23-01-2013, 10:19 AM
A Planar UWB Antenna With Signal Rejection Capability in the 4–6 GHz Band
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Abstract
The design of a compact planar antenna featuring
ultra wideband performance and simultaneous signal rejection in
the 4–6 GHz band, assigned for IEEE802.11a and HIPERLAN/2,
is presented. The design is demonstrated assuming RT6010LM
substrate with a relative dielectric constant of 10.2 and thickness
of 0.64 mm. The presented results show that the designed antenna
of 27 mm 20 mm dimensions has a bandwidth from 2.7 GHz
to more than 10 GHz excluding the rejection band. The antenna
features near omnidirectional characteristics and good radiation
efficiency.
INTRODUCTION
RECENT years have witnessed an increased interest in ultra
wideband (UWB) antennas since the adoption of UWB
technology by the U.S.-FCC in 2002 [1]. Parallel to this recent
interest in UWB antennas, research has also focused on a multilayer
dielectric substrate approach to the design of front end
modules to reduce the size of wireless transceivers [2]. In order
to achieve integration between the radio frequency (RF) front
end circuitry and a radiating structure, an UWB antenna should
preferably be of planar format. Several planarUWBantenna designs,
which have the potential to meet such requirements, were
reported in [3]–[7].
DESIGN
The configuration of a planar UWB antenna with the capability
of rejecting frequencies within the 4.0–6.0 GHz band is
illustrated in Fig. 1.
The antenna structure is assumed to be in the -plane with its
higher dimension extending along the -axis. The radiating slot
is the result of intersecting of two circles in a conductive layer
on top of the substrate. The antenna is fed with a coplanar waveguide
(CPW). The observed transition from CPW to a coaxial
probe can be regarded as via in a multilayer front end module.
A tuning slot to reject a signal within the 4–6 GHz band is introduced
in the second circle. Steps used to design this antenna
are summarized as follows.
RESULTS AND DISCUSSION
The UWB antenna with signal rejection capabilities in the
4–6 GHz band was designed assuming RT6010LM substrate
with a dielectric constant equal to 10.2, tangent loss
0.0023 and thickness of 0.64 mm, which was readily available
to the authors.
Fig. 2 shows the simulated and measured results for the
VSWR with frequency for the designed antenna (with dimensions
presented in Table I) without and with the tuning slot.
For the antenna without the tuning slot both the computed
and measured VSWR characteristics revealUWBbehavior with
bandwidth from 2.7 GHz to more than 10 GHz assumingVSWR
2 as a reference. When the tuning slot is included, the worst
VSWR occurs over a narrow sub-band within the 4–6 GHz. The
choice is dependent on the length of the tuning slot. In the passband
theVSWR characteristics of the originalUWBantenna are
only slightly affected by the presence of the tuning slot. Effect
of using tuning slot with different diameters is shown. It is clear
that position of the rejected band can be controlled by adjusting
diameter of the slot. Increasing diameter of the tuning slot shifts
the rejected band to a lower value by a proportion which is compatible
with the design (6). It has been observed that in order to
maintain the behavior of the antenna within the desired band the
center of the tuning slot should coincide exactly with center of
Circle2. The rejected bandwidth can be increased using a wider
tuning slot. This is confirmed by both simulated and measured
results, which show only small discrepancies.
CONCLUSION
In this letter, a simple design method for a planar UWB antenna
with signal rejection capabilities over the 4–6 GHz subband
has been presented. The proposed radiating element is a
slot formed by the intersection of two circles which also include
the CPW as a feed. A tuning slot is responsible for the sub-band
rejection. Computer simulations and measurements have proved
the validity of simple design formulas. The designed UWB antenna
features omnidirectional radiation pattern and good radiation
efficiency. The presented antenna configuration and its design
method should be of considerable interest to the designers
of UWB front ends employing multilayer substrates.