Waveguide Fed Spiral Antenna

The present invention relates generally to the field of transitions to and from waveguides and, more specifically, to the field of transitions from waveguide to balanced transmission lines and still more specifically, to spiral balanced transmission lines.

A printed circuit spiral antenna is a balanced transmission line wound into a spiral. Spiral antennas are very desirable for use in electronic warfare systems because such antennas have very broadband and broadbeam characteristics. The operable frequency range of a single antenna is as high as 2-18 GHz. The beamwidth of these antennas is about 90% and a set of four such antennas are commonly used for determining a signal's angles of arrival by means of amplitude comparison circuits. The balanced transmission lines that form such spiral antennas are normally matched to the commonly used coaxial transmission line by means of a balun. At higher frequencies, i.e. above 18 GHz, the size of the coaxial line must be very small to avoid moding and, hence, the balun becomes very difficult to assemble. The performance of spiral antennas at the millimeter wave frequencies is also degraded by the loss and VSWR of coaxial baluns. In general, coaxial lines are not desired for millimeter wave circuits. Thus, spiral antennas are better matched for waveguide circuits such as mixers and switches. It has been found that the optimum assembly of a millimeter wave circuit is achieved by integration of the various circuit components. Hence, it is desirable to provide a transition from the antenna that can be integrated with the normal millimeter wave circuits and hence utilize a balanced transmission line suitable for waveguide circuits.

The present invention is a printed circuit that adapts a spiral balanced transmission line to a fin-line transition and then from the fin-line medium to waveguide. The invention thus provides a smooth, low loss transition from a balanced transmission line wound into a spiral to a waveguide propagating in the TE.sub.10 mode, which is likewise a balanced transmission line. This smooth, broadband transition from the spiral balanced transmission line to the waveguide balanced transmission line extends the use of spiral antennas to the millimeter wave frequencies where waveguide circuits are needed for processing signals received by the spiral antenna. Thus, the usage of spiral antennas can be extended to the millimeter wave bands in the frequencies ranging from 30-150 GHz by adapting the output of a balanced transmission line of a spiral antenna to a waveguide transition such that the spiral antenna can be fed by a waveguide directly behind a cavity interposed between the waveguide and the spiral antenna and such that the requirement for a conventional balun can be eliminated.

Accordingly, it is the primary object of the present invention to provide a mechanism for extending the usage of spiral antennas to the millimeter wave bands of frequencies from 30-150 GHz.

It is a further object of the present invention to provide a transition from a printed circuit spiral antenna to a waveguide that eliminates the requirement for a conventional balun.

It is a concomitant object of the present invention to disclose a printed circuit spiral antenna-to-waveguide transition that is small, printed, efficient and provides a good match over an octave bandwidth.

It is a further object of the present invention to disclose a transition as described above that allows the placement of several printed circuit spiral antennas in close proximity as required for various array configurations.

It is another object of the present invention to disclose a printed circuit spiral antenna-to-waveguide transition that can be matched directly to various waveguide circuits that are commonly used for electronic warfare systems.

It is a still further object of the present invention to disclose a transition as described above that permits size reduction and optimization of various receiving and transmitting circuits and permits integration of these components thereby reducing losses and improving the operating bandwidth.

Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.

FIG. 1 is a top view of a printed circuit spiral antenna.

FIG. 2a is a cross-section of the transition of the present invention viewed through the sidewall of a waveguide that is connected by the present invention to a printed circuit spiral antenna.

FIG. 2b is an end view of the transition illustrated in FIG. 2a, viewed through the right end of FIG. 2a.

FIG. 2 c is a cross-sectional top (broad wall) view of the transition illustrated in FIGS. 2a and 2b.

FIG. 3a is a top view of the fin-line transition card of the present invention.

FIG. 3b is a side view of the fin-line transition card of the present invention.

FIG. 3c is an end view of the fin-line transition card of the present invention.

FIG. 4 is a schematic representation of a typical antenna pattern achievable with the present invention.

FIG. 1 shows an enlarged top view of a balanced, slot-line, printed circuit spiral antenna 12. The black lines represent the slots and the white lines represent conductive copper surfaces on a thin sheet of dielectric material 13. In the spiral antenna 12, the two slot-line transmission lines 14 and 16 are connected in series at point 18 and 20 at the center of the figure. The lines are open circuited at the perimeter as is illustrated. The present invention disclosed herein creates a matched broadband connection from points 18 and 20 to a standard waveguide operating in the TE.sub.10 mode or to a waveguide circuit utilizing a fin-line or slot-line transmission.

Referring now to FIGS. 2a, 2b and 2c, the spiral antenna 12 is depicted as positioned within a cylindrically shaped opening within metallic, split block housing assembly 21. The spiral conductors 18 and 20 are located on the left, or inside, of the printed circuit board 13. An E-plane or fin-line printed circuit assembly 22 has a circuit pattern on circuit board 23 as is illustrated in FIGS. 3a, 3b and 3c. The tapered conductive patterns 24 and 25 are printed on opposite sides of the circuit board 23 as is shown. As is illustrated in FIG. 3a, the circuit board 23 is tapered at the right end thereof, this tapered portion being referred to as potion 23a. Further, tapered conductive pattern 24 on the topside of circuit board 23 is further tapered at section 24a to extend along the tab portion 23a of circuit board 23. Similarly, the conductive portion 25 on the underside of circuit board 23 is further tapered and continues along the tab portion 23a to the right end of fin-line structure 22 as is illustrated in FIG. 3a. Tapered extensions 24a and 25a are then connected, respectively, to points 18 and 20 of the spiral antenna 12 by soldering or bonding.

As is illustrated in FIG. 2c, the waveguide cavity 30 has gradually tapered sidewalls 35 that lead into waveguide opening 35a. A disc 36 of rubberized absorbing material is placed over the waveguide opening 35a and contains an aperture 38 through which the tab 23a and conductive patterns 24a and 25a extend into the cavity 32. The cavity 32 is a quarter-wavelength cavity that is formed behind the spiral antenna 12 within the metallic split block assembly 21. The rubberized absorbing material 36 is used for absorbing the back radiation. The cavity aperture 38 through which the tab 23a connects with the spiral antenna circuit 22 has a diameter and length designed to eliminate propagation of the waveguide TE.sub.10 mode from the waveguide cavity 30 to the spiral cavity 32. The aperture 38 and absorber 36 thus comprise a TE mode suppressor.

The circuit pattern of the fin-line structure 22 thus tapers into the waveguide cavity 30 by gradually increasing the separation between the conductive ridges 24 and 25 that are in contact with the waveguide top and bottom walls. The circuit transition 22 thus connects the terminal points 18 and 20 of the spiral antenna 12 directly to the waveguide by means of a matched broadband transmission line without the use of balun. The present invention has shown excellent results in the 18-36 GHz frequency range and can easily be scaled for use at the higher millimeter wave frequencies. FIG. 4 is a schematic representation of a typical antenna pattern achievable with the present device.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.