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Design and Testing of Apertures in Conductive Film for
Wireless Communication 2010 AMTA Atlanta, GA
Eric K. WaltonThe Ohio State University ElectroScience Laboratory
Columbus, Ohio 43212
Charles Voeltzel Bombardier Transportation
Pittsburgh, PA 15236
Reason to put metal film in automotive windows
• SPUTTERED METAL FILMS• 80% Transparent at Visual wavelengths
• Automotive windshields• Automotive side and rear windows
• 90% Reflective at Infrared wavelengths• Blocks Sun’s heat • Lowers AC energy requirements
• Able to conduct electrical current• Deice• Defog• Release frozen WS wipers
• Can serve as an excellent AM/FM antenna
• BLOCKS RADIO SIGNALS– Cellular– GPS– Toll Collection– Intellegent Highway Information– Blue Tooth– WiFi
Reason to NOT put metal film in automotive windows
AB 32, California’s sweeping 2006 climate law
--- KILLEDMarch 26, 2010 – LA Times:California's 'cool car' rules are shelvedThe state Air Resources Board halts a plan to require clear, reflective glaze on windows
California officials abruptly halted an effort to slash the carbon footprint of automobile air conditioning.
The rules were adopted in June by the state Air Resources Board and were in the process of being finalized.
• Law enforcement officials had expressed concerns that the coating would interfere with the electronic monitoring of ankle bracelets on paroled felons.
• Wireless phone companies said it could degrade cellphone signals, including 911 calls. • Toll road operators said it could make it difficult to use the "E-Z Pass" systems
• "After listening to this input, The state Air Resources Board announced that the AB 32 'cool cars' rule-making will cease."
• AB 32, California’s sweeping 2006 climate law, requires the state to cut its greenhouse gas emissions by 15% over the next decade. The cool car rule, to be phased in between 2012 and 2016, was designed to block 60% of the sun's energy from entering the interior of a car. Once finalized, the rule would have cut 700,000 metric tons of CO2 emissions by 2020, the equivalent of taking 140,000 cars off the road.
Status Today
• Today, millions of vehicles are equipped with coated windshields, primarily in the European Union where higher light transmission requirements make heat absorbing tints less effective. In fact, some high end vehicles use coated glass in all of the windows.
• In addition to increased passenger comfort, an added benefit is that the reduced load on the air conditioner has been shown to improve gas mileage
HEATED WINDSHIELDCONCEPT
ADVANCED HEATED SOLAR CONTROL WINDSHIELD
METALFILM
BUSBAR
POTENTIAL AM/FM ANTENNA FEED POINT
WORKS GREAT! BUT … BLOCKS RF
HEATED WINDSHIELDWITH APERTURE CONCEPT
ADVANCED HEATED SOLAR CONTROL WINDSHIELD
BUSBAR
RF BLOCKAGE MITIGATEDBUT INTERUPTS HEATING CURRENT
DELETIONZONE
RF BLOCKAGE MITIGATED USING OPEN APERTURES CONFLICTS WITH HEATING CURRENT FLOW
Note the cold spots and hot spots that result.
HEATED WINDSHIELDWITH FSS GRID PATTERNS
LASER ABLATED LINES IN DELETION ZONE ARE SO THIN AND CLOSELY SPACEDTHAT THEY ARE NOT ACTUALLY VISIBLE TO THE DRIVER.
DELETIONZONE
WORKS GREAT!BUT: •POLARIZATION SENSITIVE•STRONG SIDELOBE NULLS
Sub-scale test samples (30 cm x 30 cm x 5 mm)
These were etched with square grid pattern using a CO2 Laser. The grid spacing: varied from 0.90, to 2.00 mm.
Non-deleted(reference)
HEATED PANELS(with bus bars, top and bottom)
non-conductive FSS on the left, same pattern but with additional deletion lines to control current flow on the right.
10 watts/square for 10 minutes
Intermediate test; note cold zone in both cases
OPTIMIZATION OF PERIODIC DELETION PATTERNS
• Theoretical calculations use the Periodic Method of Moments (PMM) code developed by Ohio State Univ. to design arrays of slots.
• The resulting arrays of slots behave very much like a frequency selective surface (FSS).
• A large number of possible slot array sizes and shapes were studied both theoretically and experimentally.
• In general, theoretical results (PMM for FSS) agreed with the experimental measurements.
RF transmission theoretical example
Grids, Modeled Values: PMM Code
FREQUENCY
TR
AN
SM
ISS
ION
CO
EF
FIC
IEN
T
“NO-GRID”- REFERENCE
1-40 GHz; 0.5 to 5.0 mm grid spacing
EXPERIMENTAL TEST SYSTEMS
Measurement System
300MHz – 3GHz horns
HP network analyzer
Computer interface
RF absorbing enclosure
Two FSS measurement systems were developed by the Ohio State ElectroScience Lab.
• 300Mhz – 3ghz range, two UWB horn antennas, with a short gap for samples in between them in a RF absorbing enclosure• X, Ka, Ku radar bands; three sets of Gunn oscillators and Schottky diodes were used.
Both systems proved to be very repeatable and gave results consistent with MOM modeling and experimental test range data
Gunn Oscillator
Horn Antennas
Schottky detector
X, K, Ka
COATED VS. UNCOATED
-45
-40
-35
-30
-25
-20
-15
-10
-5
0
5
800 1300 1800 2300 2800
MHz
dB
COATED
UNCOATED
Air Gap (NO GLASS)
Example experimental results
-45.00
-40.00
-35.00
-30.00
-25.00
-20.00
-15.00
-10.00
-5.00
0.00
5.00
800.00 1300.00 1800.00 2300.00 2800.00
COATED
UNCOATED
Air Gap
ARAY 1-1 V
ARAY 1-1 H
ARRAY 1-2 V
ARRAY 1-2 H
ARRAY 2-2 V
ARRAY 2-2 H
FREQ 800 – 2,800 MHz
INS
ER
TIO
N C
OE
FF
ICIE
NT
UNCOATED
AIR GAP REFERENCE
CO-POLARIZED
CROSS-POLARIZED
PROBLEM #1; POLARIZATION
SOLUTION; CREATE A POLARIZATION INSENSITIVE FSS
TYPICALLY; DUAL POL SOLUTION IS ONLY AT A SINGLE FREQUENCY
EXAMPLE OF RESULTS FOR A WIDER BAND SOLUTION
PROBLEM #2;DIRECTIONAL NULLS
SOLUTION;TAPER THE TRANSMISSION COEFFICIENT
PATTERN VS. RADIATED FIELDFOR TAPERED COEFF. APERTURE
CONCLUSIONS
• THERE ARE SEVERAL GOOD REASONS TO IMBED METAL LAYERS IN AUTOMOTIVE WINDOWS.
• THE MAIN REASON TO AVOID SUCH A METAL LAYER IS BASED ON RF BLOCKAGE.
• RF BLOCKAGE CAN BE MITIGATED BY PROPER DESIGN OF (TAPERED) APERTURES.
REFERENCES
Krause, Antennas, First Edition, McGraw-Hill, 1950
Munk, Ben A., Frequency Selective Surfaces: Theory and Design, Wiley, John & Sons, Incorporated, April 2000
Voeltzel, Charles S., “Coated Substrate Having A Frequency Selective Surface”, US Patent 6730389, 05/04/04
Voeltzel, Charles S., “Conductive Frequency Selective Surface Utilizing Arc And Line Elements” US Patent US6891517, 05/10/05
Voeltzel, Charles S., Walton, Eric, “Sidelobe Controlled Radio Transmission Region In Metallic Panel”, US Patent US6860081, 06/10/04
Voeltzel, Charles S., “Electromagnetic Compatibility of Conductive Heat Reflecting Automotive Windows”, SAE 2009-01-1143