Slot antenna assembly with tapered feedlines and shaped aperture

A slot antenna includes a substrate having a first side and a second side, a first conductive layer on the first side of the substrate, and a second conductive layer on the second side of the substrate. A first aperture is in the first conductive layer, a second aperture is in the first conductive layer, a first slotline is in the first conductive layer and in communication with the first aperture, and a second slotline is in the first conductive layer and in communication with the second aperture. A third aperture can be in the second conductive layer. A plurality of vias can be in the substrate and surrounding at least a portion of a region including the first aperture, the second aperture, the first slotline, and the second slotline, each of the vias extending through the substrate from the first conductive layer to the second conductive layer.

BACKGROUND

An ideal radio frequency (RF) antenna will radiate 100% of power available from a transmission line connected to an RF source, in the case of a transmitting antenna. Alternatively, in the case of reception, an ideal antenna will send 100% of the power captured by the antenna down a transmission line toward the receiver. To attain the 100% value there must be an exact impedance match between the transmission line impedance, and the antenna impedance. For example, an antenna transmitting RF power must have an impedance of exactly 50 ohms (Ω) as a necessary condition to attain 100% efficiency when connected to a 50Ω transmission line. However, there are other sources of inefficiency, so attaining a perfect impedance match does not guarantee maximum radiated power. When the impedance between transmission line and antenna are mismatched, a reflection occurs, and a reflected wave sends power back toward the RF source, setting up a standing wave in the transmission line. One common measurement of the magnitude of the reflection is known as Voltage Standing Wave Ratio (VSWR). VSWR is the ratio of the maximum (time averaged) voltage existing in the transmission line to the minimum voltage (the maximums located a physical distance of a quarter wavelength from the minimums within the transmission line). An ideal antenna will be perfectly matched, hence no reflected wave, and consequently no standing wave, so for a perfect match, VSWR reaches its lowest possible value, which is unity, or expressed as a ratio, 1:1. Mismatches raise the value of VSWR, so designing an antenna with a minimum value of VSWR maximizes the power that can be radiated, provided other losses are also controlled. The reciprocal case, an antenna receiving, acts in an analogous manner. In this case, a portion of the captured incident RF power is reflected back into the atmosphere when the impedance is mismatched. Impedance mismatches can be mitigated by adding impedance matching components, such as resistors and capacitors. However, these components are bulky and difficult to implement in small scale applications. Therefore, non-trivial impedance mismatching issues remain.

DETAILED DESCRIPTION

Slot antenna assemblies are disclosed. The assemblies are well-suited for compact high band conformal receive antenna applications, such as in applications where the assembly must fit into a relatively tight space of a given platform. However, it will be appreciated that the disclosed designs may benefit other RF applications as well. Example applications for the slot antenna assembly include communications equipment for vehicles (e.g., land, air, and/or sea, whether manned or unmanned), smart munitions, and stationary applications (e.g., ground stations). According to an embodiment, a slot antenna assembly includes a cavity-backed PCB assembly with an integrated radome. The PCB assembly includes conductive (metal) layers applied to a substrate. The conductive layers have apertures and coplanar waveguide transmission lines that are tapered to improve impedance matching without using additional components, such as resistors and capacitors. In some such embodiments, the antenna radome can be cast in place to the aperture plate using a mold, which reduces complexity, parts count, and the need for expensive machining operations. Numerous embodiments and variations will be appreciated in light of this disclosure.

General Overview

As noted above, impedance mismatches in an antenna can cause undesirable voltage and current reflections, which distort the signal and affects the performance of a given communications system. According to the theory of electromagnetic radiation, a perfect impedance match between an antenna and a transmission line can only be achieved at a discrete set of frequency points, and not through a band (continuum) of frequencies, at least if the antenna radiates RF power. One of the main goals of most antenna design is to minimize this mismatch through a specified band of frequencies. Impedance matching techniques incorporating devices such as resistors, capacitors, transformers, and other components are used in some designs to achieve better performance over significant bandwidths. However, the use of such discrete devices has the drawback of adding a certain amount of loss to the antenna, resulting in portions of the available power converted to heat, rather than radiating into the atmosphere, thereby reducing the radiation efficiency. Resistors are particularly noted for causing such losses. Additionally, at higher frequencies, resistors, capacitors and other impedance matching components start showing significant parasitic effects. For instance, at lower frequencies a capacitor will act substantially as an ideal capacitor from circuit theory. But at higher frequencies (when the size of the component is a substantial fraction of a wavelength), the leads of the capacitor act as inductors, the case and dielectric material act as resistors. These parasitic effects can be unpredictable, differing from individual capacitor to capacitor due to tiny manufacturing differences. Thus, parasitic effects can lead to difficult design problems and potential manufacturing yield problems. Furthermore, the use of these discrete devices add cost to manufacturing process and also add bulkiness to the antenna assembly, causing size problems if the antenna is intended to have a small form factor. For antennas with very low VSWR requirements, and large frequency bandwidths, the use of these devices might be necessary, but in light of this, it is advantageous to avoid these devices if possible.

To this end, and in accordance with various embodiments of the present disclosure, a slot antenna assembly includes, in conductive layers on a substrate, shaped apertures and tapered feedlines to reduce or eliminate the need for impedance matching components, such as resistors and capacitors. As discussed in further detail below, various aperture shapes and feedline transitions to the aperture can potentially benefit VSWR frequency characteristics and gain patterns.

Example Antenna Assembly

FIG.1Ashows an example slot antenna assembly100, in accordance with an embodiment of the present disclosure. The slot antenna assembly includes a PCB assembly102, which is positioned between a cover104and an aperture plate106. The cover104provides access to a radio frequency (RF) connector118of the PCB assembly102, while also protecting the PCB assembly102from RF interference and physical debris. The PCB assembly102has one or more apertures302and304, such described in further detail with respect toFIGS.3A and3B. The aperture plate106has an antenna aperture120shaped to match the apertures302and304in the PCB assembly102. The antenna aperture120is covered by a radome116. The radome116can be made from a dielectric material.

In some examples, the PCB assembly102is adhered to the aperture plate106with a bonding adhesive108so that the apertures302and304align with the antenna aperture120of the aperture plate. Many bonding adhesives108can be used to adhere the PCB assembly102to the aperture plate106. In some cases, the adhesive108is electrically and/or thermally conductive, flexible, and/or removable. In some embodiments, an epoxy film or an adhesive film that is designed for bonding materials with mismatched coefficients of thermal expansion, such as LOCTITE® ABLESTIK ECF561, can be used, but it will be understood that other conductive adhesive materials can be used. Other methods of attachment can also be used such as double-sided tape, snap locks, screws, lock teardrops, snap rivets, and edge holders. In some embodiments, the PCB assembly102, cover104, and aperture plate106are aligned or otherwise located with respect to each other using a pin and hole alignment system. For example, the aperture plate106can include a pin114that aligns with a hole110or recess in the cover104and a hole112through the PCB assembly102.

Some embodiments include a carbon-based, antireflection (ARC) absorber122and a spacer124positioned between the cover104and the shaped apertures302and304of the PCB assembly102. The absorber122can include a high loss dielectric or similar material, which eliminates or reduces reflection of received RF signals, after passing through the apertures302and304, on the transmission signal, causing destructive interference. The spacer124keeps the absorber122from loosening during the vibrations of operation, mechanical shock, or other interfering forces. The spacer124can be made from any low loss dielectric material, such as a material that simulates the properties of air by having a relative dielectric constant approaching 1.0. In some embodiments, the spacer124can be a closed-cell rigid, plastic based foam such as ROHACELL®. The thicknesses of the absorber122and the spacer124can vary based on the design of the slot antenna assembly100. In some embodiments, such as depicted inFIGS.1A-B, the combined thickness of the absorber122and the spacer124is approximately ⅛ to ⅜ of a wavelength of the transmitted and/or received RF signals.

The antenna100uses the RF connector118to communicate the signal that is sent or received by the PCB assembly102. The RF connector118can include, for example, a sub-miniature push-on (SMP) connector, although it will be understood that other suitable connectors can be used. The type of connector118used can depend on the application of the slot antenna assembly100and the cavity space available.

FIG.1Bis a cross-sectional side view of an assembled slot antenna assembly100. As previously described, the PCB assembly102, is positioned between the cover104, and the aperture plate106. The slot antenna102is located so that the connector118is aligned with and protrudes from the RF connector port204located in the cover104. The example PCB assembly102is further located by the hole112of the PCB assembly102which aligns with the pin114of the aperture plate106.

FIGS.2A and2Bshow two sides of the cover104. The cover104protects the PCB assembly102from debris and damage. In this example, four screw holes202are provided for attaching the cover104to the aperture plate106with screws or other types of fasteners. The cover104includes an RF connector port204, which provides access to the RF connector118of the PCB assembly102.

FIG.2Ashows the side of the cover104that is oriented towards the PCB assembly102, with respect to slot antenna assembly100. The raised cavity206provides space for the spacer124and absorber122of the slot antenna assembly100while seated inside the cavity206. In some embodiments, the hole110aligns with the pin114when cover104is attached to the aperture plate106. The cover104can be constructed out of rigid, electrically conductive materials such as aluminum, aluminum alloy, nickel iron alloy, stainless steel, steel, zinc, zinc alloy, graphite, and carbon fiber reinforced polymers, or of non-electrically conductive materials plated with an electrically conductive material.

FIGS.3A-Dshow portions of the PCB assembly102.FIG.3Ais a plan view of a first side350of a PCB300. The first side350of the PCB300includes two apertures302and304, two slotlines306and308, an RF connector314, and a series of vias316at least partially surrounding a region including the apertures302,304,334(seeFIGS.3B-C), the slotlines306,308, and the RF connector314. The slotline pair306,308form what is known as a coplanar waveguide, which excites the apertures302and304simultaneously. In some embodiments, the two slotlines306,308are mirror images of each other about a centerline. The vias316are openings extending through the PCB300that provide a Faraday cage around the apertures302,304,334, the slotlines306,308, and the RF connector314. In some examples, the vias316are approximately 1/10 of a wavelength (as transmitted or received by the antenna) apart from each other. As previously explained, the PCB assembly102also includes two holes112to align or locate the aperture plate106, PCB assembly102, and cover104.

Each aperture302and304has two ends320/326and322/328, and a width324/330that are orthogonally oriented about a lateral axis310of the substrate332and parallel to a longitudinal axis318of the substrate332. The ends320/326and322/328as well as the width324/330are aligned with one another about a second lateral axis312that is parallel to the lateral axis310of the substrate332. Note, orthogonal, as used here, does not require precise ninety-degree angles, and parallel, as applied here, does not require infinite expansion without intersection. In some examples, the width324/330of the apertures302and304is larger than each of the two ends320/326and322/328and positioned closer to the end322/328, which is located closer to the tapered feedlines306and308. The feedlines306and308are tapered along a length of the longitudinal axis318. WhileFIGS.3A and3Bshow one tapered aperture shape, other tapered aperture shapes are also possible, such as the example apertures shown inFIGS.4A-Cat402,404, and406.

The apertures302,304,402,404, and406optimize the VSWR ratio of the antenna102and thus reduce or eliminate the need for additional impedance matching elements. The angular shapes of the apertures302,304,402,404,406generate two regions of electric field on the substrate332that oscillate in phase with each other. The described regions are those on the substrate332, when receiving or sending a RF signal, where the transmission lines diverge to a nearly orthogonal angle from their original parallel state, allowing the electric fields to oscillate in phase, rather than out of phase (as in the transmission lines), thus creating the source for the radiated RF energy. In some embodiments, these isolated regions are identified through electromagnetic simulations and optimization techniques.

In some examples, the apertures302,304,402,404,406are mirror images of one another about a longitudinal axis318. For example, the shape of the aperture302is substantially the same as a shape of the aperture304mirrored across the longitudinal axis318. In some examples, the first aperture302and second aperture304are not mirror images of one another. For example, the aperture302can be larger than304. The width324can be larger than the width330. The end328can be closer to the longitudinal axis318than322. The two apertures302and304can be different shapes from one another.

An example PCB300of this type, with apertures like those described, is capable of operating within the Ka microwave band. With operation at a lower frequency of approximately 26 GHz and an upper frequency of approximately 40 GHz.

Referring again toFIG.3A, the PCB300includes at least two slotlines306and308on the first side350. Each slotline,306and308, begins at one of the two apertures302and304and terminates at the RF connection point314, which connects to the RF connector118discussed in reference toFIG.1. In some examples, the PCB300includes a plurality of circular vias316, patterned around the slotlines306and308and apertures302and304.

The angle at which the slotlines306and308approach and connect to the apertures302and304affect the slot antenna's VSWR.FIG.3AandFIGS.4A-Cshow some alternative angles of the slotline306and308connections to the apertures302,304,402,404,406. To improve the VSWR, each feedline306and308is tapered or angled along the length towards the longitudinal axis318of the PCB300. The apertures302,304and slotlines306,308are shown as substantially polygonal. However, these shapes can, for example, be curved or radiused at the corners.

In some embodiments, the width324of the aperture302along the second lateral axis312varies as a function of a distance from the first slotline306, and the width330of the second aperture304along the second lateral axis312varies as a function of a distance from the second slotline308. For example, as shown inFIGS.3A,4A,4B, and4C, the width of the apertures302,304,402,404,406varies, such that each of the apertures has a particular shape. The shape of the apertures302,304,402,404,406helps to mitigate impedance mismatches of the antenna assembly.

FIG.3Bshows a second side352of the PCB300. The second side352of the PCB300includes an aperture334that is opposite from, and thus generally aligned with, the apertures302and304. The aperture334has a shape that is similar to the shapes of the apertures302and304on the first side of the PCB300, such as described above. Additionally, the aperture334extends between the two portions having the same shapes as the apertures302and304to create one contiguous aperture on the second side352of the PCB300, such as shown inFIGS.3B-C. In some examples, the second side352of the PCB300is adhered to the aperture plate106. In some examples, the aperture plate pin110is aligned with the PCB300using the pin holes112.

The PCB300can be manufactured, for example, using an etching process or a metallization process.FIG.3Cshows a cross section along cut line C-C, which is along the longitudinal axis312depicted inFIGS.3A-B. The PCB300begins as a substrate332. The substrate332can be any dielectric material, such as duroid, ceramic PFTE, silicon or other compound III-V or II-VI materials.

If a metallization process is used, the substrate332has first and second conductive layers354and356deposited on the first side350and the second side352of the substrate332, respectively. If an etching process is used, the substrate332is purchased with complete sheets of metal on each side, and metal is etched away where it is not wanted, to form an equivalent structure. The conductive layers are typically copper, but in some embodiments can include other metals such as aluminum, nickel, gold, silver, titanium, tungsten, platinum, or other materials with comparable electrically conductive properties. Metallization can, for example, involve filament evaporation, electron-beam evaporation, flash evaporation, induction evaporation, and sputtering, or other similar processes. In some embodiments, the vias316are filled with the same material as the conductive layers354,356.

For the etching option, portions of the conductive layers354,356are etched (chemically or by use of lasers) or completely removed from the pre-metallized substrate332to form the apertures302,304, and320, and the feedlines306and308. Thus, the apertures302,304,320are in the respective conductive layers354,356.

FIG.3Dis a perspective view of the PCB assembly102, including the PCB300ofFIGS.3A-Cand the RF connector118attached to the RF connection point314of the PCB300.

FIGS.5A-Bshow an example of the aperture plate106. The aperture plate106can be flat or curved. Both a flat and curved aperture plate106, in conjunction with the radome, create the fish-eye lens effect, explained previously, which increases the antenna's FOV without having a significant effect on the recognized frequency range or VSWR ratio of the slot antenna assembly100. In this example, the aperture plate106is mounted in place using through holes502and corresponding fasteners. The fasteners can, for example, include screws with threaded through holes or any other type of attachment.

FIG.5Aalso shows an example of the outermost side of the aperture plate106. The aperture plate106includes a shaped recess504around the antenna aperture120. This shaped recess504allows the radome116to sit flush with the surface of the aperture plate106.

FIGS.5A and5Balso show an example of the antenna aperture120. The shape of the antenna aperture120can match the shape of the apertures302and304in the PCB assembly102. By matching the shape of the antenna aperture120to the shape of the apertures302and304, impedance mismatching, return loss, and/or VSWR affects are reduced.

FIG.5Balso shows an example of the innermost side of the aperture plate106, to which the PCB assembly102and cover104are attached. The innermost side has a recess506that aligns the shaped apertures302and304in the PCB assembly102with the antenna aperture120in the aperture plate106. The pins114and508align with the holes112on the PCB assembly102. As previously described with respect toFIGS.2A and2B, the cover104attaches to the aperture plate106by aligning the four cover through holes202with the aperture plate through holes510and joining them with a screw or other suitable fastener. In some embodiments the through holes may be threaded. The cover can be attached using alternative fasteners such as a turn key or latch, or the assembly may not include an attachment method and continue to operate as described.

FIGS.6A and6Bshow an example of the radome116. The radome116includes a dielectric material that presents a lower characteristic impedance than air to radio signals and is useful in impedance matching the antenna over the desired frequency band of the incoming (or transmitted) signal. The first side602of the example radome116, which is generally outward facing, has a substantially curved surface, which is intended to conform to the surface of the object in which it is installed (a conformal aperture). For example, the radome116creates a “fish-eye lens” effect in transmission and reception, which expands the slot antenna's field of view (FOV). However, it will be understood that the outwardly facing surface of the radome116can have any suitable shape, including planar (flat), in some embodiments, and that this shape can focus, or defocus (in the case of a fish-eye lens) the pattern of the antenna to some extent. The second side604of the example radome has a surface shape606that fits over and within the antenna aperture120. The radome116material is impact-resistant, which helps protect the antenna and its performance from debris, such as sleet, hail, and insects. The radome116can be made of a plastic, such as UV grade ABS, Korad capped ABS, thermoplastic polyolefin (TPO), or other suitable materials. In some examples, the radome116is cast in place including, for example, thermoformed plastic, injection molded plastic, gas assist, structural foam, custom blow molding, or any other suitable mold in place techniques. The radome is also useful in accomplishing the fish-eye lens affect described previously, due to its lower characteristic impedance to RF signals.

FIG.7shows an example of the aperture plate106ofFIG.5A, with the radome116ofFIGS.6A and6B, positioned within the shaped recess504, the first side602of the radome116facing outward.FIG.7illustrates a cast in place radome116but as previously explained other radomes can be used.

Simulated and Measured Results

FIG.8illustrates measured VSWR for two prototype slot antenna assemblies that implement the aperture design illustrated inFIGS.1A and3A-B.

FIG.9Aillustrates measured elevation gain of the example slot antenna assembly depicted inFIGS.1A,3A and3Bat different frequencies, when the example antenna is installed in a two-inch diameter pole.FIG.9Billustrates measured azimuth gain of the example slot antenna assembly depicted inFIGS.1A,3A and3Bat different frequencies, when the example antenna is installed in a two-inch diameter pole.

ADDITIONAL EXAMPLES

Numerous embodiments will be apparent in light of the present disclosure, and features described herein can be combined in any number of configurations.

Example 1 provides a slot antenna including a substrate having a first side and a second side; a first conductive layer on the first side of the substrate; a second conductive layer on the second side of the substrate; a first aperture in the first conductive layer; a second aperture in the first conductive layer; and a coplanar waveguide having a first slotline in the first conductive layer and in communication with the first aperture, and a second slotline in the first conductive layer and in communication with the second aperture, the coplanar waveguide configured to excite the first and second apertures simultaneously.

Example 2 includes the subject matter of Example 1, and further includes a plurality of vias in the substrate and surrounding at least a portion of a region including the first aperture, the second aperture, the first slotline, and the second slotline, each of the vias extending through the substrate from the first conductive layer to the second conductive layer.

Example 3 includes the subject matter of any of Examples 1-2, further including a radio frequency (RF) connector in communication with the first slotline and the second slotline.

Example 4 includes the subject matter of any of Examples 1-3, where a width of the first aperture varies as a function of a distance from the first slotline, and wherein a width of the second aperture varies as a function of a distance from the second slotline.

Example 5 includes the subject matter of any of Examples 1-4, where a shape of the first aperture is substantially the same as a shape of the second aperture mirrored across a longitudinal axis of the substrate.

Example 6 includes the subject matter of any of Examples 1-5, further including a third aperture in the second conductive layer, the third aperture being opposite from the first and second apertures.

Example 7 provides a slot antenna assembly. The slot antenna assembly includes a slot antenna having a substrate, a conductive layer on a side of the substrate, an aperture in the conductive layer, the aperture oriented about a lateral axis of the substrate, and a slotline in the conductive layer and extending adjacent to a longitudinal axis of the substrate, the slotline in communication with the aperture. The slot antenna assembly further includes an aperture plate defining an antenna aperture and a radome positioned over the antenna aperture.

Example 8 includes the subject matter of Example 7, further including a radio frequency (RF) connector in communication with the slotline.

Example 9 includes the subject matter of any of Examples 7-8, where a width of a first end of the aperture furthest from the slotline is different from a width of a second end of the aperture nearest to the slotline.

Example 10 includes the subject matter of any of Examples 7-9, where a width of the aperture varies as a function of a distance from the slotline.

Example 11 includes the subject matter of any of Examples 7-10, where the aperture is a first aperture, where the slot antenna further includes a second aperture in the conductive layer, and where a shape of the first aperture is substantially the same as a shape of the second aperture mirrored across the longitudinal axis of the substrate.

Example 12 includes the subject matter of Example 11, where the slotline is a first slotline, where the slot antenna further includes a second slotline in the conductive layer, and where a coplanar waveguide includes the first slotline and the second slotline, the coplanar waveguide configured to excite the first and second apertures simultaneously.

Example 13 includes the subject matter of any of Examples 11-12, where the conductive layer is a first conductive layer, where the side of the substrate is a first side of the substrate, and where the slot antenna further includes a second conductive layer on a second side of the substrate, and a third aperture through at least a portion of the second conductive layer.

Example 14 includes the subject matter of any of Examples 8-13, where at least a portion of the slotline is tapered along a length of the longitudinal axis of the substrate.

Example 15 includes the subject matter of any of Examples 7-14, where the slot antenna further includes a plurality of vias in the substrate and surrounding at least a portion of a region including the aperture and the slotline, each of the vias extending through the substrate.

Example 16 provides a slot antenna including a substrate having a first side and a second side; a first conductive layer on the first side of the substrate; a second conductive layer on the second side of the substrate; a first aperture in the first conductive layer, the first aperture oriented about a lateral axis of the substrate; a second aperture in the first conductive layer, the second aperture oriented about the lateral axis; a radio frequency (RF) connector; a coplanar waveguide having a first slotline in the first conductive layer and extending adjacent to a longitudinal axis of the substrate, the first slotline in communication with the RF connector and the first aperture, the coplanar waveguide further having a second slotline in the first conductive layer and extending adjacent to the longitudinal axis of the substrate, the second slotline in communication with the RF connector and the second aperture, the coplanar waveguide configured to excite the first and second apertures simultaneously; and a plurality of vias in the substrate and surrounding at least a portion of a region including the first aperture, the second aperture, the first slotline, the second slotline, and the RF connector, each of the vias extending through the substrate from the first conductive layer to the second conductive layer.

Example 17 includes the subject matter of Example 16, where a width of the first aperture varies as a function of a distance from the first slotline, and where a width of the second aperture varies as a function of a distance from the second slotline.

Example 18 includes the subject matter of any of Examples 16-17, where a width of a first end of the first aperture furthest from the first slotline is greater than a width of a second end of the first aperture nearest to the first slotline, and where a width of a first end of the second aperture furthest from the second slotline is greater than a width of a second end of the second aperture nearest to the second slotline.

Example 19 includes the subject matter of any of Examples 16-18, further including a third aperture in the second conductive layer, the third aperture being opposite from the first and second apertures.

Example 20 includes the subject matter of any of Examples 16-19, where at least a portion of the first slotline is tapered along a length of the longitudinal axis of the substrate; and where at least a portion of the second slotline is tapered along a length of the longitudinal axis of the substrate.

The foregoing description and drawings of various embodiments are presented by way of example only. These examples are not intended to be exhaustive, or to limit the invention to the precise forms disclosed. Alterations, modifications, and variations will be apparent in light of this disclosure and are intended to be within the scope of the invention as set forth in the claims.