 and N-1 to thus achieve an approximate bandwidth of (N-2): 1.

It will be seen from the foregoing that the frequency independent bandwidth of the present invention increases with increasing numbers of spiral arms and it is further noted that energization or excitation of the antenna arms may be provided by a simple power divider so as to minimize complexity of antenna energization.

DESCRIPTION OF FIGURES

The present invention is illustrated as to a single preferred embodiment thereof in the accompanying drawings wherein:

FIG. 1 is an illustration of a prior art broadband spiral antenna;

FIG. 2 is an illustration of radiation patterns of various harmonics of a spiral antenna;

FIG. 3 is a schematic illustration of a spiral antenna in accordance with the present invention;

FIG. 4 is a schematic representation of the inner ends of spiral arms of an antenna in accordance with the present invention and identifying terminal position angle .phi.;

FIG. 5 is a schematic representation of an antenna feed network for the antenna of FIG. 3; and

FIG. 6 is a schematic representation of the antenna of FIG. 3 in side elevation.

DESCRIPTION OF PREFERRED EMBODIMENT

The antenna of the present invention comprises a multiarm spiral antenna having an even number of arms from 4 to 20 or more. In FIG. 1 of the drawings there is is illustrated the prior art antenna of U.S. Pat. No. 3,624,658 having 6 spiral arms. The first and second arms are connected together and energization is applied between such connection and a connection between the fourth and fifth arms as shown. The opposite third and sixth arms of the antenna are parasitically excited. This prior art antenna produces only Mode 1 and Mode 5 radiation and in accordance with the teaching of the above-noted patent, Mode 5 radiation is suppressed by the utilization of an absorber or resistive terminations at particular antenna arm radius. This antenna has a frequency independent bandwidth of the order of 4:1. The antenna of the present invention may be physically configured in much the same manner as the antenna of FIG. 1; however, the present invention is not limited to a 6 arm spiral and, furthermore, is energized in a different manner.

There is illustrated in FIG. 2 the radiation patterns for different modes or harmonics of energy radiated from a spiral antenna as originally presented in the publication "Frequency-Independent Antennas" by Victor H. Rumsey, Academic Press, 1966, page 122. It will be seen that Mode 1 radiation comprises a single lobe extending in one direction from the source or antenna and for many purposes this type of radiation is highly desirable.

In order to preserve these desirable features of Mode 1 in a truly frequency independent manner, all other modes must be eliminated or at least suppressed to such an extent that their effect on the radiation becomes minimal.

There is illustrated in FIG. 3 an eight arm spiral antenna in accordance with the present invention and it will be seen that the arms A through H are equiangular, for example, and equally spaced apart with inner terminal points A1, B1, etc., which may lie on a circle about the center of the antenna. In FIG. 4 there are illustrated these inner terminal points in connection with an XY coordinate system having a center at the center of the antenna. A center line along the X axis of the coordinate system is taken as a reference line for measurement of terminal position angles .phi.. With an eight arm spiral antenna, as illustrated in FIG. 3, and the arms being equally spaced apart, .phi. = 0.degree. for A1, .phi. = 45.degree. for B1, etc. Obviously with a different number of arms the increase in terminal position angle between successive arms will be different. It is particularly noted that the present invention is limited to a spiral antenna having an even number of arms and this even number may be any number from 4 to 20 or more, with the example of eight illustrated herein being chosen solely for convenience.

The present invention provides an unobvious and highly advantageous energization of the spiral antenna hereof in accordance with the limitation

I.sub.n = Sin .phi..sub.n

wherein I.sub.n is the excitation current of the Nth arm and .phi..sub.n is the terminal position angle of the Nth arm.

It will be appreciated that the relative excitation currents of different arms of the antenna of the present invention are proportioned in accordance with the sines of the terminal position angles of the arms; however, the absolute value of any excitation current is, in fact, the product of the sine of the terminal position angle of the arm multiplied by a fixed arbitrary current value I. Thus the excitation current of the Nth arm of an N arm spiral antenna in accordance with the present invention is

I.sub.n = I .sup.. Sin .phi..sub.n

In the illustrated example of the present invention there are provided eight arms in the spiral with the first arm A having a terminal position angle .phi. = 0. Sin 0.degree. = 0 and thus the excitation I.sub.A = 0. With the arms of the antenna equally spaced apart, it will be seen that sin .phi..sub.B = 45.degree., sin .phi..sub.C = 90.degree., sin .phi..sub.D = 135.degree., sin .phi..sub.E = 180.degree., etc. In other words, the terminal positions of the antenna arms are separated 45.degree. apart about a circle and thus excitation of the arms will be as follows:

I.sub.A = 0

I.sub.B = 0.707

I.sub.C = 1.00

I.sub.D = 0.707

I.sub.E = 0

I.sub.F = -0.707

I.sub.G = -1.00

I.sub.H = -0.707

In this illustrated example the figures set forth above are intended to be relative, i.e., each of the numbers in the foregoing table are multiplied by I which is some predetermined basic excitation current value determining the overall power to be transmitted from the antenna. It is furthermore to be noted that in the foregoing table the negative values represent currents of opposite phase and thus, for example, the excitation currents I.sub.B and I.sub.F are 180.degree. out of phase.

With an eight arm antenna, as illustrated, energization or excitation of the arms may be very simply accomplished to attain the excitation values of the present invention. This is illustrated in FIG. 5 wherein an input power terminal 51 is shown to be connected to a 180.degree. hybrid or balun 52 which then provides currents on the two output lines 53 and 54 which are 180.degree. out of phase. Across the top of FIG. 5 there are illustrated eight terminals representing the arms A to H of the antenna and labeled with these letters. It will be seen that terminal arm A is not connected to the power supply inasmuch as the present invention, as described above, provides for zero excitation current applied to antenna arm A. The line 53 extends from the hybrid 52 to a power splitter 56 which may be wholly conventional and which has three output lines leading to antenna arm terminals B, C, and D. This power splitter 56 applies current to the antenna arms B, C and D in the proporations I.sub.B = 0.707, I.sub.C = 1.00 and I.sub.D = 0.707. The other lead 54 from the hybrid 52 extends to another power splitter 57, also of conventional design, having three output lines connected to antenna arms or terminals F, G and H. Current is split to provide excitation currents to these arms in the proportions I.sub.F = 0.707, I.sub.G = 1.00 and I.sub.H = 0.707. It will, however, be appreciated that the hybrid or balun 52 provides a 180.degree. phase difference between currents applied to the output lines 53 and 54 thereof and thus the currents to the left of the hybrid may be considered as being defined by a minus sign so that in fact I.sub.F = -0.707, I.sub.G = -1.00 and I.sub.H = -0.707. The antenna arm E is to receive no energization and thus it will be noted that terminal E is not connected to the power supply.

Power supply connections to the eight arms of the illustrated antenna are clearly of a very simple nature employing only conventional circuit elements to split the excitation currents as required by the present invention. It has been determined that with the recited excitation currents of the arms of the antenna hereof, the only possible modal regions on the spiral antenna will be 1 and N-1. For an eight arm spiral where N = 8 there then results only the possibility of Mode 1 and Mode 7 excitation. It is believed clear from FIG. 2 that Mode 7 excitation is the very minor one. This mode, however, can be completly eliminated by resistively loading the spiral at a predetermined radius. It has furthermore been determined that the approximate achievable bandwidth is (N-2): 1 and thus, in the present example of this invention where N = 8, the bandwidth is 6:1. This in itself will be appreciated to be a major improvement in the art.

Although the present invention has been described with respect to an eight arm antenna, it has been noted above that the invention is applicable with any spiral antenna having an even number of arms wherein such number lies in the range of 4 to 20 or more. A spiral antenna, in accordance with the present invention, having sixteen arms and energized as set forth above, would have a bandwidth of 14:1 and it will be appreciated that this is, in fact, substantially no bandwidth limitation at all. For practical applications it is generally considered that a bandwidth of 6:1 or more is so broad as to comprise substantially no bandwidth limitation.

It is preferable for the antenna arms of the present invention to be resistively terminated to a ground plane such as illustrated in FIG. 6 wherein resistive terminations 61 and 62 are schematically shown to extend from antenna arms to a ground plane 63. It is not, however, necessary with the present invention to employ absorbers behind the antenna even though the antenna is formed as a low profile unit, i.e., is closely spaced to a ground plane or cavity wall. The invention is equally applicable to VHF and UHF applications and will thus be seen to be highly advantageous for air-borne applications.

Although the present invention has been described with respect to a single preferred embodiment thereof, it will be appreciated that variations and modifications may be made within the spirit of the present invention. It is thus not intended to limit the present invention to the precise details of description or illustration.

Claims

What is claimed is:

1. A broadband frequency-independent spiral antenna comprising

an even number of spiral arms equally spaced circumferentially about a center and having a center line extending through the inner terminals of two opposite arms, and

means energizing said arms in accordance with the relationship I.sub.n = Sin.phi..sub.n where I.sub.n is the excitation current of the nth arm of the antenna and .phi. is the angle between the center line and the inner terminal of the nth arm of the antenna.

2. The antenna of claim 1 further defined by said means energizing said antenna arms comprising a circuit element adapted to receive electrical current and pass current onto two output lines in opposite phase, and a pair of power dividers connected one in each of said output lines and to separate antenna arms.

3. The antenna of claim 1 further defined by means resistively loading each of said antenna arms adjacent the outer ends thereof for suppressing mode 7 radiation therefrom.