VEHICLE ROOF MOUNTED SLOT ANTENNA WITH SEPARATE AM AND FM FEEDS

Abstract: A slot antenna for a motor vehicle (l0) roof (22) having a central portion (24) made of electrically non-conducting material comprises a horizontal sheet or layer of electrically conducting material attached to the roof with a looped slot (3l) adjacent the central portion of the roof dividing the sheet into inner (32) and outer (33) portions and having a total loop length of substantially one wavelength in the commercial FM broadcasting band. Antenna feed means are provided at the front centre of the slot for FM receiver apparatus (l3) and at the side centre of the slot essentially 90 degrees rotated from the front centre of the slot for AM receiver apparatus (l3). The AM and FM feeds may be connected by coaxial cable (35″,35′) to AM and FM tuners in a common radio receiver in the vehicle dashboard (l2); or separate AM and FM portions of the receiver may be installed adjacent the appropriate antenna feeds. The antenna is grounded at DC and in both the AM and FM commercial frequency bands. The horizontal sheet or layer may comprise a conducting material having a sheet resistance of l-2 ohms per square for a maximum VSWR of 5.

Description: This invention relates to a slot antenna for a motor vehicle and particularly for a non-cavity-backed single slot antenna in the roof of the motor vehicle suitable for commercial AM and FM radio reception. Such an antenna is linked with the vehicle body itself, and its characteristics are profoundly influenced by those of the vehicle body. In the prior art, most vehicle mounted slot antennas have been disclosed in the vehicle trunk lid (as, for example, in US-A-3 6ll 388) or as cavity backed antennas in the vehicle roof (as, for example, in US-A-4 229 744) for directional signal locating purposes. The roof mounting for a slot antenna is superior to a trunk mounting because of the additional height of the antenna, which improves gain in both the AM and FM bands and which also removes it from the signal "shadow" of the upper portions of the vehicle body for an improved FM reception pattern. The lack of a cavity back for the antenna greatly reduces the capacitive loading of the antenna to enable reception at commercial AM frequencies, besides eliminating the bulk of the cavity from the vehicle roof. There are several aspects of such a vehicle roof mounted slot antenna, however, which are critical to its performance but have not been shown in the prior art. A slot antenna of this type must be fed and grounded properly. There are several grounds to consider: DC ground, signal ground at AM frequencies and signal ground at FM frequencies. In addition, the optimum feed points may be different for signals in the commercial AM and FM broadcast bands. Finally, the material of the conducting members bordering the slots is also important in reducing the voltage standing wave ratio (VSWR) of the antenna. A single slot AM/FM antenna for a motor vehicle in accordance with the present invention is characterised over US-A-3611 388 by the features specified in the characterising portion of Claim l. The invention is a slot antenna for a motor vehicle. The motor vehicle forms part of the slot antenna and comprises a vehicle body comprising an electrically conducting material and having a lower body portion, a plurality of substantially vertical roof pillars defining window openings and a substantially horizontal vehicle roof with an outer conducting portion and a central portion or roof panel made of electrically non-conducting material. A horizontal sheet or layer of electrically conducting material attached to the central portion includes a looped slot dividing the sheet into inner and outer portions and having a total loop length of substantially one wavelength in the commercial FM broadcasting band. FM feed means are connected to the inner portion of the horizontal sheet at the front centre of the slot to provide signals in the commercial FM band to FM receiver apparatus; and AM feed means are connected to the inner portion of the horizontal sheet at the side centre of the slot essentially 90 degrees rotated from the front centre of the slot to provide signals in the commercial AM band to AM receiver apparatus. Means are effective to ground the outer portion of the horizontal sheet to the vehicle body at DC and at radio frequenies in the commercial AM and FM bands. The antenna may be in the form of electrically conducting film applied to the underside of a plastic resin or similar non-conducting roof panel which itself has some overlap over/under the metal portion of the vehicle roof; or it may comprise a flexible sandwich of conducting foil between two insulating layers attached to the underside of the vehicle roof and extending under the electrically metal portion thereof. The antenna produced is thus effective to act optimally in both the AM and FM commercial frequency bands. The present invention is further described, by way of example, with reference to the following de scription of preferred embodiments, and the accompanying drawings, in which:- Figure l shows a perspective drawing of a motor vehicle having a roof mounted slot antenna with a common AM and FM feed point; Figures 2a and 2b show top views of a portion of the motor vehicle of Figure l with the roof portion partially cut away to show two embodiments of the antenna in greater detail; Figure 3 shows in detail one manner of making one of the ground connections in the antenna of Figure l; Figures 4 and 5 show vertical section views through a portion of the antenna of Figure l, with Figure 4 being an enlarged view of a portion of Figure 5; Figure 6 shows a perspective view of a motor vehicle with an embodiment of a roof mounted slot antenna having separate AM and FM feed points in accordance with the present invention; Figure 7 shows a partial cutaway top view of an alternative embodiment of a roof mounted slot antenna; Figure 8 is a partial section view along lines 8-8 in Figure 7; and Figure 9 shows a portion of Figure 6 with a slightly modified alternate embodiment of an antenna having separate AM and FM feed points. Referring to Figure l, a motor vehicle l0 has a lower body portion ll including a dashboard l2 behind or within which is a standard AM-FM radio receiver l3. A plurality of roof pillars l5, l6, l7, l8, 20, 2l rise in a substantially vertical direction from lower body portion ll to support a vehicle roof 22. Vehicle roof 22 has an outer electrically conducting portion 23 typically made of steel rails connected to and supported by the roof pillars l5-2l. A non-conducting roof panel 24 made of a sheet moulded compound (SMC) plastic resin overlaps outer electrically conducting portion 23 and comes part of the way down the roof pillars, if necessary, to provide a smooth roof surface with no visible discontinuities. The centre portion of non-conducting roof panel 24, as defined by the inner boundary of outer electrically conducting portion 23, comprises an inner, non-conducting portion 25 of the vehicle roof 22. Since non-conducting roof panel 24 covers the entire roof of the motor vehicle l0 and is painted to match the remainder of the motor vehicle or covered with a vinyl top, there is no trace of the antenna in the external appearance of the motor vehicle and no wind resistance therefrom. The antenna lies just below the vehicle roof as shown in Figure 5. In this embodiment the antenna comprises a flexible sheet 26 of electrically conducting aluminium foil sandwiched between layers of insulating plastic resin. The thickness of the flexible sheet 26 is exaggerated in Figure 5 and the layers are not shown in true proportional thickness; but the Figure does show the overlap of flexible sheet 26 including its conducting layer under the outer electrically conducting portion 23 of the vehicle roof 22. The overlap extends entirely around the vehicle roof 22 as seen in Figure l, although only the sides are shown in Figure 5. A clearer and more accurate representation of the cross-section of the flexible sheet 26 than is possible in Figure 5 is shown in Figure 4. The electrically conducting layer 27 is shown at the centre of the sandwich, with insulating layers 28 attached thereto by adhesive layers 30. Electrically conducting layer 27 may be aluminum foil, although a material with a higher sheet resistance may be used to reduce the voltage standing wave ratio (VSWR) as described later with respect to the embodiment of Figures 7, 8. The electrically conducting layer 27 of the flexible sheet 26 is not continuous. There is a slot 3l which is rectangularly looped and has a width of about one quarter inch (6.4 mm) and a circumference of about one wavelength in the commercial FM band (approximately l28 inches or 3.25 metres) which divides electrically conducting layer 27 into inner 32 and outer 33 portions. The actual dimensions of the slot 3l are 39 in ches (0.99 metre) across the vehicle roof 22 and 25 inches (0.64 metre) from front to back; and the corners are rounded. Inner portion 32 and slot 3l lie entirely beneath the inner non-conducting portion 25 of the vehicle roof 22. Outer portion 33 lies partially beneath the inner non-conducting portion 25 and partially beneath the outer electrically conducting portion 23 of the vehicle roof 22. Outer portion 33 is preferably clamped tightly against the outer electrically conducting portion 23 of the vehicle roof 22 to bring the conducting surfaces as close together as possible and thus maximize the capacitive coupling therebetween. This clamping should be effectively continuous around the circumference of the antenna. The feed and ground connections of the antenna, the subject of our copending application 87305213.8, for a common AM-FM feed are shown in Figures 2a, 2b and 3. A coaxial cable 35 extends from the AM-FM radio receiver l3 across the dash area under or behind the dashboard l2 to the bottom of the right front roof pillar l5. The coaxial cable 35 is routed up roof pillar l5 to the right front corner of the vehicle roof 22 (metal roof at this location), where a portion of the outer insulation of the coaxial cable is stripped and the braided outer or ground conductor 36 is clamped to the vehicle roof 22 for electrical conduction therebetween by a clamp 37 and a screw 38. This location for the ground connection is determined from the vehicle body standing wave pattern to be a voltage null. The coaxial cable 35 further extends across the front of the vehicle roof 22 to the centre front thereof and extends from there back to the centre front of the slot 3l. The coaxial cable 35 is anchored on the outer portion 33 adjacent the slot 3l by a clamp 40; and inner conductor 4l of the coaxial cable 35 extends across the slot 3l to be attached to the inner portion 32. In the embodiment of Figure 2a, the insulation is stripped from the end of the coaxial cable 35 adjacent the slot 3l; and the clamp 40 establishes electrical communication between the braided outer conductor 36 and the outer portion 33 of the electrically conducting layer 27. In the embodiment of Figure 2b, on the other hand, a grounding strap 42 connects the right front corner of the outer portion 33 to the clamp 37. Either way, a DC ground and a signal ground at commercial AM frequencies is established to the vehicle body. As already mentioned, the outer portion 33 of the electrically conducting layer 27 lies partially beneath the inner non-conducting portion 25 and partially beneath the outer electrically conducting portion 23 of the vehicle roof 22. This overlap extends entirely around the circumference of the vehicle roof 22 and provides capacitive coupling between the outer or ground portion 33 of the electrically conducting layer 27 of the antenna and the electrically conducting portion of the vehicle body, which coupling establishes an FM signal ground for the antenna. An embodiment of the antenna in accordance with the present invention is shown in Figure 6, wherein separate feed points are provided for AM and FM reception. It has been determined, at least for some vehicle structures, that optimum FM reception with a slot as described above is obtained with a centre front feed while optimum AM reception is obtained with a side feed. Therefore, in this embodiment, dual coaxial cables 35′ and 35′′ are provided. The coaxial cable 35′ is connected at its lower end to the FM tuner of the AM-FM radio receiver l3 and is routed and connected as is the coaxial cable 35 of the previous embodiments. The coaxial cable 35′′ is connected at its lower end to the AM tuner of the AM-FM radio receiver l3 and follows coaxial cable 35′ to the top of the roof pillar l5; but it extends from there back along the side of the vehicle roof 22 and then inward therefrom as shown to feed the slot 3l at the right side thereof. The antenna thereby becomes a front fed slot antenna for FM reception and a side fed slot antenna for AM reception. This principle may be extended to other frequency bands as further testing determines the optimum feed points for CB or cellular telephone frequencies. The principle could also be used in an embodiment wherein separate AM and FM portions, 5l and 52, respectively, of the AM-FM radio receiver are physically located at feeds of the inner conductors 4l′ and 4l′′, respectively, of the slot antenna, as shown in Figure 9, with the remainder of the AM-FM radio receiver in dashboard l2. This configuration has the potential to eliminate the RF signal loss associated with the coaxial cable, permit antenna matching at each slot terminal, remove part of the AM-FM radio receiver from the dash area and reduce electromagnetic compatibility problems, depending on how much of the AM-FM radio receiver is removed to the roof area. If only the RF portions of the AM-FM radio receiver are included in AM and FM portions 5l and 52, coaxial cables would be run down to the AM-FM radio receiver l3 in the manner already shown or could be joined at some point with a splitter. If the IF and detector sections are also included, plain audio cable may be used. In either case, a tuner control cable may be required from the AM-FM radio receiver l3 to AM and FM portions 5l and 52 to control tuning therein. Another embodiment of the invention is shown in Figures 7 and 8. In this embodiment, the antenna is applied as a coating on the underside of the plastic non-conducting portion of the vehicle roof. As seen in Figure 7, a sheet moulded compound (SMC) panel 43 overlaps the top of front and side rails 60 and 6l of the outer electrically conducting portion 23 of the vehicle roof at the front and sides thereof but extends under a sheet metal rear portion 45 of the vehicle roof. The antenna is a slot 46 between inner 47 and outer 48 painted-on areas of a layer 27 of a conductive nickel coating having a sheet electrical conductivity of l-2 ohms per square (that is, per square of any size: inch, metre, etc.) in order to reduce the antenna's VSWR to an acceptable level of 5 or less (preferably 3 or less). The use of such a resistive material is a change from the conventional teaching of the prior art, in which a much higher conductivity (a material such as silver, copper, aluminium or silver paint with sheet resistance much less than 0.l ohm) is considered optimum. However, in the context of this vehicle roof mounted, non cavity backed slot antenna, the distributed resistance of the higher resistive material effectively increases the load resistance at the antenna terminals and appears to improve the electromagnetic radiation efficiency by increasing the surface impedance, which is proportional to the square root of the frequency divided by the conductivity, and the skin depth, which is inversely proportional to the square root of the frequency times the conductivity; and this increased radiation efficiency appears to more than make up for any resistive losses in the antenna. A specific example of the paint is Electrodag (R) 440, available from Acheson Colloids Co., Port Huron, MI (USA). The slot dimensions are approximately 0.006 metres wide in a rectangle l.035 metres across the car by 0.65 metres front to back. In the embodiment of Figure 7, a single inner conductor 4l′ for AM and FM reception may be provided; or separate inner conductors 4l′ for FM reception and 4l′′ for AM reception may be used, as previously described for other embodiments. Figure 8 shows a partial cross section of the rear conducting to non-conducting roof interface. The SMC panel 43 and the sheet metal rear portion 45 abut to form a generally smooth outer surface which supports a vinyl or other roof covering which covers the entire vehicle roof or that portion necessary to hide the apparatus. A portion 50 of SMC panel 43 underlies sheet metal rear portion 45 to provide structural support at the joint and extend outer painted-on area 48 of the conductive coating under portion 50 of the vehicle roof. Capacitive coupling may be improved by clamping with bolts or rivets to hold portion 50 and sheet metal rear portion 45 tightly together. If so, the spacing of the bolts or rivets should be sufficiently close as to provide essentially continuous clamping, such as every one-tenth of a wavelength of the received frequencies. This would be, for example, about every 0.229 metres (9 inches) or so. This could also be done around the remainder of the antenna to clamp portion 50 with outer painted-on area 48 against the metal roof rails comprising outer electrically conducting portion 23 of the vehicle roof. In the preceding specification and the claims which follow, radio frequencies in the commercial AM broadcasting band are frequencies assigned to commercial broadcasting at the time of filing of this application: specifically 535 kilohertz to 1605 kilohertz, inclusive. Furthermore, radio frequencies in the commercial FM band are frequencies assigned to commercial FM broadcasting at the time of the filing of this application: specifically 88.1 Megahertz to 107.9 Megahertz, inclusive. Wavelengths in the same commercial broadcasting bands refer to wavelengths corresponding to the same frequencies: specifically 2.78 metres to 3.41 metres inclusive for FM. Reference is drawn to our European patent application no. 87305213.8 (MJD/1970) filed the same day as this application, and published under the no. 0,262,755, the claims of which relate to the grounding arrangement for a single slot AM/FM antenna as described with reference to Figures 1 to 5 above. The present invention, as claimed below, relates to the dual feed arrangement as described with reference to Figures 6 to 9 above.

Claims: A single slot AM/FM antenna for a motor vehicle body having an electrically conducting lower body portion (11), a plurality of electrically conducting substantially vertical pillars (15-21) defining window openings, and a substantially horizontal roof (22) with an outer electrically conducting portion (23) and a roof panel (24) made of electrically non-conducting material; the antenna comprising a substantially horizontal layer (27) of electrically conducting material attached to the roof, the horizontal electrically conductive layer including a looped slot (31) positioned under the roof panel, the looped slot dividing the horizontal electrically conductive layer into inner (32,47) and outer (33,48) portions, the slot having a total loop length of substantially one wavelength in the commercial FM broadcasting band; and signal cable feed means (35',35") adapted for connection to AM/FM radio receiver apparatus (13), characterised in that the signal feed cable means comprises a first signal feed cable (35') connectable to an FM receiving portion of the radio receiver apparatus and a second signal feed cable (35") connectable to an AM receiving portion of the radio receiver apparatus; the first signal feed cable (35') being connected to the inner portion (32,47) of the horizontal electrically conductive layer (27) at the front centre of the slot relative to the vehicle body to provide an antenna feed for signals in the commercial FM frequency band; and the second signal feed cable (35") being connected to the inner portion (32,47) of the horizontal electrically conductive layer (27) at the side centre of the slot substantially 90 degrees rotated from the front centre of the slot to provide signals in the commercial AM frequency band. A single slot AM/FM antenna as claimed in claim 1, wherein the first signal feed cable comprises a first coaxial cable (35') adapted for connection at its lower end to the radio receiver apparatus (13) which is positioned in the lower body portion (11) of the vehicle body, the first coaxial cable being routed up one of the roof pillars (15) to the vehicle roof (22) and across the vehicle roof to the centre front of the slot (31), the inner conductor (41') of the coaxial cable being connected to the inner portion (32,47) of the layer (27) at its front centre relative to the vehicle body; and the second signal feed cable comprises a second coaxial cable (35") adapted for connection at its lower end to the radio receiver apparatus (13) which is positioned in the lower portion (11) of the vehicle body, the second coaxial cable being routed up one of the roof pillars (15) to the vehicle roof (22) and back along the side of the vehicle roof to the side centre of the slot (31), the inner conductor (41") of the second coaxial cable being connected to the inner portion (32,47) of the layer (27) at its side centre relative to the vehicle body. A single slot AM/FM antenna as claimed in claim 1, wherein the AM receiving portion (51) of the radio receiver apparatus (13) is disposed adjacent the second signal feed cable at the side centre of the slot (31), and the FM receiving portion (52) of the radio receiver apparatus (13) is disposed adjacent the first signal feed cable at the front centre of the slot.

Inventors: NAGY, LOUIS L., WOOD, PAUL W.
Applicants: GENERAL MOTORS CORPORATION
Application year: 1987
Main classification: H01Q1
IPC codes: H01Q1, H01Q13