Chapter 3
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CHAPTER‐3
Chapter3 Designing theory of multiband antenna
3. Introduction In this work, an antenna for mobile phone is designed. This design is based on Planar Inverted‐F antenna (PIFA). For this purpose, different shape antennas are studied and designed. Then a new design has been formed. Introduction of Planar Inverted Antennas has been covered in chapter‐1. Following section will describe the derivation of PIFA, different techniques for size reduction and bandwidth enhancement.
3.1
The Derivation of the PIFA [35] The inverted‐F (IFA) family of antennas presents a popular alternative for
low profile omni‐directional applications. The basis for the planar inverted‐F antenna originated from the inverted‐L antenna (ILA) (Figure 3.1). The inverted‐L antenna is an end‐feed short monopole with a horizontal wire element placed on top that acts as a capacitive load. This antenna is essentially a microstrip coaxial connection with the inner conductor in free space and bent at a right angle in such a way that the inner conductor end forms a horizontal element that is parallel with the ground plane. The outer conductor is extended to form the ground plane. The resulting structure possesses a short monopole element as a vertical element and a wire horizontal element, which is attached to the end of the monopole.
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Figure 3.1 The InvertedL Antenna. The inverted‐L antenna is an attractive alternative because of its simple layout. The design is uncomplicated and can be easily manufactured with low cost materials. Additionally, many of the electrical characteristics of the inverted‐L are similar to those of the well understood short monopole. The radiation pattern of the inverted‐L is nearly identical to that of the short monopole. A z‐directed short monopole produces a pattern that is omni‐ directional and maximum in the azimuth plane and has nulls at θ = 0°, 180° (along the z‐axis). The inverted‐L oriented with its radiating element in the z‐direction will also have a pattern that is omni‐directional in the azimuth plane. The inverted‐ L however has an additional E‐component due to the horizontal arm. The non‐zero currents along the horizontal arm cause the radiation pattern in the azimuth plane to deviate slightly from omni‐directional. The input impedance of the inverted‐L is similar to that of the short monopole: low resistance (RILA) and high reactance (XILA). Low order approximations for the input resistance and reactance of the inverted‐L are [22]:
⎛ 2π RILA = 40 ⎜ ⎝ λ
X ILA
⎞ h⎟ ⎠
2
2
h ⎞ a ⎛ ⎜1 − ⎟ T = 1 − (3.1) h ⎝ h+L⎠
h ⎞⎡ ⎛ T 3h ⎤ h −60h ⎜ 2 − La − LaT − ⎥ ⎟⎢ 3h 1 20a h+L⎠ ⎢ ⎝ 3 4 ⎥ (3.2) 4 + = − − + log 2 a 3 9h h2 L2 a + 9h 2 + 4 ⎥ (h + L) k ⎢ 2 L + ⎢ ⎥ a 4 ⎣ ⎦ ‐ 22 ‐
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Where, L is the length of the horizontal loading section, h is the height of the radiating section, a is the radius of the antenna, k = number, La = L + a and T = 1 −
2π
λ
is the wave
a . h
Complete closed form derivations for both the input resistance and reactance can be found in [22]. Equations (3.1) and (3.2) are plotted in Figure 3.2. As can be seen from Figure 3.2, the relatively low resistance and high reactance for useful antenna dimensions make the inverted‐L difficult to impedance match to typical feed lines.
Figure 3.2 InvertedL antenna input resistance and reactance from eqn. (3.1) and (3.2) [35]. As the height of the vertical element is restricted to only a fraction of a wavelength, the ILA has a very low profile structure. The horizontal element is normally set to a quarter of a wavelength. As a result, the ILA has low impedance with a magnitude similar to a short monopole. In order to increase the impedance ‐ 23 ‐
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of the ILA, another inverted‐L element must be incorporated into the original antenna, with one end of the inverted‐L element connected to the bend of the original ILA and the other end shorted to the ground plane. The new antenna now appears to have an inverted‐F configuration: hence the name inverted‐F antenna (IFA). The inverted‐F is a variation on the inverted‐L that modifies the input impedance to be nearly resistive and thus provides reduced mismatch loss. The inverted‐F antenna is known as a “shunt‐driven inverted‐L antenna‐transmission line with an open end".
Figure 3.3 The InvertedF Antenna [35] This additional inverted‐L segment adds a convenient tuning option to the original inverted‐L antenna and greatly increases the antenna usability. The location of the feed point, S, along the length of the upper element provides the impedance tuning mechanism. The input impedance behavior of the inverted‐F antenna is similar to that of a transmission line antenna of length (H +L) with its feed located at the tap point, S. This feature of the IFA allows any additional matching circuitry of the ILA to be discarded, resulting in the IFA being a more practical antenna. Despite its relatively simple design, the design of an optimal IFA is not unique. Variations in the height of the radiator, the length of the horizontal element as well as the tap point all impact the electrical performance characteristics of the IFA. ‐ 24 ‐
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The low profile of the IFA, as well as its performance with two polarizations facilitates the antenna for urban environmental use, especially in conjunction with mobile applications. One major drawback of the IFA is the lack of bandwidth. This issue can be resolved by converting the horizontal wire element into a plate. The resulting antenna, shown in Figure 3.4, is referred to as the planar inverted‐F antenna (PIFA).
Figure 3.4 The Planar InvertedF antenna (PIFA) [35]. The compact size of the PIFA makes it a suitable candidate. This unobtrusive design makes it ideally suited for mobile and handheld situations and complies with our low profile design goal. Accidental damage to the antenna via unintentional contact with other objects is avoided. The size and aspect ratio of the top radiating plate, the height of the plate above the ground plane, the size and position of the ground strap and the feed point location all have considerable impact on the electrical performance of the antenna. Additionally, the PIFA offers very high radiation efficiency and sufficient bandwidth in a compact antenna. A bandwidth of 10% can be realized with the PIFA. The design variables for this antenna are the height, width, and length of the top plate, the width and location of shorting plate, and the location of the feed wire. ‐ 25 ‐
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3.1.1 PIFA Dimensions The size of the PIFA radiating top plate can be calculated approximately using [35]:
λcenter = 4( L + W ) (3.3) Where, L and W are the length and width of the plate, respectively. The resonant frequency is also influenced by the aspect ratio of the top plate (W/L). The width of the grounding strap, S, in relation to the width of the radiating top plate is also particularly important in determining the radiating behavior of the PIFA. Figure 3.5, shows how the current flow on the surface of the top plate varies, with different top plate and grounding strap configurations.
Figure 3.5 Surface current on PIFA top plate for various top plate aspect ratios and grounding strip widths [35].
In general, a greater top plate aspect ratio will result in a lower the resonant frequency for a given grounding strap width. For an aspect ratio of W/L > 1, there is an inflection point in the resonant frequency when W‐S = L and the resonant frequency begins to increase with increasing aspect ratio. As seen in ‐ 26 ‐
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Figure 3.5, the current on the planar element generally flows to the open‐circuit edge on the long side of the top plate when W‐ S < L. When W‐S > L, the current flows to the open circuit edge along the short side of the top plate. The inflection point in the resonant frequency is attributed to this change in current flow. The inflection point can be seen clearly in the case of W = L and where S << W, the current flows almost equally along the both the W and L dimensions. Like the resonant frequency of the PIFA, the relative impedance bandwidth is affected by the design of the structure. The height of the radiating top plate, H , and the width of the grounding strap, have the greatest influence on the bandwidth of the PIFA. In general, the bandwidth increases with increasing top plate height. However, as the height of the top plate approaches the magnitude of L = W, the height begins to influence the resonant frequency. When the grounding strap width is very small, S << W, the resonant frequency is given by:
λcenter = 4( L + W + H ) (3.4) The width of the grounding strap similarly affects the bandwidth. The limiting case, where the grounding strap is the same width as the top plate, the bandwidth of the PIFA is greatest. This case corresponds to the operation of a short circuited microstrip antenna. The PIFA with a W/L = 2.0 and H/λ0 = 0.053 has a relative bandwidth of 10%. As the width of the grounding strap decreases, the relative bandwidth of the PIFA decreases. The bandwidth of a PIFA with the grounding strap width much less than the width of the top plate (S/W≤0.1) can be reduced to below 1%. There are several procedures available for designing PIFAs and many different PIFA layouts may satisfy the same design criteria.
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3.2 Some Points to be Considered, while Designing Mobile Phone Antenna: 1. Antenna size should be small 2. Operating bandwidth should be wider 3. Radiation efficiency should be higher 4. Less power consumption Many of the techniques are used to improve the performance of the antenna. Some of these techniques are for size reduction, some for bandwidth enhancement, and some for improving efficiency and power consumption. At a time, the one technique used to improve one constrain, can degrade the other one. So we have to take a good trade off between the various constrains for better performance. This work is focused to form geometry of reduced size and the wider bandwidth, with less power consumption. The techniques used for size reduction and bandwidth enhancement will be discussed in the following section.
3.3 Antenna Size Reduction Techniques [1]: The miniaturization of the microstrip antenna can be achieved using several approaches:
3.3.1 Employing
a
Dielectric
Material
of
Higher
Permittivity From equation (1.1), it is found that the radiating patch of the microstrip antenna has a resonant length approximately proportional to
1
εr
, and the use of a
dielectric substrate with a larger permittivity thus can result in a smaller physical antenna length at a fixed operating frequency. But higher ε r increases the surface wave effect which leads poor performance. i.e., low radiation efficiency and lower bandwidth. ‐ 28 ‐
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The high dielectric constant of the substrate is not very appropriate for our purpose. Therefore, the method based on capacitive loading, and the method that involves slots for longer current path along the patch edges have been chosen for bandwidth improvement.
3.3.2 Capacitive Loading of the Patch Structure (Radius of shorting pin> radius of feed probe) Another technique for size reduction is the use of an edge‐shorted patch (Reduces the resonant frequency). The shorting pin located close to the feed point results in a significant reduction in overall patch size. The typical RLC circuit used to describe a probe feed patch is loaded with a parallel LC circuit. The shorting pin effect on the input impedance of the antenna can be represented by the LC combination. “When shorting pin is located near the feeding point, a strong capacitive coupling or loading is produced due to which surface electrical current path is increased. This results in cancelling the inductive nature of a patch below resonance. Therefore, the amount of capacitive loading induced determines the ultimate reduction in size”. It makes a microstrip antenna act as a quarter‐ wavelength structure and thus can reduce the antenna’s physical length by half at a fixed operating frequency. When a shorting plate (Figure. 3.6(b)) or a shorting pin (Figure. 3.6(c)) is used instead of a shorting wall (Figure. 3.6(a)), further reductions in both the antenna’s fundamental resonant frequency and size can be obtained. However, with the use a dielectric substrate with larger permittivity and overall size reduction at a fixed operating frequency, the impedance bandwidth of a microstrip antenna is usually decreased.
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(a)
(b)
(c) Figure 3.6 Configurations of a rectangular patch antenna with (a) a shorting wall, (b) a shorting plate or partial shorting wall and (c) a shorting pin [1]. With the shorting pin technique, the antenna size reduction is mainly due to the shifting of the null‐voltage point at the center of the rectangular patch (excited at TM01 mode) and the circular patch (at TM11 mode) to their respective patch edges, which makes the shorted patches resonate at a much lower frequency. So, size is reduced and the reduction in size is limited by the distance between null‐ voltage point in the patch and the patch edge. ‐ 30 ‐
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There is also a significant dependency of resonant frequency on shorting
pin position. Also, in order to get a good matching condition, the feed position is placed closer to the shorting pin. When the feed position is away from the shorting pin, the resonant input resistance quickly increases.
3.3.3 Meandering of Radiating Patch Meandering the excited patch surface current paths, in the antenna’s radiating patch, is also an effective method for achieving a lowered fundamental resonant frequency for the microstrip antenna. For the case of a rectangular radiating patch, the meandering can be achieved by inserting several narrow slits at the patch’s non‐radiating edges. It can be seen in Figure 3‐7 that the excited patch’s surface currents are effectively meandered, leading to a greatly lengthened current path for a fixed patch linear dimension. This behavior results in a greatly lowered antenna fundamental resonant frequency, and thus a large antenna size reduction at a fixed operating frequency can be obtained.
Figure 3.7 Surface current distributions for meandered rectangular microstrip patches with meandering slits [1]. If we use shorting pin and meandering in conjunction with: The shorting pin makes the patch resonate at a much lower frequency and the narrow slots meander the patch, which increases the effective electrical length of patch. These two factors effectively reduce the required size for an antenna operated at a given frequency. With increasing slot length, the resonant frequency ‐ 31 ‐
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of the meandered patch decreases. But slot width has relatively little effect on the resonant frequency. Slots in patch produce additional capacitive reactance, while shorting pin with probe contribute inductive reactance.
3.3.4 Use of Inverted UShaped or Folded Patch Antenna Surface current lengthening for a fixed patch projection area, can also be obtained by using an inverted U‐shaped patch [Figure 3.8(a)], a folded patch [Figure 3.8(b)]. With these microstrip patches, the resonant frequency can be greatly lowered compared to a regular single‐layer microstrip antenna with the same projection area. Note that the resonant frequency is greatly lowered due to the bending of the patch surface current paths along the antenna’s resonant or excitation direction.
(a)
(b) Figure 3.8 Geometries of stacked shorted patch antennas (a) Ushaped patch (b) folded patch [1].
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3.3.5 Use of Lumped Elements Size of antenna can be reduced by using lumped elements in the antenna structure. These change the current distribution on the patch and can lower the resonant frequency.
Figure 3.9 Geometry of a compact broadband microstrip antenna with chip resistor loading [1].
3.4 Bandwidth Enhancement Techniques [1]: The bandwidth enhancement of the microstrip antenna can be achieved by using several approaches. Problem of achieving a wide impedance bandwidth (greater than or about 10% suitable for present day cellular communication system) for a compact microstrip antenna is becoming an important topic in microstrip antenna design. Broadband operation has been achieved with the use of two stacked shorted patches. The main relationships among various parameters having influence on bandwidth are follows [25]:
fu − fl 1 ∞ fr Q
L R C
(3.6)
Bandwidth = Q=
‐ 33 ‐
(3.5)
CHAPTER‐3
Q∞ 1 S
(3.7)
where, fu and fl are upper and lower frequency of bandwidth, fr is resonant frequency, Q is quality factor, R is loss component of antenna, L is inductive component of antenna, C is capacitive component of antenna, S is volume of antenna. The most frequently used method to broaden the bandwidth is to raise the height of the shorting plane i.e.; increase the volume. Bandwidth is affected very much by the size of the ground plane. By varying the size of the ground plane, the bandwidth of a PIFA can be adjusted. For example, reducing the ground plane can effectively broaden the bandwidth of the antenna system. Several slits at the ground plane edges can be inserted to reduce the quality factor of the structure (and to increase the bandwidth). Bandwidth enhancement of a PIFA can also be achieved by several efficient approaches, namely using dual resonance by additional patch that is adding capacitive load, loading dielectric with high permittivity, attaching chip resistor that is increasing loss term. In the following section all these techniques are discussed in detail.
3.4.1 Increasing the Height (h) of Substrate By increasing the height of the substrate, bandwidth can be extended (up‐to about 35%). Impedance bandwidth of PIFA is inversely proportional to the quality factor Q that is defined for a resonator [25]: Q = Energy Stored / Energy dissipated Substrates with high dielectric constant (εr) tend to store energy more than radiate it. This is equivalent by modeling the PIFA as a lossy capacitor with high εr, thus leading to high Q value and obviously reducing the bandwidth. Similarly when the substrate thickness is increased the inverse proportionality of thickness to the ‐ 34 ‐
CHAPTER‐3
capacitance (as C= ε0.Area/h) decreases the energy stored in the PIFA and the Q factor also. In summary, the increase in height and decrease of εr can be used to increase the bandwidth of the PIFA. Figure 3.10 shows the effect of substrate thickness on impedance bandwidth and efficiency for two values of dielectric constants. While the bandwidth increases monotonically with thickness, a decrease in the ε r value increases the bandwidth.
Figure 3.10 Effect of substrate thickness on impedance bandwidth and efficiency, for two dielectric constants [34].
3.4.2 Use of Chip Resistor of Low Resistance (usually on the order of 1Ω resistor is connected between patch & ground plate) As we know that bandwidth is inversely proportional to the quality factor (Q). Moreover, owing to the introduced small ohmic loss of the chip resistor, the quality factor of the microstrip antenna is greatly lowered. So bandwidth is increased.
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Figure 3.11 Geometry of a chipresistorloaded rectangular microstrip antenna with a probe feed [1]. Resistor should be placed at the edge opposite to the feed for maximum resonant frequency reduction. Antenna gain is reduced due to above technique. Bandwidth can be enhanced by considering the effect of the chassis dimension. If chassis resonates at the operating frequency of the antenna element, the bandwidth of the antenna‐chassis combination will improve considerably.
3.4.3 Using two (or more) Stacked Shorted Patches By using two (or more) stacked shorted patches and making both patches radiate at as equally as possible and having a radiation quality factor as low as possible, one can obtain enhanced impedance bandwidth for a fixed antenna volume.
(a) ‐ 36 ‐
CHAPTER‐3
(b) Figure 3.12 Shorting wall (a) Offset shorting wall (b) A common shorting wall [1].
3.5 Some other Parameters, to be Considered in PIFA Designing: 3.5.1 Resonant Frequency The resonant frequency of PIFA can be approximated with [25]: L + W = λ /4
(refer figure 3.4)
When, S/W=1 then W + H = λ /4
(3.8)
When, S=0 then L + W + H = λ /4 The resonant frequency of the PIFA is proportional to the effective length of the current distribution. The introduction of an open slot reduces the frequency, due to the fact that there are currents flowing at the edge of the shaped slot, therefore a capacitive loaded slot reduces the frequency and thus the antenna dimensions drastically. The same principle of making slots in the planar element can be applied for dual frequency operation as well. Changes in the width of the planar element can also affect the determination of the resonant frequency. The width of the short circuit plate of the PIFA plays a very important role in governing its resonant frequency. Resonant frequency decreases with the decrease in short ‐ 37 ‐
CHAPTER‐3
circuit plate width, S. Unlike micro‐strip antennas that are conventionally made of half wavelength dimensions, PIFA’s are made of just quarter wavelength. Analyzing the resonant frequency and the bandwidth characteristics of the antenna can be easily done by determining the site of the feed point, at which the minimum reflection coefficient is to be obtained.
3.5.2 Radiation Pattern The radiation pattern of the PIFA is the relative distribution of radiated power as a function of direction in space. In the usual case the radiation pattern is determined in the far‐field region and is represented as a function of directional coordinates. Radiation properties include power flux density, field strength, phase, and polarization.
3.5.3 Electric Field Distribution The dominant component of the electric field Ez is equal to zero at the short‐circuit plate while the intensity of this field at the opposite edge of the planar element is significantly large. For fields Ex and Ey, there is pointing part, which corresponds to the feed source. This means that the electric lines of force are directed from feed source to the ground plane. When the width of the short circuit plate is narrower than the planar element, the electric fields Ex and Ey start generating at all open circuit edges of the planar element. These fringing fields are the radiating sources in PIFA.
3.5.4 Current Distribution PIFA has very large current flows on the undersurface of the planar element and the ground plane compared to the field on the upper surface of the element. Due to this behavior PIFA is on of the best candidate when is talking about the influence of the external objects that affect the antenna characteristics (e.g. mobile operator’s hand/head). PIFA surface current distribution varies for different ‐ 38 ‐
CHAPTER‐3
widths of short‐circuit plates. The maximum current distribution is close to the short pin and decreases away from it. The ground surface waves can produce spurious radiations or couple energy at discontinuities, leading to distortions in the main pattern, or unwanted loss of power. The surface wave effects can be controlled by the use of photonic band gap structures or simply by choosing air as the dielectric. This solves the limitation of poor efficiency as well along with certain degree of bandwidth enhancement.
3.5.5 Ground Plane Effects It is assumed that a large ground plane is used for antenna designing. A detailed study on the effects of operating the PIFA in a reduced ground plane environment is presented in [20]. The size of the ground plane plays an important role in the behavior of the PIFA. The resonant frequency, input impedance, bandwidth and gain are all impacted when the PIFA is operated over a finite size ground plane. The resonant frequency for a PIFA with a fixed top element and grounding strap size tends to remain fairly constant for large ground plane sizes. As the ground plane size is reduced, the value of the resonant frequency oscillates around the value for the infinite ground plane case until the ground plane size reaches about 0.2λ in length. At that point, the resonant frequency is highly dependent on the size of the ground plane and it increases linearly with decreasing ground plane size. The relative bandwidth of the PIFA increases with increasing ground plane size. As in the center frequency case, the relative bandwidth oscillates around the value of the infinite ground plane case. However, the PIFA relative bandwidth exhibits a stronger dependence on ground plane size than the center frequency does for larger ground planes. It is reported in [20] that a ground plane size of at least 0.8 λ is required to achieve the desired 8% impedance bandwidth of the Cellular band. The gain of the PIFA is influenced by the size of the ground plane as well. The gain of the PIFA increases with increasing ground plane size. It then reaches a ‐ 39 ‐
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local maximum around 0.9 λ and begins to oscillate to the infinite ground plane case of nearly 5 dB. To achieve the desired 3 dB in gain for the Cellular band, a ground plane of at least 0.5 λ is required [20, 21].
‐ 40 ‐
Chapter3 Designing theory of multiband antenna
3. Introduction In this work, an antenna for mobile phone is designed. This design is based on Planar Inverted‐F antenna (PIFA). For this purpose, different shape antennas are studied and designed. Then a new design has been formed. Introduction of Planar Inverted Antennas has been covered in chapter‐1. Following section will describe the derivation of PIFA, different techniques for size reduction and bandwidth enhancement.
3.1
The Derivation of the PIFA [35] The inverted‐F (IFA) family of antennas presents a popular alternative for
low profile omni‐directional applications. The basis for the planar inverted‐F antenna originated from the inverted‐L antenna (ILA) (Figure 3.1). The inverted‐L antenna is an end‐feed short monopole with a horizontal wire element placed on top that acts as a capacitive load. This antenna is essentially a microstrip coaxial connection with the inner conductor in free space and bent at a right angle in such a way that the inner conductor end forms a horizontal element that is parallel with the ground plane. The outer conductor is extended to form the ground plane. The resulting structure possesses a short monopole element as a vertical element and a wire horizontal element, which is attached to the end of the monopole.
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Figure 3.1 The InvertedL Antenna. The inverted‐L antenna is an attractive alternative because of its simple layout. The design is uncomplicated and can be easily manufactured with low cost materials. Additionally, many of the electrical characteristics of the inverted‐L are similar to those of the well understood short monopole. The radiation pattern of the inverted‐L is nearly identical to that of the short monopole. A z‐directed short monopole produces a pattern that is omni‐ directional and maximum in the azimuth plane and has nulls at θ = 0°, 180° (along the z‐axis). The inverted‐L oriented with its radiating element in the z‐direction will also have a pattern that is omni‐directional in the azimuth plane. The inverted‐ L however has an additional E‐component due to the horizontal arm. The non‐zero currents along the horizontal arm cause the radiation pattern in the azimuth plane to deviate slightly from omni‐directional. The input impedance of the inverted‐L is similar to that of the short monopole: low resistance (RILA) and high reactance (XILA). Low order approximations for the input resistance and reactance of the inverted‐L are [22]:
⎛ 2π RILA = 40 ⎜ ⎝ λ
X ILA
⎞ h⎟ ⎠
2
2
h ⎞ a ⎛ ⎜1 − ⎟ T = 1 − (3.1) h ⎝ h+L⎠
h ⎞⎡ ⎛ T 3h ⎤ h −60h ⎜ 2 − La − LaT − ⎥ ⎟⎢ 3h 1 20a h+L⎠ ⎢ ⎝ 3 4 ⎥ (3.2) 4 + = − − + log 2 a 3 9h h2 L2 a + 9h 2 + 4 ⎥ (h + L) k ⎢ 2 L + ⎢ ⎥ a 4 ⎣ ⎦ ‐ 22 ‐
CHAPTER‐3
Where, L is the length of the horizontal loading section, h is the height of the radiating section, a is the radius of the antenna, k = number, La = L + a and T = 1 −
2π
λ
is the wave
a . h
Complete closed form derivations for both the input resistance and reactance can be found in [22]. Equations (3.1) and (3.2) are plotted in Figure 3.2. As can be seen from Figure 3.2, the relatively low resistance and high reactance for useful antenna dimensions make the inverted‐L difficult to impedance match to typical feed lines.
Figure 3.2 InvertedL antenna input resistance and reactance from eqn. (3.1) and (3.2) [35]. As the height of the vertical element is restricted to only a fraction of a wavelength, the ILA has a very low profile structure. The horizontal element is normally set to a quarter of a wavelength. As a result, the ILA has low impedance with a magnitude similar to a short monopole. In order to increase the impedance ‐ 23 ‐
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of the ILA, another inverted‐L element must be incorporated into the original antenna, with one end of the inverted‐L element connected to the bend of the original ILA and the other end shorted to the ground plane. The new antenna now appears to have an inverted‐F configuration: hence the name inverted‐F antenna (IFA). The inverted‐F is a variation on the inverted‐L that modifies the input impedance to be nearly resistive and thus provides reduced mismatch loss. The inverted‐F antenna is known as a “shunt‐driven inverted‐L antenna‐transmission line with an open end".
Figure 3.3 The InvertedF Antenna [35] This additional inverted‐L segment adds a convenient tuning option to the original inverted‐L antenna and greatly increases the antenna usability. The location of the feed point, S, along the length of the upper element provides the impedance tuning mechanism. The input impedance behavior of the inverted‐F antenna is similar to that of a transmission line antenna of length (H +L) with its feed located at the tap point, S. This feature of the IFA allows any additional matching circuitry of the ILA to be discarded, resulting in the IFA being a more practical antenna. Despite its relatively simple design, the design of an optimal IFA is not unique. Variations in the height of the radiator, the length of the horizontal element as well as the tap point all impact the electrical performance characteristics of the IFA. ‐ 24 ‐
CHAPTER‐3
The low profile of the IFA, as well as its performance with two polarizations facilitates the antenna for urban environmental use, especially in conjunction with mobile applications. One major drawback of the IFA is the lack of bandwidth. This issue can be resolved by converting the horizontal wire element into a plate. The resulting antenna, shown in Figure 3.4, is referred to as the planar inverted‐F antenna (PIFA).
Figure 3.4 The Planar InvertedF antenna (PIFA) [35]. The compact size of the PIFA makes it a suitable candidate. This unobtrusive design makes it ideally suited for mobile and handheld situations and complies with our low profile design goal. Accidental damage to the antenna via unintentional contact with other objects is avoided. The size and aspect ratio of the top radiating plate, the height of the plate above the ground plane, the size and position of the ground strap and the feed point location all have considerable impact on the electrical performance of the antenna. Additionally, the PIFA offers very high radiation efficiency and sufficient bandwidth in a compact antenna. A bandwidth of 10% can be realized with the PIFA. The design variables for this antenna are the height, width, and length of the top plate, the width and location of shorting plate, and the location of the feed wire. ‐ 25 ‐
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3.1.1 PIFA Dimensions The size of the PIFA radiating top plate can be calculated approximately using [35]:
λcenter = 4( L + W ) (3.3) Where, L and W are the length and width of the plate, respectively. The resonant frequency is also influenced by the aspect ratio of the top plate (W/L). The width of the grounding strap, S, in relation to the width of the radiating top plate is also particularly important in determining the radiating behavior of the PIFA. Figure 3.5, shows how the current flow on the surface of the top plate varies, with different top plate and grounding strap configurations.
Figure 3.5 Surface current on PIFA top plate for various top plate aspect ratios and grounding strip widths [35].
In general, a greater top plate aspect ratio will result in a lower the resonant frequency for a given grounding strap width. For an aspect ratio of W/L > 1, there is an inflection point in the resonant frequency when W‐S = L and the resonant frequency begins to increase with increasing aspect ratio. As seen in ‐ 26 ‐
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Figure 3.5, the current on the planar element generally flows to the open‐circuit edge on the long side of the top plate when W‐ S < L. When W‐S > L, the current flows to the open circuit edge along the short side of the top plate. The inflection point in the resonant frequency is attributed to this change in current flow. The inflection point can be seen clearly in the case of W = L and where S << W, the current flows almost equally along the both the W and L dimensions. Like the resonant frequency of the PIFA, the relative impedance bandwidth is affected by the design of the structure. The height of the radiating top plate, H , and the width of the grounding strap, have the greatest influence on the bandwidth of the PIFA. In general, the bandwidth increases with increasing top plate height. However, as the height of the top plate approaches the magnitude of L = W, the height begins to influence the resonant frequency. When the grounding strap width is very small, S << W, the resonant frequency is given by:
λcenter = 4( L + W + H ) (3.4) The width of the grounding strap similarly affects the bandwidth. The limiting case, where the grounding strap is the same width as the top plate, the bandwidth of the PIFA is greatest. This case corresponds to the operation of a short circuited microstrip antenna. The PIFA with a W/L = 2.0 and H/λ0 = 0.053 has a relative bandwidth of 10%. As the width of the grounding strap decreases, the relative bandwidth of the PIFA decreases. The bandwidth of a PIFA with the grounding strap width much less than the width of the top plate (S/W≤0.1) can be reduced to below 1%. There are several procedures available for designing PIFAs and many different PIFA layouts may satisfy the same design criteria.
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3.2 Some Points to be Considered, while Designing Mobile Phone Antenna: 1. Antenna size should be small 2. Operating bandwidth should be wider 3. Radiation efficiency should be higher 4. Less power consumption Many of the techniques are used to improve the performance of the antenna. Some of these techniques are for size reduction, some for bandwidth enhancement, and some for improving efficiency and power consumption. At a time, the one technique used to improve one constrain, can degrade the other one. So we have to take a good trade off between the various constrains for better performance. This work is focused to form geometry of reduced size and the wider bandwidth, with less power consumption. The techniques used for size reduction and bandwidth enhancement will be discussed in the following section.
3.3 Antenna Size Reduction Techniques [1]: The miniaturization of the microstrip antenna can be achieved using several approaches:
3.3.1 Employing
a
Dielectric
Material
of
Higher
Permittivity From equation (1.1), it is found that the radiating patch of the microstrip antenna has a resonant length approximately proportional to
1
εr
, and the use of a
dielectric substrate with a larger permittivity thus can result in a smaller physical antenna length at a fixed operating frequency. But higher ε r increases the surface wave effect which leads poor performance. i.e., low radiation efficiency and lower bandwidth. ‐ 28 ‐
CHAPTER‐3
The high dielectric constant of the substrate is not very appropriate for our purpose. Therefore, the method based on capacitive loading, and the method that involves slots for longer current path along the patch edges have been chosen for bandwidth improvement.
3.3.2 Capacitive Loading of the Patch Structure (Radius of shorting pin> radius of feed probe) Another technique for size reduction is the use of an edge‐shorted patch (Reduces the resonant frequency). The shorting pin located close to the feed point results in a significant reduction in overall patch size. The typical RLC circuit used to describe a probe feed patch is loaded with a parallel LC circuit. The shorting pin effect on the input impedance of the antenna can be represented by the LC combination. “When shorting pin is located near the feeding point, a strong capacitive coupling or loading is produced due to which surface electrical current path is increased. This results in cancelling the inductive nature of a patch below resonance. Therefore, the amount of capacitive loading induced determines the ultimate reduction in size”. It makes a microstrip antenna act as a quarter‐ wavelength structure and thus can reduce the antenna’s physical length by half at a fixed operating frequency. When a shorting plate (Figure. 3.6(b)) or a shorting pin (Figure. 3.6(c)) is used instead of a shorting wall (Figure. 3.6(a)), further reductions in both the antenna’s fundamental resonant frequency and size can be obtained. However, with the use a dielectric substrate with larger permittivity and overall size reduction at a fixed operating frequency, the impedance bandwidth of a microstrip antenna is usually decreased.
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(a)
(b)
(c) Figure 3.6 Configurations of a rectangular patch antenna with (a) a shorting wall, (b) a shorting plate or partial shorting wall and (c) a shorting pin [1]. With the shorting pin technique, the antenna size reduction is mainly due to the shifting of the null‐voltage point at the center of the rectangular patch (excited at TM01 mode) and the circular patch (at TM11 mode) to their respective patch edges, which makes the shorted patches resonate at a much lower frequency. So, size is reduced and the reduction in size is limited by the distance between null‐ voltage point in the patch and the patch edge. ‐ 30 ‐
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There is also a significant dependency of resonant frequency on shorting
pin position. Also, in order to get a good matching condition, the feed position is placed closer to the shorting pin. When the feed position is away from the shorting pin, the resonant input resistance quickly increases.
3.3.3 Meandering of Radiating Patch Meandering the excited patch surface current paths, in the antenna’s radiating patch, is also an effective method for achieving a lowered fundamental resonant frequency for the microstrip antenna. For the case of a rectangular radiating patch, the meandering can be achieved by inserting several narrow slits at the patch’s non‐radiating edges. It can be seen in Figure 3‐7 that the excited patch’s surface currents are effectively meandered, leading to a greatly lengthened current path for a fixed patch linear dimension. This behavior results in a greatly lowered antenna fundamental resonant frequency, and thus a large antenna size reduction at a fixed operating frequency can be obtained.
Figure 3.7 Surface current distributions for meandered rectangular microstrip patches with meandering slits [1]. If we use shorting pin and meandering in conjunction with: The shorting pin makes the patch resonate at a much lower frequency and the narrow slots meander the patch, which increases the effective electrical length of patch. These two factors effectively reduce the required size for an antenna operated at a given frequency. With increasing slot length, the resonant frequency ‐ 31 ‐
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of the meandered patch decreases. But slot width has relatively little effect on the resonant frequency. Slots in patch produce additional capacitive reactance, while shorting pin with probe contribute inductive reactance.
3.3.4 Use of Inverted UShaped or Folded Patch Antenna Surface current lengthening for a fixed patch projection area, can also be obtained by using an inverted U‐shaped patch [Figure 3.8(a)], a folded patch [Figure 3.8(b)]. With these microstrip patches, the resonant frequency can be greatly lowered compared to a regular single‐layer microstrip antenna with the same projection area. Note that the resonant frequency is greatly lowered due to the bending of the patch surface current paths along the antenna’s resonant or excitation direction.
(a)
(b) Figure 3.8 Geometries of stacked shorted patch antennas (a) Ushaped patch (b) folded patch [1].
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3.3.5 Use of Lumped Elements Size of antenna can be reduced by using lumped elements in the antenna structure. These change the current distribution on the patch and can lower the resonant frequency.
Figure 3.9 Geometry of a compact broadband microstrip antenna with chip resistor loading [1].
3.4 Bandwidth Enhancement Techniques [1]: The bandwidth enhancement of the microstrip antenna can be achieved by using several approaches. Problem of achieving a wide impedance bandwidth (greater than or about 10% suitable for present day cellular communication system) for a compact microstrip antenna is becoming an important topic in microstrip antenna design. Broadband operation has been achieved with the use of two stacked shorted patches. The main relationships among various parameters having influence on bandwidth are follows [25]:
fu − fl 1 ∞ fr Q
L R C
(3.6)
Bandwidth = Q=
‐ 33 ‐
(3.5)
CHAPTER‐3
Q∞ 1 S
(3.7)
where, fu and fl are upper and lower frequency of bandwidth, fr is resonant frequency, Q is quality factor, R is loss component of antenna, L is inductive component of antenna, C is capacitive component of antenna, S is volume of antenna. The most frequently used method to broaden the bandwidth is to raise the height of the shorting plane i.e.; increase the volume. Bandwidth is affected very much by the size of the ground plane. By varying the size of the ground plane, the bandwidth of a PIFA can be adjusted. For example, reducing the ground plane can effectively broaden the bandwidth of the antenna system. Several slits at the ground plane edges can be inserted to reduce the quality factor of the structure (and to increase the bandwidth). Bandwidth enhancement of a PIFA can also be achieved by several efficient approaches, namely using dual resonance by additional patch that is adding capacitive load, loading dielectric with high permittivity, attaching chip resistor that is increasing loss term. In the following section all these techniques are discussed in detail.
3.4.1 Increasing the Height (h) of Substrate By increasing the height of the substrate, bandwidth can be extended (up‐to about 35%). Impedance bandwidth of PIFA is inversely proportional to the quality factor Q that is defined for a resonator [25]: Q = Energy Stored / Energy dissipated Substrates with high dielectric constant (εr) tend to store energy more than radiate it. This is equivalent by modeling the PIFA as a lossy capacitor with high εr, thus leading to high Q value and obviously reducing the bandwidth. Similarly when the substrate thickness is increased the inverse proportionality of thickness to the ‐ 34 ‐
CHAPTER‐3
capacitance (as C= ε0.Area/h) decreases the energy stored in the PIFA and the Q factor also. In summary, the increase in height and decrease of εr can be used to increase the bandwidth of the PIFA. Figure 3.10 shows the effect of substrate thickness on impedance bandwidth and efficiency for two values of dielectric constants. While the bandwidth increases monotonically with thickness, a decrease in the ε r value increases the bandwidth.
Figure 3.10 Effect of substrate thickness on impedance bandwidth and efficiency, for two dielectric constants [34].
3.4.2 Use of Chip Resistor of Low Resistance (usually on the order of 1Ω resistor is connected between patch & ground plate) As we know that bandwidth is inversely proportional to the quality factor (Q). Moreover, owing to the introduced small ohmic loss of the chip resistor, the quality factor of the microstrip antenna is greatly lowered. So bandwidth is increased.
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Figure 3.11 Geometry of a chipresistorloaded rectangular microstrip antenna with a probe feed [1]. Resistor should be placed at the edge opposite to the feed for maximum resonant frequency reduction. Antenna gain is reduced due to above technique. Bandwidth can be enhanced by considering the effect of the chassis dimension. If chassis resonates at the operating frequency of the antenna element, the bandwidth of the antenna‐chassis combination will improve considerably.
3.4.3 Using two (or more) Stacked Shorted Patches By using two (or more) stacked shorted patches and making both patches radiate at as equally as possible and having a radiation quality factor as low as possible, one can obtain enhanced impedance bandwidth for a fixed antenna volume.
(a) ‐ 36 ‐
CHAPTER‐3
(b) Figure 3.12 Shorting wall (a) Offset shorting wall (b) A common shorting wall [1].
3.5 Some other Parameters, to be Considered in PIFA Designing: 3.5.1 Resonant Frequency The resonant frequency of PIFA can be approximated with [25]: L + W = λ /4
(refer figure 3.4)
When, S/W=1 then W + H = λ /4
(3.8)
When, S=0 then L + W + H = λ /4 The resonant frequency of the PIFA is proportional to the effective length of the current distribution. The introduction of an open slot reduces the frequency, due to the fact that there are currents flowing at the edge of the shaped slot, therefore a capacitive loaded slot reduces the frequency and thus the antenna dimensions drastically. The same principle of making slots in the planar element can be applied for dual frequency operation as well. Changes in the width of the planar element can also affect the determination of the resonant frequency. The width of the short circuit plate of the PIFA plays a very important role in governing its resonant frequency. Resonant frequency decreases with the decrease in short ‐ 37 ‐
CHAPTER‐3
circuit plate width, S. Unlike micro‐strip antennas that are conventionally made of half wavelength dimensions, PIFA’s are made of just quarter wavelength. Analyzing the resonant frequency and the bandwidth characteristics of the antenna can be easily done by determining the site of the feed point, at which the minimum reflection coefficient is to be obtained.
3.5.2 Radiation Pattern The radiation pattern of the PIFA is the relative distribution of radiated power as a function of direction in space. In the usual case the radiation pattern is determined in the far‐field region and is represented as a function of directional coordinates. Radiation properties include power flux density, field strength, phase, and polarization.
3.5.3 Electric Field Distribution The dominant component of the electric field Ez is equal to zero at the short‐circuit plate while the intensity of this field at the opposite edge of the planar element is significantly large. For fields Ex and Ey, there is pointing part, which corresponds to the feed source. This means that the electric lines of force are directed from feed source to the ground plane. When the width of the short circuit plate is narrower than the planar element, the electric fields Ex and Ey start generating at all open circuit edges of the planar element. These fringing fields are the radiating sources in PIFA.
3.5.4 Current Distribution PIFA has very large current flows on the undersurface of the planar element and the ground plane compared to the field on the upper surface of the element. Due to this behavior PIFA is on of the best candidate when is talking about the influence of the external objects that affect the antenna characteristics (e.g. mobile operator’s hand/head). PIFA surface current distribution varies for different ‐ 38 ‐
CHAPTER‐3
widths of short‐circuit plates. The maximum current distribution is close to the short pin and decreases away from it. The ground surface waves can produce spurious radiations or couple energy at discontinuities, leading to distortions in the main pattern, or unwanted loss of power. The surface wave effects can be controlled by the use of photonic band gap structures or simply by choosing air as the dielectric. This solves the limitation of poor efficiency as well along with certain degree of bandwidth enhancement.
3.5.5 Ground Plane Effects It is assumed that a large ground plane is used for antenna designing. A detailed study on the effects of operating the PIFA in a reduced ground plane environment is presented in [20]. The size of the ground plane plays an important role in the behavior of the PIFA. The resonant frequency, input impedance, bandwidth and gain are all impacted when the PIFA is operated over a finite size ground plane. The resonant frequency for a PIFA with a fixed top element and grounding strap size tends to remain fairly constant for large ground plane sizes. As the ground plane size is reduced, the value of the resonant frequency oscillates around the value for the infinite ground plane case until the ground plane size reaches about 0.2λ in length. At that point, the resonant frequency is highly dependent on the size of the ground plane and it increases linearly with decreasing ground plane size. The relative bandwidth of the PIFA increases with increasing ground plane size. As in the center frequency case, the relative bandwidth oscillates around the value of the infinite ground plane case. However, the PIFA relative bandwidth exhibits a stronger dependence on ground plane size than the center frequency does for larger ground planes. It is reported in [20] that a ground plane size of at least 0.8 λ is required to achieve the desired 8% impedance bandwidth of the Cellular band. The gain of the PIFA is influenced by the size of the ground plane as well. The gain of the PIFA increases with increasing ground plane size. It then reaches a ‐ 39 ‐
CHAPTER‐3
local maximum around 0.9 λ and begins to oscillate to the infinite ground plane case of nearly 5 dB. To achieve the desired 3 dB in gain for the Cellular band, a ground plane of at least 0.5 λ is required [20, 21].
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