Super-Resolving Biomimetic Electrically Small Antennas and Their Applications

Transcription

1 Super-Resolving Biomimetic Electrically Small Antennas and Their Applications Nader Behdad Department of Electrical and Computer Engineering University of Wisconsin-Madison IEEE Central North Carolina Chapter Friday April 1, 2011

2 Overview Review of Research at Applied EM Lab Problems of electrically-small antennas Drawing inspiration from nature to design enhanced ESAs Directional hearing in small animals The concept of super-resolving biomimetic electrically-small antenna arrays. Advanced BMAA architectures Potential applications of BMAAs: DOA estimation High-resolution, small-aperture radar systems Microwave and mm-wave imaging Summary Acknowledgements 2

3 Overview of Research at UWAEL Research at University of Wisconsin Applied Electromagnetics Laboratory Periodic Structures at RF/Microwave Frequencies: High-power-capable microwave metamaterials for high-power phased array, spatial filtering, and pulse shaping applications. High-resolution microwave lenses: Continuous aperture phased MIMO: A Hybrid analog/digital front ends for optimum agile wireless communication. Antennas for Medical Imaging Applications: Antenna arrays for microwave imaging of the breast: Early detection of breast cancer. Electrically Small Antennas: Addressing the fundamental problems of electrically small antennas: Biomimetic, super-resolving electrically-small antenna arrays. Ultra-wideband miniaturized antennas. 3

4 Periodic Structures at RF/Microwave Frequencies Goal: Develop new technologies for designing high-power, affordable phased arrays. Phased array antennas: Used in communication and radar applications. Primarily used in military systems. Excessive cost Power handling capability Our solution: Use a class of sub-wavelength periodic structures capable of handling very high power levels. Tunable microwavelenses forhigh powerphased arrays. phased arrays. HPM-capable spatial filters and FSS. Nonlinear lenses for HPM pulse shaping applications. 4

5 Periodic Structures at RF/Microwave Frequencies Tunable, spatially-fed microwave lens: Synthetic structure (a.k.a. artificial structure, metamaterial, etc.). A single high-power source feeds the aperture: Phase shift gradient of the aperture is dynamically changed. Unique implementation of the lens: No solid state phase-shifter is required for beam steering. Feed network losses are eliminated. High-power capable, Low-loss, Wideband, d and Light-weight. i 5

6 Periodic Structures at RF/Microwave Frequencies Photograph of a fabricated lens prototype operating at X band. Focusing properties of the X band lens. The x mark shows the focal point of the lens. 6

7 Non-Linear Lenses for Pulse Shaping Applications Non-linear transmission lines: Compact HPM sources, no vacuum, wide range of waveforms, Giga Watt power levels desired. Challenges with NLTLs: GW power levels 10 KA and 100KV. Limits the circuit elements that could be used. Since this power needs to be radiated: Conventional HPM source for power generation. Non-linear lens for pulse shaping & beamforming. Non-linear lens: Distributing the radiated power over large aperture alleviates the requirement to work under very high voltage or current levels. 7

8 Electrically Small Antennas 8

9 What is an Electrically Small Antenna? HF Whip Antenna What is an electrically small antenna (ESA)? One having dimensions significantly smaller than the operating wavelength For examples, this is a small antenna! What do we want from them? We want to make them work in the first place but also: Work efficiently. Over wide frequency bands. And work at very low frequencies. In some applications, we want them to be directional. What is the problem? They do none of these. 9

10 Problems of electrically-small antennas Decreasing the size of an antenna, compared to the wavelength, has some very important consequences. In ESAs, we deal with two types of limitations: Fundamental limitations and practical limitations. Fundamental limitations: Size Antenna Bandwidth (Q ). Capacity of a wireless system is directly related to the bandwidth. The antenna may not be able to cover the desired bandwidth. Size Radiation efficiency and antenna gain. This affects the signal to noise ratio of a receiver (SNR) and hence, the capacity of the system. Think of dropped calls and battery life. Despite these consequences, antenna miniaturization is a must. Why? 10

11 Why do we need to miniaturize antennas? Miniature antennas have a lot of applications. Miniature does not necessarily mean physically y small Military applications: Large physical dimensions Very low frequencies Hence, electrically small Consumer electronics applications: Not much physical volume Many individual antennas Physically and electrically small 11

12 Miniaturized UWB ESAs Wideband, artificial magnetic conductor ground plane for the UWB electrically small antenna. Dual mode, UWB ESA Concept. The antenna has two complementary (in frequency) modes of operation. 12

13 Miniaturized UWB ESAs Photograph and measurement results of the latest prototype of the dual mode UWB miniaturized antenna. The antenna has a UWB response with a lowest frequency of operation of 100 MHz. Both modes have consistent radiation characteristics. 13

14 On-Chip Antennas and Fully- Integrated Radio Systems 550 μm 550 μm LNA An X-Band Integrated Antenna (9-10 GHz) Radiation efficiency 10% Chip area: 550μm 550μm (0.3 mm 2 ) Electrical Dimensions: 0.016λ λ 9.0 GHz A 5.0 GHz Fully Integrated Super Regenerative Radio 5 Ghz On Chip Antenna Area: 600μm 670μm (0.4 mm 2 ) Electrical Dimensions: 0.01λ λ 5.0 GHz IBM0.13μm CMOS Process 14 14

15 Practical Limitations of Small Antennas Small antennas are not very directional. They receive (transmit) energy from (to) almost all directions. Example: Cell phone antennas. Physically and electrically small and non-directional. Example: Radar antennas, electrically large and very directional. This, however, is not a fundamental limitation. In theory, we can have super- directive electrically-small ll ll antenna arrays. 15

16 Superdirective Electrically Small Antennas Practical limitations for achieving super-directivity: Requires widely oscillatory excitation coefficients. Precision in synthesizing exact current distributions or excitation coefficients. Practically impossible due to the mutual coupling between elements. Applications of a super-directive ESA? High-resolution, small aperture radar systems High-resolution, miniaturized i i imaging i systems Biomedical imaging systems Miniaturized MIMO communication systems Direction of arrival estimation 16

17 Biological Antennas? An Interesting Thought: How would a biological entity evolve the capability to receive and transmit electromagnetic waves if an evolutionary environment was present in which it would have needed this capability to survive? The answer to this question could reveal new methodologies and concepts for designing small antenna systems that work within the bounds set by the laws of physics but deliver performance levels, in man-made devices, that have not been achieved to date. What would such a organism look like and how would that operate? Can we replicate this in the lab? 17

18 Examples of Interesting Biological Sensing Mechanisms Magnetoreception? Rapid motion detection with compound eyes. Electroreception: sensitivity 5 10nV/cm in sharks. Long range infrared detection: Pyrophilous Beetle (Melanophila acuminata) Directional hearing: Parasitoid Fly (Ormia Ochracea) Electroreception: sensitivity uv/cm in platypus. 18

19 Directional Hearing What is directional hearing? Essential to the survival of most animals. How does directional hearing work? Humans and most animals have two ears. Based on its direction of arrival, the sound wave arrives at one ear earlier than the other. Interaural time difference (ITD). Because of the scattering caused by the head, the intensity it of the sound received at the two ears are different. Interaural intensity difference (IID). ITD and IID are the main cues used for sound localization. In humans and large animals: Large separation between the two ears: Large ITD. Big head sizes: Large IID. Easy sound localization. 19

20 Directional Hearing in Small Animals What happens as the size of the animal decreases? Very small ITD and negligible IID. Then, small animals don t have directional hearing. Right? However, many small animals have acute directional hearing Desert Locust (Schistocerca gregaria) capabilities. Vertebrates: Frogs, reptiles, birds, etc. Insect World: Cicadas, crickets, parasitoid flies, grasshoppers. Grass frog (Rana temporaria) 20

21 Star of the Insect World in the Directional Hearing Area Parasitoid fly Ormia Ochracea Female Ormia flies rely on live male crickets for reproducing. Locate host at night by listening to the mating calls of the male crickets: Peak at 5.0 khz. Ormia has: Two ears separated by 500 μm. Corresponding to λ/140. Virtually no intensity difference The only cues are very minute ITDs. Yet the fly can resolve the DOA of the sound with a resolution of

22 How do small animals do it? ITD is too small and no IID. Nervous system cannot distinguish i the two signals. Solution: Coupled Ears. The coupled ear takes the small input time difference and converts it to a large output time (or amplitude) difference. The outputs of the coupled ears excite the auditory neurons. Enhanced time difference between the signals received by the brain It becomes easier to distinguish them. 22

23 Mechanical Model Ormia s Ears Mechanical model*: Predicts the measured frequency response of Ormia s ears. 2π λ ( ω) f ( ω) f ( ω) f ( ω) sinθ f = d λ s ( θ ) = kd sinθ y ( ω) y ( ω) = Φ ( θ ) Φin 1 2 out *R. N. Miles, D. Robert, and R. R. Hoy, "Mechanically coupled ears for directional hearing in the parasitoid fly Ormia ochracea," J. Acoust. Soc. Am. 98 (6), pp Dec

24 Electrical Model for Ormia s Ears Analogy between electrical and mechanical systems. Both circuits are coupled resonator systems. Both circuits have two inputs and two outputs. Inputs: amplitude and phase of the received sound. Outputs: t vibration amplitude F t d l it lt of tympanal membrane. Force voltage and velocity current Force current and velocity voltage Role of the circuits: Convert the small input phase difference to large output phase difference. 24

25 Biomimetic Electrically Small Antennas Analogy between hearing mechanism of animals and a two element antenna array. Two isotropic receiver antennas. Case 1: Regular array (Uncoupled ears) Same magnitude Small phase difference Φ out = Φin = kd sinθ Case 2: Biomimetic array (Coupled ears) Can have the same magnitude Enhanced output phase difference Φ out >> Φin = 2π / λ d 0 sinθ 25

26 How to quantify this I propose defining a quantity called sensitivity factor as: SF RA x sin x2 jkd θ y1 + y2 A θ jφout ( θ ) = 1+ e SFBMAA θ = = 1+ e x2 y2 B( θ ) ( θ ) = ( ) Sensitivity pattern: angular variations of sensitivity factor. Two simple power measurements are required for SF: ( ) ( ) 2 2 θ = 10log 10 y y / y SF BMAA ( ) 3dB beamwidth of the sensitivity pattern: The range over which Φ out (θ) varies in the ±90 range. SP quantifies the angular range over which a receiving BMAA composed of isotropic receivers can effectively determine the direction of arrival of an incoming EM. 26

27 Examples of Sensitivity Patterns Two isotropic i receiving i antennas BMAA 1 θ Φ with d=0.05λ 0. out = 2 tan θ3db Characteristics of Φ out : Nonlinear Φ in vs. θ. π Rapidly saturates to 180 deg. RA Φ out = 2 dsin λ ( θ) 27

28 The Concept of Virtual Aperture Expansion Virtual Aperture Expansion: Signals from the two outputs of the BMAA appear to have arrived from a regular array with a significantly larger element spacing. The concept is also observed as the spacing between the elements is increased. Comparison between regular arrays and BMAAs with large element spacing between them are shown: 28

29 BMAA Prototypes Prototype at 300 MHz. Monopoles, each 70mm long and 50 mm apart. Output currents sampled. BMAA is illuminated with plane wave in the azimuth plane. As a test case, a regular version of this array was also fabricated and tested in the same environment in the same manner. 29

30 Measurement Results Amplified output phase difference. More directional sensitivity pattern To achieve the same sensitivity pattern from a regular array, we need a 30 times larger aperture. 30

31 What is the Catch? Nothing in the universe comes for free. So, what s the catch here? Let s look at the circuit and the outputs again: d cos π sin( θ ) sin π sin( θ ) = λ0 + λ0 i 1( ω, θ ) j i ω θ 2 2 2(, ) ω ω ( 2 ) + 1 C ω + ω 1 D R R j L R j L s + c s s s 2 2 ω d ω = d cos π sin λ0 j 2 ω + C 2Rc jωls 1 R 2 ω d λ0 2 ω + D jωls 1 ω ( θ ) sin π sin( θ ) ( R ) s + s 2 There is a tradeoff between the beam-width of the sensitivity pattern and the available power at the output of the antenna. Small animals have traded the capability to hear over long distances in favor of the ability to precisely localize targets in the desired range. 31

32 What else can we do with BMAAs? 32

33 Beam Agile BMAAs Dynamically changing direction of maximum sensitivity: Phase amplifying nature Simple phase shifters will suffice. Adaptive beam width change without changing the array spacing. Adaptive beamwidth change in an electronically agile BMAA Electronic beam steering of BMAA 33

34 Animals with Multiple Ears or MIMO Ears A few interesting hearing mechanisms in animals: Multiple ears MIMO ears Concept of passive acoustic imager of environment. Example of some animals with multiple input auditory systems. Bladder grasshopper has 6 setof ears. Praying mantis has 2 set of ears but no directional hearing! 34

35 Multi-Ear BMAAs: Multiple Independently Controlled Beams Multi-element BMAAs: Multiple independently controlled sensitivity beams. Direction and beam width of each beam can be controlled individually. 35

36 Applications: High-Resolution, Small-Aperture Radar Systems Why do we use very large antennas in radar systems? Angular resolution. Range (higher gain antenna) Using BMAAs in a radar system: High-resolution small-aperture radars become possible. Such radars will have a shorter range. High-resolution, small-aperture radars: Micro UAVs, guidance and navigational systems, devices for assisting the disabled, 36

37 Applications: Miniature RF Sensors Miniature RF tracking sensor. Composed of a multibeam, BMAA with independently controlled sensitivity patterns. Can track a source of RF radiation in space. It can potentially be implemented entirely on a single chip. This sensor can also be thought of as a digital imaging device, which provides a digital image (mosaic image) of active radiation sources in the environment. 37

38 Applications: mm-wave imaging, biomedical imaging mm-wave imaging systems: Among other applications, used for screening passengers. Tradeoff between the aperture size and image resolution. BMAAs can enable: Miniature MMW imaging camera. Adaptive resolution change: Magnify areas of interest. Microwave medical imaging: Tradeoff between penetration depth and resolution. 38

39 Summary Nature provides us with numerous solutions to various challenging technical problems. Mimicking the directional hearing capabilities of small animals may offer a means of addressing one of the unresolved problems of electrically small antennas. Such biomimetic electrically small antenna arrays offer capabilities that are simply non-existent in ESAs today. They can be beneficial in a wide range of applications: Miniature direction finding systems High-resolution small aperture radars Miniaturized radio-frequency sensors High-resolution, small aperture microwave and MMW imaging systems. MIMO communication systems, etc. 39

40 Acknowledgements Thanks to all of my students whose works are featured in this presentation: Current students: Mudar Al-Joumayly Meng Li Suzette Aguilar Past students: Bin Yu (MS 2010) Thanks to the funding agencies: Wisconsin Alumni Research Foundation (WARF) accelerator program. National Science Foundation 40