VEHICLE ANTENNA

Abstract: An antenna adapted for placement in the window of a vehicle includes a flexible substrate having a pair of opposing surfaces, at least one of which is provided with electrically conductive metal traces in the form of a pattern configured to receive and transmit one or more electromagnetic signals. The pitch, trace width, and thickness of the metal traces are selected such that the antenna has an area-averaged resistivity of no greater than about 1 ohm/square for a predetermined area-averaged transmission value in the visible region.

Description: VEHICLE ANTENNA Background ofthe Invention This invention relates to providing a vehicle with an antenna to enable the vehicle to receive and transmit one or more electromagnetic signals. Mast or whip antennas mounted on the exterior of a vehicle such as an automobile have been used for receiving and transmitting electromagnetic signals, e.g., radio waves. Thin film antennas designed to be affixed to a window ofthe vehicle, e.g., the windshield, are also known. An example of such an antenna is described in Nagy et al , U.S. 5,083,135. Summary ofthe Invention In general, the invention features an antenna adapted for placement in the window of a vehicle that includes a flexible substrate having a pair of opposing surfaces, at least one of which is provided with a series of electrically conductive metal traces in the form of a pattern configured to receive and transmit one or more electromagnetic signals. The pitch, trace width, and thickness ofthe metal traces are selected such that the antenna has an area-averaged resistivity of no greater than about 1 ohm/square for a pre-determined area-averaged transmission value in the visible region. By "area-averaged" it is meant that the resistivity and transmission values represent the average values for these parameters over the entire surface area ofthe antenna to account for the fact that selected areas ofthe antenna may exhibit different values of pitch, trace width, and/or trace thickness. In preferred embodiments, the antenna has an average resistance of no greater than about 0.5 ohm/square. Moreover, the antenna preferably has an area- averaged transmission in the visible region of at least 10%. The metal traces are preferably made of copper metal. The antenna may also include metal traces adapted to ground the antenna (thereby eliminating the need to ground the antenna to a metallic portion ofthe vehicle such as the roof, space frame, or battery). To enhance environmental resistance ofthe antenna, the surface on which the metal traces are located may further include a primer on which at least a portion ofthe metal traces are located. Preferred materials for the metal traces include metal alloys (e.g., Inconel). The antenna may further include a first colored topcoat overlying at least a portion ofthe metal traces. A second colored topcoat located on the opposing side of the substrate may be included as well. The metal pattern may be configured to receive and transmit AM and FM radio signals, television signals, microwave signals (e.g., cellular telephone signals or global positioning signals), or a combination thereof. In general, the metal pattern is preferably configured in the form of a grid, with the individual grid design being selected based upon the type of signal the antenna is designed to receive and transmit, as well as requirements that may be imposed by the design ofthe particular vehicle itself. The invention also features a method for enabling a vehicle having one or more windows to receive and transmit one or electromagnetic signals that includes incorporating the above-described antenna in a window ofthe vehicle. For example, the antenna may be incoφorated in the shade band area ofthe windshield ofthe vehicle. In one preferred embodiment, the antenna is affixed to a portion ofthe surface ofthe window. In a second preferred embodiment, the antenna is incoφorated within the window. The invention provides an antenna that can be readily incoφorated in the window (e.g., the windshield) of a vehicle. The antenna can thus be located close to, e.g., the vehicle's radio, thereby mirtimizing the need for signal amplifiers which are sometimes required if the antenna is located a distance from the radio. The antenna also eliminates drag associated with mast-type antennas, thereby improving fuel economy and wind noise. By selecting the pitch, trace width, and thickness of individual metal traces to yield particular values of area-averaged resistivity and transmission, the invention provides an antenna in which resistivity is decoupled from transmission. Thus, it is possible to tailor the area-averaged resistivity and transmission ofthe antenna for a particular application. For example, it is possible to prepare an antenna that appears substantially transparent to the naked eye without compromising conductivity. Moreover, one or both sides ofthe antenna can be colored to match, e.g., the color of the vehicle to enhance the overall aesthetic appearance ofthe vehicle. By providing metal traces in the antenna for grounding the antenna, the antenna can be used in a wide variety of vehicles. Other features and advantages ofthe invention will be apparent from the foiiowing description ofthe preferred embodiments thereof, and from the claims. Brief Description ofthe Drawing Fig. 1 is a schematic representation of a grid-like pattern of metal traces suitable for use in antennas according to the invention. Description ofthe Preferred Embodiments A preferred antenna according to the invention features a flexible plastic substrate provided on one or both sides with metal traces in which the metal traces form a pattern that allows the antenna to transmit and receive one or more electromagnetic signals (e.g., AM/FM radio signals, television signals, and microwave signals, including cellular telephone and global positioning signals). In the case of an antenna adapted to transmit and receive multiple signals, individual areas ofthe antenna may be assigned to each particular signal, and configured accordingly. The antenna is designed to be placed in a window of a vehicle such as an automobile, van, or truck. A preferred location is the shade band area ofthe windshield (which may or may not be tinted). The antenna may be secured to the interior or exterior surface ofthe window, in which case it is preferably provided on one side with an adhesive. Examples of suitable adhesives include optically transparent adhesives such as transparent acrylic-based adhesives, polyvinyl butyral, and polyurethanes. The antenna may also be embedded within the window itself, e.g., during manufacture ofthe window. The puφose ofthe flexible substrate is to provide a durable surface on which the delicate, metal traces can be supported. It thus facilitates both antenna manufacture and installation. It is also preferred, for aesthetic reasons, that the substrate be substantially transparent. A preferred material for the flexible substrate is a plastic such as polyethylene terephthalate. Other suitable materials include polyethylene naphthalate, polypropylene, and polyimides. The thickness ofthe substrate typically is in the range of about 10 to about 200 microns (more preferably about 25 to about 50 microns). The material for the metal traces can be any electrically conductive metal or metal alloy. The primary considerations governing the selection ofthe metal include electrical conductivity, resistance to environmental degradation, ease of deposition and patterning, and cost. Examples of suitable metals include copper, gold, aluminum, silver, and titanium, with copper being preferred. The metal traces are in the form of a pattern selected based upon the particular signal(s) which the antenna is designed to transmit and receive. As a person of ordinary skill will readily appreciate, the particular pattern may depend on the vehicle itself (e.g., the space frame ofthe vehicle), as well as the location and orientation ofthe antenna in the vehicle and the signal frequency. In general, however, grid-like patterns are preferred in which the metal traces form a series of parallel and/or criss-crossed lines. The antenna may also include non-functional traces (i.e., traces that do not participate in receiving and transmitting the electromagnetic signal) for the puφose of optimizing the overall transmission characteristics ofthe portion ofthe vehicle window in which the antenna is incoφorated. Furthermore, the antenna may include metal traces for grounding the antenna. The pitch and width of individual traces are selected, along with the metal thickness, to yield an antenna having an area-averaged resistivity no greater than about 1 ohm/square (more preferably no greater than about 0.5 ohm/square) and an area- averaged transmission in the visible region (i.e., at 550 nm) that is at least 10%. The particular transmission value will depend, in part, on the area ofthe vehicle in which the antenna is to be incoφorated. For example, antennas incoφorated into the shade band area ofthe vehicle (or which function as the shade band itself) can tolerate lower transmission values than antennas located lower down in the window. To provide enhanced environmental resistance, as well as enhanced adhesion, a primer layer is preferably included between the substrate and the metal traces. Suitable materials for the primer include metals, metal oxides, and metal alloys that exhibit improved oxidation resistance relative to the metal used for the metal traces. Examples include chromium, nickel, and titanium (and alloys thereof), and oxides such as chromium oxide, with Inconel (an alloy of nickel, chromium, and iron) being preferred. The thickness ofthe primer layer is selected such that it is as thin as possible, but still thick enough to support a bond between the substrate and the metal traces. In general, the primer thickness ranges from about 0.003 to about 0.030 micron. One or both sides ofthe antenna may be colored to enhance the overall appearance ofthe vehicle; in the case of two-sided color, the two sides may bear the same or different colors. In addition, each individual side can be provided with more than one color. For example, the antenna can be tinted to match the exterior and/or interior ofthe vehicle. It can also be tinted to match the color ofthe shade band. Coloration is preferably achieved by using a colored ink during patterning ofthe antenna. To facilitate adhesion between the ink and the metal traces, a primer may be included between the metal traces and the ink. Once incoφorated in the vehicle, the metal traces ofthe antenna must be connected to the transmitting receiving device ofthe vehicle (e.g., the radio). In addition, the transmitting/receiving device must be connected to ground (e.g., to the metal roof of the vehicle, the space frame ofthe vehicle, or to the vehicle's battery, or to metal traces provided on the antenna itself). In order to establish such connections, conductive tape or mechanical fasteners may be used. Alternatively, a thin film connector ofthe type described in Engfer et al., U.S. 5,426,622, hereby incoφorated by reference, may be used. As described therein, such connectors feature a flexible insulator (e.g., polyethylene terephthalate) having on at least one face an electrically conductive portion (e.g., a copper or aluminum layer) and an electrically conductive adhesive (e.g., adhesives such as Creative Materials 108-10 from Creative Materials Inc. of Tyngsboro, MA or 3M 9703 from 3M Co. of St. Paul, MN) inteφosed between the electrically conductive portion and at least a portion ofthe metal traces of the antenna to establish electrical contact between the connector and the metal traces. In general, the antenna is prepared by depositing a primer layer on the substrate surface in the form of a continuous layer, followed by deposition of a metal layer (also in the form of a continuous layer). The preferred technique for both primer and metal deposition is vacuum metallization using art-recognized processes. Prior to primer deposition, the substrate surface may be treated to enhance adhesion between the primer and substrate surface. Examples of suitable priming processes include plasma treatment, corona discharge, flame priming, and flashlamp priming (as described in Ouderkirk et al., U.S. 4,822,451, hereby incoφorated by reference), with flashlamp priming being preferred. Following metal deposition, a pattern of interest is printed on the metal surface using conventional ink printing equipment such as a rotary letteφress, flexography, or screen printing. Once the ink has dried or cured, both the metal and primer are removed by exposing the article to a wet etchant such as a ferric chloride solution or sulfuric acid, an ablation source such as excimer laser, flashlamp, or accelerated plasma according to the process described in Yu et al., U.S. 5,178,726, hereby incoφorated by reference, or a combination thereof. The resulting antenna is then outfitted with a suitable connector and incoφorated on or within the desired vehicle window. The invention will now be described further by way ofthe following examples. EXAMPLES Example 1 A biaxially oriented, heat-set, polyethylene terephthalate (PET) film having a thickness of about 50 microns was flashlamp primed according to the process described in Ouderkirk et al., U.S. 4,822,451 to produce a film as described in Ouderkirk et al., U.S. 4,879,176 (hereby incoφorated by reference). Next, a primer layer of Inconel was deposited on the film by vacuum metallization (thickness = 0.01 micron), followed by a 0.14 micron thick layer of copper (also deposited by vacuum metallization). The resulting metallized article was then printed with an antenna pattem in the form of a grid as shown in Fig. 1 using a conventional rotary letteφress and ultraviolet-curable letteφress ink (UVOFOIL ink commercially available from Nobel). As shown in Fig. 1, the grid pattem features a series of horizontal and vertical metal traces arranged to transmit and receive AM and FM radio signals. The grid is characterized by the following parameters: vertical width, horizontal width, horizontal length, trace width (i.e., the width of an individual metal trace), and pitch (i.e., the trace to trace spacing). Following the printing step, the copper metal between the traces ofthe grid was etched in a sulfuric acid/hydrogen peroxide bath, after which the article was exposed to an accelerated plasma according to the process described in Yu et al., U.S. 5,178,726 to remove the Inconel primer. The plasma treatment produced an antenna having a series of 0.14 micron thick copper traces arranged in a grid characterized by a 203 micron (8 mil) pitch, 0.1 m (4 inch) vertical width, 0.89 (35 inch) horizontal length, and 0.1 (4 inch) horizontal width. The trace width to pitch ratio was 0.5, yielding a value of 44% transmission in the visible region for the antenna (0.5 multiplied by 88%, which is the percent transmission for PET in the visible region when there are two PET/air interfaces. However, the value would be different if a material having a refractive index different from that of air were to form an interface with the PET. For example, if a PET/polyvinyl butyral interface were present, the transmission through the antenna would approach 50% rather than 44%). The metal traces themselves had an optical density of 3.5, indicating that the traces themselves were essentially opaque, even though the overall antenna was substantially transmissive. Furthermore, the antenna exhibited a resistivity of 0.5 ohm/square after patterning and a resistivity of 0.25 ohm/square prior to patterning. Electrical connections to the metal traces were made by soldering a short piece of wire to copper tape having a metal-filled pressure sensitive adhesive (Scotch Brand Foil Shielding Tape #1181 commercially available from 3M Co. of St. Paul, MN). The tape was then adhered to the connector portion ofthe antenna, as shown in Fig. 1. The antenna was attached on the outside ofthe windshield of a 1985 Nissan Stanza sedan in the shade band area ofthe windshield such that the closest conducting trace ofthe antenna was four inches from the edge ofthe metal roof of the vehicle. The coaxial feed from the vehicle's radio was then connected to the wire piece described above. The coaxial shield was grounded to the metal roof of the vehicle using a direct solder connection to a 1 inch (2.54 cm) square conductive patch on the roof. Antenna testing was conducted in ah outdoor parking lot using, as a source antenna, a 100,000 watt, 1250 foot dipole antenna used by local radio stations. The power received by the car antenna from the source antenna (in decibels, dB's) was measured using a Hewlett Packard 8595E Spectrum Analyzer at a series of eight different azimuth orientations ofthe vehicle: North 135° Northeast 180° Northwest 90° South 315° Southeast 270° Southwest 360° East 225° West 45° For each orientation, six different radio station transmitter sources were used: 89.3 MHz FM; 89.9 MHz FM; 94.5 MHz FM; 104.1 MHz FM; 770 kHz AM; and 1030 kHz AM. The measured power ofthe noise floor was -100 dBm for the FM band and -105 dBm for the AM band. The measured power of each radio station was taken to be the peak power associated with the center carrier frequency ofthe radio station. The signal (in dB) is taken to be the difference between the measured peak power ofthe center carrier and the measured power ofthe noise floor. A 1995 Ford Taurus G equipped with a mast antenna was also measured for the sake of comparison. The results are shown below in Table I. Example 2 An antenna was prepared according to the procedure of Example 1 except that the grid was characterized by an 813 micron (32 mil) pitch. Example 3 An antenna was prepared according to the procedure of Example 1 except that the grid was characterized by a 406 micron (16 mil) pitch. Example 4 An antenna was prepared according to the procedure of Example 3 except that the grid was characterized by a vertical width of 0.05 (2 inch). Examples 2-4 were tested according to the procedure of Example 1 except that the test vehicle was a 1995 Ford Taurus G and the coaxial shield was connected to the battery ground terminal ofthe vehicle. Averaged over each ofthe eight orientations, the antennas in Examples 2-4 performed -0.2 dB, -0.4 dB, and -0.8 dB, respectively, compared to the performance ofthe Example 1 antenna. Example 5 A series of four samples was prepared according to the procedure of Example 1 with the following exceptions to investigate the effect ofthe horizontal length ofthe antenna: Each ofthe four samples had a grid pattem characterized by a 406 micron (16 mil) pitch and 0.05 m (2 inch) vertical width. In addition, the horizontal lengths were 0.875 (35 inch) for Sample 5a, 0.675 m (27 inch) for Sample 5b, 0.475 m (19 inch) for Sample 5c, and 0.275 m (11 inch) for Sample 5d. Each ofthe four samples was tested according to the procedure of Example 1 except that the test vehicle was a 1995 Ford Taurus G and the coaxial shield was connected to the battery ground terminal ofthe vehicle. In addition, only one orientation (northeast) was tested. Samples 5b, 5c, and 5d performed +3.2 dB, +1.7 dB, and +3.1 dB, respectively, relative to the performance ofthe Sample 5a antenna. Example 6 A series of four samples was prepared according to the procedure of Example 1 with the following exceptions: Sample 6a was vacuum metallized with 0.075 micron copper and 0.003 micron Inconel. Sample 6b was vacuum metallized with 0.075 nm copper and the Inconel primer layer was omitted. Sample 6c was not flashlamp primed, and was vacuum metallized with 0.075 micron copper and 0.003 micron Inconel. Sample 6d was not flashlamp primed, was vacuum metallized with 0.075 micron copper, and the Inconel primer layer was omitted. Each ofthe four samples was printed with an antenna pattem in the form of a grid as described in Example 1, followed by exposure to an accelerated plasma to remove primer (in the case of Samples 6a and 6c) and copper metal between the traces ofthe grid pattem. The sulfuric acid/hydrogen peroxide wet chemical etch was omitted. Sample 6a was tested according to the procedure used to test the antennas described in Examples 2-4. Averaged over each ofthe eight orientations, the antenna performance was +0.7 dB higher compared to the performance ofthe Example 1 antenna. Example 7 An antenna was prepared according to the procedure of Example 1 except that the antenna had a series of copper traces arranged in a grid characterized by a 406 micron (16 mil) pitch. In addition, printing was accomplished using a multi-color rotary press. Traces on the copper metal-bearing side ofthe PET substrate were printed with blue ink, while the unmetallized side ofthe substrate was printed in registration with the metallized side using red ink. Example 8 An antenna was prepared according to the procedure of Example 1 except that the grid was characterized by a 406 micron (16 mil) pitch. In addition, the thickness ofthe copper traces was 5 microns (resistivity = 0.01 ohm/square). The connector region shown in Fig. 1 was split into 2 portions: a left-hand portion and a right-hand portion. A 7.62 cm (3 inch) wire was used to electrically connect the coaxial shield from the vehicle's radio to the left-hand connector portion, thereby allowing this portion ofthe antenna to serve as the ground surface. The coaxial feed was connected to the right-hand connector portion. The antenna was tested according to the procedure of Example 1 except that the test vehicle was a 1995 Ford Taurus G. In addition, only one orientation (south) was tested. The antenna's performance was -15.9 dB lower than the antenna of Example 1. Other embodiments are within the following claims.

Claims: What is claimed is: 1. An antenna adapted for placement in the window of a vehicle, said antenna comprising a flexible substrate having a pair of opposing surfaces, at least one of which is provided with a series of electrically conductive metal traces in the form of a pattem configured to receive and transmit one or more electromagnetic signals, the pitch, trace width, and thickness of said metal traces being selected such that said antenna has an area-averaged resistivity of no greater than about 1 ohm/square for a pre-determined area-averaged transmission value in the visible region. 2. An antenna according to claim 1 wherein said antenna has an area-averaged resistivity of no greater than about 0.5 ohm/square. 3. An antenna according to claim 1 wherein said pre-determined area- averaged transmission value in the visible region is at least about 10%. 4. An antenna according to claim 1 further comprising electrically conductive metal traces for grounding said antenna. 5. An antenna according to claim 1 wherein at least one of said surfaces of said substrate further comprises a primer on which at least a portion of said metal traces are located. 6. An antenna according to claim 1 wherein said pattem comprises a grid. 7. An antenna according to claim 1 further comprising a first colored topcoat overlying at least a portion of said metal traces. 8. A method for enabling a vehicle having one or more windows to receive and transmit one or more electromagnetic signals comprising incoφorating in a window of said vehicle an antenna comprising a flexible substrate having a pair of opposed surfaces on at least one of which is provided electrically conductive metal traces in the form of a pattem configured to receive and transmit one or more electromagnetic signals, the pitch, trace width, and thickness of said metal traces being selected such that said antenna has an area-averaged resistivity of no greater than about 1 ohm/square for a pre-determined area-averaged transmission value in the visible region. 9. A method according to claim 14 comprising affixing said antenna to a portion ofthe surface of said window. 10. A method according to claim 14 comprising incoφorating said antenna within said window.

Inventors: BREKKEN ROGER A, GIVOT BRADLEY L, JELLUM GREGORY M, BREKKEN, ROGER, A., GIVOT, BRADLEY, L., JELLUM, GREGORY, M.
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1996
Main classification: H01Q1
IPC codes: B32B17, H01Q1