7. Liquid Crystal and Liquid Crystal Polymer based Antennas

Transcription

1 Chapter 7 7. Liquid Crystal and Liquid Crystal Polymer based Antennas 7.1 Introduction: Depending on the temperature, liquid crystal (LC) phase exists in between crystalline solid and an isotropic liquid. In this state the material can flow like a liquid but at the same time molecules have orientation order. A typical LC molecule has a rod like shape as shown in the Fig 7.1. The size of the molecule is typically is few nanometres. This shape anisotropy causes anisotropy in terms of dielectric constant. Fig 7.1 Typical Liquid Crystal Molecule and its temperature dependency Cross section of Liquid Crystal based molecule orientation with bias voltage is shown in Fig 7.2. Depending on the RF field distribution and n, LCs feature anisotropic electrical properties. Thin polyimide film is coated on the inner surface of the substrates to orient the LC molecules parallel to the surface initially. Fig 7.2 LC Molecules orientation with applied bias voltage 152

2 At this level the RF field distribution is perpendicular to the director n as shown in the Fig 7.2 and the relative permittivity and loss tangent along the short axes are effective. The orientation of the molecules will be same if the applied bias voltage is less than the threshold voltage. If applied voltage exceeds certain voltage Vmax then all molecules will be aligned parallel to the bias voltage. When voltage is released, then molecules return to the initial state due to the polimide film. Required time for this process is defined as switching time which depends on LC material, its thickness, temperature and its oriention mechanism. All other states between ε and ε equivalently continuous tunability, can be achieved by varying the applied voltage between the threshold voltage and Vmax. A general characteristic of LC that is dielectric constant and loss tangent versus bias voltage is plotted in Fig 7.3. Fig 7.3 ε r and tan δ characteristics of LC material Vs bias voltage Nematic phase state liquid crystal is a non linear dielectric material in which the dielectric constant can be changed between two extreme states that are described by the orientation of the liquid crystal molecules, either parallel or perpendicular to the exited RF field. The effective dielectric anisotropy can be defined as ε = ε - ε Where ε = Dielectric anisotropy ε = LC Permittivity with DC voltage ε = LC Permittivity without DC voltage Instead of placing a microstrip line on the top substrate, microstrip patches can be realized as antenna elements. Reconfigurable reflect arrays also can be designed based on the principle of variable patch dimensions. Instead of changing the dimensions of the metalized patch, dielectric properties of LC under the patches are tuned with a bias voltage. Although all the patches have identical physical dimensions, they have different electrical properties. This results in different 153

3 backscattered phases. Beam forming is possible as different patch lengths from a feed to the patches are compensated by the preadjusted phases of the patches. Our intension is to use the liquid crystal and liquid crystal polymer based materials as substrate materials in the design of microstrip patch antennas. By looking at the basic operation mechanism in terms of dielectric anisotropy with change in bias voltage between two electrodes, we can tune the antenna to our desired frequency and digital controllability will be in our hand. Liquid crystal substrate material can be attained with LC cavity by placing spherical spacers with small diameter between two substrates. Although antenna consists of three dielectric layers which are top substrate, LC layer and bottom substrate the thickness should be maintained low to attain compactness in size. 7.2 Liquid Crystal Polymer Material To reduce the overall size, structural complexity and the cost of RF systems, multilayered substrate materials in which passive elements are Integrated as distributed elements inside the substrate/packing layers is required. A solution for the compact integration of these passive components, potentially with embedded active chips as well is the system-on-package (SOP) approach. In SOP package, connecting passive and active components on the board and encapsulating the assembly inside of a robust package are two steps critical to reliable operation of the RF systems. The package should create an acceptable environmental seal without significantly affecting the electrical characteristics of the entire circuit. Specifically, the electrical discontinuities created from the package should create minimal reflections and minimal inductive or capacitive parasites to avoid throwing off the sensitive matching circuit of active devices. The most important material characteristics for use in SOP systems are low electrical loss and excellent seal integrity, generally specified by permeability to moisture and gases. So many engineered materials are excellent in performance, but expensive in the fabrication. For example alumina has dielectric constant 10, which makes it appropriate for so many antenna applications. In addition, alumina has a high thermal coefficient of dielectric constant, which means its dielectric constant changes significantly with temperature. This is why network analyzers, which frequently used 154

4 alumina substrates, are often left turned on continuously to ensure the dielectric. Properties of the RF substrates are stabilized. A lamination of these microwave composites and alumina materials is that none is capable of creating homogeneously laminated compact 30 integrated RF modules. In other words, these microwave boards must use other adhesive materials, which have significantly worse water and gas permeability characteristics to achieve a compact stacked configuration. Fully hermetic packaging with microwave boards is often required for military specifications or in satellites, but at much greater financial expense. The term hermetic means that zero water or gases will permeate the package. Low temperature co-fired ceramic (LTCC) is the most commonly used ceramic substrate material for compact RF-design systems. LTCC has very low electrical loss and can be used in multilayer laminated modules that have densely integrated passive and active devices stacked and connected vertically to save space and cost. The major drawback with LTCC is with its high dielectric constant which decreases the antenna radiation efficiency. 7.3 Liquid Crystal Polymer Substrate LCP has drawn much attention for its outstanding packaging characteristics. LCP is low-cost material with the best packing characteristics of any polymer has generated great interest in using it as a substrate material for mm-wave applications. A comparison of these packing characteristics Vs all other polymers are shown in Fig 7.4. Low water absorption should be there for microwave substrate materials for better stability and reliability. Generally for organic materials the range of water absorption characteristics is from 0.02% to 0.25% or more. Fig 7.4 Water and oxygen permeability of different polymers 155

5 Fig 7.4 shows the water and oxygen permeability of different polymers, in which liquid crystal polymer is having low water vapour transition. Not only for less water absorption characteristic, but also for several reasons we prefer LCP material in the design of RF and microwave modules. Some of the key features are LCP is having excellent high-frequency electrical properties, stable ɛ r and low loss tangent for frequency < 35GHz. Quasi-hermetic Low coefficient of thermal expansion(cte) Recyclable Cost is less Naturally Non-flammable(Environmentally Friendly) Flexible Multilayer all LCP laminations capabilities to create multilayer LCP RF modules Relatively low lamination processing temperature(= c) Low dielectric constant for use as an efficient antenna substrate. Fig 7.5 (a) Inset fed Microstrip antenna on LCP Substrate (b) Twisted LCP antenna (c) Bended LCP antenna (d) Rolled LCP Antenna (Courtesy by Google Images) Fig 7.5 shows Inset fed flexible liquid crystal polymer based microstrip antenna, which can be twisted, bended and rolled without affecting its performance characteristics. 156

6 7.3.1 FELIOS LCP FELIOS liquid crystal polymer is a flexible circuit board material, which has high frequency characteristics and low loss tangent after moisture absorption. Now a days in so many applications like notebook PC s and smart phones these materials are been placed instead of traditional materials. The features of this material includes a) High dimensional stability b) Good peel strength of copper foil c) Excellent high frequency properties Ultralam 3850 (Liquid Crystalline Polymer Substrate Material) Ultralam 3850 is the emerging liquid crystalline polymer circuit material from Rogers Corporation. The features like excellent high frequency properties, good dimensional stability, external low moisture absorption and flame resistant makes this as one of the potential substrate material for RF circuit design. So many benefits are associated with this material, which are listed below Excellent and stable electrical properties for impedance matching Uniformity in the thickness for maximum signal integrity Flexible material for conformal applications i.e. bends easily In humid environment it maintains the stable mechanical, electrical and dimensional properties. The applications includes high speed switches and routers, chip packing, MEM s, military satellites, radar sensors, hybrid substrates, handheld RF devices and in the design of antennas. 4.5 Frequency Vs Dielectric constant D ielec tric C ons ta nt LCP at 50 C LCP at 23 C FR4 at 50 C FR4 at 23 C Frequency Fig 7.6 Frequency Vs Dielectric constant of LCP and FR4 at 23 0 C and 50 0 C 157

7 The electrical and environmental properties of Ultralam 3850 at 10 GHz, 23 0 c are tabulated as shown in Table 7.1. Table 7.1 Electrical and Environmental Properties of Ultralam 3000 LCP material S. No Parameter Value 1 Dielectric Constant Dissipation Factor Surface Resistivity 1x10 10 Mohm 4 Volume Resistivity 1x10 12 Mohm cm 5 Dielectric Breakdown Strength 1378(3500) KV/cm(v/mil) 6 Water Absorption (23 0 c, 24 hrs) 0.04% 7.4 Package and Interconnecting Traditionally with high dielectric constant materials, introducing air cavities inside of multilayer RF modules for embedding chips or other elements creates impedance discontinuities that cause reflections and can destroy RF performance. In addition many package cavities rely on metal bonding rings around the cavity interface, which necessitates more difficult feed through solutions such as re-routing and tapering the transmission line underneath the seal. A unique possibility with LCP, because of its low dielectric constant and multilayer lamination capabilities, is to form cavities in the substrate before lamination to provide sandwiched all LCP constructions that can pass transmission lines directly through the package interface with negligible effects on the RF performance. The LCP low dielectric constant would enable the superstrate packing to accommodate chips, MEMS and other devices without any concern for the parasitic packing effects. Because of the flexibility and low cost, the LCP can be used in conformal antenna designs. 7.5 Flexibility Consideration Flexibility is one of the key factors for LCP materials usage in the design of conformal antennas. Tests were performed on the mechanical rollability and the effects of rolling on antenna performance. The procedure for performing this antenna testing included 158

8 1. Ensuring measurement repeatability when connecting/disconnecting antenna in the default flat state 2. Performing flexure testing on the antenna, rolling it on to tubes with various diameters 3. Re-measuring and observing once again the potential differences in measurement or visual structural changes. In this procedure we observed that there are very minute changes in the reflection coefficient of antenna at resonating frequency and in all the cases, there is no frequency shift is observed. 7.6 Liquid Crystal Patch Antenna People are attracting towards the development of microwave tuneable devices for various applications. To develop these devices, liquid crystals are gaining much attention because of their anisotropic behaviour, which permits to change in the resonant frequency and reflection phase. By using ferroelectric phase shifters and varactor diodes, we can implement electronic beam scanning but they will increase the system complexity and cost. (a) (b) Fig 7.7 Rectangular patch antenna on LC Material, (a) Antenna Model, (b) Side View A rectangular patch antenna is designed to operate at X band with N15 liquid crystal material as substrate with thickness of 650 µm on ground plane of dimension 42X27 mm as shown in Fig 7.7. Electrodes are connected between patch element and ground plane to apply biasing voltage. Voltage is applied through electrodes to antenna from 1V to 20 V and it is observed that dielectric constant is varied between 159

9 2.17 to 2.27 in this range. Fig 7.8 shows the change in resonant frequency with the change in bias voltage and Fig 7.9 shows change in reflection phase with change in bias voltage and dielectric constant. R e f le c t io n C o e f f ic ie n t in d B V 4 V Frequency in GHz Fig 7.8 Return loss Vs Frequency of LC antenna at two stages of voltage The simulated tuning range was 6% while the measurement is showing around 4%. Losses in the liquid crystal material are giving antenna efficiencies ranging from 30-35% at these frequencies when the liquid crystal was in unbiased state. If we consider at millimetre wave frequencies, the LC exhibits lower loss tangents with highly efficient in performance. R e f l e c t i o n P h a s e (d e g ) V LC 2.10, tand LC 2.27, tand V Frequency in GHz Fig 7.9 Frequency Vs Reflection phase of LC antenna Fig 7.10 Radiation pattern for single element LC patch antenna 160

10 Fig 7.11 Radiation pattern for 2X2 array LC patch elements Fig 7.10 shows the simulation radiation pattern of the single element liquid crystal patch antenna and 2x2 array liquid crystal patch antenna. E-plane radiation pattern seems to be directive and H-plane pattern of butterfly like in single element and nulling at for 2x2 array antenna. Table 7.2 shows the antenna parameters for single element and 2x2 array case. Gain is considerably increased with array implementation but efficiency is constant. Table 7.2 Liquid crystal antenna parameters for single element and 2x2 array S No Parameter Single Element Patch 2x2 array patch 1 Peak Directivity 6.37 db 19.5 db 2 Peak Gain 4.85 db db 3 Peak Realized Gain 4.05 db db 4 Radiated Power w w 5 Accepted Power w w 6 Incident Power w w 7 Radiation efficiency 76% 76% The results are giving strong evidence to apply liquid crystal materials in the design of antennas for tuneable applications. The existing phase shifters that are been used in millimetre wave applications, which have performance limitations can be replaced with liquid crystals material based devices. 7.7 Balanced Antipodal Vivaldi antenna (BAVA) on LCP Substrate Balance antipodal antenna is a modified version of antipodal antenna. The structure seems to be like antipodal with two chords, in which it consists of three conducting arms and the remaining part is ground plane. This balanced antipodal antenna normally contains two chords in which each chord contains two planes. Top substrate contains conducting arm on the plane and copper etching on the other plane. Bottom 161

11 substrate contains slot line on one plane and conducting arm on the other plane. The conducting path is increased in this balanced type reducing the ground plane, surface waves have been decreased and so cross polarization has been decreased. The improvement in the cross polarization performance was brought about by converting the usual antipodal Vivaldi into triple structure, by inserting additional substrate and the metallization layer, which balance the electric field distribution in flared slot. Fig 7.12 Balanced Antipodal Vivaldi Antenna (a) HFSS Simulated Model, (b) Fabricated Prototype Design Steps: By taking operating frequency (f r ), height of the substrate (h) and dielectric constant ε r, the antenna length and width can be calculated using the formula W L c f r (1) 1 r The radiating structure can be formed from intersection of quarters of two ellipses. The primary radii r1 and r2, secondary radii rs1 and rs2 can be taken as w wm r (2) w wm r2 2 2 rs1=l, rs2=0.5r2 The transmission feeder width W m with impedance Z o equal to 50 ohms can be determined by w m 120 r 162 h Z (3)

12 The antenna dimensions are listed in table 7.3 Table 7.3 Balanced Antipodal Vivaldi Antenna Dimensions S. No Parameter Value 1 Antenna Length 52 mm 2 Antenna Width 28.5 mm 3 Slot Width 8 mm 4 Slot Length 36 mm 5 Transmission Line Width, Wm 2.5 mm 6 Transmission Line Length, Lm 16 mm 7 Operating Frequency Range 6-18 GHz 8 Dielectric Material Used LCP with r =2.9 9 Feed Element Length 8 mm 10 Feed Element Width 6 mm BAVA Parameters Fig 7.13 Return Loss Vs Frequency of Balanced Antipodal Vivaldi Antenna Fig 7.14 VSWR Vs Frequency of Balanced Antipodal Vivaldi Antenna Fig 7.15 Measured Return loss Vs Frequency of Balanced Antipodal Vivaldi Antenna 163

13 Fig 7.13 shows the frequency Vs return loss of the antenna and Fig 7.14 shows the VSWR Vs frequency curve. The measured results are taken on Agilent Vector Network analyzer. Calibration is done using the standard loads supplied by the manufacturer. Fig 7.15 shows the measured return loss Vs frequency curve on network analyzer. All the measurements are carried out carefully by not disturbing the cable setup, which is necessary for accurate measurement. Fig 7.16 Simulated Radiation Pattern Plots of Balanced Antipodal Vivaldi Antenna at 6, 9, 12, 15, 16 and 18 GHz Fig 7.17 Measured radiation Pattern Plots of Balanced Antipodal Vivaldi Antenna at 6, 9, 12, 15, 16 and 18 GHz 164

14 The proposed model has reflection coefficient below -10 db through the entire frequency band i.e. from 6-18 GHz and good radiation pattern with minimised back and side lobes. Radiation patterns of the proposed model in simulation and measurement is shown in Fig 7.16 and Fig 7.17 respectively. Frequency Vs Gain Simulation Measured G a in in d B Frequency Fig 7.18 Frequency Vs Gain of Balanced Antipodal Vivaldi Antenna Maximum gain obtained through simulation is 12 db and practically it is 8.5 db. By considering all the factors, the designed balanced antipodal antenna performance is excellent in wideband applications. 7.8 Wideband tapered step antenna on LCP Substrate Performance study of wideband tapered step antenna on liquid crystal polymer substrate material is presented. Bandwidth enhancement is achieved by adding step serrated ground on the front side of the model along with the radiating patch. The radiating patch seems to be the intersection of two half circles connected back to back. The lower half circle radius is more than upper half circle radius. Fig 7.19 Wideband tapered step antenna on LCP Substrate Wideband tapered step antenna is designed on the liquid crystal polymer substrate (Ultralam 3850, ε r = 2.9) with dimensions of 20X20X0.5 mm. Coplanar waveguide 165

15 feeding is used in this model with feed line width of 2.6 mm and gap between feed line to ground plane of 0.5 mm Antenna Parameters Fig 7.20 shows the return loss curve for the LCP based wideband tapered step antenna. Antenna is resonating between 5.2 to 16.6 GHz with bandwidth of 11.4 GHz in the simulation and Fig 7.21 shows the measured result from network analyzer from GHz with bandwidth of 10.7 GHz. There is a small difference of 0.7 GHz in bandwidth is observed from simulate result due to poor quality in the SMA connector with feed line and ground plane. Fig 7.20 Simulated Return loss Vs Frequency of wideband tapered step antenna on LCP substrate Fig 7.21 Measured Return loss Vs Frequency of wideband step serrated antenna on LCP substrate Fig 7.22 shows the antenna three dimensional radiation view at 13.6 GHz. Radiation pattern of omni directional in E-plane and quasi omni directional in H-plane with low cross polarization levels can be observed from Fig

16 Fig 7.22 Three dimensional view of radiation for wideband tapered step antenna at 13.6 GHz Fig 7.23 Radiation pattern in E and H-plane of wideband tapered step antenna at 13.6 GHz Fig 7.24 Current distribution of wideband tapered step antenna at 13.6 GHz Fig 7.24 shows the simulated current distribution of the antenna at 13.6 GHz. The current intensity is maximum at radiating element and feed line towards x-direction with equal magnitude but opposite in polarity. Fig 7.25 shows the frequency Vs gain plot for the liquid crystal polymer antenna. From this result we can observe that gain is increasing up to 14 GHz and after reaching to the peak gain of 4 db, the gain is decreased at higher frequency. 167

17 Fig 7.25 Gain Vs Frequency of wideband tapered step antenna Flexibility Testing Actual reason for choosing LCP material in the design is to have a flexible model, which should not produce odd results when it is placed on different surfaces. The ability of this model is also tested by bending the antenna in different angles and in each case the reflection coefficient result is noted. The current model is placed in different tubes with various diameters for this testing R e t u r n lo s s in d B Measurement 1 Measurement 2 Measurement 3 Measurement Frequency in GHz Fig 7.26 Flexibility testing of LCP antenna with different angles By consolidating all these cases of testing we observed not much variation in the frequency of operation rather than a small shift in the frequency, which gives the potential of the LCP material in the conformal applications. Fig 7.26 shows the testing result of the antenna with different bending angles. 7.9 Comparison of different liquid crystal antennas Table 7.4 shows the comparison of the liquid crystal based antennas performance characteristics. Liquid crystal antenna can be used for tuneable applications and liquid crystal polymer antenna can be used for conformal applications. Balanced 168

18 antipodal is covering large bandwidth but thickness of the substrate is more so wideband tapered step antenna with 0.5 mm substrate thickness can be used for conformal applications. Table 7.4 Comparison of Liquid Crystal Antennas S. No Antenna Model 1 Liquid Crystal Antenna 2 Balanced Antipodal Vivaldi Antenna on LCP 3 Wideband Tapered Step Antenna on LCP Dimensions in mm Resonant Frequency in GHz Bandwidth in MHz Impedance Bandwidth % Gain in db Efficien cy % Applications 42x27x % % Tunable Applications 52x28.5x GHz % % Wideband applications 20x20x % 4 95% Conformal applications 7.10 Chapter Summary: Three models are studied namely liquid crystal antenna, liquid crystal polymer based balanced antipodal Vivaldi antenna and wideband tapered step LCP antenna. The dielectric anisotropy of liquid crystal material with change in temperature due to applied bias voltage is the base for the first model liquid crystal antenna. By applying DC voltage to electrodes connected between patch and ground plane, tunability in the resonant frequency is obtained. Peak gain of 4.85 db and efficiency of 76% is achieved from this model. A balanced antipodal Vivaldi antenna is constructed on liquid crystal polymer substrate material and it is resonating between 6 to 18 GHz with 12 GHz bandwidth. Another model of wideband tapered step liquid crystal polymer material based antenna is designed to operate between 4.8 to 15.5 GHz with bandwidth of more than 10.7 GHz. Omni directional radiation with peak realized gain of 4 db is attained from this model. Flexibility of the antenna is tested by placing the model in different tubes with different diameters and observed the constant reflection coefficient results over the frequency range. So this model on liquid crystal polymer substrate can be used in the conformal wideband applications. 169