A Ku band circularly polarized 2×2 microstrip antenna array for remote sensing applications

This paper reports the development of a Circularly Polarized (CP) microstrip patch array working at Ku band and in particular between 16.9–17.2GHz for remote sensing applications. The design is based on a sequential rotation of four linearly polarized rectangular patches and appropriate phase shifts provided by a corporate feed network. The agreement between measurements and simulations carried out using HFSS is satisfactory.


I. INTRODUCTION
The use of circular polarization(CP) in telecommunications, both in terrestrial and space borne applications, is well consolidated owing to its capability to maximize the polarization efficiency component of the link budget and its lower sensitivity to multipath clutter with respect to the linear polarized radiation.
The antenna developed in this work is intended to be used in a recent ground based microwave remote sensing application, namely Ground Based radar interferometry using Synthetic Aperture Radar(GBSAR) [1]. Printed antennas offer the possibility of low cost, low weight and scalability to high gain version [2] compared to conventional horn antennas.
Usually CP antenna arrays are composed of individual CP elements. Alternatively, linearly polarized (LP) elements can be used [3] which are in general easier to design than CP elements. The technique is based on imposing appropriate phase differences on each element in order to produce a composite CP field [4]. A 2x2 microstrip antenna array is developed here following this method.

A. Design requirements
The antenna array is intended as an antenna with a radar used for vibrations and deformations monitoring. The available in house radar device is a Continuous Stepped Frequency transceiver operating at Ku band. The apparatus is transportable and it is used in measurement campaigns in the field. Therefore lightweight components such as printed antennas are highly desirable. The frequency allocation for GBSAR [5] has a center frequency of f= 17.1 GHz and a total bandwidth of 300 MHz(16.9-17.2 GHz). The antenna gain requirements are related to the distance of operation. Typical values for the antenna gain range from 10dB to 20dB.

B. Geometry and design procedure
Exploiting the ideas in [3] and [4], four LP inset-fed half wavelength rectangular microstrip antennas are sequentially rotated (Fig.1a) and appropriate phase shift is applied on each branch of the feeding network (Fig.1b). The configuration for the case of Left Hand CP (LHCP) is shown in Fig.1. The design approach can be adjusted for Right Hand CP(RHCP) by employing rotations in the reverse sense.  A critical point in the design procedure is the achievement of a proper 90 o phase difference among the four branches of the feeding network. As a first step, the feeding network was designed as a circuit in Keysight´s Advanced Design System (ADS) and then optimized in Momentum to account for electromagnetic effects. In Fig. 2, the phase of the simulated Sparameters is plotted. The relative phase difference is 90º±1º at the design frequency of 17.1 GHz.
Subsequently, the optimized feeding network along with the inset-fed antenna patches were simulated in HFSS. For the simulated antenna array, the calculated Axial Ratio(AR) remained below 2.5 dB in the frequency band of interest.

III. ANTENNA MEASUREMENTS
A prototype array was fabricated on an Arlon 25N substrate (εr=3.38, h=0.762 mm) (Fig. 3) and measurements were carried out in an anechoic chamber. The measured impedance matching of the array is shown in Fig. 4. The return loss is better than 10 dB over a much wider bandwidth.  The measured antenna pattern at the center frequency is shown in Fig.5. If more gain is needed, the 2x2 antenna array can serve as a subarray for a larger array.
The most important metric for a CP antenna is its Axial Ratio. Measurements were carried out over frequency and angle (Fig. 6). As it can be seen, the measured broadside AR maintains values around 2.5 dB as predicted. The beamwidth is also stable.

IV. CONCLUSIONS
An LHCP 2x2 antenna array has been implemented for the Ku band by appropriate phase combinations of linearly polarized microstrip patches. The antenna array presents good impedance matching and axial ratio over the band 16.9-17.2 GHz showing promise as a good antenna candidate (in itself or as a subarray) for GBSAR applications in this band. Furthermore, the method can be adjusted to produce RHCP by reversing the sense of sequential rotation.

ACKNOWLEDGMENTS
The work has been supported by the Catalan Government under Grant 2014 SGR 1551. The work of C.Kalialakis was supported under EU H2020 Marie Sklodowska-Curie Grant Agreement 654734.