The present invention is directed to a new category of portable Ku-band satellite antenna that offers lighter weight, reduced storage volume, and similar link performance compared to existing designs. To reduce the weight and storage volume, the invention's reflector is flat and assembled like puzzle pieces.
Ku-band very small aperture terminals (VSATs) are used extensively globally for transmitting and receiving narrowband and broadband data. VSAT antennas are often over 1 meter in diameter and heavy, inhibiting the ease of portability and storage. For these reasons, there is a demand for a lighter weight and reduced storage volume antenna, without any reduction in link performance.
Fresnel zoneplate (FZP) antennas are advantageous to traditional antennas because the surface has a phase shifting property that allows the antennas to be constructed flat. The FZP is also relatively inexpensive to manufacture and install. The transportability and high gain of FZPs make them ideal for use in VSATs. The present invention uses multiple steps within each Fresnel zone to maximize gain and antenna efficiency.
It is known in the art, that the phase step directly impacts FZP efficiency. A 2-step FZP has a phase correction of 180-degrees. This translates into 40% efficiency, which is 4 dB less gain. A large-aperture (40λ+) implementation of this was modeled in Ku-band and the bandwidth found to be approximately 9% regardless of the number of minor steps (2, 4, and 8). The efficiency of a stepped reflector design improves with the number of minor steps per Fresnel zone ring.
An FZP reflector has Fresnel zone rings divided into minor steps. At the outer radius of each zone ring, there is a major step that occurs from maximum thickness to minimum thickness and is equal to an odd multiple of (λ)(90°) at the center frequency of the design, where λ is the wavelength. The minor step is the incremental zone height between major steps. A 2-step FZP only has two zone heights, whereas an 8-step FZP has eight. The larger the number of steps, the more closely the Fresnel zone rings approaches a section of a smooth parabola and the more efficient it becomes. The minor step size determines the zone correction. The coarser the step size, the more phase error is introduced by each zone ring of the FZP. The gain efficiencies are summarized in the following table:
FZP reflectors are not limited to a Fresnel zone major step of 90° (λ/4). However, the major step must provide 180° of effective phase change for the bounce paths to sum in phase at the feed horn. Therefore, the major step can be (n*90), where n=1, 3, 5, 7 . . . , but any value of n greater than 1 results in a thicker and heavier structure.
Recently, a new technology based on slight deformations on a flat metallic surface was used to obtain a geometric model of a flat FZP reflector antenna. This approach uses smoothed segments of a parabolic curve instead of flat steps. Their implementation consists of an 11.8 GHz antenna only 3 zones×2 zones in extent (280 mm×210 mm, or 11λ×8λ). The bandwidth attained by this approach is 17.6% but this is likely because there are only a few Fresnel rings in the design.
The present invention produces greater antenna gains than what is known in the art. Further the FZP reflectors in the prior art are single-tuned and can cover a standard 500-MHz wide band segment at Ku-band. However, conducting VSAT operations requires a multiple-tuned FZP. VSATs require that the antenna function over two separate 500-MHz band segments: one for receiving (RX) and one for transmitting (TX). The current invention solves this problem by designing a dual-tuned (stagger-tuned) FZP, allowing the antenna to have gain peaks at both desired frequencies.
The present invention is a new category of portable Fresnel zone plate reflector antenna that offers lighter weight, reduced storage volume, and similar link performance compared to existing non-portable designs. To reduce the weight and storage volume, the invention's antenna reflector is flexible or foldable and can be assembled like puzzle pieces rather than rigid segmented reflector shapes.
The preferred embodiment of the current invention is an 8-step dual-tuned (stagger-tuned) FZP antenna with the feed point centered. Even with a diameter of 1-meter, the innovations of the present invention keep the thickness under 1-inch and additionally allows the antenna to be folded to approximately the size of a tissue box when stored and transported. The stagger-tuned FZP divides the FZP into 8 pie-shaped sections of 45-degrees each, alternating low and high band patterns to maintain radial symmetry, as further described below.
The invention achieves the desired gain at the RX band (centered at 11.95 GHz) and the TX band (centered at 14.25 GHz), overcoming the limitations of single-tuned antennas in the art. The FZP can be divided into other numbers of “pie” pieces and achieve the similar results as further described below. It is not necessary that the proportion of FZP aperture allocated to low and high bands be 50%-50%. When the proportion is changed, the angles of “pie” pieces must be altered to balance the RX and TX gain values.
Another alternate embodiment uses an offset feed design instead of prime focus but uses the same method of a stagger-tuned FZP reflectors. The offset feed horn design provides an advantage because less black body noise is coupled into the feed horn, thus improving signal to noise ratio of the incoming signal. Alternate embodiments of multiple-tuned designs are not limited to pie-shaped segments or a perfect circular outline. For example, a hexagonal implementation of the invention will also work. Additionally, three or more frequency channels can be implemented.
A better understanding of the present invention may be had from the drawings as follows:
As previously stated, the present invention is directed to a design concept for a new category of portable Ku-band satellite antenna that will offer lighter weight, reduced storage volume, and similar link performance compared to existing portable designs.
R
n={(2nFλ0/P)(nλ0/P)2}1/2 {Equation 1}
n=nth minor ring
Rn=nth minor ring step starting radius
F=Focal distance (distance from center of FZP to phase center of feed horn, meters)
λ0=Wavelength at center frequency of design (meters)
P=4 (for reflector case with λ/4 major phase step)
n
required=−8F/λ0+{(8F/λ0)2+64(router/λ0)2}1/2 {Equation 2}
router=Outer radius of the FZP
1-meter diameter FZP centered at 11.95 GHz with a focal length of 0.75 meters. Nrequired=48.26 (round up to 49) ring steps. Given that there are 8 rings per Fresnel zone, the number of Fresnel zones is 48.26/8≈6.0.
The last parameter that is needed is the height of the nth ring step. Since there are 8 step levels in each Fresnel zone, there are 7 step increments to achieve 8 levels. Each Fresnel zone increases in step height until the major phase step λ0/2 is reached at the 8th ring. After this ring, the next ring height resets to zero and the steps sequence repeats for next Fresnel zone. The step increment is therefore:
Δstep=λ0/2/7=λ0/14 {Equation 3}
For example, at 11.95 GHz, λ0 is 0.02508 m (˜25 mm) and Δ step is 0.003584 m (3.584 mm).
Although the results from the implementation of the preferred embodiment in
Although the preferred embodiment consists of two frequency channels, three or more channels can be implemented with this method (multiple-tuned design) with a corresponding drop in gain per the allocation of the FZP aperture at each frequency band.
For the VSAT design in
An alternate embodiment can be considered other than stagger-tuning two FZP designs into distinct regions. The zone radii defined in Equation 1 can be modulated against rotational angle to periodically vary the FZP center frequency. The result would appear as a wiggled zone pattern instead of circular zones of fixed radii. The same rules off symmetry apply: the number of wiggles per rotation should be 6 or greater.
While the principles of the disclosure have been described above in connection with specific methods, it is to be clearly understood that this description is made only by way of example and not as limitation on the scope of the disclosure. Whether now known or later discovered, there are countless other alternatives, variations and modifications of the many features of the various described and illustrated embodiments, both in the process and in the device characteristics, that will be evident to those of skill in the art after careful and discerning review of the foregoing descriptions, particularly if they are also able to review all of the various systems and methods that have been tried in the public domain or otherwise described in the prior art. All such alternatives, variations and modifications are contemplated to fall within the scope of the present invention.
Although the present invention has been described in terms of the foregoing preferred and alternative embodiments, these descriptions and embodiments have been provided by way of explanation of examples only, in order to facilitate understanding of the present invention. As such, the descriptions and embodiments are not to be construed as limiting the present invention, the scope of which is limited only by the claims of this and any related patent applications and any amendments thereto. With reference again to the figures, it should be understood that the graphical representation of the system is an exemplary reference to any number of devices that may be implemented by the present invention.