Radial EBG cell layout for GPS patch antennas

Abstract

A novel radial layout for mushroom-like electromagnetic-bandgap (EBG) cells surrounding a printed circularly-polarised patch antenna is proposed. Two radial EBG configurations surrounding a circular patch are compared to a reference patch on a conventional ground plane of the same dimension. The radial shape and displacement of the EBG cells around the patch offers improvements in terms of gain and axial-ratio compared to the reference antenna and is more suitable for circular geometries compared to conventional Cartesian layouts. In particular, the distance between the patch and the surrounding EBG cells is independent of the cell period, which can be arbitrarily chosen, and the overall layout offers footprint reduction.

Full text

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Dublin Institute of Technology
ARROW@DIT
Articles
School of Electronic and Communications
Engineering
2009-06-18
Radial EBG Cell Layout for GPS Patch Antennas
Giuseppe Ruvio
Dublin Institute of Technology, Giuseppe.Ruvio@dit.ie
Max Ammann
Dublin Institute of Technology, max.ammann@dit.ie
Xiulong Bao
Dublin Institute of Technology, xiulong.bao@dit.ie
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Recommended Citation
Ruvio, G., Amman, M., Bao, X.: Radial EBG Cell Layout for GPS Patch Antennas. Electronic Letters, Vol. 45, issue 13, June 2009,
pp.663-664.

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Radial EBG Cell Layout for GPS Patch Antennas

Giuseppe Ruvio, Max J. Ammann and Xiulong Bao

A novel radial layout for mushroom-like electromagnetic-bandgap (EBG) cells
surrounding a printed circularly-polarised patch antenna is proposed. Two radial
EBG configurations surrounding a circular patch are compared to a reference patch
on a conventional ground plane of the same dimension. The radial shape and
displacement of the EBG cells around the patch offers improvements in terms of gain
and axial-ratio compared to the reference antenna and is more suitable for circular
geometries compared to conventional Cartesian layouts. In particular, the distance
between the patch and the surrounding EBG cells is independent of the cell period,
which can be arbitrarily chosen and the overall layout offers footprint reduction.
Introduction: In recent years numerous designs for EBG cells which surround printed
circularly polarised (CP) antennas have been proposed with the aim of improving the
axial-ratio (AR) performance. The addition of EBG cells reduce the edge diffraction
caused by surface waves which contributes in the far-field to cross-polarised
components [1]. Controlling these field components leads to improved AR [2]. The
benefits to CP antenna performance introduced by EBG structures have been
particularly notable in positioning systems [3]. Fractal based designs have been
proposed to achieve EBG miniaturisation for GPS [4]. In [5] a radial layout of
resonating passive printed dipoles was proposed, improving the operating bandwidth
and gain of an aperture-coupled printed-dipole antenna. A numerical investigation on
different rotationally symmetric EBG configurations surrounding a 15 GHz CP patch
antenna was recently reported [6]. Although the gain performance of radial EBG

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antennas is similar to Cartesian layouts, significant design improvement can be
achieved with rotationally symmetric EBG arrangements when combined with
circular printed patches. Compared to the conventional Cartesian arrangement of
EBG cells, a radial layout is more flexible because the distance d, between the edge
of the patch and the closest row of EBG cells can be freely chosen. This geometric
parameter is critical for impedance matching and gain optimisation which in a
conventional Cartesian layout is constrained by the EBG cell period and the antenna
radius. Thus, the radial layout offers an extra degree of freedom, which is valuable to
the antenna designer. The radial layout is also geometrically more suitable for
circular patches and circular groundplanes with a resulting reduction of the overall
footprint compared to conventional Cartesian layouts.
Configurations investigated: The reference antenna is a circular patch antenna fed
by four 50 Ω coaxial probes with a relative phase delay of 90° to excite RHCP and
shorted by a central pin to suppress higher-order modes. The patch radius is Rp=
17 mm and the probes are located 7 mm from the centre. A Taconic CER10
substrate of dimension 110 mm × 110 mm × 1.58 mm is used for all antennas. The
initial EBG structure used is the well-known mushroom-like Sievenpiper square cell
Cartesian array [7]. In order to achieve a bandgap across the GPS L1 band, the
period p = w + l of the structure was chosen to be 20.4 mm (where w = 3 mm is the
separation between adjacent cells and l = 17.4 mm is the square dimension), so that
the overall area of the cell surface is A = 302.76 mm2. The diameter of the
grounding vias is 1.3 mm. The dispersion diagram for this structure (Figure 1) shows
the bandgap to be from 1.41 GHz to 1.75 GHz. This Cartesian array was reshaped
into adjacent truncated wedge-shaped sectors to match the rotationally symmetric
layout of the circular patch antenna. The area of 302.76 mm2 was maintained which

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is independent of the radius of the patch antenna and the order of the ring to which
the cell belongs, and the spacing between cells was w=3 mm. Two different
configurations of EBG cell layouts were evaluated and compared to the reference
antenna. They are an 8-sector dual-ring and a 16-sector single-ring radial
arrangement as shown in Figure 2. A 1.3 mm diameter grounding via was centred in
each radial cell. Table 1 lists the geometric parameters.
Results: All antennas were well matched (> 10dB RL) in the GPS L1 band and the
radiation properties were measured. The measured boresight gain for the reference
patch was 3.9 dBic. Although a minor gain enhancement was found for the dual-ring
configuration compared to the reference, the single-ring 16-sector cell arrangement
presented a gain improvement of 0.6 dB. However, an improvement in boresight AR
is realised for both radial EBG configurations. The measured values are 0.75 dB,
0.45 dB and 0.25 dB for the reference, dual and single-ring EBG antennas
respectively, as shown in Figure 3. The improved gain of the single-ring configuration
is due to a greater number of sectors in the ring (16) which offers better resolution in
the radial geometry and the resultant bandgap performance, compared to the 8-
sector layout. The EBG augmented antennas also provide a significantly wider 3 dB
AR beamwidth. The measured AR beamwidths for the reference antenna was 132°,
whereas with radial EBG cells this improved to 162° for single ring to 150° for the
dual ring. The AR beamwidth is particularly important for GPS antennas because
improved reception from a larger number of satellites can accelerate the correlation
calculation. If the receiver antenna can sufficiently discriminate against indirect
counter-polarized signals at lower elevation angles, then the increased visibility of
the sky can allow faster correlation of the satellite orbital paths. Finally, figure 4
shows the measured RHCP and LHCP radiation patterns at 1.575 GHz for the three

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prototypes. It can be seen that the artificial surfaces significantly attenuate back
radiation due to the suppression of surface waves. The counter polarisation
discrimination is also improved. The measured boresight AR, gain and counter
polarisation rejection is summarised in Table 1.
Conclusion: In this paper two radial layouts of EBG mushroom-like cells have been
introduced and compared to a reference CP circular patch antenna. The EBG cells
have been shaped in order to match the rotational symmetry of the circular layout.
This arrangement allows more design flexibility compared to conventional Cartesian
layouts because the distance between the patch antenna and the first ring of EBG
cells is independent of the cell period, offering an extra degree of freedom. It also
provides a smaller footprint for circular designs. The radial configurations provide
improved axial-ratio performance over a larger beamwidth and better cross
polarisation characteristics across the hemisphere.

ACKNOWLEDGMENT
This work was supported by Science Foundation Ireland and the Research Support
Unit of the Dublin Institute of Technology.

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References
[1] F. Yang and Y. Rahmat-Samii, Microstrip Antennas Integrated with
Electromagnetic Band-Gap (EBG) Structures: A Low Mutual Coupling Design
for Array Applications, IEEE Trans. Antennas Propag., 2003, 51, (10), 2936-
2946.

M. Rahman and M.A. Stuchly, Circularly Polarised Patch Antenna with Periodic
Structure, IEE Proc. Microw. Antennas and Propag, 2002, 49, (3), 141-146.

R. Baggen, M. Martinez-Vazquez, J. Leiss, S. Holzwarth, L.S. Drioli and P. de
Maagt, Low Profile GALILEO Antenna Using EBG Technology, IEEE Trans
Antennas Propag. 2008, 56, (3), 667 – 674.

X.L. Bao, G. Ruvio, M.J. Ammann and M. John, A Novel GPS Patch Antenna
on a Fractal Hi-Impedance Surface Substrate, IEEE Antennas Wirel. Propag.
Lett., 2006, 5, 323-326.

N. Llombart, A. Neto, G. Gerini and P. de Maagt, Planar Circularly Symmetric
EBG Structures for Reducing Surface Waves in Printed Antennas, IEEE Trans.
Antennas Propag, 2005, 53 (10), 3210-3218.

G. Ruvio and M.J. Ammann, EBG Mushroom-Like Structures for Enhancing
Axial-Ratio Performance of CP Antennas, 1st International Congress on
Advanced Electromagnetic Materials for Microwaves and Optics, Rome, Italy,
2007, 727-730.

D. Sievenpiper, L. Zhang, R. Broas, N. Alexopolous and E. Yablonovitch, High-
Impedance Electromagnetic Surfaces with a Forbidden Frequency Band, IEEE
Trans Microw. Theory Tech. 1999, 47, (11), 2059-2074.
[2]
[3]
[4]
[5]
[6]
[7]

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Figure captions:
Fig 1. Dispersion diagram
Fig 2. Configurations investigated: (a) two rows of radial EBG cells in 8 sectors; (b)
one row of radial EBG cells in 16 sectors
Fig 3. Measured axial-ratio beamwidth at 1.575 GHz for the three prototypes
Fig 4. Measured RHCP and LHCP radiation pattern comparison at 1.575 GHz

Table 1. Summary of geometrical parameters and measured performance for the
different configurations investigated.

Authors’ affiliation:
Giuseppe Ruvio, Max J. Ammann and Xiulong Bao (Centre for Telecommunications
Value-Chain Research, CTVR, School of Electronic and Communications
Engineering, Dublin Institute of Technology, Ireland)

Corresponding author:

giuseppe.ruvio@dit.ie

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Figure 1

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Figure 2

(a)

(b)

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Figure 3
-90-60-30 0306090
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0


Axial-ratio [dB]
Elevation angle, θ [degrees]
Reference
Radial 2 rings 8 sectors
Radial 1 ring 16 sectors

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Figure 4
-180 -150 -120-90-60-300306090120150180
-35
-30
-25
-20
-15
-10
-5
0
5


Gain [dBic]
Elevation angle, θ [degrees]
Reference
Radial 2 rings 8 sectors
Radial 1 ring 16 sector
RHCP
LHCP

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Table 1.

Configuration
Rp
[mm]
d
[mm]
AR
(dB)
3 dB AR
beamwidth
(°)
Peak
gain
(dBic)
Xpol-
rejection
(dB)
Reference 17.4 - 0.75 132 3.9 30
2 rings of 8 sector
radial cells
17.3 5 0.45 162 4.0 36
1 ring of 16 sector
radial cells
17.4 3 0.25 150 4.5 39