Patent Publication Number: US-4367475-A

Title: Linearly polarized r.f. radiating slot

Description:
This application generally relates to linearly polarized r.f. radiating slot antenna structures. In the preferred exemplary embodiment, the structures are substantially formed by photo etching techniques commonly used for constructing microstrip antennas and printed circuits. 
     Microstrip antennas having linearly polarized radiating slots formed between the edge of a radiator plate and an underlying ground plane are well known in the art. However, although nominally linearly polarized, these microstrip radiating slots actually also produce considerable cross polarization components in the radiated field. The degree of cross polarization experienced in the radiated field will vary as function of the spatial region examined. However, in certain arrays of microstrip radiators, such cross polarization components lead to serious field pattern degradation. Sometimes, this degradation may not even permit certain array designs to be useable. 
     Now, however, it has been discovered that a new single slot, linearly polarized microstrip structure produces much lower cross polarization components in the radiated field. In actual measurements made to date, cross polarization components have been more than 20 dB less than the desired linear polarization components when using this invention. Such low cross polarization will permit array pattern synthesis in situations where conventional microstrip cross polarization components would forbid such designs. 
     The single slot linearly polarized radiator of this invention uses an odd mode resonant or non-resonant structure whereas traditional microstrip slot radiators utilize even mode structures. In either the resonant or non-resonant embodiment, the radiating slot of this invention is formed by the juxtaposed edges of two separate electrically conducting plates or areas which lie substantially within a single plane or layer (if conformed to a curved surface or the like). The thus formed radiating slot is much narrower than conventional radiating wave guide slots or the like and is also totally unshorted at the ends of the slot, contrary to the usual wave guide slot radiator structure. In the preferred embodiment, the slot is formed by conventional photo etching techniques by selectively etching away portions of a conductive layer bonded to one side of a dielectric sheet. 
     The outboard edges of the plates or areas used to define the radiating slot are shorted to an underlying ground plane so as to define an included cavity therebetween. The cavity can have a resonant dimension or, if a non-resonant dimension is utilized, the structure can nevertheless be made resonant by adding an appropriate reactive impedance (e.g. a plurality of capacitors) across the radiating slot (preferably plural discrete devices distributed along the slot). 
     While the radiating slot of this invention may be fed simply by connecting a coaxial feedline across the slot edges, a more uniformly distributed feed is preferably utilized along the slot length. In the preferred exemplary embodiment, a corporate structured microstrip feedline is formed by photo etching techniques and disposed beneath the antenna structure. Feed through structures from the branches of the corporate feedline then pass upwardly through a dielectric layer, to connections proximate one edge of the radiating slot. 
     Both the feedline connections and the lumped tuning reactance (if a non-resonant design is utilized) are preferably connected as close as possible to the respective opposed edges of the radiating slot in the preferred exemplary embodiment. However, depending upon the frequency of operation, the structure will also work when such connections are made at other locations proximate these edges as should be appreciated. 
    
    
     These and other advantages and objects of the invention will be more completely appreciated and understood by reading the following detailed description of the presently preferred exemplary embodiments of the invention taken in conjunction with the accompanying drawings of which: 
     FIGS. 1 and 1a are an exploded perspective view and a cross-sectional view respectively of the presently preferred exemplary embodiment of this invention; 
     FIGS. 2 and 3 are elevation and plan views respectively of another exemplary embodiment of this invention; 
     FIGS. 4 and 5 are elevation and plan views respectively of yet another exemplary embodiment of this invention; and 
     FIG. 6 is an exemplary equivalent circuit diagram of the equivalent reactive impedances presented by the antenna structures of the exemplary embodiments in FIGS. 1-3. 
    
    
     Referring to FIGS. 1 and 1a, a radiating slot 10 is formed by the opposed edges 12 and 14 of two separate electrically conducting plates 16 and 18 respectively. Edges 12 and 14 are juxtaposed substantially within one plane thereby defining the radiating slot 10. A conducting ground plane 20 is disposed beneath the plates 16 and 18 thus defining a cavity 22 therebetween. Cavity 22 is also defined by electrical shorts 24 and 26 extending along the length of plates 16 and 18 and connecting those plates to the underlying ground plane. As will be appreciated by those in the art, the electrical shorts 24 and 26 may comprise a series of closely spaced electrical connections through the dielectric of cavity 22 rather than a continuous connection as shown in FIG. 1. 
     The radiating slot 10 differs from the usual wave guide radiating slot in at least two respects. First of all, it should be noted that the ends of the plates defining the radiating slot are not electrically connected. That is, the plates 16 and 18 whose edges define the radiating slot are separate electrically conducting plates rather than being connected at the ends of the slot. Among other things, this permits the radiating slot of this invention to be fed more uniformly along its length. Secondly, the transverse slot dimension G is much narrower than the usual wave guide radiating slot which is often on the order of 1/2 wavelength. In the presently preferred exemplary embodiment, the transverse slot dimension G is on the order of only 0.03 to 0.1 inch at a frequency of approximately 225 mHz. 
     The cavity 22 in FIG. 1 is not self resonant. Instead, a series of discrete capacitors 28 are connected across the slot to provide a resonant antenna structure. Referring to the equivalent structure shown in FIG. 6 the parasitic capacitances C p  are normally negligible and, in any event, very much smaller than the gap capacitance C g . In turn, the gap capacitance C g  is very much smaller than the combined capacitance of capacitors 28 which have been collectively denoted as loading capacitance C L  in FIG. 6. The inductance L f  /2 results from the plates 16 and 18 as will be appreciated. Using the equivalent circuit of FIG. 6 normal r.f. circuit design calculations well known to antenna engineers, it is possible to calculate the required loading capacitance for resonance at a particular frequency with any particular structure. 
     It may be observed that by making the transverse gap dimension G extremely small, the gap capacitance C g  may become quite large and the structure could in fact be made to resonant without the external lumped capacitances 28. However, when the gap capacitance is made this large, it is difficult to control the manufacturing tolerances sufficiently to accurately tune the structure. Accordingly, the transverse slot dimension G is preferably made large enough to prevent the gap capacitance C g  from dominating in the tuned equivalent circuit shown in FIG. 6. On the other hand, the transverse slot dimension G cannot be made too wide or the parasitic capacitance C p  will dominate and it may be impossible to properly excite the structure to radiate fields as desired. 
     The non-resonant embodiment of FIG. 1 is also depicted in FIGS. 2 and 3 with a simple coaxial feedline having its outer conductor connected to edge 14 and its inner conductor connected to edge 12 at somewhere near the midpoint of the radiating slot. In the embodiment of FIGS. 2 and 3, the lumped capacitance 28 is simplified to a single capacitor. 
     However, in the preferred exemplary embodiment of FIG. 1, the lumped capacitance has been distributed in the form of several discrete capacitors 28 along the length of the radiating slot 10. Further, the r.f. feed to the slot has been similarly distributed along the length of the slot to provide a more uniform exitation of the slot. In the embodiment of FIG. 1, this feed is provided by corporate structured microstrip feedline 30 which is bonded to the underside of another dielectric layer 32. The branches 34 of the corporate structured microstrip feedline 30 are interconnected with edge 12 of the radiating slot by conductors 36 which pass through passages formed in the dielectric layer 32, and in the shorting strip 26. The r.f. feed input/output is connected as schematically shown in FIG. 1 to feed the microstrip feedline 30 with respect to the ground plane 20. 
     In one actually tested non-resonant embodiment of this invention, plates 16 and 18 were approximately 1 inch by 12 inches in dimension and the transverse slot dimension G was approximately 0.05 inch. The structure was caused to resonant at approximately 225 Mhz by the provision of eight 45-50  picofarad capacitors 28. Cavity 22 was approximately 1/4 inch in height and incuded a honeycomb dielectric spacer material so as to provide a minimum dielectric constant. However, teflon or other dielectric materials (including a vacuum) could be utilized if desired and as will be appreciated. In this actually tested exemplary embodiment, the gap capacitance C g  was probably less than one picofarad and measured cross polarization components in the radiated field were everywhere more than twenty dBs below the desired linearly polarized components. The desired linear polarization of the radiated electric fields is directed transverse to the radiating slot as should be appreciated. 
     The length dimension L of the radiated slot is normally dictated by size constraints or by desired design radiation resistance values as should also be appreciated by those skilled in the art. 
     In the preferred exemplary embodiments, the connections made to the edges 12 and 14 were formed by soldering or by rivets connected as close to the edges as possible. However, especially at lower frequencies, these connections may be made elsewhere in the vicinity of edges 12 and 14 as will be appreciated. 
     The resonant cavity design shown in FIGS. 4 and 5 is substantially identical to the embodiments of FIGS. 1-3 except that loading capacitors 28 are unnecessary because the cavity 22 is of a resonant dimension. For example, in the embodiment of FIGS. 4 and 5, the resonant cavity 22 includes a folded resonant shorted cavity having a one-fourth wavelength dimension as measured through the dielectric of the cavity from the slot around intermediate conducting plate 40 to the internal short 42. Such folded resonant cavities, per se, are also known in the art from, for example, U.S. Pat. Nos. 4,131,893 and 4,131,892 commonly assigned with this pending application. 
     In the preferred embodiment of this invention, the radiating slot is formed from opposed conductive edges by selectively etching away a portion of a first integral electrically conducting layer bonded to the first side of a dielectric sheet. Furthermore, in the presently preferred embodiment, r.f. electromagnetic signals are fed to/from the radiating slot by a corporate structured microstrip feedline which is also formed by selectively etching a second integral electrically conducting layer bonded to the second side of the dielectric sheet. The whole etched structure, in turn, is selectively connected to a conductive ground plane layer by suitable cavity-defining electrical shorts. Suitable feed through electrical connections are made through this layered structure to complete the antenna. Accordingly, this invention is believed to include the method of manufacturing such a structure as well as a method of transmitting or receiving linearly polarized r.f. signals by forming such a structure. 
     While only a few presently preferred exemplary embodiments of this invention have been specifically described in detail above, those ordinarily skilled in the art will appreciate that many modifications and variations in these exemplary embodiments may be made without departing materially from the novel and advantageous features of this invention. Accordingly, all such modifications and variations are intended to be included within the scope of the following claims.