CHAPTER - 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 22 Chapter 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 [1] for the calculation antenna dimension (L,W) at resonating frequency f0 are listed below: Effective dielectric constant  re   r    r  1  12  2  1 h2 W  Patch width: W = Patch length: L = (1) (2) (3) Extended length (ΔL) of patch due to fringing field ΔL = (4) Effective patch length: Leff = L + 2ΔL (5) 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 23 Chapter 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) 24 Chapter 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 [11], tapered line [12], 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 25 Chapter 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 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 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 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 = 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= BWNB = x100 (10) 27 Chapter If FH/ FL= 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≤ ensures good performance 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= (11) The VSWR expresses the degree of match between the transmission line and the antenna When the VSWR is to (1:1) the match is perfect and all the energy is transferred to the antenna prior to be radiated 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 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) 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 [1] In mathematical form, it can be written as D= (12) 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 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” 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 29 Chapter Fig 2.5 Comparison of various communication standards [18] 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: 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 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, watt of power spread across 1GHz of spectrum results in only 1nW of power into each hertz band of frequency Thus, UWB signals not cause significant interference to other wireless systems 30 Chapter 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 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 Security Aspects: UWB provides high level security and reliable communication 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 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 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 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 31 Chapter 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 RF MEMS PIN diode Optical switch Voltage[V] 20-100 3-5 1.8-1.9 Current [mA] 3-20 0-87 Power consumption[mW] 0.05-0.1 5-100 0-50 Switching speed 1-200 μ sec 1-100 n sec 3-9 μ sec Isolation[1-10GHz] Very high High High Loss{1-10 GHz}[dB] 0.05-0.2 0.3-1.2 0.5-1.5 (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 38 Chapter diode is used as a switch to controls the path of RF signals [73] 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 (a) (b) (c) 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 39 Chapter as metal pin length and diameter The resistance (Rs) of the I region under forward bias is given by μ μ (13) 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 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 mA, a typical PIN diode will have an RF resistance of about 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 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 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 A PIN diode is a current controlled device in contrast to a varactor diode which is a voltage controlled device 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 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 PIN diodes are used to control RF power in circuits such as switches, attenuators, modulators and phase shifters 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 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 [79] 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 = 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 42 Chapter 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 [74], Murata SMD ceramic multilayer capacitor [75] and Infineon PIN diodes [76] 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 (a) (b) Fig.2.13 (a) S-parameter vs frequency of a microstrip line (b) measurement setup 43 Chapter Ideally 50Ω microstrip line offer 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 to 10 GHz whereas measured results show that the insertion loss is better than 0.1 dB from to GHz and 0.4dB from to 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 (a) (b) Fig 2.15 (a) S-parameter vs frequency for Capacitor under Test (b) measurement setup 44 Chapter 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 (a) (b) Fig 2.17 (a) S-parameter vs frequency of a DC block under test (b) measurement setup 45 Chapter 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 to 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 (a) (a) (b) (b) Fig.2.19 (a) S-parameter vs frequency of PIN diode under test (b) measurement setup 46 Chapter When diode is forward biased by applying +5V on DC pad, then it offers minimum impedance for the flow of RF signal So, insertion loss S21 should be low as possible The simulated and measured S-parameter vs frequency is shown in Fig 2.19 The Simulated and measured results shows that insertion loss is 2.5dB for to 6.5GHz When diode is reverse biased it blocks the RF signal by offering maximum impedance In this case simulated and measured isolation loss is better than 15dB for 1-6.5 GHz The S11 parameter degrades beyond GHz So the working frequency range for PIN diode is to 6.5GHz for optimum switch operation So PIN diode is well suited as a switch for to 6.5GHz 2.6 SUMMARY This chapter describes the basic understanding of framework component to design and implement antenna solution for CR Firstly, introduction to microstrip patch antenna is described to define basic terminology of antenna parameter In the next, different 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