A slotted waveguide array (SWA) antenna is provided for emitting electromagnetic radiation. The antenna includes a base, a pair of brackets, a pair of spars, a plurality of waveguides, and a radome. The base provides longitudinal and lateral support for the antenna on a platform. The brackets are disposed at longitudinally opposite ends on the base. The spars connect the brackets and are disposed at laterally opposite ends of the base. The waveguides are disposed in stagger array. Each waveguide has a pair of broad walls and a pair of side walls that share longitudinal edges. The broad wall is wider than the side wall. The broad wall includes elliptical slots penetrating each waveguide. The stagger array arranges first and second waveguides share respective first and second longitudinal edges. The radome covers the waveguides and connects to the pair of spars.

BACKGROUND

The invention relates generally to antennas. In particular, the invention relates to high power microwave (HPM) antennas.

Efficient and directive radiation of high power radio frequency (RF) from an antenna is one of the most critical stages of an HPM system. Most recent conventional high power microwave (HPM) antennas comprise reflectors, horns, and horn arrays: deep and heavy systems unsuitable for small volumes and conforming to curved surfaces. With the constant push to package HPM systems into more compact packages, a need naturally emerged for an effective, conformal, and shallow HPM antenna.

SUMMARY

Conventional waveguide antennas yield disadvantages addressed by various exemplary embodiments of the present invention. In particular, various exemplary embodiments provide a slotted waveguide array (SWA) antenna emitting electromagnetic radiation. The antenna includes a base, a pair of brackets, a pair of spars, a plurality of waveguides, and a radome. The base provides longitudinal and lateral support for the antenna on a platform. The brackets are disposed at longitudinally opposite ends on the base. The spars connect the brackets and are disposed at laterally opposite ends of the base.

The waveguides are disposed in stagger array. Each waveguide has a pair of broad walls and a pair of side walls that share longitudinal edges. The broad wall is wider than a side wall and includes elliptical slots penetrating each waveguide. The stagger array arranges first and second waveguides share respective first and second edges. The radome covers the waveguides and connects to the pair of spars. Other various embodiments alternatively or additionally provide for the side wall featuring additional elliptical slots.

DETAILED DESCRIPTION

The disclosure generally employs quantity units with the following abbreviations: length in meters (m) or inches (″), mass in grams (g), time in seconds (s), angles in degrees (°), force in newtons (N), temperature in kelvins (K), energy in joules (J), power in watts (W), signal strength in decibels (dB) and frequencies in hertz (Hz). Supplemental measures can be derived from these, such as density in grams-per-cubic-centimeters (g/cm3), moment of inertia in gram-square-centimeters (kg-m2), electric field (voltage potential) field stress in kilovolts-per-centimeter (kV/cm) and the like.

The objective of this disclosure is to provide an antenna design to radiate at peak power levels of ≥1 GW with low dispersion and high gain from a shallow volume and curved aperture. The frequencies include microwave, so that the antenna to be described herein corresponds to emissions in the microwave portion of the electromagnetic (EM) spectrum. The antenna is designed to be conformal to any arbitrary non-planar shape. Exemplary embodiments provide an antenna that radiates linearly polarized high power microwave (HPM) pulses into free space from a microwave source through a combination of broad wall and side wall slots cut into the surfaces of waveguides while conforming to a curved aperture. The antenna array may also be enclosed in a radio frequency (RE) transparent radome and dielectrically loaded.

The present disclosure teaches a wideband slotted waveguide array (SWA) antenna comprising four rectangular waveguides rotated off-plane to be conformal to a curved aperture. A half-circle arc cylindrical Quonset hut geometry constitutes an exemplary embodiment of this design. This off-plane rotation exposes a side wall and broad wall of each waveguide, enabling both to potentially radiate. The emission frequencies correspond to the microwave spectrum with wavelengths between 1 mm (300 GHz) and 1 m (300 MHz).

Carefully positioned slots of any desired shape are cut into each of these walls. Fed at one end and shorted on the other end, the exemplary antenna employs a combination of broad and side wall slots to effectively radiate high power microwaves in the broadside direction. The development of the curved aperture SWA began in 2021 as part of a Naval Surface Warfare Center Dahlgren Division (NSWCDD) effort to develop a very shallow compact HPM antenna conformal to a curved surface.

FIG.1shows an isometric assembly view100of a Quonset hut geometry for an exemplary slotted waveguide array (SWA) antenna110. A Cartesian coordinate system or compass rose120exhibits orientation—in particular X for longitudinal, Y for lateral and Z for azimuthal. The mechanically passive antenna110has a geometry of a half-cylinder with a half-circle arc cross-section, with the axial centerline corresponding to the longitudinal (X) direction. The overall physical length of the antenna110is 150 cm and the width is 56 cm. The antenna110weighs about 180 kg when fully assembled and dielectrically loaded. This configuration is merely exemplary and not limiting.

FIG.2shows an isometric view200of an SWA panel210comprising four slotted waveguides220arranged abreast. Each waveguide220has a pair of planar broad walls (or faces)230and a pair of side walls (or longitudinal edges)240that define a channel250. The waveguides220are preferably composed of aluminum and exhibit the shape of transverse electric one-ten (TE10) mode hollow elongated rectangular tubes. Alternatively, the waveguides220can be manufactured from macromer resin such as for stereolithography and electroplated with a conductive metal, such as nickel.

Staggered elliptical slots260are disposed along the length of the planar broad walls230, penetrating through the waveguide's thickness 0.125″. The exemplary waveguide220is substantially flat (i.e., having minimal curvature). The slots260emit EM waves to directionally propagate an EM signal. The broad walls230and the side walls240extend longitudinally parallel to the X axis for the antenna110.

Artisans of ordinary skill will recognize dielectrical loading entails filling the volume within the radome310with a solid or liquid non-conductive material. For example, transformer oil can be used to raise the electric field levels the SWA antenna110can withstand without internal electrical breakdown or arcing. Artisans will also recognize that to reduce variations in antenna emission, edges are preferably smooth, including the slots260.

FIG.3shows an isometric exploded view300of components for the SWA antenna110. These components can be divided into sub-assemblies: a radome310, a waveguide composition320and a base mount330. The radome310includes a half-cylinder cover340supported internally by ribs350. The waveguide composition320includes a fore-and-aft pair of hemispherical brackets360, each with rectangular cavities365, and a port-and-starboard pair of spars370. The cover340extends to connect between the spars370. A plurality of fasteners375connect the brackets360to the waveguides220.

The base mount330comprises lateral stiffeners380that when paired form a support pad390. The base mount330rests on a floor or else a portable support platform and forms a longitudinal and lateral perimeter. The materials for the radome cover340and rib350are high density polyethylene (C2H4)n), but any low-loss plastic material would be suitable. The cover340constitutes a 0.125″ thick plastic sheet radially bent around the brackets360, and the ribs350are machined from bulk material. The stiffeners380and spars370are machined from bulk aluminum metal.

FIG.4shows a plan and quasi-isomeric views400of the waveguides220arranged as a panel210and a conformal stagger array410, such as in the waveguide composition320. The idea behind the curved aperture SWA antenna110was first conceived as an attempt to fit a panel210for a standard slotted waveguide array into an electrically small volume with curved surfaces. The waveguides220are arranged such that the linear corners of a side wall240of one waveguide220touch along a planar broad wall230of an adjacent waveguide220as an edge joint420.

At the apex of the array410, an adjacent pair of broad walls230connect together at their mutual edges. Each broad wall230has a broad normal vector430from which EM waves can radiate from its associated elliptical slots260. For reasons explained further, each side wall240has a side normal vector440from which associated elliptical slots450can radiate. The slots260and450penetrate their respective broad and side walls230and240normal to the wall surfaces. The slots260and450can be identical in size and shape.

FIGS.5A,5B,5C,5D and5Eshow isometric, plan and elevation views of the SWA antenna110with and without the cover340.FIG.5Aprovides an isomeric view of the SWA antenna110showing the cover340, brackets360and spars370.FIG.5Bprovides an isometric view of the antenna assembly510absent the cover340to reveal the waveguides220and ribs350.FIG.5Creveals an overhead plan view of the assembly510showing four waveguides220as custom inlets #1520, #2530, #3540and #4550ordered from top-to-bottom for this configuration.

FIG.5Dpresents an elevation view of the assembly510.FIG.5Epresents an underneath plan view of assembly510. The waveguides220(including #1520, #2530, #3540and #4550) are 4.54″ in length and 0.52″ in width with walls 0.125″ thick. The cover340is 4.66″ in length and 0.91″ in height. The ribs350and the stiffeners380are 0.038″ thick.

FIGS.6A,6B,6C and6Dshow elevation views600of waveguides220. The inlets have interior cross-section dimensions of 0.50″ by 0.125″. The numbered entities exhibit the same frame, but have distinct locations for their respective slots260and450. Each slot260is between 0.436″ and 0.45″ in length and 0.04″ in width. The interleaving slots260are separated from their centers by 0.55″ longitudinally and by 0.099″ laterally. The slots450are separated from their sensors by 1.10″. The slots260start their centers from the inlet (at the forward bracket360) for #1, #2, #3 and #4 at respective distances of 0.442″, 0.373″, 0.374″, and 0.442″. The slots450start their centers from the inlet for #1, #2, #3 and #4 at respective distances of 0.948″, 0.329″, 0.088″, and 0.040″. Artisans of ordinary skill will recognize that these dimensions are merely exemplary and not limiting.

FIG.7shows an elevation view700of the SWA antenna110featuring its geometric profile. The rectangular cavities365that receive the waveguides220are ordered clockwise from bottom left as #1, #2, #3 and #4 corresponding to the waveguides in view500. A hemispherical cutout710is visible as part of the aft bracket360and for scale has an exemplary hemispherical diameter of 0.65″.FIG.8shows a quasi-elevation views800of various waveguide compositions. These geometries are identified as triple810, quadruple820and quintuple830within a circular frame840. The triple geometry810features a curved waveguide850. The quadruple geometry820features the flat waveguide220. The quintuple geometry830also features a curved waveguide860, albeit narrower than the triple version waveguide850.

To conform this array410to a curved surface as circle frame840, the waveguides220could not be positioned in a standard two-dimensional plane configuration. Rotating the waveguides220with respect to one another to conform to the curved surface results in the normal vectors430of the broad walls230pointing in different directions, which has untoward directivity effects. To counteract this, elliptical slots450are cut into the newly exposed side walls240with side normal440to re-direct beams from each individual waveguide220back towards broad normal430.

The slots260and450are also carefully repositioned with respect to the longitudinal center line of the planar surfaces of the broad walls230and with respect to each other to account for the difference in mutual coupling from off-plane locations. The staggered dispositioning of slots260on the broad wall230facilitates concurrent radiation of the nearby slots450on the side wall240.

A curved aperture conformal slotted waveguide array (SWA) antenna110was simulated and optimized in commercially available computational electromagnetics (CEM) software using this disclosure's design methodology and fabricated from four dielectrically loaded rectangular half-height waveguides220for HPM applications. This exemplary antenna110possesses forty-eight elliptical slots260and450in total distributed among four waveguides220. These are distributed as eight slots260on each broad wall230and four slots450on each side wall240. Assuming a balanced in-phase input on all four rectangular ports at cavities365achieved with a four-way coaxial-to-waveguide coupler, the exemplary SWA antenna110achieves an impressive 65% aperture efficiency, which is comparable to a flat non-conformal slotted waveguide array, while occupying a much smaller confined space.

FIG.9shows a graphical view900of signal reflection plot. In particular, view900features an S-parameter plot of the S11(reflection) viewed at the coaxial input port of the coupler attached to the slotted waveguide array410. Frequency deviation910(percentage off-center) denotes the abscissa while S11signal920(dB) presents the ordinate. Data points930plotted can be connected by a curve940. The trace identifies two substantial decreases in signal of −14.6 dB at frequency minus one (−1) percent by arrow950and −16.1 dB at four (4) percent by arrow960. A local maximum indicated by arrow970is situated between these minimums of −9 dB at about one-and-a-half (1½) percent.

FIG.10shows a circular plot view1000of simulated two-dimensional (2D) radiation pattern in the H-plane1010and E-plane1020normalized to maximum gain within a radial domain1030. The H-plane plot1010features a first profile1040, while the E-plane plot1020features a second profile1050, both profiles1040and1050exhibiting a major lobe1060at 0°, a back lobe1070at 180° and side lobes1080. The major lobes1060are normalized at 0 dB. The 3 dB beam width is 15.0° in the H-plane plot1010and 36.0° in the E-plane plot1020. The back lobe1070is −12 dB down from the main lobe1060and the highest magnitude side lobes1080are −15 dB down.

The curved aperture SWA110has an 11.4% total 3 dB bandwidth with respect to maximum gain. The bandwidth is asymmetric, due to the lower cutoff frequency of the waveguide, enabling frequency tuning of 3.6% down and 7.8% up. As commonly observed in slotted waveguides, beam steering behavior naturally occurs with frequency tuning due to their dispersive properties. On the bottom end of the 3 dB bandwidth, the beam steers 8.5° back towards the feed end of the array and at the top of the bandwidth the beam steers 11° forward.

FIG.11shows an isogrid view1100of an elliptical finite element analysis (FEA) solution plot1110of the normalized electric field inside the highest stress slot. Lines1120of constant E-field show gradients in electric field stress.FIG.12shows an isogrid view1200of a rectilinear FEA solution plot1210of the normalized electric field inside the highest stress waveguide. Lines1220of constant E-field show gradients at a power level of 1 GW at the center frequency.

With a 1 GW continuous wave (CW) input at the center frequency, the maximum electric field stress approaches 160 kV/cm in the slots260and450and 125 kV/cm in the waveguides220. In air and under a static electric field these could be problematic values for electrical breakdown (threshold of ≥30 kV/cm), but the present dielectric load and a short pulse input are expected to be sufficient to prevent breakdown. In the environment outside of the radome's dielectric volume, the maximum E-field stress is expected to be 25 kV/cm, which is below the electric breakdown threshold in air.

FIG.13shows a graphical view1300of compressed ringdown style pulse and the resulting output pulse from the SWA antenna110, illustrating its dispersive properties. Time1310denotes the abscissa while amplitude1320denotes the ordinate, which is not to scale. A high frequency wave1330shown by solid line has modulation waveform1340shown by dash curve.

The input waveform1330and the resulting output waveform1340located eight wavelengths above the SWA antenna110are determined by a transient electromagnetic wave solution. The approximately four additional cycles in the output waveform1340before achieving peak voltage demonstrates the dispersion due to the waveguide nature of the SWA antenna110.

The curved aperture SWA110can also effectively radiate short pulse inputs as exhibited in plot view1300. While dispersive effects exist due to the waveguide structure of the antenna, the effects can be minimized in practice by reducing the total amount of dispersive media the pulse must travel through to reach the SWA antenna110.

Due to being a slotted waveguide array410fed from one end, this exemplary configuration also suffers from a non-zero turn-on time caused by the time required for the wavefront to travel from one end to the other and fully illuminate the waveguide220. Despite these detrimental effects, the curved aperture SWA antenna110is capable of radiating short pulses with low-to-moderate dispersion. The design for the exemplary SWA antenna110was fabricated, characterized at low power in an anechoic chamber, and tested at high power.

The final design produced by this process includes a coupling device for feeding the antenna110the input power as well as the radome310. Solid dielectric ribs350are included to structurally support the cover340. The ribs350have material removed in areas of close proximity to the radiating slots260to reduce chances of electrical breakdown and to provide additional paths for air to escape when filling the SWA antenna110with dielectric material.

The curved aperture SWA antenna110can be used as a transmitting or receiving antenna in a wide range of industries that utilize microwave radiation including but not necessarily limited to radar, communications, laboratory research and development (R&D), etc. The device has been instrumental in the success of a directed energy effort at NSWCDD. Further development of this effort may potentially lead to a program of record, resulting in a potential mass production of the device to fulfill design requirements. Additionally, the high power microwave source this antenna was designed for is early in its development and the exemplary SWA antenna may be beneficial for future applications.

The closest alternatives that are similar to the design of the exemplary curved aperture SWA are patents for high power microwave antennas, e.g., U.S. Pat. Nos. 7,535,428 and 10,103,448; as well as Chinese patents CN112086747B, CN103151620B, CN110148839B and CN114725687A. None of these disclose a non-flat aperture conformal gigawatt power handling slot array as a panel210, which as an exemplary geometry provides a novel technique for an SWA antenna110of high power microwave technology.

Additional modifications can be made to the waveguide geometry in an effort to further increase curved aperture conformity. For example, the broad walls230of the flat waveguide220can be warped into circular arc segments as arc channels850or860to match a circular cross-section frame840without perturbing the dominant TE10rectangular mode.

The waveguides220,850or860can be dielectrically loaded for power handling purposes and to reduce their size, enabling the number of waveguides220to be a tunable variable for designing the array based on wavelength and relative dielectric constant. The circular cross sections810,820and830clearly demonstrate the advantages of this design process, such as the extremely shallow profile leading to a minimal amount of volume consumed by the SWA antenna110. The volume consumed by the antenna110is low value, enabling the higher value space in the center permits inclusion of a potential HPM source, pulsed power driver, and auxiliary systems.

The curved aperture SWA antenna110chosen to be fabricated was designed for greater than 1 GW of peak power. This was achieved by loading the waveguides with and immersing them in a high strength liquid dielectric as well as the shaping of sharp edges and corners to suppress localized field enhancements.

The array410is encased in an RF transparent, structural, and weatherproof enclosure as the radome310. The SWA antenna110was carefully positioned inside the cover340to maximize the dielectric lensing effect for additional gain. The lensing is produced by the curved surface between dielectric and air, due to aperture shape and shift in relative dielectric constant.