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