CHAPTER 2 Basics of Microstrip Antennas
2.2 Basic Microstrip Antennas
2.2.3 Slot Antennas
By embedding suitable slots in the radiation patch of a microstrip antenna, enhanced bandwidth with a reduced antenna size can be obtained. This can be accomplished by increasing the number of resonances of single resonant antennas such as a half-wavelength slot. A example is illustrated in Figure 2.6. It is found that, by embedding a pair of branchlike slots of proper dimensions, the first two broadside-radiations modes TM and 10 TM of the triangular microstrip antenna can 20 be perturbed such that their resonant frequencies are lowered and close to each other to
(a)
Chapter 2 Basics of Microstrip Antennas
form a wide impedance bandwidth [11].
Figure 2.6 Geometry of the slot antenna.
2.3 Architectures of Feed
There are many configurations that can be used to feed microstrip antennas. The four most popular architectures of feed are microstrip line feed, probe feed, aperture coupling feed, and coplanar waveguide feed [10].
2.3.1 Microstrip Line Feed
The microstrip line feed network is illustrated in Figure 2.7 [10]. The microstrip line feed is easy to fabricate, simple to match by controlling the inset position and rather simple to model. However, as the substrate thickness increases, surface wave and spurious feed radiation increase, which for practical designs limit the bandwidth.
Chapter 2 Basics of Microstrip Antennas
Figure 2.7 Geometry of the microstrip line feed network.
2.3.2 Probe Feed
The probe feed is illustrated in Figure 2.8 [10]. Probe feed, where the inner conductor of the coax is attached to the radiation patch while the outer conductor is connected to the ground plane, are also widely used. The coaxial probe feed is also easy to fabricate and match, and it has low spurious radiation, However, it is also has narrow bandwidth and it is difficult to model, especially for thick substrates (h>0.02λ0).
Both the microstrip line and the probe feed possess inherent asymmetries which generate higher order modes which produce cross-polarized radiation. To overcome these problems, no contacting aperture coupling feed is used [10].
Chapter 2 Basics of Microstrip Antennas
Figure 2.8 Geometry of the probe feed network.
2.3.3 Aperture Coupling Feed
The aperture coupling feed is illustrated in Figure 2.9 [10]. The aperture coupling feed is the most difficult of all feeding methods to fabricate and it also has narrow bandwidth. However, it is somewhat easier to model and has moderate spurious radiation. The aperture coupling feed consists of two substrates separated by ground plane. On the bottom side of the lower substrate there is a microstrip feed line whose energy is coupled to the patch through a slot on the ground plane separating the two substrates. This arrangement allows independent optimization of the feed mechanism and the radiating element. Typically a high dielectric material is used for the bottom substrate. The ground plane between the substrates also isolates the feed from the radiating element and minimizes interference of spurious radiation for pattern formation and polarization purity. Typically matching is performed by controlling the width of the feed line and the length of the slot.
Chapter 2 Basics of Microstrip Antennas
Figure 2.9 Geometry of the aperture coupling feed network.
2.3.4 Coplanar Waveguide Feed
The coplanar waveguide feed is illustrated in Figure 2.10 [12]. The coplanar waveguide feed is widely used in industry and investigated in academia for a long time.
It is an important transmission line. The advantages of coplanar waveguide feed are low radiation loss, wide bandwidth, easy to integrate to MIC or MMIC, and simple to tune the characteristic impedance by changing the ratio of the slit width and the microstrip line width. In order to the coplanar architecture, it is not necessary to dig holes for parallel or series to active elements.
Chapter 2 Basics of Microstrip Antennas
(a)
(b)
Figure2.10 Geometry of the coplanar waveguide feed network
(a)Coplanar waveguide (b)Ground backed coplanar waveguide.
Chapter 3 Miniaturization Techniques of Microstrip Antennas
Chapter 3 Miniaturization Techniques of Microstrip Antennas
Compact microstrip antennas have recently received much attention and many techniques have been reported to reduce the size of microstrip antennas at a fixed operating frequency such as [11]-[14].The popular techniques include patch-meandering technique, PIFA technique, and using magneto-dielectric materials. They are described as followings:
3.1. Patch-meandering Technique
This kind of patch-meandering technique is achieved mainly by creating several meandering slits at the non-radiating edges of a rectangular patch or at the boundary of a circular patch [13]. The characteristics of patch-meandering technique are compact and broadband. Compact operation of microstrip antennas can be obtained by meandering the radiating patch. Broadband characteristic can be accomplished by increasing the number of resonances of single resonant antennas such as a
Chapter 3 Miniaturization Techniques of Microstrip Antennas
half-wavelength meandering patch.
In the following, a compact design combining the techniques of patch meandering and shorting-pin loading for a circular microstrip antenna is demonstrated. Figure.3.1 [11] shows the geometry of a short-circuited, meandered circular microstrip antenna.
The circular patch is short-circuited at the edge with a shorting pin, and three narrow slots of the same length A and width w are cut in the patch. The shorting pin makes the circular patch resonate at a much lower frequency compared with a conventional circular patch of the same size, and the narrow slots meander the patch, which increases the effective electrical length of the patch. These two factors effectively reduce the required disk size for an antenna operated at a given frequency. Figure 3.2 [11] shows the measured resonant frequency against slot length A in Figure 3.1. As A/ 2R increases, the resonant frequency decreases to a lower frequency. Thus, it is obviously realized that the patch-meandering technique is a good method for miniaturization of microstrip antennas.
Figure 3.3 [4] shows a broadband interior antenna with a meandering structure.
The broadband frequency response is shown in Figure 3.4 [4] and the current distribution in the resonant frequency of the antenna is shown in Figure 3.5 [4]. From Figure 3.5, it is obviously seen that there are three current paths in this antenna. Thus, the broadband characteristic can be arrived by proper design of the length of each current path. In this case, it is known that the broadband antenna can be designed by a meandering structure.
Chapter 3 Miniaturization Techniques of Microstrip Antennas
Figure 3.1 Geometry of a meandered circular microstrip antenna with a shorting pin.
Figure 3.2 Measured resonant frequency against slot length A in the circular patch in Figure 3.1; R =7.5 mm, d =6.5 mm. s
Chapter 3 Miniaturization Techniques of Microstrip Antennas
Figure 3.3 A broadband interior antenna of planar monopole type in handsets.
Figure 3.4 Simulated and measured return loss of a broadband interior antenna.
Chapter 3 Miniaturization Techniques of Microstrip Antennas
Figure 3.5 Current distribution of a broadband interior antenna.
3.2 PIFA Technique
Effective resonant length of a antenna can be shortened by applying PIFA technique. Figure 3.6 shows a typical design example for a probe-fed shorted patch antenna operated at dual bands of 1.8 and 2.45 GHz. A shorting strip of width 2.5 mm is used for short-circuiting the rectangular patch to the ground plane. The geometry of this structure looks like a planar inverted F. So this structure of antennas is called PIFA (Planar inverted F antenna). Between the rectangular radiating patch and the ground plane is an air substrate of thickness 9.6 mm. The rectangular patch has dimensions of 36 × 16 mm , an L-shaped slit of width 1 mm and total length 40 mm is cut in the 2 rectangular patch for achieving an additional operating band at 2.45 GHz (the industrial, scientific, medical [ISM] band); the lower operating band at 1.8 GHz is mainly controlled by the dimensions of the rectangular patch [11]. The simulation and measurement results are shown in Figure 3.7. [11] IE3D is the simulator. In this example, it can be seen that the longer patch length corresponds to the lower resonant
Chapter 3 Miniaturization Techniques of Microstrip Antennas
frequency is about 4
λ of the lower center frequency. In conventional cases, the
resonant length is about 2
λ of the center frequency. In the same manner, the shorter
patch length corresponds to the higher resonant frequency is about 4
λ of the higher
center frequency. It can be obviously seen that the antenna size is greatly decreased. So the PIFA technique is the very popular design guide for the designs of the mobile handsets. It is widely used in the new thin and small handsets.
From Figure 3.7, the bandwidth of the lower resonant is 302 MHz (1588-1890 MHz). The wide bandwidth is just because the multiple current paths can be selected by a roomy radiating element.
Figure 3.6 Geometry of a probe-fed shorted patch antenna for broadband and dual frequency operations. The dimensions given in the figure are in millimeters.
Chapter 3 Miniaturization Techniques of Microstrip Antennas
Figure 3.7 Measured and simulated return loss of the probe-fed shorted patch antenna shown in Figure 3.3 with a ground-plane size of 18 × 80 mm . 2
3.3 Using Magneto-dielectric Materials
The magneto-dielectric material is utilized for broadening the bandwidth and minimizing the size of antenna. Figure 3.8 [14] shows the design of meander line antenna that is developed to get broadband characteristic with small size. This is done by accompanied with using magnetic material as the dielectric substrate. [14] In this design, the changing factors are permeability and permittivity. The Figure 3.9 [14]
shows the return loss against permeability of pure magnetic antenna. It can be seen that the resonant frequency will be changed by changing the permeability. It is not only used for reducing the sizes, but also for adjusting resonant frequency. In the same manner, The Figure 3.10 [14] shows the return loss of broadband antenna against dielectric constant. The resonant frequency will be changed by different permittivity. It also can
Chapter 3 Miniaturization Techniques of Microstrip Antennas
be used to reduce sizes and adjust resonant frequency. This design is also a broadband structure. So the magneto-dielectric material is used to achieve broadband and miniaturization characteristics.
Figure 3.8 Structure of the meander line antenna using magneto-dielectric material.
Chapter 3 Miniaturization Techniques of Microstrip Antennas
Figure 3.9 Return loss against permeability of pure magnetic antenna (Antenna parameters: L1 = 0.5 mm, L2 = 1 mm, L3 = 9.5 mm, L4 = 4mm,W1 = 1.8 mm, W2 = 1 mm, W3 = 35 mm, W4 = 50 mm,
μ” = 2, ε” = 2).
Figure 3.10 Return loss of broadband antenna against dielectric constant.
Chapter 3 Miniaturization Techniques of Microstrip Antennas
3.4 Comparison of Three Miniaturization Techniques
3.4.1 Advantages of Three Miniaturization Techniques
In this chapter, three miniaturization techniques are introduced. Patch-meandering technique can be obtained a very small size and broadband bandwidth by proper design.
Its advantages are low profile, small size, low cost, easily fabricated. Comparing with conventional structures, effective resonant length can be reduced nearly a half by PIFA technique. Its advantages are low profile, small size, low cost, easily fabricated and it can be also arrived the broadband characteristic by proper design. Different characteristics of antennas will appear by using magneto-dielectric materials. Its advantages are low profile, small size, broad bandwidth, adjustable frequency.
3.4.2 Disadvantages of Three Miniaturization Techniques
The disadvantages of patch-meandering technique are low gain, hard to design.
Because of coupling effect in the adjacent lines, the energy will be storing in the coupling capacitance. The radiation efficiency will decrease to a low level. The disadvantages of PIFA technique are hard to design, low radiation efficiency, low gain.
Because the current flows on PCB, the energy loss and lower radiation efficiency will occur. The disadvantages of using magneto-dielectric materials are more expensive, hard to fabricate, low gain.
Chapter 4 The Proposed Broadband Antenna
Chapter 4 The Proposed Broadband Antenna
The proposed antenna is suitable multi-band operations covering the bands of GPS (1575.42 MHz), GSM (1710–1880 MHz), PCS (1850–1990 MHz), 3G (1920–2170 MHz), WLAN (2400–2484 MHz), and WiMAX (2500-2690 MHz). The antenna is a planar antenna on a substrate, which is small enough to be installed in mobile handsets.
Chapter 4 The Proposed Broadband Antenna
4.1 Design Concept of the Proposed Antenna
Figure 4.1 The purpose of each design step.
The design purpose of each major step is shown in Figure 4.1. In step one, the main work is to determine the rough dimension of the antenna according to the required center frequency. In step two, the broadband characteristic was created. In step three, the work is to match the desired frequency band by adjusting the geometrical parameters of the antenna. In step four, coupling effects between the ground plane and the antenna was discussed. In all steps, FR4 with thickness of 0.8 mm is used for computing by HFSS. Width of the feed line is 1.5 mm.
Chapter 4 The Proposed Broadband Antenna
Step One : Determine the rough dimension of the antenna according to the required center frequency
The resonant (center) frequency of the simple planar rectangular antenna shown in Figure 4.2 is given by [16]
respectively, as shown in Figure.4.2. The wanted center frequency will be achieved by properly tuning Lr and Wr. The computed results using Eq. (4.1) and HFSS are compared in Table 4.1 (Up to 4GHz) for different combinations of Lr and Wr. Here, W = 50 mm, L = 45 mm and G = 1.6 mm are fixed for simplification. The comparison shows that Eq. (4.1) at least determines one of resonant frequencies with reasonable accuracy. Therefore, Eq. (4.1) is proposed as a rough model for determining the resonant frequency of the simple patch antenna. Sizes of Lr and Wr will affect the bandwidth of the planar rectangular antenna. Figure 4.3 (a), (b) and (c) show its return loss versus frequency for Wr= 25mm, 35 mm and 45 mm, respectively. In each figure, it is found that the bandwidth is increased withLr. It is also found that when Wr=25 mm, more than one resonant frequency may be created. In Table 4.1, the one of specific size of Lr and Wr, which are underlined, represents its resonant frequencies in the range of 1.7 - 1.8 GHz to fulfill the desired bands.
Chapter 4 The Proposed Broadband Antenna
Figure 4.2 The geometry of the simple planar rectangular antenna.
1.0 1.5 2.0 2.5 3.0 3.5 4.0
Chapter 4 The Proposed Broadband Antenna
Figure 4.3 The computed return loss versus frequency of the simple planar rectangular antennas (a) Wr =25 mm (b) Wr =35 mm (c) Wr =45 mm.
Chapter 4 The Proposed Broadband Antenna
Table 4.1 Resonant frequency comparison using Eq. (4.1) and HFSS.
Computed resonant frequency
Chapter 4 The Proposed Broadband Antenna
Step Two : Broaden the bandwidth
In step two, the chosen antennas (the underlined one) shown in Table 4.1, are shaped into the one as shown in Figure 4.4, which is called the planar binomial antenna [17]. The binomial antenna shows ultra-wideband characteristics. The formula of the binomial curve is shown below [17].
4 1 1 simplification. The wanted bandwidth will be achieved by properly tuning Wb1 and
1
Lb . From the computed return loss versus frequency as shown in Figure 4.5, it is found that the bandwidth is increased with Wb1. From the computed covering frequency band using HFSS listed in Table 4.2 (Up to 4GHz) for different sizes of Wb1 and Lb1, it is found that the operational frequency range is increased with Wb1. The low bound of the operational frequency range becomes smaller when Lb1 increases. By comparing between Figure 4.3 and Figure 4.5, it is found that a binomial curve structure provides with a wider bandwidth than a simple rectangular one.
Chapter 4 The Proposed Broadband Antenna
Figure 4.4 The geometry of the planar binomial antenna.
1.0 1.5 2.0 2.5 3.0 3.5 4.0
Figure 4.5 The computed return loss versus frequency of the planar binomial antennas with Lb1=25 mm
.
Chapter 4 The Proposed Broadband Antenna
Table 4.2 Computed bandwidths of the planar binomial antennas.
Chapter 4 The Proposed Broadband Antenna
Step Three : Shaping the antenna
In step three, a planar binomial antenna is shaped into the one as shown in Figure 4.6, which is called the planar hybrid-binomial antenna. Current mainly flow in edges of an antenna. Frequency response is changed severely by modifying a shape of edges.
Therefore, some edges of the selected planar binomial antenna are selected to cut off.
Another binomial formula is selected to cut the end edge of the selected planar binomial antenna in order to sustain the broadband characteristic. The formula is shown below [17] respectively, as shown in Figure.4.6. W = 50 mm and L = 45 mm are fixed in this step.
G = 1.6 mm and Wb2 =25 mm are fixed for simplification. The wanted bandwidth will be achieved by properly tuning L . From the computed return loss versus frequency b2 shown in Figure 4.7, it is found that the bandwidth is decreased with larger L . As b2
2
L is varied from 5 to 20 mm, the lower bound of the covering frequency band will b
shift to a higher value. The components of high frequencies will also cut off. Although a stop frequency band is created in high frequencies, the lower bound of the covering frequency band shift to a higher value. This is not my desired result. So we make a second adjustment.
Chapter 4 The Proposed Broadband Antenna
Figure 4.6 The geometry of the planar hybrid-binomial antenna.
1.0 1.5 2.0 2.5 3.0 3.5 4.0
Figure 4.7 The computed return loss versus frequency of the first adjustment,
2 25
Chapter 4 The Proposed Broadband Antenna
In the second adjustment, we would like to cut off frequency response higher than 2.9 GHz by cutting some parts of a planar binomial antenna and also compensate lower frequency response of the one. The geometry of the modified antenna is shown in Figure 4.8 which is called the planar eagle-shaped antenna. Here, W and c L are the c width and length of the cutting area, respectively, as shown in Figure.4.8. G = 1.6 mm are fixed for simplification. The wanted bandwidth will be achieved by properly tuning W and c L . From the computed return loss versus frequency shown in Figure 4.9, it is c found that the bandwidth is decreased with larger L . The resonant frequency will also c shift to a lower value with larger W . From Figure 4.9, we find that there are two c purposes by tuning W and c L . One is to compensate the components of lower c frequencies; another is to cut off the components of higher frequencies. The computed current distribution of the unmodified antenna in f = 1.575 GHz is shown in Figure 4.10.
From Figure4.10, it is found that the current mainly flows along edges of the antenna and the strength of the current magnitude is stronger near the feed point. The computed current distribution of the modified antenna in f = 1575 MHz is shown in Figure 4.11.
From Figure 4.11, it is found that the current flows along the cutting area. The miniaturization is achieved due to increasing resonant length formed by the cutting area.
Chapter 4 The Proposed Broadband Antenna
Figure 4.8 The geometry of the planar eagle-shaped antenna.
1.0 1.5 2.0 2.5 3.0 3.5 4.0
Chapter 4 The Proposed Broadband Antenna
Figure 4.9 The computed return loss versus frequency of the second adjustment (a) Wc =2 mm(b) Wc =6 mm.
Figure 4.10 The computed current distribution of the unmodified antenna;
Chapter 4 The Proposed Broadband Antenna
Figure 4.11 The computed current distribution of the modified antenna;
f=1.575GHz, phase=320 degree.
G is a gap between the ground plane and the antenna as shown in Figure.4.9. In the following discussion, the influence on frequency response will be studied by properly tuning G.Wc =4mm and Lc =6 mm are fixed in this discussion. From the computed return loss versus frequency shown in Figure 4.12, it is found that the resonant frequency will shift to a lower value with larger G. It is simply because that there is a coupling effect between the ground plane and the antenna. The coupling effect is created by two currents flowing on the ground plane and the antenna.
Chapter 4 The Proposed Broadband Antenna
Figure 4.12 The computed return loss versus frequency of the planar eagle-shaped antenna by varying G.
4.2 Comparison of the Computed and Measured Results
After some numerical computations, size of the proposed antenna has been fine tuned and determined as shown in Figure 4.13. The proposed antenna was fabricated using FR4 substrate material. The radiation element is copper. A photograph of the proposed antenna is shown in Figure 4.14. Agilent 8719ET is used for measuring the return loss of the proposed antenna. In Figure 4.15, it is found that the computed and measured return losses versus frequency are close. The computed operating frequency range is 1.49 GHz to 2.78 GHz and the measured operating frequency range is 1.54 GHz to 2.73 GHz. The computed and measured bandwidths are 1.29 GHz and 1.19 GHz,
After some numerical computations, size of the proposed antenna has been fine tuned and determined as shown in Figure 4.13. The proposed antenna was fabricated using FR4 substrate material. The radiation element is copper. A photograph of the proposed antenna is shown in Figure 4.14. Agilent 8719ET is used for measuring the return loss of the proposed antenna. In Figure 4.15, it is found that the computed and measured return losses versus frequency are close. The computed operating frequency range is 1.49 GHz to 2.78 GHz and the measured operating frequency range is 1.54 GHz to 2.73 GHz. The computed and measured bandwidths are 1.29 GHz and 1.19 GHz,