Saurabh Sanghai (1) *, Maxim Ignatenko (1), Kim Hassett (2) and Dejan S. Filipović (1)

Transcription

1 Antenna Measurement Techniques Association 36 th Annual Meeting and Symposium October 12-17, 2014 Saurabh Sanghai (1) *, Maxim Ignatenko (1), Kim Hassett (2) and Dejan S. Filipović (1) (1) University of Colorado, Boulder, Colorado Antenna Research Group ARG (2) Nearfield Systems Inc. - NSI Acknowledgement: This work has been sponsored by the Office of Naval Research under grant # N

2 Introduction Vertical Half-Loop Antenna Inverted-L Antenna Scaled Model Design Measurements Summary 2

3 HF frequencies : point to point communication from 0 to 1000s of km Ionosphere Skywave NVIS Tx/Rx Ground wave Three modes of communication: 1. Ground wave 2. Sky wave (long range propagation) 3. Near Vertical Incidence Skywave (NVIS) Ground NVIS antenna pattern Near Vertical Incidence Skywave (NVIS) communication: 2-10 MHz High incidence angles Any polarization Beyond line-of-sight 3

4 Goal is to design On The Move vehicular NVIS antenna Low Cost Low Profile 24 khz Bandwidth Simple Challenges: HF antennas have large physical size Narrow bandwidth Different grounds cause detuning Radiation pattern limits use of all the HF propagation modes Assault Amphibious Vehicle (AAV) High Power Requirements: 400 Watts Power Handling 24 khz bandwidth Low profile 4

5 Q: What is the minimum size of a lossless antenna that may achieve 24 khz BW at 2 MHz? Analytical Results: Numerical Validation: Antennas are much larger than AAV! Loop Dipole 39m AAV Spherical antenna 14m 23m 0.71 [3] 2 24kHz Estimation: / 1.5 where [1, 2] [3] References: [1] M. Gustaffson et al., Physical Limitations on Antennas of Arbitrary Shape, Proc. Royal Society A, (2007) [2] J.L. Volakis et al., Small Antennas: Miniaturization Techniques and Applications, McGraw-Hill, (2010) [3] A.D. Yaghjian and S.R. Best, Impedance, Bandwidth, and Q of Antenna, IEEE Trans. Antennas Propag., (2005) Obtaining 24 khz bandwidth at 2 MHz within a vehicle platform is very challenging! 5

6 Shakespeare [1] Cobham [2] Stealth 9400c [3] MHz 160 Watts VSWR: < 3.5: MHz 150 Watts VSWR: < 2.5: MHz 150 Watts VSWR: < 1.2:1 [1] Shakespeare Military Antenna Products Brochure, pp-15. Harris OTM Loop [4] 2-30 MHz 150 Watts VSWR: < 2:1 [4] [2] Codan 9350 [5] 2-30 MHz 125 Watts VSWR: < 1.5:1 [5] hf_ssb_antennas_9350.html [3] nf.php?id=372&id_categories=136 Stealth 9420[6] 3-16 MHz 200 Watts VSWR: < 1.3:1 [6] ads/stealth-antennas/9420-datasheet.pdf 6

7 1.5m 1.5m [2] 0.8~1m [1] 1.7~2.4m 2-30 MHz Power 150W Gain > MHz VSWR < 2.0:1 Bandwidth N/A 2-30 MHz Power 150W Gain > MHz VSWR < 2.5:1 Bandwidth > 3.5kHz Since antennas are electrically small, they have very low gain at 2 MHz (~ -25 dbi). These performances are reference point for our research. [1] [2] [3] 7

8 Introduction Vertical Half-Loop Antenna Inverted-L Antenna Scaled Model Design Measurements Summary 8

9 Electrical Performance Propagation Mode Frequency Range Polarization IEEE gain at zenith (25cm profile, PEC ground) NVIS 2 10 MHz Linear in zenith direction 2MHz 4.7 dbi 5MHz 3.9 dbi 10MHz -2.8 dbi 4.8m 0.25m 0.9m Features: Linear Polarization Detuning of the two separate antennas potentially increases bandwidth Easy to tune each antenna Total gain [dbi] (PEC ground plane) Challenge: Very small input resistance 9

10 Opening width 29.49m ( 5 MHz) 0.5m 38.4m 3.9 MHz 15m 5 MHz 7.7m 1.5m Start from a large wideband TEM horn Rescale to fit a vehicle and load by a loop Run parametric studies Make two HMD Make antenna less visible and mount on more detailed AAV Mount on a rough AAV and do parametric studies 10

11 Geometry: (profile 25cm) vs with tracks no tracks Antenna is a strip (width 0.08m) Backward extension is 0.25m AAV is steel Antennas are Cu 24kHz 3kHz Effect of tracks on performance is minor 11

12 Introduction Vertical Half-Loop Antenna Inverted-L Antenna Scaled Model Design Measurements Summary 12

13 Electrical Performance Propagation Mode Frequency Range Polarization IEEE gain at zenith (80cm profile) NVIS 2 10 MHz Horizontal in zenith direction 2MHz -4.6 dbi 5MHz 1 dbi 10MHz -1 dbi 2 m 6 m 1.3 m Features: Linear Polarization Offset Feeding improves match and gain Easy to tune, simple design Total gain [dbi] (PEC ground plane) Challenge: Achieve Better Bandwidth 13

14 16 m H 1. Vary Height and Length L 2. Vary Antenna Bend Radius R 3. Introduce Loop / Coils Start from traditional vertical whip antenna Tilt for lower profile and Conform with vehicle Geometry Run parametric studies Feed 1.3 m L = 8 m H = 1.3 m Feed Feed Introduce Rear Mount Configuration with lower profile Mount on detailed AAV in Rhino Configuration and Introduce offset feeding Mount on rough AAV with Rhino Configuration 14

15 Geometry: (profile 50cm) vs 50 Ω Perfect Match no tracks with tracks no tracks with tracks Antenna is a wire (radius 8 mm) 24 KHz Offset Feed at 6m AAV is steel Antennas are Cu 3 KHz no tracks with tracks no tracks with tracks Still need some extra loss to get 3 khz bandwidth at 2 MHz 15

16 Introduction Vertical Half-Loop Antenna Inverted-L Antenna Scaled Model Design Measurements Summary 16

17 3D CAD Model: Model Segmentation: Non-Critical Features Complex Features Non-Critical features are eliminated Complex features are simplified Model is segmented Individual model parts are printed Upper Hull Lower Hull Tracks 18

18 3D Printer: MODEL Material Used Layer resolution Accuracy Max model size MakerBot Replicator PLA 100 microns 0.1 mm 25x20x15 cm Model Specifications: Rafts: ON Infill: 15% Number of Shells: 2 Print Resolution: High Layer Resolution: 100 microns Total Model Cost: $10 : 3D Printed model parts Integrated 3D Printed Model 19

19 Copper Plating: Model is polished before plating Surface is sealed to get a non-porus base Conducting adhesive is sprayed on the model Model is electroplated with 1 Oz. Copper Antenna Deployment: Scaled model antennas are fabricated with cable Antenna mounted onto AAV with copper supports Copper support harnessed onto vehicle with copper tape for easy replacement and model reuse Ferrite choke is used to suppress the return currents 19

20 Introduction Vertical Half-Loop Antenna Inverted-L Antenna Scaled Model Design Measurements Summary 20

21 Measurement scaling factor is 50x Frequency range of interest: MHz Model Relevance: 2 10 MHz (in full scale) Measurements Setup: With ferrite beads Without ferrite beads Ground plane : 1.2m x 1.2m (0.4 λ x 0.4 λ) Made from aluminum foil coated insulation foam 21

22 To validate simulation results To establish confidence in the scaled modelling process Inverted-L Half-loop Resonant antenna. Goal: to see the resonance and compare measurements with modeling. Loop has no resonant frequencies of interest. Goal: test the capabilities to measure impedance of small antennas 22

23 Model Parameters: AAV and antenna are made of Cu PEC infinite ground plane Half-loop: 2.7cm 7.9cm coax cable 2.9cm TEM port is at reference plane position Simulation results can be directly compared to measurements! 1.2cm 0.9cm Results for infinite ground plane and 1.2 x 1.2 m 2 ground are very similar. No results for finite ground are shown. wave port VNA reference plane Cable model: Total length of shield is 13.4cm for halfloop and 17.5cm for inverted-l. Inverted-L: 3.9cm 11.8cm coax cable 2.8cm 1.2cm 1.0cm 2.21mm 1.7mm 0.51mm wave port VNA reference plane copper PTFE ( r = 2.008) 23

24 Smith chart meas., no ferrite meas., ferrite FEM Resistance Reactance Port 1 Averaging feature turned on to filter out noise Very good agreement between measurements and modeling! 24

25 Resistance Port 1 Reflection Coefficient FEM ~0.25dB Measurements (with and without ferrite) Reactance <3% difference in linear scale Averaging feature turned on to filter out noise Very good agreement between measurements and modeling! 25

26 Introduction Vertical Half-Loop Antenna Inverted-L Antenna Scaled Model Design Measurements Summary 26

27 Design and modelling of low profile, low cost vehicular NVIS antennas for 24 khz bandwidth is described Evolution of the half-loop antenna and the inverted-l antenna to adapt to the vehicular platform of interest is shown Antenna performance for the two configurations of half-loop and the inverted-l is simulated Scaled model design and fabrication process is described in detail 3D printing and electroplating is used to realize the scaled model with advantages of lower cost (5x cheaper), lighter weight and faster turn in time as compared to traditional CNC machining options Scaled model measurements are performed to validate simulation results Good agreement between the measurement and simulations establishes confidence in the computational as well as scaled modelling technique 27

28 Questions Booth 18