CHAPTER - 2 BASIC FRAME WORK FOR ANTENNA DESIGN INTRODUCTION Antenna is a primary and necessary component of all wireless communication systems. It enables the transition of energy between a guiding device, such as coaxial line or a waveguide to the free-space. It transforms the electric energy to electromagnetic energy and vice versa. By the IEEE Standard Definitions, antenna is defined as “a means for radiating or receiving radio waves." In a transceiver system, the antenna is the final block in the transmission region and is the first block in the receiving region. So, the fundamental understanding of antenna parameter and working are prerequisite to develop an antenna solution for smart systems. So, in this chapter basics of microstrip patch antenna are elaborated to define the common antenna parameter and terminology. In order to complement the next chapters the design description for UWB technology, UWB antenna and reconfigurable antenna are also detailed. As PIN diode is the back-bone of this research work which is used as switching element, therefore a detailed description, working principle and characterization of PIN diode is also described in this chapter.
2.1. MICROSTRIP PATCH ANTENNA Microstrip antenna is the best choice for modern wireless and mobile applications due to many considerable advantages like simple, lightweight, simple and economical and compatible with MMIC designs etc. [1-5]. The basic shape of microstrip antenna can be
rectangle, square, ellipse, circle, triangle, ring, pentagon, or their complex variations to meet particular design demands [1-6].
Fig. 2.1 Basic Rectangular Patch antenna (b) Patch antenna showing E- field distribution
Chapter 2 A basic rectangular patch is considered here to understand the basics of antenna which consists of ground plane, dielectric substrate and radiating patch as shown in Fig. 2.1(a). This rectangular microstrip patch antenna (L,W) is designed on a substrate with relative dielectric constant =
and substrate with height = h. The CAD formulae  for the calculation antenna
dimension (L,W) at resonating frequency f0 are listed below: Effective dielectric constant re r 1 r 1 1 12 2
Patch width: W = Patch length: L =
Extended length (ΔL) of patch due to fringing field ΔL =
Effective patch length: Leff = L + 2ΔL
If the input impedance of antenna is 50Ω at a particular frequency then antenna will efficiently matched with the input impedance (50 Ω) of input port. For efficient antenna design, impedance distribution should be known so that antenna can be easily matched to 50 Ω impedance. To study the impedance distribution over a patch it is necessary to study the electric and current distribution. The feed probe couples electromagnetic energy in and or out of the patch as shown in Fig. 2.1(b). The electric field is zero at the center of the patch, maximum on one edge and reverses its direction on opposite edge. This field distribution continuously reverses its direction according to the instantaneous phase of the RF signal. Fig. 2.2 shows the current, voltage and impedance behavior in the radiating patch; the current (magnetic field) is maximum at the center of patch and minimum on the opposite sides of patch, while the voltage (electrical field) is zero in the center and maximum on one edge and reverses its direction (minimum) on opposite edge.
Fig. 2.2 Voltage, Current and Impedance distribution along patch resonant length
Chapter 2 Hence the distribution of impedance is minimum at the center and maximum on both edges of patch. So there is a point lie inside the surface of radiating patch where the impedance is 50Ω. The simplest method for impedance matching is to locate the position of 50 Ω point on the antenna surface and connect the input RF port at this point. The input impedance of rectangular microstrip patch antenna is calculated by Transmission line model. The equivalent transmission line model of a microstrip fed rectangular patch [1-2] is shown in Fig. 2.3 which consists of a parallel-plate transmission line connected with two radiating slots (apertures), each of width W and height h, separated by a transmission line of length L. Each radiating slot of microstrip patch antenna is represented as a parallel equivalent admittance Y=G + jB.
Fig. 2.3 Transmission line model for rectangular patch antenna as radiating slot [1-2]
Since both slots are identical, the total resonant input impedance [1-2] becomes Zin=1/2 G. The conductance (G) of single radiating slot-1 is associated with the power radiated and is given by eq. (6) (6) Where W = patch width and λ= resonant wavelength. B is susceptance due to energy stored in the fringing field near the edge of the patch and given by eq. (7) (7) If G12 is the mutual conductance between two slots, Jo is Bessel function of first kind than (8) So, the total input impedance is given by eq. (9) (9)
Chapter 2 So using formula given in eq. (9) input impedance for microstrip patch antenna can be accurately calculated. This is more reliable method to calculate the input impedance of a rectangular patch antenna. After calculating the input impedance, it should be matched to 50Ω because the facts that almost all the microwave sources and lines are manufactured with 50Ω characteristic impedance [6-8]. After calculating the input impedance various impedance matching techniques can be applied. These impedance matching techniques can be categorized in two broad categories i.e. distributed method and lumped element method. In distributed method [9-12], impedance matching can be done by doing some structural modifications through the use of stubs [9-10], quarter wave transformer , tapered line , balun and active components as shown by Fig. 2.4. The main advantage of distributed method is that there is no requirement to modify the geometry of radiating structure. Therefore, radiation performance of the radiating structure is independent to the matching network and results in easy design optimization. However, this method increases the size of antenna and not recommended for the design of practical array systems. Also system efficiency degrades due to the increase in spurious radiation losses from extra circuitry of matching network.
Fig. 2.4 Matching techniques (a) Distributed impedance (b) Lumped element
In the second method, a lumped network [13-14] is introduced to realize impedance matching between antenna and feed structure. This method can be implemented either by inserting a separate network without changing the antenna structure or by etching slots or notch in the antenna geometry as indicated in Fig. 2.4(b). The most important advantage of placing the impedance matching network between antennas and feeding structure is the enhancement in the impedance bandwidth. This method allows incorporating last minute design change by allowing freedom in choosing the values of discrete components, independently.
Chapter 2 2.1.1. Antenna Feeding Techniques Microstrip patch antennas can be excited by a number of methods [1-5]. These methods can be categorized into two types: Contacting and Non-contacting. In the contacting method, the RF power is fed directly to the antenna using a connecting part such as a Microstrip line/Coaxial Cable. In the non-contacting method, electromagnetic field coupling is provided to transfer the RF power between the microstrip line and the patch such as aperture coupling and proximity coupling. The four most popular feeding techniques for microstrip patch antenna are: coaxial feeding, Microstrip feeding, proximity feeding and aperture feeding. Each is explained below briefly: (a) Coaxial feeding: It is one of the basic techniques used in feeding microwave power to the antenna. The coaxial cable is connected to the antenna such that it’s outer conduct or is attached to the ground plane while the inner conductor is soldered to the metal patch. Coaxial feeding is simple to design, easy to fabricate, easy to match and have low spurious radiation. However, coaxial feeding has the disadvantages of requiring high soldering precision. There is difficulty in using coaxial feeding with an array since a large number of solder joints will be needed. Coaxial feeding usually gives narrow bandwidth and when a thick substrate is used a longer probe will be needed which increases the surface power and feed inductance. (b) Microstrip feeding: In Microstrip feed, the patch is excited by a microstrip line that is located on the same plane as the patch. In this technique, impedance matching is required between patch and 50 Ω feed line. The main disadvantage of this technique is that antenna suffers from narrow bandwidth and the introduction of coupling between the feeding line and the patch which leads to spurious radiation. (c) Proximity coupled feeding: In this type, feeding is conducted through electromagnetic coupling that takes place between the patch and the Microstrip line. The patch antenna is located on the top of the upper substrate and the Microstrip feeding line is located on the top of the lower substrate. The two substrates can be chosen different than each other to enhance antenna performance. The proximity coupled feeding reduces spurious radiation and increase bandwidth. However it needs precise alignment between the two layers in multilayer fabrication. (d) Aperture coupled feeding: It is a non contacting feed; the feeding is done through electromagnetic coupling among antenna and the microstrip line through the slot etched in the ground plane. It consists of two substrate layers with common ground plane in between the two substrates, the Microstrip patch antenna is on the top of the upper substrate and the 26
Chapter 2 Microstrip feeding line on the bottom of the lower substrate and there is a slot cut in the ground plane. The slot can be of any size or shape and is used to enhance the antenna parameters. The two substrates can be chosen different than each other to enhance antenna performance. The aperture feeding reduces spurious radiation. It also increases the antenna bandwidth, improves polarization purity and reduces cross-polarization. 2.1.2 Basic Antenna Parameters An antenna is a device that converts a guided electromagnetic wave on a transmission line to a plane wave propagating in free space. Thus, one side of an antenna appears as an electrical circuit element, while the other side provides an interface with a propagating plane wave. Antennas are inherently bi-directional which can be used for transmitting and receiving as well. The power, gain and the directivity define the ability of the antenna to concentrate energy in a particular direction. Some important antenna parameters [15-16] concerning the radiation performance of antenna are described here. 1. Resonant Frequency (fr): The antennas are tuned to work at one particular frequency and are operative only over a range of frequencies centred on this frequency, called the resonant frequency. So, when driven at its resonant frequency, large standing waves of voltage and current are excited in the antenna elements. These large currents and voltages radiate the intense EMW, so the power radiated by the antenna is maximum at the resonant frequency. 2. Reflection coefficient (RL): is a measure of effectiveness of power delivered to antenna. If the power incident on the antenna is Pin and the reflected power from the antenna to the source is Pref. The degree of mismatch between the reflected and incident power is given by Reflection coefficient = 3. Bandwidth (BW): It is defined as the range of operating frequencies within which the performance of the antenna conforms to a specified standard. BW is the difference of either side of frequencies in accordance to the center frequency where the antenna characteristics such as radiation pattern, polarization, gain, are close to those values which have been found at the center frequency. The BW of a UWB antenna can be demarcated as the relation of the upper to lower frequencies of acceptable operation. The BW of a narrowband antenna is the percentage of the frequency difference over the center frequency. So, it can write in terms of equations as under: BWWB=
Chapter 2 If FH/ FL= 2 then antenna is assumed to be UWB. The method of trying how capably an antenna is operating over the required range of frequencies is to calculate its VSWR. A VSWR≤ 2 ensures good performance. 4. Voltage Standing Waves Ratio (VSWR):- The antenna will operate efficiently when the maximum transfer of power must take place between the transmitter and the antenna. The Maximum power transfer can only take place when the impedance of the antenna is matched to that of the transmitter. The VSWR can be expressed as SWR=
The VSWR expresses the degree of match between the transmission line and the antenna. When the VSWR is 1 to 1 (1:1) the match is perfect and all the energy is transferred to the antenna prior to be radiated. 5. Antenna Efficiency (η): The radiation efficiency of an antenna is defined as the ratio of the power radiated by the antenna to the power at its input terminals. It is a measure of how efficiently an antenna radiates its input power as RF energy. When given in terms of a percentage, an antenna efficiency of 0% means all power absorbed by the antenna at its input is effectively lost within the device and no useful radiation occurs. An efficiency of 100% refers to a perfectly radiating antenna wherein all power absorbed at the input is radiated. 6. Gain (G): The gain of an antenna is a measure of the ability to focus power into a narrow angular region of space. If an antenna is transmitting with a positive gain is used as a receiving antenna, it will also have the same positive gain for receiving. The energy propagated in the direction compared to the energy that would be propagated if the antenna were Omni-directional are said to be gain of antenna. It is related to directivity and efficiency by Gain (G) = directivity (D) * efficiency (η) 7. Directivity (D): The ratio of the radiation intensity (U) in a given direction from the antenna to the radiation intensity of an isotropic antenna (U0) is known as the directivity D of an antenna . In mathematical form, it can be written as D=
8. Radiation Pattern: The Radiation Pattern of an antenna is a 3-dimensional graphical representation of the relative strengths of the fields emitted by the antenna. It can also be thought as the locus of points around the antenna which have the same electric field. The pattern consists of a main lobe and several minor lobes. These minor and side lobes are 28
Chapter 2 always unwanted because they represent wasted energy for transmitting antennas and potential noise sources for receiving antennas. The radiation pattern is determined in the farfield region and is represented as the power radiated or received by an antenna in a function of the angular position and radial distance from the antenna. The two or three dimensional pattern of spatial distribution of radiated energy can be constructed using multiple twodimensional patterns. For a linearly polarized antenna, performance is often described in terms of its principal E- and H-plane patterns. The E-plane is defined as “the plane containing the electric field vector and the direction of maximum radiation,” and the H-plane as “the plane containing the magnetic-field vector and the direction of maximum radiation”. 9. Polarization: It is the property of an electromagnetic wave describing the time-varying direction and relative magnitude of the electric-field vector. According to the electric field vector behavior polarization may be classified as linear, circular, or elliptical. 10. Input impedance (Zin): It is the impedance presented by an antenna at its terminals and can be written as: Zin = Rin + jXin where Zin is the antenna impedance at the terminals, Rin is the antenna resistance which consisting of radiation resistance Rr and the loss resistance RL. The imaginary part Xin is the antenna reactance and represents the power stored in the near field. The power associated with the radiation resistance is the power actually radiated by the antenna, while the power dissipated in the loss resistance in the form of heat is due to dielectric or conducting losses. 2.2 UWB TECHNOLOGY Since, FCC declared a bandwidth of 7.5GHz (from 3.1GHz to 10.6GHz) designated as UWB spectrum platform i.e. wireless communications for public uses [17-19], the UWB technology is rapidly advancing as a short range high-speed high data rate wireless communication technology. UWB is defined as any wireless plan that occupies either a fractional bandwidth greater than 20% or more than 500 MHz of absolute bandwidth. This technology has been engaged into our daily lives with minimal interference. This technology is an unlicensed service that can be used anywhere, anytime, by anyone. UWB communications transmit signal without interfering with other traditional narrow bands operating in the same frequency band. Fig. 2.5 displays the behavior between Emitted signal powers versus frequency in GHz. UWB signal is noise-like signal with low energy density, hence its detection is quite difficult. Additionally, the “noise-like” UWB signal has a particular shape compared to real noise signal (no shape). So, it is almost unfeasible for real noise signal to destroy the UWB pulse
Fig. 2.5 Comparison of various communication standards 
because interference would have to spread uniformly across the entire spectrum to obscure the pulse. UWB pulse behaves as a wideband noise source for other NB systems operating in that frequency range; but it doesn’t affect them because of its low signal power. It only increases the SNR requirement of those systems. By using PN (Pseudo Random) codes UWB system can be made undetectable for hostile receivers and can be protected from jamming. Hence, UWB is possibly the most safe and secure means of signal transmission. The unique characteristics of UWB technology present a more powerful solution to wireless broadband than other technologies [17-19]. The UWB devices operate by employing a series of very short electrical pulses that result in very wideband transmission bandwidths. In addition, UWB signals can run at high speed and low power levels. It also enables various types of modulation scheme to be employed, including on–off keying, pulse-amplitude-modulation, pulse-position-modulation, phase-shift-keying, as well as different receiver types such as the energy detector, rake, and transmitted reference receivers. Another strong candidate for UWB is multicarrier modulation by using orthogonal frequency division multiplexing (OFDM). The unique characteristics of Ultra Wide band technology are listed below: 1. Capacity: Since UWB has an ultra wide frequency bandwidth, so a huge capacity as high as hundreds of Mbps or even several Gbps can be obtained. 2. Low power transmission: UWB systems operate at extremely low power transmission levels. By dividing the power of the signal across a huge frequency spectrum, the effect upon any frequency is below the acceptable noise floor. For example, 1 watt of power spread across 1GHz of spectrum results in only 1nW of power into each hertz band of frequency. Thus, UWB signals do not cause significant interference to other wireless systems.
Chapter 2 3. Fading Robustness: It is channel fading resistant, due to the large number of resolvable multipath components. Wide band nature of the signal helps it in avoiding the problem of time varying amplitude fluctuations. It is also immune to Multipath Delays where various version of same signal appear at the receiver which have undergone a variety of diffraction, reflection, scattering effects as time delay introduced is generally more than the signal duration. 4. Short Range: Its normal range of operation is within 10m, so its power requirement is low and interference with other short range devices is less. It comes under WPAN protocol. 5. Security Aspects: UWB provides high level security and reliable communication. 6. Low Cost: UWB system has low cost and low complexity because it does not modulate and demodulate a complex carrier waveform, so it does not require components such as mixers, filters, amplifiers and local oscillators. 7. Large Bandwidth: The FCC allocated an absolute bandwidth more than 500 MHz up to 7.5 GHz which is about 20% up to 110% fractional bandwidth of the center frequency. This large bandwidth spectrum is available for high data rate communications as well as radar and safety applications. 8. Very Short Duration Pulses: Ultra-wideband pulses are typically of nanoseconds or picoseconds order. Transmitting the pulses directly to the antennas results in the pulses being filtered due to the properties of the antennas. Due to using UWB systems those very short duration pulses, they are often characterized as multipath immune or multipath resistant. 9. Resolution: High resolution localization, due to the very short pulse duration. 10. Multiple accesses: UWB technology provides multiple access capabilities, due to the wide bandwidth of transmission. 11. Target Detection: UWB antenna is used as target detection in RADAR. All these unique features of UWB technology make it suitable for many different applications such as geo positioning, radar and sensor applications e.g. vehicular, marine, GPR, imaging, wall-imaging, sense-through-the-wall (STTW), surveillance systems etc. 2.2.1 UWB Antenna Design Challenges UWB antennas exhibit very large bandwidth compared to general antennas [20-26]. There are two criteria available, for identifying when an antenna may be considered as UWB. A definition given by DARPA says that a UWB antenna has a fractional bandwidth greater than 0.25. Whereas, the United States Federal Communications Commission (FCC), places this
Chapter 2 bandwidth limit to 0.2. Additionally, the FCC provides an alternate definition whereby an UWB antenna any antenna may have a bandwidth greater than 500 MHz. There are several known antenna topologies that are said to achieve broadband characteristics, such as the horn antenna, biconical antenna, helix antenna and bowtie antenna. All these antennas have been proven to have excellent broadband characteristics, but they are large, non-planar and physically obtrusive, therefore ruling them out as a possibility for use with small UWB integrated electronics. Another antenna design approach is to use frequency independent antenna which uses Babinet’s Equivalence Principle of duality and complementarity for meeting the requirements of very wide impedance bandwidth. The Archimedian spiral antenna, logarithmic spiral antenna, fractal antenna are used for UWB operation because they possess small size, light weight and thin shape for portable devices. The design of a UWB antenna is very difficult, because the fractional bandwidth is actually big, and antenna must cover multiple octave bandwidths in order to transmit pulses that are of the order of a nanosecond in duration. Since data may be contained in the shape of the UWB pulse, antenna pulse distortion must be kept to a minimum value. A non-dispersive characteristic in time and frequency domain, provides narrow pulse duration to enhance a high data throughput. Antennas in the frequency domain are typically characterized by radiation pattern, directivity, impedance matching, and bandwidth. The following are important challenges in designing UWB antennas. 1. UWB antenna must possess ultra wide frequency bandwidth. 2. The performance of a UWB antenna is required to be consistent over the entire operational band. Ideally, antenna radiation patterns, gains and impedance matching should be stable across the entire band. Sometimes, it is also demanded that the UWB antenna provides the band-rejected characteristic to coexist with other narrowband devices and services occupying the same operational band. 3. UWB antenna must possess directional or omni-directional radiation properties depending on the practical application. Omni-directional patterns are normally desirable in mobile and hand-held systems. For radar systems and other directional systems where high gain is desired, directional radiation characteristics are preferred. 4. UWB antenna needs to be small enough to be compatible to the UWB unit especially in mobile and portable devices. It is also highly desirable that the antenna's feature should be low profile and compatible for integration with PCB.
Chapter 2 5. UWB antenna should be optimal for the performance of overall system. For example, the antenna should be designed such that the overall device (antenna and RF front end) complies with the mandatory power emission mask given by the FCC or other regulatory bodies. 6. UWB antenna is required to achieve good time domain characteristics. For the narrow band case, it is approximated that an antenna has same performance over the entire bandwidth and the basic parameters, such as gain and return loss, have little variation across the operational band. In contrast, UWB systems often employ extremely short pulses for data transmission. The UWB antenna design is the major dimension in the progress of UWB technology. The main challenge in UWB antenna design is achieving the wide impedance bandwidth while still maintaining high radiation efficiency. UWB antenna should be designed focussing on various parameters such as frequency of operation, substrate height, dielectric constant to be used. To cater to all these requirements the microstrip antenna has been gaining popularity. Owing to its narrow bandwidth, many solutions have been introduced, which offer the impedance bandwidth across the entire UWB range. Some of these solutions are described in next sections. 2.2.2 UWB Antenna Design Techniques There are many methods for broadening the impedance bandwidth of antenna. Different techniques are applied for good impedance matching over the UWB range which includes different combination of specially designed patch or feed line with partial or optimized ground plane. The descriptions of some techniques are given below: (a) Combination of different patch geometry with partial ground: UWB operation can be achieved by using either partial ground or CPW feed in different shapes of patch structures. By using different shapes of the patch, accommodate multimode surface current waves, which in turn lead to resonating at multiband frequencies and finally widen the impedance bandwidth [20-29]. The geometries shown in Fig.2.6 includes step rectangular patch in which steps are slotted from original patch structure, circular, octagon monopole, U-shaped monopole, knight’s helm shape monopole and two steps circular monopole are used to modify the impedance bandwidth.
Fig. 2.6 Broad Banding methods by varying patch geometry
(b) Combination of different feeding structure with slotted ground The shape of feed line is optimized for broad banding the UWB behaviour. Addition of different shape stub and wide slot under the feed line also enhance the bandwidth. Fig. 2.7 shows the different shaped stub that can be used for broad banding antenna like T-shaped stub, three offset stub, fan shaped stub, rectangular [29-31]. Fig. 2.7(b) shows the different shape slot under feed line like tapered, circular hexagon, semicircular, elliptical to enhance the impedance bandwidth.
Fig. 2.7 Broad banding method by (a) varying Feed and stub (b) various shapes of wide-slots
(c) Modified partial ground structure: The structure of partial ground plane can be optimized to make a broad band antenna [32-37]. This method improves the gain and
Chapter 2 bandwidth, reduces the reflection coefficient. Different ground structure are used for board banding like notched, saw tooth, rounded, trapezoid form as shown in Fig. 2.8.
Fig. 2.8 Broad banding methods by modifying ground structure [38-43]
2.3. RECONFIGURABLE ANTENNA In modern times, Wireless devices are not limited to one standard and can operate at multiple frequencies. Multi-mode terminals have received great attention and have increased in popularity because by this single terminals or devices could have many applications such as, GPS, GSM, WLAN, Bluetooth, etc. Reconfigurable antennae have their ability to modify fundamental characteristics, including operating frequency, impedance bandwidth, radiation pattern, and polarization or even a combination of these features in real time [38-42]. Reconfigurable antenna have the potential to add substantial degrees of freedom and functionality to mobile communication and phase array systems by allowing us for spectrum reallocation in multi-band communication systems, dynamic spectrum management, therefore reducing the number and size of antenna in a system. Compared with conventional antennas, reconfigurable antennas have more advantages for example, saving energy; reducing the number of antennas thus reducing the mutual interferences between them. Different types of reconfigurable antenna help to overcome the problem like spectrum congestion, interference between different users etc. Reconfigurable antenna support new desired capabilities [43-49] to cope with extendable multiservice and multi standard, multiband operation, as well as with efficient spectrum and power utilization. Some important consequences of reconfigurable antennas are: 1. Allows spectrum reallocation and dynamic spectrum management,
Chapter 2 2. Meets flexible multi-radio wireless platform requirements i.e. multiple services in a single device. 3. Reduced number of antennas in the system resulting reduced overall device size and cost. 4. Provides good isolation between different wireless standards and bands. 5. Reconfigurability in antenna radiation patterns achieves spatial diversity for interference cancellation. 6. Frequency reconfigurable antennas are useful to support many wireless applications, where they can reduce the size of the front end circuitry. 7. Polarization reconfigurable antenna are valuable to solves the various problems like signal fading due to multipath propagation, sensitivity of signals to transceiver antenna orientation, limited channel capacity; security etc. 8. Radiation pattern reconfigurable antennas are useful to improve the coverage area and system performance by redirecting the main beam. 9. Low front end processing circuits as there is no need for front end filtering, good out-ofband rejection. 10. Enable cost-effective SDR, MIMO and Cognitive Radio implementations. 11. Compact reconfigurable antenna allows efficient radio implementations in reduced formfactor mobile platforms. 2.3.1 Classification The reconfigurable antenna can be classified into four different categories viz. frequency reconfigurable antenna, radiation pattern reconfigurable antenna, polarization reconfigurable antenna, and combination of any two. (a) Frequency Reconfigurable antenna: A radiating structure that is able to change its operating frequency by hoping between different frequency bands is called frequency reconfigurable antenna [38-42]. This is achieved by producing some tuning in the antenna length. Frequency reconfigurable antennas have attracted significant attention due to their ability to cover multiple frequency bands so significantly reduce the number of antenna required for multi-mode communication. (b) Radiation pattern Reconfigurable antenna: A radiating structure which is able to tune its radiation pattern and maintain its operating frequencies is called radiation pattern reconfigurable antenna [43-49]. Manipulation of an antenna’s radiation pattern can be used to avoidance of noise source, improved security by directing signals only toward intended users, avoidance of signal fades, improved beam steering capability of phased array systems, and 36
Chapter 2 increased diversity gain. So there is a great demand for pattern reconfigurable antennas in the fields of wireless communications, satellite communications, radars, etc. The antenna is designed to be able to reconfigure its radiation pattern during real time operation such that it maintains its broad pattern in the absence of interferences, and is capable of narrowing its pattern beam width, when the interfering signals arrive at the antenna, to suppress these undesired signals as much as possible. (c) Polarization Reconfigurable Antenna: A radiating structure that can change its polarization (horizontal/vertical, slant 450, left hand or right-hand circular polarized, etc.) is called polarization reconfigurable antenna [50-58]. (d) Bandwidth Reconfigurable Antenna: A radiating structure that can change its bandwidth from NB to WB in different frequency range and vice -versa according to user's requirement is called as bandwidth reconfigurable antenna [59-60]. 2.3.2 Reconfigurable Methods Reconfigurablity of antenna is normally achieved in one of four ways as summarized below. (a) Electrical Method: In this method, the switching or tuning of antenna to redirect their surface currents is achieved by means of PIN diodes, GaAs FETs, MEMS devices or varactors [61-66]. The MEMS devices have the advantage of very low loss, but the disadvantages are high operating voltage, high cost and lower reliability than semiconductor devices. GaAs FET used in switching mode, with zero drain to source bias current, have low power consumption but poorer linearity and higher loss. PIN diodes can achieve low loss at low cost, but the disadvantage is that in the ON state there is a forward bias dc current, which degrades the overall power efficiency. Varactor diodes have the advantage of providing continuous reactive tuning rather than switching, but suffer from poor linearity. One of the major advantages of such components is their good isolation and low-loss property. The incorporation of switches increases the complexity of the antenna structure due to the need for additional bypass capacitors and inductors which will increase the power consumption of the whole system. The activation of such switches requires biasing lines that may negatively affect the antenna radiation pattern and add more losses. (b) Optical Method: In this method, an optical switch is integrated to reconfigure the antenna structure. These switches are integrated into the antenna structure without any complicated biasing lines which eliminates unwanted interference, losses, and radiation pattern distortion [67-69]. The activation or deactivation of the photoconductive switch by shining light from the laser diode does not produce harmonics and inter-modulation distortion 37
Chapter 2 due to their linear behavior. Despite all these advantages, optical switches exhibit lossy behavior and require a complex activation mechanism. Table 2.1 shows a comparison of the characteristics for the different switching techniques used on electrically (RF-MEMS/PIN diodes) and optically reconfigurable antennas. Table 2.1 Comparisons of different switching scheme Electrical property
1-200 μ sec
1-100 n sec
3-9 μ sec
(c) Physical Method: Antennas can also be reconfigured by physically altering the radiating structure by using mechanical motor. The tuning of the antenna is achieved by a structural modification of the antenna radiating parts. The importance of this technique is that it does not rely on any switching mechanisms, biasing lines, or optical fiber/laser diode integration rather it use some mechanical rotational part (stepper motor). The main drawback of mechanical switching is bulkier structure of antenna and hard to implement in small devices. (d) Material Based Method: Antennas are also made reconfigurable by changing the substrate characteristics, using special materials such as liquid crystals, or ferrites and Meta material [70-72]. These materials have property to change their relative electric permittivity or magnetic permeability under different operating conditions. In fact, a liquid crystal is a nonlinear material whose dielectric constant can be changed under different voltage levels, by altering the orientation of the liquid crystal molecules. As for a ferrite material, a static applied electric/magnetic field can change the relative material permittivity/permeability. In the present research work electrical method is used to achieve reconfigurablity of antenna in which PIN diode is used as a switching element. So complete functioning, working principle of PIN diodes is important to elaborate and it is discussed in the next section. 2.4 PIN DIODE This section presents a general overview of PIN diode, its operating condition as a RF switch and its characterization to form an adequate basis for the subsequent chapters. PIN
Chapter 2 diode is used as a switch to controls the path of RF signals . A PIN diode is constructed by sandwiching a wide, intrinsic semiconductor region between a P-type semiconductor and an N-type semiconductor region. The P-type and N-type regions are typically heavily doped because they are used for ohmic contacts. The wide intrinsic region is in contrast to an ordinary PN diode. The wide intrinsic region makes the PIN diode an inferior rectifier (the normal function of a diode), but it makes the PIN diode suitable for attenuators, photo detectors, and high voltage power electronics application. Drawing of a PIN diode chip is shown in Fig. 2.9 (a). The PIN diode is generally constructed using a PIN chip that has a thicker I-region, larger cross sectional area. The PIN diode has small physical size compared to a wavelength, high switching speed, and low package parasitic reactance; make it an ideal component for the use in RF applications. The performance of PIN diode primarily depends on chip geometry and the nature of the semiconductor material used in the finished diode, particularly in the I region.
Fig. 2.9 (a) Cross section of a basic diode (b) Forward bias (c) Reverse bias
2.4.1 Equivalent Circuit parameters When the diode is forward biased, it can represent as basic electrical characteristics of series resistance (RS), and a small Inductance. If the PIN diode is reverse biased, there is no stored charge in the I-region and the device behaves like a Capacitance (CT) shunted by a parallel resistance (RP) as shown in Fig 2.9(c). These equivalent circuit parameters are defined in detail as follows. a) Under forward bias PIN diode behaves as a current controlled resistor when forward biased. The equivalent circuit for the forward biased is shown in Fig. 2.9(b) which consists of a series combination of the series resistance (Rs) and a small Inductance (L). The Rs inversely proportional to the stored charge Q = If τ where If is the forward current and τ is the recombination time or carrier lifetime and Inductance (L) depends on the geometrical properties of the package such
Chapter 2 as metal pin length and diameter. The resistance (Rs) of the I region under forward bias is given by μ
W = I-region Width, If = forward bias current, τ = minority carrier lifetime, μ , μ = electron and hole mobility The eq. (13) is valid for frequencies higher than the transit time of the I-region (f in MHz and W in microns). At lower frequencies, the PIN diode rectifies the RF signal just as any PN-junction diode. b) Under Reverse Bias The reverse bias equivalent circuit consists of a parallel combination of capacitance (CT) and resistance (Rp). The defining equation for CT is (14) Which is valid for frequencies above the dielectric relaxation frequency of the I-region, i.e. Where = dielectric constant of silicon, A =diode junction area, = resistivity of silicon. At frequencies much lower than ƒ, the capacitance characteristic of the PIN diode resembles a varactor diode. Due to changes and variations in the capacitance PIN diode switches have low frequency limitations. 2.4.2 Working Operation of PIN diode A switch is an electrical component for opening and closing the connection of a circuit or for changing the connection of a circuit device. An ideal switch exhibits zero resistance to current flow in the ON state and infinite resistance to current flow in the OFF state. A practical switch design exhibits a certain amount of resistance in the ON state and a finite resistance in the OFF state. A PIN diode obeys the standard diode equation for low frequency signals. At higher frequencies, the diode looks like an almost perfect (very linear, even for large signals) resistor. When the diode is forward biased, the carrier concentration is much higher than the intrinsic level carrier concentration in I region. Due to this high level injection level, the electric field extends deeply (almost the entire length) into the region. This electric field helps in speeding up the transport of charge carriers from P to N region, which results in faster operation of the diode, making it a suitable device for high frequency operations. Diode 40
Chapter 2 doesn't turn off until the stored charged removed and I region provide plenty of store charge at low DC voltage. So
Qs>>> QRF (IRF / ω)
Stored charge >>> RF induced charge
(QRF added or removed from the I-region cyclically by the RF current). At high frequencies the stored carriers within the intrinsic layer are not completely swept by the RF signal or recombination because there is not enough time to remove the stored charge so always ON in negative cycle also (not rectify in case of PN diode at low bias condition). Under zero or reverse bias, PIN diode has a low capacitance and very high impedance which resist the flow of RF signal. Under a forward bias of 1 mA, a typical PIN diode will have an RF resistance of about 1 Ω, making it a good RF conductor. Consequently, the PIN diode makes a good RF switch. At RF frequency the PIN diode resistance is governed by the DC bias applied. In this way it is possible to use the device as an effective RF switch or variable resistor for an attenuator producing far less distortion than ordinary PN junction. 2.4.3 Important Features of PIN diode 1. A microwave PIN diode is a semiconductor device that operates as a variable resistor at RF and microwave frequencies. The value of resistance varies from 1Ω (ON) to10 kΩ (OFF) depending on the amount of DC current ﬂowing through it. 2. Due to lightly doped I layer it has high carrier life time, high breakdown voltage low junction capacitance, high switching speed and poor reverse recovery time. 3. A PIN diode is a current controlled device in contrast to a varactor diode which is a voltage controlled device. 4. When the forward bias control current of the PIN diode is varied continuously, it can be used for attenuating, leveling, and amplitude modulating an RF signal. 5. When the control current is switched on and off, or in discrete steps, the device can be used for switching, pulse modulating, and phase shifting an RF signal. 6. PIN diodes are used to control RF power in circuits such as switches, attenuators, modulators and phase shifters. 7. High voltage current controlled RF resistor for RF attenuator and switches. 2.5. CHARACTERIZATION OF PIN DIODE AND ITS BIASING COMPONENT PIN diodes are often used as a switch that controls the path of RF signals. The fundamental parameters that describe PIN diode switch performance are: Isolation and 41
Chapter 2 Insertion loss. Physically, Isolation is a measure of the RF power through the switch that is not transferred to the load, when the switch is OFF. But practically, isolation is a measure of how effectively a PIN diode switch is turned OFF. Insertion Loss (IL) is measure of transmission loss through the physical structure of a PIN diode switch. This is a measure of large values of bias current plus RF current may flow through the switch structure, causing significant ohmic loss under the ON state . Working operation of PIN diode as a switch can be easily explained by Fig. 2.10 (a). To bias the PIN diode accurately, it is necessary to provide some degree of isolation between DC signal and the RF signal. Otherwise, RF current can flow into the power supply's output impedance, causing unfavourable effect to the efficient operation of the power control circuit. The DC bias supply is isolated from the RF circuits by inserting an RF inductor in series with the bias line and a RF by-pass capacitor, in shunt with the power supply output impedance the RF control circuit. In Ansoft HFSS simulation, PIN diodes are modeled using lumped RLC boundary PIN diodes. For forward bias, Infelon diode is modelled as a forward resistance of 2.1 Ω, and lead inductance= 0.6 µH as shown in Fig. 2.10 (b) and in reverse bias it is modeled as a reverse parallel resistance = 3 KΩ, capacitance = 0.17 pF and lead inductance = 0.6 µH as shown in Fig. 2.10 (c). The simulated S-parameter for PIN diodes is shown in Fig. 2.11. It is observed that in ON condition, insertion loss is 0.1 dB from 1GHz to
Fig.2.10 The biasing circuit of PIN diode (b) equivalent circuit in ON state (c) OFF state
Fig.2.11 S-parameter of PIN diode in OFF and ON condition
Chapter 2 8 GHz hence diode would offer low impedance and acts as short circuit for RF signal. When PIN diode is OFF; insertion loss is greater than 18 dB as shown in Fig. 2.11; hence it exhibits high impedance so there is no propagation of power from source to load terminal. It is important to figure out the insertion loss of each component when actually embedded in the fabricated prototype. In this research Coil Craft Inductor , Murata SMD ceramic multilayer capacitor  and Infineon PIN diodes  are used in testing. To find out the working behaviour and frequency response of these components, some prototype for each biasing component is fabricated and tested. The detail for the characterization of each component is described as follows. 2.5.1 Testing of a Simple 50Ω Microstrip line A simple microstrip structure is designed on 20x20mm2 sized FR4 substrate having thickness 1.57 mm and relative dielectric constant
= 4.4. The schematic for microstrip line,
electric field distribution over it and the fabricated photograph are shown in Fig. 2.12. The characteristic impedance of microstrip through line is 50Ω. Fig. 2.13 shows the simulated and measured S-parameter of microstrip line.
Fig.2.12 (a) 50Ω Microstrip Line (b) E-field Distribution (c) fabricated photograph
Fig.2.13 (a) S-parameter vs. frequency of a microstrip line (b) measurement setup
Chapter 2 Ideally 50Ω microstrip line offer 0 dB insertion loss for RF signal but practically there is some insertion loss. In simulation, value of insertion loss is 0.5dB which is nearly constant from 1 to 10 GHz whereas measured results show that the insertion loss is better than 0.1 dB from 1 to 6 GHz and 0.4dB from 6 to 9 GHz. 2.5.2 Testing of SMD Capacitor as a RF bypass element The RF bypass/DC Block capacitor offer minimum resistance for RF signal but blocks the DC signal. Therefore when a capacitor is placed in between the 50 Ω microstrip line, it should bypass the RF with minimum loss. The 30pF SMD Ceramic Multilayer Capacitor is used for characterization of capacitor. To figure out the insertion loss of a 30 pF capacitor; it is mounted in the 0.5 mm wide gap of 50 Ω microstrip line as shown in Fig. 2.14. In HFSS simulation, capacitor is assigned as 30 pF using lumped boundaries condition. Fig. 2.14 (a) shows the geometry of the proposed layout, electric field distribution and the fabricated photograph. Simulated and measured S-parameters are shown in Fig.2.15.
Fig. 2.14 (a) Layout for Capacitor under test (b) E-field Distribution (c) fabricated photograph
Fig. 2.15 (a) S-parameter vs. frequency for Capacitor under Test (b) measurement setup
Chapter 2 Simulated value of insertion loss is 0.5dB from 1GHz to 7GHz whereas measured value is 0.2 dB from 1-7 GHz. So, the SMD capacitor is used to bias the PIN diode and also to isolate the different DC voltage regions while maintaining RF signal continuity from 1-7 GHz. 2.5.3 Testing of Coil Craft Inductor as a RF choke element Now, to check the behaviour of inductor in the circuit, a prototype for DC block bias line has been designed as shown in Fig.2.16. The Inductor and capacitor used here for characterization are coil craft Inductor L=0.3μH, Murata 0.3nF ±5% 50V dc Dielectric SMD Ceramic Multilayer Capacitor. The prototype consists of two SMD capacitors in 0.5mm wide gap and two inductors connected with bias pads via thin interconnecting lines. From this we are checking that RF capacitors offer minimum impedance to RF signal in the presence of DC voltage and the inductor offer low impedance towards DC signal. When +5V signal is applied on DC pad, the SMD capacitor will offer minimum impedance for RF signal but maximum impedance for DC signal.
Fig. 2.16 (a) RF choke under test (b) E-field Distribution (c) fabricated photograph
Fig. 2.17 (a) S-parameter vs. frequency of a DC block under test (b) measurement setup
Chapter 2 So, the transmitted RF signal must have low insertion loss. Fig. 2.17 show the simulated as well as measured S-parameter for it which clearly shows that insertion loss is better than 2dB and return loss is better than 13dB from 1 to 7 GHz. So, it can be is conclude that presence of DC voltage don’t affect the transmission of RF signal if the value of capacitor and inductor is chosen accurately for specific frequency range. 2.5.4 Testing of Infineon PIN diode as a RF switch The prototype to characterize PIN diode as a switch is fabricated and tested as shown in Fig. 2.18. The biasing scheme is very simple, requiring only two RF choke coil and a two DC blocking capacitors. The PIN diode used for characterization is Infineon BAR-64-02,with operating parameter: Diode reverse voltage =150V, total breakdown current = 5μA, Diode forward current = 100mA, operating voltage = 1.1V, forward current at 1.1 V = 50mA, Diode capacitance at 0.17pF, Reverse parallel resistance = 3KΩ, Forward Resistance =2.1Ω, Insertion loss =0.16dB, Isolation =22dB.
Fig.2.18 (a) PIN diode under test (b) E-field Distribution (c) fabricated photograph
Fig.2.19 (a) S-parameter vs. frequency of PIN diode under test (b) measurement setup