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AP1109-1121.R2
1
Abstract—A dual-band omnidirectional circularly polarized
antenna is proposed. The antenna comprises back-to-back
microstrip patches fed by a coplanar waveguide. A very low
frequency ratio of 1.182 has been achieved, which can be easily
tuned by adjusting four lumped capacitors incorporated into the
antenna. An analysis of the omnidirectional circular polarization
mechanism as well the dual band operation is provided and
confirmed by numerical and experimental data. Key parameters
to tune the resonant frequencies and the axial ratio have been
identified. The prototype antenna provides omnidirectional
circular polarization in one plane with cross polar isolation better
than 12 dB for both frequency bands.
Index Terms—Aperture coupled antennas, Circular
Polarization, Microstrip antennas, Multifrequency antennas.
I. I
NTRODUCTION
IRCULARLY POLARISED (CP) antennas have gained
much attention due to their improved immunity to
multipath distortion and polarization mismatch losses
including those caused by Faraday rotation. These properties
are desirable for satellite communications, WLAN and RFID
systems. However the number of compact CP antennas with
omnidirectional patterns is still very small. These antennas are
needed for multi-user communication in indoor environments
with many scatters (e.g. RFID or WLAN systems). In satellite
communication, where one traditionally expects the beam to
be directed towards the sky, the orientation of the antenna
cannot be a priori determined for many applications, making
omnidirectional antennas desirable. Moreover, many modern
systems require dual-band operation with relatively low
frequency ratios (i.e. the ratio between GPS L1 and L2 bands
is 1.28; between L1 and Galileo E6, it is 1.232), which
introduces additional complexity to the design.
To the best of our knowledge, there are no works in the
open literature reporting dual-band omnidirectional CP
antennas. The first approach to provide omnidirectional CP
(for single frequency) involved an array of radiating elements,
located around a common centre [1]-[4]. These solutions are
somewhat large and more complex to manufacture than planar
Manuscript received September 9, 2011. Revised June 13
th
2012. This
work was supported by the Science Foundation Ireland under Grant Number
09/SIRG/I1644.
A. Narbudowicz, X. L. Bao and M. J. Ammann are with the Antenna &
High Frequency Research Centre, Dublin Institute of Technology, Dublin 8,
Ireland (phone: +353-1402-4905; fax: +353-1402-4690; e-mail: max.ammann
@ dit.ie).
structures. Attempts to reduce the number of radiators
degraded the performance, with shouldering and dipping in the
radiation pattern [2], [3]. A lower profile antenna was
proposed in [5], where omnidirectionality was achieved by
superposition of four horizontally polarized stubs and a
vertically polarized mushroom structure.
There is very little reported work introducing
omnidirectional CP behavior using planar structures. The first
one employs epsilon negative material and its zeroth order
resonance to replace a mushroom in [5] by a completely
planar structure [6]. It exhibits good omnidirectional axial-
ratio (AR), however the use of five combined elements and
zeroth order resonance makes it difficult to modify the antenna
for dual band operation.
Another paper introduces the basic concept of using back-
to-back coupled patches [7]. The AR in the plane of
omnidirectionality for the planar structure is kept within a
4 dB limit and the S
11
is -11.19 dB for the CP frequency.
There is however around 6 dB variation in the CP radiation
pattern and no gain value reported.
Many techniques have been proposed to implement dual-
frequency patch antennas for unidirectional CP, including
stacked patches [8], dual-negative materials [9], slots cut in
the patch [10]-[12] or complex annual-ring structures [13].
From the above-mentioned patch structures, the slot loaded
rectangular patch was chosen for our design due to its
simplicity and relative compactness (compared to the stacked
patch). Slots can be used to perturb the current distribution,
shifting the higher modes down in frequency and modifying
their radiation pattern to be broadside. Typically there is a slot
along each radiating edge of the antenna [10], [12]. Such
configuration employs a third resonant mode (TM
300
),
generating an unwanted TM
200
mode that needs to be
suppressed. As typical CP patch antennas employ two
orthogonal modes, a total of four slots would be required using
this technique.
In this paper the first dual-frequency antenna with
omnidirectional performance is proposed. The omnidirectional
right-handed CP is achieved by employing back-to-back
coupled patches. The use of a novel feed, a conducting strip
connected between patches and a reduced ground plane size
allows better impedance matching and reduced overall size,
compared to [7]. Slots with capacitive loading provide dual-
band operation, however in contrast to previous work, only
two slots are used in each patch. This configuration supports
TM
200
(and respectively orthogonal TM
020
) mode rather than
TM
300
, simplifying the design.
Dual-band Omnidirectional
Circularly Polarized Antenna
Adam Narbudowicz, Student Member, IEEE, Xiu Long Bao, Senior Member, IEEE, and Max J.
Ammann, Senior Member, IEEE
C