This document presents the design of a novel wideband circularly polarized antenna for UHF RFID reader applications. The antenna uses an air-substrate truncated patch structure to achieve circular polarization. It consists of a radiating patch, suspended microstrip feed line, and ground plane with no dielectric. Four feeding probes connected to a feed network are used to generate quadrature signals and excite circular polarization on the patch. Simulation results show the antenna achieves an impedance bandwidth of 840-960 MHz and axial ratio bandwidth where AR is less than 3. The antenna has a gain over 8 dB and is suitable for universal UHF RFID applications due to its wide bandwidth and simple design.

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A Novel Wideband Circularly Polarized Antenna for Worldwide
UHF Band RFID Reader Applications
Mukhtar F. Mukhtar*, Ayman G. Sobih ** Abd El Aziz A. Mitkees ***
*(Department of Electronic, MTC University, Cairo, Egypt)
** (Department of Electronic, MTC University, Cairo, Egypt)
*** (Department of Electronic, MTC University, Cairo, Egypt)
Abstract
This paper presents A Novel Wideband Circularly Polarized Antenna for Worldwide UHF Band RFID Reader
Applications with simple, compact structure. An air-substrate truncated patch antenna is used to achieve the
circularly polarized characteristic. The impedance and AR bandwidth of proposed antenna are 840-960 MHz
(VSWR less than 1.5) and (AR less than 3), respectively. Proposed antenna has a good radiation performance
with gain more than 8 dB. And it is a good choice for UHF RFID applications.
Keywords— RFID, UHF, Circular polarize, Unidirectional
I. INTRODUCTION
A considerable amount of research towards
improving existing RFID systems and incorporating
new ones into different applications, where at the
moment the cost of implementation is a limiting
factor, is currently taking place. Antennas for RFID,
known as reader and tag antennas, play an important
role in the performance of these systems since the
key features of locating, identifying, and extracting
information from an object rely on their design and
performance; consequently, the whole system
effectiveness is greatly determine by them. It is for
this reason that, among the research in this area,
antennas receive special attention always with the
aim of improving one or several parameters such as
the impedance matching, gain, axial ratio,
polarization and, definitely, reducing their size and
cost.
Microstrip patch antennas usually have a
dielectric between the patch itself and the ground
plane, which reduces the efficiency of the antenna.
Using a dielectric with low permittivity and low loss
tangent is one way of improving the efficiency of the
antenna and widening the bandwidth. Generally foam
is used as dielectric substrate with the lowest
dielectric constant for patch antennas. Foam has a
dielectric constant of 1.03 and very low dielectric
loss factors (tan δ=0.0008), but the surface of the
foam is not well defined, which makes it impractical
to deposit material directly on it. Foam is also a
porous material. Therefore, the idea of using air as
the dielectric is investigated here. Air is a dielectric
with permittivity of 1 and with loss tangent of 0.
With air as dielectric, the bandwidth is improved
because the fields at the edges of the patch are less
confined. This thesis consists of analyzing a
rectangular air-spaced patch antenna. The analysis
consisted of designing two different patches using
two different literatures; two patch antennas were
simulated and built; the patch antennas parameters
were simulated and the results were compared with
published literatures. Patch antennas studies have not
specifically investigated the air spaced case. Reported
predictions indicate that for a microstrip patch
antenna to deliver good results a dielectric of at least
2.2 should be used to build the antenna [1], [2].
In this paper, a broadband sequentially fed
CP stacked patch antenna has been presented for
universal UHF RFID applications. By using a simple
feeding structure and combining several band
broadening techniques, the optimized antenna has
achieved the desired performance over the UHF band
of 818–964 MHz or 16.4% with the gain of more
than 8.3 dBic, AR of less than 3 dB, return loss of
less than 15 dB, and 3-dB AR beamwidth of larger
than 75 . Therefore, this universal design can be
applied to all the UHF RFID applications worldwide.
The remainder of this paper is organized as
follows. Section II describes the geometry of the
proposed antenna. The simulated results, analysis,
and discussion are presented in Section III. The
validation of the proposed antenna in RFID system
applications is exhibited in Section V. Finally, a
conclusion is drawn in Section VI
II. ANTENNA CONFIGURATION
A singly – fed circular polarization may be
regarded as one of the simplest radiators for exciting
circular polarization and is very helpful in situations
where the space do not allow to accommodate dual-
orthogonal feeds with a power divider network. This
technique generally radiates linear polarization; but
RESEARCH ARTICLE OPEN ACCESS

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in our study case we want to achieve a circular
polarization, so we are going to talk of some
techniques used to achieve this goal.
However, the single-fed single-patch CP
antenna in its simple form has inherently narrow AR
and impedance bandwidths of 1%–2% [7]. To
improve the bandwidth, a variety of CP antennas
have been studied, wherein the bandwidth of AR,
impedance matching, and gain have been enhanced,
e.g., by modifying the radiator shape, designing
feeding structures, and optimizing antenna or array
configurations [8]–[17]. Usually, a CP antenna with
the hybrid feed features a wide AR bandwidth, but
suffers a complicated structure, expansive
manufacture, and increased antenna size.
In these papers, the antenna needs to reduce
cost, size and complexity. Many scholars have done a
series of work in the reader antenna design. In order
to increase the impedance bandwidth, and reduce the
antenna dielectric loss; air based microstrip patch
structure is used. Broaden band techniques, such as
parasitic patch, aperture-coupling feeding structure
and so on are shown in [3]-[7]. Besides, to realize the
circularly-polarized characteristic, truncated patch
and novel feeding structures are used to design [8]-
[9]. This paper gives a simple UHF RFID
circularlypolarized reader antenna design; an air-
substrate truncated patch antenna is used to achieve
circularly-polarized characteristic. Compared with
the structures stated above, the proposed antenna has
a simple and compact structure with wide impedance
and axial ratio bandwidths. Thus, the requirement for
a universal UHF RFID reader include: enough wide
bands to cover all UHF RFID frequencies ranges,
perfect performance and lost cost.
By using two coax feeds separated by 90°
which generate fields that are orthogonal to each
other under the patch, as well as outside the patch.
Also with this two-probe arrangement, each probe is
always positioned at a point where the field generated
by the other probe exhibits a null; therefore there is
very little mutual coupling between the two probes.
To achieve circular polarization, it is also required
that the two feeds are fed in such a manner that there
is 90° time phase difference between the fields of the
two, this is achieved through the use of a 90° hybrid.
To achieve circular polarization, the magnitude of the
axial ratio must be unity while the phase must be
±90°. This is can be achieved by both the numerator
and denominator are of equal magnitude and 90° out
of phase [4].
The circular polarization radiation of the
proposed antenna is excited by four cylinder probes
which transmit four signals that have equal amplitude
with a quadrature phase difference (0° or 360°±,
90°±, 180°±, and 270°±) generated from the feed
network (6).
Fig. 1 shows the configuration of the proposed
antenna. The antenna comprises three layers of
conductor, which include main radiating patch, a
suspended microstrip feed line, and a finite-size
ground plane. Air substrate is used in this
configuration to achieve higher gain, broader
bandwidth, and lower cost.
The microstrip feed line of a width of 24
mm is suspended above the ground plane
( 250 250mm mm ) at a height of (5 mm). One
end of the feed line is connected to an RF input,
while the other one is open circuited, which
simplifies the antenna structure.
(a)
(b)
Fig. 1 shows the configuration of the proposed
antenna
The main radiating patch of
(156 156mm mm ) and with a truncation 1L of
24.5 mm at two diagonal corners is placed above the
feed line at spacing of 2 20h mm .
The main patch is fed by four probes which
are connected to the microstrip line. The probes are
of diameter of 2.2d mm , and positioned along
the microstrip feed line with adequate distance to
create the
0
90 phase lag between the probes and
brings into sequential rotation of current on the
radiation patch for CP radiation.
The prototype and detailed dimensions are
shown in Fig. 2. The truncated patch, feed line, and
ground plane are all made of copper and fixed using

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plastic spacers. Four metallic screws are used as the
probes to connect the microstrip feed line and the
main patch. A coaxial cable is directly connected to
the microstrip feed line to simplify the assembly of
the antenna, where the coaxial cable is split into two
wires (screen and core) and the wires are soldered to
the suspended feed line and the ground plane
separately.
Feed line Network
The probes are of diameter of 1.2 mm, and
transmit four signals that have equal amplitude with
quadrature phase difference (0±, 90±, 180±, and
270±) generated from the feed network. In our
design, mechanism of every probe-feed radiating
patch is similar with the technique of common
coaxial probe-feed patch antenna [4], thus all of the
probes need to be connected to about 50- microstrip-
line, in succession to about 100Ω microstrip-line,
ultimately by a stepped impedance transformer to a
50Ω coaxial cable. As shown in Fig. 1(a), the feed
network is placed over the ground plane with a
distance h.
The feed network is composed by a stepped
impedance transformer, 90± delay line, and 180±
delay lines as shown in Fig. 2(a), and brings into
sequential rotation of current on the radiation patch
for CP radiation. To avoid low radiation efficiency,
100Ω isolation resistor is omitted in the feed network
[9]. By properly selecting the positions of four feed
points and carefully optimizing the stepped
impedance transformer, the input resistance and
reactance can be easily controlled to achieve a wide
impedance bandwidth.
Fig.3.Feed line Network
Figure 1. Configuration of the proposed antenna.
Top view (Dimensions in cm).
Figure 2. (a) Equivalent-circuit model of the feed
network.
The extension of length in square patch has
been obtained from oblique thin slot cut values. Fig 3
shows the top view of the feed line network for the
proposed antenna. The length and width of the feed
line network has been calculated from diagram Fig 4.
0 04 4 83.33C f mm  
Using ADS line Cal (Z=100 Ω) W=24.213400mm,
L=83.275700mm
III. RESULTS AND DISCUSSION
With aid of simulation by the Ansoft HFSS
13 3-D EM simulator, which is based on the finite-
element method (FEM), the antenna is optimized and
then prototyped. A photograph of the fabricated
antenna with circularly polarized characteristic is
shown in Fig. 2(b).

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To better understand the influence of the
parameters on the performance of the antenna, only
one parameter at a time will be varied, while others
are kept unchanged unless especially indicated.
Fig. 3(a) shows the simulated return loss of
the antenna. The simulated return loss is 31.9280 dB
at 921 MHz. Fig. 3(b) exhibits the simulated AR. The
simulated gain is illustrated in Fig. 3(c). The antenna
exhibits the simulated gain with a peak gain of 9.3
dBic at 921 MHz.
(a)
(b)
(c)
Fig. 3. Simulated and measured results of the
proposed antenna. (a) Return loss. (b) AR. (c)
Gain.
IV. CONCLUSIONS
Broadband sequentially fed CP stacked
patch antenna has been presented for universal UHF
RFID applications. By using a simple feeding
structure and combining several band broadening
techniques, the optimized antenna has achieved the
desired performance over the UHF band of 818–964
MHz or 16.4% with the gain of more than 8.3 dBic,
AR of less than 3 dB, return loss of less than 15 dB,
and 3-dB AR beamwidth of larger than 75 .
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