Patent Publication Number: US-8120543-B2

Title: Transmission line slot antenna

Description:
FIELD OF THE INVENTION 
     This invention pertains generally to the field of antennas and more specifically to slot antennas formed by a slotted cylinder transmission line that can be of non-uniform cross-section. 
     BACKGROUND 
     Slot antennas have been widely investigated over the past 60 years. Slot antennas can be divided into two groups: slot antennas in a screen and boxed-in slot antennas. Slot antennas in a screen are wideband antennas that radiate in two directions. Boxed-in slot antennas are narrow band antennas that radiate in only one direction. The narrow bandwidth of operation of boxed-in antennas is achieved by cutting a radiating slot in a wall of a resonant cavity tuned to resonate on the TE 10  mode. The resonant cavity substantially increases the size of the antenna, even in a space saving configuration presented in U.S. Pat. No. 6,307,520, and makes the antenna narrowband. The prior art of slot antennas failed to combine in one design wide bandwidth of operation with the directional property of boxed-in slot antennas. 
     SUMMARY OF THE INVENTION 
     The conductive top surface of the antenna&#39;s ground plate has a slot of length L and width W with the width W less than the length L. A feed has its ground reference terminal connected to one side of the slot and a signal terminal connected to the other side of the slot. A conductive cylindrical screen of one or more sections running lengthwise along the slot is attached along the bottom surface of the ground plate. Each of the conductive cylindrical screen sections has the first and the second edge conductively connected to the top surface of the ground plate along opposing respective sides of the slot and is tuned to support the fundamental mode (H 00 ) of a slotted cylinder transmission line formed by the screen sections and the ground plate with the slot. The cylindrical screens in this configuration can have an arbitrary cross-section. The cylindrical screens can be non-uniform in the longitudinal direction. 
     Various aspects, advantages, features and embodiments of the present invention are included in the following description of exemplary examples thereof, which description should be taken in conjunction with the accompanying drawings. All patents, patent applications, articles, other publications, documents and things referenced herein are hereby incorporated herein by this reference in their entirety for all purposes. To the extent of any inconsistency or conflict in the definition or use of terms between any of the incorporated publications, documents or things and the present application, those of the present application shall prevail. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The various aspects and features of the present invention may be better understood by examining the following figures, in which: 
         FIGS. 1   a - 1   e  show various configurations of the ground plate with the slot and screens forming slotted cylinder transmission line; 
         FIGS. 2   a - 2   c  illustrate embodiments with differing lengths of transmission line; 
         FIGS. 3   a - 3   i  show various details of how the feed line can be connected; 
         FIGS. 4   a - 4   e  illustrate embodiments where the transmission line is made up of several segments; 
         FIGS. 5   a - 5   g  show various embodiments of how the transmission line can be conductively connected to the conductive top surface of the ground plate; 
         FIG. 6   a  is a particular embodiment and some relevant dimensions; and 
         FIGS. 6   b - 6   d  give the response of the embodiment of  FIG. 6   a.    
     
    
    
     DETAILED DESCRIPTION 
     The slot antenna presented here is, according to various aspects, formed by a section of non-uniform slotted cylinder transmission line with either open or closed ends, variable cross section configuration, and variable direction of the transmission line. Further, the transmission line can be conformal to the space available for the antenna. The antenna also can include a ground plate to form a radiation pattern of the antenna predominantly in a hemisphere. The body of the transmission line can be non-uniform and can also include windows through which a coaxial cable or an micro-strip line reaches the excitation point at the mid-point of the slot or its vicinity, for a single frequency operation mode, or at a point off the midpoint, for a dual frequency operation mode. Open ends of the transmission line can also be used by a feeding network to access the excitation point of the antenna. The body of the transmission line can also have a number of windows or non radiating slots for technological purposes. The length of the radiating slot of the antenna can be made longer than the transmission line length. 
     As noted in the Background, the prior art of slot antennas failed to combine in one design wide bandwidth of operation with the directional property of a boxed-in slot antenna. The antenna presented here addresses the need for a wideband slot antenna radiating in one direction. The proposed antenna design can be used in sector antennas, directional panel antennas or antenna arrays. The presented antenna includes a section of a slotted cylinder transmission line (or, alternately, open cylindrical waveguide, open cylindrical waveguide with longitudinal slot, or slotted cylindrical waveguide) with the slot shorted at both ends and a ground plate overlapping with a part of the transmission line. The slot width W, the transmission line shape, and the cross-section area are all chosen to provide conditions for the antenna to operate at a wavelength λ in the band λ TE &lt;λ&lt;λc, where λc is the cut-off wavelength of the basic mode of operation of a slotted cylinder transmission line (H 00 ) and the λ TE  is the cut-off wavelength of the first propagating mode in the cylinder without slot. The λc of a slotted cylinder transmission line is a function of the slot width W, the shape and the area of the transmission line cross-section. 
     Considering the existence and frequency of an H 00  mode, the distance around the screen (or screens) and the cross section area are the two important factors that ultimately define conditions for the H 00  mode&#39;s existence. The shape of the cross section, however, can also be important. For example, take the ease of a cylinder with a square cross-section. When the cylinder is compressed without changing the perimeter length, the H 00  mode will vanish at some point. In general, the resonance frequency of the H 00  mode in a cylinder with a longitudinal slot can be calculated only numerically. In very specific cases, e.g. when the cross-section of the cylinder is a circle, some simple analytical formulas can be derived. The conformal application case considered here encompasses a broad family of configurations. Thus, the full analysis of the presented antenna design can only be done numerically. The physics of the effect studied here can be effectively illustrated on a simplified case where the cross-section of the cylinder is a circle and the slot is narrow. In H-mode, the currents flowing in the azimuthal direction prevail. Thus, the surface of the cylinder is an inductive loop and the edges of the slot form a capacitor. Consequently, the resonance frequency depends on two factors: the capacitance of the capacitors formed by the edges of the slot, and the inductance of current on the surface of the cylinder. 
       FIGS. 1   a - 1   e  show several variations on the basic configuration of the slot antenna to illustrate different arrangements of the open cylinder conductive screen that form the transmission line.  FIG. 1   a  shows the antenna  100  with the ground plate  101 , the conductive top surface, and the slot  103 . The cross-section of the slotted cylinder transmission line  105  is formed by a conductive screen that is conductively connected to the top surface of the ground plate. The shape of the cross-section of the slotted cylinder transmission line  105  can be arbitrary. The body of the transmission line  105  can be of various configurations. In particular cases presented in the  FIGS. 1   b  and  1   c , the slotted cylinder transmission has a rectangular cross-section as well as more general shapes. The area of any cross section of the line may be a variable function along the line length, as in  FIG. 1   b . The body of the transmission line can be bent as in the  FIG. 1   c  or “shifted” as in the  FIG. 1   d . In most configurations, the transmission line  105  has an axis running lengthwise along the length of the slot, so that the axis of transmission line sections are substantially (that is, more or less) parallel to the length of the slot. However, the slot edges need not be parallel to the transmission line body, as in  FIG. 1   e . In general, the slot can be tilted, askewed to relative direction of the cylindrical screen section axes, or even zigzagged. The slot should not be arranged in a perpendicular direction relatively to the cylinder axes. Additionally, depending on the space available for the antenna, the slot in the antenna can be of a straight or of a bent type as in  FIGS. 1   a  and  1   c . This versatility for the arrangement of the screen (or screens) forming the body of the transmission line  105 , allowing the transmission line to conform to the available space, is one of the practical advantages of the design. 
     Although shown open in the figures, the ends of the screen (or screens) forming the slotted cylinder transmission line  105  can also be closed. Such an antenna will typically be in a plastic enclosure to prevent dust and moisture accumulation. In other cases, the end-walls can keep the inside clean and free of insects. For example, for a single section screen longer than the slot, these end-walls can be conductive or non-conductive and may also include an opening for the antenna feed if needed. For the case of multiple screen sections, the gaps between adjacent section could be filled, for example, with a thin dielectric layer or film, or the line itself could even be a flexible film with conductive sections. Also, the slot itself may be covered or filled by a dielectric layer (as  113  in  FIG. 3   f  discussed below). 
     The transmission line in the presented antenna has an arbitrary cross section and may or may not include conductive ends. The components of the antenna presented here are arranged to support the fundamental radiation waveguide mode, H 00 , of the open cylindrical waveguide formed by the screen, the ground plate, and the slot. In prior art, a rectangular waveguide is used, and it is assumed that the rectangular waveguide supports the TE 10  mode that propagates in the direction orthogonal to the slot. In some prior art, the electromagnetic waves propagate in the direction normal to the ground plane and the slot. In U.S. Pat. No. 6,307,520, the box is positioned in such way that the TE 10  mode waves propagate in the direction that is still orthogonal to the slot, but parallel to the ground plate. The antenna presented here uses the “open cylindrical waveguide” (also know in the literature as “open cylindrical waveguide with longitudinal slot” and “slotted cylindrical waveguide”), and the fundamental mode of this waveguide referred to as H 00 . In this mode, the slot defines the direction of the H 00  mode, with waves in the waveguide propagating in the direction along the waveguide axis, i.e. in the direction of the slot. 
     Similarly, in the prior art, the length of the slot  103  has been shorter than or equal to the length of the cavity formed by the transmission line. In the antenna presented here, the length of the slot, L, can be shorter, equal or longer than the transmission line length, L tr , as shown respectively in the  FIGS. 2   a ,  2   b  and  2   c.    
     In  FIGS. 2   a - 2   c , the feeding network  107  of the antenna is shown. The feed has its ground reference terminal connected to the conductive top surface of the ground plate  101  on one of the sides of the slot  103  and the signal terminal connected to the other of the sides of the slot  103 . In these figures, the excitation point of the antenna is shown as located either in the mid-point of the slot or in its neighborhood. As discussed below, in some cases the excitation point may be positioned off center. The antenna can be excited either by a coaxial or a micro-strip (or “m-strip”) transmission line. 
       FIGS. 3   a - 3   d  show some of the possibilities for excitation by a coaxial line  107 . In  FIGS. 3   a  and  3   b , the coaxial line  107  is placed on the top and on the bottom of the ground plate  101 . One of the terminals is then attached on one of the long sides of the slot  103  and the other terminal is attached on the opposite side. The coaxial line  107  can reach the excitation point of the antenna  100  either through a window in the body of the transmission line  105  (as in  FIG. 3   c ) or through one of the ends of the transmission line  105  (as in the  FIG. 3   d ) when the coaxial line is positioned below the ground plane as shown in the  FIG. 3   b . When the transmission line has ends, a window would similarly be introduced for the arrangement in  FIG. 3   d . When the coaxial line  107  is placed right on the top of the ground plane, as shown in the  FIG. 3   a , such a window is not needed. In both cases, the outer conductor of the coaxial line  107  can be electrically connected to the first slot edge and the inner conductor, after crossing the slot  103 , is electrically connected to the opposite side of the slot. 
     Excitation of the antenna by a micro-strip line can use a dielectric layer on the ground plane of the antenna, as shown in  FIGS. 3   e  and  3   f . In  FIGS. 3   e  and  3   f , the ground plate  101  has a conductive top layer  111  and a dielectric layer  113 . The dielectric layer  113  can overlap with the slot  103  cut in the ground plane  101 , as in  FIG. 3   f , where the slot is only cut in the conductive layer  111 . The slot can also be cut through the ground plane&#39;s dielectric layer  113 , as shown in  FIG. 3   e . If the slot is free of the dielectric layer  113 , as in the  FIG. 3   e , the micro-strip  109  is electrically connected to the opposite side of the slot  103  by a thin conductor  121 , which is soldered or otherwise attached to the micro-strip line  109  after crossing the slot  103  and passing through a hole  123  in the dielectric and the ground plate it is soldered to its upper side. If the slot is filled with the dielectric, the micro-strip  109 , after crossing the slot  103 , can be shorted to the ground plane by a via  123  as in the  FIG. 3   f , or extend for approximately quarter wavelength after crossing the slot and remain open as in the  FIG. 3   i . As with the coaxial case, the micro-strip  109  can pass though a hole in the side of the screen  105 , as in  FIG. 3   g , or enter through the end as in  FIG. 3   h.    
     Looking at the example presented in  FIG. 3   f  in more detail, the micro-strip line is on the lower surface of the dielectric layer  113 , which is on the lower side of the ground plate  101 . The micro-strip line  109  goes under the slot  103  to the opposite side and connected to the via (for example, a hole drilled through the extended micro-strip, dielectric layer and the ground plate and filled with a conductive material) that goes through the dielectric layer, thereby connecting the micro-strip with the ground plane on its conductive upper side. The micro-strip line  109  should not touch the cylindrical screen sections  105 . The choice of the type of excitation (shorted, extended open) depends on the desired bandwidth for operation of the antenna. The resonant nature of the quarter wave stub results in a narrower band of operation of the antenna compared with the shorted micro-strip excitation. 
     In the discussion above, the feed has mainly been connected to the ground plate near the center of the long sides of the slot. Having the excitation point at the mid-point of the slot, or in its vicinity, will result in the antenna&#39;s response being most sharply peaked at a single frequency. This configuration with the excitation point at the mod-point of the slot is often preferred for use in a single frequency operation mode. Instead of the symmetrical arrangement of the feed just described, the attachment point can be shifted from the center of the slot. The configuration with an off-center or an asymmetrical feed can be used when a response on multiple different frequencies is desired. 
     The conductive screen  105  forming the transmission line can be partitioned into several segments. Some parts of the conductive screen can be removed as shown in the  FIG. 4   a  ( 105   a ,  105   b ) and  FIG. 4   b  ( 105   a ,  105   b ,  105   c ). The number of sections of the transmission line can vary from 2 to a large number depending on manufacturing restrictions or space limitations, with the sections preferably of the same, or very similar, shapes tuned to support the same radiation waveguide mode H 00 . Although in many configurations the axis of each section will run along the slot, the position of each section with respect to the slot is not necessarily the same— FIGS. 4   c  ( 105   a ,  105   b , and  105   c ). As discussed above, the sections need not necessarily be parallel to the slot.  FIG. 4   d  shows the bottom view of the antenna with a three section transmission line whose top section is shifted to the right and whose bottom section is shifted to the left the slot.  FIG. 4   e  shows the bottom view of the antenna with two section transmission line whose top and bottom sections are shifted to the left of the slot. The actual position of the transmission line sections and their shapes can be determined based on specific design requirements. The ends can be closed. 
     As also discussed above, the transmission line is conductively connected to the conductive top surface of the ground plate. The electrical connection of a transmission line  105  and the ground plate  101 , in case of the absence of a dielectric layer, can be done by means of soldering, welding, screwing, bolting or riveting parts as respectively shown in  FIG. 5   a ,  5   b ,  5   c , or  5   d , with the various conductive connectors shown as  151 . When the ground plate is formed on a dielectric layer as in  FIGS. 3   e  and  3   f , the electrical connection of the transmission line body  105  to the conductive top surface  111  of the ground plate can be done through holes drilled in the ground plate layer  111  and dielectric  113 , as in  FIG. 5   e  or  FIG. 5   f , or through slots in the dielectric and the ground plate, as demonstrated in  FIG. 5   g.    
     The minimum number of points at which the transmission line body  105  is electrically connected to the ground plate along each side of the slot is equal to 2, for a total 4 contact points. In the case when the transmission line body is composed of several sections, the number of contact points for each section can be 1 or more on each side. With the contact points on each side of the transmission line, the antenna will operate largely the same as a one-section antenna with continuously soldered/welded parts. More generally, the number of contact points can vary and can be greater than 4. Taking into account that the slot length L is preferably greater than the half wavelength in the free space and less than one wavelength, the distance between connection points for the case of 4 points on each side is about 0.2 times the wavelength, which is much larger than what was considered acceptable in the prior art designs. 
       FIGS. 6   b ,  6   c , and  6   d  show the behavior of a specific example of the antenna as shown in  FIG. 6   a . The arrangement here is a design for a single operating frequency peaked at 5.75 GHz. The measured antenna gain is 8.6 dBi. Other designs could be tuned to other operating frequencies, such as from 2.4000 GHz to 2.4835 GHz for a WiFi antenna application. 
       FIG. 6   b  shows the measured return loss of the antenna shown in the  FIG. 6   a  with ground plate  101  dimensions of X=45 mm and Y=65 mm. The ground plate is formed of RO4350B high frequency laminate with the slot  103  cut through the upper layer of 1 oz of copper and the 30 mill dielectric layer with width W=2.1 mm and length L=41 mm. The transmission line  105  consists of two sections made of 0.1 mm brass shim, shifted with respect to the slot. The sections&#39; dimensions are A=18 mm, B=15 mm and the depth of the transmission line is 11 mm. The distance between the sections is C=8 mm and the shift between the sections is D=5 mm. The upper transmission line section is shifted to the right by 2.5 mm of the center, and the lower section is shifted to the left by 2.5 mm. The transmission line length is L tr =38 mm. The transmission line body  105  is connected to the copper layer on the upper surface of the laminate by copper rivets  151 , four on each side of the slot, with 8 mm distance between them. The slot  103  is excited by a micro-strip line  109  etched on the bottom side of the laminate. The micro-strip is electrically connected to the ground plate by a copper wire that is soldered on one side to the micro-strip and, after crossing the slot and passing through a hole  123 , is soldered to the other side of the ground plate. On its other end, the micro-strip line is connected to a surface mount end-launch SMA connector  161 . The transmission line is open on both ends, and the transmission line dimensions are below critical dimensions of the TE 10  rectangular waveguide mode in the whole of the 5 GHz band. It should be noted that in this configuration the slot length L is greater then the transmission line length L tr . The antenna operates on the lowest mode of the slotted cylinder transmission line (H 00 ). 
       FIG. 6   c  and  FIG. 6   d  show the measured far field patterns in the E and H planes. The relatively high level of the front-to-back ratio, which is equal to −12 dB, is defined by the ground plate size X˜λ. The front-to-back ratio drops down significantly for a larger ground plate size in the E plane (perpendicular to slot). The S 11 &lt;−10 dB bandwidth of the antenna is ˜1 GHz. 
     Although the invention has been described with reference to particular embodiments, the description is only an example of the invention&#39;s application and should not be taken as a limitation. Consequently, various adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as encompassed by the following claims.