Patent Publication Number: US-6222494-B1

Title: Phase delay line for collinear array antenna

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority of European Patent Application No. 98305164.0, which was filed on Jun. 30, 1998. 
     FIELD OF THE INVENTION 
     The present invention relates to a delay line, and particularly but not exclusively to a feeding delay line in a collinear antenna array. 
     BACKGROUND OF THE INVENTION 
     In a wireless local area network (WLAN) a number of wireless access points (APs) form the wireless infrastructure, and wireless hosts communicate with each other via the wireless APs. The wireless hosts may be stationary or may roam around. Such a system is similar to any cellular network system. 
     A requirement for antennas at a wireless access point, or in a base station of a cellular network, is that the radiation must be omni-directional in the azimuth plane, in order to give an equal chance of access to all mobiles around it. There is a continuing desire for higher gain, omni-directional antennas, in particular for wireless APs, so as to extend the cell size in a cellular network and/or increase communication reliability of cells. However, such improvements need to be achieved whilst minimizing the cost, size and technical complexity of the antennas. 
     A good example of an omni-directional antenna is the well-known half wavelength dipole antenna which has a so-called “donut” shaped radiation pattern providing good omni-directional coverage. Such well-known half-wavelength dipole antenna&#39;s have a signal gain of 2 dBi, which can be insufficient for the desired large cell size/good communication reliability required or wireless AP antennas. A gain of 5 dBi can provide substantial improvements in omni-directional coverage. 
     The 2 dBi gain of a half-wavelength dipole antenna can be increased by “squashing” the “donut” radiation pattern across its vertical cross-section, thus changing it from the “donut” shape of a well-known half-wavelength dipole antenna to a “squashed donut”, being flatter and larger in the azimuth plane. 
     Theoretically, such a pattern modification can be obtained, for example, by means of a couple of ordinary half-wavelength dipoles vertically stacked on top of each other to form a collinear array and fed in phase. However, the implementation of such an antenna can be troublesome primarily due to difficulties in arranging the feeding for the array elements in such a way as to avoid disturbing the radiation pattern. Known solutions to the problem of providing a feeding network in the collinear array add to the cost, size, or technical complexity of the antenna, which is undesirable. 
     It is therefore an object of the present invention to provide a feeding arrangement suitable for use in a collinear array antenna which can be implemented in a collinear array without unduly increasing the technical complexity thereof, which minimizes interference with the radiation pattern of the antenna, and which does not unduly add to the physical size of the antenna. 
     SUMMARY OF THE INVENTION 
     Thus, in one aspect of the present invention there is provided a delay line formed on an insulating sheet and having an input and an output, and comprising a single spiral revolution conductive strip coupled between the input and output. 
     There is thus provided a compact delay line suitable for use in an antenna array feeder stage. 
     The single spiral revolution conductive strip may comprise in one preferable embodiment: first to fifth conductive strips connected end-to-end in series, the first and third conductive strips being opposite to one another, the third and fifth conductive strips being opposite to one another and the second and fourth conductive strips being opposite to one another. Preferably the first and third conductive strips are parallel, the third and fourth conductive strips are parallel, and the second and fourth conductive strips are parallel. 
     The end of the first conductive strip not connected to the second conductive strip may be connected to the input by a sixth conductive strip. The end of the fifth conductive strip not connected to the fourth conductive strip may be connected to the output by a seventh conductive strip. 
     The single spiral revolution strip may comprise in another preferable embodiment: a first conductive strip coupled at one end to the input; a second conductive strip connected at one end to the other end of the first conductive strip and orientated at approximately 90° thereto; a third conductive strip connected at one end to the other end of the second conductive strip and orientated at approximately 90° thereto in a direction opposite to that of the first conductive strip: a fourth conductive strip connected at one end to the other end of the third conductive strip and orientated at approximately 90° thereto in a direction opposite to that of the second conductive strip; and a fifth conductive strip connected at one end to the other end of the fourth conductive strip and orientated at approximately 90° thereto in a direction opposite to that of the third conductive strip, and coupled at the other end thereof to the output. 
     The first conductive strip may be coupled to the input by a sixth conductive strip connected at one end to the other end of the first conductive strip and orientated at approximately 90° thereto in a direction opposite to that of the second conductive strip. The fifth conductive strip may be coupled to the output by a seventh conductive strip connected at one end to the other end of the fifth conductive strip and at its other end to the output, and orientated at approximately 90° relative to the fifth conductive strip in a direction opposite to the fourth conductive strip. 
     The first to sixth conductive strips are preferably formed on a first side of the insulating sheet, and the seventh conductive strip ( 50 ) is preferably formed on a second side of the insulating sheet. 
     Preferably, the third conductive strip is longer than the first conductive strip, the fourth conductive strip is shorter than the second conductive strip, the fifth conductive strip is shorter than the third ( 44 ) conductive strip, and the output is located opposite the input ( 34 ). 
     The present invention further provides an antenna array comprising at least one feeder stage including a single spiral revolution conductive strip delay line. 
     In another aspect of the present invention there is provided a collinear antenna array formed on an insulating sheet comprising: a first end fed dipole antenna system for a radio frequency generator having an operating wavelength L, comprising: on a first side of an insulating sheet a first and a second quarter wavelength conductive strip in end-to-end connection; on a second side of the insulating sheet a third quarter wavelength conductive strip, overlying the first quarter wavelength conductive strip, a fourth quarter wavelength conductive strip having a longer arm spaced from and parallel to the third quarter wavelength conductive strip and a shorter arm connected to the third quarter wavelength conductive strip and a fifth quarter wavelength conductive strip having a longer arm spaced from and parallel to the third quarter wavelength conductive strip, symmetrical with the fourth quarter wavelength conductive strip, and a shorter arm connected to the third quarter wavelength conductive strip; and means to connect said radio frequency generator between the end of the third quarter wavelength conductive strip remote from the connection to the fourth quarter wavelength conductive strip, and the corresponding end of the first quarter wavelength conductive strip, whereby the second and fourth quarter wavelength conductive strips form a linear dipole antenna; wherein the collinear antenna array further comprises; a feeder stage including a delay line having an input and an output and a single spiral revolution conductive strip coupled therebetween, the delay line input being connected to the end of the second quarter wavelength conductive strip remote from the first quarter wavelength conductive strip; and a monopole comprising a conductive strip having one end connected to the output of the delay line. 
     There is thus provided a collinear antenna array having a simple feeding network implementation, and an overall smaller size due to the feeder arrangement provided by the delay line having a compact size. 
     The collinear antenna array may further comprise an auxiliary antenna orientated orthogonal to the collinear antenna array. Thereby selection antenna diversity is achieved by means of a small extra antenna. The auxiliary antenna may be a bent-notch antenna. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described with reference to a preferred embodiment with reference to the accompanying drawings, in which: 
     FIG. 1 is a plan view of a printed sleeve antenna; 
     FIG. 2 is a schematic illustrating the RF currents in the aprts of the antenna of FIG. 1, 
     FIG. 3 is a plan view of a modified printed sleeve antenna also illustrating the RF currents therein; 
     FIG. 4 is a plan view of a collinear antenna array including the modified printed sleeve antenna of FIG. 3 and a phase delay line according to the present invention; 
     FIG. 5 is as detailed view of the phase delay line of FIG. 4; and 
     FIG. 6 is a plan view of the collinear antenna array of FIG. 4 with an auxiliary antenna. 
    
    
     DETAILED DESCRIPTION 
     Reference is now made to the drawings, in which like reference numerals identify similar or identical elements. FIGS. 1 and 2 illustrate an end fed dipole antenna system as described in U.S. Pat. No. 5,598,174. Such an end fed dipole antenna system utilizes a particularly advantageous feeding technique which provides an end fed dipole which operates as if it were center fed. The delay line according to the present invention can be combined with such an antenna to construct a compact collinear array antenna having high performance, as discussed hereafter. 
     FIG. 1 illustrates an antenna system, indicated generally as  10 , which comprises first and second conductive strips  12 ,  14  formed on an insulating layer or sheet  16 , such as a printed circuit board (PCB). Conductive strips  12 ,  14  are on the lower side of the PCB as viewed in FIG. 1, and are therefore shown in dashed outline. Each conductive strip is L/4 in length where L is the wavelength of operation, and the conductive strips are connected end-to-end. The end of conductive strip  12  which is remote from conductive strip  14  is connected to one side of a radio frequency (RF) generator  18  operating at the wavelength L. 
     On the upper side of the insulating layer  16  are third and fourth conductive strips  22 ,  24 ; conductive strip  22  is straight and of length L/4 and has one end connected to the other side of the RF generator  18 . Conductive strip  24  is essentially “L” shaped, the longer arm of the L lying parallel to and spaced from conductive strip  22 , and the shorter arm being connected to the opposite end of conductive strip  22  to that end of conductive strip  22  connected to the generator. Adjacent strips  22 ,  24  is a fifth conductive strip  26  perpendicular to the other four conductive strips. Conductive strip  26  is of relatively small size and provides a suitable connection for unbalanced feed means such as a coaxial feed cable (not shown) which connects the RF generator  18  to the antenna. It will be appreciated that with this arrangement, the provision of a true ground plane, which would need to be of much greater size than strip  26 , is unnecessary. Conductive strip  22  overlies conductive strip  12 , i.e., the conductive strips  22 ,  12  are in register but are separated by the thickness of PCB  16 . PCB  16  advantageously follows the general elongated outline of the strips but is of slightly greater area. 
     In FIG.2, both sides of the PCB  16  are shown in a schematic view. Above the chain dashed line are conductive strips  12 ,  14 , and below the chain dashed line are conductive strips  22 ,  24  and conductive strip  26 . While conductive strips  12 ,  14  are shown to be thinner than conductive strips  22 ,  24 , this is for clarity of illustration only; the conductive strips in practice may be of equal width. 
     It is well known in antenna theory that for optimum performance the RF currents in each arm of a linear dipole, e.g. that formed by conductive strips  12 ,  14 , must be of equal amplitude and phase, that is the dipole must be balanced. This is easily achieved if the dipole is center fed from a balanced source. However, the dipole often has to be connected to an unbalanced source (e.g. a coaxial cable or a microstrip line) which creates the need for a balun. Moreover the RF signal has to be brought to the center of the dipole (i.e. the junction between conductive strips  12  and  14 ) in a way that will not disturb the RF current distribution in the dipole itself. 
     The placement of conductive strip  22  underneath conductive strip  12  forms a transmission line that transfers the signal from the RF generator  18  to the junction of conductive strips  12  and  14 . Therefore the RF currents I 12  and I 22 , in conductive strips  12  and  22  respectively, are of equal amplitude and opposite phase. In such an arrangement, conductive strip  14  attached to conductive strip  12  can be regarded as a L/4 monopole with respect to the virtual ground positioned at the end of conductive strip  22  underneath the junction of conductive strips  12  and  14 . It can be assumed that the RF generator has moved to the other end of the line formed by conductive strips  12 ,  22  and has one of its outputs connected to conductive strip  14  and the other floating. In order to ensure that the arrangement operates as a monopole (which is by definition an unbalanced antenna) fed from an unbalanced RF source  18 , the effect of a ground plane has to be present and the other (floating) end of the RF generator  18  has to be connected to it. This results in the injecting into this ground plane of an RF current, equal in amplitude and opposite in phase to the RF current  114  in conductive strip  14 . 
     The effect of the presence of an infinite ground plane (ideal current sink) at this point (i.e. at the end of conductive strip  22  positioned below the junction of conductive strips  12  and  14 ) is achieved by placing conductive strip  24  parallel to conductive strip  22  and connecting it to conductive strip  22  (at the junction of strips  12  and  14 ). The L/4 length of conductive strip  24  forms, with respect to strip  22 , an open quarter wavelength transmission line and therefore, as seen by monopole  14  (and transferred RF generator  18 ) appears as an infinitely large ground plane since conductive strip  24  is terminated at a position of zero current and maximum voltage of the standing wave. The result is that the RF currents I 24  and I 14 , in conductive strips  24  and  14  respectively, are of equal amplitude and orientation, as in the case of a center fed dipole, while the unbalanced RF generator  18  appears to feed unbalanced monopole antenna  14 , through a microstrip line formed by conductive strips  12 ,  22 . The RF currents I 12  and I 22  cancel out each other in terms of radiation, while currents I 14  and I 24  act together as a center fed dipole. More precisely the currents in conductive strips  14  and  24  are distributed in the same way as in the arms of a center fed dipole, creating its effect of a true dipole-like radiation pattern. Although the system operates as if it were center fed, the dipole  14 ,  24  is in fact end fed (through line  12 ,  22 ), and thus has the convenience of an end fed antenna. 
     If a physical ground plane is provided at the end of conductive strip  22 , closer to the actual location of the RF generator  18  (e.g. conductive strip  26 ), it will be almost free of (unbalanced) ground currents since these are redirected to strip  24 , effectively radiating associated energy to the air. This feature of antenna  10  that prevents the occurrence of unbalanced ground currents on the ground plane associated to the antenna feeding point, is important for hand held radio devices since it can lead to significant improvements in RF efficiency. 
     In the preferred embodiment of the present invention, the end-fed dipole antenna of U.S. Pat. No. 5,598,174 described hereinabove with reference to FIGS. 1 and 2 is modified and used as part of a collinear array antenna. The end-fed dipole antenna of FIGS. 1 and 2 is modified, as shown in FIG.  3  and described further hereinafter, in order to improve the symmetry of the radiation pattern, which feature becomes more important in constructing an antenna array. 
     In FIG. 3 elements which correspond to elements shown in FIGS. 1 and 2 are identified by like reference numerals. FIG. 3 shows the PCB  16  from the opposite side shown in FIG. 1, i.e. the underside. The grey areas are on the upperside of the PCB and the white (or clear) areas on the underside the underside being visible in FIG.  3 . FIG. 3 shows the first and second conductive strips  12  and  14 , and the third and fourth conductive strips  22 ,  24 . In addition a sixth conductive strip  28  is provided, essentially “L”-shaped and symmetrical with conductive strip  24  about conductive strips  12  and  22 . 
     The provision of strip  28 , symmetric to strip  24 , improves the symmetry of the radiation pattern of the printed sleeve antenna. In operation an RF current I 28  flows in conductive strip  28 . To ensure the radiation pattern of the antenna in FIG. 3 is omnidirectional and maximized in the azimuth plane, the RF currents I 14 , I 24  and I 28  must be in phase. 
     FIG. 4 illustrates how the adapted end-fed dipole antenna of FIG. 3 is further modified to form a collinear array incorporating a delay line in accordance with the present invention. Once again, like reference numerals denote like elements. 
     The end of the conductive strip  14  remote from the conductive strip  12  is connected through an interconnection comprising a delay stage  30  to a conductive strip  32  of length L/2 forming a half-wavelength monopole. The delay stage, or delay line  30  acts as a feeder delay stage in the arrangement of FIG.  4 . 
     In order for the antenna of FIG. 4 to operate as a collinear array, the delay stage  30  must let approximately half of the total incident RF power from the RF source  18  be fed directly to the top element  32  of the collinear array. This is required to achieve a desired gain of 5 dBi, which is approximately twice the half-wavelength dipole power gain of 2 dBi. 
     The delay stage  30  must also delay the RF current supplied to the top element  32  of the collinear array by 180°, because only then will the RF currents I 14  and I 32 , in conductive strips  14  and  32  respectively, be in phase. The RF currents I 14 , I 24 , I 28  and I 32  must all be in phase to maximize the radiation pattern in the azimuth plane and ensure the desired 5 dBi power gain. 
     The delay stage  30  according to the preferred embodiment of the present invention is shown in greater detail in FIG.  5 . The specific arrangement of the delay stage  30  shown in FIG. 5 is for the specific implementation of the collinear array as discussed hereinabove, and this specific implementation is presented for illustrative purposes only to facilitate an explanation of the present invention. As discussed hereinafter, the delay stage of the specific embodiment may be modified and adapted according to the desired application, whilst still applying the principals of the present invention. 
     As illustrated in both FIGS. 4 and 5, the delay stage  30  has an input  34  and an output  36 . The delay stage input  34  is connected to the end of the conductive strip  14  remote from the conductive strip  12 , and the delay stage output  36  is connected to one end of the conductive strip  32  forming the half-wavelength monopole. 
     The delay stage  30  comprises a conductive strip, generally designated as  31 , which is formed in a single spiral revolution. That is, the single spiral revolution conductive strip  31  turns completely, once, through 360°. The single spiral revolution conductive strip  31  is comprised of five conductive strips connected end-to-end in series which are shaped to form the single spiral revolution The single spiral revolution conductive strip  31  comprises a first conductive strip  40 , a second conductive strip  42 , a third conductive strip  44 , a fourth conductive strip  46 , and a fifth conductive strip  48 . 
     The first  40 , second  42 , third  44 , fourth  46  and fifth  48  conductive strips are arranged such that the first  40  and third  44  conductive strips are substantially parallel and opposite to one another, the third  44  and fifth  48  conductive strips are substantially parallel and opposite to one another, and so that the second and fourth conductive strip  42  and  46  are substantially parallel and opposite to one other, the first to fifth conductive strips thereby forming a single spiral revolution conductive strip  31 . 
     By positioning the first conductive strip approximately parallel to the third conductive strip, the third conductive strip approximately parallel to the fifth conductive strip, and the second conductive strip approximately parallel to the fourth conductive strip, the RF currents in the respective conductive strips cancel each other out in terms of electromagnetic radiation, which is essential for the correct operation of the delay stage. Although ideally the respective conductive strips should be precisely parallel, it will be appreciated by one skilled in the art that an imperfect arrangement of the first to fifth conductive strips may still enable the delay stage  30  to operate within acceptable tolerances for the application. 
     In the preferred embodiment of FIGS. 4 and 5, the delay stage  30  thus comprises a conductive strip comprising a first conductive strip  40  coupled at one end to the delay stage input, a second conductive strip  42  connected at one end to the other end of the first conductive strip  40  and orientated at approximately 90° thereto, a third conductive strip  44  connected at one end to the other end of the second conductive strip  42  and orientated at approximately 90° thereto in a direction opposite to the first conductive strip  40 , a fourth conductive strip  46  connected at one end to the other end of the third conductive strip  44  and orientated at approximately 90° thereto in a direction opposite to that of the second conductive strip  42 , and a fifth conductive strip  48  connected at one end to the other end of the fourth conductive strip and orientated at 90° thereto in a direction opposite to that of the third conductive strip  44 , the other end of the fifth conductive strip  48  being coupled to the output  36  of the delay stage  30 . 
     The third conductive strip  44  is preferably approximately equal in length to the combined length of the first  40  and fifth  48  conductive strips to achieve ideal current balancing. Similarly the fourth conductive strip  46  is approximately equal in length to the second conductive strip  42 . However in practice, to achieve the single spiral revolution shape, the fourth conductive strip  46  is shorter than the second conductive strip  42 . 
     In the preferred embodiment of the present invention, the first conductive strip  40  is coupled to the input  34  of the delay stage  30  by a sixth conductive strip  38 , which is preferably orientated at approximately 90° to the first conductive strip  40  in a direction opposite to the second conductive strip  42 . In such preferred embodiment, the first to fifth conductive strips  40  to  48  are formed on one side of the insulating sheet together with the sixth conductive strip  38 . A seventh conductive strip  50  is provided on the other side of the insulating sheet, and couples the fifth conductive strip  48  to the output  36  of the delay stage  30 . The seventh conductive strip  50  is connected to the fifth conductive strip  48  by a via  52  through the insulating sheet  16 . The insulating sheet is preferably also provided with a via  54  to couple the end of the seventh conductive strip  50  connected to the output of the delay stage to the conductive strip  32  on the first side of the insulating sheet forming the half wavelength monopole. 
     It will be appreciated that, in an alternative arrangement, the seventh conductive strip  50  may be formed on the first side of the insulating layer and the third conductive strip  44  formed on the second side of the insulating layer, interconnections being provided to connect the appropriate ends of the second  42  and fourth  46  conductive strips. 
     FIG. 5 shows the dimensions, in millimeters, of the preferred implementation of the delay stage  30  of the invention for application in the collinear array of FIG. 4, wherein a 180° phase delay and 50% power feed is required at a frequency of operation of 2.4 to 2.5 GHz. 
     It will be appreciated by one skilled in the art that the single spiral revolution conductive strip delay line of the present may be utilized in antenna arrays requiring multiple feeds. For example in a collinear array having three antennas two feeder delay stages are required. The first feeder delay stage feeding ⅔ of the total incident RF power to the second and third antennas of the array, and the second feeder delay stage feeding ½ of the ⅔ power fed to the third antenna of the array. 
     Thus it will be appreciated by one skilled in the art that the specific dimensions of the single spiral revolution conductive strip delay line can be experimented with to achieve the required performance characteristics (phase delay, power feed) for a particular application whilst maintaining the single spiral revolution shape. 
     In a further modification, the collinear array of FIG. 4 is adapted to include an extra antenna on the insulating sheet  16  thereby to provide a means for selection antenna diversity. FIG. 6 shows a bent notch antenna implemented in the small ground plane  26  of the collinear array. Once again, like reference numerals are used in FIG. 6 for elements corresponding to elements shown in other figures. The half wave monopole formed by conductive strip  32  is not shown in FIG. 6 for reasons of clarity. 
     The bent notch antenna is indicated generally by numeral  60  in FIG.  6  and is represented diagrammatically by the white ‘L -shape’ gap in the shading representing the ground plane  26  formed on the underside of the insulating layer. The bent notch antenna  60  comprises two portions  60   a  and  60   b  forming the ‘L-shape’. The bent notch antenna  60  is an ordinary notch antenna bent into two sections in order to reduce the occupied surface. The total length of the notch in the specific application of FIG. 6 is approximately L/4, L being the operating wavelength. An antenna diversity switch  62  is also provided in the ground plane  26 , and receives the RF feed to the antenna system from a cable attachment  66 . The feeding line “enters” the notch at such a point that the input impedance is close to 50 ohm. The antenna diversity switch is an SPDT (single pole double terminal), low distortion switch. 
     The antenna diversity switch  62  includes a switch connection  64  which can switch between two switch contacts  74  and  76 . Switch contact  74  provides the RF feed via a microstrip line  68  to the colliner antenna discussed hereinabove, and switch contact  76  provides the RF feed via microstrip line  78  to the notch antenna  60 . The microstrip line  78  is connected to the bent notch antenna feeding point. 
     The specific implementation of the bent notch antenna in the ground plane of the collinear array antenna to provide an auxiliary antenna to thereby give selection antenna diversity will be within the skills of one knowledgeable in the art. The provision of the bent notch antenna as the auxiliary antenna provides a compact collinear antenna array having selection antenna diversity. 
     Whereas the collinear array both transmits and receives, the addition of a notch antenna provides an auxiliary antenna for receiving only. As the auxiliary antenna is not used for transmission, then it is not required to have the careful design and high power gain of the collinear array. 
     The provision of the auxiliary antenna enables the antenna system to provide selection antenna diversity. As is well-known, antenna diversity switching circuitry is provided to enable the auxiliary antenna to be switched on when the signal received by the collinear array is weak.