Mutual Coupling Reduction between Asymmetric Reflectarray Resonant Elements

Received Dec 11, 2017 Revised Mar 7, 2018 Accepted Mar 14, 2018 A physically asymmetric reflectarray element has been proposed for wide band operations. The dual resonant response has been introduced by tilting one side of the square path element. The numerical results have been analyzed in the frequency band between 24GHz to 28GHz where a reflection phase range of more than 600° has been achieved. The proposed asymmetric element can produce mutual coupling with adjacent elements on a reflectarray. This effect has been monitored by placing the elements in a mirror configuration on the surface of reflectarray. The single unit cell element results have been compared with conventional 4 element unit cell and proposed mirroring element configuration. The proposed mirroring element technique can be used to design a broadband reflectarray for high gain applications. Keyword:


INTRODUCTION
The array of microstrip patches printed on a flat grounded dielectric substrate and fed by a horn describes the architecture of a reflectarray antenna. It can scan its beam to wide angles by maintaining low profile design. The phased arrays and parabolic reflectors are its main competitors [1]. The main drawback associated with phased arrays is their complex design which also introduces additional losses especially at higher frequencies. Parabolic reflectors are also difficult to design at millimeter waves for high gain performance. On the other hand, the design simplicity of reflectarrays makes it suitable for high frequency operations. However the narrow bandwidth with low gain performance is its main operational limitation. This issue is even more challenging at higher frequencies such as millimeter waves due to shorter operational wavelengths [2]- [4]. The performance of a reflectarray antenna can be improved by suitably designing its unit cell patch element in an infinite array approach. Its bandwidth can be improved by enhancing the unit cell reflection phase range while its gain depends on the losses contributed by its unit cell [5], [6]. The conventional patch elements such as square and circular can provide low loss performance however their bandwidth is limited due to their reflection phase range narrower than a full 360° cycle [7]. Some extraordinary efforts are required to enhance the reflection phase range of a unit cell beyond a conventional 360° phase swing. This can be done by introducing an extra resonance in the reflection response of the unit cell element. However, this phenomenon could also produce extra losses and mutual coupling [8], [9]  1883 between the adjacent elements. Mutual coupling can be reduced by increasing the distance between the elements [1]. However, it limits the gain performance and increases the profile of the reflectarray. Figure 1. Asymmetric reflectarray unit cell element Therefore in this work an asymmetric reflectarray unit cell element as shown in Figure 1, has been proposed to enhance its bandwidth performance at 26GHz. The performance of the unit cell element has been analyzed in an infinite array approach by enclosing it within magnetic and electric boundaries. An extra resonance has been produced by tilting one side of a square patch element from 90° to 83.48°. The full parametric studies of the unit cell has already been presented in [10]. The resonance behavior of a single patch unit cell and a 4 element unit cell has been thoroughly examined with 0.254 mm thick Rogers 5880 substrate. A technique has been introduced to limit the mutual coupling between the elements which is caused by the asymmetry in the physical design of the elements.

RESULTS AND DISCUSSIONS
The selected design of unit cell element has been investigated using Finite Integral Method (FIM) with infinite array approach. It has been analyzed that, by tilting one side of the square patch element offers broadband resonant features. The tilting of the one side of the element creates asymmetry between two sides and introduces two different lengths for the same element at opposite sides. These two lengths are aligned in the same direction of the incident electric field as shown in Figure 2(a), creating two different values of reflected electric field (E1 and E2). These two different magnitudes of reflected electric field are the main reason behind the dual resonance response of the element which can be seen from Figure 3(a). As the single element is analyzed with infinite array approach, this dual resonance response should be applicable for any number of elements on a reflectarray. However, when a 4 element unit cell comprising of same elements has been calculated, its reflection response deteriorated. This can clearly be observed from Figure 3

. Reflection loss and reflection phase curves of reflectarray unit cells (a) Comparison between single and 4 element unit cells (b) Comparison between single unit cell and unit cell with mirroring the elements
The reason behind that is the tilted electric field (E 2 ) which generates two more electric components which are E 2x and E 2y . E 2y is already in the same direction of E 1 whereas E 2x is in the orthogonal direction to it. The E 2x components of all elements in the reflectarray are added up together due to the same direction to generate mutual coupling as shown in Figure 2(a). This mutual coupling can change resonance frequency, reflection phase and can introduce high cross polarization with high side lobe levels for a reflectarray. The mutual coupling effect can also be observed from Table 1, where reflection response of each type of element is recorded. It can be seen from Table 1 that, the resonant frequency of 4 element unit cell changed from 26GHz to 25.7GHz. Its maximum reflection loss value has also been increased up to 2.9 dB as compared to 1.6 dB of single unit cell. The mutual coupling also affects and increases the slope of reflection phase of 4 element unit cell by 505/GHz. A smoother reflection phase curve with low slope ensures a wider bandwidth performance. This shows that, introduction of mutual coupling between the elements can increase the loss, decrease the bandwidth and can drastically affect the gain and bandwidth performance of a full reflectarray as a whole.
This issue of mutual coupling can be avoided by physically mirroring the orientation of the elements as shown in Figure 2(b). The same technique is used in [11] where the elements with stub lines are used. However in this work, this technique is applied on the physically asymmetric elements. By doing so, all E 2x components which are responsible for creating mutual coupling are canceled with each other due to opposite directions. This concept can be proved by analyzing Figure 3(b) where mirror elements are showing the same resonant response as of a single unit cell element. Table 1 also confirms the elimination of mutual coupling where reflection response of unit cell with mirror orientation coincides with the reflection response of single unit cell. The reflection loss and reflection phase curves of both unit cells are exactly same which can provide ease of creating a large reflectarray.

CONCLUSION
The bandwidth of a square patch reflectarray element can be enhanced by introducing asymmetry in its design with a tilted side. This effect can introduce two narrowly separated resonances with reflection phase ranging beyond 600° mark. The asymmetrical designs of elements on a reflectarray can introduce mutual coupling effects which can completely alter the resonance behavior of the elements. This effect can