The antenna presented in this article will be developed for satellite communications onboard systems based on the recommendations ITU-R S.580-6 and ITU-R S.465-5. The antenna consists of printed elements grouped in an array, this terminal works in a frequency band from 7.25 up to 8.4 GHz (14.7% of bandwidth), where both bands, reception (7.25 - 7.75 GHz) and transmission (7.9 - 8.4 GHz), are included simultaneously. The antenna reaches a gain about 31 dBi, and it has a radiation pattern with a beamwidth smaller than 10° and a dual circular polarization. The antenna has the capability to steer in elevation from 90° to 40° electronically and 360° in azimuth with a motorized junction

Exposito Dominguez, Gonzalo

Fernández González, José Manuel

Padilla de La Torre, Pablo

Sierra Castañer, Manuel

Servicio de Coordinación de Bibliotecas de la Universidad Politécnica de Madrid

Electronic Steering Antenna Onboard for Satellite 
Communications in X Band.  
G. Expósito-Domínguez(1), P. Padilla-Torre(2), J. M. Fernández-González(1) and M. Sierra-Castañer(1). 
(1)Radiation Group. Signals, Systems and Radiocommunications Dept.Universidad Politécnica de Madrid. 
Ciudad Universitaria. 28040. Madrid, Spain. 
{gexposito, jmfdez, m.sierra.castaner}@.gr.ssr.upm.es 
(2)Department of Signal Theory Telematics and Communications, Universidad de Granada. 
Periodista Daniel Saucedo Aranda. 18071. Granada, Spain. 
pablopadilla@.ugr.es 
 
Abstract— The antenna presented in this article will be 
developed for satellite communications onboard systems based on 
the recommendations ITU-R S.580-6 [1] and ITU-R S.465-5 [2]. 
The antenna consists of printed elements grouped in an array, this 
terminal works in a frequency band from 7.25 up to 8.4 GHz 
(14.7% of bandwidth), where both bands, reception (7.25 – 7.75 
GHz) and transmission (7.9 – 8.4 GHz), are included 
simultaneously. The antenna reaches a gain about 31 dBi, and it 
has a radiation pattern with a beamwidth smaller than 10º and a 
dual circular polarization. The antenna has the capability to steer 
in elevation from 90º to 40º electronically and 360º in azimuth with 
a motorized junction. 
Index terms— Steering antenna, X band. 
I. INTRODUCTION 
The aim of this work is the development of an onboard 
satellite communications system in X band. This printed 
antenna [3] has the following capabilities: transmitting 
multimedia data, thanks to its broadband capacity, full 
steering direction (360º azimuths and 50º elevation) get by 
electronic and mechanically systems and direct 
synchronization from the mobile vehicle by signal processing 
tracking.  
The gain offered by the antenna will allow the plane to 
communicate from every longitude or latitude of the earth. 
Thorough calculations were made in order to assure coverage 
under most probable weather conditions. Frequency bands, 
transmission and reception, are interchangeable by changing 
the feeding connectors. The radome will be integrated in the 
fuselage of the structure and will provide weatherproof to the 
system. A scheme is shown in Figure 1.  
 
 
Fig. 1  Usage Stage. 
II. SPECIFICATIONS AND ARCHITECTURE 
The antenna operates with left hand circular polarization in 
reception band (7.25-7.75 GHz) and right hand circular 
polarization in transmission band (7.9-8.4 GHz). The key 
element is a resonant double stacked patch, which is fed by a 
miniaturized 90º branch-line coupler to obtain the desired 
circular polarizations (RHCP or LHCP) and ensures adequate 
port coupling isolation. 
A rectangular structure has been selected formed by an 
array [4] of 16x24 elements. These elements are treated in two 
different ways: 
A. Rows 
First grouped in rows, separated a distance between them of 
0.85λ. In case of using an uniform excitation the side lobe 
levels exceed the limit of the recommendations mentioned 
before. Due to that restriction, a progressive reduction of the 
feeding amplitude from the centre to the edges of each row 
will be implemented with a passive unbalance feeding 
network. 
B. Columns 
Thus secondly, 16 rows of 24 elements are grouped and a 
separation of 0.5λ. The steering direction of the main beam is 
achieved due to the phase shifting feeding between each row. 
The terminal will be integrated in the fuselage and because of 
that, it will be needed to steer close to endfire directions. 
Therefore to avoid the closer angles to endfire the planar 
structure is raised. With the installation of a wedge (qtilt=30º) 
the final steering direction will be from 90º - 40º. 
Being the antenna final dimensions: 33cm x 85cm x 20cm. 
III. ANTENNA DESIGN 
One of the main features of the system is its big bandwidth. 
The radiating element (Fig.2) enables to cover 1.15 GHz, 
therefore, Tx and Rx bands are covered simultaneously. 
Another of the main features is the capability for 
electronically steering, which is get by an active feeding 
network with phase shifters. The last part included in this 
section will show the passive feeding network. 
A. Radiating element 
The radiating element is composed of two stacked patches, 
the upper one is fed by electromagnetic coupling, and the 
bottom one, is fed by two via holes (Fig.3) which get the 
circular polarization. By using a miniaturized hybrid coupler 
in two stages, we get to cover the frequency band needed, to 
use two circular polarizations (RHCP and LHCP) at the same 
time, and to insulate the both channels (Tx and Rx). 
 
 
Fig. 2  Radiating element. 
In (Fig.3) it can be seen a radiating element layer view. In 
that figure, we can see how the feeding network and hybrid 
couplers feed each patch. The structure is composed of an 
inverted microstrip, a common ground plane and a microstrip 
on top. one foam layer to move apart the two patches, and a 
microstrip without ground plane. 
 
 
Fig. 3  Radiating element layer view. 
The substrate permittivity is 2.17 in order to get a good 
radiation of the antenna. The thickness of  the different layers 
has been optimize to get the proper radiation pattern and 
beamwidth. The dimensions of the patches and hybrid 
couplers have been calculated for low reflection coefficient 
and insertion losses. 
 
Fig. 4  Radiating element S-parameters. 
S-parameters of radiating element are shown in (Fig.4). The 
reflection coefficient is S11 <-15 dB and the hybrid gets an 
isolation of S21 <-13.5. The isolation between the two 
frequency bands (7.25 – 7.75 GHz) and (7.9 – 8.4 GHz) will 
assure that the transmission power do not affect the receiving 
signal. 
 
 
Fig. 5  Radiation pattern of the radiating element f=7.825GHz, ϕ=0º. 
The CP/XP is higher than 19 dB in the middle of the 
frequency band (fc= 7.825 GHz), the directivity is over 8 dBi 
(Fig.5). 
Furthermore, in (Fig.6) it can be seen the purity of the 
circular polarization, in the centre of the working bands, 
fc1=7.5 GHz and fc2=8.15 GHz. Axial ratio value is above 3 
dB for ∆q=70º. 
 
 
Fig. 6  Axial ratio of the radiating element. 
B. Active feeding network 
The active feeding network is presented in (Fig.7) and 
(Fig.8). Each subsystem will be the responsible for adapting 
the signal in amplitude, noise or phase. In both cases, 
transmitter and receiver, the subsystem in charge for beam 
steering will be the phase shifters, these, can be digital or 
analogical, but in both cases TTD (true-time-delay) in order to 
cover the whole frequency operation band. 
 
 
Fig. 7  Transmitter diagram. 
In receiver system, another filter is added after the antenna 
in order to minimize the power transmitted, coupled in the 
receiver system. 
 
 
Fig. 8  Receiver diagram. 
The steering range needed in elevation goes from 10º until 
60º above the horizon. However, the plane in which the 
antenna is supported is tilted, thus, the new steering range will 
be from 40º to 90º. Electromagnetic simulations (in dashed 
line) and the analytic simulations (continuous line) for the 
different steering directions are presented in (Fig.9). The 
simulations were carried out with a structure of 16x1 elements. 
I would like to emphasize, the directivity losses of 3 dB 
between the perpendicular steering direction (90º, or 
broadside) and the worst steering angle case (40º). This is due 
to the non isotropic properties of the radiating element and the 
array effect. 
 
 
Fig. 9  Radiation pattern of 16x1 array ϕ=0º. 
The feeding amplitude and phase coefficients were 
calculated (Fig.10) for reducing the side lobe levels, and get 
the desired steering angles.  
Further developments in the project, will include a genetic 
algorithm. This algorithm will calculate in real time the best 
coefficients. 
 
Fig. 10  Amplitude and phase of the active network elements. 
C. Passive  feeding network 
To construct an unbalanced feeding network is the only 
way to reduce the side lobe level. Therefore the array 1x4 
shown in (Fig.11) is a sub array that will compound the final 
antenna.  
 
 
Fig. 11  Array patch antenna 1x4. 
The radiation pattern is shown in (Fig.12), it has a 
beamwidth of 30º and a directivity of 14 dBi. Thus, 6 sub 
arrays in horizontal will be group, each sub array will be fed 
with an specific amplitude in order to obey the ITU-R 
required. 
 
 
Fig. 12  Array patch antenna 1x4 radiation pattern H plane, f=7.825GHz. 
IV. PROTOTYPE 
The available space in the passive feeding network is one 
problem for this kind of antenna, because the vertical 
separation between the elements is 20.68 mm (0.5λ @ 7.25 
GHz). For this reason, a miniaturized hybrid coupler was 
design. Basically, the reduction consists in substitute each line 
in the model of a branch line for its equivalent quarter – 
wavelength transmission line of T- model [5]. 
  
 
Fig. 13  Equivalent quarter – wavelength transmission line of T-model. 
 
 Za=
Z0
tanθa
  (1) 
 Ybtanθb=
2
Zatan2θa
  (2) 
 
A hybrid coupler was designed, simulated and constructed 
following these guidelines. Final dimensions are less than 
8mm x 10mm (Fig.14). 
 
 
Fig. 14  Miniaturized hybrid coupler photograph. 
Electromagnetic optimizations and simulations were carried 
out. These simulations and measurements can be seen in 
(Fig.15) module of S-parameters and (Fig.16) phase of S-
parameters. 
 
Fig. 15  Hybrid coupler S-parameters (module) m=measured, s= simulated. 
Reflection coefficient is S11 < -15 dB and isolation is S41 < -
15 dB) and transmission coefficients approximately are -3.5 
dB (S21, S31 =-3.5 ±0.5 dB) (Fig.15). 
In (Fig.16) it can be seen the difference between the phases 
of S21 and S31 is ~90º ± 5º. 
 
Fig. 16  Hybrid coupler S-parameters (phase) m=measured, s= simulated. 
V. CONCLUSIONS 
In this work a printed antenna with electronic steering 
capability for onboard systems has been presented. Its parts 
have been developed in order to get wideband, two circular 
polarizations (RHCP and LHCP) and a good isolation between 
them. In addition a reduction of the passive feeding network 
was carried out and the measurements were presented. 
Further results and prototypes of this antenna will be 
presented in EuCAP 2011, Rome. 
ACKNOWLEDGMENT 
The simulations done in this work has been realized using 
CST Microwave Studio Suite 2010 under a cooperation 
agreement between Computer Simulation Technology (CST) 
and Universidad Politécnica de Madrid. We kindly thank the 
company NELTEC S.A. for giving the substrates, in which the 
prototypes were built, freely. This work is been supported by 
an UPM grant CH/003/2011, and the CROCANTE project 
with reference TEC2008-06736- C03-01. 
REFERENCES 
[1] ITU-R Recommendation S.580-6 (2004). Radiation diagrams for use as 
design objectives for antennas of earth stations operating with 
geostacionary satellites. 
[2] ITU-R Recommendation S.465-5 (1993), Reference earth-station 
radiation pattern for use in coordination and interference assessment in 
the frequency range from 2 to about 30 GHz. 
[3] J.R. James, P.S. Hall, “Handbook of Microstrip Antennas”, IEE 
Electromagnetic waves series, Peter Peregrinus Ltd, 1989. 
[4] D.M. Pozar, D.H. Schaubert,”Microstrip Antennas: The analysis and 
design of Microstrip Antennas and Arrays”. IEEE Press, 1995. 
[5] Ching-Wen Tang, Ming-Guang Chen, “Synthesizing Microstrip 
Branch-Line Couplers With Predetermined Compact Size and 
Bandwidth”, IEEE Trans on Microwave Theory and Techniques, vol. 
55, no.9, pp. 1296-1934, September 2007. 
[6] A. Garcia-Aguilar, J.M. Inclán-Alonso, L. Vigil-Herrero, J.M. 
Fernández-González, M. Sierra-Pérez, "Printed Antenna For Satellite 
Communications", 2010 IEEE International Symposium on Phased 
Array Systems &Technology, Boston, EE.UU, pp. 529-535, Oct. 2010. 
[7] A. Garcia-Aguilar, J.M. Inclán-Alonso, J.M. Fernández-González and 
M. Sierra-Pérez, "Printed Antenna For Satellite Communications", 4th 
European Conference on Antennas and Propagation (EuCAP 2010), 
Barcelona, España, pp. 1-5, Apr. 2010. 


Electronic Steering antenna onboard for satellite communications in X band.

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Electronic Steering antenna onboard for satellite communications in X band.

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