Abstract:
An antenna arrangement for use over a metallic surface in motor vehicles is adapted to improve reception in the AM frequency range. An FM loop includes two conductor sections separated by a capacitor. An AM probe extends from one of the conductor sections near an antenna feed line connector and isolates AM signals by an L-C circuit between the probe and the connection to the conductor section. Omnidirectional reception is achieved by a coplanar WB probe, FM probe and AM coiled loop between them, mounted to a substrate.

Description:
BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The invention relates to antenna arrangements for installation under dielectric covers, and more particularly to antennas for use in motor vehicle structures. 
   2. Description of the Related Art 
   It is common for motor vehicles such as cars, trucks, tractors, recreational vehicles and the like to use several antennas for such purposes as cellular telephones, CB, global positioning system (GPS), weatherband (WB), and the standard AM/FM radio. This proliferation of antennas is attended by special problems such as finding an appropriate mounting position for non-interfering operation as well as such inconveniences as high-speed antenna noise or “whistle.” Attempts have been made in the prior art to avoid external antennas and incorporate them into windowpanes and roof panels and the like. 
   Non-conducting materials such as fiberglass are now commonly used particularly in the construction of truck cabs in order to save weight. However, the use of such a dielectric material presents a problem for antenna designers, since most antennas require a ground plane provided by the metallic vehicle body for efficient operation. One solution to this problem is provided in U.S. Pat. No. 5,402,134 where a flat plate antenna module is disclosed. The module comprises one or more antenna loops formed from thin conductor strips arranged on a dielectric substrate. The substrate readily fits between the headliner of a truck cab and a nonconductive roof panel. 
   The solution in the &#39;134 patent works well over a nonconductive surface, but there is a noticeable degrading of performance in applications were the antenna module is disposed over a conductive surface, such as in a tractor where the headliner is strengthened with metal. Also, the manufacture of antenna modules with conductive ink is expensive. 
   Another common solution, especially for vehicles using the common AM/FM and WB frequencies is providing three receiving wires of different lengths laid horizontally parallel or flared from a single attachment point. The AM receiving wire would typically be 105 inches long, the FM 30 inches, and the WB 16 inches. These lengths are normally effective to render the wires (at least for FM and WB) about a quarter wavelength at midband. But such an antenna arrangement proves to be highly directional and subject to other noise generating sources that may be nearby, including wiring harnesses, etc. 
   SUMMARY OF INVENTION 
   These and other problems of the prior art are overcome in accordance with this invention of an antenna arrangement comprising a first loop having first and second conductor sections of substantially equal length, and a capacitor. Each conductor section has an antenna feed line connector on one end, and the other end is connected to one side of the capacitor. The length of the first and second conductor sections is sufficient to resonate in the FM frequency range. The capacitor has a predetermined value of capacitance to block frequencies in the AM range. 
   A probe, coplanar with the first loop, is connected to the first section, and has sufficient length to effectively function as a dipole AM antenna. Preferably, an L-C circuit is mounted between the probe and the first section to isolate AM loading from the first loop. 
   In one embodiment of the invention, the first loop is square, and both the loop and the probe are formed of multiple segments. Typically, the segments are normal to each other. Ideally, the probe comprises four segments, and each of the segments is parallel to a conductor section. 
   In another aspect of the invention, a second probe is coplanar with the first loop, connected to the first section and has a length sufficient to resonate at the weatherband frequency. An L-C circuit can be disposed between the second probe and the first section. 
   Preferably the probe and/or the second probe are connected to the first section near the antenna feed line connector, although connection to the first section near the capacitor is equally possible. 
   Another aspect of the invention has an antenna module comprising a planar dielectric substrate. A first loop is mounted to the substrate and has first and second conductor sections of substantially equal length, and a capacitor. Each conductor section has one end with an antenna feed line connector and the other end connected to one side of the capacitor. The length of the first and second conductor sections is sufficient to resonate in the FM frequency range and the capacitor has a predetermined value of capacitance to block frequencies in the AM range. A probe is mounted to the substrate substantially coplanar with the first loop, connected to the first section, and having sufficient length to effectively function as a dipole AM antenna. 
   In a further aspect of the invention, an antenna module comprises a planar dielectric substrate, a first probe mounted to the substrate, a second probe mounted to the substrate normal to and shorter than the first probe; and a loop mounted to the substrate coplanar with the first and second probes, and within the angle formed between the first and second probes. The first and second probes and the loop are each connected to a single feed point and the loop comprises multiple turns, no turn extending beyond the length of either the first or second probes. 
   Preferably, each turn of the loop and the first and second probes are equidistantly spaced from each other. The antenna arrangement can further have a ground lead connected to the feed point. 
   Typically, the first probe has sufficient length to resonate in the FM frequency range, the second probe has sufficient length to resonate in the WB frequency range and the loop has sufficient length to effectively function as an AM antenna. Because of the interaction among them, the length of each of the probes and the loop is less than one quarter wavelength of its corresponding frequency midrange. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     In the drawings: 
       FIG. 1  is a plan schematic view of an antenna arrangement according to the invention; 
       FIG. 2  is a plan schematic view of an alternate construction of the antenna arrangement of  FIG. 1 ; 
       FIG. 3  is a plan schematic view of a third embodiment of an antenna arrangement according to the invention; and 
       FIG. 4  is a plan schematic view of a fourth embodiment of an antenna arrangement according to the invention. 
       FIG. 5  is a plan view of a fifth embodiment of an antenna arrangement according to the invention. 
   

   DETAILED DESCRIPTION 
   A flat antenna arrangement  10  according to the invention is illustrated in FIG.  1 . The arrangement  10  comprises an FM loop  12  and an AM probe  14 . The FM loop  12  has two separate conductor sections  16 ,  18 , together forming a square loop having a total length of approximately one wavelength in the FM frequency range. A capacitor  20  is connected between the two conductor sections  16 ,  18 , preferably in one corner of the square loop  12 . Diametrically opposite the capacitor  20  on the square loop  12  is a feed point  22  that is adapted to connect to an antenna feed line (not shown) in the conventional manner. 
   The capacitor  20  may be a discrete capacitor, preferably having a typical value of 50 Pico farads. Ideally, the capacitor will have a value such that the capacitor  20  presents essentially a low impedance connection at the FM frequency, and a substantial impedance in the AM frequency range. 
   The conductor section  16  has portions  24 ,  26  extending at right angles to each other between the feed point  22  and the capacitor  20 . In a similar manner the conductor section  18  has portions  28  and  30  extending at right angles to each other past the capacitor  20 . With each portion  24 ,  26 ,  28  and  30  being approximately 32 inches long, and the capacitor  20  having a value at about 50 Pico farads, the whole square FM loop  12  will resonate in the FM band (from 88 to 108 MHz). 
   The position and value of the capacitor  20  effectively makes the conductor sections  16 ,  18  into the equivalent of two legs of an AM antenna. It has been found that when the foregoing structure is placed over a conductive surface, the performance of the antenna module in AM reception is significantly degraded. In accordance with the invention, the AM probe  14  is connected to the portion  24  near the feed point  22  to enhance AM reception, even over a conductive plane. The AM probe  14  comprises four segments  32 ,  34 ,  36 , and  38  disposed inside the square loop  12 , and coplanar with it, with each segment being respectively parallel to the portions  24 ,  26 ,  28 , and  30 . The AM probe  14  is isolated from the square FM loop  12  by a resonant L-C circuit  40  connecting an end of the segment  32  to a feed point  42  near the feed point  22  of the FM loop  12 . The circuit  40  is preferably self-resonant at about 100 MHz to prevent the AM probe  14  from loading the FM loop  12 . A coil  44  can optionally be added between the segments  34  and  36  to electrically lengthen the AM probe  14  and further enhance AM reception. 
   It will be understood that while the FM loop  12  and the AM probe  14  are illustrated in  FIG. 1  as essentially square, the relative configurations can be circular, provided that the relative circumferences are selected to resonate at the appropriate frequency range. An alternative arrangement is illustrated in  FIG. 2  where like components are identified with like numerals. It will be seen that the only difference between the configuration of FIG.  2  and the configuration of  FIG. 1  is the location of the feed point  42  where the AM probe  14  connects to the FM loop  12 . In  FIG. 2 , the feed point  42  is located nearer to the capacitor  20  than to the feed point  22 . 
   The actual construction of an arrangement  10  in accordance with the invention can take many different forms. Typically the feed point  22  will be adapted to connect to a high impedance cable (normally RG  62 ). With respect to the arrangement  10  itself, it is important only that the FM loop  12  and the AM probe  14  be coplanar. The FM loop  12  and the AM probe  14  can be formed of single wires mounted and held in place so as to be coplanar. Alternatively, the arrangement  10  can be formed of wires or conductive ink on a dielectric substrate to form a module. It is possible to the weave wires into a web, or to sew wires onto a dielectric substrate. A preferred and low-cost construction is to use a corrugated polypropylene substrate for some rigidity, and affix to it an arrangement  10  such as that illustrated in  FIGS. 1-2 . 
   An enhancement to the invention is illustrated in  FIG. 3. A  weatherband probe  50  extends from the feed point  52  on the FM loop  12  and runs parallel to one of the segments  24  or  30 . The length of the weatherband probe  50  is selected to resonate in the weatherband frequencies (162 MHz). The weatherband probe  50 ′ can alternatively extend at an angle relative to the portion  24  as illustrated in phantom. The weatherband probe  50  can be isolated from the FM loop  12  and by using a resonant L-C circuit  54 . 
     FIG. 4  illustrates an alternative embodiment of the invention that has been found to provide nearly omnidirectional reception in the AM/FM and WB frequencies. A flat antenna arrangement  60  comprises an FM probe  62 , a WB probe  64  and an AM coiled probe  66 . All three connect to a single feed point  68  that is adapted to connect to an antenna feed line  70  in a conventional manner. The FM probe  62  and WB probe  64  are disposed at 90 degrees with respect to each other, and intermediate the two probes  62 ,  64  is the AM coiled probe  66 . The AM coiled probe  66  is a coil wherein each turn is preferably equidistantly spaced from another. Also, preferably, the probes  62 ,  64  are spaced from the AM coiled probe  66  the same distance as the turns of the coil are spaced from each other. Optionally, a ground connection  72  can be provided from the feed point  68  to improve performance as needed. 
   It has been found that with the antenna arrangement  60 , interaction between the probes  62 ,  64  and the coiled probe  66  reduces the actual length of the components necessary to optimize reception in the respective frequency ranges. For example, good performance has been achieved where the FM probe  62  is 19 inches long, the WB probe  64  is 14 inches long, and the total length of the AM coiled probe  66  is 112 inches. It is noted that no single side of the AM coiled probe  66  is longer than an adjacent probe  62 ,  64 . 
   As with the first embodiment, the actual construction of the arrangement  60  can take many different forms. One such embodiment is illustrated in FIG.  5 . Typically the feed point  68  will be adapted to connect to a high impedance cable  70  (normally RG  62 ). With respect to the arrangement  10  itself, it is important only that the FM probe  62 , the WB probe  64  and the AM coiled probe  66  be coplanar. Each can be formed of single wires mounted and held in place so as to be coplanar. In  FIG. 5 , the arrangement  60  is formed of wires or conductive ink on a dielectric substrate  74  to form a module. The ground connection  72  is a separate lead adapted to be connected to a ground source in conventional manner. It also is possible to weave wires into a web, or to sew wires onto a dielectric substrate, or to produce conductive traces on a printed circuit board. As well, an optional coil  76  can be added to the AM coiled probe  66  to electrically lengthen it and further enhance AM reception (see FIG.  4 ). 
   While the invention has been specifically described in connection with certain specific embodiments thereof, it is to be understood that this is by way of illustration and not of limitation, and the scope of the appended claims should be constructed as broadly as the prior art will permit.