Patent Publication Number: US-2022236310-A1

Title: Over-the-air measurement system and method of testing a device under test over-the-air

Description:
FIELD OF THE DISCLOSURE 
     Embodiments of the present disclosure generally relate to an over-the-air (OTA) measurement system for testing a device under test. Further, embodiments of the present disclosure relate to a method of testing a device under test over-the-air. 
     BACKGROUND 
     In the state of the art, over-the-air (OTA) measurement systems for testing a device under test are known. Typically, so-called compact antenna test ranges (CATR) are used to test the respective device under test, wherein the compact antenna test range has a testing chamber, e.g. an anechoic chamber, in which the device under test is placed for testing purposes. The measurement system typically provides a quiet zone within the testing chamber, which is associated with a testing location of the device under test. At least one reflector is used that is located in a beam path established between the test location and a respective antenna in order to increase the traveling distance of the electromagnetic waves, thereby ensuring far-field conditions at the test location, which relates to the quiet zone. 
     The device under test may relate to a modern communication device, for instance a user end device such as a mobile phone. Modern communication devices use telecommunication standards having large frequency bandwidths. Therefore, the respective measurement systems have to cover the entire frequency range for testing purposes, which may be used by the device under test. In other words, the measurement system has to cover a large frequency range. Usually, multiple feed antennas are located inside the chamber of the measurement system in order to cover the respective frequency range. For instance, the wide band operation is necessary for testing the device under test with respect to (telecommunication) standards like 3GPP, CTIA and/or FCC. 
     For testing the respective standards, the reflector typically provides sufficient operational bandwidth, but the feed antenna used has constraints. In some embodiments, the feed antenna has to have characteristics such as a stable phase center, low sidelobes and stable beamwidth which in turn result in a narrow bandwidth of the feed antennas. 
     Accordingly, it is necessary to provide several different feed antennas for covering the frequency range at which the respective device under test is tested. One solution relates to mounting multiple feed antennas to a positioner that moves each feed antenna in to the phase center, resulting in optimal quiet zones respectively. However, the positioner is necessary which increases the costs of the measurement system. 
     Alternatively, it is known to use offset feed antennas, namely feed antennas located offset. Thus, at least a second feed antenna is located offset from a focal point of the reflector. However, this results in an angled wave front at the quiet zone, namely the testing location, thereby shifting the quiet zone away from the center. For instance, an offset of about 5 cm results in a 30-100% shift of the quiet zone out of the center. Hence, a positioner is necessary again, wherein the positioner is associated with the device under test in order to re-position the device under test. This ensures that the device under test is always located within the respective quiet zone. However, this solution also requires a positioner that increases the costs of the measurement system. 
     Accordingly, there is a need for an over-the-air measurement system that can be provided in a cost-efficient manner while ensuring wideband testing of a device under test. 
     SUMMARY 
     The present disclosure provides examples of an over-the-air (OTA) measurement system for testing a device under test. In an embodiment, the OTA measurement system comprises a plurality of feed antennas, a test location for the device under test, and a reflector array with a main reflector and a sub-reflector. The plurality of feed antennas face the sub-reflector. The reflector array is located such that a signal path is established between the plurality of feed antennas and the test location via the sub-reflector and the main reflector. The sub-reflector has at least one focal point. The plurality of feed antennas comprises a first feed antenna and at least one second feed antenna. The first feed antenna is associated with the focal point of the sub-reflector. The at least one second feed antenna is located offset from the focal point of the sub-reflector. 
     Accordingly, a special offset feed antenna arrangement is provided that comprises the at least one second feed antenna located offset with respect to the focal point with which the first feed antenna is associated. In some embodiments, the first feed antenna is located exactly at the focal point or nearby the focal point, namely within unavoidable deviations. However, the at least one second feed antenna is intentionally located offset from the focal point. 
     In any case, the respective feed antennas of the special offset feed antenna arrangement, namely the first feed antenna and the at least one second feed antenna, are directed towards a dual reflector system comprising the main reflector and the sub-reflector, namely the reflector array. 
     Hence, off-orthogonal angles can be effectively reduced by about 50% due to the special offset feed antenna arrangement and the reflector array provided. In some embodiments, the at least one second feed antenna can be located offset by a distance comprising up to about 10 cm, up to 10 cm, etc., within the special offset feed antenna arrangement due to the specific reflector array having the main reflector and the sub-reflector. 
     Generally, this allows to use several feed antennas orientated towards the reflector array while simultaneously ensuring far-field conditions at the (same) test location since the quiet zone is not shifted significantly. In other words, the center of the quiet zone does not move (significantly). Thus, it is not necessary to provide a feed antenna positioner for the feed antenna(s) and/or a positioner for the device under test, as the quiet zone, for example its center, remains substantially stationary, namely at the same location, for example with regard to its center. Accordingly, the quiet zone is not shifted due to the special offset feed antenna arrangement, namely the respective offset arrangement of the at least one second feed antenna. 
     The plurality of feed antennas may comprise more than one second feed antenna. In some embodiments, several additional feed antennas or rather a set of additional feed antennas may be provided that are directed towards the sub-reflector while being located offset from its focal point. The plurality of feed antennas may relate to a set of second feed antennas that are located offset from the focal point in a substantially similar manner, e.g. by a similar or rather at least comparable distance, namely within a range of ±1 cm. 
     An aspect provides that the first feed antenna is configured to emit electromagnetic waves in a first frequency band. The at least one second feed antenna is configured to emit electromagnetic waves in a second frequency band. The second frequency band is different from the first frequency band. Hence, a first test signal may be generated by the first feed antenna in a first frequency band. A second test signal is generated by the at least one second feed antenna in a second frequency band. The second frequency band is different from the first frequency band. 
     Accordingly, the different feed antennas are used to cover a certain frequency sub-range of the entire frequency range to be tested with regard to the device under test. The respective frequency bands associated with the different feed antennas may be complementary. 
     The first feed antenna may cover a frequency band that is associated with a frequency band associated with 5G-New Radio (5G NR), e.g. Frequency Range 2 (FR2), namely frequencies from 23 GHz up to 53 GHz. In some embodiments, the first feed antenna may be used for FR2 in-band testing at frequencies from 23 GHz to 44 GHz. 
     The at least one second feed antenna may cover a frequency range from 40 GHz to 200 GHz. In some embodiments, a set of second feed antennas is provided that are located offset from a center at which the first feed antenna is located. The focal point of the sub-reflector and the center may coincide with each other. In some embodiments, the several second feed antennas establishing the set of second feed antennas may each cover different frequency ranges which however together cover the frequency range from 40 GHz to 200 GHz. 
     According to another aspect, a central frequency of the first frequency band is lower than a central frequency of the second frequency band. For instance, the first feed antenna may cover the frequency band associated with the frequency band FR2 such that its central frequency is about 34 GHz. The central frequency of the at least one second feed antenna may relate to 50 GHz, 115 GHz or rather 75 GHz, for example. 
     For instance, a set of three second feed antennas is provided, wherein the three different second feed antennas each cover a different frequency range. The set of the second feed antennas together cover a frequency range from 40 to 140 GHz. 
     In some embodiments, one of the three second feed antennas may cover a frequency range from about 40 to about 60 GHz, another second feed antenna covers a frequency range from about 60 to about 90 GHz and a further second feed antenna covers a frequency range from about 90 to about 140 GHz. Moreover, an optional fourth second feed antenna may be provided that covers a frequency range from about 140 to about 200 GHz. Accordingly, the set of second feed antennas together covers a frequency range from about 40 to about 140 GHz or rather about 40 to about 200 GHz in some embodiments. 
     Generally, the several second feed antennas are located on a substantially circular ring around the center of the special offset feed antenna arrangement. The center of the special offset feed antenna arrangement is associated with the focal point of the sub-reflector. In other words, the location of the first feed antenna corresponds to the center of the special offset feed antenna arrangement. 
     The substantially circular ring may have a deviation from a perfect circular ring that amounts to about ±1 cm in some embodiments. 
     In other words, the several second feed antennas are located on a curve enclosing the center of the special offset feed antenna arrangement by a substantially similar distance, for example the same distance with a deviation of about ±1 cm. 
     A further aspect provides that the OTA measurement system comprises at least one third feed antenna. The at least one third feed antenna is located offset from the focal point, wherein a distance between the focal point and the at least one third feed antenna is bigger than a distance between the second antenna and the focal point. Thus, the at least one third feed antenna is located more offset with respect to the focal point of the sub-reflector compared with the second feed antenna. Hence, two different kinds of feed antennas are provided that located offset from the focal point in a different manner. 
     In a similar manner, a set of third feed antennas may be provided. The third feed antennas are located in a substantially similar manner offset from the focal point of the sub-reflector. 
     In some embodiments, the at least one third feed antenna is configured to emit electromagnetic waves in a third frequency band. The third frequency band is different from the first frequency band and from the second frequency band. Accordingly, the entire feed antenna arrangement ensures that wideband measurements of the device under test can be performed accordingly since different frequency bands are associated with different locations of the different types of feed antennas. 
     For instance, a central frequency of the second frequency band is higher than a central frequency of the third frequency band. Additionally or alternatively, a central frequency of the first frequency band is higher than a central frequency of the third frequency band. Thus, the third frequency band may be associated with a lower frequency sub-range to be tested. 
     In some embodiments, the middle portion of the overall frequency range to be tested, namely the frequency band FR2, is associated with the focal point of the sub-reflector, whereas the at least one second feed antenna associated with the higher frequency sub-range to be tested is located offset from the focal point, and whereas the at least one third feed antenna associated with the lower frequency sub-range to be tested is located more offset from the focal point compared to the at least one second feed antenna. 
     Generally, the several third feed antennas, namely the set of third feed antennas, may be located on a curve that is more distanced from the focal point, namely the center at which the first feed antenna is provided, than a curve associated with the set of second feed antennas, namely the several second feed antennas. The third feed antennas may cover a frequency range from about 6 to about 40 GHz. 
     The respective curves may be substantially circular rings, thereby ensuring that the respective feed antennas are distanced by a certain radius. However, the curves may also deviate from a perfect circular shape by a certain deviation, e.g. a deviation of about ±1 cm. 
     In any case, it is ensured that the at least one second feed antenna, for example the several second feed antennas, operated at higher frequencies is located closer to the center of the special offset feed antenna arrangement, namely the focal point of the sub-reflector, compared to the at least one third feed antenna, for example the several third feed antennas operated at lower frequencies. 
     Generally, the at least one second feed antenna operated at higher frequencies has a smaller antenna aperture compared with the at least one third feed antenna operated at lower frequencies. Since the at least one second feed antenna is located closer to the center of the special offset feed antenna arrangement, namely the focal point of the sub-reflector, than the at least one third feed antenna, it is ensured that the respective offsets from the focal point have a lower impact on a shift of the quiet zone. 
     Another aspect provides that the main reflector is located between the sub-reflector and the test location. The main reflector faces the test location and the sub-reflector such that the signal path is established between the plurality of feed antennas and the test location via the sub-reflector and the main reflector. Put differently, the feed antennas facing the sub-reflector emit electromagnetic waves that impinge on the sub-reflector that reflects the waves towards the main reflector which in turn reflects the waves towards the test location, thereby establishing the signal path between the plurality of feed antennas and the test location. 
     Thus, the electromagnetic waves emitted by the plurality of feed antennas is reflected at least twice by the antenna array comprising the main reflector and the sub-reflector. The respective reflectors of the reflector array are shaped differently, thereby ensuring the reflective characteristics required in order to create the quite zone at the test location in the intended manner. 
     For instance, the sub-reflector facing the plurality of feed antennas is established as a non-paraboloid reflector. For instance, the sub-reflector is established by an ellipsoid or a hyperboloid. Hence, the non-paraboloid reflector, namely the sub-reflector, may have two or more focal points, namely the focal point associated with the first feed antenna and at least one further focal point. 
     The at least one second feed antenna and/or the at least one third feed antenna may be associated with the at least one further focal point. However, the at least one second feed antenna and/or the at least one third feed antenna is not associated with the focal point at which the first feed antenna is located. 
     Further, the main reflector being located between the sub-reflector and the test location is established as a paraboloid reflector. The main reflector has only a single focal point that is directed towards the center of the test location or rather the center of the quite zone. 
     A further aspect provides that the OTA measurement system comprises a positioner for the device under test. The positioner is configured to position the device under test in a predetermined manner within the test location. Therefore, it is still ensured that the device under test can be re-positioned in order to follow a shifted quite zone, thereby ensuring optimum test conditions when performing the OTA measurements. However, the OTA measurement system, for example the feed antenna arrangement, ensures that it is not necessary to re-position the device under test with regard to a large frequency range. In some embodiments, the positioner may comprise one or more linear and/or angular stages. 
     Further, the OTA measurement system may comprise an RF-shielded chamber. The RF-shielded chamber encloses the plurality of feed antennas, the test location for the device under test, and the reflector array. The RF-shielded chamber relates to a testing chamber of the OTA measurement system, which encompasses the respective components of the OTA measurement system used for testing the device under test. The RF-shielded chamber may also be called anechoic chamber as it is ensured that no interfering signal from the outside interact with the components located in the chamber, thereby ensuring that those signals from the outside do not have an influence or rather impact on the respective measurements performed. 
     Moreover, the OTA measurement system may comprise measurement equipment. The measurement equipment is configured to conduct OTA measurements. The measurement equipment may be connected with the feed antennas and a test terminal associated with the test location such that a connection can be established with the device under test. Accordingly, it is ensured that the device under test located at the test location can be measured appropriately with regard to its far-field characteristics by controlling the feed antennas appropriately and by evaluating signals received by the measurement equipment. Depending on the test scenario, signals received from the feed antennas or rather signals received from the device under test are evaluated accordingly. 
     In some embodiments, a quite zone is established by the plurality of feed antennas together with the reflector array. The test location is located within the quite zone. Therefore, it is ensured that the device under test can be tested with regard to its far-field characteristics accurately. 
     The OTA measurement may comprise a device under test that is located in the test location. Therefore, the OTA measurement system itself may already comprise the device under test. 
     Embodiments of the present disclosure also provides a method of testing a device under test over-the-air. In an embodiment, the method comprises the steps of: 
     providing a device under test; 
     providing an OTA measurement system for testing the device under test over-the-air, wherein the OTA measurement system comprises a plurality of feed antennas, a test location for the device under test, and a reflector array with a main reflector and a sub-reflector, the plurality of feed antennas facing the sub-reflector, the reflector array being located such that a signal path is established between the plurality of feed antennas and the test location via the sub-reflector and the main reflector, the sub-reflector having at least one focal point, the plurality of feed antennas comprising a first feed antenna and at least one second feed antenna, the first feed antenna being associated with the focal point of the sub-reflector, and the at least one second feed antenna being located offset from the focal point of the sub-reflector; 
     placing the device under test at the test location; and 
     performing an over-the-air measurement on the device under test. 
     The method can be performed by one or more of the OTA measurement systems described above. In some embodiments, the characteristics and advantages mentioned above also apply to the method in a similar manner. 
     Generally, the special offset feed antenna arrangement may comprise up to 8 feed antennas nested in three different layers, namely the first feed antenna located at the center of the special offset feed antenna arrangement as well as seven feed antennas located offset from the focal point of the sub-reflector, in particular wherein the seven feed antennas located offset from the focal point of the sub-reflector are nested in two different layers, e.g. on two different curves around the center. For instance, four second feed antennas are provided that are located closer to the center than three third feed antennas. 
     According to an embodiment, an inner curve associated with the second feed antennas is provided that is distanced from the focal point by 3 to 5 cm, for example by 3.75 cm, whereas an outer curve is provided that is distanced from the focal point by 8 to 10 cm, for example by 10 cm, wherein the outer curve is associated with the third feed antennas. 
     Moreover, the second feed antennas are operated at frequencies higher than the frequencies of the third feed antennas. 
     Generally, the main reflector may have an edge treatment, thereby improving the reflective properties of the main reflector, for example at its edges. 
     Further, the sub-reflector may also have an edge treatment in order to also improve its reflective properties in its edge area. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and many of the attendant advantages of the claimed subject matter will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  schematically shows an overview of an over-the-air (OTA) measurement system according to an embodiment of the present disclosure; 
         FIG. 2  schematically shows a more detailed view on an overview of a special offset feed antenna arrangement used by the over-the-air (OTA) measurement system shown in  FIG. 1 ; 
         FIG. 3  schematically shows an overview of field distributions for the FR2 co-polarization patterns indicating the influence of the offset on the field distributions; and 
         FIG. 4  schematically shows an overview of field distributions for the FR2 cross-polarization patterns indicating the influence of the offset on the field distributions. 
     
    
    
     DETAILED DESCRIPTION 
     The detailed description set forth below in connection with the appended drawings, where like numerals reference like elements, is intended as a description of various embodiments of the disclosed subject matter and is not intended to represent the only embodiments. Each embodiment described in this disclosure is provided merely as an example or illustration and should not be construed as preferred or advantageous over other embodiments. The illustrative examples provided herein are not intended to be exhaustive or to limit the claimed subject matter to the precise forms disclosed. 
       FIG. 1  shows an over-the-air (OTA) measurement system  10  for testing a device under test  12 . The OTA measurement system  10  comprises a testing chamber  14  established by an RF-shielded chamber so as to shield an interior of the chamber  14  from interfering and disturbing signals. In the testing chamber  14 , the device under test  12  is located at a test location  16 . 
     The OTA measurement system  10  also comprises measurement equipment  18  that is used for testing the device under test  12 . In the shown embodiment, the measurement equipment  18  is connected with a test terminal  20  in order to establish a connection with the device under test  12  so as to receive signals from the device under test  12  that can be evaluated by the measurement equipment  18 . The measurement equipment  18  is connected with a feed antenna array  22  that comprises a plurality of feed antennas  24 ,  26 , namely a first feed antenna  24  and at least one second feed antenna  26 , as will be described later in more detail when referring to  FIG. 2 . 
     The OTA measurement system  10  further includes a reflector array  28  that comprises a main reflector  30  and a sub-reflector  32 . As shown in  FIG. 1 , the reflector array  28 , for example its reflectors  30 ,  32 , is arranged within the testing chamber  14  such that a signal path  34  is established between the plurality of feed antennas  24 ,  26  and the test location  16  via the sub-reflector  32  and the main reflector  30 . Accordingly, the sub-reflector  32  and the main reflector  30  together relate to a specific dual-reflector system. 
     In some embodiments, the feed antennas  24 ,  26  face the sub-reflector  32  of the reflector array  28  such that electromagnetic waves emitted by the feed antennas  24 ,  26  impinge on the sub-reflector  32  that reflects the electromagnetic waves towards the main reflector  30  that faces the test location  16 . Accordingly, the electromagnetic waves impinging on the main reflector  30  are reflected towards the test location  16 , thereby establishing a quiet zone  35  at the test location  16 . Hence, the device under test  12  is located within the quiet zone  35  at the test location  16 , thereby ensuring far-field conditions for testing purposes. 
     The sub-reflector  32  facing the plurality of feed antennas  24 ,  26  has a focal point  36  indicated by the dotted line in  FIG. 1 . Accordingly, the first feed antenna  24  is associated with the focal point of the sub-reflector  32 . In some embodiments, the first feed antenna  24  is located exactly at the focal point  36  or nearby the focal point  36 , namely within unavoidable deviations. In contrast thereto, the at least one second feed antenna  26  is located offset from the focal point  36  of the sub-reflector  32 . Specifically, the second feed antenna  26  is intentionally located offset from the focal point  36  to which the first feed antenna  24  is assigned. 
     In the shown embodiment, the sub-reflector  32  is established as a non-paraboloid reflector having a further focal point besides the focal point  36 . Hence, the sub-reflector  32  may have more than one focal point. For instance, the sub-reflector  32  is established by an ellipsoid or a hyperboloid. 
     In contrast thereto, the main reflector  30  located between the sub-reflector  32  and the test location  16  is established as a paraboloid reflector having a single focal point that is associated with the test location  16 . 
     Accordingly, the respective antenna array  22  comprising the several feed antennas  24 ,  26  relates to a special offset feed antenna arrangement  38  since the at least one second feed antenna  26  is (intentionally) located offset from the reflector on which the electromagnetic waves emitted impinge directly, namely the sub-reflector  32 . 
     An embodiment of the antenna array  22 , namely the special offset feed antenna arrangement  38 , is shown in more detail in  FIG. 2 . 
     The antenna array  22  comprises the first feed antenna  24  associated with the focal point  36  of the sub-reflector  22 . In some embodiments, the center of the first feed antenna  24  coincides with the focal point  36 . The first feed antenna  24  is configured to emit electromagnetic waves in a first frequency band, namely a frequency band associated with Frequency Range 2 (FR2), e.g. frequencies from 23 GHz up to 53 GHz. In some embodiments, the first feed antenna  24  can be used for FR2 in-band testing of the device under test  12  at frequencies ranging from 23 GHz to 44 GHz. 
     Furthermore, the antenna array  22 , namely the special offset feed antenna arrangement  38 , comprises a set of second feed antennas  26  that are distanced from the focal point  36 , e.g. the center of the antenna array  22  or rather the special offset feed antenna arrangement  38 , by a substantially equal distance labelled by d1. Small deviations from this distance may occur as shown for the lower second feed antennas  26  that are also displaced in horizontal direction by a certain distance. 
     Alternatively, the lower second feed antennas  26  may be displaced in vertical orientation by a smaller distance than d1 such that all second feed antennas  26  are located on a (perfect) circle around the focal point  36 , which has a radius corresponding to distance d1. This is illustrated by the dashed lines in  FIG. 2 . 
     Generally, the second feed antennas  26  together cover a frequency range from 40 GHz to 140 GHz, e.g. one of the second feed antennas  26  covers the frequency range from 40 GHz to 60 GHz, another second feed antenna  26  covers the frequency range from 60 GHz to 90 GHz, and a further second feed antenna  26  covers the frequency range from 90 GHz to 140 GHz. 
     Optionally, a fourth second feed antenna  26  may be provided that coves a frequency range from 140 GHz to 200 GHz such that all second feed antennas  26  cover a frequency range from 40 GHz to 200 GHz. 
     Accordingly, the at least one second feed antenna  26 , for example each of the second feed antennas  26 , is configured to emit electromagnetic waves in a second frequency band that is different from the first frequency band, for example higher than the first frequency band. Accordingly, a central frequency of the first frequency band is lower than a central frequency of the second frequency band. 
       FIG. 2  also shows that the antenna array  22 , namely the special offset feed antenna arrangement  38 , has at least one third feed antenna  40  that is also located offset from the focal point  36 . A distance between the focal point  36  and the at least one third feed antenna  40  is bigger than the distance between the second antenna  26  and the focal point  36 . 
     In the shown embodiment, a set of third feed antennas  40  is provided, namely three different third feed antennas  40  each covering a different frequency range, namely from 6 GHz to 8 GHz, from 8 GHz to 15 GHz and from 15 GHz to 23 GHz. 
     Alternatively, a plurality (e.g., four) of the third feed antennas  40  or rather differently designed third feed antennas  40  may be provided, thereby covering a frequency range from 6 GHz to 40 GHz in total. 
     For instance, the set of third feed antennas  40  may comprise a single third feed antenna  40  that covers a frequency range from 6 GHz to 40 GHz and two additional feed antennas  40  associated with a frequency range from 8 GHz to 15 GHz and from 15 GHz to 23 GHz, respectively. 
     Accordingly, the at least one third feed antenna  40  is configured to emit electromagnetic waves in a third frequency band that is different from the first frequency band and from the second frequency band. In some embodiments, a central frequency of the second frequency band is higher than a central frequency of the third frequency band. Additionally or alternatively, a central frequency of the first frequency band is higher than a central frequency of the third frequency band. Therefore, the respective feed antennas  24 ,  26 ,  40  together cover a frequency range from 6 GHz to 140 GHz or optionally from 6 GHz to 200 GHz. 
     Accordingly, the different feed antennas  24 ,  26 ,  40  are used to cover a certain frequency sub-range of the entire frequency range used for testing the device under test  12 . The frequency bands associated with the different feed antennas  24 ,  26 ,  40  are complementary, but may overlap slightly. 
     Generally, the plurality of second feed antennas  26  or rather third feed antennas  40  are located offset from the focal point  36  in a substantially similar manner, e.g. by a similar or rather at least comparable distance, namely within a range of ±1 cm for example. 
     In some embodiments, the several second feed antennas  26  are located on a substantially circular ring around the center of the special offset feed antenna arrangement  38 . However, the substantially circular ring may have a deviation from a perfect circular ring that amounts to about ±1 cm. Thus, the several second feed antennas  26  are located on a curve enclosing the center of the special offset feed antenna arrangement  38  by a substantially similar distance, for example the same distance with a deviation of about ±1 cm. For instance, the curve associated with the second feed antennas  26  has a distance d1 between 3 cm and 4 cm. 
     The same applies for the several third feed antennas  40  that are also located on a substantially circular ring around the center of the special offset feed antenna arrangement  38 . However, the substantially circular ring may have a deviation from a perfect circular ring that amounts to about ±1 cm. Thus, the several third feed antennas  40  are located on a curve enclosing the center of the special offset feed antenna arrangement  38  by a substantially similar distance, for example the same distance with a deviation of about ±1 cm. For instance, the curve associated with the third feed antennas  40  has a distance d2 between 8 cm and 10 cm. 
     The distance associated with the second feed antennas  26  is smaller than the distance associated with the third feed antennas  40 . Hence, the second feed antennas  26  are located closer to the center of the special offset feed antenna arrangement  38  or rather the focal point  36  than the third feed antennas  40 . 
     The second feed antennas  26  are operated at higher frequencies such that their respective apertures are smaller compared to the ones of the third feed antennas  40  that are operated at lower frequencies. Therefore, the second feed antennas  26  are located closer to the center of the special offset feed antenna arrangement  38 , thereby providing an overall compact antenna array  22 . 
     Accordingly, the first set of offset feed antennas, namely the second feed antennas  26 , is located closer to the center antenna, namely the first feed antenna  24 , while operating at higher frequencies (therefore smaller antenna aperture) than the second set of antennas furthest from the center antenna. The second set of antennas furthest from the center antenna correspond to the third feed antennas  40  that are operated at the lowest frequencies of all feed antennas  24 ,  26 ,  40  of the antenna array  22 . 
     In  FIG. 2 , a table is also shown indicating that the specific offset feed antenna arrangement  38  together with the reflector array  28  ensure that a shift of the quiet zone  35  can be neglected compared to systems known in the state of the art since a wavefront of emitted electromagnetic waves is angled by 3.5 degrees in case of an offset of 10° cm for the feed antennas contrary to the prior art resulting in a wavefront angled by 7.4 degrees, requiring a re-positioning of the antenna array  22  and/or the device under test  12  in order to ensure proper testing conditions. 
     Accordingly, off-orthogonal angles can be effectively reduced by about 50% due to the special offset feed antenna arrangement  38  and the reflector array  28 . 
     This is also illustrated in  FIGS. 3 and 4  providing overviews of the respective field distributions. 
     Even though a re-positioning of the device under test  12  is not necessary due to the small shift of the quiet zone,  FIG. 1  shows that a positioner  42  for the device under test  12  is provided. The positioner  42  is configured to position the device under test  12  in a predetermined manner within the test location  16 . In some embodiments, the positioner  42  can be used for certain test scenarios. In some embodiments, the positioner  42  includes one or more controllable linear and/or angular stages. 
     The OTA measurement system  10  is used for testing the device under test  12 . Accordingly, the different feed antennas  24 ,  26 ,  40  may be used for generating different test signals, namely a first test signal, a second test signal and a third test signal respectively. The different test signals are associated with different frequency ranges as discussed above, thereby ensuring that the device under test  12  can be tested in a wideband range appropriately. 
     The present application may reference quantities and numbers. Unless specifically stated, such quantities and numbers are not to be considered restrictive, but exemplary of the possible quantities or numbers associated with the present application. Also in this regard, the present application may use the term “plurality” to reference a quantity or number. In this regard, the term “plurality” is meant to be any number that is more than one, for example, two, three, four, five, etc. The terms “about,” “approximately,” “near,” etc., mean plus or minus 5% of the stated value. For the purposes of the present disclosure, the phrase “at least one of A and B” is equivalent to “A and/or B” or vice versa, namely “A” alone, “B” alone or “A and B.”. Similarly, the phrase “at least one of A, B, and C,” for example, means (A), (B), (C), (A and B), (A and C), (B and C), or (A, B, and C), including all further possible permutations when greater than three elements are listed. 
     The principles, representative embodiments, and modes of operation of the present disclosure have been described in the foregoing description. However, aspects of the present disclosure which are intended to be protected are not to be construed as limited to the particular embodiments disclosed. Further, the embodiments described herein are to be regarded as illustrative rather than restrictive. It will be appreciated that variations and changes may be made by others, and equivalents employed, without departing from the spirit of the present disclosure. Accordingly, it is expressly intended that all such variations, changes, and equivalents fall within the spirit and scope of the present disclosure, as claimed.