Abstract:
A wideband meander line loaded antenna is provided with a capacitive feed to lower the reactance of the meander line antenna such that at lower frequencies the antenna reactance goes negative to cancel out the reactance of the meander line and distributed capacitance. The resultant lowering of the low frequency cut-off for the antenna permits the antenna to be used, for instance, in cellular phone applications in which not only are the cellular frequencies accommodated by the antenna, but also PCS and GPS frequencies as well. With the capacitive, feed the low frequency cut-off is lowered by as much as 30% over standard meander line loaded antennas.

Description:
This application claims the benefit of Provisional Application No. 60/290,874, filed May 14, 2001. 
    
    
     FIELD OF INVENTION 
     This invention relates to wideband antennas and more particularly to a method and apparatus for lowering the low frequency cut-off of meander line loaded antennas. 
     BACKGROUND OF THE INVENTION 
     Meander line loaded antennas are described in U.S. Pat. No. 5,790,080 issued to John T. Apostolos on Aug. 4, 1998 and incorporated herein by reference. The purpose of the meander line is to increase the effective length of the antenna such that compact antennas may be designed for use for instance in cellular phones where the real estate for the antenna is limited. 
     With the decrease in size of wireless handsets, it is only with difficultly that one can design an antenna which will fit within the margins of the case of the wireless handset and still be usuable in dual or trimode phones which span the 830 MHz and the 1.7 and 1.9 Mz bands. Now that GPS receivers are sometimes included in wireless handsets it is important that the antenna also be able to receive the GPS frequency of 1.575 GHz. 
     As illustrated in U.S. Pat. No. 6,323,814 issued to John T. Apostolos on Nov. 27, 2001 and incorporated herein by reference, an improvement over Apostolos&#39; original patent includes a wideband version in which the meander line loaded antenna has a wide instantaneous bandwidth. In this particular antenna the feed to the antenna is through a meander line coupled between the signal source and a plannar conductor extending orthogonally from the ground plane for the antenna. This configuration offers an instantaneous bandwidth of 7:1 and has been implemented in a so called quadrature arrangement in which there are two pairs of meander line antennas arranged in opposition. The opposed pairs are orthogonally arranged to enable circular polarization. 
     As described in this latter, patent, the meander line is connected in series between a signal source and a plannar top conductor which is spaced from the ground plane such that the signal from the meander line is directly connected to the top plate. The result for such a feed for the meander line loaded antenna is that the low frequency cut-off of the antenna is determined by the fact that the meander line loaded antenna reactance with a shorted meander line is positive at the lower frequencies, which when added to the meander line and distributed capacity reactance results in a high VSWR at frequencies, in one embodiment, below 860 megahertz, thus limiting its usefulness in the cellular band which is centered around 830 megahertz. It is noted that in this type of antenna the drive is fed through the meander line and then to the top plate. Moreover, a quadrature arrangement is possible with this meander line design and is desirable when the antenna is mounted to the roof of a truck cab because of the circular polanization provided by the quadrature design. 
     SUMMARY OF THE INVENTION 
     Rather than having the meander line connected in series with the feed for the top plate, in the subject invention the feed from the signal source is spaced from one end of the top conductor thus to provide a capacitive feed. In this case, the meander line runs from this capacitive feed point parallel to underside of the top plate where it is folded back and then is connected by a transmission line to the underlying ground plane. The result is the shifting of the low frequency cut-off by more than 20% over the direct feed wide bandwidth meander line loaded antenna described above. 
     The reason for the shifting of the low frequency cut-off is the fact that the meander line loaded antenna reactance with a shorted meander line goes negative at lower frequencies. This reactance is subtracted from the meander line and distributed capacitive reactance such that at the peak of the meander line and distributed capacitive reactance waveform the reactance at the peak is cancelled by the negative going meander line antenna reactance. 
     With cancelled reactance at the lower frequencies, the VSWR of the antenna is decreased so that the antenna is now able to operate approximately 20-30% lower in frequency than the direct feed meander line loaded antenna described in U.S. Pat. No. 6,323,814. 
     In order to provide such an antenna, a horizontal top plate is coupled to the ground plane plate through a slow wave meander line and a transmission line. Signals are coupled to and from the horizontal plate through a low impedance capacitive feed. The meander line is a so-called slow meander line which has one or more loops and is connected to the top plate along the edge at which the capacitive coupling exists. The capacitive feed, in one embodiment, includes a vertical plate having an edge which is parallel to and adjacent to one edge of the top plate, with a gap existing therebetween. The slow meander line as described in the above patent is one in which signals travel down it at speeds less than the speed of light due to the differing impedances in the line. 
     A cap may be attached to the top plate to extend over the capacitive gap and downwardly to increase capacitance between the vertical plate and the top plate at lower frequencies. 
     Mounting the antenna on a finite ground plate conductor generates currents in the conductor which enhance loop mode antenna radiation at low frequencies relative to antenna dimensions to provide a volumetrically efficient antenna suitable for cell phone applications. Also the low frequency cut-off is lowered due to the meander line running from the capacitive feed point of the antenna to the ground plane. Thus there is a series connection to ground through the meander line, or opposed to having a series connection through the meander line to the signal source as was the case in the prior direct connection design. 
     While the subject capacitively fed meander line loaded antenna is discussed in connection with its use in cell phone applications involving not only the cellular phone band of 830 megahertz but also the PCS bands of 1.7-1.9 gigerhertz, such a capacitance fed meander line antenna also has application in a variety of different antennas designed for wideband applications. 
     One such application is a both vertically and circularly polarized antenna to be mounted on trucks, with the circular polarization generated by a quad arrangement of elements. The application is for a combined GPS, PCS and cellular antenna which performs both a wireless communication function with its vertical polarization and acts to receive GPS satellite signals with its circular polarization. 
     It has thus been found that the capacitive feed along with the loading technique described above extends the low frequency capability of the meander line loaded antenna, thus to effectively increase its already wide bandwidth. 
     In summary, a wideband meander line loaded antenna is provided with a capacitive feed to lower the reactance of the meander line antenna such that at lower frequencies the antenna reactance goes negative to cancel out the reactance of the meander line and distributed capacitance, the resultant lowering of the low frequency cut-off for the antenna permiting the antenna to be used, for instance, in cellular phone applications in which not only are the cellular frequencies accommodated by the antenna, but also PCS and GPS frequencies as well. With the capacitive feed the low frequency cut-off is lowered by as much as 30% over standard meander line loaded antennas. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features of the subject invention will be better understood in conjunction with the Detailed Description in combination with the Drawings, of which: 
     FIG. 1 is a diagrammatic illustration of a wireless handset with a low-frequency cut-off meander line loaded antenna; 
     FIG. 2 is a diagrammatic representation of a prior art wideband meander line loaded antenna showing the connection of the signal source through the meander line to the top plate of the antenna; 
     FIG. 3 is a graph showing the combined reactance of the antenna of FIG. 2 in which the reactance is additive at the lower frequencies thus raising the VSWR; 
     FIG. 4 is a diagrammatic representation of the subject capacitively feed meander line loaded antenna in which the signal source has an output which is capacitively feed to upper plate of the antenna, with the meander line extending from the edge of the upper plate to which the input signal is capacitively coupled and down the upper plate in folded spaced relationship thereto; 
     FIG. 5 is a graph showing the cancellation of the reactances for the antenna of FIG. 4 which lowers the cut-off frequency of the antenna. 
     FIG. 6 is a bottom and perspective view of the antenna of FIG. 1 showing the meander line as viewed from the bottom of the top plate, with a dielectric spacer between the meander line and the top plate; 
     FIG. 7 is a side view of the antenna of FIG. 4 showing the capacitative coupling to the top plate, also showing a capacitative cap to increase the capacitance; 
     FIG. 8 is an end view of the antenna of FIG. 7 from the end opposite to the capacitive feed point; 
     FIG. 9 is a top view of the antenna of FIG. 7 showing the placement of the meander line and the capacitive feed plate in dotted outline; and, 
     FIG. 10 is a top view of a further embodiment of the subject invention showing a quadrature antenna which may be both vertically and circularly polarized. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1, a wireless handset  10  is provided with the subject antenna  12 . This wideband antenna is contained in an upper compartment  14  of the handset and provides in one embodiment for trimode operation in both analog, digital cellular and PCS frequencies. It is important to be able to provide the wireless handset with a wide bandwidth antenna which covers all of the frequencies and bands that the handset is to transmit and receive on. 
     Referring to FIG. 2, in the prior art in order to provide a wide bandwidth antenna, a loaded meander line antenna is provided in which a signal source  20  is coupled on one side to a ground plane  22  and through a meander line  24  to a top plate  26  which is parallel to ground plane  22 . The loading in this case is provided by a transmission line  28  connected between top plate  26  and ground plane  22 . 
     While the wide bandwidth meander line loaded antenna of FIG. 2 operates appropriately across a wide bandwidth, it&#39;s low-frequency cut-off, as can be seen from FIG. 3 is determined by several reactances. 
     As can be seen in FIG. 3, there is a reactance associated with the meander line plus the distributed capacity associated with the meander line which is shown by waveform  30 . As can be seen waveform  30  has a peak  32  below a low-frequency cut-off point  34 , illustrated by the corresponding arrow. 
     Also associated with the antenna of FIG. 2 is a meander line loaded antenna reactance illustrated by dotted waveform  34  which is the antenna reactance with a shorted meander line. As can be seen, above the low-frequency cut-off  32  there is cancellation of the two different reactances such that the standing wave ratio is close to 1 to 1 for frequencies above the low-frequency cut-off. 
     However, below the low-frequency cut-off it can be seen that the two reactances associated with waveforms  32  and  34  are additive, thus increasing the VSWR. 
     Referring now to FIG. 4, rather than feeding the meander line in series with the a top plate, in the subject invention a capacitive coupling is utilized in which a vertical plate  40  from a signal source  42 , serves to couple the energy from the signal source  22  to an end  44  of top plate  46 . It will be appreciated that a folded meander line  48  is electrically coupled to the edge of the plate at point  44  and is in turn shunted to ground through a transmission line  50  at the other end of the meander line. 
     The result of so doing is to shift the low-frequency cut-off to the left in the FIG. 5, graph such that the reactance illustrated by dotted line  34 , rather than being positive at lower frequencies, now goes negative as illustrated at  52 , which negative reactance is subtracted from the positive reactance at peak  32 . It has been found that this minimizes the VSWR and thus provides an antenna whose wideband characteristics are not altered, but whose low-frequency cut-off is lowered by as much as 30%. 
     Because of the use of the meander line, a compact antenna is provided which can be used in the relatively small confines of a wireless handset or, for that matter, in any place if which the low-frequency cut-off of such a compact antenna is desired. 
     Antenna  12  is further described in reference to FIGS. 6-9, collectively, unless otherwise stated. Antenna  12  generally includes a top plate  114 , which is coupled to a ground plane plate  115  through a meander line  116  and a transmission line  118 . Signals are coupled through this structure by means of a feed plate  120  connected to a signal source/receiver  122 . Optionally included is a capacitance enhancing cap or plate  124 . 
     Top plate  114  is generally rectangular or square and is substantially parallel to ground plane plate  115 . Ground plane plate  115  is finite and much larger than top plate  114 . The finite limitation of ground plane plate  115  allows the antenna  12  to induce currents therein and causes ground plane plate  115  to function as a radiating element. 
     Top plate  114  is connected to ground plane plate  115  through meander line  116  and transmission line  118 . Meander line  116  is a slow wave meander line as generally described in U.S. Pat. No. 5,790,080 mentioned above. 
     Slow-wave meander line  116  generally includes a low impedance section  126  and a high impedance section  128 . Low impedance section  126  is connected to top plate  114  at one end  130  and is mounted to top plate  114  by means of a dielectric member  132 . High impedance section  128  is connected to low impedance section  126  at the other end  134  thereof, and is physically mounted to low impedance section  126  by a second dielectric member  136 . In this manner, top plate  114  and the thickness and dielectric constant of each of the dielectric members  32  and  136  function to determine the impedances of sections  126  and  128 . 
     Meander line  116  generally has an overall characteristic impedance equal to the square root of the product of the two impedances of sections  126  and  128 . Physically, low impedance section  126  nominally extends from one end of top plate  114  for approximately three-quarters of the length of plate  114 , while high impedance section  128  nominally extends for approximately one-quarter of the length of top plate  114  back to approximately the median point thereof. As shown in FIG. 8, meander line  116 , as well as transmission line  118  are nominally centered along the width of top plate  114 . 
     Transmission line  118  connects the high impedance section  128  to ground. 
     Further structural support may be provided to top plate  114  and meander line  116  by means of dielectric material. Such support may include a wall or other structure extending up from ground plane plate  115  or it may include a dielectric member located between top plate  114  and feed plate  120 . Also, the entire antenna, including ground plane plate  115  may be located in a housing of dielectric material to which the antenna elements are attached. 
     Feed plate  120  is generally the same width as top plate  114  and extends along the entire edge  140  of top plate  114 . Feed plate  120  is not DC connected to top plate  114 , but is only located proximal thereto to provide capacitive coupling of signals to and from top plate  114 . 
     This capacitive coupling may optionally be enhanced by the presence of a capacitance plate  124 , which is connected to top plate  114 . Capacitance plate  124  has an orthogonal member  142 , which extends parallel to feed plate  120 . The length of orthogonal member  142  is along feed plate  120  and its spacing therefrom determines the capacitance created therebetween, which capacitance may be adjusted through these characteristics. 
     In application, the present antenna  110 , with finite ground plane plate  115  may be used as a cell phone antenna, wherein ground plane plate  115  would be oriented vertically while the phone is in use; and antenna structure  12  would extend away from the user&#39;s head. In this configuration, the present antenna generates current in the ground plane and radiates signals which are properly vertically polarized for cell phone applications; whereas, without the finite ground plane plate  115  the antenna structure might only radiate in a monopole mode at relatively low cell phone frequencies given the dimensions of antenna  12  and thereby have a signal null extending out horizontally. Inclusion of finite ground plane plate  115  enhances loop mode radiation, thereby avoiding the monopole null. In this manner a small antenna structure, relative to the applicable wavelengths, is provided for vertically polarized cell phone use, or other similarly restricted applications. 
     By way of example, one version of antenna  12  was constructed having a height of 0.06″, and a length and width of 1.25″, and it had a useful instantaneous bandwidth of 800 MHz to 6000 MHz. 
     FIG. 10 shows a quadrature arrangement of antenna  12  with antenna elements  110   a . Elements  110   a  differ from antenna  12  in that they have a top plate  114   a  which has a triangular shape so that the elements  110   a  may be arranged in quadrature. Each of elements  110   a  includes a feed plate  120 , a slow-wave transmission line  116 , and a transmission line to ground (not shown) which are substantially identical to those of antenna  12 . Elements  110   a  are all mounted on a single ground plane plate  115   a . By this arrangement, the elements  110   a  may be fed in quadrature by known techniques to produce circularly polarized signals. 
     Having now described a few embodiments of the invention, and some modifications and variations thereto, it should be apparent to those skilled in the art that the foregoing is merely illustrative and not limiting, having been presented by the way of example only. Numerous modifications and other embodiments are within the scope of one of ordinary skill in the art and are contemplated as falling within the scope of the invention as limited only by the appended claims and equivalents thereto.