Phased array antenna device

Abstract

A phase array antenna device (2) includes a plurality of antenna elements (5) arranged in a spatial distribution designed to allow the phase array antenna device (2) to transmit and receive superimposed radio frequency signals in different directions. Each antenna element (5) is located within a corresponding unit cell (1) of the phase array antenna device (2), whereby the unit cells (1) are arranged adjacent to each other in a non-overlapping manner. A feed circuit for transmitting antenna signals between a common feed point and a corresponding antenna element (5) includes a plurality of antenna element transmission line segments (4) and corresponding plurality of phase shifting devices (7), each segment extending into the antenna element. The phase array antenna device (2) includes several feed transmission line segments (8), whereby each feed transmission line segment (8) includes more than two transition structures (6) distributed along the feed transmission line segment, whereby each transition structure (6) provides signal coupling between the feed transmission line segment (8) and the corresponding antenna element transmission line segment (4) to connect several dedicated antenna element transmission line segments (4) to the same feed transmission line segment (8).

Description

Technical Field

[0001] This invention relates to a phase array antenna device having a plurality of antenna elements arranged in a spatial distribution designed to allow the phase array antenna device to transmit and receive superimposed radio frequency signals in different directions, wherein each antenna element is located within a corresponding unit cell of the phase array antenna device, and the unit cells are arranged adjacent to each other in a non-overlapping manner; a feed circuit for transmitting antenna signals between a common feed point and the corresponding antenna elements, wherein the feed circuit includes a plurality of antenna element transmission line segments, each transmission line segment extending into the antenna element; and a plurality of phase shifting devices, wherein for each antenna element, a corresponding phase shifting device is arranged along the corresponding antenna element signal transmission line extending into the antenna element. Prior Technology

[0002] Phased array antenna devices operated using radio frequency (RF) signals allow the transmission of RF electromagnetic beams that can be electronically manipulated to point in different directions without moving the antenna device. Similarly, many phased array antenna devices also allow for amplification of the receiving sensitivity of RF waves from a particular direction without moving the antenna device.

[0003] In most phased array antenna devices, radio frequency (RF) current from the transmitter is fed to individual antenna elements with the correct phase relationship, causing the RF waves from the individual antenna elements to superimpose and add together to increase the radiation intensity in the desired direction and cancel out the radiation intensity in the undesired direction. To control the phase relationship between individual antenna elements, power from the transmitter is fed to a plurality of antenna elements via devices called phase shifters, which electronically change the corresponding phase of the antenna signal. For each antenna element, the correct phase relationship relative to the other antenna elements is defined and preset by a corresponding phase shifting device, thereby generating a superimposed beam of RF waves, which, as a superposition of all RF waves from all antenna elements, has a peak intensity in the preset direction.

[0004] Typically, a phased array antenna device consists of many small antenna elements, sometimes including more than a thousand antenna elements arranged in a predetermined spatial distribution. For many phased array antenna devices, a large number of antenna elements are arranged in a matrix spatial distribution within a plane. The minimum size of an antenna element is usually approximately λ / 2, where λ is the wavelength of the radio frequency signal to be transmitted or received by the phased array antenna device.

[0005] For many phased array antenna devices, each antenna element is arranged within a unit cell, whereby the unit cell defines a small region in a plane dedicated to the corresponding antenna element arranged within that plane. This plane can be divided into multiple unit cells, each containing one antenna element and typically also including similar patterns of other electrodes or elements. The unit cells thus cover the plane in a non-overlapping but adjacent manner, and are typically arranged in a matrix shape. Generally, the unit cell has no structural limitations but can be considered as the region surrounding an antenna element with repeating patterns of electrodes and other elements. The extension of a unit cell in a given direction is equal to the distance between adjacent antenna elements in that direction.

[0006] For each unit cell, the corresponding antenna element is connected to the control unit via a segmented antenna element transmission line. In the case of a large number of unit cells with antenna elements, the space requirement for the corresponding number of antenna element transmission line segments becomes enormous, significantly limiting the available space for the antenna elements.

[0007] To reduce the total length of antenna element transmission line segments required for individual connection to each antenna element, many phased array antenna devices include a common feed network starting from a common feed point. This network has a small number of first common feed transmission line segments, each of which branches into two separate second common feed transmission line segments. This branching can be repeated several times, resulting in a common feed network with cascaded common feed transmission line segments, until after N branching stages, the final total number of common feed transmission line segments equals the required number of antenna element transmission line segments extending to the corresponding antenna element.

[0008] However, to allow for cost-effective fabrication of such cascaded common feed circuits, all common feed transmission line segments are arranged on the same surface of the substrate layer. Since electronic manipulation of phase shifters typically requires bias lines to apply an electric field to the tunable dielectric material, these bias lines must also be connected to each phase shifter, i.e., the unit cell. However, any intersections between common feed transmission line segments and other common feed transmission line segments or with bias lines should be avoided. Avoiding all such intersections typically requires very long bias line lengths between the control unit and the corresponding antenna element. Therefore, such cascaded common feed circuits impose several limitations on the design of phase array antenna devices and the arrangement of unit cells and corresponding antenna elements. Furthermore, if intersections or overlaps of common feed transmission line segments are avoided, the total length of the resulting common feed transmission line segments used for signal transmission between the common feed point and the antenna element will be considerable. Longer transmission line segments also lead to increased signal strength loss.

[0009] Therefore, a more efficient and space-saving signal transmission connection arrangement is needed between the common feed point and each antenna element. Summary of the Invention

[0010] This invention relates to the phased array antenna device described above, wherein the phased array antenna device includes a plurality of feed transmission line segments, wherein each feed transmission line segment includes more than two transition structures distributed along the feed transmission line segment, wherein each transition structure provides signal coupling to a corresponding antenna element transmission line segment, thereby connecting a plurality of dedicated antenna element transmission line segments to the same feed transmission line segment. In contrast to a common feed transmission line segment that branches into two secondary common feed transmission line segments, a feed transmission line segment does not branch into two secondary transmission line segments, but instead includes more than two transition structures, wherein each transition structure allows signal coupling between the feed transmission line segment and the antenna element transmission line segment. Therefore, a single feed transmission line segment is connected to and feeds a plurality, and possibly a large number, of antenna element transmission line segments. This significantly reduces the space required to connect each unit cell with a corresponding antenna element to the control unit of the phased array antenna device. Furthermore, it facilitates the control and actuation of each unit cell using a common signal transmission along a common feed transmission line segment.

[0011] According to an advantageous embodiment of the invention, each feed transmission line segment extends along or through more than two unit cells and includes a transition structure for each of the more than two unit cells. Therefore, the distance between the feed transmission line segment providing signal transmission connectivity to the control unit and each corresponding antenna element is relatively short, which also reduces the space requirement of the antenna element transmission line segments, each connecting the feed transmission line segment to its corresponding antenna element.

[0012] According to another aspect of the invention, each feed transmission line segment extends along a straight line. Typically, antenna elements, and therefore unit cells, are spatially arranged in a matrix shape. For such a matrix arrangement, the path of the feed transmission line segment can be a straight line extending between two adjacent rows of unit cells, or a straight line passing through many unit cells within the matrix-shaped unit cell arrangement. Feed transmission line segments extending along straight lines also reduce unwanted electromagnetic radiation emissions caused by bends or corners within the transmission line path.

[0013] According to an advantageous embodiment of the invention, the feed transmission line segment is implemented as a microstrip transmission line with a linear microstrip electrode arranged at a distance from a ground electrode. The microstrip line and transition structure used to couple the signal to the antenna element transmission line segment are easy to manufacture. Furthermore, the ground electrode required for the microstrip line may be useful in providing a back shield that prevents electromagnetic radiation emissions from being emitted away from the intended direction and towards the back side of the unit cell arrangement.

[0014] In yet another, and also advantageous, embodiment of the invention, the feed transmission line segment is implemented as a differential pair transmission line, wherein two similar differential pair electrodes extend along the feed transmission line segment. The differential pair transmission line does not require a ground electrode, which allows for more options in the design of the phased array antenna device. For example, the ground electrode can be placed at any distance from the radiating element, regardless of the feed transmission line segment. Furthermore, signal transmission along the differential pair transmission line is less affected by interfering electromagnetic radiation emissions, which occur within the phased array antenna device and cannot be completely avoided. Moreover, designing the antenna element transmission line segment as a differential pair transmission line is also considered advantageous. The transition structure required for signal coupling between the feed transmission line segment and the antenna element transmission line segment then does not require changing the type of transmission line from a microstrip transmission line to a differential pair transmission line.

[0015] A highly advantageous aspect of the invention is that each antenna element transmission line segment can be implemented as a differential pair transmission line, wherein two similar differential pair electrodes extend along the antenna element transmission line segment, thereby electrically isolating at least one of the two differential pair electrodes of the antenna element transmission line segment from the corresponding feed transmission line segment. Since at least one of the two differential pair electrodes of the antenna element transmission line segment is not electrically connected to the feed transmission line segment, it is possible to apply a potential difference to the two differential pair electrodes of the antenna element transmission line, independent of any potential or potential difference of the feed transmission line segment. Therefore, it is possible to utilize phase shifting devices with tunable dielectric material, wherein the tunable dielectric material is arranged between or near the two differential pair electrodes of the antenna element transmission line, and it is possible to apply an individual bias voltage to each phase shifting device. This allows for a very simple design and operation of the antenna elements and phase shifting devices within each unit cell.

[0016] According to an aspect of the invention, the transition structure includes two linear transition electrodes, and further includes an overlapping section, wherein a portion of at least one of the two linear transition electrodes extends parallel to each other, but at a distance from the feed transmission line segment, for signal coupling from the feed transmission line segment to the antenna element transmission line segment, whereby each of the two linear transition electrodes extends into a corresponding one of the two differential pair electrodes of the antenna element transmission line segment. Therefore, the two linear transition electrodes can be designed and manufactured as corresponding end segments of the corresponding differential pair electrodes of the antenna element transmission line segment of the differential pair transmission line. The length of the overlapping section, and particularly the length of the parallel-extending linear transition electrodes at a distance from the feed transmission line segment, can be adapted to provide sufficient coupling, but as short as possible, in order to reduce the space required for the transition structure. Because at least one of the two linear transition electrodes is not electrically connected to the feed transmission line segment, there is no need for, for example, vias or interconnect electrode structures providing electrical connections between different surfaces of the substrate layer, which allows for simple and cost-effective manufacturing and a space-saving design of the transition structure.

[0017] To provide a very cost-effective and space-saving design for the transition structure, one of the two linear transition electrodes is designed as a balun-type linear transition electrode, providing a 180° phase difference relative to the other linear transition electrode. The balun-type linear transition electrode includes a U-shaped delay section, which provides a simple means of providing a 180° phase difference for signal transmission segmented along the antenna element transmission line.

[0018] The feed transmission line segments with several and possibly a large number of transition structures allow signal coupling between the feed transmission line segments and the corresponding large number of antenna element transmission line segments. This results in a topology with a very small footprint, which is required for the unit cell that includes the corresponding antenna elements, but provides very high performance and efficiency as well as a favorable signal-to-noise ratio compared to conventional phased array antenna devices known in the prior art. Simple Explanation of the Diagram

[0019] The invention will be more fully understood and further features will become apparent upon reference to the following detailed description and accompanying drawings. The drawings are merely representative and are not intended to limit the scope of the claims. In fact, those skilled in the art will appreciate upon reading the following specification and viewing the current drawings that various modifications and variations can be made without departing from the innovative concept of the invention. Similar portions depicted in the drawings are referred to by the same reference numerals.

[0020] [Figure 1] illustrates a schematic top view of a matrix arrangement of unit cells with several rows of unit cells, thereby coupling each antenna element along a row of unit cells to a feed transmission line segment designed as a microstrip transmission line. [Figure 2] illustrates a schematic cross-sectional view through a portion of the unit cell shown in Figure 1. [Figure 3] illustrates a schematic top view of a matrix arrangement of unit cells similar to that shown in Figure 1, whereby the feed transmission line segments extending toward the corresponding antenna elements and the antenna element transmission line segments are designed as microstrip transmission lines. [Figure 4] illustrates a schematic cross-sectional view through a portion of the unit cell shown in Figure 3. [Figure 5] illustrates a schematic top view of a matrix arrangement of unit cells similar to those shown in Figures 1 and 3, whereby the feed transmission line segments are designed as microstrip transmission lines, and the corresponding antenna element transmission line segments are designed as differential pair transmission lines. [Figure 6] illustrates a schematic cross-sectional view through a portion of the unit cell shown in Figure 5. [Figure 7] illustrates a schematic top view of a transition structure that provides signal coupling between microstrip transmission lines, and [Figure 8] illustrates a schematic top view of a transition structure that provides signal coupling between a microstrip transmission line and a differential pair transmission line. Implementation

[0021] Figure 1 illustrates a schematic top view of the matrix arrangement of unit cells 1 within the phase array antenna device 2. The matrix arrangement of unit cells 1 includes several rows 3 of unit cells 1, whereby adjacent rows 3 of unit cells 1 are positioned with a small offset in the direction of row 3. However, such offset is not mandatory for the matrix arrangement of unit cells 1.

[0022] Each non-overlapping unit cell 1 includes an antenna element transmission line segment 4 extending toward the antenna element 5. The antenna element 5, schematically illustrated in Figure 1, is designed as a bowtie dipole antenna. The antenna element transmission line segment 4 extends from a transition structure 6 located near the boundary of the unit cell 1, along several bends toward the antenna element 5 located near the center of the unit cell 1. At least a portion of the antenna element transmission line segment 4 is used as a phase shifting device 7.

[0023] For each row 3 of a unit cell 1, a feed transmission line segment 8 extends along the corresponding row 3 and passes through all unit cells 1 within that row 3. Within each unit cell 1, the feed transmission line segment 8 passes through a corresponding transition structure 6. Within the transition structure 6, a portion of the radio frequency signal transmitted along the feed transmission line segment 8 is coupled to the corresponding antenna element transmission line segment 4 and transmitted along that segment towards the antenna element 5 of the corresponding unit cell 1. An exemplary design of such a transition structure 6 is illustrated in Figure 8.

[0024] For each unit cell 1, the individual phase shift of the radio frequency (RF) signal transmitted along the antenna element transmission line segment 4 is preset by the corresponding phase shifting device 7. RF signals emitted from each antenna element 5 are superimposed on each other, resulting in a peak intensity of the superimposed RF signal emitted from the phase array antenna device 2. The direction of the peak intensity can be preset and modified by individually controlling and presetting the phase shift of each RF signal from each antenna element 5 (i.e., from each unit cell 1). Similarly, by applying the correct phase shift to each input RF signal, the sensitivity of receiving RF signals from a specific direction relative to the plane defined by the matrix arrangement of the unit cells 1, received by the antenna elements 5, and transmitted along the antenna element transmission line segment 4 toward the corresponding transition structure 6, and fed into the common feed transmission line segment 8, is enhanced. The bias voltage line required for applying and controlling the individual phase shift of each phase shifting device 7 must individually connect each phase shifting device 7 to the bias voltage control unit. Such a bias voltage line is not depicted in the figure, but it can extend in a strip region parallel to the feed transmission line segment 8, thereby the strip region is arranged between the corresponding feed transmission line segment 8 and the column of the antenna element transmission line segment 4 adjacent to the feed transmission line segment 8 but connected to another feed transmission line segment 8 on the opposite side of the antenna element 5.

[0025] Each feeder transmission line segment 8 is connected to a common control unit 9 via a common feeder network 10. The common feeder network 10 includes a cascaded arrangement of common feeder transmission line segments 11, whereby, starting from the control unit 9, each common feeder transmission line segment 11 branches into two consecutive common feeder transmission line segments 11, until, after the final branch, the corresponding consecutive common feeder transmission line segment 11 extends into the corresponding feeder transmission line segment 8.

[0026] Due to the feed transmission line segment 8, the number and total length of the continuous common feed transmission line segments 11 required for signal transmission between the common control unit 9 and each antenna element 5 are significantly reduced. Since each common feed transmission line segment 11 requires some space and a minimum distance to other signal transmission elements, such as antenna element transmission line element 4 with phase shifting device 7, this results in a matrix arrangement of the unit cell 1 and therefore a more compact and space-saving design of the phase array antenna device 2.

[0027] Figure 2 illustrates a cross-sectional view of a portion of the unit cell 1 shown in Figure 1. The phase array antenna device 2 includes a first substrate layer 12 for a feed transmission line segment 8 and two second substrate layers 13 for antenna element transmission line segments 4 and phase shifting devices 7. The two second substrate layers 13 are made of glass, and the first substrate layer 12 may also be made of glass or any other suitable dielectric material. The feed transmission line segment 8 is designed as a microstrip transmission line, having a linear microstrip electrode 14 at a first surface 15 of the first substrate layer 12 and a planar ground electrode 16 at a second surface 15' opposite the first surface 15. One of the second substrate layers 13 may be in direct contact with the planar ground electrode 16, or may be arranged at a distance from the planar ground electrode 16 using an interruption layer of, for example, air or a solid dielectric material, as exemplarily illustrated in Figure 2.

[0028] Each transition structure 8 provides signal coupling between the feed transmission line segment 8 and the corresponding antenna element transmission line segment 4, which is designed as a differential pair transmission line with two differential pair electrodes 17, 18 arranged between two second substrate layers 13 at opposing but facing surfaces 19, 20. The volume between the two second substrate layers 13 is filled with a tunable dielectric material, such as a tunable liquid crystal material 21. Applying a potential difference between the two differential pair electrodes 17, 18 results in an electric field affecting the tunable dielectric material, which causes a preset phase shift of the radio frequency signal transmitted along the antenna element transmission line segment 4, which also acts as a phase shifting device 7. By presetting an individual phase shift for each antenna element 5 of the unit cell 1, the direction of the peak intensity of the superimposed radio frequency signal transmitted from the matrix arrangement of the antenna elements 5 can be preset and adapted to provide enhanced signal communication between the phase array antenna device 2 and any other communication device that transmits or receives radio frequency signals compatible with the superimposed radio frequency signal of the phase array antenna device 2.

[0029] Figures 3 and 4 illustrate another embodiment of the phase array antenna element 2. Both the feed transmission line segment 8 and the antenna element transmission line segment 4 are designed as microstrip transmission lines. Therefore, only two second substrate layers 13 are required. The planar ground electrode 16 and the linear microstrip electrode 14 of the feed transmission line segment 8 are arranged at opposing but facing surfaces 19 and 20 between the two second substrate layers 13. The linear microstrip electrode 14 of the feed transmission line segment 8 and the linear microstrip electrode 22 of the antenna element transmission line segment 4 are arranged at the same surface 19, thereby placing the plate-shaped ground electrode 16 on the other surface 20. The volume between the two second substrate layers 13 is filled with a tunable dielectric material, such as a tunable liquid crystal material 21. Figure 7 illustrates an exemplary design of the transition structure 6 for coupling radio frequency signals between the feed transmission line segment 8 and the corresponding antenna element transmission line segment 4. For each unit cell 1 and the corresponding antenna element 5 designed as a patch antenna and not shown in Figures 3 and 4, since the linear microstrip electrode 22 is not electrically connected to the feed transmission line segment 8, the correct phase shift can be preset by applying a corresponding potential difference between the linear microstrip electrode 22 and the plate-shaped ground electrode 16 of the antenna element transmission line 4.

[0030] Figures 5 and 6 illustrate another embodiment of the phase array antenna device 2, which has only two second substrate layers 13, whereby the feed transmission line segment 8 is designed as a microstrip transmission line, and thus the antenna element transmission line segment 5 is designed as a differential pair transmission line.

[0031] Figure 7 schematically illustrates an exemplary embodiment of the transition structure 6, which can be used to couple radio frequency signals between two microstrip transmission lines. The linear microstrip electrode 14 of the feed transmission line segment 8 extends along a straight line. The end segment 23 of the linear microstrip electrode 22 of the antenna element transmission line segment 4 forms a linear transition electrode and extends parallel to the linear microstrip electrode 14 of the feed transmission line segment 8 but at a certain distance from the linear microstrip electrode 14. The length of the parallel segment of the linear microstrip electrode 22 is adjusted and preset to provide strong signal coupling of the radio frequency signal between the linear microstrip electrode 14 of the feed transmission line segment 8 and the linear microstrip electrode 22 of the antenna element transmission line segment 4.

[0032] Figure 8 schematically illustrates another exemplary embodiment of the transition structure 6, which allows for the coupling of radio frequency signals between a microstrip transmission line and a differential pair transmission line. The end segment 24 of the first linear differential pair electrode 17 forms a linear transition electrode and extends parallel to, but at a distance from, the linear microstrip electrode 14 of the feed transmission line segment 8 (and preferably at another substrate). For clarity, the first linear differential pair electrode 17 is illustrated using dashed lines. After the end segment 24, the first linear differential pair electrode 17 extends along a U-shaped delay path, resulting in a 180° phase shift relative to the signal coupled to the second linear differential pair electrode 18. The U-shaped delay path can also be considered as part of the linear transition electrode of the transition structure 6. The second linear differential pair electrode 18 can be connected to or coupled to the linear microstrip electrode 14 of the feed transmission line segment 8 with or without electrical connection. Figure 8 illustrates the electrical connection, which is designed as a branch from the linear microstrip electrode 14 of the feed transmission line segment 8 to the branch linear differential pair electrode 18 of the antenna element transmission line segment 4.

[0033] 1: Unit cell 2: Phased array antenna device 3: line 4: Antenna element transmission line segmentation 5: Antenna components 6: Transition Structure 7: Phase Shifting Device 8: Segmentation of power supply transmission line 9: Public Control Unit 10: Common feeder network 11: Common power supply transmission line segmentation 12: First substrate layer 13: Second substrate layer 14: Linear microstrip electrode 15: First Surface 15': Second surface 16: Grounding electrode 17: Electrode 18: Electrode 19: Surface 20: Surface 21: Adjustable liquid crystal material 22: Linear microstrip electrode 23: terminal section 24: terminal section

Claims

1. A phase array antenna device (2) having a plurality of antenna elements (5) arranged in a spatial distribution, the spatial distribution being designed to allow the phase array antenna device (2) to transmit and receive superimposed radio frequency signals in different directions, wherein each antenna element (5) is positioned within a corresponding unit cell (1) of the phase array antenna device (2), and wherein the unit cells (1) are arranged adjacent to each other in a non-overlapping manner; having a feed circuit for transmitting antenna signals between a common feed point and the corresponding antenna element (5), wherein the feed circuit includes a plurality of antenna element transmission line segments (4), each segment extending into the antenna element (5); and having a plurality of A phase shifter (7) is arranged for each antenna element (5) along the corresponding antenna element signal transmission line (4) extending into the antenna element (4). The phase array antenna device (2) comprises a plurality of feed transmission line segments (8), wherein each feed transmission line segment (8) comprises more than two transition structures (6) distributed along the feed transmission line segment (8), wherein each transition structure (6) provides signal coupling between the feed transmission line segment (8) and the corresponding antenna element transmission line segment (4), thereby connecting a plurality of dedicated antenna element transmission line segments (4) to the same feed transmission line segment (8).

2. The phase array antenna device (2) according to claim 1, wherein, Each of the feed transmission line segments (8) extends along or through more than two unit cells (1) and includes a transition structure (6) for each of the more than two unit cells (1).

3. The phase array antenna device (2) according to claim 1 or 2, wherein, Each of the power supply transmission line segments (8) extends along a straight line.

4. The phase array antenna device (2) according to claim 1 or 2, wherein, The feed transmission line segment (8) is implemented as a microstrip transmission line, wherein the microstrip line is arranged at a certain distance from the ground electrode (16).

5. The phase array antenna device (2) according to claim 1 or 2, wherein, The feed transmission line segment (8) is implemented as a differential pair transmission line, wherein two similar differential pair electrodes extend along the feed transmission line segment (8).

6. The phase array antenna device (2) according to claim 1 or 2, wherein, Each of the antenna element transmission line segments (4) is implemented as a differential pair transmission line, wherein two similar differential pair electrodes (17, 18) extend along the antenna element transmission line segment (4), thereby electrically isolating at least one of the two differential pair electrodes (17, 18) of the antenna element transmission line segment from the corresponding feed transmission line segment (8).

7. The phase array antenna device (2) according to claim 6, wherein, The transition structure (6) includes two linear transition electrodes, and the transition structure also includes an overlapping section, wherein a portion of at least one of the two linear transition electrodes is parallel to the feed transmission line segment (8) but extends at a certain distance from the feed transmission line segment (8) for coupling a signal from the feed transmission line segment (8) to the antenna element transmission line segment (4), wherein each of the two linear transition electrodes extends to a corresponding one of the two differential pair electrodes (17, 18) of the antenna element transmission line segment (4).

8. The phase array antenna device (2) according to claim 7, wherein, One of the two linear transition electrodes is designed as a balanced-to-unbalanced converter type linear transition electrode, which provides a 180° phase difference relative to the other linear transition electrode.

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