Hexagonal FSS for GLONASS/GPS antenna
with improved axial ratio
Evgeny R. Gafarov, Yury P. Salomatov, Member, IEEE

Abstract—The new lattice of frequency-selective surface (FSS)
with a triangular arrangement of hexagonal elements for highprecision antennas GLONASS/GPS is presented. In the highprecision navigation systems necessary to reduce the effect of
multipath interference. Typically, bulky and heavy choke ring
structures have been used to reduce the effect of multipath
interference. The antenna presented here consists of a microstrip
patch antenna immersed in a frequency selective surface.
FSS substrate reduces the effects of multipath interference by
blocking the propagation of surface waves and, consequently,
reduces multipath interference. The hexagonal shape of the FSS
elements is selected based on polarization of the antenna, a
good axial ratio in the operating frequency band L1 is shown. The
advantage of printed FSS substrates is that they can be realized in
low-weight, low-price, and low-profile fashion.
Index terms—Frequency selective surface,
interference, choke ring, microstrip antenna.
multipath
I. INTRODUCTION
A
N IMPORTANT feature of the industry for high
accuracy positioning system, such GLONASS, GPS,
GALILEO, and other, now consist of the fact that the main
source of positioning errors is the effect of multipath
interference. The signal received by the antenna can be split
into a direct and reflected component. Surface waves appear in
many situations involving antennas [1]. On a finite ground
plane, surface waves propagate until they reach an edge or
corner, where they can radiate into free space. The result is a
kind of multipath interference or “speckle” which can be seen
as ripples in the radiation pattern [2]. This phenomenon can't
be resolved with a software signal processing, but may be dealt
with by using special antennas.
Typically, in order to reduce multipath interference choke
ring structure was used. But despite the good properties of the
multipath rejection, choke ring has disadvantage such as highweight, complexity of manufacturing and non low-profile
structure. In the construction of modern satellite navigation
antennas such disadvantages is undesirable.
As it is known, one of development areas of frequencyselective surfaces is the technology of building structures with
high surface impedance in narrow frequency range [3]. The
device is built on this technology allows to suppress the
Manuscript received June 10, 2011. This work was supported by Federal
Target Program under Grant 2011-1.3.2-215-009/16.
Evgeny R. Gafarov, Yury P. Salomatov are with the Siberian State
University, IEPaR, Russia (e-mail: slazen@mail.ru)
radiation in certain directions from the antenna, to increase
gain and to correct radiation pattern shape. Microstrip antenna
with FSS has advantages allowing it to replace the choke ring
structure.
In the process of research microstrip antenna with FSS
model the emphasis focused on axial ratio. In previously work
square element with square arrangement in lattice was
discussed [4]. Such a structure fit all the characteristics for use
in systems GLONASS/GPS except one - the axial ratio. In
considering the various forms of the elements FSS best
characteristics for the hexagonal element with a triangular
arrangement in lattice were found. The axial ratio in operating
frequency band L1 = 1,565-1,615 GHz and elevation angles in
range ±75° and azimuth angles 0°-360° more than 0,73.
Moreover, FSS substrate is performed on material with
dielectric constant 3,27 (Rogers TMM 3), this fact allows to
reduce substantially the size of slow-wave structure.
II. DESIGN FEATURES
Fig. 1 shows that antenna consist of microstrip patch - (b),
on the substrate - (a) and frequency selective surface - (c).
Microstrip patch is performed on a substrate with a dielectric
constant 6. To provide the right hand circular polarization in
the operating frequency band with a good axial ratio two
points of feeding with a phase shift 90° were used. In addition,
the microstrip patch in the center shorted on the ground plane
by means of via pin in order to suppress higher modes of
oscillations.
The FSS substrate consists of hexagonal elements with
triangular arrangement in the lattice. Each element is shorted
to the ground plane by metal probe to form LC circuit with
surrounding elements. Whole lattice based on this concept.
The FSS substrate is realized by material Rogers TMM 3. The
dimensions of hexagonal cells and the gaps between them
optimized to work in a range of L1. Merging microstrip patch
antenna and frequency selective surface is important part of
investigation. As shown in figure 2 diameter of the cutout D =
65 mm. is selected on the basis of maximum axial ratio and the
corresponding cutting elements up to moment where maximum
10
a
0
b
Gain (dB)
c
-10
-20
R
-30
-40
-180
-120
-60
0
Theta (°)
60
120
180
RHCP
Cross-polarisation
Fig. 3. Gain patterns. Right hand circular polarization (RHCP) – solid and
cross-polarization – dashed line for the 4 cuts: phi = 0°; 45°; 90°; 135°.
Frequency 1,575 GHz.
D
1,0
H
0,9
Axial ratio
Fig. 1. Design of frequency selective surface with immersed patch antenna.
c
0,8
0,7
0,6
0,5
1,54
a
b
1,56
1,58
1,60
1,62
1,64
Frequency (GHz)
Curves: Phi = 0°, 45°, 90°, 135°;
Theta = -75°, -40°, 0°, 40°, 70°
envelope line
Fig. 4. Simulated axial ratio versus frequency.
the gap along the element, describing the optimal performance
of the element FSS and compactness, weight and size.
III. SIMULATION RESULTS
Fig. 2. Zoomed fragment of patch antenna with FSS.
length of the gap between the elements is saved. All adjacent
elements are joined with a metal ring and shorted on to the
ground plane by means of metal pins.
Such an artificial metal wall divides the area of immersed
patch and the FSS substrate in terms of the minimum reflection
coefficient. Furthermore, metal pins are located along cutting
radius to provide uniform circular current along whole lattice.
This fact results in improvement the axial ratio. The maximum
overall size of the lattice is R = 279 mm. Height of substrate
for patch and FSS equals H = 6 mm. Trimming the external
shape FSS is based on the fact to save the maximum length of
In order to assess the multipath rejection properties of the
FSS substrate and axial ratio, the side lobe level (or radiation
in backside direction for multipath rejection) of the antenna
has been evaluated for different frequencies and different phi cuts.
Figure 3 shows the radiation pattern of patch antenna with
FSS to the four corners of the azimuth at a frequency of 1,575
GHz for the right-hand circular and cross – polarization. Gain
in operating frequency band better than 8 dB, the level of
cross-polarization less than -23 dB. Due to the FSS lattice the
radiation level in backside direction from antenna is not more
than -21 dB in different phi - cuts. This is characterized by a
very good multipath rejection of antenna system.
Figure 4 shows the dependence of axial ratio versus
frequency for each partition plane for azimuth angles phi = 0°,
45°, 90°, 135°, five values elevation angles theta = ±75°, ±40°,
0° were investigated. The envelope line (dashed) shows the
minimum values for which the axial ratio for investigated
angles in operating frequency band of at least 0,73. For
comparison, axial ratio for square lattice FSS no more than 0,5
in L1 band [4].
IV. CONCLUSION
Investigation has shown that the hexagonal FSS can
improve axial ratio of microstrip antenna compared with the
square FSS. Without decreasing the multipath rejection (side
lobe level -21 dB as well as for square lattice) microstrip
antenna with hexagonal FSS showed a good axial ratio better
than 0,73 (-2,7 dB) in the operating frequency band and wide
range of azimuth and elevation angles. Further improvement of
axial ratio is only possible with the use the multilayer
structures of complex shape. Weight and height of choke ring
up to 10 times greater than FSS lattice. It can be concluded
that combining FSS technology with antennas can result in
low-weight, low-cost, and low-profile alternatives as compared
to current solutions.
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