Broadband planar dipole antenna structure and associated methods

An antenna structure includes an omnidirectional broadband planar dipole antenna including a first electrically conductive disc defining a first dipole antenna element, and a second electrically conductive disc defining a second dipole antenna element in parallel with and spaced apart from the first dipole antenna element. A dual-line antenna feed structure may be connected to the planar dipole antenna and includes a first conductor electrically connected to the first conductive disc adjacent a peripheral edge thereof, and a second conductor electrically connected to the second conductive disc adjacent a peripheral edge thereof. The planar antenna may provide vertical polarization transmission and reception, and it may not require a ground plane. The antenna may use printed circuit construction like microstrip patch antennas. Operation may provide a single band antenna of broad bandwidth, a multiple band antenna, or any combination thereof. Responses may include single, multiple, or the Chebyshev polynomial tuning.

FIELD OF THE INVENTION

The present invention relates to the field of antennas, and more particularly, this invention relates to omnidirectional radiation, planar broadband antennas, microstrip patch antennas, horizontal antennas and vertical polarization, multiple tuning, and related methods.

BACKGROUND OF THE INVENTION

Modern communications systems are increasing in bandwidth requirements, causing greater needs for broadband antennas. Nature, in the present physics may impose fundamental limitations on instantaneous gain bandwidth relative to antenna size and shape. The thin ½ wave wire dipole antenna can have 3 dB gain bandwidth of 13 percent and 2.0 to 1 VSWR bandwidth of only 4.5 percent. This is often not adequate. Broadband dipoles are an alternative to the wire dipole. These preferably utilize cone radiating elements which are better fitted to wave expansion, rather than thin wires. A biconical dipole having, for example, a conical flare angle of ½π radians has essentially a high pass filter response, from a lower cut off frequency. Such an antenna provides higher bandwidth, and a response of 10 or more octaves may be achieved.

In current, everyday communications devices, many different types of conical antennas, such as biconical dipoles, conical monopoles and discone antennas are used in a variety of different ways. These antennas, however, are sometimes expensive or difficult to manufacture and flat planar antennas may be preferable. Antenna shapes may be classified as linear, planar or 3 dimensional.

Many applications, such as land mobile, may require thin planar antennas with vertical polarization when mounted in a horizontal plane. Such antennas can be planar monopoles, sometimes known as microstrip “patch” antennas. The advantages of these antennas including printed circuit manufacture, being mountable in low profile, and having high gain and efficiency have made them the antennas of choice in many applications. However, microstrip patch antennas typically are efficient only in a narrow frequency band. They are poorly shaped for wave expansion, such that microstrip antenna bandwidth is proportional to antenna thickness. Bandwidth can even approach zero with vanishing thickness (for example, see Munson, page 7-8 “Antenna Engineering Handbook”, 2nd ed., H. Jasik ed.).

Simple antennas can provide quadratic “single dip” frequency responses, akin to resonant circuits. For instance, a center fed ½ wave wire dipole has an impedance response similar to a series resonant circuit plus a resistor. Multiple tuning has been described as a way to increase instantaneous gain bandwidth from small, simple antennas. In multiple tuning, an antenna may exhibit a rippled frequency response of many “dips” and “peaks”, corresponding to staggered resonances in frequency. Wheeler has shown that multiple tuned antennas can provide up to 3π the bandwidth of single tuned antennas: H. Wheeler, “The Wide-Band Matching Area for a Small Antenna”, IEEE Trans. Antennas and Propagation, Vol. AP-31, No. 2 March 1983.

External impedance compensation networks, e.g. of the inductor capacitor (LC ladder) type, may be used to increase bandwidth by multiple tuning single tuned narrowband antennas. The LC network may connect at the antenna driving points between the antenna and the feedline, and the antenna becomes the final resonant section and a load, to a cascade of resonant filter sections. It may be preferable however to obtain the multiple tuned broadband responses directly from the antenna structure, without external compensation networks, for ease of manufacture, power handling and efficiency.

Filter theory may be applied to antenna responses, and multiple tuned frequency responses tailored to polynomials. For example, a Butterworth polynomial may be used for minimal ripple or a Chebyshev polynomial for maximum bandwidth to a controlled ripple.

The bent stacked slot antenna (BSSA) achieves a relatively wide bandwidth and small size and makes use of a center strip of a middle patch as an integrated impedance matching unit. An example of such an antenna is described in the European published patent application EP 795926. However, a disadvantage with the BSSA type of antenna is the relatively narrow bandwidth.

U.S. Pat. No. 5,003,318, to Berneking et al. entitled “Dual Frequency Microstrip Patch Antenna With Capacitively Coupled Feed Pins” describes a planar ground plane antenna with two coaxial feeds or ports. Two separate antennas are collocated in space, each single tuned.

U.S. Pat. No. 6,501,427 to Lilly et al. entitled “Tunable Patch Antenna” is directed to a patch antenna including a segmented patch and reed like MEMS switches on a substrate. Segments of the structure can be switched to reconfigure the antenna, providing a broad tunable bandwidth. Instantaneous bandwidth may be unaffected however.

U.S. Pat. No. 7,126,538 to Sampo entitled “Microstrip antenna” is directed to a microstrip antenna with a dielectric member disposed on a grounded conductive plate. A patch antenna element is disposed on the dielectric member.

U.S. Pat. No. 7,109,926 to du Toit entitled “Stacked patch antenna” discloses a stacked antenna, including a lower patch which may include a coplanar microstrip capable of feeding the stacked antenna and an upper patch which may include a slot-like part thereon and coupled to the upper patch. This antenna also requires a ground plane.

There is a need for a relatively thin or horizontally planar antenna that has a wider instantaneous bandwidth, is more omnidirectional, is for vertical polarization transmission and reception and/or does not require a ground plane.

SUMMARY OF THE INVENTION

In view of the foregoing background, it is therefore an object of the present invention to provide a planar antenna that has a wider bandwidth, is more omnidirectional, is for vertical polarization transmission and reception when horizontally disposed and/or does not require a ground plane.

This and other objects, features, and advantages in accordance with an example of an embodiment of the present invention are provided by an antenna structure including a planar dipole antenna comprising a first conductive disc defining a first dipole antenna element, and a second conductive disc defining a second dipole antenna element in parallel with and spaced apart from the first dipole antenna element. A dual-line antenna feed structure may be connected to the planar dipole antenna and includes a first conductor electrically connected to the first conductive disc adjacent a peripheral edge thereof, and a second conductor electrically connected to the second conductive disc adjacent a peripheral edge thereof. The antenna structure advantageously provides omnidirectional broadband operation. The antenna may also have vertical symmetry, provide balanced dipole operation, and may eliminate the need for a ground plane.

A conductive rod may be connected to and extend between the first and second conductive discs along central axes thereof. The first and second conductive discs may have substantially the same diameter. Also, the first dipole antenna element may also include a third conductive disc in parallel with the first conductive disc, and adjacent to and having a smaller diameter than the first conductive disc. And, the second dipole antenna element may include a fourth conductive disc in parallel with the second conductive disc, and adjacent to and having a smaller diameter than the second conductive disc. The first dipole antenna element may further include a fifth conductive disc in parallel with the third conductive disc, and adjacent to and having a smaller diameter than the third conductive disc. The second dipole antenna element may further include a sixth conductive disc in parallel with the fourth conductive disc, and adjacent to and having a smaller diameter than the fourth conductive disc.

A dielectric block may contain the dual-line antenna feed structure and the planar dipole antenna. The dielectric block preferably includes a plurality of dielectric layers, and each of the first and second conductive discs may comprise a plated conductive layer on a respective dielectric layer. Also, each of the first and second conductors of the dual-line antenna feed structure may comprise a plated conductive via through at least one dielectric layer.

Another aspect is directed to a method of making the antenna structure. The method may include providing a planar dipole antenna comprising a first conductive disc defining a first dipole antenna element, and a second conductive disc defining a second dipole antenna element in parallel with and spaced apart from the first dipole antenna element. The method may also include connecting a dual-line antenna feed structure to the planar dipole antenna and comprising electrically connecting a first conductor to the first conductive disc adjacent a peripheral edge thereof, and electrically connecting a second conductor to the second conductive disc adjacent a peripheral edge thereof.

The first and second conductive discs may have substantially the same diameter. Providing the planar dipole antenna may include providing the first dipole antenna element with a third conductive disc in parallel with the first conductive disc, and adjacent to and having a smaller diameter than the first conductive disc. The second dipole antenna element may be provided with a fourth conductive disc in parallel with the second conductive disc, and adjacent to and having a smaller diameter than the second conductive disc. Providing the planar dipole antenna may further include providing the first dipole antenna element with a fifth conductive disc in parallel with the third conductive disc, and adjacent to and having a smaller diameter than the third conductive disc, and providing the second dipole antenna element with a sixth conductive disc in parallel with the fourth conductive disc, and adjacent to and having a smaller diameter than the fourth conductive disc.

The method may also include providing a dielectric block containing the dual-line antenna feed structure and the planar dipole antenna. Furthermore, providing the dielectric block may include providing a plurality of dielectric layers. Providing the planar dipole antenna may comprise forming each of the first and second conductive discs as a plated conductive layer on a respective dielectric layer. Connecting the dual line antenna feed structure may comprise forming each of the first and second conductors as a plated conductive via through at least one dielectric layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring initially toFIGS. 1 and 2, a planar dipole antenna structure10will be described that has a wider bandwidth, is omnidirectional, is for vertical polarization transmission and reception and/or does not require a ground plane. The broadband planar dipole antenna12includes a first conductive disc14defining a first dipole antenna element20(i.e. one-half of a dipole), and a second conductive disc16defining a second dipole antenna element22(i.e. the other half of the dipole) in parallel with and spaced apart from the first dipole antenna element20. The first and second conductive discs14,16may have substantially the same diameter. Also, the discs are generally or substantially circular in the illustrated embodiments, but they also could be generally oval, elliptical or even rectangular with some changes in expected performance.

A dual-line antenna feed structure18may be connected to the planar dipole antenna12and includes a first conductor24electrically connected to the first conductive disc14adjacent a peripheral edge thereof, and a second conductor26electrically connected to the second conductive disc16adjacent a peripheral edge thereof. For example, the dual-line antenna feed structure18may be a coaxial cable having an inner conductor and an outer conductor in surrounding relation thereto. The second conductor26may also be connected, e.g. soldered, to extend along a radius of the second conductive disc16. The end point of the coax cable, and the attachment point of the center conductor define two antenna driving points6,8, which together form an antenna port4, as is common.

A conductive rod28may be connected to and extend between the first and second conductive discs14,16along central axes thereof. The rod28acts as a shunt to cause a low resistance, e.g. about 50 ohm, driving point resistance for the planar dipole antenna12.

Also, the first dipole antenna element20may also include a third conductive disc32in parallel with the first conductive disc14, and adjacent to and having a smaller diameter than the first conductive disc. And, the second dipole antenna element22may include a fourth conductive disc34in parallel with the second conductive disc16, and adjacent to and having a smaller diameter than the second conductive disc. The first dipole antenna element20may further include a fifth conductive disc36in parallel with the third conductive disc32, and adjacent to and having a smaller diameter than the third conductive disc. The second dipole antenna element22may further include a sixth conductive disc38in parallel with the fourth conductive disc34, and adjacent to and having a smaller diameter than the fourth conductive disc.

Now referring toFIG. 3, another example of a planar dipole antenna structure40that is broadband, omnidirectional and vertically polarized will be described. The broadband planar dipole antenna42includes a first conductive disc44defining a first dipole antenna element50, and a second conductive disc46defining a second dipole antenna element52in parallel with and spaced apart from the first dipole antenna element50. The first and second conductive discs44,46may have substantially the same diameter.

A dual-line antenna feed structure58may be connected to the planar dipole antenna42and includes a first conductor64electrically connected to the first conductive disc44adjacent a peripheral edge thereof, and a second conductor66electrically connected to the second conductive disc46adjacent a peripheral edge thereof. For example, again, the dual-line antenna feed structure58may be a coaxial cable having an inner conductor and an outer conductor in surrounding relation thereto. The second conductor66may also be connected, e.g. soldered, to extend along a radius of the second conductive disc66.

A conductive rod68may be connected to and extend between the first and second conductive discs44,46along central axes thereof. The rod68acts to cause a low driving point resistance, e.g. about 50 ohms, shunt feed for the planar dipole antenna42.

One method of construction for the planar dipole antenna structure10can be to thread center rod28, and to screw threaded center rod28into threaded holes in conductive discs14,32,36,16,34,38. By threading center rod28, nuts and spacer washers or jamb nut techniques may also be used, as are common. The invention is not however limited as to only threaded or nut and bolt construction, and welding or even metal plated plastic construction are feasible.

A theory of operation for a 2 disc version of the present invention will now be described. Broadband planar dipole antenna12radiates by separation of charge between conductive discs14,16. The distal, outer surfaces of conductive discs14,16function as the dipole half elements. The rims of conductive discs14,16may also be considered as slot dipole half elements from which diffractive type radiation occurs due to separation of charge. In general there is no circular motion of charge or loop radiation as the discs convey radial currents only, and separation of charge is greatest between the conductive disc rims. When the overall height of dipole antenna structure10is thin, fundamental or first resonance occurs for disc diameters of about ¼ wavelength.

Continuing the 2 disc theory of operation, the proximal, interior faces of conductive discs14,16function as transmission line conductors to each other, that are radial, balanced, and microstrip. Conductive rod28acts as a short circuit between the conductive discs14,16, and impedance matching transmission line stub is formed in place by the combination of conductive rod28(the short) and conductive discs14,26(the transmission line conductors). This in situ transmission line refers the antenna driving point resistance to a lower value, i.e. a shunt feeding approach. In general, closer spacing between conductive rod29and the driving points6,8provide a lower driving resistances and wider spacings provide higher resistances. Resistance is preferably adjusted to provide 50 ohms in common practice, for coaxial cable.

Although the driving points6,8are not at the geometric center of planar dipole antenna structure10, the realized azimuth (H field plane) radiation patterns are circular or nearly so. Conductive rod28need not be located at the geometric center of conductive discs14,16, and it may be in fact be located at the disc rims, and the driving points6,8located to the disc center or elsewhere. A method is in fact provided in which the driving points6,8and conductive rod28may be located as desired for impedance matching. In general, the closer the discs are together, the further apart driving points6,8will need be from the shorting post/conductive rod28to obtain 50 ohms resistance at resonance.

A non-limiting example of the planar 2 disc dipole antenna42is outlined by the following table:

Two Disc Example Of The InventionParameterSpecificationAntenna TypePlanar Dipole/SlotDipoleOverall Thickness Of Antenna0.049λDimension A: Brass Disc Diameter0.32λ eachDimension B: Brass Center Post0.038λDiameterDimension D: Brass Disc Spacing,0.042λBetween Proximal FacesDimensions C, E: Disc Thickness0.0038λ eachAspect Ratio (Diameter To Height)6.5 to 1Feedpoint LocationDriven Between Rims OfDiscsFeed Point Impedance50 ΩVSWR1.2 to 12 to 1 VSWR Bandwidth7.0 Percent3 dB Gain Bandwidth21.9 PercentAzimuth Radiation PatternCircular/OmnidirectionalElevation Radiation PatternSin2θ Two Petal Rose(Like ½ Wave Dipole)Lobes In Disc Planes,Nulls Normal To DiscsGain+2 dBi with balunResponse ShapeApproximatelyQuadratic
As can be appreciated by those in the art, the 2 to 1 VSWR bandwidth of the two disc example of the present invention exceeds that of a thin diameter wire half wave dipole by about 55 percent and in about 1/10 the height. In addition, an exact 50 ohm driving point resistance is obtainable.

Referring toFIG. 1, a non-limiting example of a 4 disc embodiment of planar dipole antenna42is outlined in the following table.

A theory of operation for a 4 or more disc planar dipole antenna structure10will now be described. Radiation, as in 2 disc versions, is by separation of charge/dipole moment. The interior surfaces of conductive discs14,32,36,16,34,38function as transmission lines and the exterior surfaces as dipole radiating elements, as the spaces between discs are evanescent to waves. The diameter of the conductive discs14,32,36,16,34,38set the frequencies of the response poles (VSWR dips, gain peaks) and the spacing between the conductive discs14,32,36,16,34,38sets the depth of the passband ripple. When antenna structure10is thin overall, discs are about ¼ wavelength in diameter at their respective operating frequencies.

Now referring toFIGS. 4 and 5, an example of a multilayer printed circuit embodiment of a planar dipole antenna structure70that is broadband, omnidirectional and vertically polarized will be described. The antenna structure70includes a dielectric block80having a plurality of dielectric layers82, and a planar dipole antenna72. A first conductive disc74is plated on a first dielectric layer of the plurality of dielectric layers82and defines a first dipole antenna element92. A second conductive disc76is plated on a second dielectric layer of the plurality of dielectric layers82and defines a second dipole antenna element94in parallel with and spaced apart from the first dipole antenna element92. The first and second conductive discs74,76may have substantially the same diameter.

A dual-line antenna feed structure96may be connected to the planar dipole antenna72and includes a first conductive via97through the dielectric layers82and electrically connected to the first conductive disc74adjacent a peripheral edge thereof, and a second conductive via98through the dielectric layers82and electrically connected to the second conductive disc76adjacent a peripheral edge thereof.

The first dipole antenna element92may also include a third conductive disc100plated on a third dielectric layer of the plurality of dielectric layers82and in parallel with the first conductive disc74, and adjacent to and having a smaller diameter than the first conductive disc. The second dipole antenna element94may include a fourth conductive disc102plated on a fourth dielectric layer of the plurality of dielectric layers82and in parallel with the second conductive disc76, and adjacent to and having a smaller diameter than the second conductive disc.

The first dipole antenna element92may further include a fifth conductive disc104plated on a fifth dielectric layer of the plurality of dielectric layers82and in parallel with the third conductive disc100, and adjacent to and having a smaller diameter than the third conductive disc. The second dipole antenna element94may further include a sixth conductive disc106plated on a sixth dielectric layer of the plurality of dielectric layers82and in parallel with the fourth conductive disc102, and adjacent to and having a smaller diameter than the fourth conductive disc.

TheFIG. 5printed circuit embodiment of the present invention provides broad bandwidth by staggering the diameters and resonances of the conductive discs76,74,82,100,106,104. Dielectric layers82provide dielectric loading to the printed circuit embodiment and a reduction in size. Dielectric layers82may include dielectric materials only, magnetic materials only, or they may include magnetodielectic materials having both magnetic and dielectric properties.

Radiation patterns of the present invention will now be considered.FIG. 6is a series of graphs illustrating theoretical radiation patterns relating to the planar dipole antenna ofFIG. 1, based on electrically small antenna theory. The patterns are those of the elemental dipole, which can be similar to realized radiation patterns of present invention.FIG. 7is a graph illustrating a measured XY azimuth plane radiation pattern of a prototype of the planar dipole antenna ofFIG. 1. Units are in decibels with respect to isotropic (dBi) and the source antenna was linearly polarized.FIG. 8is a graph illustrating a measured YZ elevation plane radiation pattern of a prototype of the planar dipole antenna ofFIG. 1. Again, units are in decibels with respect to isotropic (dBi) and the source antenna was linearly polarized. TheFIG. 8pattern is an E plane cut and theFIG. 7pattern an H plane cut. Measured radiation patterns include ripple contribution from coaxial cable common mode currents, as a balun was not used for sake of economy. Cross polarized radiation was generally low.

FIG. 9is the VSWR response of an especially thin 4 disc embodiment of the present invention, illustrating a double tuned response. This example provided a VSWR ripple level of 2 to 1 and the overall thickness was λ/41.

Biconical dipoles, such as those described in U.S. Pat. No. 3,618,017, to Carter and entitled “Shortwave Antenna” may typically require height dimensions of about ⅓ wavelength. As can be appreciated by those in the art, the present invention operates at dimensions well below the lower cutoff frequency of similar height biconical dipoles. In fact, the present invention may be made arbitrarily thin, with tradeoffs in instantaneous gain bandwidth.

A method of making an antenna structure10,40,70includes providing a planar dipole antenna12,42,72comprising a first conductive disc14,44,74defining a first dipole antenna element20,50,92, and a second conductive disc16,46,76defining a second dipole antenna element22,52,94in parallel with and spaced apart from the first dipole antenna element. The method may include connecting a dual-line antenna feed structure18,58,96to the planar dipole antenna and comprising electrically connecting a first conductor24,64,97to the first conductive disc adjacent a peripheral edge thereof, and electrically connecting a second conductor26,66,97to the second conductive disc adjacent a peripheral edge thereof.

Providing the planar dipole antenna12,42,72may include providing the first dipole antenna element with a third conductive disc32,100in parallel with the first conductive disc, and adjacent to and having a smaller diameter than the first conductive disc. The second dipole antenna element may be provided with a fourth conductive disc34,102in parallel with the second conductive disc, and adjacent to and having a smaller diameter than the second conductive disc. Providing the planar dipole antenna may further include providing the first dipole antenna element with a fifth conductive disc36,104in parallel with the third conductive disc, and adjacent to and having a smaller diameter than the third conductive disc, and providing the second dipole antenna element with a sixth conductive disc38,106in parallel with the fourth conductive disc, and adjacent to and having a smaller diameter than the fourth conductive disc.

The method may also include providing a dielectric block80containing the dual-line antenna feed structure96and the planar dipole antenna72. Furthermore, providing the dielectric block80may include providing a plurality of dielectric layers82. Providing the planar dipole antenna may comprise forming each of the conductive discs74,76,100,102,104,106as a plated conductive layer on a respective dielectric layer82. Connecting the dual line antenna feed structure may comprise forming each of the first and second conductors97,98as a plated conductive via through at least one dielectric layer82.

Although this invention is primarily directed towards providing a single continuous frequency band antenna with broad bandwidth and a controlled passband ripple, the present invention also includes a method, wherein conductive discs may be configured to provide a single wideband operating band, a multiplicity of separate operating bands widely spaced, or any combination. For instance the passband gain peaks (which correspond to VSWR ripple valleys) may be specified as individual discrete operating bands. Conductive discs14,32,36and16,34,38may be tuned widely apart by adjustment of dimensions, to form widely separated discrete operating bands. The discrete operating bands may be dissonant, harmonically related or both.

As can be appreciated by those in the art, a rippled passband antenna can be a single or multiple band antenna simply by choice of operating specification.

It has been found in practice that the large diameter discs may also be located at the outside, and the smaller ones at the inside with useful results. It is also possible to tie the disc edges together with a hookup wire across the driving points to vary coupling and passband ripple.

The present invention is not so limited as to require symmetry between discs in the upper and lower half spaces, and it can be modified to form an image equivalent or ground plane type antenna for operation on conductive surfaces, such as say a large metal roof. To accomplish this, referring now toFIG.1, conductive discs16and34and38would be omitted, and conductive rod28attaches onto the ground plane with the ground plane then replacing the lower half of the antenna structure. As the driving point resistance is halved by the ground plane, it is advisable to adjust the impedance match by raising the height of disc16slightly by lengthening conductive rod28. As can be appreciated by those in the art, the ground plane embodiment of the present invention is merely the omission of discs34,38accompanied by the notable enlargement of disc16, which forms the “ground plane”. It is even possible to form , for example, an “inverted ground plane” antenna, by the enlargement of disc6and the omission of discs32,36.

In the present invention, asymmetry between upper and lower discs may also be used to tailor the elevation plane radiation patterns, e.g. for concentrating most of the radiation into a single half space.

The number of conductive discs that may be included in embodiments of the present invention is non-limiting, and 8, or even 12 conductive discs may be included. Thus, the present invention may include a type of periodic antenna.

A multiple tuned planar dipole has been described then, providing increased bandwidth over prior art single tuned dipoles. The Chebyshev polynomial type frequency response can provide 4 to 3π more bandwidth than prior art quadratic response antennas, by the incorporation of multiple discs and specification of passband ripple. Accordingly, the present invention provides a vertically polarized horizontally planar omnidirectional antenna of broad instantaneous gain bandwidth and thin dimension. The present invention may operate as a multiple band antenna, and operate with or without a ground plane.