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Saurabh Sanghai(1)*, Maxim Ignatenko(1), Kim Hassett(2) and Dejan S. Filipović(1)
Antenna Research Group – ARG
Acknowledgement: This work has been sponsored by the Office of Naval Research under grant # N00014-13-1-0494
Antenna Measurement Techniques Association36th Annual Meeting and Symposium
October 12-17, 2014
(1)University of Colorado, Boulder, Colorado
(2)Nearfield Systems Inc. - NSI
2
Introduction
Vertical Half-Loop Antenna
Inverted-L Antenna
Scaled Model Design
Measurements
Summary
3
HF frequencies : point to point communication from 0 to 1000s of km
Three modes of communication:1. Ground wave2. Sky wave (long range propagation)3. Near Vertical Incidence Skywave (NVIS)
Ionosphere
GroundTx/Rx
Ground wave
NVIS
Skywave
Near Vertical Incidence Skywave (NVIS) communication:• 2-10 MHz• High incidence angles• Any polarization• Beyond line-of-sight
NVIS antenna pattern
Requirements:
• 400 Watts Power Handling• 24 kHz bandwidth• Low profile
Assault Amphibious Vehicle (AAV)
4
Goal is to design On The Move vehicular NVIS antenna
SimpleLow Cost
Low Profile
24 kHzBandwidth
High Power
Challenges:
• HF antennas have large physical size• Narrow bandwidth• Different grounds cause detuning• Radiation pattern limits use of all the HF
propagation modes
AAV
Spherical antenna
Loop
Dipole
14m
23m
39m
5
[1] M. Gustaffson et al., “Physical Limitations on Antennas of Arbitrary Shape,” Proc. Royal Society A, (2007)
0.71 60
1.5
/
[3] A.D. Yaghjian and S.R. Best, “Impedance, Bandwidth, and Q of Antenna,” IEEE Trans. Antennas Propag., (2005)
[2] J.L. Volakis et al., “Small Antennas: Miniaturization Techniques and Applications,” McGraw-Hill, (2010)
[3]
[1, 2]
Q: What is the minimum size of a lossless antenna that may achieve 24 kHz BW at 2 MHz?
Antennas are much larger than AAV!
Estimation:
Analytical Results:
References:
where
Obtaining 24 kHz bandwidth at 2 MHz within a vehicle platform is very challenging!
24kHz
Numerical Validation:
2
0.71[3]
6
Stealth 9400c [3]
4.5-22 MHz 150 Watts VSWR: < 1.2:1
Codan 9350 [5]
2-30 MHz 125 Watts VSWR: < 1.5:1
2-30 MHz 150 Watts VSWR: < 2.5:1
Shakespeare 120-99 [1] Cobham 3160-99 [2]
Harris OTM Loop [4] Stealth 9420[6]
3-16 MHz 200 Watts VSWR: < 1.3:1
[3]https://www.stealth.ae/plugins/custompages/detinf.php?id=372&id_categories=136
[5]https://hf-ssb-tranceiver.at-communication.com/en/codan/hf_ssb_antennas_9350.html
[6]http://www.cntlwire.com/dealer_downloads/stealth-antennas/9420-datasheet.pdf
[2]http://www.masergroup.com/downloads/Cobham%20Half-Loop%20Demo%20Report%20Nov-Dec%202012.pdf
2-30 MHz 150 Watts VSWR: < 2:1
[4] http://rf.harris.com/media/RF-3134-AT003-5%20Web_tcm26-19129.pdf
1.6-30 MHz 160 Watts VSWR: < 3.5:1
[1]www.Shakespeare-military.com, Shakespeare Military Antenna Products Brochure, 2010. pp-15.
7
[1]
1.5m1.5m
• 2-30 MHz• Power 150W• Gain > -24 dBi @ 2 MHz• VSWR < 2.0:1• Bandwidth N/A
[2] http://www.masergroup.com/downloads/Cobham%20Half-Loop%20Demo%20Report%20Nov-Dec%202012.pdf[1] http://rf.harris.com/media/RF-3134-AT003-5%20Web_tcm26-19129.pdf
[3] http://www.hfindustry.com/meetings_presentations/presentation_materials/2010_feb_hfia/presentations/HFIA_loop_presentation.pdf
Since antennas are electrically small, they have very low gain at 2 MHz (~ -25 dBi).These performances are reference point for our research.
1.7~2.4m
• 2-30 MHz• Power 150W• Gain > -26 dBi @ 2 MHz• VSWR < 2.5:1• Bandwidth > 3.5kHz
[2]0.8~1m
8
Introduction
Vertical Half-Loop Antenna
Inverted-L Antenna
Scaled Model Design
Measurements
Summary
Electrical PerformancePropagation Mode NVIS
Frequency Range 2 – 10 MHz
Polarization Linear in zenith direction
IEEE gain at zenith (25cm profile, PEC ground)
2MHz 4.7 dBi5MHz 3.9 dBi10MHz -2.8 dBi
Features:• Linear Polarization• Detuning of the two separate antennas
potentially increases bandwidth• Easy to tune each antenna
Challenge:• Very small input resistance
Total gain [dBi]
9
(PEC ground plane)
4.8m
0.25m
0.9m
10
0.5m15m
38.4m
/4 @ 5 M
Hz
/2 @ 3.9 MHz
Opening width 29.49m(/2 @ 5 MHz)
Start from a large wideband TEM horn
Rescale to fit a vehicle and load by a loop
7.7m
1.5m
Run parametric studies
Mount on a rough AAV and do parametric studies
Make antenna less visible and mount on more detailed AAV
Make two HMD
11
24kHz
3kHz
Effect of tracks on performance is minor
Geometry:
Antenna is a strip (width 0.08m)
Backward extension is 0.25m
(profile 25cm)
AAV is steelAntennas are Cu
vswith tracksno tracks
12
Introduction
Vertical Half-Loop Antenna
Inverted-L Antenna
Scaled Model Design
Measurements
Summary
6 m
2 m
1.3 mElectrical PerformancePropagation Mode NVIS
Frequency Range 2 – 10 MHz
Polarization Horizontal in zenith direction
IEEE gain at zenith (80cm profile)
2MHz -4.6 dBi5MHz 1 dBi10MHz -1 dBi
Features:• Linear Polarization• Offset Feeding improves match and gain• Easy to tune, simple design
Challenge:• Achieve Better Bandwidth
Total gain [dBi]
13
(PEC ground plane)
Start from traditional vertical whip antenna
Tilt for lower profile andConform with vehicle Geometry
Run parametric studies
Mount on rough AAV with Rhino
Configuration
Mount on detailed AAV in Rhino Configuration and Introduce
offset feeding
Introduce Rear Mount Configuration with lower
profile
16 m
H = 1.3 m
L = 8 m
Feed
Feed
Feed
1.3 m
2. Vary Antenna Bend RadiusH L
R
3. Introduce Loop / Coils
1. Vary Height and Length
14
15Still need some extra loss to get 3 kHz bandwidth at 2 MHz
Geometry:
Antenna is a wire (radius 8 mm)
Offset Feed at 6m
(profile 50cm)
AAV is steelAntennas are Cu
vsno trackswith tracks
no trackswith tracks
no trackswith tracks
no trackswith tracks
3 KHz
24 KHz
50 Ω – Perfect Match
16
Introduction
Vertical Half-Loop Antenna
Inverted-L Antenna
Scaled Model Design
Measurements
Summary
Non-Critical FeaturesComplex Features
3D CAD Model: Model Segmentation:
Upper HullLower Hull
Tracks
• Non-Critical features are eliminated• Complex features are simplified• Model is segmented • Individual model parts are printed
18
MODEL MakerBotReplicator
Material Used PLA
Layer resolution 100 microns
Accuracy 0.1 mm
Max model size 25x20x15 cm
• Rafts: ON
• Infill: 15%
• Number of Shells: 2
• Print Resolution: High
• Layer Resolution: 100 microns
• Total Model Cost: $10
:
3D Printer:
Model Specifications: 3D Printed model parts
Integrated 3D Printed Model19
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 0.081” cable
• Antenna mounted onto AAV with copper supports
• Copper support harnessed onto vehicle with copper tapefor easy replacement and model reuse
• Ferrite choke is used to suppress the return currents
20
Introduction
Vertical Half-Loop Antenna
Inverted-L Antenna
Scaled Model Design
Measurements
Summary
21
Measurement scaling factor is 50x
Frequency range of interest: 100 – 600 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
22
Inverted-L
Resonant antenna.
Goal: to see the resonance and compare measurements with modeling.
Half-loop
Loop has no resonant frequencies of interest.
Goal: test the capabilities to measure impedance of small antennas
To validate simulation results
To establish confidence in the scaled modelling process
23
Model Parameters:
2.7cm7.9cm
1.2cm 0.9cm
2.9cm
Half-loop:
Cable model:
wave portVNA reference plane
• AAV and antenna are made of Cu• PEC infinite ground plane
0.51
mm
1.7m
m
2.21
mm
copper
PTFE(r = 2.008)
Total length of shield is 13.4cm for half-loop and 17.5cm for inverted-L.
Inverted-L:
wave portVNA reference plane
2.8cm
1.0cm1.2cm
11.8cm
3.9cm
TEM port is at reference plane position• Simulation results can be directly
compared to measurements!
Results for infinite ground plane and 1.2 x 1.2 m2 ground are very similar. No results for finite ground are shown.
coax cable
coax cable
24
Resistance
Reactance
Smith chart
Very good agreement between measurements and modeling!
FEM
meas., no ferritemeas., ferrite
Port 1
Averaging feature turned on to filter out noise
25
~0.25dB
<3% difference in linear scale
Resistance
Reactance
Reflection Coefficient
FEM
Measurements (withand without ferrite)
Port 1
Averaging feature turned on to filter out noise
Very good agreement between measurements and modeling!
26
Introduction
Vertical Half-Loop Antenna
Inverted-L Antenna
Scaled Model Design
Measurements
Summary
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