Abstract:
An anechoic chamber ( 100 ) is provided with a system for introducing wireless communication signals into the anechoic chamber ( 100 ) for establishing wireless signaling with equipment under test ( 122 ), without interfering with signals from the equipment under test ( 122 ) that are to be measured in the anechoic chamber ( 100 ). The system comprises a low reflection cross-section cable ( 236 ) that extends through the chamber ( 100 ) to a weakly radiating small antenna ( 242, 502 ) that is positioned proximate the equipment under test ( 122 ).

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates in general to radio frequency testing.  
       BACKGROUND OF THE INVENTION  
       [0002]     Radio frequency anechoic chambers are used to provide controlled repeatable environments for performing radio frequency radiation tests. Radio frequency anechoic chambers are meant to approximate unbounded regions of free space for electromagnetic waves and are meant to provide environments in which radio frequency radiation tests can be made without introducing errors caused by reflected waves or standing waves.  
         [0003]     One type of test performed in radio frequency anechoic chambers is the measurement of power radiated from a piece of radio frequency equipment (termed the Equipment Under Test, EUT) as a function of polar and azimuth angle. Such a test allows a complete characterization of the spatial dependence of electromagnetic waves radiated by the EUT. The floor, ceiling and walls of radio frequency anechoic chambers are tiled with radio frequency absorbers that are provided to substantially diminish reflections and standing waves. The EUT is supported away from the absorbing walls, ceiling and floor, in order to make measurements. Elevating the EUT with a support also allows a measurement antenna to be moved so as to view the EUT from a wide range (nearly  180  degrees) of polar angle. To avoid large disturbances of the radio frequency fields emitted by the EUT, the support is made from dielectric materials as opposed to metal. An improved test stand that is suitably used in anechoic chambers is covered in co-pending patent application Ser. No. 10/805996.  
         [0004]     One measure of the quality of an anechoic chamber is the magnitude of unwanted reflections. In an ideal anechoic chamber, there are no reflections. One way to measure the level of unwanted reflections of radio frequency waves in an anechoic chamber that is configured for measuring radiated radio frequency wave power as a function of polar angle, is to install a transmitting antenna that radiates uniformly as a function of polar angle (e.g., a horizontally oriented dipole) at a center of rotation and rotate a receiving antenna over a large range of polar angle with respect to the transmitting antenna while measuring the power received by the receiving antenna. In an ideal radio frequency anechoic chamber there would be no variation in the measured field. The variation that occurs is termed “ripple”. Ripple can arise from a variety of sources.  
         [0005]     Certain tests performed in anechoic chambers call for wireless communication to be maintained between the EUT and a test equipment transceiver. For example, to simulate real use, the power radiated from the EUT is suitably measured while the EUT is exchanging signals with a test equipment transceiver. In order for a test equipment transceiver to be able to communicate wirelessly with an EUT, an antenna that is connected to the test equipment transceiver is placed in the anechoic chamber. In certain commercial systems, a spiral antenna is placed in an anechoic chamber. Placing an additional antenna in an anechoic chamber has the drawback that the additional antenna will partly reflect signals emitted by the EUT, thereby increasing the ripple in the anechoic chamber. Thus, there is a desire to maintain a wireless link between an EUT and a test equipment transceiver without introducing an antenna that will cause substantial reflections and increase the amount of ripple. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0006]     The present invention will be described by way of exemplary embodiments, but not limitations, illustrated in the accompanying drawings in which like references denote similar elements, and in which:  
         [0007]      FIG. 1  is a cut-away view of a radio frequency anechoic chamber according to an embodiment;  
         [0008]      FIG. 2  is a cut-away side view of a test stand, along with half of a U-shaped swing arm used to support a measurement antenna in the radio frequency anechoic chamber shown in  FIG. 1 ;  
         [0009]      FIG. 3  is a sectional view of a portion of a low reflection cross-section cable used in the radio frequency anechoic chamber shown in  FIG. 1 ;  
         [0010]      FIG. 4  is a sectional view of an end of the low reflection cross-section cable shown in  FIG. 3  including a small sub-resonance monopole antenna; and  
         [0011]      FIG. 5  shows an end portion of the low reflection cross-section cable shown in  FIG. 3  including a small sub-resonance loop antenna according to an alternative embodiment. 
     
    
     DETAILED DESCRIPTION  
       [0012]     As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. Further, the terms and phrases used herein are not intended to be limiting; but rather, to provide an understandable description of the invention.  
         [0013]     The terms a or an, as used herein, are defined as one or more than one. The term plurality, as used herein, is defined as two or more than two. The term another, as used herein, is defined as at least a second or more. The terms including and/or having, as used herein, are defined as comprising (i.e., open language). The term coupled, as used herein, is defined as connected, although not necessarily directly, and not necessarily mechanically.  
         [0014]      FIG. 1  is a cut-away view of a radio frequency anechoic chamber  100  according to an embodiment. The anechoic chamber  100  is bounded by a metal outer skin  102 . The metal outer skin  102  serves to block radio frequency or other electromagnetic interference emanating from outside the anechoic chamber  100  from entering the anechoic chamber  100  and corrupting measurements taken in the chamber  100 . In the embodiment shown in  FIG. 1 , the anechoic chamber  100  is box shaped, and the metal outer skin  102  comprises a ceiling  104 , a floor  106 , and four walls  108  (two of which are partially visible in  FIG. 1 ). Alternatively, the anechoic chamber  100  has a different shape such as another prism shape (e.g., pentagonal, hexagonal), a cylindrical shape or a hemispherical shape, for example.  
         [0015]     Within the metal skin  102  is an inner enclosure  110  that is made from plywood or some other material. A second metal skin (not shown) is optionally included inside the inner enclosure  110 . In the embodiment shown in  FIG. 1 , the inner enclosure  110  comprises a floor  112 , a ceiling (not visible in  FIG. 1 ), a left wall  114 , a right wall  116 , a back wall  117 , and a front wall (not visible in  FIG. 1 ).  
         [0016]     The inner enclosure  110  is lined with radio frequency absorbing material. In the embodiment shown in  FIG. 1 , the radio frequency absorbing material takes the form of pyramidal radio frequency absorbers  107  that are tiled on the inner surfaces of the inner enclosure  110 . The pyramidal radio frequency absorbers  107  serve to reduce reflections and standing waves in the chamber  100 .  
         [0017]     A test stand  118  is located in the middle of the floor  112  of the inner enclosure  110 . The test stand  118  is more fully described below with references to  FIG. 2 . As shown in  FIG. 1 , the test stand  118  supports an EUT  122  (e.g., a cellular telephone) and a “phantom head”  120 . The phantom head  120  is a hollow model of a human head that is filled with a fluid that approximates the electrical characteristics of a real person&#39;s head. A cellular telephone is often operated while held adjacent to a user&#39;s head. The phantom head  120  is used to simulate the effect of a person&#39;s head on the radio frequency field generated by a cellular telephone. As shown, the cellular telephone EUT  122  is mounted to the phantom head  120 . Rubber bands, packaging tape, adhesives or special mounts are suitable for attaching a cellular telephone EUT  122  to the phantom head  120 .  
         [0018]     A U-shaped swing arm  124  is disposed in the chamber  100 . The swing arm  124  includes a horizontal beam  126  that is connected perpendicularly at opposite ends to a first radial arm  128 , and a second radial arm (not visible in  FIG. 1 ). The horizontal beam  126  extends through a first arcuate slot  130  in the left wall  114  of the inner enclosure  110 , and a congruently shaped second arcuate slot  132  in the right wall  116  of the inner enclosure  110 . The first radial arm  128  is disposed between the right wall  116  of the inner enclosure  110  and the outer skin  102 . Similarly, the second radial arm (which parallels the first radial arm, but is not visible in  FIG. 1 ) is disposed between the left wall  114  of the inner enclosure  110  and the outer skin  102 . Ends of the two radial arms  128  remote from the horizontal beam  126  extend to a virtual pivot axis  134 , about which the swing arm  124  rotates. The virtual pivot axis  134  intersects an axis  135  through the test stand  118 . At the virtual pivot axis  134 , the first radial arm  128  is connected to a first shaft  136 , and the second radial arm (not shown) is coupled to a second shaft (not shown). The first shaft  136  extends through the outer skin  102  and is supported by a bearing  138 . The second shaft (not shown) is suitably similarly supported. The first shaft  136  is coupled to a first stepper motor  140  that is used to rotate the swing arm  124  and thereby control the polar angle of the horizontal beam  126  with respect to the EUT  122 .  
         [0019]     The horizontal beam  126  supports a measurement antenna  142  that is typically used to receive signals from the EUT  122 . The measurement antenna  142  can be swung through a range of polar angles about the EUT  122  mounted on the test stand  118 .  
         [0020]     Although not shown in  FIG. 1 , the test stand  118  is suitably supported on a rotation mechanism  222  ( FIG. 2 ) located at the floor  112  of the inner enclosure  110 . The rotation mechanism  222  allows the azimuth of the test stand  118  carrying the EUT  122  to be varied relative to the measurement antenna  142 . Thus, radio frequency signals emitted by the EUT  122  over a range of polar and azimuth angles can be measured in the chamber  100 .  
         [0021]     Radio frequency instrumentation  144  is located outside the chamber  100 . The radio frequency instrumentation  144  comprises a spectrum analyzer  146  (or other equipment suitable for making radio frequency measurements) that is coupled to the measurement antenna  142  by a first coaxial cable  148  that includes a first segment of low reflection cross-section cable  150  that extends along the first radial arm  128  and the horizontal beam  126  of the swing arm  124 . Using the first segment of low reflection cross-section cable  150  reduces unwanted reflections in the chamber  100 . The radio frequency instrumentation  144  also includes a test equipment transceiver  152  that is coupled by a second coaxial cable  154  to a small, sub-resonance monopole antenna  242  ( FIG. 2 ). If the EUT is a cellular telephone, a base station simulator is suitably used as the test equipment transceiver  152 . The radio frequency instrumentation  144  is capable of operating at a frequency of operation of the EUT  122 .  
         [0022]     A master controller  156  (e.g. a personal computer equipped with test instrument interfacing hardware and software) is coupled to the radio frequency instrumentation  144 , the first stepper motor  140 , and to a second stepper motor  228  ( FIG. 2 ) of the rotating mechanism  222  ( FIG. 2 ) that controls the azimuth of the test stand  118 . The master controller  156 , although not essential, allows testing to be automated.  
         [0023]      FIG. 2  is a cut-away side view of the test stand  118  shown in  FIG. 1 , along with half of the U-shaped swing arm  124  used to support the measurement antenna  142  in the radio frequency anechoic chamber  100 . The test stand  118  comprises a lower vertical support column  202 , a middle vertical support column  204 , and an upper support member  206 . Aspects of the test stand  118  are covered in co-pending application Ser. No. 10/805,996. The lower vertical support column  202  includes a lower tube  208 , and an upper tube  210 . The upper tube  210  can be inserted to different depths in the lower tube  208  in order to set the height of the test stand  118 . A dielectric collar  218  and a plurality of dielectric bolts  220  are used to set the insertion depth of the upper tube  210  into the lower tube  208 . A bottom end  212  of the lower tube  208  is attached to a mounting flange  214  with the aid of a plurality of gussets  216 . The lower vertical support column  202  and middle vertical support column  204  suitably comprise hollow fiberglass tubes. The upper support member  206  suitably comprises a block of expanded polystyrene.  
         [0024]     The mounting flange  214  is disposed on the rotation mechanism  222 . The rotation mechanism  222  comprises a first gear  224  (e.g., a ring gear) that is attached to the mounting flange  214  with a plurality of fasteners  215  and a second gear  226  (e.g., a pinion gear) that is coupled to the second stepper motor  228  and engaged with the first gear  224 . A bearing assembly  230  rotatably supports the first gear  224  and the test stand  118 .  
         [0025]     The second coaxial cable  154  includes a first portion  232  that extends from the test equipment transceiver  152  to a rotary coaxial cable joint  234 . The rotary coaxial cable joint is suitably positioned on the axis  135  of the test stand  118  near the bottom end  212  of the lower tube  208 . A second segment of low reflection cross-section cable  236  extends from the rotary coaxial cable joint  234  up through the lower vertical support column  202 , into the middle vertical support column  204 , out of a side hole  238  in the middle vertical support column  204  and to the upper support member  206 . An end  240  of the second segment of low reflection cross-section cable  236  is suitably attached (e.g., with packaging tape, not shown) to the upper support member  206 .  
         [0026]     The small sub-resonance monopole antenna  242 , which has a length that yields a resonant frequency substantially above the frequency of the EUT  122 , extends from the end  240  of the second segment of low reflection cross-section cable  236 . The small sub-resonance monopole antenna  242  is suitably less than ⅛ of the free space wavelength corresponding to the frequency of the EUT  122 . Although the radiative efficiency of the small sub-resonance monopole antenna  242  is low (at the frequency of the EUT  122 ) due to the small size, given the close proximity of the antenna  242  to the EUT  122 , the antenna  242  is able to effectively exchange wireless signals with the EUT  122 . Significantly, the small sub-resonance monopole antenna  242  has a relatively small perturbative effect on fields radiated by the EUT  122 , so that the small sub-resonance monopole antenna  242  does not greatly increase the ripple of the anechoic chamber  100 . Because the small sub-resonance monopole antenna  242  rotates with the EUT  122  (i.e., maintains a fixed relationship with the EUT  122 ), there is a no risk of losing communication due to the EUT moving into an antenna pattern null of the small sub-resonance monopole antenna  242  or vice versa. The fixed relationship facilitates the use of the small sub-resonance monopole antenna  242 .  
         [0027]     A pyramidal absorber  244  of the type used to line the chamber  100 , but which has been trimmed near the base to fit within the lower tube  208 , is positioned within the lower tube  208 . The pyramidal absorber  244  in the lower vertical support column  202  serves to dampen interaction between the EUT  122 , and the rotation mechanism  222 , which might otherwise degrade the ripple performance of the chamber  100 . The second segment of low reflection cross-section cable  236  extends through a longitudinal hole  246  in the pyramidal absorber  244 . A sheet of microwave absorbing material  248  that is cut to form a series of upwardly extending narrow tapered portions is used to line the middle vertical support column  204 . The microwave absorbing material  248  also serves to improve the ripple performance of the anechoic chamber  100 .  
         [0028]      FIG. 3  is a sectional view of a portion of the low reflection cross-section cable  150 ,  236  used in the radio frequency anechoic chamber  100  shown in  FIG. 1 . The low reflection cross-section cable  150 ,  236  comprises a coaxial cable  302  that includes a tubular outer conductor  304  and an inner conductor  306  that is arranged coaxially within the tubular outer conductor  304 . A cylindrical insulator  308  is disposed between the outer conductor  304  and the inner conductor  306 .  
         [0029]     In  FIG. 3 , one of a series of cylindrical resonators  310  (also referred to as chokes) that are arranged one after another along the low reflection cross-section cable  150 ,  236  is shown. The resonators  310  serve to reduce reflections from the low reflection cross-section cable  150 ,  236 , and thereby improve the ripple performance of the anechoic chamber  100 . Each of the resonators  310  comprise a cylinder  312  having an open first end  314  and a second end  316  that is closed by a circular wall  318  that includes a central hole  320  through which the coaxial cable  302  passes. Solder or other conductive joint material  322  is used to establish electrical contact between the tubular outer conductor  304  of the coaxial cable  302  and the circular wall  318  at the central hole  320 . A cylindrical dielectric plug  324  is positioned within each resonator  310 . The cylindrical dielectric plug  324  includes an axial bore  326  that is aligned with the central hole  320  in the circular wall  318 . The coaxial cable  302  extends through each axial bore  326  in each cylindrical dielectric plug  324 .  
         [0030]     The cylindrical dielectric plug  324  extends out of the open first end  314 . A protruding length  328  of each cylindrical dielectric plug  324  serves to maintain spacing between successive cylindrical resonators  310 . Spacing the cylindrical resonators  310  serves to control a parasitic capacitance between the open first end  314  of one cylindrical resonator  310  and the second end  316  of an adjacent cylindrical resonator  310  so as to control the effect of the parasitic capacitance on the effective electrical length of the cylindrical resonators  310 .  
         [0031]     The resonators  310  are dimensioned and spaced so as to support a one-quarter wave coaxial mode resonance (including the effect of the fringe capacitance and dielectric loading) at a frequency nominally equal to the operating frequency of the EUT. Note that the outer conductor  304  of the coaxial cable  302  serves as an inner conductor of the cylindrical resonators  310 .  
         [0032]     Although, not wishing to bound by any particular theory of operation, it is believed that the cylindrical resonators  310  reduce reflections from the low reflection cross-section cable  150 ,  236  by creating an open circuit (at the operating frequency of the EUT  122 ) at approximately every quarter-wavelength along the cable&#39;s  150 ,  236  exterior, effectively breaking the cables  150 ,  236  into a series of short (less than quarter-wave) isolated conductors that are poor radiators.  
         [0033]      FIG. 4  is a sectional view of the end  240  of the second segment of low reflection cross-section cable  236  shown in  FIG. 2  including the small sub-resonance monopole antenna  242 . As shown in  FIG. 4 , the small sub-resonance monopole antenna  242  is formed by extending the inner conductor  306  beyond the outer tubular conductor  304 . The cylindrical resonators  310  are positioned on the coaxial cable  302  such that the open first end  314  of the cylindrical resonator  310  closest to the small sub-resonance monopole antenna  242  faces the antenna  242 .  
         [0034]     According to an alternative embodiment rather than extending the inner conductor  306  to form the low reflectance sub-resonance monopole antenna  242 , the coaxial cable  302  is terminated with a first connector (e.g., an SMA coaxial connector) and the low reflectance sub-resonance monopole is attached to a second connector that mates with the first connector.  
         [0035]      FIG. 5  shows an end portion of the second segment of low reflection cross-section cable  236  shown in  FIG. 2  including a loop antenna  502  according to an alternative embodiment. The loop antenna  502  is substantially smaller than would be required to support resonance at the operating frequency of the EUT  122 . However, since the loop antenna  502  is to be positioned proximate the EUT  122  wireless signaling can be established between the EUT  122 , and the loop antenna  502 . The small size of the loop antenna  502  helps to minimize the ripple in the anechoic chamber  100 .  
         [0036]     As used herein the term ‘radio frequency’ is used in a general sense and includes all frequencies that are used for wireless communications and that are radiated and received using antennas.  
         [0037]     While the preferred and other 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 of ordinary skill in the art without departing from the spirit and scope of the present invention as defined by the following claims.