A HYBRID ANTENNA MEASUREMENT CHAMBER

TECHNICAL FIELD

The present disclosure relates to test equipment for antenna systems and wireless devices in general. There are also disclosed systems and methods for measuring the performance of antenna systems and for testing wireless devices.

BACKGROUND

Over-the-air (OTA) characterization of antenna systems evaluates the impact of hardware, wave propagation, and signal processing on the overall antenna system. Challenges in OTA characterization come from the global shift towards multi-antenna systems (as in multiple-input multiple-output, MIMO, antenna systems, or massive MIMO systems), advanced signal processing (e.g. hybrid analog-digital beamforming), and much higher levels of integration between individual components (e.g., antennas, power amplifiers, filters, etc.). Conductive testing (i.e. not wirelessly) of antenna systems is not sustainable for a large number of antenna ports, and is even more difficult with massive MIMO systems, especially when operated at higher frequency bands, e.g., at mmWave. High levels of integration also make conductive testing challenging. Furthermore, the global shift towards multi-antenna systems requires the antenna systems to be evaluated in terms of many different performance figures-of-merits (FOMs).

Today, there are no antenna measurement systems or techniques that support all required performance FOMs in order to test their conformance to international standards using a single measurement setup or even a single antenna measurement chamber. Moreover, standardized measurement chambers and techniques are in many cases complex, and hence not so cost- efficient. Therefore, there is a need for antenna measurement systems and techniques that are flexible, and cost and time effective.

SUMMARY It is an object of the present disclosure to provide improved measurement chambers and measurement methods for measuring performance of an antenna under test.

This object is at least in part obtained by a measurement chamber for measuring performance of an antenna under test, AUT, arranged inside the measurement chamber. The measurement chamber comprises a radio frequency reflective surface arranged circumferentially about a main axis A of the measurement chamber. The reflective surface is arranged to form a waveguide extending along the main axis A. The waveguide has two waveguide openings at respective ends of the waveguide. The measurement chamber further comprises a mode generator antenna arranged inside the measurement chamber. The mode generator antenna is arranged to transmit and/or to receive a radio frequency signal to/from the AUT via at least a part of the waveguide. The radio frequency signal comprises a plurality of different- order propagating modes. The mode generator antenna and radio frequency scattering properties inside the measurement chamber are arranged to present a selected mode configuration at the AUT.

The disclosed measurement chamber enables an increased flexibility, compared to known techniques, of generating and reconfiguring any desired antenna testing conditions in a single measurement environment, i.e. , chamber, while at the same time keeping the measurement time and costs down. For example, the measurement chamber enables characterization of antenna systems in anechoic environment, the environment of a reverberation chamber, or anything in between in a single measurement system. More specifically, the measurement chamber allows for different testing conditions ranging from isotropic multipath environment to a single line of sight environment, and anything in between. This allows for testing the AUT in many different scenarios, e.g. different beamforming and multiple-input multiple- output antenna system scenarios. The measurement chamber is capable of characterizing the AUT in at least total radiated power, total isotropic sensitivity, effective isotropic radiated power, effective isotropic sensitivity, error vector magnitude, adjacent channel leakage ratio, and spectrum emission mask. Furthermore, the measurement chamber has high power handling capabilities.

According to aspects, at least one delimiting surface of the measurement chamber forms a body with a through hole along the main axis (A), wherein the at least one delimiting surface of the measurement chamber constitutes the waveguide

This provides a cost effective and easy to assemble waveguide. For example, two walls, the ceiling, and the floor of a chamber may be covered or coated with electrically conductive material, such as metal (e.g. aluminum or copper).

According to aspects, at least one delimiting surface of a reverberation chamber forms a body with a through hole along the main axis A, where the at least one delimiting surface of the reverberation chamber constitutes the waveguide. In that case, the two remaining walls of the reverberation chamber may comprise radio frequency absorbent material

This way, a normal reverberation chamber may be converted into the measurement chamber.

According to aspects, a waveguide opening comprises a section with blinds. The blinds are arranged to reflect radio frequency signals when in a closed state and to let radio frequency signals pass through when in an opened state.

Preferably, radio frequency absorbent material is arranged such that radio frequency signals incident on the section with blinds is attenuated when the blinds are in the opened state. The blinds may be partially open. This way, the amount of absorption of the radio frequency absorbent material may be controlled, i.e. the amount of attenuation of the electromagnetic signal incident on the section with blinds is controlled. Such controllable attenuation could be used for having the measurement chamber operating mostly like a reverberation chamber, but with some attenuation to give the measurement chamber much higher power capabilities (over a normal reverberation chamber), which is an advantage.

According to aspects, at least one delimiting surface of a reverberation chamber forms a body with a through hole along the main axis A, where the at least one delimiting surface of the reverberation chamber constitutes the waveguide. In that case, the two remaining walls of the reverberation chamber may comprise one or more holes arranged to pass the radio frequency signal to/from the AUT and the mode generator antenna. This way, particular scattering properties of the middle compartment can be separated from wave propagation towards/from the AUT/mode generator antenna.

According to aspects, the one or more surfaces in the measurement chamber outside of the waveguide comprise radio frequency absorbent material. This way, the AUT and mode generator antenna can be placed in anechoic compartments while a middle compartment presents an environment similar to that in a reverberation chamber. The benefits of a reverberation chamber can then be combined with the benefits of deterministic wave propagation directions given by an anechoic environment. According to aspects, the measurement chamber comprises one or more mode stirrers.

The mode stirrer (also called tuner) can reduce the inhomogeneity of standing waves in a cavity, which is advantageous when the measurement chamber is operating at or close to a normal reverberation chamber. The mode stirrer may also constitute the optional reflective element used to present a selected mode configuration at the AUT. The mode stirrer may be curved or adapted for certain propagations/directions and may be movable or reconfigurable. The mode stirrer may cover a whole wall, ceiling or floor. This way, the waveguide dimensions of the waveguide may be reconfigurable. According to aspects, the waveguide openings may be closed off by respective modular wall sections. In that case, the AUT and/or the mode generator antenna is arranged on the one or more modular wall sections.

The modular wall sections can be used to quickly swap between different mode generator antennas arranged in different wall sections and/or between different AUTs arranged in different wall sections.

The above object is also at least in part obtained by a method for measuring performance of an antenna under test, AUT, arranged inside a measurement chamber. The method comprises configuring a radio frequency reflective surface circumferentially about a main axis of the measurement chamber. The reflective surface is arranged to form a waveguide extending along the main axis, where the waveguide has two waveguide openings at respective ends of the waveguide. The method further comprises configuring a mode generator antenna inside the measurement chamber. The mode generator antenna is arranged to transmit and/or receive a radio frequency signal to the AUT via at least a part of the waveguide. The radio frequency signal comprises a plurality of propagating modes. The method also comprises arranging the mode generator antenna and the radio frequency scattering properties inside the measurement chamber to present a selected mode configuration at the AUT. This can be done by extracting the propagation properties of the measurement chamber, e.g. via computer simulations and/or via routine experimentation. By knowledge of the propagation properties, that is, chamber modes, of the measurement chamber, the mode generator antenna can be constructed to couple to the modes in whatever way is advantageous for the testing to be done. The method further comprises measuring performance of the AUT.

The methods disclosed herein are associated with the same advantages as discussed above in connection to the different measurement devices. There are furthermore disclosed herein control units adapted to control some of the operations described herein.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled person realizes that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will now be described in more detail with reference to the appended drawings, where Figures 1-4 schematically illustrate example measurement chambers.

Figure 5 is a flowchart illustrating methods.

DETAILED DESCRIPTION

Aspects of the present disclosure will now be described more fully with reference to the accompanying drawings. The different devices and methods disclosed herein can, however, be realized in many different forms and should not be construed as being limited to the aspects set forth herein. Like numbers in the drawings refer to like elements throughout.

The terminology used herein is for describing aspects of the disclosure only and is not intended to limit the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.

Challenges of OTA characterization of antenna systems come from the global shift towards multi-antenna systems, more advanced (digital) signal processing, and much higher levels of integration between individual components and sub-systems. Multi-antenna systems and signal processing allows for various beamforming and MIMO techniques, such as: beamforming, which serves single users by directing the energy of the electromagnetic signal toward the user; generalized beamforming, which serve single users by sending the same data stream in different directions and possibly forming zeros (nulls) in the directions of other users; single-user MIMO (SU-MIMO), which increases data rates by transmitting several data streams to a user; multi-user MIMO (MU-MIMO), which simultaneously serve multiple users.

Performance testing of such multi-antenna systems generally requires complicated test methods and there are many critical figures of merit (FOM) for performance of the antenna system. Especially relevant FOMs are: the total radiated power (TRP) in transmit mode; the total isotropic sensitivity (TIS) in the receive mode; the effective isotropic radiated power (EIRP) in transmit mode; the effective isotropic sensitivity (EIS) in receive mode; the error vector magnitude (EVM), the adjacent channel leakage ratio (ACLR); and the spectrum emission mask (SEM). Many of these FOMs are dependent on, i.a, noise, interfering signals, nonlinear distortion and impedances, e.g., the antenna impedance presented to a nonlinear component connected to it. It is noted that many other FOMs may also be relevant.

There are no known measurement systems that can support all required measurement FOMs using a single measurement setup nor a single antenna measurement chamber, i.e. , a do-it-all chamber. The subsequent use of multiple conventional chambers is too expensive to satisfy all the impending needs of antenna characterization. Furthermore, a multi-room measurement campaign is time-consuming. FOM measurements are typically done at many occasions, as in, e.g., production testing, conformance testing, performance testing, device characterization during the design phase, etc.

The state-of-the-art antenna measurement methods typically utilize large- scale anechoic chambers (ACs). One example is indoor anechoic chamber (IAC), where a measurement probe antenna is placed in the radiating near field region of the antenna under test (AUT). The method can be used to perform the most comprehensive far- and near-field tests. A drawback is that the test range is relatively large compared to a second example: the compact antenna test range (CATR), which is similar to IAC but is more compact due to quasi-optical transformations, typically performed through reflector surfaces. A third example is the plane wave synthesizer, PWS (or plane wave generator, PGS), which synthesize a plane wave via an array antenna directly illuminating the DUT. This room can be compact as well but requires careful calibration and is not the most cost-effective solution. Reverberation chambers (RCs) represent a relatively new and more cost-effective measurement technology, especially for TRP and TRP-based figures-of-merit, as compared to ACs, but can be utilized for a limited number of measurement metrics and propagation scenarios. At least one limitation of the AC-based methods is that only a single angle-of-arrival (AoA)/angle-of-departure (AoD) can be emulated. This problem is resolved when measuring in RCs, which create a so-called Rayleigh channel.

The herein disclosed measurement chamber 100 enables an increased flexibility, over known techniques, of generating and reconfiguring any desired antenna testing conditions in a single measurement environment, i.e. , chamber, while at the same time keeping the measurement time and costs down. For example, the measurement chamber 100 enables characterization of antenna systems in an anechoic environment, the environment of a reverberation chamber, or anything in between in a single measurement system. More specifically, the measurement chamber 100 allows for different testing conditions ranging from isotropic multipath environment (as in conventional RC) to a single line of sight environment (as in conventional AC), and anything in between. This allows for testing the AUT in many different scenarios, e.g. different beamforming and MIMO scenarios. The measurement chamber 100 is capable of characterizing the AUT in at least TRP, TIS, EIRP, EIS, EVM, ACLR, and SEM. The measurement chamber is enabled by generating a selected mode configuration at the AUT, where the corresponding complex-valued fields in the AUT test zone are controlled (power, incident angles, time delay). The mode configuration at the AUT is related to how the measurement chamber is arranged. A basic example is that a simple rectangular waveguide can support ordinary rectangular modes of electromagnetic wave propagation. By changing the shape or adding elements in the chamber, these modes can be shaped to arrange for a desired mode configuration at the AUT. Another example is a circular waveguide which supports other modes than the rectangular ones. In whatever arrangement the chamber is configured, locally around the AUT, the mode configuration at hand can be decomposed in local plane waves presented to the AUT. By doing so, the AUT can be tested under the plane wave conditions desired for the specific measurement or test to be made.

Figure 1 shows an example measurement chamber 100 for measuring performance of an antenna under test, AUT, 110 arranged inside the measurement chamber. The measurement chamber comprises a radio frequency reflective surface 122 arranged circumferentially about a main axis A of the measurement chamber 100. The reflective surface 122 is arranged to form a waveguide extending along the main axis A. The waveguide has two waveguide openings 121 at respective ends of the waveguide. The measurement chamber 100 further comprises a mode generator antenna 130 arranged inside the measurement chamber. The mode generator antenna is arranged to transmit and/or to receive 131 a radio frequency signal to/from the AUT 110 via at least a part of the waveguide. The radio frequency signal comprises a plurality of different-order propagating modes. The mode generator antenna 130 and radio frequency scattering properties inside the measurement chamber 100 are arranged to present a selected mode configuration at the AUT.

The selected mode configuration at the AUT 110 is achieved by generating propagating modes that interact with radio frequency scattering properties inside the measurement chamber 100. The plurality of different-order propagating modes is propagating inside the waveguide. In addition to the radio frequency reflective surfaces 122 of the waveguide, there may be an optional reflective element inside the chamber, such as a movable reflective plate. In other words, the plurality of different-order propagating modes is generated explicitly by utilizing the scattering properties of the reflective waveguide walls and elements inside the chamber to present a selected mode configuration at the AUT 110. Hence, to present a selected mode configuration at the AUT, particular signal(s) from the mode generator antenna 130 are selected in conjunction with the scattering properties inside the measurement chamber. This can be done by extracting the propagation properties of the measurement chamber, e.g. via computer simulations and/or via routine experimentation. By knowledge of the propagation properties, that is, chamber modes, of the measurement chamber, the mode generator antenna can be constructed to couple to the modes in whatever way is advantageous for the particular testing to be done. Whatever way is advantageous may, e.g., be found via computer simulations and/or via routine experimentation.

The radio frequency reflective surfaces 122 of the waveguide and optional reflective element inside the chamber preferably comprise a good electrical conductor, such as aluminum, copper, or brass. Metalized polymers or electrically conductive polymers are also possible.

Preferably, the one or more signals from the mode generator antenna 130 and the scattering properties inside the measurement chamber are reconfigurable. This way, many different mode configurations may be presented to the AUT 110. The scattering properties inside the measurement chamber may be reconfigurable by, e.g., having a wall of the waveguide be movable or by having the reflective element inside the chamber be movable. The mode generator antenna can be a single antenna, a co-located antenna array, a plurality of co-located antenna arrays, and/or a plurality of distributed antennas. The mode generator antenna may comprise an antenna array wherein all the radiating elements are not spaced equally relative to each other. Figure 1 shows a measurement chamber 100 with the mode generator antenna 130 and the AUT 100 arranged at the opposite sides of the measurement chamber. Figure 2 shows a measurement chamber with the mode generator antenna 130 arranged at two opposite sides of the measurement chamber. Reconfiguring the one or more signals from the mode generator antenna may be done by adjusting relative phases and amplitudes transmitted by individual radiating elements (when there is a plurality of them). This is similar to beamforming and/or any MIMO functionality in a conventional array antenna. Reconfigurability may also be achieved by changing the position of the mode generator antenna. This can mean to move all radiating elements in an array equally or to move all or some radiating elements differently.

Some example scenarios the measurement chamber 100 is capable of characterizing the AUT under are: a plane wave emitted in the broadside direction of the mode generator antenna; a plane wave emitted in the off- broadside direction of the mode generator antenna; multiple plane waves emitted in different off-broadside directions of the mode generator antenna at different time intervals; characterization in the proximity of other active devices (antennas, transceivers, etc.) that may interfere with the AUT. All of these scenarios represent different selected mode configuration at the AUT.

The AUT may comprise multiple antennas, as in a co-located array or as in a distributed system. The AUT may comprise a transceiver, as in a radio base station, a point-to-point radio, or a handheld device. It is also noted that several AUTs may be tested at the same time, e.g. a plurality of handheld devices. The chamber may further comprise a movable pedestal 230 (shown in Figure 2) which the AUT can be placed upon.

According to aspects, at least one delimiting surface of the measurement chamber forms a body with a through hole along the main axis (A), wherein the at least one delimiting surface of the measurement chamber constitutes the waveguide. A particular example is a rectangular chamber wherein two walls arranged at opposite sides of the main axis A, the floor, and the ceiling constitute the waveguide. Example dimensions of a rectangular measurement chamber 100 could be a 2 m x 2 m cross section and a length of 3 m. Such measurement chamber may have other shapes too, such as curved walls, or a circular, elliptical, or polygon shape circumferentially arranged along the main axis A. Other shapes forming a waveguide are also possible. According to aspects, at least one delimiting surface of a reverberation chamber forms a body with a through hole along the main axis A, where the at least one delimiting surface of the reverberation chamber constitutes the waveguide. In that case, the two remaining walls of the reverberation chamber may comprise radio frequency absorbent material. This way, a reverberation chamber may be converted into the measurement chamber 100. A reverberation chamber is a cavity resonator with a high Q-factor and with minimum absorption of electromagnetic energy. A particular example is a rectangular reverberation chamber wherein two walls arranged at opposite sides of the main axis A, the floor, and the ceiling constitute the waveguide. Other shapes forming a waveguide are also possible.

A radio frequency absorbent material generally comprises lossy materials that attenuate transmission and reflection of electromagnetic radiation. As such, a radio frequency absorbent material should be neither a good electrical isolator (as in, e.g., rubber) nor a good electrical conductor (as in, e.g., copper). An example of a radio frequency absorbent material is a foam material loaded with iron and/or carbon. Radio frequency absorbent material can be resonant, i.e. a particular frequency is attenuated (e.g. 25 GHz), or broadband, i.e. a span of frequencies is attenuated (e.g. 1 GHz to 50 GHz). The attenuation of electromagnetic radiation in a direction is dependent on the thickness of the radio frequency absorbent material in the same direction. One example of attenuation per length is 10 dB/cm at 2 GHz. Another example is 150 dB/cm at 30 GHz. These two examples of attenuation per length could be applicable to the disclosed measurement chamber 100.

A waveguide opening may comprise a section with blinds. The blinds are arranged to reflect radio frequency signals when in a closed state and to let radio frequency signals pass through when in an opened state. The section may be part of or the whole waveguide opening 121. Preferably, radio frequency absorbent material is arranged such that radio frequency signals incident on the section with blinds is attenuated when the blinds are in the opened state. The blinds preferably comprise a good electrical conductor, such as aluminum, copper, or brass. The blinds may comprise rectangular sheets similar to normal window blinds. However, any shape that may be arranged to reflect radio frequency signals when in a closed state and to let radio frequency signals pass through when in an opened state is possible. The blinds may be partially open. This way, the amount of absorption of the radio frequency absorbent material may be controlled, i.e. the amount of attenuation of the electromagnetic signal incident on the section with blinds is controlled. Such controllable attenuation could be used for having the measurement chamber operating mostly like an RC, but with some attenuation to give the measurement chamber much higher power capabilities (over a normal RC), which is an advantage.

According to aspects, at least one delimiting surface of a reverberation chamber forms a body with a through hole along the main axis A, where the at least one delimiting surface of the reverberation chamber constitutes the waveguide. In that case, the two remaining walls of the reverberation chamber may comprise one or more holes 423 arranged to pass the radio frequency signal to/from the AUT 110 and the mode generator antenna 130. This way, particular scattering properties of the middle compartment can be separated from wave propagation towards/from the AUT/mode generator antenna. In the example of Figure 4, the measurement chamber 100 comprises a hole at respective sides of the waveguide along the axis A. In the figure, the two holes 423 constitute the two waveguide openings 121 . The holes 423 may optionally comprise blinds. The mode generator antenna may comprise distributed units where some are arranged inside the waveguide and some are arranged outside the waveguide, i.e. on the other side of the hole 423. The surface extending in the same plane as the hole has a radio frequency reflective surface facing into the waveguide. It is noted that any number of holes on each respective wall is possible. Through blinds, reflective doors, reflective movable plates, or the like, the holes may be reconfigurable. Thus, the number of holes and their respective position may be reconfigurable.

The one or more surfaces in the measurement chamber 100 outside of the waveguide may comprise radio frequency absorbent material. For example, if the measurement comprises holes 423, as in Figure 4, all surfaces outside of the waveguide may be covered in radio frequency absorbent material. This way, the AUT and mode generator antenna can be placed in anechoic compartments while a middle compartment presents an environment similar to that in a reverberation chamber. The benefits of a reverberation chamber can then be combined with the benefits of deterministic wave propagation directions given by an anechoic environment.

The measurement chamber 100 may comprise one or more mode stirrers 210, 220. The mode stirrer (also called tuner) can reduce the inhomogeneity of standing waves in a cavity, which is advantageous when the measurement chamber is operating at or close to a normal RC. The mode stirrer may also constitute the optional reflective element used to present a selected mode configuration at the AUT. The mode stirrer could comprise a flat rectangular metal plate. Other shapes are also possible. The mode stirrer may be curved or adapted for certain propagations/directions and may be movable or reconfigurable. The mode stirrer may cover a whole wall, ceiling or floor. This way, the waveguide dimensions of the waveguide may be reconfigurable.

The waveguide openings 121 may be closed off by respective modular wall sections. In that case, the AUT 110 and/or the mode generator antenna 130 may be arranged on the one or more modular wall sections. Example of such wall sections are shown in Figures 3 and 4. The walls, ceilings, and floors of the wall sections may optionally be clad completely in a radio frequency absorbent material. The modular wall sections can be used to quickly swap between different mode generator antennas arranged in different wall sections and/or between different AUTs arranged in different wall sections.

As mentioned previously, the measurement chamber 100 is, according to different aspects, capable of characterizing the AUT in at least TRP, TIS, EIRP, EIS, EVM, ACLR, and SEM. Below follow examples of how such characterization could be achieved. As also mentioned previously, the measurement chamber 100 enables characterization of antenna systems in an anechoic environment, the environment of a reverberation chamber, or anything in between in a single measurement system. Presenting the environment of a reverberation chamber could be achieved by, e.g., the closing of the aforementioned blinds, or by having a reverberation chamber comprising one or more holes 423 constituting the waveguide, wherein the holes 423 may be closed. Presenting an anechoic environment could be achieved by, e.g., arranging radio frequency absorbent material on the one or more surfaces in the measurement chamber 100 outside of the waveguide.

To measure TRP or TIS, one could configure the measurement chamber 100 to act as a reverberation chamber and measure TRP or TIS according to well know procedures of measuring TRP or TIS in a reverberation chamber.

To measure EIRP, one could configure the measurement chamber 100 to an anechoic environment, and then configure the mode generator antenna so that the different modes of the chamber can be resolved. The AUT can then be configured to radiate (i.e. to couple) optimally for whichever mode is desired and then measured using the mode generator antenna. If the results are sought for in plane wave basis, the mode generator antenna is excited in a manner to resolve a particular mode.

To measure EIS, one could configure the measurement chamber 100 in the same way as for EIRP, but wherein the AUT 110 is transmitting with decreasing output power according to normal the EIS measurement procedure.

To measure EVM, one could arrange the measurement chamber 100 in the same way as for EIRP. The AUT 110 is then set to transmit know symbols and these are decoded using the mode generator antenna. Any directional dependence of EVM can be resolved in the same manner as directions are resolved in EIRP measurements.

To measure ACLR or SEM, one could arrange the measurement chamber 100 in the same way as to measure TRP.

Figure 5 is a flowchart illustrating methods. There is illustrated a method for measuring performance of an antenna under test, AUT, 110 arranged inside a measurement chamber 100. The method comprises configuring S1 a radio frequency reflective surface 122 circumferentially about a main axis A of the measurement chamber 100. The reflective surface 122 is arranged to form a waveguide extending along the main axis A, where the waveguide has two waveguide openings 121 at respective ends of the waveguide. The method further comprises configuring S2 a mode generator antenna 130 inside the measurement chamber. The mode generator antenna is arranged to transmit and/or receive 131 a radio frequency signal to the AUT 110 via at least a part of the waveguide. The radio frequency signal comprises a plurality of propagating modes. The method also comprises arranging S3 the mode generator antenna 130 and the radio frequency scattering properties inside the measurement chamber 100 to present a selected mode configuration at the AUT. This can be done by extracting the propagation properties of the measurement chamber, e.g. via computer simulations and/or via routine experimentation. By knowledge of the propagation properties, that is, chamber modes, of the measurement chamber, the mode generator antenna can be constructed to couple to the modes in whatever way is advantageous for the testing to be done. The method further comprises measuring S4 performance of the AUT.