Broadband test antenna with low passive intermodulation

Abstract

A broadband test antenna (14) for performing PIM measurements has a plurality of antenna components (20) and a plurality of insulating layers (44), wherein each of the antenna components (20) engages with at least one other of the antenna components (20), wherein between engaging antenna components (20) one of the insulating layers (44) is located so that the engaging antenna components (20) are galvanically insulated from one another but coupled capacitively to one another. Further, a test equipment (10) is provided.

Description

[0001] Broadband test antenna with low passive intermodulation Technical Field

[0002] The invention relates to a broadband test antenna as well as to a test equipment for measuring passive intermodulation (PIM) of a device under test.

[0003] Background Mobile communication antennas are used in many frequency bands and are typically designed to receive and to transmit radio frequency signals in a frequency duplexing system (FDD) at the same time slot. In such antennas, already smallest nonlinearities in the antenna, e.g. based on material and contact distortions, can produce passive intermodulation products, which may fall into the receiving band. This is typically the case with the third order PIM product.

[0004] Such PIM products in the receiving band degrade the sensitivity level of the antenna. To avoid such degradation a test equipment is required capable of measuring the PIM level of a device under test, in particular an antenna under test, and confirming the specification level. Most desirably, every antenna that is manufactured is tested.

[0005] To measure such PIM level of a DUT, often the reflected PIM is measured at the device or antenna port(s). Nevertheless, for better evaluation of such devices or antennas, the transmitted PIM may also be measured. In doing so, the location of the source generating the PIM may be detected more easily. By means of an induced PIM, the surface of the reflector and the quality of the radiating elements may be evaluated.

[0006] For PIM measurements, test equipment with test antennas are required to enable the measurement of the transmitted PIM level and / or to send a two tone or multi-tone test signal to the DUT and to produce induced PIM in the antenna structure.

[0007] A PIM test procedure is described as example in US9455792. Here a reflection measurement with phase shifting in the signals is disclosed.

[0008] In US9728846 an antenna with low PIM values is disclosed. The antenna is designed for normal transmit and receive PIM level, typically in a distributed antenna system (DAS) which fulfills the typical specified PIM level of -153 dBc and shows a typical broad pattern characteristic. The basic structure discloses a radiator based on a related ground plane.

[0009] Nevertheless, antennas according to the prior art solutions still generate PIM in the test equipment and the test antenna itself, deteriorating measurement sensitivity.

[0010] Summary

[0011] It is an object of the present invention provide a test antenna and a test equipment with an extremely low PIM level. For this purpose, a broadband test antenna, in particular for performing PIM measurements, is provided. The test antenna comprises a plurality of antenna components and a plurality of insulating layers. Each of the antenna components engages with at least one other of the antenna components, wherein between engaging antenna components one of the insulating layers is located so that the engaging antenna components are galvanically insulated from one another but coupled capacitively to one another.

[0012] By galvanically insulating the antenna components from one another, sources of PIM at the interfaces between engaging antenna components are eliminated. In consequence, the PIM level of the test antenna is significantly reduced, allowing for very sensitive PIM measurements.

[0013] For example, the insulating layer prevents galvanic coupling between the engaging components.

[0014] In particular, each antenna component is galvanically insulated from each of the other antenna components.

[0015] In an aspect, the insulating layers are insulating foils, layers of lacquer and / or hardcoatings, in particular hard coated oxide layers, allowing for reliable insulation.

[0016] The insulating foil may be glued to an antenna component using an adhesive.

[0017] In an embodiment, the insulating layer has a thickness of equal to or less than 100 pm, in particular equal to or less than 50 pm, providing an excellent capacitive coupling of engaging antenna components.

[0018] In order to achieve reliable insulation, the respective insulating layer may be applied to at least one of the engaging antenna components at least on an engaging portion of the surface engaging with the other one of the engaging antenna components. In an aspect, the antenna components are made of a conductive material, in particular metal, and / or manufactured by subtractive manufacturing, in particular machining.

[0019] In particular the antenna components have a surface roughness of Rz 4 (mean roughness depth), reducing PIM sources at the antenna components themselves.

[0020] For improved measurement performance, the test antenna may be a dual polarized antenna.

[0021] Further, the test antenna may a horn antenna, e.g. a quad-ridged-hom antenna, or a Vivaldi antenna.

[0022] In an embodiment, at least one of the antenna components is a back cavity component having a frame portion and providing a cavity within the frame portion. This way, a cavity of the antenna is provided easily.

[0023] For achieving a planar surface, at least one of the antenna components may be a flange component engaging with the at least one back cavity component.

[0024] The flange component is in particular flat.

[0025] In an aspect, at least two, in particular at least four of the antenna components are ridge components arranged in a pair of one first ridge component and one second ridge component opposite to one another, in particular wherein the ridge components engage with the at least one back cavity component and / or the at least one flange component, providing a high quality beam.

[0026] For example, the ridge components of the same pair form a tapered slot between them.

[0027] The ridge components of two pairs may be arranged in a space efficient cross configuration. The ridge components may be fixed to the flange component and / or to the back cavity component via an adhesive or screws, in particular plastic screws.

[0028] In an aspect, the first ridge component comprises a channel extending through the first ridge component, allowing a feeding of the pair of ridge components.

[0029] The channel is, for example, open towards the second ridge component and / or the channel extends laterally with respect to the radiation direction.

[0030] In order to allow a simple feed, the test antenna may comprise a coaxial cable having an outer conductor and an inner conductor, wherein the outer conductor is electrically coupled to the first ridge component and the inner conductor extends through the channel and is electrically coupled to the second ridge component.

[0031] For example, the electrical coupling is a galvanic coupling, e.g. by soldering, or a capacitive coupling.

[0032] In a further aspect, one of the antenna components is a reflector, also eliminating PIM sources with respect to the reflector.

[0033] In an embodiment, at least one, in particular at least four of the antenna components are horn components, each forming a sidewall of a horn, in particular a horn encompassing the ridge components. This way, a horn test antenna is provided.

[0034] For simplifying manufacturing of the horn components, adjacent horn components may engage with one another, in particular wherein the horn components engage with the at least one back cavity component and / or the at least one flange component.

[0035] For example, each sidewall engages with a respective one of the ridge components. The horn encompasses the ridge components laterally with respect to the radiation direction of the antenna.

[0036] For above mentioned purpose, further a test equipment for performing PIM measurements at a device under test is provided. The test equipment comprises at least one test antenna as described above, in particular wherein the test equipment comprises an anechoic chamber in which the at least one test antenna is located.

[0037] The features and advantages described with respect to the test antenna also apply to the test equipment and vice versa.

[0038] In an embodiment, the test equipment comprises a conveyor for conveying a device under test through a beam of the test antenna, in particular wherein the conveyor extends at least partially through the anechoic chamber. This way, each antenna manufactured may be tested inline.

[0039] Brief Description of the Drawings

[0040] Further features and advantages will be apparent from the following description as well as the accompanying drawings, to which reference is made. In the drawings:

[0041] Fig. 1 shows schematically a test equipment according to an embodiment of the invention with a test antenna according to an embodiment of the invention,

[0042] Fig. 2 shows the broadband antenna of Figure 1 isolated in a perspective view,

[0043] Fig. 3 shows a back cavity component of the antenna of Figure 2 in a perspective top view,

[0044] Fig. 4 shows two pairs of the ridge components of the antenna of Figure 2 in a perspective bottom view,

[0045] Fig. 5 shows a cross-section of the antenna of Figure 2, Fig. 6 shows a second embodiment of a test antenna according to the invention in a perspective view,

[0046] Fig. 7 shows parts of the antenna of Figure 6 in a perspective bottom view, and

[0047] Figs. 8-11 show various antenna components of the antenna of Figure 6.

[0048] Detailed Description

[0049] Figure 1 shows a test equipment 10 for performing passive intermodulation (PIM) measurements at a device under test 12.

[0050] The test equipment 10 comprises at least one broadband test antenna 14 for performing the PIM measurements, an anechoic chamber 16 and a conveyor 18.

[0051] In the shown embodiment, the conveyor 18 is a conveyor belt conveying the device under test 12.

[0052] It is also conceivable that the conveyor 18 is any other means of conveying the device under test 12, for example a robot arm or a carriage.

[0053] The test antenna 14 is located in the anechoic chamber 16, in the shown embodiment on a ceiling of the anechoic chamber 16 defining the measurement area.

[0054] The conveyor 18 extends through the anechoic chamber 16. The anechoic chamber 16 is open at two ends but apart from that surrounds the conveyor 18 providing a shielded measurement area.

[0055] The device under test 12 is in particular an antenna under test, for example an antenna for mobile communication base stations.

[0056] The conveyor 18 is configured to convey the device under test 12 through the beam of the test antenna 14 and thus through the measurement area. Figure 2 shows the test antenna 14 in a perspective view.

[0057] The test antenna 14 is, in the shown embodiment, a dual polarized quad- ridged-hom antenna having a plurality of antenna components 20.

[0058] A Vivaldi antenna is also conceivable.

[0059] In the first embodiment, the test antenna 14 comprises as antenna components 20 four ridge components 22, a back cavity component 24 and a reflector 26.

[0060] The antenna components 20 may be made of conductive material, in particular metal.

[0061] For example, the antenna components 20 are manufactured by subtractive manufacturing, in particular machining, and the surface roughness of the antenna components 20 is, for example, below Rz 4 (mean roughness depth).

[0062] The back cavity component 24 is best seen in Figure 3 and comprises a frame portion 28 enclosing a cavity 30 laterally with respect to the radiation direction R of the test antenna 14.

[0063] Further, on each of the four sides of the frame portion 28 a rib 32 extends into the cavity, more precisely towards the center of the cavity 30.

[0064] The frame portion 28 has, on its upper surface and on its lower surface, engaging portions 34 for engagement with the ridge components 22 or for engagement with the reflector 26, respectively.

[0065] On the upper surface of the back cavity component 24, screw holes are provided.

[0066] The ridge components 22, which are also shown in Figure 4 in a bottom view, are arranged in pairs so that the test antenna 14 comprises two pairs of ridge components 22. Within each pair, a first ridge component 36 and a second ridge component 38 are present.

[0067] The ridge components 22 of a pair are aligned with one another and arranged opposite to one another so that a tapered slot is formed between the ridge components 22 of a pair.

[0068] As best seen in Figure 4, the ridge components 22 of the two pairs of ridge components are arranged in a cross configuration meaning that the ridge components 22 form a cross when seen in a bottom view.

[0069] Figure 5 shows a cross section through a pair of ridge components 22, the back cavity component 24 and the reflector 26 in an assembled state of the test antenna 14.

[0070] As can be seen, a channel 40 is provided in the first ridge component 36.

[0071] The channel 40 extends laterally with respect to the radiation direction R of the test antenna 14 and fully through the first ridge component 36. Thus, the channel 40 is open towards the second ridge component 38 and, on the outer side, towards the environment.

[0072] In the second ridge component 38, a recess 42 is provided.

[0073] The recess 42 extends from the lower side of the second ridge component 38 and is aligned with and open towards the channel 40 of the first ridge component 36.

[0074] In the assembled state shown in Figure 5, the antenna components 20 engage with one another. This is explained exemplarily for the back cavity component 24 engaging with the reflector 26 but holds true for the other antenna components 20 as well. The two antenna components 20, i.e. the back cavity component 24 and the reflector 26, are, however, not in direct physical contact but an insulating layer 44 of the test antenna 14 is located between them.

[0075] For example, two antenna components 20 may be regarded as engaged with one another if they are, apart from the separation due to the insulating layer 44 and a possible adhesive, directly attached to one another.

[0076] The insulating layer 44 is applied onto the engaging portion 34 of the back cavity component 24 or the respective engaging portions of the reflector 26. The engaging portions 34 of the antenna components 20 are the portions that would be in contact with each other if the insulating layer 44 (and a possible adhesive) were not present.

[0077] In Figure 5, the insulating layers 44 are shown exaggerated for illustration purposes. The thickness of the insulating layer 44 is, for example, equal to or less than 100 pm, in particular equal or less than 50 pm.

[0078] In the shown embodiment, the insulating layer 44 is an insulating foil applied, e.g. glued, to the engaging portions 34 of the respective antenna component 20.

[0079] It is also conceivable that the insulating layer 44 is a layer of a lacquer, a hard coating (e.g. a hard coated oxide layer) or a combination thereof.

[0080] Most notably, the insulating layer 44 is provided fully between the engaging antenna components 20, meaning that the engaging antenna components 20 are electrically isolated from one another, i.e. the insulating layer 44 prevents galvanic coupling. To this end, at least one of each corresponding engaging portions 34 of the respective two engaging antenna components 20 is fully covered by the insulating layer 44. Nevertheless, the engaging antenna components 20 are capacitively coupled to one another.

[0081] In the same way, the four ridge components 22 engage with the back cavity component 24, meaning that respective insulating layers 44 are located between the ridge components 22 and the back cavity component 24, i.e. their respective engaging portions 34.

[0082] Further, for a mechanical fixation of the ridge components 22 to the back cavity component 24, the ridge components 22 are screwed to the back cavity component 24 by means of nonconductive screws, for example plastic screws, using the screw holes provided.

[0083] Thus, each of the antenna components 20 is galvanically insulated from any other antenna component 20.

[0084] For feeding the test antenna 14, the test antenna 14 comprises two coaxial cables 46, namely one coaxial cable 46 for each of the pair of ridge components 22, i.e. each polarization.

[0085] Each coaxial cable 46 comprises an outer conductor 48, an inner conductor 50 and an insulator 52 between the inner conductor 50 and the outer conductor 48.

[0086] As can be seen in Figure 5, the inner conductor 50 and the insulator 52 extend through the channel 40 of the first ridge component 36.

[0087] At the inner end of the channel 40, i.e. the end facing the second ridge component 38, the inner conductor 50 extends further across the gap between the ridge components 36, 38 and into the recess 42 of the second ridge component 38. There, the inner conductor 50 is electrically coupled to the second ridge component 38, for example by soldering. It is also conceivable that the coupling between the inner conductor 50 and the second ridge component 38 is provided capacitively.

[0088] Within the channel 40, the insulator 52 insulates the inner conductor 50 from the first ridge component 36.

[0089] The outer conductor 48 does not extend through the channel 40 but is electrically coupled to the first ridge component 36. The coupling may be provided galvanically, for example by soldering, or capacitively.

[0090] The test antenna 14 obtained in this way has, for example, a width between 30 cm and 40 cm and a height between 40 cm and 60 cm. The test antenna 14 may be configured to emit and receive radiofrequency electromagnetic waves in the frequency range between 600 MHz and 6 GHz.

[0091] During use of the test antenna 14, test signals are fed to the test antenna 14 by the coaxial cables 46 and emitted as electromagnetic waves towards the device under test 12 and / or signals from the device under test 12 are received by the test antenna 14.

[0092] This is possible as all of the antenna components 20 are electrically coupled. However, as the electric coupling is a capacitive coupling and the antenna components 20 are galvanically insulated from one another, the passive intermodulation (PIM) generated by the test antenna 14 itself is very low, in particular below 165 dBc at a power of 2x40 W.

[0093] Thus, very sensitive PIM measurements may be performed using the test antenna 14 as the measurements are not disturbed by the PIM of the test antenna 14 itself.

[0094] Figures 6 to 11 show a second embodiment of a test antenna 14 according to the invention corresponding substantially to the first embodiment. Thus, in the following, only the differences are discussed and the same and functionally the same components are labeled with the same reference signs.

[0095] The test antenna 14 of the second embodiment is also a dual polarized antenna, but a quad-ridged-hom antenna.

[0096] Figure 6 shows the antenna in a perspective view, Figure 7 shows the test antenna 14 in a view without the back cavity component 24, and Figures 8 to 11 show various antenna components 20 of the test antenna 14 in isolation.

[0097] In addition to the back cavity component 24, the ridge components 22 and the reflector 26, the test antenna 14 of the second embodiment comprises four horn components 54 and a flange component 58 as antenna components 20.

[0098] The horn components 54 are shown in isolation in Figure 8 and form a sidewall of a horn 56. The horn components 54 engage with one another forming the horn 56. As can be seen in Figures 6 and 7, the horn components 54 engage at the edges of the horn 56 with one another.

[0099] In the middle of each of the horn components 54, the horn components 54 also engage with a respective one of the ridge components 22 (cf. Fig. 9) in a way that the ridge components 22 are located inside the horn 56.

[0100] As explained with respect to the first embodiment, between each of the horn components 54 and the ridge components 22 engaging with one another and between two horn components 54 engaging with one another, an insulating layer 44 is provided.

[0101] The flange component 58 is shown Figure 7 and, in isolation, in Figure 10.

[0102] The flange component 58 is flat and located between the horn components 54 and the back cavity component 24. The flange component 58 encompasses the lower ends of the ridge components 22 laterally. Due to the connection to the coaxial cables 46 of the first ridge component 36, the flange component 58 is a two-piece component.

[0103] As shown in Figure 7, the assembly of the horn 56, the ridge components 22, and the flange component 58 has a flat surface which engages with the top surface of the back cavity component 24 (shown in Figure 11).

[0104] The flange component 58 engages with the back cavity component 24, the ridge components 22, and the horn components 54, of course always with the insulating layer 44 between the respective antenna components 20.

[0105] Further, in contrast to the first embodiment, the ridge components 22 and the horn 56 are not screwed to the back cavity component 24 but glued to the back cavity component 24 using an adhesive.

[0106] It is also conceivable that screws are used for fixing the back cavity component 24 to the remaining antenna components 20.

[0107] It is further conceivable that in the first embodiment the antenna components 20 are attached to one another using an adhesive.

Claims

Claims1. Broadband test antenna, in particular for performing PIM measurements, comprising a plurality of antenna components (20) and a plurality of insulating layers (44), wherein each of the antenna components (20) engages with at least one other of the antenna components (20), wherein between engaging antenna components (20) one of the insulating layers (44) is located so that the engaging antenna components (20) are galvanically insulated from one another but coupled capacitively to one another.

2. Antenna according to claim 1, characterized in that the insulating layers (44) are insulating foils, layers of lacquer and / or hardcoatings, in particular hard coated oxide layers.

3. Antenna according to claim 1 or 2, characterized in that the insulating layers (44) have a thickness of equal to or less than 100 pm each, in particular equal to or less than 50 pm.

4. Antenna according to any of the preceding claims, characterized in that the respective insulating layer (44) is applied to at least one of the engaging antenna components (20) at least on an engaging portion (34) of the surface engaging with the other one of the engaging antenna components (20).

5. Antenna according to any of the preceding claims, characterized in that the antenna components (20) are made of a conductive material, in particular metal, and / or manufactured by subtractive manufacturing, in particular machining, in particular wherein the antenna components (20) have a surface roughness of less than Rz 4.

6. Antenna according to any of the preceding claims, characterized in that the test antenna (14) is a dual polarized antenna and / or that the test antenna (14) is a horn antenna or a Vivaldi antenna.

7. Antenna according to any of the preceding claims, characterized in that at least one of the antenna components (20) is a back cavity component (24)having a frame portion (28) and providing a cavity (30) within the frame portion (28).

8. Antenna according to claim 7, characterized in that at least one of the antenna components (20) is a flange component (58) engaging with the at least one back cavity component (24).

9. Antenna according to any of the preceding claims, characterized in that at least two, in particular at least four of the antenna components (20) are ridge components (22) arranged in a pair of one first ridge component (36) and one second ridge component (38) opposite to one another, in particular wherein the ridge components (22) engage with the at least one back cavity component (24) and / or the at least one flange component (58).

10. Antenna according to claim 9, characterized in that the first ridge component (36) comprises a channel (40) extending through the first ridge component (36).

11. Antenna according to claim 10, characterized in that the test antenna (14) comprises a coaxial cable (46) having an outer conductor (48) and an inner conductor (50), wherein the outer conductor (48) is electrically coupled to the first ridge component (36) and the inner conductor (50) extends through the channel (40) and are electrically coupled to the second ridge component (38).

12. Antenna according to any of the preceding claims, characterized in that one of the antenna components (20) is a reflector (26).

13. Antenna according to any of the preceding claims, characterized in that at least one, in particular at least four of the antenna components (20) are horn components (54), each forming at least one sidewall of a horn (56), in particular a horn (56) encompassing the ridge components (22).

14. Antenna according to claim 13, characterized in that more than one horn component (54) is provided and adjacent horn components (54) engagewith one another, in particular wherein the horn components (54) engage with the at least one back cavity component (24) and / or the at least one flange component (58).

15. Test equipment for performing PIM measurements at a device under test (12), comprising at least one test antenna (14) according to any one of the preceding claims, in particular wherein the test equipment (10) comprises an anechoic chamber (16) in which the at least one test antenna (14) is located.

16. Test equipment according to claim 15, characterized in that the test equipment (10) comprises a conveyor (18) for conveying the device under test (12) through a beam of the test antenna (14), in particular wherein the conveyor (18) extends at least partially through the anechoic chamber (16).

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