Patent Publication Number: US-6906676-B2

Title: FSS feeding network for a multi-band compact horn

Description:
BACKGROUND OF THE INVENTION 
   1. Statement of the Technical Field 
   The inventive arrangements relate generally to methods and apparatus for horn antennas, and more particularly to horn antennas which can operate in multiple frequency bands. 
   2. Description of the Related Art 
   Conventional electromagnetic waveguides and horn antennas are well known in the art. A waveguide is a transmission line structure that is commonly used for microwave signal transmission. A horn antenna (horn) is a particular type of waveguide which is optimized for wireless propagation and reception of RF signals through free space. Horns typically have a conical or pyramidal shape. In a common configuration, a narrow end of the horn is operatively connected to one end of a feed structure, which itself includes a waveguide. A feed probe is usually installed at an end of the feed structure which is opposite of the end of the feed structure that is joined with the horn. The feed probe couples the horn to transmit and/or receive circuitry and is typically optimized for operation in the particular frequency band for which the horn is designed to operate. 
   The horn and feed structure typically include a material medium that confines and guides propagating electromagnetic waves. In a horn or feed structure, as with most other waveguides, a “mode” is one of the various possible patterns of propagating electromagnetic fields. Each mode is characterized by frequency, polarization, electric field strength, and magnetic field strength. Each horn configuration can form different transverse electric and transverse magnetic modes. Since horns are generally designed to have a static geometry, the operational frequency and bandwidth of conventional horns are limited. 
   To overcome the frequency and bandwidth limitations of horns, International Patent Application No. PCT/GB92/01173 assigned to Loughborough University of Technology (Loughborough) proposes that a frequency selective surface (FSS) can be used within a horn to influence the frequency response. An FSS is typically provided in one of two arrangements. In a first arrangement, two or more layers of conductive elements are separated by a dielectric substrate. The elements are selected to resonate at a particular frequency at which the FSS will become reflective. The distance between the element layers is selected to create a bandpass condition at a fundamental frequency at which the FSS becomes transparent and passes a signal. The FSS also can pass harmonics of the fundamental frequency. For example, if the fundamental frequency is 10 GHz, the FSS can pass 20 GHz, 30 GHz, 40 GHz, and so on. 
   Alternatively, FSS elements can be apertures in a conductive surface. The dimensions of the apertures can be selected so that the apertures resonate at a particular frequency. In this arrangement, the FSS elements pass signals propagating at the resonant frequency. Any other electromagnetic waves incident on the FSS surface are reflected from the surface. In a multi-band horn, the FSS can form a second horn within a first horn wherein the second horn and the first horn are tuned to different frequencies. 
   Hence, it would be desirable for a multi-band horn antenna to have multiple feed probes, with at least one probe being optimized for the operational frequency of each horn within the multi-band antenna. However, the coaxial nature of the feed structures in the multi-band horn antenna proposed by Loughborough prevents multiple feed probes from being incorporated into the multi-band horn antenna in a conventional fashion. Accordingly, there exists a need for a feed structure having multiple feed probes which can be used with a multi-band horn antenna having FSS&#39;s. 
   SUMMARY OF THE INVENTION 
   The present invention relates to a feed structure for a horn antenna. The feed structure can include a first waveguide and a second waveguide having a first portion at least partially disposed within the first waveguide. The second waveguide also can include a second portion intersecting a first wall of the first waveguide. The first wall can include a first frequency selective surface at an intersection of the first wall and the second portion of the second waveguide. The first waveguide can be operatively coupled to a first horn section and the second portion can be operatively coupled to a second horn section. 
   The first portion of the second waveguide can include at least one wall having a second frequency selective surface. A first feed probe can be disposed within the first waveguide, wherein RF signals generated by the first feed probe are reflected by the first frequency selective surface. The RF signals generated by the first feed probe can also propagate through the second frequency selective surface. 
   The feed structure can also include a second feed probe disposed within the second portion of the second waveguide, wherein RF signals generated by the second feed probe can propagate through the first frequency selective surface or be reflected by the second frequency selective surface. 
   The feed structure can further include a third waveguide including a first portion at least partially disposed within the second waveguide, a second portion intersecting a wall of the second waveguide, and a third portion intersecting at least one of the first wall and a second wall of the first waveguide. The second waveguide wall can include a third frequency selective surface at an intersection of the second waveguide and the second portion of the third waveguide. The first or second wall of the first waveguide can include a fourth frequency selective surface at an intersection of the third portion of the third waveguide and the first or a second wall. 
   The third waveguide can be operatively coupled with a third horn section. The first portion of the third waveguide can include at least one wall having a fifth frequency selective surface. The feed structure of the third waveguide can further include a first feed probe disposed within the first waveguide, wherein RF signals generated by the first feed probe propagate through the third and fifth frequency selective surfaces or are reflected by the fourth frequency selective surface. A second feed probe can also be disposed within the second portion of the second waveguide, wherein RF signals generated by the second feed probe propagate through the fifth frequency selective surface or are reflected by the third frequency selective surface. 
   Finally, a third feed probe can be disposed within the third portion of the third waveguide, wherein RF signals generated by the third feed probe propagate through the third and fourth frequency selective surfaces or are reflected by the fifth frequency selective surface. 
   A present invention also relates to a waveguide combining network. The waveguide combining network can include a first waveguide having at least a first wall and a second waveguide intersecting the first wall. The first wall can include a first frequency selective surface at the intersection of the first wall and the second waveguide. RF signals propagating within the second waveguide can propagate through the first frequency selective surface and RF signals propagating within the first waveguide can be reflected by the first frequency selective surface. 
   Further, the waveguide combining network also can include a third waveguide intersecting the first wall and/or a second wall of the first waveguide. A second frequency selective surface can be provided at the intersection of the third waveguide and first wall and/or the second wall. RF signals propagating within the third waveguide can propagate through the second frequency selective surface and RF signals propagating within the first waveguide can be reflected by the second frequency selective surface. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a multi-band horn antenna having an alternate waveguide arrangement that is useful for understanding the present invention. 
       FIG. 2  is a cross sectional view of a waveguide assembly of the multi-band horn antenna of  FIG. 1  taken along section lines  2 — 2 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An embodiment of the present invention concerns a feed structure for use with a multi-band horn antenna. The feed structure includes a waveguide combiner network which can include a plurality of waveguides that are operatively coupled to antenna horns. For example, a waveguide can be provided for each of a plurality of coaxial horns. Further, a feed probe can be provided for each of the waveguides. Accordingly, the feed probes can be optimized for the horns with which they are used. 
   Frequency selective surfaces (FSS&#39;s), which are known in the art, can be selectively incorporated into walls of the waveguides such that the waveguides are reflective at selected frequencies and transparent at other selected frequencies. For instance, a particular waveguide can be reflective for RF signals generated by a feed probe associated with the waveguide, yet be transparent to RF signals generated by feed probes associated with other waveguides within the feed structure. Accordingly, the waveguides can be disposed in a desired arrangement, for example coaxially, with little or no adverse affect on feed structure performance. Moreover, the waveguides also can intersect one another without significant loss of performance. 
   Referring to  FIG. 1 , a multi-band horn antenna  100  having a multi-band feed structure  105  is shown. The multi-band feed structure  105  can include a plurality of waveguides, for example a first waveguide  110 , and a second waveguide  115  at least partially disposed within the first waveguide  110 . The multi-band feed structure  105  also can include additional waveguides. For instance, a third waveguide  120  can be at least partially disposed within the second waveguide  115 , and a fourth waveguide (not shown can be disposed within the third waveguide, and so on. Importantly, the invention is not limited to a specific number of waveguides; any number of waveguides can be used. Further, each of the waveguides can be operatively connected to a horn. For example, waveguide  110  can be operatively connected to a horn  130 , waveguide  115  can be operatively connected to a horn  135 , and waveguide  120  can be operatively connected to a horn  140 . Further, horns can be provided, for example in the instance there are more than three waveguides. 
   In one embodiment, output portions  111 ,  116 ,  121  of the waveguides  110 ,  115 ,  120 , respectively, can be coaxially arranged, as shown. This arrangement is beneficial for use with coaxially positioned horns. In another arrangement, output portions  116 ,  121  of second and third waveguides  115 ,  120  can be positioned side by side within the first waveguide  110 , for example when associated horns are provided that have a side by side configuration. Still, the invention is not so limited and other waveguide arrangements can be provided. Notably, the waveguides  110 ,  115 ,  120  can be rectangular, squared, cylindrical, elliptical or any other shape which would support a guided wave. Moreover, the horns  130 ,  135 ,  140  can be conical, pyramidal or any type of feed horn that can be receive a guided wave from the waveguides. 
   A cross sectional view of the multi-band feed structure  105  taken along section lines  2 — 2  is shown in FIG.  2 . Each of the waveguides can be provided with one or more feed probes. For instance, a first feed probe  211  can be provided with the first waveguide  110 , a second feed probe  216  can be provided with the second waveguide  115 , and a third feed probe  221  can be provided with the third waveguide  120 . The feed probes  211 ,  216 ,  221  can be used to generate RF signals within the waveguides. For example, the feed probes can be connected to circuitry which supplies RF signals to, or receives RF signals from, the feed probes. In the preferred arrangement, each of the feed probes  211 ,  216 ,  221  can be optimized for the operational frequency of the waveguide and horn with which it is associated. Significantly, the present invention is not limited to any particular feed probe configuration. For example, the linear vertical, linear horizontal and circular polarization feed probes can be used. 
   The multi-band feed structure  105  can provide excellent horn feed characteristics for the multi-band horn antenna  100 . In particular, the waveguides  110 ,  115 ,  120  can be arranged so that each feed probe  211 ,  216 ,  221  can be isolated within its associated waveguide  110 ,  115 ,  120 , respectively. In consequence, coupling and interference between the feed probes  211 ,  216 ,  221  is minimized. Coupling between the waveguides  110 ,  115 ,  120  also is minimized. 
   As shown, a portion  217  of waveguide  115  can intersect with a portion  212  of waveguide  110  at intersection  280 . Further, a portion  222  of waveguide  120  can intersect with a portion  218  of wave guide  115  at intersection  282 , and a portion  223  of waveguide  120  can intersect with a portion  219  of waveguide  110  at intersection  284 . Each of the waveguides can include a plurality of surfaces to enable proper waveguide operation, even though the waveguides can have portions which are coaxially positioned and other portions which intersect one another. 
   For instance, the first waveguide  110  can include conductive surfaces, dielectric surfaces, FSS&#39;s, or a combination of such surfaces. In one arrangement, waveguide walls (walls)  230 ,  235  can be conductive. Wall  240  can comprise conductive portions  242  and FSS portions  244 ,  246 . FSS portion  244  can be disposed at the intersection  280  of the waveguide  110  and the waveguide  115 . FSS portion  244  can be configured to reflect RF signals generated by the feed probe  211  and pass RF signals generated by the feed probe  216 . Similarly, FSS portion  246  can be disposed at the intersection  284  of waveguide  110  and waveguide  120 . FSS portion  246  can be configured to reflect RF signals generated by feed probe  211  and pass RF signals generated by feed probe  221 . 
   Waveguide  115  can include walls  248 ,  250 ,  252 ,  253  and  254 . Walls  252  can be conductive. Wall  254  can include a portion  258  which intersects waveguide  120  at the intersection  282 , and a remaining non-intersecting portion  256 . Walls  248 ,  250 ,  253  and portion  256  of wall  254  can be FSS&#39;s which pass RF signals generated by feed probe  211 , but are reflective to RF signals generated by feed probe  216 . Portion  258  of wall  254  also can be an FSS. Portion  258 , however, can pass RF signals generated by the feed probe  211  and RF signals generated by feed probe  221 . Further, the portion  258  can be reflective to RF signals generated by feed probe  216 . 
   Lastly, waveguide  120  can include waveguide walls  260 ,  262 ,  264 ,  266 ,  268 . Walls  264  can be conductive. Walls  260 ,  262 ,  266 ,  268  can be FSS&#39;s which are reflective to RF signals generated by feed probe  221 , and pass RF signals generated by feed probes  211 ,  216 . 
   Accordingly, RF signals generated by feed probe  211  can be propagated to horn  130  via waveguide  110  with minimal interference from waveguides  115 ,  120 . Likewise, RF signals generated by feed probe  216  can be propagated to horn  135  via waveguide  115  with minimal interference from waveguides  110 ,  120 . Finally, RF signals propagated by feed probe  221  can be propagated to horn  140  via waveguide  120  with minimal interference from waveguides  110 , 115 . In consequence, each feed probe  211 ,  216 ,  221  can be optimized for the operational frequency of the waveguide  110 ,  115 ,  120  and horn  130 ,  135 ,  140 , respectively, with which it is associated. 
   In a preferred arrangement, each FSS is optimized for the angle of incidences of RF signals to which the FSS should be passive or reflective. In particular, walls  248 ,  254 ,  260 ,  268  can be optimized for their orientation within the multi-band feed structure  105 , for example with respect to feed probes  211 ,  216  and walls  230 ,  240 . Similarly, walls  250 ,  253 ,  262 ,  266  can be optimized for their orientation within the multi-band feed structure  105 . For example, the FSS&#39;s of walls  250 ,  253 ,  262 ,  266  can be selected for optimal performance for an RF signal having an angle of incidence upon the walls  250 ,  253 ,  262 ,  266 . 
   While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as described in the claims.