Sleeve monopole antenna with spatially variable dielectric loading

A dielectric loaded sleeve monopole antenna has a dielectric loading within the sleeve enables stable impedance in a dynamic operating environment. The use of a dielectric filling in the sleeve portion of the antenna enables tight control of the input impedance over frequency establishing stable broadband operation in challenging operating environments. The effective dielectric constant inside the sleeve of the antenna is designed to exhibit spatial variability. As a result, the sleeve essentially acts as an impedance transformer enhancing control over the input impedance to the antenna. The spatial variability in the dielectric filling may be realized as arrangements of single or multiple dielectric materials machined to synthesize the desired effective dielectric properties.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention generally relates to antennas, and more specifically to the sleeve monopole antenna with dielectric loading.

Background of the Related Art

Distributed antenna systems (DAS) include a plurality of antennas distributed throughout a particular coverage area. DAS solutions are generally deployed to provide wireless coverage in areas that cannot be covered by a single access point. This is generally due to structures in the coverage area that would impede the wireless signal generated by the antenna at the access point from reaching all users within the coverage area. Some examples include office buildings, university campuses, and stadiums.

An antenna is generally impacted by objects in close proximity to the antenna especially when the object falls within the antenna's near field. Nearby objects can cause difficulties in impedance matching making it necessary to consider the operating environment in the antenna design. This can be challenging for DAS networks where the antenna mounting locations are compromised due to physical space limitations or city and government regulations. The resulting mounting locations can place antennas in close proximity to support structures or other infrastructure that can make it difficult to achieve satisfactory antenna performance. These mounting locations can also force the antennas into positions where people may pass through the nearfield of the antenna. The human body is largely composed of water and exhibits a high dielectric constant. As a result, people moving through the nearfield of an antenna can have an impact on the input impedance to the antenna. Furthermore, antenna size can be limited where the antenna is constrained to fit within a given volume, and limitations in the ability to impedance match the antenna may result. The effect of objects within the nearfield of an antenna is further compounded for omnidirectional antennas that are affected by obstructions in multiple directions. Outdoor DAS networks may present additional challenges where inclement weather can create dynamic operating environments. For example, antennas mounted near concrete structures may need to consider the loading effects of the concrete. This becomes a challenge when the concrete is exposed to water, i.e. rain or snow, as the concrete absorbs water due to its porosity. As a result, the dielectric properties of the concrete can be impacted which can, in turn, impact the loading effects on a nearby antenna. Broadband DAS networks are also challenging due to the need to maintain antenna performance over a broad frequency range. Lower frequencies have longer wavelengths than higher frequencies, and as a result, the electrical distance of an object to an antenna varies with frequency. Objects that may not have a significant impact to the antenna at higher frequencies may become problematic at lower frequencies.

As an example, U.S. Patent App. No. 62/347,801 discloses a thin, dual band stadium DAS antenna where the antenna is mounted on stadium railing near the concrete of the stadium steps. The '801 application is hereby incorporated by reference. As a result of the mounting location and size limitations, the low band antennas in the '801 application suffer from the difficulties in impedance matching and warrant a broadband impedance matching solution.

Antennas currently are metallic loaded, as shown for instance in “A Sleeve Monopole Antenna with Wide Impedance Bandwidth for Indoor Base Station Applications,” to Y. S. Li et al., Progress in Electromagnetics Research C., Vol. 16, pp. 223-232, 2010, “Design of a wideband sleeve antenna with symmetrical ridges,” Peng Huang et al., Progress in Electromagnetics Research letters, Vol. 55, pp. 137-143, 2015, and “A novel wideband sleeve antenna with capacitive annulus for wireless communication applications,” Progress in Electromagnetics Research C, Vol. 52, pp. 1-6, 2014. Those antennas are costly to fabricate and complicated to assemble.

An improvement in DAS antennas is desired whereby the antenna can maintain sufficient performance over a broad frequency range in challenging operational environments.

SUMMARY OF THE INVENTION

The present invention details a sleeve monopole antenna with spatially variable dielectric loading and a limited size ground plane to address the aforementioned difficulties in distributed antennas systems. The antenna generally consists of a sleeve approximately λ/4 in length extending distally from a ground plane where the sleeve and ground plane are in electrical contact. The ground plane is limited to approximately λ/6 in diameter corresponding to the '801 application and extends in the opposite direction of the sleeve approximately λ/12 in length. The sleeve surrounds a primary radiating element that also extends distally from a ground plane generally λ/4 beyond the end of the sleeve. The size and shape of the primary radiating element, sleeve, and ground along with the characteristics of the material filling the area between the sleeve and primary radiating element make the sleeve monopole a robust antenna element with the ability to achieve a good impedance match in challenging operating environments. When the input impedance matches the impedance of the network feeding the antenna, less energy is reflected from the antenna input and more energy is allowed radiated from the antenna. As a result, the system becomes more efficient, and less power is required by the transmitter to achieve a desired power level at the receiver. Furthermore, the radiation characteristics of the antenna make it well suited for DAS networks where omnidirectional radiation is desired.

The sleeve monopole antenna inherently provides some immunity to its operational environment due to the sleeve shielding the feed point of the antenna. A dielectric material between the sleeve and the primary radiating element provides an additional tuning parameter so the antenna has the ability to maintain an acceptable impedance match in challenging operational environments. Furthermore, spatial variations in the effective dielectric constant between the sleeve and the main radiator offers enhanced control of the input impedance to the antenna over approaches where a dielectric filler may be homogeneous or nonexistent. The spatial variation of the material allows the sleeve to function similar to a broadband impedance transformer enabling acceptable impedance matching over frequency. Synthesis techniques to realize the effective dielectric constant(s) are also disclosed.

These and other objects of the invention, as well as many of the intended advantages thereof, will become more readily apparent when reference is made to the following description, taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In describing a preferred embodiment of the invention illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, the invention is not intended to be limited to the specific terms so selected, and it is to be understood that each specific term includes all technical equivalents that operate in similar manner to accomplish a similar purpose. Several preferred embodiments of the invention are described for illustrative purposes; it being understood that the invention may be embodied in other forms not specifically shown in the drawings.

The present invention details a dielectric loaded sleeve monopole exhibiting broadband operation in challenging operational environments. The sleeve monopole is an uncomplicated yet robust antenna that can be configured to operate over broad bandwidths. For purposes of the present invention, an antenna exhibiting a −10 dB return loss over a 25% or greater fractional bandwidth is considered to be broadband. The antenna in the preferred embodiment is omnidirectional in nature and designed to operate, for example, over the cellular frequency bands from 696-960 MHz (˜33% fractional bandwidth). The antenna is suited for DAS antenna systems where the antenna is designed to operate with omnidirectional radiation characteristics. However, as those skilled in the art can appreciate, the radiation pattern for the antenna in its operating environment will likely differ from the free-space radiation pattern depending on the operating environment and the objects in close proximity to the antenna. From an impedance matching perspective, the antenna is well-suited for operation in challenging environments where impedance matching techniques beyond those of the traditional sleeve monopole antenna are required.

The sleeve monopole inherently exhibits some immunity to its operating environment due to the feed point of the antenna being shielded by the sleeve. Dielectric loading within the sleeve of the antenna adds a degree of freedom in tuning the antenna and enhances the designer's ability to control the input impedance. Furthermore, spatial variation in the dielectric loading material opens yet another degree of freedom over traditional approaches improving control over the input impedance to the antenna. Any suitable machined dielectrics can be utilized, which is a simple, low cost approach and improves on metallic loading.

With respect toFIG. 1A, the general structure of the dielectric loaded sleeve monopole antenna5is illustrated in accordance with a non-limiting example embodiment of the invention. As shown, the antenna5includes a primary radiating element or radiator100, a sleeve110, and an RF ground structure120. The antenna further includes a dielectric loading140between the sleeve110and the primary radiator100along with a coaxial feed cable130to supply RF signal to the antenna.

The primary radiator100can be, for example, a solid elongated rod having a generally cylindrical shape with a circular cross-section. The radiator100is conductive and made of metal. The radiator100has a proximal end102and a distal end104opposite the proximal end102.

The sleeve110is a hollow tube composed of a material with substantially high conductivity. Copper is the material of choice in the preferred embodiment, for example, due to the ability to solder to copper. The sleeve110surrounds the entire dielectric loading140along with the distal end104of the primary radiator100. The sleeve110is elongated and in the shape of a cylinder, and has a proximal end112and a distal end114. The proximal end112and the distal end114are both open. The radiator100is at least partly received in the sleeve110. As shown, the distal portion (for example, approximately the entire distal half) of the radiator100including the distal end104, is received in the sleeve110. The distal end104of the radiator100is nearly fully received into the sleeve110, so that the distal end104of the radiator100is nearly flush with the distal end114of the sleeve110. There is a small gap or distance between the distal end104of the radiator100and the distal end114of the sleeve110, so that the distal end104of the radiator is slightly recessed from the distal end114of the sleeve110. As further illustrated, the radiator100is substantially centrally located within the sleeve110so that the radiator100is concentric with the sleeve110.

In one example embodiment, the RF ground120is in the shape of a cap that is a circular cylinder. The ground structure120has a circular side128, a proximal end122that is closed and a distal end124that can be opened or closed. The closed proximal end122forms a flat top surface126that provides a small RF ground plane for the primary radiator100. Like the sleeve110, the RF ground120is also composed of copper in a preferred example embodiment. The top surface126of the RF ground120is also in direct contact with the distal end114of the sleeve110such that the two are electrically shorted. The side128of the RF ground120extends away from the flat top surface126in the opposite direction from the sleeve110and primary radiator100. The radiator100can extend substantially orthogonally from the ground structure120. That is, the longitudinal axis of the radiator100can be substantially orthogonal to the center axis of the ground structure120. The radiator100is orthogonal to the portion of the RF ground where the cable attaches, as shown inFIGS. 1, 3-6. As further shown, there is a small space or gap101between the distal end104of the radiator100and the top surface126of the ground structure120, so that the radiator100does not come into contact with the ground structure120.

An opening or hole129extends through the RF ground structure120, and for example can extend centrally through the middle of the ground structure120. In an alternative embodiment, the ground structure120can be hollow, and the hole129can extend only through the top126of the ground structure120. The coaxial feed cable130extends through the entire ground structure120via the hole129. Thus, the cable130extends from outside of the ground structure120into the ground structure120at the distal end124, through the hole129, and exits out of the proximal end122of the ground structure120. In this way, the cable130provides an RF signal to the antenna5.

The cable130has an outer jacket132and a center conductor134. The outer jacket132of the coaxial feed cable130is in electrical contact with the RF ground120and the center conductor134of the coaxial feed cable130is in electrical contact with the primary radiator100. The outer jacket132is metal and there is insulation between the outer jacket132and the conductor134(e.g., Teflon (PTFE)). In the example embodiment shown, the outer jacket132of the coaxial feed cable130is soldered directly to RF ground120, and the center conductor134of the coaxial feed cable is soldered directly to the primary radiator100. The outer jacket132can be soldered to the RF ground structure120(e.g., at the bottom surface of the RF ground structure120) and terminate at the top surface122of the ground structure120. The center conductor134extends beyond the top surface122of the ground structure120and into the distal end114of the sleeve110where it couples with the distal end104of the radiator100.

In an example embodiment, the distal end104of the primary radiator100may include a substantially centrally located slight recession or hole and the center conductor134of the coaxial feed cable130can be inserted and subsequently soldered to the recession to provide a reliable connection between the radiator100and the cable conductor134. Other suitable configurations can also be provided to provide a reliable connection between the radiator100and the cable conductor134. For example, the primary radiator100may include additional structure such as a tab whereby the center conductor134of the coaxial feed cable130may be attached. The inclusion of additional structure on the primary radiator100may result in an offset of the coaxial feed cable130and, correspondingly, the hole in RF ground120. This may further necessitate modification of the dielectric loading material in order to allow clearances for the additional structure on the primary radiator100.

The space103between the sleeve110and primary radiator100will likely possess an effective dielectric constant for design and analysis purposes. To achieve enhanced tuning with this antenna, a variable dielectric constant is provided in the sleeve of the antenna. The sleeve110can be completely filled with a material whose dielectric constant varies in the Z-direction. Alternatively, a variable effective dielectric constant can be achieved by utilizing very common, cheap dielectric materials. The effective dielectric constant is achieved by loading the sleeve with materials that, in some cases, only partly fill the gap103between the sleeve110and the primary radiator100. Therefore, we can essentially achieve any dielectric constant in a low-cost approach.

The space103may be entirely filled with a dielectric loading140, including in the gap101between the radiator100and the ground structure130. The dielectric loading140is designed to give an effective dielectric constant that varies with distance from the RF ground120. In other words, the effective dielectric constant exhibits a Z-dependence as indicated inFIG. 1Bwhere εeffis written to exhibit some functional dependence on the variable Z with respect to the coordinate system shown inFIG. 1B; wherein for the εeff(z) the (z) indicates that εeffis some function of z. The effective dielectric constant at the distal end104of the primary radiator100where the sleeve110attaches to RF ground120is different than the effective dielectric constant at the opposite proximal end104of the radiator100and the distal end114of the sleeve110. The change can vary gradually from one end to the other or it could be stepped (FIGS. 3, 4, and 6). The most important thing is that there is some change from one end to the other. A gradual change works best for most applications, but a stepped change might be more economical and easier to make (e.g., dielectric pucks with varying outer radii (FIG. 4) fabricated over some solid chunk of dielectric with some exotic contour to achieve the desired effective dielectric constant within the sleeve).

The gap101serves as a parameter to adjust the electrical performance (impedance match) of the antenna. In addition, the gap101ensures that the primary radiator100is not inadvertently shorted to the RF ground structure120, which would render the antenna inoperable. In one embodiment, the gap101can be about 0.06 inches, but any suitable gap can be provided (greater or smaller than 0.06 inches) based on the dimensions of the primary radiator100, the sleeve110, and the loading material140.

As those skilled in the art can appreciate, the permittivity for a given material is represented as
ε=ε0εr
where ε0is the permittivity in a vacuum (8.854*10-12 F/m), and εris the relative permittivity, or dielectric constant, for the material. The dielectric constant can be thought of as a scaling factor to represent the material permittivity relative to that of free space. The dielectric constant generally has some frequency dependence, but it remains fairly constant for typical dielectric materials at lower RF frequencies and frequencies used for mobile communications. As a result, the frequency dependence is neglected here.

Further note that the permittivity is generally complex where the imaginary part describes the loss associated with the material. The complex permittivity is written as
ε=ε′−jε″
where ε′ and ε″ are the real and imaginary parts of the permittivity respectively. The dielectric loss tangent for a material is defined as

tan⁢⁢δ=ɛ″ɛ′
and describes the amount of loss associated with the material. Materials exhibiting a low tan δ exhibit little energy lost due to the material.

The effective dielectric constant (εeff) generally refers to the dielectric constant observed by electromagnetic waves travelling through an inhomogeneous transmission medium where the fields are exposed to two or more materials with different dielectric constants. The effective dielectric constant consolidates the effects of multiple materials into a single dielectric constant for the given transmission medium. The use of the effective dielectric constant opens a new degree of freedom in tuning this antenna so that better return loss can be achieved over wider frequency bands given the limitations and operating conditions of the antenna for the present invention (space/volume limitations and mounting close to concrete or other structures with reference to the antenna of the '801 application). This facilitates impedance matching when the antenna electrically couples to objects in its environment which can modify the input impedance to the antenna.

Some examples of transmission media that are characterized by an εeffare microstrip, stripline with dissimilar materials, and partially filled coaxial cable where the space between the inner and outer conductors is filled by a combination of multiple dielectric materials. In the present invention, the field structure in the sleeve portion of the antenna is found to be very similar to coaxial cable; therefore, it makes sense to characterize the effective dielectric constant in the sleeve portion of the antenna in a similar manner.

The partially loaded coaxial cable configuration for the antenna5is illustrated inFIG. 2where one loading example configuration (series configuration) is shown inFIG. 2Aand a different example loading configuration (parallel configuration) is shown inFIG. 2B. Referring toFIG. 2A, the coaxial cable has an inner conductor200, an outer jacket210, a first dielectric material layer220, and a second dielectric material layer230. Both the center conductor200and outer jacket210are composed of materials with high electrical conductivity such as copper. The first and second dielectric material layers220,230are each composed of a material having a different dielectric constant. The first dielectric material220and second dielectric material230fill the space between the center conductor200and outer jacket210. As shown inFIG. 2A, the two dielectric materials220,230are arranged such that the first dielectric material220with εr1and tan δ1completely surrounds the center conductor200of the cable. And the second dielectric material230with εr2and tan δ2completely fills the space between the first dielectric material220and the outer jacket230of the cable.

Thus, the cable has a central conductor200, a first dielectric material layer220surrounding the central conductor200, a second dielectric material layer230surrounding the first dielectric material layer230, and an outer jacket210surrounding the second dielectric material layer230. The first dielectric layer220has a different dielectric material than the second dielectric layer230and can also have different thicknesses. In one example embodiment, the central core200, first and second dielectric layers220,230, and outer jacket210each have a circular cross-section and are concentrically arranged with respect to each other.

In this configuration, the capacitances associated with the two dielectric layers220,230are in series since all vectors describing the electric field pass through the first dielectric material layer220and then the second dielectric material layer230. Hence, the cable has an effective dielectric constant can be calculated as

With respect toFIG. 2B, the first and second dielectric materials240,250are arranged in a parallel configuration. Here, the first dielectric material240completely fills a first portion of the space between the center conductor200and the outer jacket210. That is, the first dielectric material layer240extends the entire distance from the center conductor200to the outer jacket210. But the first dielectric material layer240only partially extends around the central conductor200and outer jacket210. The first dielectric material layer240has an inner surface242that conforms to the outer surface of the center conductor200, and an outer surface244that conforms to the inner surface of the outer jacket210. In the embodiment shown, the first dielectric material layer240surrounds approximately seventy-five percent (75%) of the inner conductor200and extends approximately seventy-five percent (75%) around the inside of the outer jacket210.

The second dielectric material layer250completely fills the remaining portion of the space between the center conductor200and the outer jacket210. The second dielectric material layer250has an inner surface252that conforms to the outer surface of the center conductor200, and an outer surface254that conforms to the inner surface of the outer jacket210. In the embodiment shown, the second dielectric material layer250surrounds approximately twenty-five percent (25%) of the inner conductor200and extends approximately twenty-five percent (25%) around the inside of the outer jacket210.

In this case, the capacitances associated with the two dielectric layers240,250are said to be in parallel since a vector describing the electric field can occupy either the first dielectric layer240or the second dielectric layer250depending on where the electric field vector is taken within the transmission line. Thus, an effective dielectric constant can be calculated as
εeff=αεr1+(1−α)εr2
where α is the percent at which the first dielectric material240fills the space between the center conductor200and the outer jacket210. For example, if the first dielectric material240fills 35% of the space between the center conductor200and the outer jacket210, then α is 0.35. in one embodiment, values range from α=0 to α=1, though any value can be utilized depending on where you are in the sleeve of the antenna.

With respect toFIGS. 3A-3B, one example by which to realize spatial variability in the effective dielectric constant within the sleeve110is illustrated. Referring momentarily to FIG.1B, the dielectric material140can be a single homogeneous layer of material having a proximal end142and a distal end144. Or as shown inFIGS. 3A-3B, the dielectric material can be formed by multiple layers, for example five layers300-340. Thus, the area between the sleeve110and the primary radiator100is completely filled with multiple dielectric material layers300-340stacked in a manner that achieves a variable dielectric constant. Since the space between the sleeve110and the primary radiator100is completely filled, the effective dielectric constant for each layer300-340is simply equal to the dielectric constant of the material used for each layer300-340.

As illustrated, five layers300-340are shown, each having a different dielectric constant, namely: a first layer300exhibits εr1and tan δ1, a second layer310exhibits εr2and tan δ2, a third layer320exhibits εr3and tan δ3, a fourth layer330exhibits εr4and tan δ4, and a fifth layer340exhibits εr5and tan δ5. The various layers300-340extend from the proximal end112of the sleeve110to the distal end114of the sleeve110, with the first layer300being at and flush with the distal end114of the sleeve110and the fifth layer340being at and flush with the proximal end112of the sleeve110, as shown.

There may be more or fewer than five layers; however, there should be at least two layers to realize spatial variation in the effective dielectric constant between the sleeve110and the primary radiator100. Two or more layers may be composed of the same material exhibiting the same dielectric constant. For example, the first layer300and the second layer310may be high-density polyethylene (HDPE) so the effective dielectric constant is εeff≈2.3 from the bottom side of the first layer300through the top side of the second layer310. However, all layers of this particular embodiment should not be composed of the same material as there would be no spatial variability in the effective dielectric constant within the sleeve. Furthermore, the individual layers300-340may be of different thicknesses or they may be the same thickness. The total dielectric loading material(s) may extend the full length of the sleeve110, or it may only encompass a portion of the total height of the sleeve110.

In one example embodiment, the largest value of dielectric constant is at the bottom of the sleeve110, and the smallest value of dielectric constant is at the top of the sleeve110. This is to get the best impedance match over frequency so that the input impedance is transformed to match the capacitive loading at the end of the sleeve portion. The layers are preformed before fitting down into the sleeve. In a sequence of assembly steps: (1) The sleeve and ground are attached (soldered). (2) The bottom layer is placed inside the sleeve to serve as the spacer between the primary radiator100and the RF ground120. (3) The center conductor of the coaxial cable130is attached to the primary radiator100(soldered). (4) The outer jacket132of the coaxial cable130is soldered to the RF ground structure120. (5) The remaining dielectric materials are fit over the primary radiator100, and into the sleeve110.

The layers may be bonded to one another, the sleeve110, and/or the primary radiator100. Ideally, the layers (other than the bottom layer) are bonded to each other and then fit down into the sleeve110over the primary radiator100where they are bonded to the top of the bottom layer. The bottom layer may be bonded to RF ground. If the layers are not bonded, there should be some mechanical support structure that attaches to the sleeve and/or the primary radiator that fixes the layers in place. If such a mechanical support structure is used, it should be non-metallic and possess a low dielectric constant (<3).

Turning toFIGS. 4A-4D, alternative examples for the realization of spatially variable effective dielectric constant within the sleeve110are presented. The approaches illustrated inFIGS. 4A-4Dare similar to that shown inFIG. 3; however, the layers ofFIGS. 4A-4Dmay or may not all have the same dielectric constant value. If all layers have the same dielectric constant, then the dielectric material between the sleeve110and the primary radiator100may be machined from a single dielectric material. Since there is additional machining to control the shape of the dielectric(s), spatial variation can be achieved. As inFIG. 3, the total dielectric loading material(s) may extend the full length and width of the sleeve110, or it may only encompass a portion of the total length of the sleeve110.

In one particular embodiment as shown inFIGS. 4A, 4B, the space between the sleeve110and the primary radiator100is filled with five layers of dielectric materials where the first layer400exhibits εr1and tan δ1, the second layer410exhibits εr2and tan δ2, the third layer420exhibits εr3and tan δ3, the fourth layer430exhibits εr4and tan δ4, and the fifth layer440exhibits εr5and tan δ5. There may be more or fewer than five layers. Each layer400-440is machined with an inner contour or surface and an outer contour or surface where the inner contour of each layer400-440conforms to the outer contour or surface of the primary radiator100and the outer contour of each layer is allowed to vary. The outer contour of each layer400-440is constant for the full height of the layer so that the effective dielectric constant between the sleeve110and the primary radiator100varies in a stepped manner. That is, each layer is of uniform dimensions (i.e. the outer radius (or inner radius) of each individual layer does not vary with distance from RF ground). Thus, each layer is circular with a center opening, but each have a different diameters. Air fills the remaining space around the layers.

Furthermore, one or all layers400-440may exhibit the same dielectric constant. If two or more neighboring layers400-440exhibit the same dielectric constant, the multitude of layers may be machined from a single homogenous dielectric material. If all layers400-440are machined to have the same geometry, the dielectric constants of at least two of the layers400-440should differ in order to achieve spatial variation in the effective dielectric constant. In an alternative embodiment, the layers400-440may be machined in such a way that the outer contour of each layer is not constant. For example, each layer could be machined where the outer contour exhibits a maximum radius and a minimum radius so that the effective dielectric constant varies within each layer. The dielectric material used should exhibit a dielectric constant between εr≈2-6 with a loss tangent tan δ≤0.01. The effective dielectric constant for the approach inFIGS. 4A, 4Bmay be calculated as a series combination of the loading material(s) and air.

In all scenarios, the layers (or any dielectric filler materials) are preformed and then fit down in the sleeve. This would follow the same assembly sequence outlined above with respect toFIGS. 3A-B. The layers may be adhered to the primary radiator100using a bonding agent that has a sufficient working time to allow assembly of the antenna. Otherwise, the layers may be bonded to one another, and fixed in place using a mechanical support that attaches to the sleeve110and/or the primary radiator100. This support should be non-metallic and made of plastic material that has a relatively low dielectric constant (preferably <3). Alternatively, the bottom layer can be bonded to the RF ground120, and the remaining layers can be subsequently bonded together. The thickness need not be rigidly defined, but the effective dielectric constant should generally decrease from the bottom of the sleeve to the top of the sleeve. This generally results in the layers getting thinner as they approach the top of the sleeve, but the thickness is determined by the material chosen for each layer and the desired effective dielectric constant. If all of the layers400-440are composed of the same material, the full collection of layers may be machined from a single piece of homogeneous material.

In another embodiment as shown inFIGS. 4C-4D, the space between the sleeve110and the primary radiator100is filled with five layers of dielectric materials where the first layer401exhibits εr1and tan δ1, the second layer411exhibits εr2and tan δ2, the third layer421exhibits εr3and tan δ3, the fourth layer431exhibits εr4and tan δ4, and the fifth layer441exhibits εr5and tan δ5. There may be more or fewer than five layers. Each layer is machined with an inner contour and an outer contour where the outer contour of each layer conforms to the inner contour of the sleeve110and the inner contour of each layer is allowed to vary. The inner contour of each layer is constant for the full height of the layer so that the effective dielectric constant between the sleeve110and the primary radiator100varies in a stepped manner.

Furthermore, one or all layers401,411,421,431,441may exhibit the same dielectric constant. If two or more neighboring layers exhibit the same dielectric constant, the multitude of layers may be machined from a single homogenous dielectric material. If all layers are machined to have the same geometry, the dielectric constants of at least two layers should differ in order to achieve spatial variation in the effective dielectric constant. In an alternative embodiment, the layers may be machined in such a way that the outer contour of each layer is not constant. For example, each layer could be machined where the inner contour exhibits a maximum radius and a minimum radius so that the effective dielectric constant varies within each layer. The dielectric material used should exhibit a dielectric constant between εr≈2-6 with a loss tangent tan δ≤0.01. The effective dielectric constant for the approach inFIGS. 4C-4Dmay be calculated as a series combination of the loading material(s) and air. The layers are shown with the smallest thickness at the top layer441and the largest thickness at the bottom layer401. That arrangement is practical because it is easier to achieve an effective dielectric constant that decreases with distance from RF ground. However, the layers can be arranged in any suitable manner, such as the bottom layer401having the smallest thickness, or the layers having varying degrees of thickness, as long as spatial variation in the effective dielectric constant can be achieved.

The layers401-441may be adhered to the sleeve110, or they may be adhered to one another and fixed in place mechanically with some attachment to the sleeve110. This configuration would be advantageous overFIGS. 4A-4Bif the primary radiator100possesses a small diameter, which could make it difficult to precisely drill each layer400-440and maintain alignment within the sleeve110in the embodiment ofFIGS. 4A-4B. The advantage of the embodiment ofFIGS. 4A-4Bis that the layers400-440provide mechanical support to the primary radiator100. Without this support (as inFIGS. 4C-4D), some structure could be provided to hold the central radiator100upright and in the center of the sleeve110. For example, this structure could be a plastic piece that sits at the distal end of the sleeve110attached to the sleeve110and the primary radiator100that fixes the primary radiator100in a position relative to the sleeve110.

The layers401-441may be adhered to the sleeve110using a bonding agent that has a sufficient working time to allow assembly of the antenna. Otherwise, the layers may be bonded to one another, and fixed in place using a mechanical support that attaches to the sleeve110and/or primary radiator100. This support should be non-metallic and made of some plastic material that has a relatively low dielectric constant (preferably <3). Alternatively, the bottom layer can be bonded to RF ground, and the remaining layers can be subsequently bonded together. Also, if all of the layers401-441are composed of the same material, the full collection of layers may be machined from a single piece of homogeneous material. In addition, while the layers ofFIGS. 3-4are shown directly adjacent to and touching one another, two or more of the layers can be spaced apart from one another.

Another example embodiment of the antenna5is illustrated inFIGS. 5A, 5B, 5Cand is a variation of the approach outlined inFIG. 4A. The sleeve110is approximately 3.1 inches in length, or approximately λ/4 at the highest operating frequency (960 MHz) where λ is the free-space wavelength. The primary radiator100extends approximately 3.3 inches past the end of the sleeve110, and RF ground extends slightly less than 1″ from the base of the sleeve110. As indicated inFIG. 1A, there is a spacing101between the top of the RF ground120and the distal end104of the primary radiator100. In one example embodiment, this spacing101is set to 0.06″ but can be adjusted for impedance matching. Approximate minimum and maximum dimensions are as follows. The sleeve110can be approximately 2.9″-3.1″, the monopole extension past the end of the sleeve110can be 2.9″-3.6″, and the space101can be 0.054″-0.066″. Note that these dimensions may be able to vary further if measures are taken to tune the antenna5for the specific dimensions. These minimum and maximum dimensions basically capture tolerance analysis whereby the antenna should still perform as intended without a redesign of the antenna.

In order to maintain this spacing101and improve manufacturability, the dielectric loading material is split into an upper member or piece500and a lower member or piece510. In the preferred embodiment, the upper piece500and lower piece510of the dielectric loading material are both made of machined polytetrafluoroethylene (PTFE), or Teflon with εr≈2.1 and tan δ≈0.001. The spatial variability is realized in a manner similar to the approach outlined inFIG. 4Awhere the upper piece500has an outer contour of the Teflon that varies linearly in a conical fashion from the base of the sleeve110to the top of the Teflon loading material. The total height of the Teflon material is approximately 2.9″. In one embodiment, the upper piece500does not extend the full length of the sleeve110, to provide the best impedance match with the Teflon. The widest end of the upper piece500can be positioned at the proximal end114of the sleeve110. This provides the best impedance matching for the antenna5by transforming the input impedance to match the capacitive loading at the end of the sleeve110.

As further indicated inFIGS. 5B, 5C, the primary radiator100includes a tab106extending from the base parallel to the top of RF ground120. This tab106includes a hole108through which the center conductor134of the coaxial feed cable130is passed and soldered to make electrical contact. The tab106can extend outward from the side of the radiator100at the distal end of the radiator100and can be flat. The cable130is offset within the ground member120to align the center conductor134with the hole108in the tab106.

In order to accommodate the tab106and solder attachment for the coaxial center conductor134, the distal end of the dielectric loading material upper piece500is machined with a void502as shown inFIG. 5. The radius of the void502should be large enough to accommodate the tab106on the primary radiator100, but not as large as the inner radius of the sleeve110. The height of the void502should only be large enough to accommodate the height of the tab106and the center of the coaxial feed cable130extending through the tab101with some clearance (tens of mils is desired). In an example embodiment, the height of the void502is approximately 0.125″.

As a result of the void502, an air gap exists between the dielectric loading material lower piece510and a portion of the dielectric loading material upper piece500. This air gap reduces the effective dielectric constant in the region of the solder attachment between the center conductor of the coaxial feed cable130and the tab101on the main radiator100but is necessary for manufacturability. The dielectric loading material upper piece500and lower piece510may be bonded together using a non-conductive epoxy.

In yet another embodiment, the layers of dielectric material may be drilled to achieve an effective dielectric constant as indicated inFIGS. 6A, 6B. Similar toFIG. 3, the antenna is shown with five layers of dielectric materials where the first layer600exhibits εr1and tan δ1, the second layer610exhibits εr2and tan δ2, the third layer620exhibits εr3and tan δ3, the fourth layer630exhibits εr4and tan δ4, and the fifth layer640exhibits εr5and tan δ5. There may be more or fewer than five layers. Each layer is drilled with one or more holes602of a particular diameter where all the holes602in a given layer are the same diameter so that the dielectric constant is uniform for each layer. Of course, the holes602can have different diameters to achieve an effect similar toFIGS. 4, 5, which provides more freedom in synthesizing a desired effective dielectric constant in each layer. The holes in different layers may be the same diameter, or they may be different diameters depending on the material and the desired dielectric constant for each layer. In general, the holes602extend completely through the entire layer600-604, and are drilled with their axes aligned parallel to the longitudinal axis of the primary radiator100.

The holes achieve an effective dielectric constant. By removing some of the material, the effective dielectric constant seen by the antenna is reduced compared to if there were no holes. This is another means of achieving an effective dielectric constant as opposed toFIGS. 3 and 4. This approach would be suited for an additive manufacturing approach (3D printing) where the fill factor can be precisely controlled and each layer is not a completely solid piece of material. An additive manufacturing approach might be preferred here to drilling the materials. Depending on the materials and the hole diameters/spacing, it could be difficult to accurately drill the holes as desired. The holes offer more of a range for dielectric constant than the approach ofFIG. 3. The embodiment ofFIG. 3is limited to the dielectric constant of the material that is being utilized. However, by drilling holes into a puck of dielectric material, a lower dielectric constant can be achieved that might offer better performance for the antenna. For example, for a puck of material with a dielectric constant of 3, drilling holes could provide a dielectric constant of about 2.75.

In an example embodiment, all of the layers600-640may have the same dielectric constant, and the dielectric loading may be machined from a single homogenous dielectric material where the holes602are subsequently drilled to synthesize the desired effective dielectric constant. Similar to the approaches outlined inFIGS. 3 and 4, the total dielectric loading material(s) may extend the full length of the sleeve, or it may only encompass a portion of the total height of the sleeve110. The effective dielectric constant for each layer600-640of the configuration illustrated inFIG. 6may be calculated as a parallel combination of air and the dielectric material in which the holes are drilled. A volumetric fill factor should be used to compute the effective dielectric constant for each layer. The dielectric material used should exhibit a dielectric constant between εr≈2-6 with a loss tangent tan δ≤0.01.

Note that the aforementioned methods by which to realize a spatially variable dielectric constant within the sleeve portion of the antenna are subtractive manufacturing examples. That is, material is cut away, or otherwise removed, from a larger solid piece of material to achieve the end result. However, the variable dielectric constant may also be realized by additive manufacturing, such as 3D printing and 3D printed materials. For example, the approach ofFIG. 6is suited for 3D printing where solid chunks of material are not required, but the fill factor of a given layer can be precisely controlled to achieve a desired dielectric constant.

As an illustrative example of the antenna placement and performance,FIGS. 7A, B show the antenna5of the preferred embodiment operating in close proximity to a concrete structure700. For example, the concrete structure700represents the steps of a stadium where this antenna5is a practical solution for mobile communications. The antenna5can be mounted, for example, to a railing located in close proximity to the concrete steps. The primary difficulty in the illustrated operating environment is that the loading effects of the concrete must be take into account in the antenna design. Since the concrete structure700lies within the nearfield of the antenna, the dielectric properties of the concrete play a role in the antenna input impedance. Furthermore, concrete is porous and can absorb water. As a result, the dielectric properties of the concrete may change considerably depending on the weather for outdoor environments. Research has shown that the dielectric constant of concrete can change from εr≈4 with tan δ≈0.01 for dry concrete to εr≈15 with tan δ≈0.12 for concrete saturated with water. The spatially variable dielectric loading within the sleeve of the antenna enables stable impedance with dramatic changes in the concrete dielectric properties.

The predicted impedance and return loss for the antenna configuration inFIGS. 7A, 7Bare shown inFIGS. 7C, 7D. InFIG. 7C, the input impedance for dry concrete701is compared against the input impedance for wet concrete702on the Smith chart. The further away the two curves are from the center of the Smith chart, the worse the impedance match is to the antenna. The center of the Smith Chart indicates a perfect impedance match. The two curves as shown indicate a very good impedance match for the antenna in the presence of the concrete over the operating band. Furthermore, the two curves overlay quite well for dry concrete and for wet concrete indicating stable input impedance with different levels of water absorption by the concrete.

It is further noted that the variable dielectric loading acts as an impedance transformer providing additional impedance matching capability between the feed point of the antenna (where the coaxial cable attaches to the primary radiator100) and the end of the sleeve110. The use of the variable dielectric loading (impedance transformer) enables the antenna to achieve a better impedance match over a broader bandwidth than the antenna without variable dielectric loading. For example, the antenna of the preferred embodiment with variable dielectric loading exhibits a −15 dB return loss bandwidth of approximately 56%. The best case antenna without variable dielectric loading is found to achieve a −15 dB return loss bandwidth of approximately 44%.

The variable dielectric constant provides enhanced tuning capability enabling the antenna to achieve a better impedance match over a broader band than the antenna with single-material dielectric loading or the antenna without any loading (only air between the sleeve and primary radiator). Even with drastic changes in the dielectric constant of the concrete, the impedance match to the antenna remains very good. This is partly due to the nature of the sleeve monopole. The sleeve shields the feed point of the antenna where the antenna impedance is most sensitive to changes. As a result, the antenna inherently possesses some immunity to changes in its environment. The variable dielectric loading provides enhanced tuning capability over the traditional sleeve monopole further enhancing the ability to achieve broadband impedance matching with a small ground plane in a dynamic environment.

InFIG. 7D, the return loss plot also indicates a stable impedance match where the return loss for dry concrete703is compared against the return loss for wet concrete704. Both curves indicate return loss better than −15 dB and overlay reasonably well. With a −10 dB return loss, only 10% of the power delivered to the antenna is reflected back from the antenna meaning that 90% of the power is available to radiate from the antenna. With a −15 dB return loss, only approximately 3% of the power delivered to the antenna is reflected back from the antenna meaning that nearly 97% of the power is available to radiate from the antenna.

Within this specification embodiments have been described in a way which enables a clear and concise specification to be written, but it is intended and will be appreciated that embodiments may be variously combined or separated without departing from spirit and scope of the invention. It will be appreciated that all features described herein are applicable to all aspects of the invention described herein. Thus, for example, although the series and parallel cables are only shown and described with respect toFIG. 2B, that feature can be utilized in any of the embodiments ofFIGS. 1, 3-7.

The description uses several geometric or relational terms, such as circular, rounded, stepped, parallel, concentric, and flat. In addition, the description uses several directional or positioning terms and the like, such as top, bottom, base, lower, distal, and proximal. Those terms are merely for convenience to facilitate the description based on the embodiments shown in the figures. Those terms are not intended to limit the invention. Thus, it should be recognized that the invention can be described in other ways without those geometric, relational, directional or positioning terms. In addition, the geometric or relational terms may not be exact. For instance, walls may not be exactly perpendicular or parallel to one another but still be considered to be substantially perpendicular or parallel because of, for example, roughness of surfaces, tolerances allowed in manufacturing, etc. And, other suitable geometries and relationships can be provided without departing from the spirit and scope of the invention.

Within this specification, the terms “substantially” and “about” mean plus or minus 20%, more preferably plus or minus 10%, even more preferably plus or minus 5%, most preferably plus or minus 2%.

The foregoing description and drawings should be considered as illustrative only of the principles of the invention. The invention may be configured in a variety of shapes and sizes and is not intended to be limited by the preferred embodiment. Numerous applications of the invention will readily occur to those skilled in the art. Therefore, it is not desired to limit the invention to the specific examples disclosed or the exact construction and operation shown and described. Rather, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.