26-07-2012, 02:55 PM
Compact and Broadband Microstrip Antennas
[attachment=29085]
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, microstrip
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, dualpolarized,
circularly polarized, and gain-enhanced operations have been reported over
the past several years. In addition, various novel broadband microstrip antenna designs
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)
The microstrip-line-fed planar inverted-L (PIL) patch antenna is a good candidate
for compact operation. The antenna geometry is shown in Figure 1.6. When the antenna
height is less than 0.1λ0 (λ0 is the free-space wavelength of the center operating
frequency), a PIL patch antenna can be used for broadside radiation with a resonant
length of about 0.25λ0 [24]; that is, the PIL patch antenna is a quarter-wavelength
structure, and has the same broadside radiation characteristics as conventional halfwavelength
microstrip antennas. This suggests that at a fixed operating frequency,
the PIL patch antenna can have much reduced physical dimensions (by about 50%)
compared to the conventional microstrip antenna.
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 decreased
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 microstripantennas and result in an enhanced impedance bandwidth.
COMPACT DUAL-FREQUENCY MICROSTRIP ANTENNAS
Compact microstrip antennas with dual-frequency operation [32] have attracted much
attention. The two operating frequencies can have the same polarization planes [7]
or orthogonal polarization planes [33]. One of the reported compact dual-frequency
designs with the same polarization planes uses the first two operating frequencies of
shorted microstrip antennas with a shorting pin [34–36], and the obtained frequency
ratios between the two operating frequencies have been reported to be about 2.0–3.2
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 operation
have been reported. Figure 1.14 shows a typical compact dual-polarized microstrip
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 microstrip
antenna shown in Figure 1.14 which is better than that of the corresponding
conventional square microstrip antenna without embedded slots.
[attachment=29085]
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, microstrip
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, dualpolarized,
circularly polarized, and gain-enhanced operations have been reported over
the past several years. In addition, various novel broadband microstrip antenna designs
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)
The microstrip-line-fed planar inverted-L (PIL) patch antenna is a good candidate
for compact operation. The antenna geometry is shown in Figure 1.6. When the antenna
height is less than 0.1λ0 (λ0 is the free-space wavelength of the center operating
frequency), a PIL patch antenna can be used for broadside radiation with a resonant
length of about 0.25λ0 [24]; that is, the PIL patch antenna is a quarter-wavelength
structure, and has the same broadside radiation characteristics as conventional halfwavelength
microstrip antennas. This suggests that at a fixed operating frequency,
the PIL patch antenna can have much reduced physical dimensions (by about 50%)
compared to the conventional microstrip antenna.
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 decreased
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 microstripantennas and result in an enhanced impedance bandwidth.
COMPACT DUAL-FREQUENCY MICROSTRIP ANTENNAS
Compact microstrip antennas with dual-frequency operation [32] have attracted much
attention. The two operating frequencies can have the same polarization planes [7]
or orthogonal polarization planes [33]. One of the reported compact dual-frequency
designs with the same polarization planes uses the first two operating frequencies of
shorted microstrip antennas with a shorting pin [34–36], and the obtained frequency
ratios between the two operating frequencies have been reported to be about 2.0–3.2
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 operation
have been reported. Figure 1.14 shows a typical compact dual-polarized microstrip
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 microstrip
antenna shown in Figure 1.14 which is better than that of the corresponding
conventional square microstrip antenna without embedded slots.