_applied sciences_  
_Article_  
Dual-Band Nested Circularly Polarized Antenna Array for 5G Automotive Satellite Communications  
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LEO satellite communications at orbital heights of d = 150... 2000 km have broadened the research interest due to their relevance for a seamless worldwide coverage. Such links provide attractive features such as smaller path loss, higher area capacity, lower latency, lower power consumption, and reduced launching costs when compared with higher-orbit satellite constellations [2]. However, satellite connectivity poses severe challenges with respect to the link budget and the user equipment (UE) terminal antennas [3]. The first consideration refers to the potential frequency ranges enabling efficient 5G LEO satellite communications. Different spectral ranges at low (S-band), mid (Ku-band), and high (Ka-band) frequencies were compared by the authors previously in terms of selected key-performance indicators (KPI) such as uplink (UL) and downlink (DL) data rates, UE antenna size, terminal costs, and chipset availability for a given satellite constellation [4]. The low-band spectrum is considered well suited for 5G applications due to its moderate technical complexity, attractive link budget, and affordable costs; on the contrary, the high-band spectrum promises high data rates and compact terminal sizes. The authors deduced that both the S- band and Ka- bands seem promising candidates for effective 5G LEO satellite communications, especially in view of their compatibility with 3 GPP standards. The second step concerns suitable antenna terminals, offering compact designs that are integrable into the vehicles, and simple to fabricate for faster acceptance in the automotive mass market. The availability of high-gain mega constellations opens the potential to employ low-profile UE antennas with moderate gain and without the necessity of beam tracking [4]. Accordingly, compact and low-profile antennas become indispensable and could possibly benefit from the increasing popularity of conformal plastic-embedded antennas for automotive applications. High-gain antennas with dynamic beam tracking for satellite communications were developed in [5; 6] but they do not appear feasible for automotive mass-market applications due to their power consumption, costs, complexity, and volume. A promising technique to embed less complex antennas invisibly into plastic parts of passenger cars was presented in [7; 8] for separate TN and NTN functionality; this approach suffers from polarization loss as the antennas were linearly polarized (LP), while circular polarization (CP) is more appropriate to mitigate multipath propagation in satellite communication links [9]. Microstrip patch antennas are the first choice for this application because of their ease of manufacturing and compactness. The excitation of circular polarization over sufficiently wide bandwidths and angular ranges requires a thorough design of the feed network for the arrays. Sequential rotation is a promising approach as it offers wide impedance bandwidth and 3- dB axial- ratio bandwidth [10].