Patent Grant
7791552
The field of the invention is in antennas and, in particular, in the field of reflectarray antennas used for communication with earth-orbiting satellites.
The reflectarray is an alternative to directly-radiating phased array antennas and promises higher efficiency at reduced cost. A key advantage of reflectarray antennas over conventional phased arrays is elimination of the complex beam-forming manifold and costly transmit/receive modules. The reflectarray is also reciprocal—the same aperture can be used for transmit and receive functions. In 1963, Berry, Malech and Kennedy introduced this new class of antennas that utilized an array of elementary antennas as a reflecting surface.
In 1976, Phelan patented (U.S. Pat. No. 3,949,407) a scanning reflectarray based on interleaved Archimedian spiral antennas. Spiral arms were interconnected with diode switches. The spirals are inherently circularly polarized over a broad bandwidth. (i.e., the far-field phase shift from a circularly polarized radiator is proportional to the apparent physical rotation of the radiator.)
In 1978 Malagisi proposed a microstrip reflectarray. In a microstrip reflectarray, stubs aligned with the desired polarization direction and of varying length are attached to the elements to effect phase shift. Incident energy from the primary feed propagates down the stub, where it reflects from the open (or short) end, and re-radiates with a delay corresponding to twice the electrical length of the stub.
A circularly polarized microstrip reflectarray with a 55% efficiency was reported by Huang and Pogorzelsk in an article entitled Ka-Band Microstrip Reflectarray with Elements Having Variable Rotation Angles, IEEE Transactions On Antennas and Propagation, Vol. 46, No. 5, May 1998. The antenna used square patches with identical stubs but varying rotation angles. Huang discloses a means of achieving cophasal far-field radiation for a circularly polarized microstrip relfectarray with elements having variable rotation angles. Two Ka-band half-meter microstrip reflectarrays were fabricated and tested. One of the arrays was of conventional design having identical patches with variable length microstrip phase delay lines attached. The other array had identical square patches with identical microstrip delay lines but different element rotation angles. The element with variable rotation angles resulted in better performance according to Huang.
In 2000, Romanofsky and Miranda disclosed a scanning reflectarray antenna based on thin-film ferroelectric phase shifters. None of these technologies provided a practical or cost effective means to replace parabolic reflectors intended for communications with geostationary satellites. The current state-of-practice is to use a solid parabolic reflector which must be physically pointed directly at the satellite in order to establish a communications link.
U.S. Pat. No. 6,081,235 to Romanofsky et al. disclosed a corrugated feed horn attached to nonmetallic struts situated at the virtual focus of the antenna. Further, the '235 patent to Romanofsky et al. states that “[t]he incident circularly polarized signal is absorbed by each element of the relectarray, routed through the stubs, which are in turn connected to the phase shifters, and re-radiated with a phase shift equal to twice the electrical length of the stubs-coupled electrical lines arrangement. By varying the bias across the coupled lines of each element, the appropriate phase shift can be attained, for electronic scanning without any physical movement of the antenna to produce the desired beam steering.” The row-column steering concept is a way to cut manufacturing cost but limits field of view. In the '235 patent the cellular array compensates for the spherical phase from the feed by tuning ferroelectric phase shifters.
U.S. Pat. No. 6,384,787 to Kim et al. discloses a flat reflectarray antenna utilizing a polarization twist function and predetermined phase shifts to provide a directed narrow beamwidth signal as set forth in col. 1 lns. 5-8. It is apparent that Kim et al. does not apply to circular polarization, cellular implementation, or thick, high dielectric constant substrates.
Several concepts for reflectarrays have been proposed but the usual context has been as a replacement for a parabolic dish that is mechanically pointed to a target or a as a competitor to directly radiating Gallium Arsendie Monolithic Microwave integrated Circuit phased arrays.
A reflectarray comprises a flat surface with diameter D, containing M×N integrated phase shifters (i.e. delay transmission lines) and M×N patch radiators with inter-element separation d, that is illuminated by a single feed at a virtual focus located a distance F from the surface such that F/D≈1. This value of F/D is a reasonable compromise between feed gain (and blockage) for proper illumination and modulo 2π effects. A priori settings of all phase shifters (i.e. delay transmission lines) are used to compensate for the spherical wave-front from the feed. The computer code calculates these compensation factors based on measured and/or theoretical feed information. That is, in order for the reflectarray to emulate a parabolic surface, the phase shifters are adjusted to compensate for the increasing path length from the aperture center towards the perimeter. The modulated signal from the feed passes through the reflect-mode phase shifters (i.e. delay transmission lines) and is re-radiated as a focused beam in essentially any preferred direction in the hemisphere in front of the antenna as in a conventional phased array. Of course the physics insofar as inter-element spacing, mutual coupling, scan loss, etc. is concerned is the same as for a conventional array that uses a transmission line manifold to distribute the signal among the M×N elements.
The actual field in beam direction Uo consists of the desired re-radiated field from the patch elements, scattered fields from the ground plane and phase shifters, and possibly a direct field from the feed.
For example, a radar cross-sectional measurement of a 208 passive element was made to determine the E-field for a non-optimal selection of a dielectric constant and thickness. The 208 passive element reflectarray was constructed using a non-optimal selection of a 0.79 mm thick substrate with a dielectric constant of 2.2. Microstrip π radian delay lines on every other passive element were oriented such that they would be sensitive only to vertical polarization. The array obverse (patch side) was designed to place main beams at ±30 degrees at 19 GHz. Undesirable scattered energy from the ground plane at boresight was nearly as prominent as the desired beams (at ±30°) because of the non-optimal selection of a dielectric constant. The image of the feed will be projected normal to the reflectarray surface because of scattering, primarily from the ground plane. The array reverse (ground plane only) indicated the image pattern of the feed horn.
In practice, the aperture gain must be much greater than the feed gain to mitigate this effect. Reduced cross-polarization is achieved by choosing an appropriate dielectric constant and thickness of the substrate material equal to the guided wavelength divided by four such that the cross-polarization scattered from the elemental radiators on the front surface interferes destructively with the cross-polarized signal reflected from the ground plane on the back surface.
The program was written in MathCAD and accepts a cellular reflectarray pointing direction (θ and φ) as inputs as well as a file containing measured phase data from the microwave feed horn. The code calculates the a priori settings of the passive (transmission line) phase shifters to compensate for the spherical distortion of the feed. (i.e., energy from the feed illuminates the middle of the reflectarray prior to the rim of the reflectarray). This process causes the cellular reflectarray to emulate a conventional parabolic reflector. The code then calculates the additional incremental delay required for each patch (or alternatively radiating element) in order to form a cophasal beam in essentially any preferred direction in the hemisphere in front of the array. A matrix corresponding to the individual M×N elements of the cellular reflectarray such that each entry is the associated delay in degrees is produced. A matrix corresponding to the actual elemental M×N transmission line physical lengths is also produced. The actual number of elements is truncated for a practical circular aperture of diameter D inscribed inside the rectangular aperture defined by M×N
A method of manufacturing a cellular reflectarray antenna is disclosed and claimed. The reflectarray antenna is arranged in an m by n matrix of passive elements. Each of the elements has a delay transmission line associated therewith which contributes to the formation of a narrow cophasal beam. The method of manufacturing the antenna includes the step of determining a delay for each of the m by n elements of the cellular reflectarray antenna. The step of determining a delay for each of the in by n matrix of elements includes several sub-steps, namely, determining the longitude and latitude of the location in which the reflectarray antenna will operate, determining elevation and azimuth angles of the reflectarray with respect to an orbiting satellite, converting the elevation and azimuth angles to spherical, Cartesian and then a rotated coordinate system to obtain theta (θ) and phi (φ), converting theta (θ) and phi (φ) to theta0 (θ0) and phi0 (φ0) expressed as radians, determining Δβm,n for a given ρ equal to d/λ for a specific array where d is the inter-element spacing and λ is the wavelength, determining Δφm,n for a given radius from the central element and/or from measured data from the feed horn, and, determining a delay for each of said m by n matrix of elements as a function of Δβm,n and Δφm,n.. An additional step of converting the delay calculated in degrees to a stub line length (inches or millimeters) is also desirable so that the relectarray can be easily manufactured using printed circuit board techniques including photolithography.
A cellular reflectarray antenna for communicating with a satellite wherein a plurality of printed passive antenna elements are arranged in an m by n matrix such that each of the printed passive elements includes a phase delay φm,n is disclosed and the phase delay is accomplished by adding stubs of sufficient length and geometry for the particular application. The delay lines, sometimes referred to as stubs, are oriented in the space between elements of the matrix. The phase delay is φm,n,←mod(phasem,n,,360), where phasem,n←−360+Δβm,n·(180÷π)−Δφm,n., where Δβm,n:=mod [−2·π·ρ·[m·(sin(θo)·cos(φo))+n·(sin(θo)·sin(φo))],2·π] and where Δφm,n is selected from the group of a mathematical function of the radius from a central printed passive element of the array (look-up table) and/or it is selected from measured data. The phase delay φm,n is a delay line emanating from said printed passive element and it has a distinct length and width depending on the material used and the frequency of operation of the array.
It is an object of the present invention to enable use of a flat reflectarray having a plurality of passive elements thereon with specific delay lines interconnected to each of the passive elements such that the delay lines provide spherical wave front compensation from the feed horn and pointing vector compensation enabling the manufacture of a specific reflectarray for an area at or within a specified distance from a given longitude and latitude.
It is an object of the present invention that the delay lines provide spherical wave front compensation where Δφm,n is determined and selected from the group of a mathematical function of the radius from a central printed passive element of the array (i.e., a look-up table) and/or it is selected from measured data.
It is an object of the present invention that the delay lines provide spherical wave front compensation where Δφm,n is determined by a mathematical function dependent on the radius from a central printed passive element of the array, the frequency of operation, and the distance of the feed horn (i.e., a look-up table).
It is an object of the present invention that the delay lines provide spherical wave front compensation where Δφm,n is determined by the combination of a mathematical function dependent on the radius from a central printed passive element of the array, the frequency of operation, and the distance of the feed horn (i.e., a look-up table) and interpolated measured data.
It is an object of the present invention that the delay lines provide pointing vector delay calculations, namely, Δβm,n:=mod [−2·π·ρ·[m·(sin(θo)·cos(φo))+n·(sin(θo)·sin(φo))],2·π], where ρ is d/λ (d is inter-element spacing and λ is the wavelength at operation), and (θ0), (φ0) are pointing angles from the specific reflectarray location to an orbiting satellite.
It is an object of the present invention to provide a bidirectional dual broadband television and internet reflectarray which is: unobtrusive in use, easily and quickly installed through simple orientation due north and parallel to the earth, inexpensive, and not removable from the cell in which it is originally installed.
It is an object to manufacture a relectarray antenna having a plurality of antenna elements arranged in a matrix of elements ranging from a small M×N matrix to a matrix having greater than 10,000 elements.
It is an object of the present invention to use high impedance microstrip lines printed on the same substrate as the radiators/patches and the microstrip delay lines are in intimate contact with the radiators/patches, however, other types of transmission lines may be used such as coplanar or suspended striplines.
These and other objects of the invention will be better understood when reference is made to the Brief Description of the Drawing, Description of the Invention and Claims which are set forth below.
The drawings will best be understood by referring to the Description of the Invention and Claims which follow hereinbelow.
The cellular reflectarray antenna is intended to replace conventional parabolic reflectors that must be physically aligned to a particular satellite in geostationary orbit. Specifically, the cellular reflectarray antenna is designed for a certain geographic location defined by latitude and longitude that is called a “cell”. A particular cell may occupy approximately 1,500 square miles. Other cell sizes are specifically contemplated herein and may be necessary for high±latitudes. The cellular reflectarray antenna designed for a particular cell is simply positioned such that an index aligns to magnetic North and the antenna surface is level (parallel to the level ground). A given cellular reflectarray antenna will not operate in any other cell because the delay lines for the individual elements are specific to that cell.
The specific design and fabrication of the reflectarrray for a specific latitude and longitude (i.e. a zip code) inherently prevents pirating dish receiver systems since the antenna will only operate for the latitude and longitude for which is was designed. That is, the site specific antenna thwarts relocation of the system for the purpose of avoiding subscription fees.
The design avoids the need for a highly skilled installer to mechanically point the antenna. The technique also offers an inherent benefit since the equipment will not operate outside of its designed cell space. Next generation “Direct TV” markets are expected to operate at Ka-band frequencies and provide asymmetric duplex communications to enable very wideband (e.g. MBPS) internet access in addition to conventional or high definition television programming.
Geostationary satellites occupying several orbital slots near 101 degrees West will provide service to North America. Subscribers to these new services will require ground terminals with significantly larger apertures than had been used previously for Ka-band Direct TV, which provided only downlink entertainment programming. The state-of-the-art technology for these consumer ground stations is a parabolic reflector antenna system, colloquially referred to as a dish antenna system. To meet link requirements these parabolic reflectors will need to be at least 26 inches in diameter. At 29 GHz the corresponding beamwidth is about 0.9 degrees. The industry has several legitimate concerns with the current approach to subscriber ground stations. The techniques described herein are not limited to a particular continent or application and may be used in any geographic zone and with any Geostationary satellite.
Consumers may be reluctant to install unsightly and bulky antenna systems on their properties. There is no way for the state-of-the-art technology to blend into landscapes or rooflines. Because of the narrow beamwidth, dish alignment will be particularly difficult and necessitate that a highly trained technician install the reflector perhaps from a difficult location like a roof top. Specialized equipment might be required for alignment. At even moderately Northern latitudes the dish will be pointed at acute angles from Zenith. Wind loading and wind gusts are likely to induce enough vibration to misalign the antenna beam and cause signal loss. Again this problem arises because of the narrow beamwidth.
Consequently, the Direct TV and satellite industry desires subscriber ground terminals that are: aesthetically pleasing, easily aligned such that a typical consumer can install his or her own antenna system, and flat such that wind loading is no longer an issue.
A given reflectarray supplied to a subscriber contains an index indicating how to align (point) the reflectarray to magnetic North. The subscriber requires only knowledge of magnetic North from his or her location. This knowledge is satisfied with a simple compass and the reflectarray antenna is aligned accordingly therewith. The only orientation requirement is that the reflectarray is level (i.e. parallel to the ground). Transmission lines integrated with the elemental radiators are used to induce circular polarization and provide the proper electrical delay to achieve the required phase shift for that element to contribute effectively to forming a collimated antenna beam in the direction of the geostationary satellite. If transmission lines are affixed to orthogonal edges of an elemental radiator and one line is electrically 90 degrees longer than the other, the reflected signal from the feed will be polarized in the same sense as the feed. The signal scattered from the elemental radiators and ground plane will be oppositely polarized by virtue of the reversal of propagation direction.
The cellular reflectarray can transmit and receive circular or linear polarization. If one side of the passive element has a delay 90° longer than the orthogonal side the patch will radiate the same sense circular polarization as the feed. If the orthogonal edges have the same delay length then the re-radiated signal will be oppositely polarized. A conventional rectangular waveguide can be used at the feed if only linear polarization is required and this means that a transmission line stub may be attached to one side of the patch in the “x” direction for horizontal polarization or in the “y” direction for vertical polarization
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Consider the E-field pattern shown in
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In practice, the aperture gain must be much greater than the feed gain to mitigate this effect. The solution to the problem illustrated and described in connection with
The software code/process accepts a subscriber's zip code as input and automatically generates the appropriate phase shifter settings of each reflectarray elemental radiator so that the antenna beam is directed to the appropriate satellite for that subscriber's geographic location. The code generates time delays for each passive element of the array. First, a spherical wave front phase delay, Δφm,n., determined as defined below for each element m,n. In the example given below the inter-element spacing is 0.296 inches which is λ/2 and corresponds to a frequency of 19.95 GHz. Next, the pointing vector phase delay, Δβm,n,, for each element m,n is determined by the Δβm,n, expression below where m,n are elements, ρ is d (inter-element spacing) divided by λ, and θ and φ are pointing angles. Use of the look up table for Δφm,n involves a given element m, n position and an inter-element spacing constant. φm,n can be determined from a look-up table created mathematically (i.e., from calculations) based on the geometry of the feed and its spacing from the reflectarray as well as the size of the array, inter-element spacing and the frequency of operation. Alternatively the look-up table can be modified by interpolating transformed measured data which forces the phase delay to be zero at the central element. All other elements are transformed as well. Still, alternatively, purely transformed measured data comprises the look-up data or a combination of the calculated look-up data and the interpolated transformed measured data may be used. The delay is determined as given below and elsewhere herein and is always between 0° and −360°. Here ρ corresponds to an element's radial distance from the central element. For each element, ρ is calculated and compared to the radial distance range at each category of the lookup table. This method serves to cluster the elements into annular bands wherein the elements grouped into a given band are nominally within plus or minus a selected deviation from the middle of the annulus band, for example, plus or minus 25 degrees.
Δβm,n:=mod [−2·π·ρ·[m·(sin(θo)·cos(φo))+n·(sin(θo)·sin(φo))],2·π]
Finally, the delay in degrees or radians is converted into a length. The length is then printed along with the elements m,n which form a reflectarray capable of cophasal transmission and reception of circularly or linearly polarized electromagnetic waves. The number “215.5” in the expression is a constant for a given, guided wavelength, substrate dielectric constant and thickness. The 215.5 converts delay (phase shift) to a physical line length based on frequency and substrate dielectric constant and thickness. The example given above is just an example and in practice there will be on constants derived for different applications. The “effective” dielectric constant (some electric field is in air and some in the substrate) is designated as ∈e. The wavelength λ is the speed of light “c” divided by [frequency (f) times √∈e. The line length, “1” is then delay/360 times lambda.
The invention has been set forth by way of example only and those skilled in the art will readily recognize that many changes may be made to the invention without departing from the spirit and scope of the claims which are set forth below.
The invention described herein was made by an employee of the United States Government, and may be manufactured and used by the government for government purposes without the payment of any royalties therein and therefor.
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