Patent Application
20240405437
The following disclosure relates generally to antenna design and fabrication and, more specifically, to the design of wideband, low-profile, end-fire radiating antennas suitable for conformal mounting to conducting surfaces.
High bandwidth communications and data transfer capabilities that can operate over a wide frequency range are crucial in the modern world, especially in combat situations where traditional communication links are unlikely to be available.
A variety of RF/microwave antenna types exist that achieve multi-octave bandwidth across segments of the 100 MHz to 100 GHz frequency range. Examples include cavity backed two and four-arm spirals, Vivaldi notch radiators, and multi-layer dielectric rod antennas. Some of these antennas are suitable for conformal mounting to conductive surfaces, but have their peak radiation direction normal to the mounting surface. In applications such as aircraft, belly-mounted antennae are configured to see directly ahead or directly behind the host aircraft, however, it would be desirable for the antenna to be both suitable for conformal mounting to conductive surfaces and to radiate most of its energy ‘end-fire’, that is, in a direction parallel to the mounting surface.
What is needed, therefore, is a wideband, conformally mounted antenna that radiates most of its energy in a direction parallel to the mounting surface, preferably one that is relatively low-profile and that can survive direct exposure to high shock and high-velocity environments that may be encountered, for instance, on an aircraft.
Disclosed herein is an adaptation to the well-established dielectric rod antenna (DRA), a type of surface wave antenna, that applies electromagnetic image theory to realize a wideband, end-fire antenna for low-profile, conformal applications. By combining an imaged, uniform thickness section of a DRA (see Dielectric Image Line) with an imaged transverse electromagnetic (TEM) feed, and an imaged radiation taper, significant performance benefits are obtained.
An overview of the prior art's working principles is given and then applied to the disclosure. Methods are described for extending bandwidth through structural and feed mechanism enhancements. A means of incorporating a second operating mode that radiates with a polarization orthogonal to the primary mode is also presented.
Implementations of the approach described above may include a method or process, a system or apparatus, a kit, or computer software stored on a computer-accessible medium. The details or one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims.
The features and advantages described herein are not all-inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been selected principally for readability and instructional purposes and not to limit the scope of the inventive subject matter.
These and other features of the present embodiments will be understood better by reading the following detailed description, taken together with the figures herein described. The accompanying drawings are not intended to be drawn to scale. For purposes of clarity, not every component may be labeled in every drawing.
As a preliminary matter, as used herein, image theory and related terminology should be understood to refer to the theory that states that for antennas mounted over or near a ground plane (a very large, perfectly conducting plane), virtual sources (images) can be place below the ground plane to account for reflections from the ground plane. After introducing the properly arranged image sources, the electromagnetic field above the ground plane can be considered as a sum of the electromagnetic fields, due to the real sources (above the ground plane) and the image sources (below the ground plane), with the ground plane removed. This image theory can only be applied above the ground plane, but not below the ground plane; below the ground plane, the electromagnetic field is strictly zero.
Furthermore, as used herein, ‘imaged’ means cutting an original structure in half and attaching an electrical conductor to the split plane to maintain the same fields above the split as were present with the original structure.
Still further, modes of electromagnetic radiation are used herein to describe the field pattern of the propagating waves and are analogous to the normal modes of vibration in other systems, such as mechanical systems. Specifically in the case of hybrid electromagnetic (HEM) modes, both the electric and magnetic fields have a component in the direction of propagation. They can be analyzed as a linear superposition of transverse electric (TE) and transverse magnetic (TM) modes.
Now referring to the present disclosure, the present disclosure teaches new, wideband, multi-mode, end-fed, low-profile, end-firing antenna designs that can be mounted conformally to a metallic body, or ground plane.
HE11 mode excitation is commonly achieved by tapering the feed region 100 of the DRA comprising the TEM horn 108 and applying a two-wire line feed 112, such as a pair of strip conductors or differential twin strip feed, to a launcher 110 disposed on the tapered surfaces of the TEM horn 108, with the separation between the conductors gradually growing to the full DRA diameter. By exciting each of the wires of the two-wire line feed 112 180 degrees out of phase in an ‘odd-mode’ or ‘differential’ arrangement, the predominant electric field orientation is in a plane containing the center lines of the wires. If the flare angle is relatively small and the length of the tapered portion of the feed region 100 is a guided wavelength or more, then the excited field in the feed region 100 will be approximately transverse electromagnetic (TEM) and this will effectively excite the desired HE11 mode in the uniform rod section, i.e. the guide section 102. This mechanism of inducing a particular mode may be referred to as the launch method. The field polarization of the fundamental mode and the radiated energy is defined by the orientation of the two-wire line feed 112, as described above. This radiated energy has its peak amplitude along the axis of the DRA and is referred to herein as an ‘end-fire’ configuration.
The bandwidth over which the uniform portion of the DRA, i.e. the guide section 102, carries only the fundamental HE11 mode can be defined as fhigh-flow, where fhigh corresponds to the guide wavelength that is approximately equal to the DRA diameter within the guide section 102 and where flow is that frequency where the impedance mismatch of the feed reaches 2:1 Voltage Standing Wave Ratio (VSWR). At lower frequencies, the launcher 110 becomes electrically too short and ineffectively excites the HE11 mode in the rod. At frequencies above fhigh, higher order modes form on the DRA, corrupting the resulting radiation patterns. The operating bandwidth of a practical TEM-fed, uniform DRA is approximately 2:1, e.g. 2-4 GHz.
In general, the fields of the HE11 mode exist both within the guide section 102 of the DRA and in the surrounding air region. The higher the frequency of operation, the more the fundamental mode fields concentrate within the guide section 102 of the DRA and have their wave velocity slowed by the dielectric 200. At lower frequencies, or when the DRA diameter is reduced, the fields extend more in the air region outside the DRA and the wave velocity approaches that of air. At the limit, when the DRA diameter becomes very small, relative to the guide wavelength, the wave detaches completely from the DRA and radiates; this is why the radiation region 104 of embodiments is tapered to nearly a point. The length of the radiation region 104 impacts directivity of the DRA. A short radiation region 104 will have relatively low directivity while the phasing up of progressive radiation across a longer radiation region 104 increases directivity.
The DRA described above works satisfactorily when there are no conducting bodies in the immediate vicinity of the antenna. However, if this antenna were situated within a wavelength of a conducting body, such that a long side of the guide section 102 was parallel to the conducting surface, serious field disruptions would be expected in the feed region 100, He11/guide sections 102, and radiation regions 104.
According to features of the present description, to avoid this disruption in applications where a broadband, end-fire antenna is needed on the skin of a conducting body, embodiments herein exploit the image theory of electromagnetics and place one half of the DRA directly on a conducting ground plane 300, such embodiments being herein referred to as wedge radiators.
The aforementioned imaged embodiments functions because, assuming the conducting ground plane 300 extends to infinity in all directions and the full-rod geometry and feed is split along the plane of magnetic symmetry (i.e. the DRA is sliced in half, with a half disposed directly on the conducting ground plane 300), then the field pattern in the space containing the half-rod geometry will be identical to the full-rod case without a conducting ground plane 300. Unfortunately, in practice the conducting ground plane 300 is finite sized and the radiating fields diffract somewhat at the boundaries of the ground conductor, creating deviation in the radiation pattern from that of the un-imaged case. This deviation is typically acceptable, however, if the ground plane termination is at least a few wavelengths away from the antenna extent.
Given its similar geometry to existing DRAs, the imaged wedge radiator of
As with the DRA, the primary electric field polarization of the He11 mode and the final radiation pattern are determined by the feed polarization, which is normal to the image ground plane. Consistent with the DRA, there are many parameters available in the wedge radiator design to establish the required performance, such as the TEM horn 328 length, launcher 340 length, microstrip transmission line 302 final width, He11 mode region/guide section 322 length & thickness, radiation taper region 324 length, radiation taper region 324 minimum thickness, the dielectric constant of the dielectric 350, and others, as would be known to one of ordinary skill in the art. A three-dimensional electromagnetic field solver is an effective means of assessing and optimizing the performance of a given design.
To achieve an operating bandwidth greater than 2:1, multiple layers of dielectric 350 are incorporated in the structure of embodiments, with each layer being referred to herein as a core 400 or core dielectric 400. Cores, as used herein, should be understood to refer to blocks of material, of any suitable geometry, that are contained, at least partially, within an antenna in accordance with embodiments of the present disclosure.
The fields of the fundamental He11 mode will concentrate toward the center of a DRA, also referred to as central portion, if it has a core component comprising one or more cores of relatively higher dielectric constant material. This action of the fields increases fundamental-mode bandwidth by constraining higher frequency energy to the core dielectric 400, as long as the guide wavelength remains greater than the core 400 diameter. When the guide wavelength drops below the core 400 diameter, higher order modes become possible. In principle, adding a properly designed core dielectric 400 doubles the operating bandwidth and enables a two-layer antenna to operate with predominant, fundamental mode fields over a 4:1 frequency range.
The following points address modifications and variations to the single layer wedge radiator that could also be applied to multi-layer instantiations.
The TEM mode created in the feed region 320 effectively excites the fundamental He11 mode when the latter is tightly coupled to the dielectric 350 body of the wedge radiator, which occurs at higher frequencies in the operating band. At lower frequencies, the He11 mode is more loosely coupled to the dielectric 350 body of the wedge radiator and greater mode mismatch is produced at the end of the microstrip feed 302. This mechanism results in reflections in the microstrip feed 302 and the lower limit of the operating band.
In a similar vein,
To this point in the description of the wedge antenna, the polarization of the radiated field has been normal to a conducting ground plane 300. Applications exist that would benefit from a polarization perpendicular to this.
Lastly,
In such embodiments, the higher dielectric constant 400 of the embedded, relatively smaller wedge antenna creates a material discontinuity at its interface with the relatively larger host wedge antenna that supports an electromagnetic wave mode along the guide section 322′ and radiation taper region 324′, in the same manner as the air to dielectric boundary of the host wedge antenna. The embedded wedge antenna of such embodiments operates higher in frequency than the host wedge antenna, given the first order principle that antenna operation scales inversely with size (i.e. mode excitation and conduction are dependent on the antenna's geometry, relative to operating wavelength).
Notably, the frequency increase of the embedded wedge antenna does not scale in a 1:1 manner with size because the higher dielectric makeup of the embedded wedge 400 and its non-air boundary environment, the portion of the guide section 322 and radiation taper region 324 of the host wedge antenna adjacent to the embedded wedge antenna, allows a longer wavelength (e.g. a lower frequency) to exist on the guide section 322′ and radiation taper region 324′ of the embedded wedge antenna than would be possible with the host dielectric material 350 and its air boundary environment. For at least these reasons, the embedded wedge antenna is able to operate over a range of frequencies that overlaps with the host wedge antenna, with some part of the frequency range above that of the host wedge antenna's own range and some within that range.
Furthermore, the embodiment of the host wedge antenna shown in
In embodiments, different feed lines excite the host wedge connector 800 and the embedded wedge connector 802 at different frequencies.
In embodiments, a diplexer 806 is used to take a single feed line, in embodiments a broadband feed line, and divert higher frequency signals on it to the embedded wedge antenna's underside connection 802 and lower frequency signals to the host wedge connector's underside connection 800. In this way, the embedded wedge embodiment of
The foregoing description of the embodiments of the present disclosure has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the present disclosure to the precise form disclosed. Many modifications and variations are possible in light of this disclosure. It is intended that the scope of the present disclosure be limited not by this detailed description, but rather by the claims appended hereto.
A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made without departing from the scope of the disclosure. Although operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results.
