Cavity-backed spiral antenna with mode suppression

BACKGROUND OF THE INVENTION 20

This invention relates to spiral antennas and more particularly to planar spiral antennas having unidirectional radiation patterns.

The planar equiangular spiral antenna is described in the IRE Transactions on Antennas and Propagation, vol. AP-7, No. 2, April 1959, pp. 181-187. This antenna provides circularly polarized, single lobe, bidirectional radiation, normal ίο> the plane of the antenna. This antenna is particularly useful in airborne direction finding systems since it can be mounted flush with the skin of the aircraft. ³⁰The bidirectional radiation of the spiral antenna is the principal feature limiting its usefulness. A common technique for obtaining unidirectional radiation from the planar spiral antenna is to back it with a coaxial cavity. In order to obtain broad band operation, the bottom of ³® the cavity may be stepped to make the height of the cavity backing the active regions of the spirals substantially equal to a quarter-wavelength at the operating frequency. This causes reflected radiation in the cavity and waves radiated in the same direction to be in-phase in the plane ” of the spirals so that the waves combine constructively. A single four-arm spiral antenna may be utilized in a sum and difference monopulse direction finding system. The two· pairs of spirals are fed so that they provide sum and difference mode beams. Signals received while operating in the sum and difference modes are processed to determine the direction of the signal source. In order to obtain accurate direction finding information, the difference between the peak amplitudes of the sum and difference _gobeams must remain constant as frequency is varied. Also, the axial ratio of the antenna beams must remain substantially equal to 0 db as frequency is varied. The axial ratio of the antenna beam is defined as the ratio of the strength of the horizontal electrical field to the strength of _ggthe vertical electrical field. Although the planar spiral antenna theoretically has an unlimited bandwidth, the cavity backed spiral has a limited useful bandwidth since the cavity will simultaneously support the dominant mode as well as higher order modes of propagation of electro- θθ magnetic waves when the cavity size is large in terms of wavelength. This will cause deterioration of the radiation pattern and a decrease in the antenna gain in the vicinity of cavity resonance. Unless the higher order modes are suppressed, the useful bandwidth is limited to approxi- _ggmately 3:1 for a two arm spiral and 2:1 for a 4-arm spiral operating in the sum and difference mode. A previous attempt to solve this problem was to back the spirals with a cavity filled with a material that absorbs all the radiation in the cavity. The obvious disadvantage of this tech- „„ nique is the high loss that is involved and the resultant ¹low efficiency of the antenna.

An object of this invention is the provision of an improved spiral antenna providing unidirectional radiation.

Another object is the provision of a broadband planar unidirectional spiral antenna.

SUMMARY OF THE INVENTION

The cavity backed spiral antenna embodying this invention has a tubular resistance card in the cavity to prevent simultaneous excitation and propagation of a plurality of modes in the cavity.

DESCRIPTION OF DRAWINGS

FIG. 1 is a top view of a unidirectional spiral antenna embodying this invention;

FIG. 2 is a section taken along the line 2—2 in FIG. 1;

FIG. 3 is a top view of a second embodiment of this invention;

FIG. 4 is a section along the line 4—4 of FIG. 3;

FIG. 5 is a section view of a third embodiment of this invention; and

FIG. 6 is a section view of a fourth embodiment of this invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to FIG. 1, this antenna comprises four planar equiangular spiral conductive patterns 11, 12, 13 and 14 on a dielectric disc 15. Although equiangular spiral patterns are shown in FIG. 1, they could also be other types of spirals such as the Archimedes spiral. The spiral patterns 11 to 14 may be formed by conventional printed circuit techniques from the copper clad on a Teflon or fiber glass base. In practice, the conductive patterns are approximately 0.005 inch thick. They are shown much thicker than this in the figures, however, for illustrative purposes. Alternatively, the spirals may be cut from a thin sheet of copper and bonded to a dielectric base.

The lengths L of the spiral patterns are representable as tn J pt L \“P/ J (1) which reduces to

L = [a-²+ip/2(_p-_P()) (2) where p and φ are the conventional polar coordinates, dp and άφ are the differentials of the variables p and Φ, p₀is the initial value of the radius vector, and a is a contant that controls the expansion rate of the pattern. The radius vectors p_x and p₂ define the inner and outer edges, respectively, of pattern 11 and are representable as

Pi = /ce“* (3) and

Ρ2 = Κ<?“⁽*~^Η=Ζ<_ρ1 ¢4) where p! and p₂ are shown in FIG. 1, κ is a positive constant which determines the size of the terminal region of pattern 11, δ is a fixed angle between the radius vectors Pi and p₂ that determines the width of pattern 11, e is the base of Naperian logarithm, and K is defined as

Pi (5)

Thus, the edges of pattern 11 are identical curves with one curve rotated through the fixed angle δ with respect to the other. It is this rotation angle that gives the spiral a finite width.

The radius vectors defining the inner and outer edges of the other patterns are also defined by Equations 3 and 4 respectively, except that the radius vectors associated with patterns 12, 13 and 14 are advanced by tt/2, tt and 3ir/2 radians, respectively, over that shown in these equations. The exact values of the angle δ, the arm lengths L and

3,555,554 ³ the constants a and k are a function of the desired characteristics from the antenna.

The planar spiral conductive patterns are backed by a cylindrical coaxial cavity 16. The dielectric disc 15 supporting the spirals is rigidly secured to the top 17 of side wall 18 of the cavity by bonding and screws (not shown). The center conductor of the coaxial cavity is a post 19 which is formed, such as by machining or casting, in the bottom wall 20 of the cavity. The post 19 extends over the full height of the cavity.

The spirals are fed by a beam-forming network comprising coaxial transmission lines 21 to 24 and a microwave phase shifter (not shown). The coaxial cables are symmetrical about the center of the antenna and extend through associated bores in post 19. The outer conductors of the transmission lines are electrically connected to post 19 such as by soldering. The inner conductors 21' to 24' of the transmission lines are electrically connected to the ends of the associated patterns 11 to 14, respectively, at the center of the antenna.

The cross section of the coaxial cavity is the same as that of a cylindrical waveguide and will support higher order waveguide modes when the cavity diameter is large with respect to operating wavelength. For this reason, the cavity will simultaneously support the dominant mode and a number of higher order modes of propagation of electromagnetic signals in the cavity at some high frequency. This multimode propagation and cavity resonances causes the antenna radiation patterns to deteriorate. As a result of this operation, this antenna has a useful bandwidth in the order of 2:1.

In accordance with this invention, a tubular resistance card 27 is located in the cavity coaxial with post 19. The resistance card is rigidly secured in a cylindrical groove 28 in the bottom of the cavity by bonding. The resistance card may, by way of example, be a metal film resistance card comprising a thin film 29 of pure metal approximately 50 millionths of a inch thick evaporated on a dielectric substrate 30 such as Mylar. The thicknesses of members 29 and 30 are greatly enlarged in the drawings for illustrative purposes. Such a resistance card is manufactured by Filmohm Corporation, New York, N.Y. Alternatively, a resistance card comprising a graphite impregnated dielectric sheet may be employed. The wall of tube 27 is normal to the base 20 or the cavity. It is believed that the resistance card introduces a small loss in the cavity that causes suppression of the higher order modes and in this manner prevents deterioration of the performance of the antenna.

An embodiment of this invention for operating over a broad band of frequencies is illustrated in FIGS. 3 and 4. This antenna comprises two planar equiangular spiral conductive patterns 31 and 32 formed on a dielectric disc 34. The spirals are fed by a beam forming network comprising a pair of rigid coaxial transmission lines 35 and 36 (see FIG. 4) which are symmetrically located with respect to the center of the antenna. The center conductors 37 and 38 of the associated transmission lines 35 and 36 are electrically connected to the ends of patterns 31 and 32, respectively, adjacent the center of the antenna.

Spirals 31 and 32 are backed by a cylindrical coaxial cavity 41 having a center conductor that is a post 42. Cylindrical steps 43 and 44 are formed in the bottom 45 of the cavity, such as by machining or casting, coaxial with post 42. Feed lines 35 and 36 extend through bores 47 and 48, respectively, in post 42 and the bottom of the cavity. The outer conductors of the feed lines are electrically connected to the center post 42 and wall of the cavity such as by soldering.

In operation, only small portions of spirals 31 and 32 comprise the active regions of the antenna at a particular operating frequency. As the frequency is increased, the active regions shift toward the center of the antenna. The heights, diameters, and number of steps in the cavity are selected so that the heights of the cavity under the active regions of the spirals are approximately one-quarter wavelength at the operating frequencies of the antenna. This causes radiation in the cavity that is reflected back to the plane of the spirals to be in-phase with and constructively combine with radiation for the spirals.

Cylindrical sleeves 50 and 51 of resistance card are located in the cavity to prevent excitation and propagation of higher order modes in the cavity. The sleeves are supported on shoulders 54 and 55 of the associated steps 43 and 44 to which they are secured by bonding and are oriented normal to the bottom 45 of the cavity. The heights and diameters of sleeves 50 and 51 are determined empirically.

A section view of an alternate embodiment of this invention for operating over a broad band of frequencies is illustrated in FIG. 5. The spirals in FIG. 5 are the same as those in FIGS. 3 and 4. Primed reference characters refer to similar components in FIGS. 3 and 4. The cavity 41' backing spirals 31' and 32' has a conically shaped bottom 45' that causes the height of the cavity to vary linerally from a maximum at side wall 46' to a minimum determined by step 57. The cylindrical sleeves 50' and 51' of resistance card are rigidly secured in cylindrical grooves in the bottom of the cavity by bonding.

A section view of another embodiment of this invention for operating over a broad band of frequencies is illustrated in FIG. 6. The spirals in FIG. 6 are also the same as those in FIGS. 3 and 4. Double primed reference characters refer to similar components in FIGS. 3 and 4. The height of the cavity 41 backing spirals 31 and 32 is periodically varied abruptly by steps 43 and 44 therein. The resistance card sleeves 50 and 51 extend over the full height of the associated steps. Sleeves 50 and 51 each extend the same distance above the height of the bottom 45 of the cavity.

By way of example, an antenna similar to that illustrated in FIG. 6 that was actually built and tested had the following dimensions and characteristics.

Spiral patterns 31 and 32:

Length L—37 inches

5—60° k—0.045 inch a—0.093

Cavity 41:

Diameter—3.5 inches

Height—0.8 inch

Post 42 diameter—0.30 inch

Step 43:

Diameter—2.0 inches

Height—0.35 inch

Step 44:

Diameter—1.1 inches

Height—0.55 inch

Post 42:

Diameter—0.30 inch

Resistance card 50:

Height—0.7 inch

Loss—377 ohms/square

Resistance card 51:

Height—0.75 inch

Loss—377 ohms/square

This antenna operated over a frequency band of greater than 10:1 and had a voltage standing wave ratio of less than 3:1, an axial ratio of less than 2 db, and a gain at X band frequencies of greater than 5 db.