Abstract:
A meander line loaded antenna is provided with a ferrite donut surrounding a portion of the meander line so as to effectively lower the operating range of the meander line loaded antenna by as much as 30%. At the lower frequencies the ferrite material introduces a minimal loss of 2 to 3 dB, whereas at the higher frequency range very little current passes through the meander line thus eliminating any effect of the ferrite on antenna performance. The utilization of the ferrite surrounding a meander line element permits the use of the miniaturized antenna and size for size reduces the low frequency cut off of the antenna.

Description:
FIELD OF INVENTION  
         [0001]    This invention relates to meander line loaded antennas and more particularly to the use of ferrite to reduce the low frequency cut off of the antenna.  
         BACKGROUND OF THE INVENTION  
         [0002]    Whether the antenna is a capacitive feed type meander line loaded antenna or a standard meander line loaded antenna, these antennas are characterized by their small size and their wide band performance.  
           [0003]    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 real estate for the antenna is limited or in military applications where it is important to be able to provide a compact antenna for surveillance and communications in which the desired frequency range is 30 MHz to 200 MHz.  
           [0004]    For cellular applications 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 useable 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.  
           [0005]    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.  
           [0006]    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 MHz, thus limiting its usefulness in the cellular band which is centered around 830 MHz.  
           [0007]    For military applications, while antennas have been designed to operate between 50 MHz and 200 MHz it is important to lower the low frequency cut off to 30 MHz and still maintain the small size of the antenna. 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 polarization provided by the quadrature design.  
           [0008]    While capacitive feed meander line antennas have been effective in lowering the low frequency cut off of meander line loaded antennas, there is still a need to operate at even lower frequencies without enlarging the antenna. Whether utilizing a conventional meander line loaded antenna which has an ultra wide bandwidth, or when using a capacitively coupled meander line loaded antenna which in turn has a lowered low frequency cut off, it is desirable to even further lower the low frequency cut off for such antennas, making possible an operating range down to, for instance, 30 MHz in an antennas whose range typically goes from 50 MHz to 200 MHz.  
           [0009]    In short, while present techniques permit the lowering of the low frequency cut off of such antenna systems, it is still desirable to have these antennas be able to operate at lower and lower frequencies, yet not increase their size.  
         SUMMARY OF THE INVENTION  
         [0010]    Whether utilizing a conventional meander line loaded antenna such as described as above or one having a capacitive feed which even further lowers the low frequency cut off of a meander line loaded antenna, in the subject invention the low frequency cut off of these meander line loaded antennas may be lowered still further by as much as 30% by surrounding one of the meander line loaded antenna elements with a ferrite core, usually in form of a toroidal donut. The effect of surrounding one of the elements with a toroidal ferrite core is in effect to provide a series inductor which in turn effectively lengthens this particular element. The lengthening of this particular element in turn results in a lower frequency response such that the VSWR for the antenna at this lower frequency is identical to that at higher frequencies.  
           [0011]    There is however an insertion loss at the low frequency end of these antennas, but the insertion loss is generally less than 2 to 3 dB. It is noted that the effect of the toroidal ferrite core is dependant upon current through the meander line. At the high frequency end the current through the meander line is virtually non-existent, meaning that the effect of the ferrite core is completely eliminated. The reason is that at the higher frequencies the antenna basically relies on the capacitance between the upper plate and the ground plane, leaving meander line as if it were not connected.  
           [0012]    The result of utilizing the toroidal ferrite cores is either to lower the low frequency cut off of an existing meander line loaded antenna, or to permit even further shrinking of the meander line loaded antenna for the original frequency range intended.  
           [0013]    The shrinking of the antenna is important especially when these antennas are utilized in hand held wireless devices or, for instance, when they are utilized in remotely piloted vehicles such as the Predator or other such unmanned vehicles used for surveillance. Size is important because it is critical that the antennas not create aerodynamic drag or in fact dictate an increase in the size of the vehicle.  
           [0014]    For instance, when these vehicles are used for electronic surveillance, it is important to be able to cover a wide frequency band in order to detect signals of interest as the remotely piloted vehicle flies over a given area. These remotely piloted vehicles can in some instances be driven by electric motors and are hand launched at the battlefield. Thus the ability to micro miniaturize antennas operating as far down as 30 MHz is exceedingly important, especially in the electronic surveillance field.  
           [0015]    In summary, a meander line loaded antenna is provided with a ferrite donut surrounding a meander line element so as to effectively lower the operating frequency of the meander line loaded antenna by as much as 30%. At the lower frequencies the ferrite material introduces a minimal loss of 2 to 3 dB, whereas at the higher frequency range very little current passes through the meander line, thus eliminating any effect of the ferrite on antenna performance. The utilization of a ferrite donut surrounding a meander line element permits the use of the miniaturized antenna at the lower and lower frequencies, and size for size reduces the low frequency cut off of the antenna. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    These and other features of the subject invention will be better understood in connection with the Detailed Description in conjunction with the Drawings, of which:  
         [0017]    [0017]FIG. 1A is a diagrammatic representation of a standard meander line loaded antenna having a toroidal ferrite core surrounding one of the elements thereof;  
         [0018]    [0018]FIG. 1B is a diagrammatic illustration of the conventional meander line loaded antenna of FIG. 1A, illustrating the effective lengthening of one element of the meander line through the use of the toroidal ferrite core by effectively putting an inductor in series with the ferrite surround element;  
         [0019]    [0019]FIG. 2A is a diagrammatic illustration of one embodiment of a capacitively fed meander line loaded antenna having a toroidal ferrite core surrounding a downwardly directed portion of the meander line which is electrically connected to the ground plane;  
         [0020]    [0020]FIG. 2B is a diagrammatic illustration of the capacitively fed meander line loaded antenna of FIG. 2A illustrating the effective lengthening of the downwardly directed portion of the meander line by effectively introducing a series connect inductor in the element surrounded by ferrite;  
         [0021]    [0021]FIG. 3 is a perspective view of the capacitively fed meander line loaded antenna of FIG. 2A illustrating the position of the toroid about the downwardly projecting portion of the meander line;  
         [0022]    [0022]FIG. 4 is a schematic diagram of the effect of adding a toroidal ferrite core about the downwardly projecting element of the antenna of FIG. 3, indicating a series connected inductor as being the effect of adding the toroid;  
         [0023]    [0023]FIG. 5 is a graph showing VSWR versus frequency for an antenna without the toroid versus one with the toroid, showing that without the toroid the low frequency cut off is around 50 MHz, whereas with toroid the low frequency cut off is around 33 MHz;  
         [0024]    [0024]FIG. 6 is a perspective view of a conventional meander line loaded antenna having a top plate which is 32″ by 32″, with a meander line segment being a ⅜ th  inches wide; and, FIG. 7 is a diagrammatic illustration of the horizontally running segment of the meander line of FIG. 6 being provided with a toroidal donut of ferrite material which is ½″ long with a ½″ in inside diameter.  
     
    
     DETAILED DESCRIPTION  
       [0025]    Referring now to FIG. 1A, a meander line loaded antenna is shown having a meander  10  connected between a signal source  12  coupled to a ground plane  14  and a top plate  16  parallel to ground plate  14 . The antenna has an upstanding feed conductor  20  coupled to one end  22  of a section  24  of meander line  10  which has an upstanding section  26  and a folded back section  28  which is in turn coupled to plate  16  at its distal end  30  by an upstanding portion  32 . As is usual the meander line is composed of elements having different impedances which are effectively used to lengthen the circuit and thus reduce the overall size of the antenna. As shown in this Figure, a flap  34  is disposed over end  36  of feed  20  at end  38  of top plate  16 .  
         [0026]    The configuration shown in FIG. 1A comprises a wide bandwidth meander line loaded antenna which can be designed to have a low frequency cut off of 50 MHz and an high frequency cut off over 200 MHz.  
         [0027]    Size-for-size in order to lower the low frequency cut off of the standard meander line loaded antenna of FIG. 1A, a toroidal ferrite donut  40  surrounds a portion of meander line element  24 , the operation of which is to effectively lengthen that particular element from electrical point of view while not in any way altering the overall size of the structure.  
         [0028]    Referring to FIG. 1B, in which like elements have like reference characteristics with respect to FIG. 1A, the utilization of the ferrite core in essence lengthens element  24  as illustrated by the serpentine line  42  such that the effective length  44  of this meander line element is increased, sometimes as much as 30%. The increase of this particular meander line segment or section contributes to the lowering of the low frequency cut off of the antenna.  
         [0029]    Ferrite may also be utilized in a capacitive feed antenna such as the one shown diagrammatically in FIG. 2A. Here meander line  10  is feed capacitively by the capacitance between end  36  of feed  20  and end  38  of top plate  16 . The capacitance feed for this antenna can result in significant lowering of the low frequency cut off of the antenna. However, if it is desired to even further lower the low frequency cut off of the capacitively fed antenna, meander line section  24  which is disposed downwardly as illustrated at  50  and is connected to ground plane  14  as illustrated is surrounded by a toroidal donut  52  which is spaced from or insulated from ground plate  14  by insulator  54 .  
         [0030]    Referring to FIG. 2B, in which like elements have like reference characteristics with respect to FIG. 2A, it can be seen that meander line segment is effectively lengthened as illustrated by the serpentine line  56 , with the effective lengthening illustrated at  58 .  
         [0031]    Referring to FIG. 3, in perspective this capacitively fed meander line loaded antenna is shown with the sections of meander line  10  as illustrated. Here it can be seen that the toroidal ferrite donut  52  surrounds the downward projecting portion  50  of meander line element  24 , with FIG. 4 showing electrically that portion of the meander line as having a downwardly directed portion  50  and an inductor  60  connected between portion  50  and ground plane  14 .  
         [0032]    The result in terms of VSWR, with and without the use of a toroid, is illustrated in FIG. 5. Here it can be seen that in a graph of VSWR versus frequency line  80  describes the VSWR of the antenna without a toroid, whereas line  82  describes the VSWR when utilizing the toroid.  
         [0033]    It can be seen that the low frequency cut off of the antenna in one embodiment is above 50 MHz, whereas the low frequency cut off of the self-same antenna with the use of the toroid is approximately at 33 MHz.  
         [0034]    Referring to FIG. 6, a conventional meander line loaded antenna for use between 50 MHz and 200 MHz has a top plate  84  which is 32″ by 32″. Meander line  10  has width of ⅜″ such that its sections  24 ,  26 ,  28  and  32  have this width. Also feed  20  has this same width, with feed  20  having an extending tab  21  and an insulating mounting member  23 . The ground plane segment of the antenna  14  has at least this 32″ by 32″ footprint.  
         [0035]    Referring to FIG. 7, a ferrite toroid  90  surrounds a portion of section  24 .  
         [0036]    In this embodiment, for a frequency range of 30-150 MHz, one uses a T-50 ferrite core with permeability of 6. The outer diameter is 0.5 inches, with the inner diameter being 0.3 inches and the height being 0.19 inches. Inserting the toroid around the meander line results in an inductance of 0.16 microhenry. This inductance is in series with the meander line. The reactance of the meander line at 50 MHz is about 60 ohms. This normally cancels out the antenna capacitive reactance at 50 MHz, which is −60 ohms. Adding the toroid increases the meander line reactance from 60 to 90. This lowers the minimum usable frequency from 50 MHz to 30 MHz since the antenna reactance at 30 MHz is −90 ohms.  
         [0037]    One method for reducing the lowest frequency of operation from 50 MHz to 30 MHz is as follows:  
         [0038]    Measure the reactance of the antenna at 30 MHz. In one case the reactance of the antenna is −90 ohms capacitive.  
         [0039]    Measure the meander line reactance. In the above case, the meander line reactance is measured at +60 ohms inductive at 30 MHz. One therefore needs to add 30 ohms of inductive reactance to the meander line at 30 MHz. One then looks up in tables of toroid design as in the ARRL handbook for the right combination of size and permeability of the toroid. Assuming ½ turn, this is 0.16 microhenries. This yields an additional 30 ohms of inductive reactance. The above is how the dimensions and permeability of the toroid are determined.  
         [0040]    Referring back to FIG. 7, it is noted that the antenna current as illustrated by double-ended arrow  92  goes up as the frequency of source  12  is decreased, thereby in essence activating the ferrite toroidal core. As the frequency at which the antenna is to operate is increased the amount of current in section  24  markedly decreases such that at, for instance, at the 200 MHz end of the band for this antenna there is virtually no current flowing through the toroid and its effect is minimal at best.  
         [0041]    What can be seen is that through the utilization of ferrite in a toroidal form surrounding a meander line element, the element is effectively elongated by in effect placing an inductor in series with the element.  
         [0042]    For a meander line loaded antenna which has multiple meander lines or multiple meander line sections, in order to lower the low frequency cut off of such an antenna, the toroidal donut is placed about that meander line section which corresponds to the longest element of the meander line. There is, however, no reason why the toroidal cannot be placed around any meander line section the length of which is to be extended, thus to provide great flexibility in the design of the meander line loaded antenna. Also, meander lines in general can be effectively lengthened through the use of the toroidal ferrite core in any application that a meander line might be used.  
         [0043]    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.