Abstract:
A cylindrically conformable antenna is formed on a flexible substrate and preferably comprises a complex pattern coupled to a first feedline and, spaced-apart from the complex pattern, a patch that floats electrically. The complex pattern preferably is a fractal pattern, deterministic or otherwise, but need not be a fractal. The shape, size, and position of the patch relative to the complex pattern, as well as the complex pattern itself, produces multiple frequency bands of interest. These bands may be varied by varying the relative parameters associated with the patch and complex pattern. The resultant antenna is substantially smaller than conventional antennas for the same frequency band, has a natural 50 Ω feed impedance and performs substantially as well as larger conventional antennas.

Description:
RELATION TO PREVIOUSLY FILED APPLICATION 
   Priority is claimed to applicant&#39;s U.S. provisional patent application Ser. No. 60/066,689, filed Nov. 22, 1997, and entitled “Cylindrical Conformable Antenna on a Planar Substrate”. 
   FIELD OF THE INVENTION 
   The present invention relates to miniaturized antennas suitable for communication systems including cellular telephones and more particularly to reducing the size of such antennas while still providing an acceptable antenna loading mechanism. 
   BACKGROUND OF THE INVENTION 
   Attempts have been made in the prior art to miniaturize antennas for communications.  FIG. 1A  for example depicts an end-loaded shortened dipole antenna  10  with a meander-line counterpoise  20 . A commercially available antenna  10  such as shown in  FIG. 1A  suitable for cellular telephony is marketed by Radio Shack Corp. The size of antenna  10  may be compared to the enlarged U.S. quarter, shown in  FIG. 1B , the enlargement being the same for  FIGS. 1A and 1B . A common resonant frequency for the prior art antenna of  FIG. 1A  is about 870 MHz. 
     FIG. 1C  depicts antenna  10  used with a cellular telephone  30 . While antennas such as antenna  10  do function, they are several cm in length or must be pulled-out to a length of several cm. This length makes the antenna and/or cellular telephone (or other transceiver device) somewhat vulnerable to breakage. Clearly a smaller version of a cellular telephone-type antenna would be beneficial. 
   As described in the following sections, fractal patterns are preferably used with the present invention. By way of further background, applicant refers to and incorporates herein by reference his PCT patent application PCT/US96/13086, international filing date 8 Aug. 1996, priority date 9 Aug. 1995, entitled “Fractal Antennas and Resonators, and Loading Elements”. 
   SUMMARY OF THE INVENTION 
   The present invention provides an antenna configuration comprising a flexible substrate having spaced-apart first and second surfaces. A conductive pattern is formed on the first surface, the pattern preferably defining complex geometry such as a fractal of first or higher iteration. One portion of the complex pattern defines a feed-point to which RF energy may be coupled or received. (Preferably the other feed-point will be a groundplane associated with the environment with which the antenna is used, for example the interior shell of a cellular telephone.) The frequency characteristics of the antenna may be tuned by varying the iteration and/or shape of the fractal. 
   More preferably, tuning is facilitated by disposing a conductive patch spaced-apart by about the substrate thickness from the complex pattern. The patch may be a small square or rectangle or other shape. The patch “floats” electrically in that it is not directly coupled to any feedline. Instead, the patch acts as a capacitive load that can capacitive couple various locations in the complex pattern. The preferably dielectric substrate couples RF current through the substrate thickness. RF current in the complex pattern on the first surface differs in magnitude from location to location at the through-substrate coupling regions. 
   On one hand, the complex geometry on the first surface contributes an inductive loading. On the other hand, the patch on the second surface contributes a capacitive loading. In combination, the two loading effects produce a monopole that is dimensionally small physically yet is an efficient radiator of RF energy and exhibits a multi-band frequency characteristic. Multiple frequency bands of interest may be produced and tailored by the size, configuration, and/or position of the patch relative to the complex pattern, as well as by the complex pattern itself. If desired, the patch can be formed on a separate layer of substrate that is slid or otherwise moved about relative to the location of the complex pattern, to tune characteristics of the antenna. 
   The preferably flexible substrate(s) may be partially rolled to form a semi-cylindrical or cylindrical shape. The conformally rolled substrate (with complex pattern and patch on the spaced-apart surfaces) may then be inserted into a cylinder and used to replace the “ducky” or “stubby” antenna commonly used in cellular telephone or transceiver applications. 
   Other features and advantages of the invention will appear from the following description in which the preferred embodiments have been set forth in detail, in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  depicts a miniaturized cellular telephone antenna, according to the prior art; 
       FIG. 1B  depicts a U.S. quarter, enlarged to the same scale as the prior art antenna of  FIG. 1A ; 
       FIG. 1C  depicts a communications transceiver equipped with a prior art antenna such as that shown in  FIG. 1A ; 
       FIG. 2A  depicts an exemplary complex pattern suitable for the present invention, here a first iteration Minkowski fractal; 
       FIG. 2B  depicts another exemplary complex pattern suitable for the present invention, here a third iteration Sierpinski fractal ribbon; 
       FIG. 3A  depicts a preferred embodiment of the present invention in a preliminary stage of formation; 
       FIG. 3B  depicts the embodiment of  FIG. 3A  with the substrate partially rolled; 
       FIG. 3C  depicts the embodiment of  3 B with the substrate inserted within a cylindrical form; 
       FIG. 4A  depicts a communications transceiver equipped with an external antenna, according to the present invention; 
       FIG. 4B  depicts a communications transceiver equipped with an internal antenna, according to the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   As will be described, the present invention comprises a substrate having first and second surfaces spaced-apart by the typically sub-mm substrate thickness. A complex pattern of conductive material is formed on the first surface, for example a first or higher iteration fractal pattern.  FIG. 2A  depicts an exemplary such pattern  40 -A, namely a first iteration Minkowski fractal geometry having an RF feed-point  45 .  FIG. 2B  depicts another exemplary such pattern  40 -B, here a third iteration Sierpinski ribbon, again with an RF feed-point  45 . For ease of comparison, the geometries of  FIGS. 2A and 2B  are drawn to the same scale as what is depicted in  FIGS. 1A and 1B . 
   If fractal configurations are employed, other fractal patterns may include (without limitation) Koch, Cantor, torn square, Mandelbrot, Caley tree, monkey&#39;s swing, and Julia. Thus  FIGS. 2A and 2B  depict but two exemplary complex patterns, but other patterns including deterministic and non-deterministic fractals, and non-fractal geometries may instead be used. 
   Fractal patterns comprise at least a first motif and a first replication of that first motif. Fractals of iteration greater than two may be defined as also including a second replication of the first motif such that a point chosen on a geometric figure represented by said first motif will result in a corresponding point on both the first replication and the said second replication of the first motif. Further, there will exist at least one non-straight line locus connecting each such point. The definition of a greater than first order fractal may be said to require that replication of the first motif is a change selected from a group consisting of (a) a rotation and change of scale of the first motif, (b) a linear displacement translation and a change of scale of said the motif, and (c) a rotation and a linear displacement translation and a change of scale of said the motif. 
   Turning now to  FIG. 3A , complex pattern  40  (which is understood to include without limitation first or higher order fractals, (deterministic and non-deterministic) or non-fractal configurations is formed on first surface  50  of substrate  60 . The pattern of  FIG. 3A  may also be described as a stubbed open-loop configuration. 
   Substrate  60  is preferably a dielectric material, for example the polymeric material sold under the trademark Mylar®, polyester, etc. having a thickness of less than 1 mm. In  FIG. 3A , the length and width of dielectric substrate  60  are perhaps 18 mm ×12 mm, although other dimensions could instead be used. 
   Complex pattern  40  may be formed using a variety of techniques. Substrate  60  may for example be double-sided flexible printed circuit board, in which case pattern  40  may be formed using conventional pattern and etching techniques. Alternatively, pattern  40  could be printed or sprayed or sputtered onto substrate  60  using electrically conductive paint. The advantage of using a fractal configuration for pattern  40  is that the effective area required for the pattern is reduced, although the perimeter length of the pattern is increased. A portion  45  of pattern  40  is used as an RF feed-point, whereat a lead from RF cable may be attached. 
   Two embodiments are shown simultaneously in  FIG. 3A . In one embodiment, patch  80  is formed on second surface  70  of substrate  60 . If patch  80  is rectangular in shape, typical dimensions for use at cellular telephone frequencies are perhaps about 10 mm × about 3 mm. Patch  80  is formed from electrically conductive material and may be created by depositing or spraying or painting conductive paint (or the like), or by etching away from surface  70  all conductive material except patch  80 . At noted, patch  80  floats in that no direct electrical connections are made to it. The geometry, size, and/or location of patch  80  relative to complex pattern  40  is varied to alter characteristics of the overall antenna to be formed. In practice, the desired relationship between complex pattern  40  and patch  80  may be determined in a laboratory environment by trial and error. However once determined, the resultant double-sided substrate configuration may then be mass produced at relatively low cost. Patch  80 ′, for example, shows a different location relative to complex pattern  40  relative to patch  80 . Thus, if patch  80 ′ is used, a different antenna characteristic can result than if patch  80  were instead used. 
   Note in  FIG. 3A  that an optional second substrate  90  is shown, whose upper surface  100  contains an electrically conductive patch  80 ″. Assume now that neither patch  80  or  80 ′ is present (although if desired, one or more such patches could be present). Patch  80 ″ essentially abuts second surface  70  of substrate  60 . In this embodiment, fine tuning of the overall antenna can readily be accomplished by sliding substrate  90  relative to substrate  60 , circularly and/or linearly as indicated by the two sets of double-arrowed lines. In this fashion, patch  80 ″ can be oriented in an optimum location by moving one substrate relative to the other. Once an optimum location and/or orientation (e.g., rotary movement) is determined, the substrates can be secured one to the other using clamps, adhesive, or other attachment mechanisms. 
   In  FIG. 3B , substrate  60  is shown in the process of being curved, which is one advantage of a flexible substrate. In this embodiment, a patch  80  is shown fabricated on second side  70  of the substrate. In  FIG. 3C  substrate  60  has been conformed to an almost closed cylindrical shape and is depicted as being inserted into a closed cylinder  90 . A gap  110  may exist if substrate  60  does not close fully upon itself, but the presence or absence of such a gap is not important. A rolled or cylindrically shaped antenna system  130  lends its readily to functioning as a substitute for the stub or ducky type antennas  10  used with communication transceivers  30 , as depicted in  FIG. 1C . 
   If desired, patch  80 ,  80 ′, or  80 ″ (or more than one patch) may in fact be formed on the interior surface of cylinder  90 . This permits a mechanism for tuning the resultant antenna system  130 , namely by rotating and/or laterally moving substrate  60  relative to cylinder  90 . For example, micro-threads might be formed such that substrate  60  screws into cylinder  90 . A fine veneer mechanism may also (or instead) be formed to facilitate fine tuning, if desired. 
   In  FIG. 3C , a feedline  140  (e.g., 50 Ω coax) is shown coupled to feed-point  45  and to a ground plane  120 . In practice, ground plane  120  may be the interior shell of the electronic device with which antenna  130  is used. For example, in the embodiment of  FIG. 4A , the electronic device is a cellular telephone or transceiver  30  (which may be similar to that shown in  FIG. 1C ), and ground plane  120  may be a metal plate or perhaps metallic paint sprayed on a portion of the interior housing of device  30 . 
   In  FIG. 4A , an antenna system  130  according to the present invention is shown protruding from the housing of device  30 . However in stark contrast to antenna  10  shown in  FIG. 1C  (whose overall length may be 70 mm), the overall length of antenna  130  will be perhaps 15 mm (for cellular telephone frequencies). Indeed, as shown in  FIG. 4B , antenna  130  is sufficiently small to be mounted inside the housing of device  30 . As such, antenna  130  is immune to damage from being broken off device  30 , in contrast to antenna  10  in  FIG. 1C . 
   The present invention has been found to provide a natural approximately 50 Ω feed impedance, thus obviating the need for matching transformers, stubs, or the like. Further, the present invention provides an omni-directional gain and bandwidth that is substantially identical to the performance of conventional antenna  10  in  FIG. 1C , notwithstanding that the present invention is substantially smaller than antenna  10 . 
   Although the preferred embodiment has been described with respect to use with a cellular telephone communication system, those skilled in the art will appreciate that applicant&#39;s fractal antenna system may be used with other systems, including without limitation transmitters, receivers, and transceivers. 
   Modifications and variations may be made to the disclosed embodiments without departing from the subject and spirit of the invention as defined by the following claims.