06-09-2013, 04:48 PM
Compact and Broadband Microstrip Antennas
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INTRODUCTION
Conventional microstrip antennas in general have a conducting patch printed on a
grounded microwave substrate, and have the attractive features of low profile, light
weight, easy fabrication, and conformability to mounting hosts [1]. However, mi-
crostrip antennas inherently have a narrow bandwidth, and bandwidth enhancement
is usually demanded for practical applications. In addition, applications in present-day
mobile communication systems usually require smaller antenna size in order to meet
the miniaturization requirements of mobile units. Thus, size reduction and bandwidth
enhancement are becoming major design considerations for practical applications
of microstrip antennas. For this reason, studies to achieve compact and broadband
operations of microstrip antennas have greatly increased. Much significant progress
in the design of compact microstrip antennas with broadband, dual-frequency, dual-
polarized, circularly polarized, and gain-enhanced operations have been reported over
the past several years. In addition, various novel broadband microstrip antenna de-
signs with dual-frequency, dual-polarized, and circularly polarized operations have
been published in the open literature. This book organizes and presents these recently
reported novel designs for compact and broadband microstrip antennas.
COMPACT MICROSTRIP ANTENNAS
Many techniques have been reported to reduce the size of microstrip antennas at
a fixed operating frequency. In general, microstrip antennas are half-wavelength
structures and are operated at the fundamental resonant mode TM01 or TM10 , with
a resonant frequency given by (valid for a rectangular microstrip antenna with a thin
microwave substrate)
COMPACT BROADBAND MICROSTRIP ANTENNAS
With a size reduction at a fixed operating frequency, the impedance bandwidth of a
microstrip antenna is usually decreased. To obtain an enhanced impedance bandwidth,
one can simply increase the antenna’s substrate thickness to compensate for the de-
creased electrical thickness of the substrate due to the lowered operating frequency,
or one can use a meandering ground plane (Figure 1.7) or a slotted ground plane
(Figure 1.8). These design methods lower the quality factor of compact microstrip
antennas and result in an enhanced impedance bandwidth.
By embedding suitable slots in a radiating patch, compact operation with an en-
hanced impedance bandwidth can be obtained. A typical design is shown in Figure 1.9.
However, the obtained impedance bandwidth for such a design is usually about equal
to or less than 2.0 times that of the corresponding conventional microstrip antenna. To
achieve a much greater impedance bandwidth with a reduction in antenna size.
COMPACT DUAL-POLARIZED MICROSTRIP ANTENNAS
Dual-polarized operation has been an important subject in microstrip antenna design
and finds application in wireless communication systems that require frequency reuse
or polarization diversity. Microstrip antennas capable of performing dual-polarized
operation can combat multipath effects in wireless communications and enhance
system performance. Designs of compact microstrip antennas for dual-polarized op-
eration have been reported. Figure 1.14 shows a typical compact dual-polarized mi-
crostrip antenna fed by two probe feeds [17]. Antenna size reduction is achieved by
having four bent slots embedded in a square patch. Results [17] show that, with the
use of an FR4 substrate (thickness 1.6 mm and relative permittivity 4.4), good port
decoupling (S21 less than −35 dB) is obtained for the compact dual-polarized mi-
crostrip antenna shown in Figure 1.14 which is better than that of the corresponding
conventional square microstrip antenna without embedded slots.
COMPACT MICROSTRIP ANTENNAS WITH ENHANCED GAIN
It is generally observed that when the antenna size is reduced at a fixed operating
frequency, the antenna gain is also decreased. To obtain an enhanced antenna gain,
methods involving the loading a high-permittivity dielectric superstrate [40, 49] or
an amplifier-type active circuitry [50, 51] to a compact microstrip antenna have been
demonstrated. For the former case, with the antenna’s projection area unchanged or
even smaller, the antenna gain can be enhanced by about 10 dBi [49]. For the latter
case, the radiating patch is modified to incorporate active circuitry to provide an
enhanced antenna gain, and an extra antenna gain of 8 dBi in L-band operation has
been reported [50].
[attachment=57704]
INTRODUCTION
Conventional microstrip antennas in general have a conducting patch printed on a
grounded microwave substrate, and have the attractive features of low profile, light
weight, easy fabrication, and conformability to mounting hosts [1]. However, mi-
crostrip antennas inherently have a narrow bandwidth, and bandwidth enhancement
is usually demanded for practical applications. In addition, applications in present-day
mobile communication systems usually require smaller antenna size in order to meet
the miniaturization requirements of mobile units. Thus, size reduction and bandwidth
enhancement are becoming major design considerations for practical applications
of microstrip antennas. For this reason, studies to achieve compact and broadband
operations of microstrip antennas have greatly increased. Much significant progress
in the design of compact microstrip antennas with broadband, dual-frequency, dual-
polarized, circularly polarized, and gain-enhanced operations have been reported over
the past several years. In addition, various novel broadband microstrip antenna de-
signs with dual-frequency, dual-polarized, and circularly polarized operations have
been published in the open literature. This book organizes and presents these recently
reported novel designs for compact and broadband microstrip antennas.
COMPACT MICROSTRIP ANTENNAS
Many techniques have been reported to reduce the size of microstrip antennas at
a fixed operating frequency. In general, microstrip antennas are half-wavelength
structures and are operated at the fundamental resonant mode TM01 or TM10 , with
a resonant frequency given by (valid for a rectangular microstrip antenna with a thin
microwave substrate)
COMPACT BROADBAND MICROSTRIP ANTENNAS
With a size reduction at a fixed operating frequency, the impedance bandwidth of a
microstrip antenna is usually decreased. To obtain an enhanced impedance bandwidth,
one can simply increase the antenna’s substrate thickness to compensate for the de-
creased electrical thickness of the substrate due to the lowered operating frequency,
or one can use a meandering ground plane (Figure 1.7) or a slotted ground plane
(Figure 1.8). These design methods lower the quality factor of compact microstrip
antennas and result in an enhanced impedance bandwidth.
By embedding suitable slots in a radiating patch, compact operation with an en-
hanced impedance bandwidth can be obtained. A typical design is shown in Figure 1.9.
However, the obtained impedance bandwidth for such a design is usually about equal
to or less than 2.0 times that of the corresponding conventional microstrip antenna. To
achieve a much greater impedance bandwidth with a reduction in antenna size.
COMPACT DUAL-POLARIZED MICROSTRIP ANTENNAS
Dual-polarized operation has been an important subject in microstrip antenna design
and finds application in wireless communication systems that require frequency reuse
or polarization diversity. Microstrip antennas capable of performing dual-polarized
operation can combat multipath effects in wireless communications and enhance
system performance. Designs of compact microstrip antennas for dual-polarized op-
eration have been reported. Figure 1.14 shows a typical compact dual-polarized mi-
crostrip antenna fed by two probe feeds [17]. Antenna size reduction is achieved by
having four bent slots embedded in a square patch. Results [17] show that, with the
use of an FR4 substrate (thickness 1.6 mm and relative permittivity 4.4), good port
decoupling (S21 less than −35 dB) is obtained for the compact dual-polarized mi-
crostrip antenna shown in Figure 1.14 which is better than that of the corresponding
conventional square microstrip antenna without embedded slots.
COMPACT MICROSTRIP ANTENNAS WITH ENHANCED GAIN
It is generally observed that when the antenna size is reduced at a fixed operating
frequency, the antenna gain is also decreased. To obtain an enhanced antenna gain,
methods involving the loading a high-permittivity dielectric superstrate [40, 49] or
an amplifier-type active circuitry [50, 51] to a compact microstrip antenna have been
demonstrated. For the former case, with the antenna’s projection area unchanged or
even smaller, the antenna gain can be enhanced by about 10 dBi [49]. For the latter
case, the radiating patch is modified to incorporate active circuitry to provide an
enhanced antenna gain, and an extra antenna gain of 8 dBi in L-band operation has
been reported [50].