Abstract:

Steerable Risley Prism Antennas with Low Side Lobes in the Ka Band

Bradley J Tame, Nathan A Stutzke Ball Aerospace & Technologies Corp., Broomfield, CO

The Risley Prism antenna is a steerable beam antenna that is light weight, compact, conformal if desired and can be used on a wide variety of platforms. Small mobile platforms have limited real estate and this antenna technology is a viable solution . Some potential applications include UAV's, Humvee's and other ground mobile vehicles as well as larger platforms such as ships.

Introduction: Risley prism beam steering has been used in optics for many years. The typical optical system consists of two independently rotatable prisms which deflect a laser beam via refraction, as shown in Figure 1. When both prisms are aligned with maximum thickness in the same direction, maximum steering occurs. When the two prisms are oriented 1800 opposite to each other, the beam passes straight through. The beam may be steered anywhere in e between the two extremes by controlling the relative rotation of the two prisms. The beam is steered in <p by rotating the prism pair together.

Broadside Scan

Transmitted Plane Wave Phase Front is corrected by 2nd Prism producing a Broadside Scan

.... -.-!'..--Prism 2

Maximum Scan »,... Transmitted Plane Wave

/,.... Phase Front is further tilted /).,... by 2nd Prism Scanning the /),., Beam

/) ...•.

rism 1 __ ::::.-.-.... EO

Incident Plane Wave .. r·l .... r .. r·r .. l .... r··r .. r.. Phase Front

(Normal Incidence)

EO Incident Plane Wave

.. T·l .... r .. r·r .. T .... rl .. ·T .. · Phase Front (Normal Incidence)

Figure 1. Risley prism beam steering.

The optical refraction steering equations are shown below in Figure 2. for a classical Risley prism pair that has two identical prisms of index n. The pair can be used to direct a laser beam

into any elevation angle e and azimuthal angle �, limited only by the prism wedge angle u.

Given an azimuthal rotation <p' between the prisms, it can be shown that for normal beam incidence, the direction cosines k3 of the directed beam are given. These formulae give the direction cosines in terms of prism orientation. The Risley steering requires the inverse, so given the target elevation and azimuthal angles, one obtains the required prism orientations. For a two­

prism beam director there are always two possible prism orientations for the same target angles [1]. Similar equations are used in the microwave refraction.

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[GX] [COS;sin8]

K]7 = sin;sin9 =

i3z cos(J

psina+cos;'sina [ �1-n2 +r2(;')-r(;')] sin;'sina[ �1-n2 +r2(;') -r(;')]

(1+ pcosa)+cosa[ �1-n2 +r2(;') -r(;')]

p=.Jn2 -sin2a -cosa r(;') = cosa + p( cos] a + cos;' sin2 a)

Figure 2 Optical Lens Steering Formulas

In a Ball Aerospace built optical system for laser communications, three Risley prism lenses are used to steer an optical beam with 100 Jlrad precision [1]. Ball Aerospace has applied this technology to microwave beams and will discuss the two lens planar Risley Prism architecture. This dual lens Risley architecture has several advantages over conventional gimbaled-dish antennas as it is light weight, low-profile, low cost, mechanically robust and does not require an RP rotary joint. Several lens types are possible and are discussed in this paper.

Lenses:

Risley prism lens models were developed with Ansoft High Frequency Structural Simulator (HFSS). This modeling tool provides accurate predictions of the lens performance. The resulting lens is an optimized balance of ohmic loss, scattering performance, and sidelobe performance. The lens matching layers are also simulated to optimize the overall efficiency of the lens

providing the lowest predicted insertion loss. Using modeling software such as HFSS is a highly efficient way to reduce design time. Once confidence is gained in the model by correlating the model and measured prototype data. The model is then used to simulate multiple lens configurations. The optimized configuration is then built and tested.

Several Risley prism lens types were considered including dielectric wedges, stepped/zoned dielectrics, Vivaldi radiators as well as printed circuit lenses. Each of the aforementioned lens types has its advantages and disadvantages. The plain dielectric wedge offers the purest pattern, lowest sidelobe option. The scattering in the wedge design drives the sidelobe levels. Optimizing the lens scattering performance will ultimately provide the overall lens system with the lowest sidelobe performance [2]. The dielectric wedge is made out of high dielectric material such as ceramic and works well at higher frequencies where the lens is thin and the diameter is small. At lower frequencies the lens becomes much thicker and the diameter much larger making the lens heavy and more difficult to use. The stepped and zoned lens approaches provide a lower lens profile and smaller mass. These lenses have higher sidelobe levels due to the higher additional scattering in the stepped/zoned lens [3]. These lenses also work well at higher frequencies due to their size and weight. The printed circuit (with micro strip patch) and Vivaldi lenses are a good compromise at lower frequency. . A good impedance match, element

efficiency, phase match and non-radiating delay lines are crucial to achieving a low loss lens design [2].

Fig. 2 shows representative wedge elements and a typical gain pattern. These have been

prototyped for the Ka band which can utilize smaller aperture sizes while maintaining sufficient gain to close line-of-sight high data rate links [2].

or,

(a) (b) (c) (d) Fig. 2. Dielectric Risley prism (a) wedge (b) stepped wedge (c) circuit (d) Ka band scan pattern

System and Mechanical Design:

In the two lens system, two identical Risley prism lenses are placed parallel to each other and centered in the near field of the RF feed antenna. The RF feed remains stationary so no RF rotary joint is required. This results in reduced insertion loss and improved reliability. Each lens is independently rotated in a common rotational axis about the center of rotation of the feed to redirect the incoming RF energy by refraction. Algorithms derived from the equations listed earlier are used to control motors that mechanically rotate each lens (on bearings) independently to a known location. Once each lens is at this location, RF energy from a planar feed antenna is refracted through each of the lenses to the calculated angles thereby controlling the beam

position.

This type of motor driven antenna system is light weight and consumes very little power. The mechanical drive system is simple, light, and robust compared to a traditional multi-axis gimbal system. Unlike dish antennas, a Risley prism antenna system can be conformal mounted. This dual lens Risley configuration has a field-of-view conical scan volume of 60° to 70° and can readily steer through boresight without any keyhole effects or discontinuities in the antenna

pattern. It can easily be designed for low radar cross section (ReS) performance as well.

Conclusion:

The Risley Prism microwave antenna architecture provides a unique directional antenna solution that can be implemented with several different lens types. The advantages and disadvantage of each of these lens types were outlined. For a given platform the driving antenna requirements will dictate the lens type and motor sizing requirements. Risley prism antennas are a good alternative to dish antennas providing a lower profile alternative, lower cost and better overall reliability .

References:

[1] Miroslaw Ostaszewski et al, "Risley Prism Beam Pointer", Proc. of SPIE Vol. 6304 630406-10 2006

[2] Santanu Daset aI, "New Approaches to Directional Antenna Technologies for Unmanned, System Communications" IEEE AP-S 2010

[3] Antenna Beam Steering Techniques Using Dielectric Wedges, lEE Proceedings Vol. 136 Pr. H, No. 2, 2 April 1989