Communications: radio wave antennas – Antennas – Slot type
Reexamination Certificate
2002-05-14
2003-11-04
Ho, Tan (Department: 2821)
Communications: radio wave antennas
Antennas
Slot type
C343S767000
Reexamination Certificate
active
06642898
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates to a fractal cross slot antenna, and more particularly to a fractal cross slot antenna having reduced size, and bandwidth enhancement with a small slot width. When arrayed these features enable reduced element-to-element coupling.
BACKGROUND OF THE INVENTION
The Global Positioning System (GPS) has begun to permeate every aspect of the military and commercial sectors, with new applications being proposed each day. For the military, GPS has become a significant, enabling technology for the present and future war fighter. This technology is becoming part of almost every aspect of the military and is forming the foundation for new paradigms in wartime tactics. As a result, the U.S. military is increasingly utilizing GPS.
There are a number of challenges associated with designing and producing good antenna elements and arrays for military GPS and commercial applications. Size, performance, cost, and weight are all generally significant issues when designing for a military application (war fighter, aircraft, submarine, ship, etc.). When working with antennas, these requirements can be mutually exclusive. For instance, optimum antenna performance is predicated upon a given antenna size and many techniques used to reduce the size of the antenna require a trade-off of some, or all, of other antenna requirements.
With proliferation of GPS, and the desire to outfit more and varied types of platforms, comes a need for small, low cost, lightweight GPS antenna elements and conformal arrays. In order to produce a low profile, reduced size, conformal GPS array, there is needed small, slim elements that can be spaced less than ½ wavelength apart within an array without a significant degradation in individual element performance. These requirements limit the element type options, and often the possible array configurations.
Most existing GPS array designs utilize microstrip patch antenna elements. These elements are attractive because of relatively simple designs that exhibit a low profile, and have well understood performance characteristics. Often these patch elements, and associated arrays, are fabricated using expensive microwave substrate materials such as Duroids (PTFE), Alumina, and TMM. While these materials provide excellent low loss mediums, they can add significant cost and weight to the final design. In addition, the narrow band (High Q) response of the patches coupled with material and manufacturing tolerances can lead to elevated element and array costs.
One element option having a low profile, low cost, light weight as an alternative to the patch element is the cross slot. While the cross slot tends to be overlooked because of its relatively directive radiation pattern, the cross slot provides one of the few conformal alternatives to the patch. A more directive radiation pattern may prove to be a benefit for the auxiliary elements in a reduced size (smaller than optimal electrical size) Controlled Reception Pattern Antenna (CRPA) array. More cross slot elements can be packed closer together without excessive element-to-element coupling. In addition, the cross slot has the benefit of allowing the elements to be somewhat “interleaved”—which further aids in “packing” the elements within the array. However, challenges with the cross slot design still exist. One significant challenge is the difficulty in reducing the size of the element with dielectric loading and still maintain adequate feed-slot coupling.
The most common way to reduce the size of an element operating at high RF or microwave frequencies is to load it with a material that has a high permittivity or dielectric constant. This dielectric “loading” reduces the propagation velocity for a wave in that medium, and consequentially, the element's effective electrical length. The basic relationship between the wavelength in the dielectric (&lgr;
d
) and the wavelength in air (&lgr;
O
) is given by equation (1).
λ
d
:=
λ
o
ϵ
eff
(
1
)
Where (&egr;
eff
) is the effective relative dielectric constant—which takes into account the dielectric constant of the material and the associated electromagnetic field distribution.
While dielectric loading can effectively reduce the size of the element, it does come at a price. One must consider the changes in electrical properties associated with a given amount of dielectric loading. At a minimum, dielectric loading reduces the bandwidth and efficiency of an antenna (as well as adding weight and cost). The amount of bandwidth and efficiency lost will depend upon the material properties of the dielectric chosen, and the amount of reduction attempted. For very narrow band elements, such as microstrip patches, the loss of bandwidth coupled with manufacturing and material tolerances can be a real production problem. For this reason, a broadband, reduced size element that requires no (or less) dielectric loading could be a real plus.
Published studies describe how fractal concepts can be applied to antenna elements as a means to reduce the effective (tip-to-tip) length of elements, alter the antenna input impedance, and/or enhance antenna bandwidth without a significant reduction in element performance. Conceptually, the fractal “bending” facilitates a more efficient “packing” of the conductor and gives rise to a distributed reactive loading.
When an antenna element is placed within a multiple element array, the element performance will be altered due to the presence of the other elements. This alteration, which is seldom for the better, can include perturbations in the current distribution and radiated field of an element, as well as a significant change in the input impedance of the element. This element interaction is generally characterized by measuring how much of the signal of one element is coupled into adjacent elements. This quantity, termed mutual coupling, gives an indication of how much the performance of an element will be affected by the presence of the adjacent elements. As the mutual coupling increases, the performance of the elements and an array will steadily degrade.
Typically, elements within an array are spaced at least ½ wavelength apart. There are a number of reasons for this spacing. First, and most basic, most resonant elements are close to ½ wavelength in size. If two adjacent elements are put closer than the size of an element, they will physically touch. The second is that even if the element is made smaller such that it does not physically touch and can be moved closer, the mutual coupling between two adjacent elements increases as the spacing decreases. Element-to-element spacing of ½ wavelength or greater tends to provide acceptable coupling levels in most designs. While somewhat design dependent, coupling values of −15 to −20 dB or better are preferred.
Fractal antenna elements might in some cases aid in the reduction of mutual coupling by reducing the element size and, in the case of the fractal slot, by confining the element fields to a narrow slot width. Gianvittorio and Rahmat-Samii (J. P. Gianvittorio and Yahya Rahmat-Samii, “Fractal Loop Elements in Phased Array Antennas: Reduced Mutual Coupling and Tighter Packing”, IEEE, 2000) show how a 5-element array of small fractal loop elements could be used to reduce the mutual coupling effects to facilitate a larger scan volume. It is also possible that in certain cases the meandering of the fractal elements may provide a form of “random” element clocking, thus contributing to lower mutual coupling.
SUMMARY OF THE INVENTION
The single slot type of antenna is a variation of the basic dipole antenna. Each side of the slot acts as one node of an elementary dipole. The length and separation dimensions of the slot are selected to maximize performance (fraction of a wavelength).
A fractal cross slot antenna has two orthogonal intersecting fractal crossed slots in a cavity backed conductive element where each leg of each slot is excited by an RF signal from a feed providing four RF inputs of 0°, 90°
Baker & Botts L.L.P.
Ho Tan
Raytheon Company
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