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AUTHOR PROOF: ELECTRONICS LETTERS Vol. 51 No. 9 1
Circularly Polarized Solar Antenna for
Airborne Communication Nodes
Oisin O’Conchubhair, Adam Narbudowicz, Patrick McEvoy
and Max J. Ammann
A circularly polarized solar cell antenna consisting of four sequentially
rotated printed inverted-F antennas is proposed. Four multicrystalline
silicon solar cells act as the ground plane and the antenna is suitable for
low power airborne communication nodes and wireless sensor networks.
The antenna design was developed to allow 100% insolation of the cells
when directly facing a light source. The low-profile antenna minimises
shadowing of the solar cell for oblique angle insolation.
Introduction: Low-altitude airborne communication nodes are
envisaged for improved wireless communications over remote areas and
to support emergency services responding to large scale natural
disasters [1]. Short-term systems can be deployed rapidly to the lower
troposphere, while longer-term systems are placed in the upper
troposphere or stratosphere. Networked nodes are to communicate with
each other and with ground users. Entirely solar powered, each node
makes use of increased solar energy availability at altitude to charge
lithium batteries for low light conditions [2]. TETRA, WiFi and
WiMAX radios have been tested on tethered balloons [3], with TETRA
achieving 9 km but with WiFi achieving higher data rates at 1 km. The
proliferation of WiFi enabled consumer devices also makes it a viable
aid in emergency situations. Airborne antenna orientation can be widely
variable, so circular-polarisation (CP) is considered due to reduced
polarization mismatch losses.
Most work to date has focused on antenna designs which have potential
for solar integration. With an aim to integrate a CP antenna with a small
satellite solar panel, two linearly-polarized meshed patch antennas were
prototyped using a substitute Rogers laminate [4]. While a 5.15 dBi
gain at 2.47 GHz was achieved, it expected that a lossy solar cell
substrate would degrade the gain performance. The 2% S11 bandwidth
covers WiFi channels 9-14. The CP bandwidth of 0.61% has 3-dB
beamwidths of 20° and 80° in the principle planes.
Only one paper presents the integration of CP antennas with solar cells.
A 3.87 GHz amorphous silicon solar cell was prototyped with an
integrated crossed-slot antenna [5]. The slot was excited by a microstrip
line beneath the slot. A 3.7 dBi gain was achieved using a reflector
10 mm behind the feed line, with a 10.8 mm overall device thickness.
The antenna achieves a 3.1% CP bandwidth that covers the 2.6%
impedance bandwidth. The beamwidths were 46° and 71° in the
principle broadside planes.
This paper presents the integration of four low-profile printed
inverted-F antennas (IFAs) with four multicrystalline solar cells suited
to airborne CP communications. The antennas are sandwiched between
neighbouring solar cells to minimise the profile and solar shadowing.
The antenna is designed to achieve CP over the WLAN channels 2.4 -
2.45 GHz. The solar panels on a high altitude aerial balloon are
mounted almost perpendicular to the earth’s surface ensuring optimal
solar cell insolation during shorter winter days [1]. Mounting the IFAs
between the solar cells in these panels provides ideal antenna
orientation for communication between aerial nodes.
Proposed Antenna Configurations: Four multicrystalline solar cells are
arranged to form a square ground plane for the 322 mm wide antenna
over a lightweight support. Each solar cell has a length and width of
156 mm and consists of three elements, a latticed anode front contact,
an aluminium rear contact cathode layer and a multicrystalline silicon
material between the contacts. The 0.1 mm wire lattice layout allows
the maximum insolation of the semiconductor material while
maintaining an adequate surface contact to yield the maximum power
output. Two perpendicularly oriented 2 mm wide bus-bars, 74.18 mm
apart, interconnect the 57 electrode wires.
Fig. 1 CP solar antenna configuration
The antenna uses the cathode layer beneath the solar cells instead of a
metal ground plane thus reducing the aerial vehicle weight. A copper
strip connects the rear contacts of each high power cell providing an
output of over 12 W, considerably more than a-Si alternatives [5]. The
arrangement is shown in Fig. 1.
The IFA dimensions are HA = 6 mm, TA = 1 mm, LA = 22.65 mm,
LC = 8.67 mm and LS = 8.96 mm, shown in Fig. 2. Each antenna is
positioned between a pair of solar cells without the need for expensive
customised cells. This location allows for 100% insolation of the cell
when directly facing a light source, with 30% less shadowing than the
meshed patch [4]. While the low-profile of the antenna minimises
shadow casting from light sources at low oblique angles, this has lower
impact than the loss of light intensity at low angles.
A rat race coupler and two branchline couplers are employed to provide
sequentially rotated phases for CP. The simulations were carried out
using CST Microwave Studio. Identical antennas positioned above a
copper ground plane and fed using the same transmission line
configuration were simulated for comparison.
Results and Discussion: The feed line network has measured insertion
losses of -6.9 dB, -6.6 dB, -6.8 dB and -6.5 dB from ports 2, 3, 4 and 5,
respectively. The measured phase offset between ports 2 and 3 was
89.5º, between ports 3 and 4 was 90.0º, between ports 4 and 5 was 89.7º
and between ports 5 and 2 was 90.9º.
The simulated and measured results for each antenna are given in Table
I. The simulated and measured S11 for each antenna is shown in Fig. 3
and isolation is shown in Fig. 4. The measured bandwidth was slightly
less than simulated by 0.3 percentage points due to manufacturing
discrepancies in the assembly of the four antennas and the solar panel.
Fig. 2 Printed inverted-F antenna parameters
Table 1: Integrated Element Results
Frequency
(GHz)
-10dB Bandwidth
(%)
Simulated Solar Antenna 1
2.45
1.7
Measured Solar Antenna 1
2.45
1.6
Simulated Solar Antenna 2
2.46
1.8
Measured Solar Antenna 2
2.45
1.5
Simulated Solar Antenna 3
2.45
1.7
Measured Solar Antenna 3
2.45
1.5
Simulated Solar Antenna 4
2.46
1.8
Measured Solar Antenna 4
2.45
1.6
Simulated Copper Antenna
2.42
1.7
AUTHOR PROOF: ELECTRONICS LETTERS Vol. 51 No. 9 2
Fig. 3 Measured and simulated S11 for each antenna
Fig. 4 Measured and simulated isolation between antennas
When comparing a simulated setup using a solid copper ground plane
instead of multicrystalline silicon, the wider bandwidth is attributed to
losses in the silicon. Bandwidths can be further increased with higher
antennas. In addition, the is resonance is slightly higher in the solar case
due to an electrical shortening of the antenna, where the silicon surface
is proud compared to the copper surface.
Simulated and measured isolation between antennas is better than 10 dB
in all cases for 2.4 - 2.45 GHz. The simulated axial-ratio is below 3 dB
for 2.33 - 2.58 GHz. The measured axial-ratio is below 3 dB for
2.29 - 2.63 GHz. Axial-ratio results are shown in Fig. 5.
The feed line is configured for left hand circular polarization (LHCP),
shown in Fig. 6. Measured LHCP gain for the solar antenna is 3.7 dBic
at boresight while RHCP gain is -10.2 dBic. Gain results are shown in
Table II and no significant degradation in radiation properties of the
solar antenna is observed compared to the copper case. The measured
beamwidth for the solar antenna was 51º and 58º in the XZ and YZ
planes respectively. Radiation patterns are shown in Fig. 6. The antenna
can be re-configured for RHCP, linear polarizations and beam switching
by changing the phase arrangement. Linear polarizations are achieved
by feeding two opposing elements in the array; simulations show the
antenna beamwidth to be 114° in the plane of the inactive elements and
51° in the plane of the active elements.
Fig. 5 Measured and simulated axial ratio
TABLE II: Antenna Radiation Results at 2.45 GHz
Antenna
LHCP
Boresight
Gain (dBic)
RHCP
Boresight
Gain (dBic)
Axial Ratio
Bandwidth
(%)
LHCP
Beamwidth (º)
X-Z
Y-Z
Meas Solar
3.7
-10.2
13.8
51
58
Sim Solar
4.0
-13.5
10.4
61
61
Sim Cu
4.0
-12.7
8.3
61
61
Fig. 6 CP Gain X-Z (left) and Y-Z (right)
Conclusion: The first integration of sequentially rotated IFA antennas
with a high power photovoltaic solar panel is reported. Locating the
antennas between solar cells in a 12 W solar panel results in greater
power generation than previous solar CP designs and avoids costly
customised solar cells [5]. The configuration ensures 100% solar cell
insolation when directly facing light sources, while the low-profile
minimises shadowing due to oblique angle light sources.
Beam switching and polarization reconfiguration can be achieved by
adjusting the phase between antenna elements, allowing the antenna to
cover a larger area than other CP integrations.
Acknowledgments: This work was part funded by the Irish Higher
Education Authority under PRTLI Cycle 5 as part of the
Telecommunication Graduate Initiative and by Science Foundation
Ireland grant number 13/TIDA/I2746.
O. O’Conchubhair, A. Narbudowicz, P. McEvoy and M.J. Ammann
(Antenna & High Frequency Research Centre, School of Electrical and
Electronic Engineering, Dublin Institute of Technology, Dublin,
Ireland)
E-mail: oisin.oconchubhair@mydit.ie
References
1. “Project Loon”, http://www.google.com, [online].
Available: http://www.google.com/loon/ [Accessed Dec. 12, 2014].
2. “Prospects for Commercial Stratospheric Industrialisation at the
Threshold of Space”, athenalab.com, Oct. 16, 2010. [online]. Available:
http://www.athenalab.com/Prospects_for_commercial_stratospheric_ind
ustrialization.pdf . [Accessed Dec. 15, 2014].
3. A. Vilhar, A. Hrovat, T. Javornik, M. Mohorčič, “Experimental
Analysis of Wireless Temporary Networks Deployed by Low Altitude
Platforms” IEEE 18th International Workshop on Computer Aided
Modelling and Design of Communication Links and Networks
(CAMAD), 2013, pp. 238-242.
4. T. Yasin, R. Baktur, "Circularly Polarized Meshed Patch Antenna for
Small Satellite Application," IEEE Antennas and Wireless Propagat.
Lett., 12, pp. 1057-1060, 2013.
5. S. Vaccaro, J.R. Mosig, P. de Maagt, "Two advanced solar antenna
"SOLANT" designs for satellite and terrestrial communications," IEEE
Trans on Antennas and Propagat. 51, (8), pp. 2028-2034, 2003.
