# Flat Array Antennas for Ku-Band Mobile Satellite Terminals

Authors: Roberto Vincenti Gatti; Luca Marcaccioli; Elisa Sbarra; Roberto Sorrentino
Journal: International Journal of Antennas and Propagation
 (2009)

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## Abstract

This work presents the advances in the development of two innovative flat array antennas for Ku-band mobile satellite terminals. The first antenna is specifically conceived for double-deck trains to allow a bi-directional high data rate satellite link. The available circular surface (diameter 80 cm) integrates both a transmitting and a receiving section, operating in orthogonal linear polarizations. The TX frequency range is fully covered while the RX bandwidth is around 1 GHz arbitrarily allocated on the DVB range depending on requirements. The beam is steered in elevation through a phased array architecture not employing costly phase shifters, while the steering in azimuth is mechanical. Active BFNs allow excellent performance in terms of EIRP and G/T, maintaining extremely low profile. High antenna efficiency and low fabrication cost are ensured by the employment of innovative SIW (Substrate Integrated Waveguide) structures. The second antenna, receiving-only, is designed for radio/video streaming services in mobile environment. Full DVB coverage is achieved thanks to cavity-backed patches operating in double linear polarization. Two independent broadband active BFNs allow simultaneous reception of both polarizations with full tracking capabilities and a squintless beam steering from20∘ to 60∘ in elevation. A minimum gain of 20 dBi and G/T >−3 dB/K∘ are achieved, while maintaining extremely compact size and flat profile. In the design of both antennas fabrication cost is considered as a driving factor, yet providing high performance with a flat profile and thus resulting in a great commercial potentiality.


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## Body

## 1. Introduction

In the last few years an increasing interest has been
addressed to  many satellite services at Ku-band being proposed or already
operative, ranging from digital radio and TV broadcast to broadband internet
services. Potential applications and markets especially regard mobile
environments (cars, trains, boats, airplanes), though some particular
applications may also involve fixed users (e.g., flat wall-mounted antennas for
digital TV and radio for environmental impact reduction).The success of new services and the expansion of those already present
depend to a great extent on the user terminal costs and performance and
therefore on the employed antenna, which becomes a critical component of the
whole system. Since most of the applications refer to mass consumer markets, the
first driving factor for the antenna design and development is the fabrication
cost, that must be maintained as low as possible. A compact size is then
needed for an easy placement on mobile vehicles, and for many applications a
flat profile is mandatory to maintain aerodynamic properties of the vehicle as
well as to allow the integration of the antenna itself. Finally high
performance must be achieved in terms of EIRP and G/T in such a way as to
ensure high data bit rate links, as well as the automatic beam steering to
maintain the link in mobile environment. The most critical issue for an
effective design achieving such stringent requirements is indeed the
identification of the most suitable technology. The development of
high-performance and low-cost critical components (e.g., LNA’s, rotary joint,
radiating element, etc.) is furthermore essential as well as the
implementation of a cost-effective architecture for the BFN and control system.
The major limiting factor for mobile
terminal development is indeed represented by the beam steering approach and
the number of electronic components employed. To that end RF Microtech has
recently proposed [1] an alternative approach for beam steering, not employing
phase shifters, based on a switched beam architecture to obtain electronic beam
steering in the elevation plane, while mechanical beam steering is performed in
the azimuthal plane. This approach allows a considerable reduction of the
overall system complexity and fabrication costs being the BFN realized in
microstrip technology in the form of a Rotman lens, still maintaining extremely
high performance through the use of LNAs. The use of COTS in this case is the
most convenient solution due to the extremely wide market of such components,
already employed in all standard DVB receivers (LNB’s for reflector antennas),
making the cost of a single LNA as low as 2€. None of the
solutions present on the market or under development seem to retain all the
required features, being either too costly or too bulky especially for what
concerns the profile. In this work two innovative antenna systems for mobile satellite
terminals at Ku-band are presented, retaining all the above mentioned characteristics.
The constituting elements of such antennas will be described in detail, and the
achievable performance as well as the great potential of the proposed solutions
will be illustrated.


## 2. Electronic Beam Scanning RX/TX Flat Antenna

The first antenna is specifically
conceived for double-deck trains to allow a bidirectional high data
rate satellite link, though with proper adjustments it could be also employed also
in other kind of vehicles. The first and most important specification to be
satisfied is the flat profile (<5 cm), together with a limited area
(<80 cm) compliant with the roof of a double-deck train. The research
group at the University of Perugia headed by professor Roberto Sorrentino is
active in the field of flat beam scanning antennas since 2000. This research
activity has  produced along the years a number of interesting and innovative
solutions for mobile satellite terminal antennas operating up to Ka-band [2–7]
and has spurred the development of a unique cad tool for the design of slotted
waveguide technology [8]. In 2005 the University of Perugia was involved in ESA Project “Satellite Access for European Trains” (SAET, Contract N° 14272/00/NL/US) for a feasibility study of a bidirectional high-performance Ku-band flat antenna for mobile terminals to be placed on the roof of double deck trains [9]. In this framework the
maximum achievable performance was assessed, and slotted waveguide technology was
identified as the most suitable technology for the realization of the flat
array antenna. This result has been supported by the design,
fabrication, and test of a proof-of-concept slotted waveguide array antenna. The
tests have confirmed the validity of the electromagnetic modelling and the
design tools as well as the feasibility of the proposed solution. The
proof-of-concept antenna is represented by a dielectric-filled slotted
waveguide array made of 32 waveguides with 10 slots each, and it is realized in
aluminium. It has been designed and optimized to present the best performance
in the scanning range from 30° to 50° in elevation (E-plane). Measured results
confirm the high performance achievable with this technology, giving an overall
antenna efficiency of about 80%.With
the conclusion of SAET Project, a self-financed R&D activity has been
carried out aimed at the development of a ready-to-market product. To that
purpose the spin-off company RF Microtech has been  founded in 2007. What follows is the result of
this activity. Table1 shows the expected performance for the final antenna.Table 1
Expected RX/TX antenna performance.
ParameterUnitTX sectionRX sectionCentre frequencyGHz14.12511.575Bandwidth at VSWR<1.5MHz800800Polarization—Linear HLinear VMin gain (at25° elev.)dBi31.231.8Side lobe leveldB−12−12Cross polarizationdB−25−25Scan angles (elevation)Degree25°÷50°25°÷50°Scan angles (azimuth)Degree360°360°EIRP (ITU-R S.728.1)dBW32—G/T (at25° elev)dB/k°—10Size (D×H)Cm80 × 5The final shape of the
antenna will be circular (diameter 80 cm), as shown in Figure3. A prototype
antenna with the same performance of the final version but with a rectangular
shape is now under construction. A picture of the RX panel is shown in Figure 4.
The prototype antenna consists of two radiating panels for reception and
transmission, respectively, each subdivided in four sections made of 32
centre-fed slotted waveguides with 8 slots each. Longitudinal slots are used
for the receiving section while transverse slots are employed for the
transmitting antenna.  Substrate integrated waveguide (SIW) technology is
employed, with strong impact on cost, ease of fabrication, thickness, and
weight. Subarrays of the same line are fed in phase through the lowloss
waveguide BFN shown in Figure 5, connected to a 32-way active Rotman lens
realized in printed technology (see Figure 6) for beam steering in elevation,
while mechanical beam steering is performed in the azimuthal plane. The overall
thickness is around 3 cm and total weight <20 kg. 12 dB-gain and 1 dB-NF LNAs
covering the full DVB frequency range have been integrated in the RX section,
while low-cost monolithic PA’s are integrated in the TX printed circuit. The
use of active BFNs allows high antenna efficiency and therefore high
performance with a compact size and a low cost. Experimental results of the
complete prototype are expected in the next few months.Typical scenarios
of applications: (a) mobile vehicles such as cars, trains, boats, and trains; (b) example of strong environmental impact that could be reduced with the
employment of flat wall-mounted antennas for DVB and radio reception.
(a)(b)SAET RX prototype
antenna, 32×10
slotted waveguide array: (a) measurement setup; (b) measured radiation
patterns.
(a)(b)Figure 3
Final version size and geometry.Figure 4
RX panel realized in SIW technology. The array is made of 32×32 slots, with a size of approximately 40×80 cm.Figure 5
Fabricated waveguide combining network.Fabricated active Rotman lens (a); schematic
3D view of the final antenna (b).
(a)(b)


## 3. Receiving-Only Broadband Dual Polarization Flat
Antenna

The
second antenna under development is a receiving-only circular array with full DVB
frequency range coverage (10.7÷12.75 GHz) with simultaneous dual polarization. Main features are summarized in Table 2.Table 2
Expected antenna performance.
Operative Frequency10.7÷ 12.75 GHzPolarizationDual
linear V/HScanning angleElevation:20°÷60°Azimuth:0°÷360°3 dB beamwidth>6° in azimuth>10° in elevationGain>20 dBiG/T>−2 dB/k°Thickness<1
cmSize<30 cmEstimated price<500€(min 10.000 units)The antenna is fully
realized in printed technology, the radiating element being a cavity-backed
slot-fed patch antenna. Two parallel broad-band BFNs are employed, one for each
polarization, realized with two strip-line layers and a microstrip layer where
an active BFN, integrating LNAs at subarray level, is realized to allow
electronic beam steering in the elevation plane. The overall architecture and
the radiating element are shown in Figures7 and  8. Full polarization tracking is possible, since
the two orthogonal linear polarizations (V and H) are available at the same
time. For the same reason a simultaneous double circular polarization is also
possible (LHCP and RHCP) using simple passive circuitry. Two antennas with
different diameters (22 cm and 30 cm) have been designed, employing 144 and 296
elements, respectively. The overall thickness is <1 cm. Beam steering is
performed without the use of phase shifters and extremely low-cost active
components are employed for the LNAs, thus resulting in a very competitive
solution, potentially applicable also for larger diameters. The adopted beam
steering architecture, together with the full DVB coverage, the simultaneous
double polarization, and the low fabrication costs makes the proposed antenna
system an attractive solution also as a flat wall-mount panel for fixed
satellite terminals.Antenna Architecture. BFN realized with two
strip-line layers and a microstrip layer. Beam scanning is electronic in
elevation and mechanical in azimuth.
(a)(b)Figure 8
Dual polarization radiating element.


## 4. Conclusions

In this
work two innovative products under development at RF Microtech have been
presented. The proposed antenna systems include unique features which ensure a
flat profile, reconfigurability, and high performance at a low cost. Prototypes
of both solutions will be available for testing in the next few months.



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*Source: 836074-2009-05-10.xml*