16 Chapter One Advantages: Planer (and can be made conformal to shaped surface) Low profile Ease of integration with microstrip technology Can be integrated with circuit elements Ability to have polarization diversity(can easily be designed to have vertical,horizontal,right-hand circular(RHCP),or left-hand circular (LHCP)polarizations) Lightweight and inexpensive Disadvantages: Narrow bandwidth(typically less than 5%),requiring bandwidth- widening techniques Can handle low RF power ·Large ohmic loss The most common microstrip antenna is a rec ngular patch.The rectangular patch antenna is approximately a one-half wavelength long section of rectangular microstrip transmission line.When air is the antenna substrate,the length of the rectangular microstrip antenna is app roximately one-half of a free-space wa avelength.If the antenna is loaded with a diel as its substrate the l ength of the antenn decreases as the relative dielectric constant of the substrate increases The resonant length of the antenna is slightly shorter because of the extended electric fringing fields,which increase the antenna's electrical length slightly.The dielectric loading of a microstrip antenna affects both its radiation pattern and impedance bandwidth.As the dielectric nstant of the substrate incre a bandwidth decrease This increases the antenna's factor and.therefore.decreases the impedance bandwidth. Feeding Methods: Coaxial probe feeding Microstrip transmission line Recessed microstrip line Aperture coupling feed- Proximity-couped microstrip line feed (no direct contact between the feed and the patch)
16 Chapter One Advantages: ■ Planer (and can be made conformal to shaped surface) ■ Low profile ■ Ease of integration with microstrip technology ■ Can be integrated with circuit elements ■ Ability to have polarization diversity (can easily be designed to have vertical, horizontal, right-hand circular (RHCP), or left-hand circular (LHCP) polarizations) ■ Lightweight and inexpensive Disadvantages: ■ Narrow bandwidth (typically less than 5%), requiring bandwidthwidening techniques ■ Can handle low RF power ■ Large ohmic loss The most common microstrip antenna is a rectangular patch. The rectangular patch antenna is approximately a one-half wavelength long section of rectangular microstrip transmission line. When air is the antenna substrate, the length of the rectangular microstrip antenna is approximately one-half of a free-space wavelength. If the antenna is loaded with a dielectric as its substrate, the length of the antenna decreases as the relative dielectric constant of the substrate increases. The resonant length of the antenna is slightly shorter because of the extended electric fringing fields, which increase the antenna’s electrical length slightly. The dielectric loading of a microstrip antenna affects both its radiation pattern and impedance bandwidth. As the dielectric constant of the substrate increases, the antenna bandwidth decreases. This increases the antenna’s Q factor and, therefore, decreases the impedance bandwidth. Feeding Methods: ■ Coaxial probe feeding ■ Microstrip transmission line ■ Recessed microstrip line ■ Aperture coupling feed10–11 ■ Proximity-coupled microstrip line feed (no direct contact between the feed and the patch12)
Fundamentals of Antennas 17 Bandwidth can be increased using the following techniques: Using thick and low permittivity substrates Introducing closely spaced parasitic patches on the same layer of the fed patch(15%BW) Using a stacked parasitic patch(multilayer,BW reaches 20%) Introducing a U-shaped slot in the patch(to achieve 30%BW)13 Aperture coupling(10%BW,high backlobe radiation)10-11 Aperture-coupled stacked patches(40-50%BW achievable)14 L-probe coupling The size of the patch antenna can be reduced by using the following techniques: Using materials with high dielectric constants ■Using shorting walls ■Using shorting pins To obtain a small size wide-bandwidth antenna,these techniques can be combined. 1.2.2 Suspended Plate Antennas Asuspended plate antenna(SPAs)is defined as a thin metallic conductor bonded to a thin grounded dielectric substrate,as shown in Figure 1.8. Suspended plate antennas have thicknesses ranging from 0.03to0.12 is the wa velength corresp g to the minimur well-matched impedance bandwidth)and a low relatived quency of the stant of about 1.SPAs have a broad impedance bandwidth and unique radiation performance. The use of thick dielectric substrates is a simple and effective method to enhance the impedance bandwidth of a microstrip patch antenna by reduc- ing its unloaded Q-factor.As the im edance band surface wave lose which reduc idth increa s,however sradiation efficiency.To sup press the surface waves a low permittivity of the substrate is required. Advantages. ■Easy to fabricate ■Not expensive Large bandwidth ■No surface waves
Fundamentals of Antennas 17 Bandwidth can be increased using the following techniques: ■ Using thick and low permittivity substrates ■ Introducing closely spaced parasitic patches on the same layer of the fed patch (15% BW) ■ Using a stacked parasitic patch (multilayer, BW reaches 20%) ■ Introducing a U-shaped slot in the patch (to achieve 30% BW)13 ■ Aperture coupling (10% BW, high backlobe radiation)10–11 ■ Aperture-coupled stacked patches (40–50% BW achievable)14 ■ L-probe coupling15 The size of the patch antenna can be reduced by using the following techniques: ■ Using materials with high dielectric constants ■ Using shorting walls ■ Using shorting pins16 To obtain a small size wide-bandwidth antenna, these techniques can be combined. 1.2.2 Suspended Plate Antennas A suspended plate antenna (SPAs) is defined as a thin metallic conductor bonded to a thin grounded dielectric substrate, as shown in Figure 1.8. Suspended plate antennas have thicknesses ranging from 0.03 l1 to 0.12 l1 (ll is the wavelength corresponding to the minimum frequency of the well-matched impedance bandwidth) and a low relative dielectric constant of about 1. SPAs have a broad impedance bandwidth and unique radiation performance.17 The use of thick dielectric substrates is a simple and effective method to enhance the impedance bandwidth of a microstrip patch antenna by reducing its unloaded Q-factor. As the impedance bandwidth increases, however, surface wave losses also increase, which reduces radiation efficiency. To suppress the surface waves a low permittivity of the substrate is required. Advantages: ■ Easy to fabricate ■ Not expensive ■ Large bandwidth ■ No surface waves
18 Chapter One Side view Patch Patch Ground plane Feedina probe Ground plane 1Suspended plate antenna patch fed by a Lhaped Disadvantages: High cross polarization Thick Antenna compared to conventional microstrip antenna Feeding Methods: Coaxial probe (poor matching BW is only 8%) Bandwidth can be improved using these techniques:17 Long U-shaped slot,cut symmetrically from the plate (BW is 10%-40%) L-shaped probe(BW reaches 36%)15 T-shaped probe(BW reaches 36%) A half-wavelength feeding strip A center-fed SPA with a symmetrical shorting pin Stacked suspended plate antenna20 1.2.3 Planer Inverted-L/F Antennas A planar inverted-L/F antenna is an improved version of the monopole antenna.The straight wire monopole is the antenna with the most basic form.Its dominant resonance appears at around one-quarter of the oper- ating wavelength.The height of has restricted their application to instances where a low-profile design is
18 Chapter One Disadvantages: ■ High cross polarization ■ Thick Antenna compared to conventional microstrip antenna Feeding Methods: ■ Coaxial probe (poor matching BW is only 8%) Bandwidth can be improved using these techniques:17 ■ A dual probe-feeding arrangement consisting of a feed probe and a capacitive load18 ■ Long U-shaped slot, cut symmetrically from the plate (BW is 10%–40%) ■ L-shaped probe (BW reaches 36%) 19 ■ T-shaped probe (BW reaches 36%) ■ A half-wavelength feeding strip ■ A center-fed SPA with a symmetrical shorting pin ■ Stacked suspended plate antenna20 1.2.3 Planer Inverted-L/F Antennas A planar inverted-L/F antenna is an improved version of the monopole antenna. The straight wire monopole is the antenna with the most basic form. Its dominant resonance appears at around one-quarter of the operating wavelength. The height of quarter-wavelength has restricted their application to instances where a low-profile design is necessary. 17 W L Ground plane Patch Feeding probe Ground plane Patch h Side view Figure 1.8 Suspended plate antenna patch fed by an L-shaped probe
Fundamentals of Antennas 19 Figure 1.9 shows the geometry of a narrow-strip monopole with a horizontal bent portion,and the planer inverted-F antenna(PIFA)is shown in Figure 1.10. APIFAcan be considered a kind of linear inverted-Fantenna(IFA) with the wire radiator element replaced by a plate to expand band- width. Advantages: ■Reduced height Reduced backward radiation Moderate to high gain in both vertical and horizontal polarizations Disadvantages: Narrow bandwidth The shorting post near the feeding probe of the PIFA antennas is a good method for reducing the antenna size,but this results in the narrow impedance bandwidth. (a) (b) aaniCectenota2t6Tmoopemonopol Figure 1.10 Geometry of the PIFA
Fundamentals of Antennas 19 Figure 1.9 shows the geometry of a narrow-strip monopole with a horizontal bent portion, and the planer inverted-F antenna (PIFA) is shown in Figure 1.10. A PIFA can be considered a kind of linear inverted-F antenna (IFA), with the wire radiator element replaced by a plate to expand bandwidth. Advantages: ■ Reduced height ■ Reduced backward radiation ■ Moderate to high gain in both vertical and horizontal polarizations Disadvantages: ■ Narrow bandwidth The shorting post near the feeding probe of the PIFA antennas is a good method for reducing the antenna size, but this results in the narrow impedance bandwidth. (a) (b) h h w w Figure 1.9 Geometry of a (a) narrow-strip monopole with a horizontal bent portion (b) monopole Figure 1.10 Geometry of the PIFA l 1 l 2 w h
20 Chapter One Bandwidth can be improved using these techniques: Using thick air substrate Using parasitic resonators with resonant lengths close to the resonant frequency Using stacked elements22 Varying the size of the ground plane23 The size of the PIFA can be reduced using these techniques: Using an additional shorting pin24 Loading a dielectric material with high permittivity Capacitive loading of the antenna structure2 Using slots on the patch to increase the antenna's electrical length27 1.2.4 Planer Dipoles/Monopoles Dipoles and monopoles are the most widely used antennas.The mono- pole is a straight wire vertically installed above a ground plane;it is ndhas ndirctionl radian in the horizontal mpedance bandwidth planer elements can be used to replace the wire elements. Planer designs with different radiator shapes have been widely used in which the bandwidth reaches 70%.These shapes include Circular (BW from 2.25-17.25 GHz) ■Triangular Elliptical (BW from 1.17-12 GHz) Rectangular (BW of 53%) ■Ring ·Trapezoidal(80%BW) Roll monopoles(more than 70%BW) The square planar monopole with trident-shaped feeding strip(shown in Figure 1.11)was introduced with a bandwidth of about 10 GHz (about 1.4-11.4 GHz).This bandwidth is three times the bandwidth obtained using a simple feeding strip. A compact wideband cross-plate monopole antenna (shown in Figure 1.12)has been proposed.This antenna has a cross-sectional area only 25%that of a corresponding planar cross-plate monopole antenna and can generate omnidirectional or near omnidirectional
20 Chapter One Bandwidth can be improved using these techniques: ■ Using thick air substrate ■ Using parasitic resonators with resonant lengths close to the resonant frequency21 ■ Using stacked elements22 ■ Varying the size of the ground plane23 The size of the PIFA can be reduced using these techniques: ■ Using an additional shorting pin24 ■ Loading a dielectric material with high permittivity25 ■ Capacitive loading of the antenna structure26 ■ Using slots on the patch to increase the antenna’s electrical length27 1.2.4 Planer Dipoles/Monopoles Dipoles and monopoles are the most widely used antennas. The monopole is a straight wire vertically installed above a ground plane; it is vertically polarized and has an omnidirectional radian in the horizontal plane. To increase the impedance bandwidth of the monopole antenna, planer elements can be used to replace the wire elements.28 Planer designs with different radiator shapes have been widely used in which the bandwidth reaches 70%. These shapes include17 ■ Circular (BW from 2.25–17.25 GHz) ■ Triangular ■ Elliptical (BW from 1.17–12 GHz) ■ Rectangular (BW of 53%) ■ Ring ■ Trapezoidal (80% BW) ■ Roll monopoles (more than 70% BW) The square planar monopole with trident-shaped feeding strip (shown in Figure 1.11) was introduced29 with a bandwidth of about 10 GHz (about 1.4–11.4 GHz). This bandwidth is three times the bandwidth obtained using a simple feeding strip. A compact wideband cross-plate monopole antenna (shown in Figure 1.12) has been proposed.30 This antenna has a cross-sectional area only 25% that of a corresponding planar cross-plate monopole antenna and can generate omnidirectional or near omnidirectional