Low profile tri-filar, single feed, circularly polarized helical antenna

Information

  • Patent Grant
  • 6816127
  • Patent Number
    6,816,127
  • Date Filed
    Tuesday, July 15, 2003
    21 years ago
  • Date Issued
    Tuesday, November 9, 2004
    19 years ago
Abstract
A low-profile, tri-filar, helix antenna having circular polarization (CP) includes a single feed, in the absence of an internal feed network. The antenna includes three metal, bent, quarter-wave monopoles that are physically positioned at 0, 120, and 240 degrees, respectively, on a top flat surface of the antenna. One of the monopoles is directly-fed, and the other two monopoles are parasitically coupled to the directly-fed monopole. Metal perturbations on one or both of the two parasitic monopoles control their coupling-phase to the directly-fed monopole. One of the parasitic monopoles couples at positive 120 degrees to the directly-fed monopole, and the other parasitic monopole couples at negative 120 degrees to the directly-fed monopole. Various perturbation options generate this CP phasing. One of the parasitic monopoles can have a capacitive shunt, and the other parasitic monopole can have a series inductance, or only one parasitic monopole can include a perturbation, either capacitive or inductive, depending on the sense of the CP that is desired. The three monopoles are supported by a dielectric substrate, or they are free-standing. A ground plane is provided directly under the three monopoles.
Description




FIELD OF THE INVENTION




This invention relates to the field of radio communications, and more specifically to spiral or helical antennas for use in wireless communication devices and systems.




BACKGROUND OF THE INVENTION




Small, low profile, circular polarized (CP) antennas are used in the mobile communication industry, usually for satellite communication. As the demand for mobile handsets increases, there is a growing need for antennas of this type, and especially for low cost GPS antennas.




One solution to providing a low profile CP antenna is a patch or microstrip antenna. In order to achieve circular polarization, patch antennas need to be a half wavelength long. A patch antenna's free-space half wavelength is usually too long for the compact space that is provided within a wireless communications device, of which a mobile handset is an example. As a result, the physical size of such a patch antenna must be reduced dramatically, using ceramics having a high dielectric constant. However, the use of ceramics having a high dielectric constant increases antenna cost, and also reduces the efficiency of the patch antenna.





FIG. 1A

shows a standard-technology, dielectric-loaded, ceramic-body, hexagonal patch antenna


10


that is tuned to the global positioning system (GPS) frequency (1575.42 MHz, referred to as L


1


) wherein a high dielectric constant (er=40) ceramic body portion


11


was used to reduce the physical size of patch antenna


10


to less than one inch, which size is usually desirable for mobile wireless communication applications.





FIG. 1B

shows the frequency/magnitude characteristic of antenna


10


, wherein antenna


10


included a ceramic body portion


11


, a top-located metal radiating/receiving surface


12


that lies in the X-Y plane of

FIG. 1B

, an off-center feed conductor


13


that extends in the Z-direction in FIG.


1


B and was connected to a metal-plated top surface


12


, a vertex-to-vertex dimension


14


of about 0.88 inch, and a flat-to-flat dimension


15


of about 0.72 inch.




For wireless communications systems that can tolerate relatively large antennas, the following CP antennas are standard solutions: (1) single helix antennas which have a single feed and are typically a few wavelengths tall, (2) multi-filar helix antennas that have a 90 degree hybrid and are that are typically a few wavelengths tall, (3) crossed dipole antennas that have a 90-degree hybrid and are typically a quarter wavelength tall over a ground plane, or (4) spiral antennas that have a single balanced feed and are typically a quarter wavelength tall over a ground plane.




SUMMARY OF THE INVENTION




This invention provides a small, low-profile, tri-filar helix antenna, which can have either linear polarization or CP, the antenna being provided with a single feed, and the antenna having no internal feed network.




Antennas in accordance with the invention include three metallic, bent, quarter wave monopoles, wherein only one of the monopoles is fed, and wherein the other two monopoles are parasitically coupled to the fed-monopole.




The three bent monopoles of the invention are physically positioned at 0, 120, and 240 degrees, respectively. The three monopoles are self-supporting, or they are supported on a relatively flat dielectric surface. Only one of the three monopoles is fed, for example using an inductive shunt match. The other two monopoles are strongly coupled to, and parasitically feed from, the directly-fed monopole. The two parasitic monopoles are fed at phases that are controlled by the incorporation of, or by the non-incorporation of, metal perturbations within the two parasitic monopoles.




In order to induce linear polarization, no metal perturbations are used within the two parasitic monopoles, and the two parasitic monopoles are coupled at positive 120 degrees to the directly-fed monopole.




In order to induce CP, one of the two parasitic monopoles couples at positive 120 degrees to the directly-fed monopole, and the other parasitic monopole couples at negative 120 degrees to the directly-fed monopole. A metal perturbation on a given parasitic monopole operates to offset the resonant frequency of that parasitic monopole, which in turn affects the phase of coupling of that parasitic monopole to the fed-monopole.




Various metal perturbation options are available in order to generate the phase of this coupling to the directly-fed monopole. One of the parasitic monopoles can have a capacitive shunt, and the other parasitic monopole can have a series inductance, or only one parasitic monopole can have a metal perturbation, either a capacitive perturbation or an inductive perturbation, depending on the sense of the CP that is desired.




The three monopoles in accordance with the invention can be physically supported by a dielectric substrate member, or the three monopoles can be constructed of a material that renders the monopoles free-standing. A metallic ground plane is desirable directly under the three monopoles.




Antennas in accordance with the invention find utility as replacements for a dielectrically-loaded, single feed, CP patch antenna.




Antennas in accordance with the invention do not require dielectric loading. Hence, antennas in accordance with the invention are a less expensive choice for narrow band CP applications.




Antennas in accordance with an embodiment of the invention include three bent quarter wave monopoles, wherein only one of the bent monopoles is fed, and wherein the other two bent monopoles are parasitically coupled to the fed-monopole.




The bent monopoles were, for example, physically positioned at 0, 120, and 240 degrees, respectively. Only one of the bent monopoles was fed, for example with an inductive shunt match. The other two bent monopoles were excited parasitically from the fed-monopole with phases that were controlled by the incorporation of, or by the non-incorporation of, perturbations on or within the two parasitically-fed monopoles.




In antennas constructed and arranged in accordance with the invention the magnitude of the above-described parasitic coupling was relatively large (for example about −6 dB), and this relatively large parasitic coupling between the directly excited monopole and the two parasitic monopoles provided that the antenna generated a symmetric radiation pattern. This relatively large parasitic coupling also effectively acts as a feed network to the two parasitically coupled monopoles, and allows the antenna to have just one of the monopoles directly excited. This relatively large parasitic coupling is, to a large extent, controlled by the width of a capacitive gap that existed between the two parasitic monopoles and the fed-monopole.




In summary, the present invention provides a small, low-profile, single feed, linear polarized or CP, tri-filar, helix antenna having three bent quarter wave monopoles that are physically positioned at about 0, 120, and 240 degrees, respectively. The outer perimeter of the antenna can be a hexagon, or it can be circular, it can approach a circular shape, or it can have a number of sides equal to 6×N where N is an integer that is greater then zero.




Linear antenna polarization is produced when no perturbations are provided for either of the two parasitic monopoles, in which case both of the parasitic monopoles are excited parasitically in-phase at positive 120 degrees.




In order to produce CP, metal perturbations are applied to the two parasitic monopoles in order to generate a positive 120 degree parasitic coupling in one of the parasitic monopoles, and to in order to generate a negative 120 degree parasitic coupling in the other of the two parasitic monopoles.




Various perturbation options can be used to generate the above phasing. For example, one of the two parasitic monopoles can include a capacitive shunt, and the other parasitic monopole can include an inductive shunt. Or, only one of the two parasitic monopoles can be provided with a perturbation, either a capacitive perturbation or an inductive perturbation, depending on the sense of the CP that is desired.




The reactive-capacitance or reactive-inductance perturbations can be provided either by shaping the metal legs of the parasitic monopoles, or by connecting discrete capacitive or inductive electrical components to the parasitic monopoles.




With only one monopole directly fed, the large coupling between this directly-fed monopole and the two parasitic monopoles acts as a feed network to the two parasitic monopoles. It is desirable that all three monopoles be fed with equal RF energy levels in their resonant condition, such that the three monopole antenna will generate three symmetric radiation patterns.




In practice it is desirable that one half of the RF energy that is provided as an input to the directly-fed monopole be coupled to the two parasitic monopoles, and that the other half of this RF energy be radiated into free space.




If coupling from the directly-fed monopole to the two parasitic monopoles is significantly larger than this one-half amount, each of the three monopoles may act as a poor radiator, and the efficiency of the three monopole antenna may be reduced. If the coupling from the directly-fed monopole to the two parasitic monopoles is significantly smaller than this one-half amount it may be difficult to parasitically excite the two parasitic monopoles in order to generate CP.











BRIEF DESCRIPTION OF THE DRAWINGS





FIG. 1A

shows a standard-technology, dielectric-loaded, ceramic-body, hexagon-shaped patch antenna that is tuned to the GPS frequency, wherein a high dielectric constant ceramic body member is used to reduce the physical size of the antenna.





FIG. 1B

shows the frequency/magnitude characteristic of the antenna of FIG.


1


A.





FIG. 2

is a top perspective view of a three-monopole, single-feed, antenna in accordance with the invention, the antenna having no metal perturbations associated with the antenna's three generally identically shaped quarter wave metal monopoles, this figure showing a precursor geometry that pertains to other embodiment of the invention.





FIG. 3

is a top view of three-monopole antenna of

FIG. 2

showing that the antenna's top surface includes three coplanar and quarter wave metal monopole patterns that are physically located at 0-degrees, 120-degrees and 270-degrees, respectively, about the top surface of the antenna.





FIG. 4

is an exploded view that shows the physical positioning and the three-dimensional shape of the three metal patterns that form the three monopoles of the

FIG. 2

antenna.





FIG. 5

shows the magnitude of the parasitic coupling (i.e. about −6 dB) between FIG.


2


's directly-fed monopole and FIG.


2


's two parasitically-fed monopoles, wherein about one half of the power that is feed to the directly-fed monopole is coupled to the two parasitically-fed monopoles.





FIG. 6

shows the phase of the coupling (i.e. about plus 120 degrees) that exists between the three monopoles of FIG.


2


.





FIG. 7

shows the linear polarization pattern of the

FIG. 2

antenna, this figure also showing the antenna's Directivity Pattern (dB) versus Theta at 1580 MHz, and the central axis of the antenna.





FIG. 8

shows the three metal monopoles of a dielectric-supported, single-feed CP antenna in accordance with the invention wherein a metal capacitive perturbation or stub is provided on one of the parasitic monopoles, and wherein a single-feed to the antenna is provided by way of the inductive shunt that forms a portion of a directly-fed monopole.





FIG. 9

shows the three metal monopoles of an antenna in accordance with the invention wherein one of the antenna's two parasitic monopoles includes an inductive metal perturbation that is provided by widening the inductive shunt that is provided at the base of this parasitic monopole.





FIG. 10

shows the Antenna Directivity Pattern (dB) versus Theta at about 1587.5 MHz for an antenna in accordance with the invention, this figure showing the right-hand and the left-hand radiation patterns at the center frequency of about 1587 MHz for a CP antenna wherein the axial ratio is nearly perfect at 0 dB.





FIG. 11

shows a Smith chart for an antenna in accordance with the invention.





FIG. 12

shows the VSWR bandwidth for an antenna in accordance with the invention.





FIG. 13

shows a two-shot-molded tri-arm, single feed, helix antenna in accordance with the invention that is tuned to about 1.575 GHz.





FIG. 14

shows an embodiment of the invention wherein a single-fed, three monopole helix antenna includes three stamped-metal monopole elements that are mounted onto the outer surface of an injection molded plastic and non-conductive support member.





FIG. 15

shows a single-feed, three-monopole helix antenna in accordance with the invention that is made from a printed circuit board that includes three etched areas, each of the three etched areas including a metal monopole element tuned for about 1.575 GHz, wherein the antenna is tuned for CP by using a series inductance (i.e. a wider inductive shunt) in one of the parasitically-fed monopoles.





FIG. 16

shows an antenna in accordance with the invention wherein the antenna's dielectric support is provided by a printed circuit board whose top surface is etched to provide three metal monopole elements, and wherein the printed circuit board includes a through-hole or a via through which a feed conductor is threaded and then connected to the directly-fed monopole.











DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT





FIG. 2

is a top perspective view of a single feed, three-monopole antenna


20


in accordance with the invention.




Antenna


20


includes a plastic or ceramic, electrically non-conductive, rigid, dielectric, and hexagon-shaped support member


21


having a planar top surface


22


, a planar bottom surface that is generally parallel to top surface


22


, and six side walls


23


-


28


that extend downward and generally perpendicular to top surface


22


. As mentioned above, shapes other than hexagonal can be provided within the spirit and scope of the invention.





FIG. 3

is a top view of three-monopole antenna


20


. As seen in

FIGS. 2 and 3

, the top surface


22


of non-conductive support member


21


carries three coplanar, bent, quarter wave and metal monopole patterns


29


-


31


that are physically located at 0-degrees, 120-degrees and 270-degrees, respectively about top surface


22


, as is perhaps best shown in FIG.


3


.





FIG. 4

is an exploded view that shows the three-dimensional shape of the three metal patterns that form the three bent and quarter wave monopole patterns


29


-


31


. In this embodiment of the invention, but without limitation thereto, the three quarter wave monopoles


29


-


31


have a generally identical three-dimensional geometric shape.




As seen in

FIGS. 2 and 4

, the base of each of the three quarter wave monopole metal patterns


29


-


31


includes a metal inductive-shunt portion


32


-


34


, respectively, that respectively lie on the three side walls


24


,


26


and


28


of non-conductive support member


21


. The inductance-value of a shunt portion


32


-


34


is determined by the distance between the antenna's feed and the point of grounding the shunt portion, and by the vertical height of the metal loop that forms the shunt portion.




A capacitive shunt portion is formed by adding metal at a location that is close to the ground, in parallel with the shunt inductance portion, so that RF currents flow across the capacitive portion and the inductance portion.




In this embodiment of the invention, none of the three metal quarter wave monopole patterns


29


-


31


shown in

FIGS. 2-4

includes a metal perturbation, and as shown in

FIGS. 2-4

, a single feed is provided for three-monopole antenna


20


by way of an electrical conductor


35


that electrically connects to the inductive shunt


34


that is a portion of the base of monopole metal pattern


31


. Thus, antenna


20


provides linear polarization, monopole


31


is a fed-monopole and monopoles


29


and


33


are parasitic monopoles that are parasitically fed by fed-monopole


31


.




The physical location at which conductor


35


connects to inductive shunt


34


operates to control the inductance value of the inductive shunt.




With reference to monopole


29


as shown in

FIG. 3

, note that each of the three bent monopoles


29


-


31


includes two linear metal sections


17


and


18


that are connected by a bending metal section


19


.




The angle


16


of inclination of one linear section


17


to the other linear section


18


is not critical, and in this embodiment of the invention angle


16


was about 60 degrees.




As perhaps best seen in

FIG. 3

, the three monopoles


29


-


31


form a helix about the central axis


36


of antenna


20


, wherein axis


36


extends generally perpendicular to the top surface


22


and the bottom surface of support member


21


.




In an embodiment of the invention non-conductive support member


21


had a low dielectric constant of about 3.1, the vertex-to-vertex dimension


37


of antenna


20


was about 0.88 inch, the flat-to-flat dimension


38


was about 0.72 inch, the height


30


was about 0.28 inch, and the coupling between fed-monopole


31


and the two parasitic monopoles


29


and


30


was about −6 dB.




The invention's parasitic coupling between monopole elements is to a relatively large extent controlled by the width of a capacitive-gap that exists between the top of the three monopole elements


29


-


31


, that is by the two generally parallel edges that are formed by the end of one monopole element and the generally middle section of an adjacent monopole element.




The bottom surface of non-conductive support member


21


carries a hexagon-shaped metal ground plane member


40


that cooperates with the metal monopoles


29


-


31


in a well known manner.




As described above relative to

FIGS. 2-4

, the present invention provides a small, low-profile, single feed, tri-filar helix antenna


20


that can be constructed and arranged to provide either linear polarization or CP.




It is now useful to consider a precursor-antenna-geometry that can be used to determine the construction and arrangement of antenna


20


as shown in

FIGS. 2-4

.




As used herein, the term “precursor-antenna-geometry” means an antenna like antenna


20


of

FIGS. 2-4

wherein the precursor-antenna has not as yet been converted to the single feed antenna


20


that is above-described, and wherein the precursor-antenna has not as yet been converted to either linear polarization or CP.




Conversion of the precursor-antenna to a single feed antenna


20


as above-described is accomplished by providing a relatively strong coupling between the three monopoles


29


-


31


of the precursor-antenna, to thereby allow the monopoles of the precursor-antenna to be parasitically excited.




The behavior of the precursor-antenna is determined by feeding each of its three monopoles by way of an individual inductive shunt, i.e. in the precursor-antenna each of the three monopoles is provided with its own individual feed and feed port.




The existence of these three feed ports for the precursor-antenna, and the use of computer simulation, provides a prediction of the coupling that exists in the precursor-antenna between its three monopoles. The final design of a

FIGS. 2-4

single-feed antenna


20


depends upon the magnitude of this coupling.




An input match that is achieved in this manner is shown in

FIG. 5

wherein the magnitude of the coupling between one directly-fed monopole and two parasitically-fed monopoles is about −6 dB at the center frequency of about 1.575 GHz. This input match is controlled by the physical shape of the inductive shunt


32


-


34


that is provided at the base of the three monopoles of the precursor-antenna.




Each monopole of the precursor antenna is a single bent quarter wave monopole, and each monopole has an input impedance that is much lower than the typical 50 ohm signal that is sent to the precursor-antenna. Hence a matching component is necessary.




Energy that is fed to any one of the three monopoles must get past that monopole's feed point before the feed energy can couple to the other two monopoles, or before this feed energy can radiate from that monopole into space. Hence this matching impedance structure of the precursor-antenna is an important portion of the final design of an antenna in accordance with the invention.




The magnitude of this coupling, approximately −6 dB from a given monopole to each of the other two monopoles, is shown in

FIG. 5

wherein about one half of the power that is feed to any given monopole is coupled to the other two monopoles. The magnitude of this coupling is to a large extent controlled by the width of a capacitive gap that exists between the adjacent edges of the three monopoles that reside on the top surface of the non-conductive support member.




In the final design of the single-feed antenna


20


of

FIGS. 2-4

, wherein only monopole


31


is directly fed or excited, it is necessary to induce a relatively large coupling between this directly excited monopole


31


and the two parasitic monopoles


29


and


30


. This relatively large coupling effectively acts as a feed network to the two parasitic monopoles


29


and


30


, thereby allowing the single-feed antenna


20


to have just one of its three monopoles


29


-


31


directly excited. When all three of the monopoles


29


-


31


receive equal amounts of feed energy in the resonance condition, single-feed antenna


20


generates symmetric radiation patterns.




It has been found that a desirable design of a three-feed precursor-antenna provides that about one half of the feed energy that is provided to each of its three monopoles couples to the other two monopoles, whereas and the other half of the feed energy that is provided to each of the three monopoles radiates into free space.




When the coupling that is provided by the design of the three-feed precursor-antenna is larger than this, each monopole tends to be a poor radiator into free space, and the efficiency of the antenna suffers.




When the coupling that is provided by the design of the three-feed precursor-antenna is less than this, it is difficult for a given monopole to parasitically excite the other two monopoles, and it is difficult for the antenna to generate CP.




In the final design of the single-feed antenna


20


of

FIGS. 2-4

, when no metal perturbations are provided on one or more of the two parasitic monopoles


29


and


30


, the phase of the coupling between the three monopoles


29


-


31


is about +120 degrees, as is shown in FIG.


6


. The value of this coupling phase is important in order to induce either linear polarization or CP. Metal perturbations on one or more of the two parasitic monopoles


29


and


30


offsets the resonance frequency of the one or more parasitic monopole


29


/


30


, and also affects the phase of the coupling between the three monopoles.




As described above,

FIGS. 2-4

show a single-feed, tri-filar, linear polarized helix antenna


20


in accordance with the invention wherein antenna


20


exploits a strong monopole-coupling that was determined by way of the above-described investigation of a precursor-antenna wherein each of the three monopoles of the precursor-antenna were fed.




In order to generate linearly polarization, only one of the three monopoles of the

FIGS. 2-4

antenna


20


needs to be directly-fed (i.e. monopole


31


in FIGS.


2


-


4


), the two other +120 degree coupled monopoles (i.e. monopoles


29


and


30


having no perturbations in

FIGS. 2-4

) are coupled to the directly-fed monopole in order to generate linear polarization, and the directly-fed monopole is fed by way of an inductive shunt match (i.e. inductive shunt


34


in

FIGS. 2-4

.




The linear polarization radiation pattern of such a tri-arm, single-feed, no-perturbation, helix antenna


20


is shown in

FIG. 7

, wherein this figure shows the antenna's Directivity Pattern (dB) versus Theta at 1580 MHz, and wherein this figure also shows the central axis


36


of the

FIGS. 2-4

single-feed antenna


20


. The

FIG. 7

radiation patterns are “patch-like”, following the Ludwig 3rd definition, and the antenna's polarization is parallel to axis


36


through the antenna's feed point


34


and the geometric center of antenna


20


.




In order to induce CP, one of the two parasitic monopoles


29


or


30


needs to couple to fed-monopole


31


at positive 120 degrees, and the other parasitic monopole


29


or


30


needs to couple to fed-monopole


31


at negative 120 degrees.




This construction and arrangement in accordance with the invention generates an electric field that rotates uniformly with time around the outer perimeter of antenna


20


. Various metal perturbation options can be used within the spirit and scope of this invention in order to generate this +120-degree/−120-degree phasing of the two parasitic monopoles.





FIG. 8

provides a non-limiting example of a dielectric-supported, single-feed CP helix antenna


20


in accordance with the invention wherein a metal capacitive perturbation or stub


50


is provided on FIG.


4


's parasitic monopole


31


, and wherein the single-feed


35


to antenna


20


is provided by way of the inductive shunt


33


that forms a portion of directly-fed monopole


30


.




In this embodiment of the invention monopole


30


is the directly-fed monopole, whereas the two monopoles


29


and


31


are the two parasitically-fed monopoles.




Capacitive stub


50


lies on the bottom surface of FIG.


4


's non-conductive support member


21


in a manner so as to be electrically insulated from FIG.


4


's ground plane


40


.




Capacitive stub


50


forms a bottom metal portion of monopole


31


that extends inward and parallel to the top metal portion of monopole


31


that lies on the top surface


22


of non-conductive support member


21


.





FIG. 9

shows another embodiment of the invention wherein one of the parasitic monopoles


30


of a dielectric-supported CP antenna


20


of the type shown in

FIGS. 2-4

includes an inductive metal perturbation that is provided by lowering the series inductance of the inductive shunt


33


that is provided at the base of that one parasitic monopole


30


.




This lower of the inductance of the inductive perturbation occurs by virtue of the fact that a wider metal conductor that is positioned where electrical current is a maximum provides less inductance than does a thinner metal conductor.




More specifically, and with reference to

FIG. 9

, monopole


31


is the directly-fed monopole, monopole


29


is a parasitically-fed monopole, and monopole


30


is a parasitically-fed monopole that includes an inductive metal perturbation that is provided by a relatively wide (see dimension


60


) metal inductive shunt


33


.




In other embodiments of the invention, both of the parasitic monopoles may include metal perturbations. For example, one parasitic monopole may include a capacitive stub such as shown in

FIG. 8

, and the other parasitic monopole may include a series inductance as shown in FIG.


9


.




In addition, a relatively small change in the shape of a parasitic monopole, at its base and/or at its top portion, will create a metal perturbation that changes the phase of the coupling to the directly-fed monopole.




In addition, the sense of the CP can be reversed by switching a perturbation from one parasitic monopole to the other parasitic monopole.





FIG. 10

shows an Antenna Directivity Pattern (dB) versus Theta at about 1587.5 MHz for an antenna


20


in accordance with the invention, this figure showing the right-hand and the left-hand radiation patterns at the center frequency of about 1587 MHz for the CP antenna


20


, wherein the axial ratio is nearly perfect at 0 dB.





FIG. 11

shows a Smith chart, and

FIG. 12

shows the voltage-standing-wave-ratio (VSWR) bandwidth for such an antenna


20


in accordance with the invention.




Antennas


20


in accordance with the invention provide better efficiency, as is typical with most antennas, when antenna


20


is wider (see dimensions


37


and


38


of

FIG. 2

) or taller (see dimension


39


of FIG.


2


). Better efficiency is achieved using taller antennas


20


, under the constraint that the above-described strong coupling between the three monopoles is maintained. Providing a taller antenna


20


in accordance with the invention may become impractical after a certain height


39


has been achieved due to the fact that the directly-fed monopole becomes so efficient that it, in itself, radiates most of its energy into space before a portion of this energy can be coupled to the two parasitically-fed monopoles. In this limiting height case, antennas


20


in accordance with the invention may function as a single bent monopole antenna.




Spacing the three bent quarter wave monopoles of an antenna


20


in accordance with this invention farther away from each other may increase the efficiency of the antenna. However, again the above-described monopole coupling must be maintained. That is, the three monopoles must be physically close enough so that significant coupling occurs.




Note that antennas in accordance with this invention, using very little dielectric loading, have the same small physical size as the highly dielectrically loaded patch antenna


10


that is shown in FIG.


1


A. Hence antennas in accordance with this invention can replace dielectrically-loaded, single-feed CP patch antennas of the same physical size, and antennas in accordance with the invention do not require dielectric loading. Thus antennas in accordance with the invention are a less expensive choice, especially for narrow band CP applications.




Various manufacturing methods can be used to produce single-fed, tri-filar helix antennas in accordance with this invention.




For example, and with reference to

FIGS. 2-4

, antenna


20


can be made using a two-shot molding process wherein a first-shot of a polymer material is used to form the major portion of non-conductive support


21


, and wherein a second-shot of a different polymer material is used to form those portions of antenna


20


that correspond to the metal portions of antenna


20


. These portions of the second polymer material are then treated in a well known manner to facilitate the deposition or plating of metal onto these second polymer portions.




Other manufacturing techniques include a two-shot molding process wherein metal monopole elements are placed on the top and on the bottom of a molded polymer member in order to create a low-dielectrically loaded antenna; insert molding of an antenna having the above described metal portions; a hybrid antenna that includes an etched printed circuit board (PCB) and stamped metal portions; a completely PCB antenna; and an antenna that includes free-standing metal portions.





FIG. 13

shows a two-shot-molded tri-arm helix antenna


65


in accordance with the invention that is tuned to about 1.575 GHz.




In a non-limiting embodiment of the invention antenna


65


had a flat-to-flat dimension


73


of about 0.88 inch, a vertex-to-vertex dimension


74


of about 1.01 inch, and a height dimension


75


of about 0.30 inch.




Antenna


65


includes three not-plated plastic portions


69


-


72


and three metal-plated plastic portions


66


-


68


. Not-plated plastic portions


69


-


72


provide the mechanical support for antenna


65


. Metal plated portions


66


-


68


comprise three metal antenna monopoles as above described, one monopole of which is directly-fed, and the other two of which are parasitically-fed monopoles. Plated metal portions


66


-


68


are nearly fully covered by a metal in order to reduce dielectric loss and in order to reduce dielectric loading, which would reduce the bandwidth of antenna


65


.




One advantage of the

FIG. 13

embodiment of the invention is that “lossy” plastics can be used without reducing the efficiency of antenna


65


. Another advantage is that two-shot molding achieves tight mechanical tolerances for the construction and arrangement of antenna


65


.





FIG. 14

shows an embodiment of the invention wherein a single-feed, three monopole helix antenna


80


includes three stamped-metal monopole elements


81


-


83


that are mounted onto an injection-molded plastic and non-conductive support member


84


. Tri-monopole helix antenna


80


is tuned to about 1.575 GHz. In accordance with the above description, one of the three metal monopoles is directly-fed, and the other two of the metal monopoles are parasitically-fed from the directly-fed monopole.




An advantage of antenna


80


is low cost in that after plastic molding, metal-stamping and metal-bending tools are made, antenna


80


can be manufactured from a sheet metal and a plastic material that are relatively inexpensive.





FIG. 15

shows a single-feed, three-monopole helix antenna


88


that is made from a PCB


86


that includes three etch dielectric areas


87


-


89


areas. Each of the three etched dielectric areas


87


-


89


includes a stamped-metal monopole element


90


-


92


that is tuned for about 1.575 GHz. As before, one of the three monopoles


90


-


92


is directly-fed, and the other two monopoles are parasitically-fed from the directly-fed monopole. Antenna


88


is tuned for CP by using a series inductance (i.e. a wider inductive shunt) at the base of the directly-fed monopole.




In a non-limiting embodiment of antenna


85


, the flat-to-flat dimension


93


was about 0.76 inch, the vertex-to-vertex dimension


94


was about 0.27 inch, and the height dimension


95


was about 0.27 inch.




An advantage of FIG.


15


's antenna


88


is that mechanical support is provided by thin PCB


86


, and as a result the material-cost of antenna


88


is minimized. Another advantage to the

FIG. 15

antenna is that etching provides tight tolerances on the top surface of the antenna.





FIG. 16

shows an antenna


96


in accordance with the invention wherein the antenna's dielectric support function is provided by a PCB


97


whose top surface


98


is etched to provide three metal quarter wave monopole elements


99


-


101


. PCB


97


includes a through-hole or via


102


through which a feed conductor


103


is threaded and then connected to monopole


101


at location


104


. Thus, monopole


101


is the directly-fed monopole, and monopoles


99


and


101


are parasitically-fed from directly-fed monopole


101


.




Antenna


96


can be tuned for CP by providing a discrete reactive electrical element in series with feed conductor


103


.




Antenna


96


provides an advantage in that little or no capital cost or specialized tooling is required. As a result, the tuning of antenna


96


can be integrated into each individual antenna platform.




In summary, it can be seen that the present invention provides a small, low-profile, tri-filar, single-feed, helix antenna that includes three bent quarter wave metal monopoles. The three bent-monopoles are positioned at about 0 degrees, 120 degrees, and 240 degrees about the top surface of the antenna. The perimeter of the antenna that supports the three bent-monopoles can be a hexagon, or it can be another shape such as a circle or a shape that approaches a circle. Only one of the three quarter wave monopoles is fed, for example, with an inductive shunt match, and the other two monopoles are excited parasitically from the fed-monopole. Linear polarization is produced when no metal perturbation is applied to the two parasitic monopoles, such that both parasitic monopoles are parasitically excited in phase at positive 120 degrees. In order to produce CP, metal perturbations are applied to at least one of the two parasitic monopoles in order to generate positive 120 degrees coupling in one of the two parasitic monopoles and negative 120 degrees in the other of the two parasitic monopole. Various metal perturbation options are available in order to generate this phasing: For example, one of the parasitic monopoles can have a capacitive shunt, and the other parasitic monopole can have an inductive shunt. Or, only one parasitic monopole need be provided with a metal perturbation, either capacitive or inductive, depending on the sense of the CP that is desired. These electrically reactive metal perturbations can be provided by either shaping the metal legs or metal base of the parasitic monopoles, or by the electrical connection of discrete electrically reactive components in series with the parasitic monopoles.




In order to induce CP operation, the resonant frequency of one or both of the above-described parasitic monopole elements is shifted such that one of the parasitic elements couples at positive 120 degrees to the directly-fed monopole element, and such that the other of the parasitic elements couples at negative 120 degrees to the directly-fed monopole element.




In order to induce this CP operation, a perturbation can be provided on one or both of the above-described parasitic monopoles, and various perturbations options can be used to generate the phasing that is required for CP. In

FIG. 8

a perturbation


50


is provided as an extended portion of parasitic monopole


31


. In

FIG. 9

parasitic monopole


30


is provided with a perturbation in the form of a wide shunt


33


(see width dimension


60


).




More generally, relatively small changes in the geometric shape of a parasitic monopole can create a perturbation that controls the phasing of the parasitic monopole relative to the directly-fed monopole. Non-limiting geometric-shape examples are increasing a monopole-length as in

FIG. 8

, and/or increasing a monopole-width as in

FIG. 9

, both of which change the phase of the coupling of a parasitic monopole to the directly-fed monopole.




The perturbation options or means available to generate the phasing that is required for CP operation including, but are not limited to, increasing or decreasing a length or width of a parasitic monopole, and/or providing one of the parasitic monopoles with a capacitive perturbation shunt as the other parasitic monopole is provided with an inductance perturbation.




Only one parasitic element need have a perturbation, either capacitive or inductive, depending upon the sense of the CP that is desired.

FIG. 8

shows the use of a capacitive perturbation


50


that extends inward from the base of parasitic monopole element


31


.

FIG. 9

shows an inductive perturbation that is formed by a wide portion


60


at the base of parasitic monopole element


30


, it being noted that wide metal conductors that are positioned where current is at a or near maximum have less inductance than thinner metal conductors.




Thus, CP can be induced by perturbations that are associated with one or more of the two parasitic monopoles, including, but not limited to, in

FIG. 8

providing a metal perturbation


50


at the end of parasitic monopole


31


, in

FIG. 9

providing a metal perturbation at the end of parasitic monopole


30


by providing of a relatively wide metal shunt


33


, and more generally by changing the geometric shape of a parasitic monopole at any position along the length of the parasitic monopole, such that the phase of the parasitic monopole's coupling to the directly fed monopole results in CP operation.




While the present invention has been described with respect to certain preferred embodiments of the invention, modifications and variations may be employed without departing from the spirit and scope of the present invention as set forth in the following claims.



Claims
  • 1. A method of making a single-feed, three-monopole, circularly polarized, helix antenna, comprising the steps of:providing a first, a second and a third monopole element, each of said monopole elements having a first and a second end portion; physically positioning said first, second and third monopole elements at about 0-degrees, 120-degrees, and 240-degrees, respectively, around a central axis such that said first end portions of said first, second and third monopole elements are located physically adjacent to said central axis, and such that said first, second and third monopole elements are relatively strongly coupled; connecting an antenna-feed to said first monopole element such that said first monopole element is a directly-fed monopole element, and such that said second and a third monopole elements are parasitically-fed from said first monopole element; providing perturbation-means on at least one of said second and third monopole elements; and controlling said perturbation-means on said at least one of said second and third monopoles in a manner to produce a plus 120 degree coupling of said second monopole element to said first monopole element, and in a manner to produce a minus 120 degree coupling of said third monopole element to said first monopole element.
  • 2. The antenna of claim 1 including the step of:controlling a geometric shape of said perturbation-means.
  • 3. The antenna of claim 1 including the step of:providing said first, second and third monopole elements as quarter wave monopole elements.
  • 4. The antenna of claim 1 including the step of:providing that said relatively strong coupling between said first, second and third monopole elements results in about one-half of feed-energy applied to said antenna feed being radiated from said first monopole element into space, as a remaining portion of said feed-energy is parasitically coupled to said second and third monopole elements.
  • 5. The method of claim 1 wherein said relatively strong coupling has a magnitude of about −6 dB.
  • 6. The method of claim 1 including the step of:locating said perturbation-means on at least one of said second and third monopole elements generally at said second end of said at least one of said second and third monopole elements.
  • 7. The method of claim 1 including the step of:physically positioning said first, second and third monopole elements in a generally common plane.
  • 8. A method of making a circularly polarized antenna, comprising the steps of:providing a first, a second and a third quarter-wave monopole element, each of said monopole elements having a first end portion and a second end portion; physically positioning said first, second and third monopole elements at about 0-degrees, 120-degrees, and 240-degrees, respectively, in a common plane and around a central axis that extends generally perpendicular to said common plane such that said first end portions of said first, second and third monopole elements are located physically adjacent to each other and to said central axis, and such that said first, second and third monopole elements are relatively strongly coupled; connecting an antenna-feed to said first monopole element such that said first monopole element is a directly-fed monopole element, and such that said second and a third monopole elements are parasitically-fed from said first monopole element; providing a perturbation generally at said second end of at least one of said second and third monopole elements; and controlling a geometric shape of said metal perturbation on said at least one of said second and third monopoles in a manner to produce a plus 120 degree parasitic coupling of said second monopole element to said first monopole element, and in a manner to produce a minus 120 degree parasitic coupling of said third monopole element to said first monopole element.
  • 9. The antenna of claim 8 including the step of:providing that said relatively strong coupling between said first, second and third monopole elements results in about one-half of feed-energy applied to said antenna feed being radiated from said first monopole element, as a remaining portion of said feed-energy is parasitically coupled to said second and third monopole elements.
  • 10. The method of claim 9 wherein said relatively strong coupling has a magnitude of about −6 dB.
  • 11. In a helical antenna having three monopole elements, a first of which is directly-fed, and a second and third of which are parasitically coupled to said directly fed monopole element, an improvement comprising:means for shifting a resonant frequency of at lease one of said second and third monopole elements in a manner such that one of said second and third monopole elements couples to said directly-fed monopole element at positive 120 degree and such that another of said second and third monopole elements couples to said directly-fed monopole element at negative 120 degrees.
  • 12. The improvement of claim 11 wherein said means for shifting a resonant frequency of at least one of said second and third monopole elements comprises:a capacitive perturbation associated with said one of said second and third monopole elements and/or an inductive perturbation associated with said another of said second and third monopole elements.
CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. non-provisional patent application Ser. No. 10/314,685, filed on Dec. 9, 2002 now U.S. Pat. 6,738,026 and entitled LOW PROFILE TRI-FILAR, SINGLE FEED, HELICAL ANTENNA.

US Referenced Citations (3)
Number Name Date Kind
6639559 Okabe et al. Oct 2003 B2
6697025 Koyanagi et al. Feb 2004 B2
6759990 Rossman Jul 2004 B2
Continuation in Parts (1)
Number Date Country
Parent 10/314685 Dec 2002 US
Child 10/604371 US