A dual band circularly polarized SIW interleaved antenna array

The design of a new dual band slot antenna for Substrate Integrated Waveguide (SIW) technology is presented. The antenna element operates in two bands suitable for emerging High Throughput Satellite Systems terminals; namely the K band(19.2-20.2GHz) and the Ka Band (29-30 GHz). The inclined slotted antenna elements are first designed separately for each band and then in conjunction. These slot antenna pairs can be subsequently utilized for the formation of circularly polarized interleaved antenna arrays. Simulated axial ratio and impedance bandwidth cover the required bandwidths of 1GHz in each band.


INTRODUCTION
High Throughput Satellite Communications Systems in the Ka band are expected to fulfil the European Digital Agenda for broadband everywhere by 2020 [1]. There are several satellite systems in development that aim to deliver broadband communications. For example, the Inmarsat Global Xpress [2] has already started offering Ka Band satellite communications on mobile platforms. Global Xpress is a GEO (Geostationary Earth Orbit) system that is going to utilize 3 satellites and another one for redundancy. The frequency plan for the system follows the FDD(Frequency Division Duplex) approach (Uplink 29-30 GHz, downlink 19.2-20.2GHz).
Mobile platforms can include a lot of different scenarios; from cars to manpack. Earth stations in terms of satellite communications are categorized as Onboard Aircrafts (AES), Onboard Vessels (ESV) and Mobile Platforms (ESOMPs). ESOMPs in particular can be used in a harmonized way in terms of regulatory constraints according to a recent report by CEPT/ECC [3].
Considering the different frequency bands for uplink and downlink, the reflector antennas currently in offer utilize dual band feeders. However, reflector antennas are bulky for a widespread adoption and lightweight antennas are needed [4]. Planar antennas can an offer an attractive alternative. Antenna designs operating in K-Ka bands have been presented in the literature mostly as reflectarrays [5]- [6].
In this work, planar antenna elements are proposed based on SIW technology. SIW is considered as a technology that offers the benefits of waveguide technology along with the integration potential of planar technology [7]. The slots pair introduced in [8]- [9] for waveguides, is adapted for SIW. For the array topology, an interleaved arrangement [10][11] is adopted.
Section II presents the slot antenna geometry and the interleaved array. Section III describes the design procedure. Section IV presents some preliminary results on the antenna elements. Conclusions and future work are drawn in Section V.

A. Single Band Slot Pair
The antenna is composed by two slots orthogonal to each other [8]. The geometry along with the relevant parameters are shown in Fig.1. Slots with the same dimensions (L,W) are placed a distance ds apart along the waveguide length and are offset by x in the lateral dimension.

B. Dual Band Slot Pairs
The conceptual steps in forming the elementary antenna to operate in two bands is shown in Fig. 2. Each band can be designed separately and then combined to form the dual band antenna.

C. Interleaved Array
Having the element of Fig. 2 in place, arrays can be formed (Fig.3). Furthermore, since the antenna element can be considered as two separate antenna elements, interleaved arrays are also possible.

III. DESIGN PROCEDURE
The design procedure for the determination of the slot dimensions follows [8] utilizing the SIW-waveguide equivalence [7]. The simulations were carried out using a widely used simulator (ANSYS HFSS v.15.1). The procedure is shown in Fig.4. The center frequency of each band is used as the design frequency. A typical design plot is shown in Fig.5 for the K band considering that the most important parameter that controls the slot excitation strength is the length. As described in Fig.4, the magnitude S21 is recorded as a function of slot length L. For each slot length, several values of S21 are possible under combinations of the other four geometry parameters. The procedure is repeated separately for each band. When the two single band pairs are brought together, each pair acts as a scattering element for the other. As a final step for the interleaved array, the design is done with both pairs present. The aim is to bring as close as possible the two pairs (Fig.2) The initial dimensions of the slots pair for each band are given in Table I with reference to Fig.2. The substrate used is Arlon (εr=3.38, h=0.51mm, tanδ=0.0025). The SIW width is 6.27mm with vias of diameter of 0.6 mm and spacing 1.4mm. In the traveling wave array design, each element requires a different value of S21 in order to synthesize a Taylor or other aperture distributions [12]. Towards this end, design plots such as the one in Fig.5 permit the mapping of S21 to slot length for each array element.

IV. PRELIMINARY PERFORMANCE RESULTS
Axial ratio results are shown for the two bands in Fig.6a for the K-band and in Fig. 6b for the Ka band. Overall the axial ratio is below the 3 dB throughout the bands of interest. Correspondingly for the impedance, the S11 plots for the two bands are shown in Fig. 8. In both bands, good matching is achieved. For comparison, the axial ratio when the slot pair operates isolated is included.

V. CONCLUSIONS
The design of slot antennas that operate in the K and Ka bands has been presented. The antenna element for each band consists of inclined orthogonal slot pairs that generate circular polarization. Preliminary results on axial ratio and impedance satisfy the 2GHz total bandwidth requirements. These elements can be used to form interleaved subarrays that can satisfy requirements for mobile platforms. Results on a linear interleaved array will be presented in the conference.