Reflectarray Antennas: An Intuitive Explanation of Reflection Phase Behavior

Harish Rajagopalan and Yahya Rahmat-Samii

Department of Electrical Engineering, University of California Los Angeles, Los Angeles, CA-90095 USA harish@ee.ucla.edu, rahmat@ee.ucla.edu

Abstract

This paper presents the concept of reflectarray in a novel way, utilizing a graphical visualization tool. This approach greatly facilitates understanding of the working of the reflectarray antenna, and provides an intuitive explanation for the reflectarray behavior. The reflectarray element is characterized in a unit cell environment (periodic) and true reflectarray environment for its reflection phase performance. By understanding the reflection phase response of the element itself in these two environments, one can gain great insight into the reflectarray reflection phase phenomenon. By analyzing and evaluating the incident, scattered, and total fields, the element behavior can be studied in detail and the overall reflectarray performance can be validated. This reflection phase approach can also be potentially used to perform diagnostic tests on reflectarrays and identify existing flaws in the design and thus help in enhancing the design of reflectarray antenna.

1. Introduction A microstrip reflectarray consists of an array of radiating isolated elements printed on a grounded dielectric substrate [1–3]. These antennas are typically illuminated by a feed (horn antenna). They convert a spherical wavefront produced by a feed into a desired wavefront using a suitable phasing mechanism. By proper manipulation of these phase response curves (design S-curves), one can generate any scattered wavefront. Typically, for large reflectarrays with many elements, infinite array model is used for analysis of the reflectarray element. Floquet modes are used and the problem is reduced to a periodic unit cell approach. The design curves (reflection phase curves) typically relate the reflection phase of the reflectarray element to some geometrical feature of the element. Many approaches have been studied in the past to provide different phasing schemes for a microstrip reflectarray. These include variable size microstrip patches [4], identical microstrip patch elements loaded with stubs of variable length [2] and microstrip patches having variable rotational angles for circular polarization [5]. This analysis will aid in fundamental understanding of the reflectarray, help facilitate the design of the reflectarray antenna effectively, and also help in potential diagnostic approaches. Fig. 1(a) shows the reflectarray element in a unit cell environment. The wave is incident from plane I and the incident, scattered, and total fields are analyzed at plane II (immediately above the element). When the incident wave impinges on the reflectarray element, ideally (no losses in the structure) all the energy is scattered back. As shown in Fig. 1(b), this back scattered energy consists of the following three components: the re-radiated component due to the element, the specular component due to the dielectric-backed ground plane, and the scattered component from the top surface of the element.

(a) (b)

Fig. 1. (a) Unit cell model for reflectarray element. (b) All the components of the back-scattered energy from the reflectarray element.

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2. Reflectarray Element: Dual Resonance Patch-Ring Element

In order to get a complete phase swing of 360°, a dual resonance element is chosen. The element is a patch loaded ring (patch-ring). The patch resonance and the ring resonance are the two resonances inherent to the structure thus enabling a phase shift of greater than 360°. The frequency of operation is fixed at 13.4GHz. A 62 mil (1.574mm) thick Rogers substrate (RT Duroid 5880) with dielectric constant of 2.2 and loss tangent of 0.0009 backed by a ground plane is used for the analysis. The unit cell dimensions are fixed at 11mm x 11mm (0.5λ0 x 0.5λ0). The element is illuminated by a normally incident plane wave polarized in the x-direction. The width of the ring, the gap between the patch and the ring and the patch size are optimized to obtain the phase response shown in Fig. 2(a). The dual resonance is clearly observed with the sharp slope resonance (first resonance) corresponding to the ring resonance and the second resonance (point 1 in Fig. 2(a)) showing the patch resonance. To get a more in-depth understanding into the working of the element, the incident and total E-fields (magnitude and phase) are calculated at plane II. The scattered fields are extracted from the incident and total fields. Once all the fields are available, the incident fields are subtracted from the scattered fields. Fig. 2(b) shows amplitude and phase difference between the scattered and incident fields for the x, y, and z components for point 1 in Fig. 2(a). This indicates that the edges of the elements radiate and provide the expected phase shift. In particular, it is the radiating edges of the patch of the patch-ring element that provide the phase shift of 0° in this case. Fig. 2(c) shows amplitude and phase difference between the scattered and incident fields for the x, y, and z components for point 2 in Fig. 2(a). It is the edges of the ring of the patch-ring element that provide the phase shift of -120° (240°) in this case. This method is very powerful in understanding the reflection phase mechanism and also identifying the important geometrical parameters of the element that contribute to the reflection phase shift.

(a)

(b) (c)

Fig. 2. (a) Reflection phase response of the patch-ring element in a unit cell environment. (b) Amplitude and phase difference between the scattered and incident fields for the x, y, and z components for point 1 (patch radiation). (c) Amplitude and phase difference between the scattered and incident fields for the x, y, and z components for point 2 (ring radiation).

3. Reflectarray Environment: Reflection Phase Analysis

For the reflectarray design, receive mode analysis is used [6]. In this mode, one reflection phase curve based on normal incidence is used for the design. This is a good approach in which the angles of incidence are less than 45°. The reflectarray element has to provide phase shift to offset the path delay from the feed to the reflectarray surface. A WR62 waveguide probe is used to illuminate the reflectarray. The probe is placed 3.5″ from the reflectarray surface. The system is a symmetric center-fed system. The reflectarray radius is 4.125″ and F/D is 0.42. Fig. 3(a) shows the expected phase shift needed for beam collimation. After the design of the reflectarray, the incident, scattered, and total fields are analyzed immediately above the reflectarray surface. Fig. 3(b) shows the phase difference between the scattered and incident fields for the x-component (co-polarization direction). It can be clearly seen that the actual phase shifts are provided by the patch-ring element radiating edges. By comparing Figs. 3(a) and 3(b), it is apparent that the designed reflectarray does provide the required phase shift. Fig. 3(c) shows the beam collimation in the two planes. The simulated directivity is about 25dB. The patterns are different in the two planes due to asymmetric tapering caused by the probe. Fig. 3(d) shows the wavefront in the plane perpendicular to the reflectarray surface. The incident field from the waveguide has an expected spherical phase front. The scattered field from the reflectarray surface has a planar wavefront, demonstrating that the elements convert the incoming spherical wave into planar wavefront.

(a) (b)

(c) (d)

Fig. 3. (a) Required phase shift at the reflectarray surface. (b) Actual phase shift provided by the reflectarray elements in true reflectarray environment. (c) Radiation pattern for the reflectarray in two principal planes. (d) Scattered planar wavefront from the reflectarray surface.

4. Diagnostics using Graphical Visualization

In order to validate the effectiveness of this technique as a useful diagnostic tool, the following study is conducted. Two different patch-type reflectarrays are built (for simplicity variable size patch approach is used). Both the reflectarrays are designed using the above mentioned receive mode technique. One of the reflectarrays is designed with defects in which the patch sizes on two concentric rings are kept constant to provide 0° phase shift instead of the actual required phase shift. Actual phase shifts provided by the elements in these two arrays are studied; these are obtained by plotting the phase difference between the scattered and incident fields for the x-component. In order to amplify and identify the defective region of the reflectarray, the phase difference plot of the good reflectarray is subtracted from the phase difference plot of the defective reflectarray. Fig. 4(a) shows

the defective region encompassed within the two concentric circles. Back projection holography using measured near-field data is performed (setup shown in Fig. 4(b)) and the phase is computed at the reflectarray surface for both original and defective designs. Fig. 4(c) shows the phase subtraction plot obtained by subtracting the holographic image (containing phase information) of the original reflectarray from the defective one. The defective region can be conclusively identified in this case and corresponds well to the expected phase difference obtained from simulation shown in Fig. 4(a).

(a) (b) (c)

Fig. 4. (a) Simulated phase subtraction plot between the defective and the original reflectarray for the x-component. (b) Measurement setup of the reflectarray system in the UCLA spherical near-field chamber. (c) Measured phase subtraction plot between the defective and the original reflectarray for the x-component.

5. Conclusion

This paper describes a novel method to analyze, design, and understand the reflectarray antenna behavior using a graphical visualization tool. A dual resonance patch-ring element is described and analyzed in a unit cell environment. The mechanism of reflection phase shift from the element is studied using this graphical procedure. A reflectarray is designed and the elemental phase shifts are analyzed in true reflectarray environment. It is observed that this procedure can also be used to verify the validity of the reflectarray design. In order to use this analysis for diagnostics, good and defective reflectarray designs are analyzed and fabricated. It is shown that this approach of reflection phase can successfully predict the defective regions of the bad reflectarray which is also confirmed through measurements.

6. Acknowledgments

The authors would like to thank Dr. Shenheng Xu (UCLA antenna lab) for his valuable suggestions.

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