Orthogonally polarized dual-band printed circuit antenna employing radiating elements capacitively coupled to feedlines

Abstract: A dual polarized printed circuit antenna operating in dual frequency bands. A first array (3) of radiating elements radiates at a first frequency, and a second array (5) of radiating elements radiates at a second, different frequency. Separate power divider arrays (2, 4) are provided for each array of radiating elements, and the overall structure is provided in a stacked configuration.

Description: BACKGROUND OF THE INVENTION This invention relates to another improvement in a series of inventions developed by the present inventors relating to printed circuit antennas having their elements capacitively coupled to each other, and in particular, two antennas wherein the feed to the radiating elements is coupled capacitively, rather than directly. The first in this series of inventions, invented by one of the present inventors, resulted in U.S. Patent No. 4,761,654. An improvement to the antenna disclosed in that patent is described in EP-A2-0 271 458. The antenna described in the foregoing U.S. patent and European patent application permitted either linear or circular polarization to be achieved with a single feedline to the radiating elements. The antennas disclosed included a single array of radiating elements, and a single array of feedlines. One of the improvements which the inventors developed was to provide a structure whereby two layers of feedlines, and two layers of radiating elements could be provided in a single antenna, enabling orthogonally polarized signals to be generated, without interference between the two arrays, the later published U.S. Patent No. 4,929,959 describes such a structure. Having developed the dual-band orthogonally polarized antenna, various experiments have been conducted with different shapes of radiating elements, and antenna configurations. EP-A2-0 342 175 and corresponding later published U.S. Patent No. 4,926,189 disclose such an array employing gridded antenna elements. The work on dual polarized printed antennas resulted in the provision of an array which could operate in two senses of polarization, a lower array of the antenna being able basically to "see through" the upper array. The improvement represented by the present invention is to extend that concept. It is known from GB-A-2219143 and EP-A2-0342175 (U.S. Patent 4.926,189) to provide a five-layer printed circuit antenna structure, each higher layer being capacitively coupled to a preceding layer, to yield an antenna capable of radiating at two different senses of polarization. Further, it is known from US-A-4,450,449 and US-A-4,605,932 to provide a multiple-antenna structure which operates at different frequencies, but whose elements are not capacitively coupled to each other. The gain characteristics of the antenna structures disclosed in the last-mentioned U.S. patents are not specified. SUMMARY OF THE INVENTION In view of the foregoing, it is one object of the present invention to provide a high-performance, light weight, low-cost dual-band planar array. The inventors have determined that employing certain types of antenna elements for the upper and lower arrays enables operation at two different, distinct frequency bands from a single radiating array structure. According to the present invention a dual polarized printed circuit antenna comprising a ground plane, a first power divider array disposed over and capacitively coupled to said ground plane, a first array of radiating elements disposed over and capacitively coupled to said first power divider array, a second power divider array disposed over and capacitively coupled to said first array of radiating elements , and a second array of radiating elements disposed over and capacitively coupled to said second power divider array, characterized in that said first array of radiating elements comprises an array of radiating elements having a first size and being so configured as to operate within a first frequency band, and said second array of radiating elements comprises an array of radiating elements having a second size that is larger than said first size and being so configured as to operate within a second frequency band that is at least 1 GHz lower than said first frequency band, and wherein said second array of radiating elements have a gain that is at least 3.0 dB less than a gain of said first array of radiating elements throughout said first frequency band, and said first array of radiating elements have a gain that is at least 3.0 dB less than a gain of said second array of radiating elements throughout said second frequency band. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows an exploded view of the dual frequency antenna of the invention; and Figures 2-8 show graphs of the measured performance of a sixteen-element dual band array. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to Figure 1, the inventive structure, as described also in EP-A2-0 342 175 and in the later published U.S. Patents Nos. 4,929,959 and 4,926,189, comprises five layers. The first layer is a ground plane 1. The second layer is a high frequency power divider 2, with the individual power divider elements disposed at a first orientation. The next layer is an array of high frequency radiating elements 3. These three layers together define the first operating band array B1, in which layers 1 and 3 form the ground plane for the power divider 2. The operating frequency of the array is dictated by the dimensions of the radiating elements and the power distribution network. The array of high frequency elements 3 will have physically smaller radiating slots than those used in the low frequency array. The principal controlling factor in the resonant frequency of the slot is the outer dimension (radius or side) of the element. This dimension is inversely proportional to the operating frequency. As a rule of thumb, for a circularly-shaped element, the diameter is approximately one-half of the operating wavelength; for a square or rectangularly-shaped element, a side (longer side for a rectangle) is approximately one-half the operating wavelength. Those of working skill in this field will appreciate that the actual dimensions may vary somewhat, according to the earlier-stated prescriptions. The power divider 2 may consist of impedance transforming sections at the tee junctions where the power split is performed. These transforming sections typically are λ/4 in length, where λ refers to the wavelength at the operating frequency. The transformer length also will be inversely proportional to the operating frequency. Disposed above the high frequency elements 3 is a low frequency power divider array 4, with the individual power divider elements disposed orthogonally with respect to the elements of the power divider 2. Above the low frequency power divider 4 is a second array of radiating elements 5, these elements 5 being low frequency radiating elements. The layers 3-5 together form a second operating band array B2, wherein the layers 3 and 5 provide the ground plane for the power divider 4. The element designs in layers 3 and 5 are designed appropriately to minimize both radiation interaction between the lower and upper arrays, and coupling between the two power distribution networks. As discussed previously, the physical size of the elements in the layer 5 will determine the operating frequency. The elements of the low frequency array 5 will be larger than those of the high frequency array 3. Transformer sections within the low-frequency power divider network will be longer than those used in the high frequency divider, but otherwise the divider networks may be very similar in design. All of the layers 1-5 may be separated by any suitable dielectric, preferably air, for example by providing Nomex honeycomb between the layers. The structure depicted in Figure 1 shows the design and construction for a dual-band linearly polarized flat-plate array. Linear polarization is dictated by the radiating elements. Circular polarization may be generated by choosing the appropriate elements with perturbation segments as described, for example, in EP-A2-0 271 458. Later published U.S. Patent No. 4,929,959 also shows examples of such elements. The measured performance of a 16-element dual band linear array is depicted in Figures 2-8. For one sense of polarization, the band of interest is 11.7-12.2 GHz, and for the other, orthogonal sense of polarization, the band of interest is 14.0-14.5 GHz. Figure 2 shows the input return loss for both senses of polarization (in each instance, the input match is very good over a broad band, as can be seen from the figure). Figure 3 shows the corresponding radiation gain for each polarization. As shown in the Figure, both senses of polarization radiate very efficiently and over a broad band, and the radiation efficiency of each is comparable. Figure 4 shows the port-to-port or array network isolation. The isolation is sufficiently high to ensure that the two arrays are virtually decoupled, and operate as required in an independent manner. Figures 5-8 show a corresponding on axis swept cross polarization and radiation patterns for each frequency band, demonstrating the efficiency of the radiating array, and the low radiated cross polarization.

Claims: A dual polarized printed circuit antenna comprising a ground plane (1), a first power divider array (2) disposed over and capacitively coupled to said ground plane (1), a first array of radiating elements (3) disposed over and capacitively coupled to said first power divider array (2), a second power divider array (4) disposed over and capacitively coupled to said first array of radiating elements (3), and a second array of radiating elements (5) disposed over and capacitively coupled to said second power divider array (4), characterized in that said first array of radiating elements (3) comprises an array of radiating elements having a first size and being so configured as to operate within a first frequency band, and said second array of radiating elements (5) comprises an array of radiating elements having a second size that is larger than said first size and being so configured as to operate within a second frequency band that is at least 1 GHz lower than said first frequency band, and wherein said second array of radiating elements have a gain that is at least 3.0 dB less than a gain of said first array of radiating elements throughout said first frequency band, and said first array of radiating elements have a gain that is at least 3.0 dB less than a gain of said second array of radiating elements throughout said second frequency band. An antenna as claimed in claim 1, wherein said first and second power divider arrays (2;4) comprise respective power divider arrays for feeding said first and second arrays of radiating elements (3; 5) at frequencies within said first and second frequency bands, respectively. An antenna as claimed in claim 1, wherein said first and second power divider arrays (2;4) comprise tee junctions and impedance transforming sections, the impedance transforming sections of said second power divider array (4) being longer than the impedance transforming sections of said first power divider array (2). An antenna as claimed in claim 1, wherein said first frequency band is 14.0-14.5 GHz, and said second frequency band is 11.7-12.2 GHz.

Inventors: SORBELLO ROBERT MICHAEL, ZAGHLOUL AMIT IBRAHIM, SORBELLO, ROBERT MICHAEL, ZAGHLOUL, AMIT IBRAHIM
Applicants: COMSAT CORP, COMSAT CORPORATION
Application year: 1990
Main classification: H01Q5
IPC codes: H01Q21, H01Q13, H01Q25, H01Q1, H01Q5