Technical Field of the Invention

The present invention relates to a structure for shielding an antenna from multipath (multiply scattered waves) and/or electromagnetic interference such as radio interference.

The invention is particularly though not exclusively applicable to shielding antennas of global navigation satellite system (GNSS) reference stations from multipath and/or interference.

Background of the Invention

Antennas mounted in proximity to other objects (trees, buildings, etc.) not only receive electromagnetic waves such as radio waves that have traveled a straight path from a desired sender to the antenna, but also scattered waves that have been scattered at the objects in the proximity. Signals resulting from scattering are commonly referred to as multipath. Multipath presents a significant background to the desired signal which corresponds to radio waves that are directly incident on the antenna phase center without previous scattering. In the case of an antenna of a GNSS reference station (ground station), such background deteriorates the accuracy with which e.g. satellite clocks, satellite positions, satellite delay code biases, and ionospheric model data from phase and code values of signals received from GNSS satellites can be determined, and thus deteriorates the quality of any GNSS service data that is provided by the GNSS reference station to mobile GNSS receivers. Consequently, in view of positioning accuracy, it is desirable to have as good as possible control of the received multipath. On the other hand, scattering of a radio wave off a given object depends on, among others, an angle of incidence of the radio wave on the object, a shape, material or surface structure of the object, and moreover can be time- dependent. Therefore, parameters of the scattered wave are difficult to predict even if the parameters of the incident wave are known. Moreover, existence of multiple objects in the proximity of an antenna renders a prediction of the parameters of scattered radio waves incident on the antenna phase center virtually impossible. Consequently, satisfactory mitigation of the impact of multipath by signal processing techniques at present is impossible.

A further source of background is given by interference from other sources that emit electromagnetic waves at wavelengths comparable to the wavelength of the desired signal. Also interference as source of background is difficult to control by means of signal processing techniques, for similar reasons as discussed above.

In practice, as indicated above, the radio-frequency environment of multipath and interference is a key GNSS system performance driver, so that effective control of both multipath and interference is to be pursued.

A first known strategy to reduce the impact of multipath and interference is to provide a large antenna having selectivity with respect to the elevation angle or direction. Thereby, the antenna may be adjusted to receive electromagnetic waves arriving at the antenna from a desired elevation angle or direction, so that a sensitivity of the antenna with respect to unwanted signals arriving from other elevation angles or directions is reduced. However, providing a spatially extended antenna with directional sensitivity has negative impact on the phase center stability of the antenna, that is, the phase stability over angle and frequency. Another known strategy to reduce the impact of multipath is to mount the antenna at a position without any nearby objects, e.g. on a radio mast remote from any trees or buildings. However, depending on the size and weight of the antenna, or due to other limitations, such mounting of the antenna may not be possible. Moreover, this strategy is not capable of reducing the impact of interference. A further strategy for reducing the impact of multipath as employed by prior art US 2005/0225474 Al is illustrated in Fig. 8, which is a top view on a skin 902 of a fuselage of an aircraft on which an antenna 901 is mounted. In order to avoid multipath, the skin 902 of the fuselage surrounding the mounting position of the antenna 901 is covered with a microwave absorbent

material 903. Thereby, microwaves that are incident on the skin 902 of the fuselage surrounding the antenna 901 are attenuated before being scattered onto the antenna phase center. Nevertheless, microwaves scattered from more remote portions of the skin 902 of the fuselage are not attenuated before being scattered onto the antenna 901, so that suppression of the background level by this strategy is limited. In addition, this strategy is not applicable to reducing a background resulting from interference.

Summary of the Invention

It is an object of the present invention to overcome the limitations of the prior art discussed above. It is another object of the invention to provide a shielding structure for an antenna that shields the antenna from electromagnetic interference and/or multipath.

In view of the above objects, the present invention proposes a shielding structure having the features of claim 1. Preferred embodiments of the invention are described in the dependent claims. According to a preferred embodiment of the invention, a shielding structure for shielding an antenna from radio interference comprises a shielding element having a base part and an upper part arranged on the base part, a cavity for arranging the antenna therein being defined by an inner wall portion formed inside the upper part and an upper surface of the base part, wherein the inner wall portion and the upper surface of the base part are structured to absorb radio waves incident to the cavity, and means for mounting the antenna, said means being disposed with regard to the cavity such that the antenna is arranged within the cavity in its mounted state.

The shielding structure according to the present invention effectively mitigates the effect of both interference and multipath on an antenna mounted on the means for mounting the antenna inside the shielding structure. As has been realized by the present inventors, the angle of arrival (with respect to the local horizon) of unwanted signals such as interference and multipath typically is below a certain elevation. Accordingly, by embedding the antenna in a geometrical structure having components structured to absorb radio waves, a local radio-frequency environment, as observed at the location of the antenna, can be modified.

Modification of the local radio-frequency environment is achieved by the shielding structure according to the present invention in two conceptual steps: (1) Isolation of the antenna phase center from the local uncontrolled and external radio frequency environment, for all angles of arrival below a certain elevation, and (2) synthesis of a local controlled and internal radio frequency environment for all angles of arrival below the certain elevation. On the other hand, the external radio frequency environment for all angles of arrival above the certain elevation is not affected by the inventive shielding structure.

Isolation is achieved via reflection, diffraction and absorption of the local uncontrolled and external radio frequency environment (i.e. interference and multipath). Synthesis of a local controlled radio frequency environment is achieved by suppressing reflections at the inner surface of the inventive shielding structure. Thus, unwanted signals are prevented from arriving at the antenna phase center, and a known local radio frequency environment at the antenna phase center is provided that is independent of the unwanted signals.

Thus, by providing the inventive shielding structure, the background signal level can be lowered significantly. In the case of the antenna being the antenna of a GNSS reference station (GNSS ground station), the system performance of the GNSS reference station, corresponding to the quality and accuracy of service data provided to mobile GNSS receivers is drastically increased. Preferably, the shape of the cavity is rotationally symmetric with respect to an axis perpendicular to the base part, the inner wall portion comprises a first inner wall section, a lower end of which being disposed on the base part, and a second inner wall section, an upper end of which defining a circular opening of the cavity, and a first angle formed between the first inner wall section and the base part is larger than a second angle formed between the second inner wall section and an imaginary plane extending in parallel to the base part.

According to this configuration, an opening is provided that has a smaller diameter than a lower part of the cavity. Thus, unwanted signals from outside of the shielding structure are efficiently attenuated. On the other hand, desired signals arriving from a large angle of arrival with respect to the local horizon are not attenuated and can arrive at the antenna phase center substantially unimpededly. Further, the second angle may be configured so that for any part of the second inner wall section, a reflected (scattered) component of a radio signal incident from outside of the cavity to the part of the second inner wall section is reflected away from a position of the antenna in its mounted state, towards a part of the inner wall portion or towards the base part. It is preferred that the second angle is in the range from 0 degrees to 30 degrees. It is further preferred that the first angle is in the range from 60 degrees to 90 degrees. Thus, signals that are incident to the cavity of the inventive shielding structure that do not directly arrive at the antenna are efficiently attenuated by the absorptive surface structure of the inner walls of the cavity. By the inventive choice of the second angle, these signals are reflected at least once at either the inner wall portion or the upper surface of the base part before arrive at the antenna, and therefore are attenuated by scattering at the absorptive surface structure of the inner wall portion and the upper surface of the base part. In other words, multipath originating from reflection of signals inside the cavity can be effectively suppressed.

In a preferred embodiment, the upper part comprises an annular top portion which partly covers the upper part and has a tapering configuration, wherein a top surface of the top portion is inclined towards the center of the shielding element. A particular advantage is achieved if the top surface intersects the second inner wall section at an acute angle.

By providing the annular top portion having the top surface that is inclined towards the center of the shielding element, a threshold angle for the angle of arrival with respect to the local horizon is defined, from below of which radio waves cannot enter the cavity of the shielding structure. Radio waves having a smaller angle of arrival than the threshold angle are either reflected at a part of the outer wall of the shielding structure or at a part of the top surface.

Further, the top surface may be subdivided into a plurality of annular sub- surfaces by one or more concentric gaps, the one or more concentric gaps extending in parallel to the second inner wall section. Preferably, the top portion of the upper part comprises a plurality of annular absorbing elements stacked along their thickness direction, wherein gaps are formed between adjoining absorbing elements, and the top surface is formed at least partially by the radially inward end faces of the absorbing elements. Accordingly, a radio wave having an angle of arrival below the threshold angle that is reflected at the top surface is refracted at the annular inner edges of the radially inward end faces of the absorbing elements, and thereby effectively absorbed.

It is of particular advantage if a third angle formed between the top surface and an imaginary plane extending in parallel to the base part is configured such that radio signals incident from an elevation angle with respect to an imaginary plane comprising the base part that is larger than a

predetermined first threshold angle can enter the cavity, and such that radio signals incident from an elevation angle that is smaller than the predetermined first threshold angle cannot enter the cavity, at least not without being attenuated. Preferably, the third angle is in the range from 2 degrees to 10 degrees.

GNSS satellites orbiting earth are configured to emit radio signals towards earth so that at each given position on earth's surface GNSS signals arrive at a receiver antenna at an angle of arrival with respect to the local horizon that is above a predetermined threshold. By adjusting the geometric parameters of the inventive shielding structure, the shielding structure may be adjusted to allow passage of radio signals arriving at the antenna from an angle of arrival above the predetermined threshold, while blocking radio signals arriving at the antenna from an angle of arrival below the predetermined threshold. Thus, only the desired signals, i.e. radio signals emitted from the GNSS satellites are received by the antenna, while unwanted signals such as multipath and interference are blocked.

In a further preferred embodiment, a ratio between an inner diameter of the opening and a maximum inner diameter of the cavity is in the range from 0.35 to 0.7. Preferably, a ratio between a height of the cavity from the upper surface of the base part to the opening and a maximum inner diameter of the cavity is in the range from 0.22 to 0.45. Further preferably, a ratio between a height of a portion of the cavity defined by the second inner wall section and a height of the cavity from the upper surface of the base part to the opening is in the range from 0.15 to 0.3. As discussed above, the inventive shielding structure has a property that signals incident to the cavity that do not directly arrive at the antenna are attenuated by being scattered at least once at the absorptive surface structure of the bounding surface of the cavity. By providing either or all of the above ratios of the physical dimensions of the shielding structure, attenuation of signals reflected inside the cavity is further improved.

A further advantage is achieved if a maximum inner diameter of the cavity is in the range from 9λ to 15λ, wherein λ is a wavelength corresponding to an operating frequency of the antenna.

Accordingly, the inventive shielding structure is large compared to the wavelength corresponding to the operating frequency of the antenna. Therefore, a back-reaction between the shielding structure and the antenna can be avoided, so that the shielding structure may be used with all kinds of different antenna designs. Thus, no re-design of an antenna to be provided with the inventive shielding structure is required. On the other hand, the geometric structure of the inventive shielding structure has been optimized to achieve all the above advantageous effects at a minimum overall size and therefore, weight. In addition, the shielding structure may further comprise a plurality of pyramidal radio-frequency absorbers provided on the base part.

Provision of pyramidal absorbers on the base part results in a very efficient absorption of radio waves that are incident on the upper surface of the base part. Thereby, reflection of radio waves at the base part towards the antenna is avoided, and multipath resulting from internal reflection of radio waves inside of the shielding structure is prevented. Brief Description of the Drawings Fig. 1 illustrates a vertical cut along the central axis of a shielding structure for an antenna according to a first embodiment of the invention;

Fig. 2 illustrates a vertical cut along the central axis of a shielding structure for an antenna according to a second embodiment of the invention;

Fig. 3 illustrates angular parameters of the shielding structure according to the embodiments of the present invention;

Fig. 4 illustrates dimensional parameters of the shielding structure according to the embodiments of the present invention;

Fig. 5 illustrates a top view and a slant view of the shielding structure according to the embodiments of the present invention;

Fig. 6 illustrates a top view and a slant view of a component of the shielding structure according to the embodiments of the present invention;

Fig. 7 illustrates a top view and a slant view of another component of the shielding structure according to the embodiments of the present invention; and

Fig. 8 illustrates a prior art example of an antenna mounted on a fuselage of an aircraft.

Detailed Description of the Invention

Preferred embodiments of the present invention will be described in the following with reference to the accompanying figures, wherein in the figures, identical objects are indicated by identical reference numbers. It is understood that the present invention shall not be limited to the described embodiments, and that the described features and aspects of the embodiments may be modified or combined to form further embodiments of the present invention. - lo in the following, reference will be made exemplarily to radio waves and radio antennas. However, the present invention shall not be limited to the application to radio waves and radio antennas but is also applicable to other kinds of electromagnetic waves and antennas configured for receiving such electromagnetic waves.

Moreover, unless stated otherwise, in the following, the term

"interference" or "unwanted signals" shall relate to both multipath and interference within the meaning as discussed above. Whenever it is stated that unwanted signals may not reach an antenna or are blocked by the inventive shielding structure, it is understood that these unwanted signals are sufficiently attenuated with respect to the desired signals. It is particularly understood that the average level of suppression is at least -17 dB. Fig. 1 illustrates a vertical cut along the central axis of a shielding structure for an antenna according to a first embodiment of the invention. The shielding structure 1 has a base part 100 and an upper part 200 arranged on or around the base part 100. The upper part 200 is rotationally symmetric with respect to an imaginary axis perpendicular to the base part 100 that intersects the base part 100 at its central position. This axis will be referred to as the central axis of the shielding structure 1 in the following. Likewise, the base part 100 is rotationally symmetric with respect to the central axis. As will be described below, also the further components of the shielding structure 1 are arranged so that the overall shielding structure 1 is rotationally symmetric with respect to the central axis.

The upper part 200 has an inner wall portion 210 formed on the inside of the upper part 200 and an outer wall portion 500 formed on the outside of the upper part 200. The outer wall portion 500 has a substantially cylindrical shape and serves as a mechanic support of the upper part 200 and protects the shielding structure l from mechanical damage. The outer wall portion 500 has such a thickness and is formed from such a metallic material that it is capable to provide a required mechanical strength, depending on the field of application of the shielding structure (e.g. an atmospheric exposure of the shielding structure). In particular, the outer wall portion 500 is configured to sustain a maximum wind load that the shielding structure 1 is expected to be exposed to. Preferably, the outer wall portion 500 has a thickness of about 3mm and is formed from aluminum. The inner wall portion 210 is made from panels of a material absorbing at radio frequencies (absorbing material), such as Eccosorb ^® ANW77 by Emerson & Cuming, a polyurethane foam absorber, that are provided on the inner surface of the outer wall portion 500.

The base part 100 comprises a metal supporting structure not shown in the figure and a layer of the absorbing material provided on the upper surface of the metal supporting structure. A cavity 50 is formed and bounded by the inner wall portion 210 of the upper part 200 and an upper surface of the base part 100. As follows from the above description, the entire bounding surface (i.e. inner wall) of the cavity 50 is covered by the absorbing material and is rotationally symmetric with respect to the central axis of the shielding structure 1. A circular opening 55 is situated at the upper end face of the cavity 50. However, the circular opening 55 may be covered by a non-metallic and non-absorbing material, such as a plastic cover, to protect the cavity from moisture ingress e.g. through rain.

In the following, it is understood that an upper or lower end of a cylindrical face corresponds to a circular line bounding the face from above or below, respectively. Likewise, a radially inner or outer end of an annular surface is understood to correspond to an inner circumference or to an outer

circumference of the annular surface, respectively. The inner wall portion 210 is build up from a cylindrical first inner wall section 210A situated towards the lower end of the inner wall portion 210 and, joined to the first inner wall section 210A, a conical second inner wall section 210B towards the upper end of the inner wall portion 210. The upper end of the first inner wall section 210A and the lower end of the second inner wall section 210B join in a circular line lying in an imaginary plane extending in parallel to the base part 100. The lower end of the first inner wall section 210A is disposed on or connected to the base part 100, wherein a first angle δ is formed between the first inner wall section 210A and the base part 100. The upper end of the second inner wall section 210B defines the circumference 55A of the circular opening 55 of the cavity 50. A second angle γ is formed between the second inner wall portion 210B and an imaginary plane extending in parallel to the base part 100, so that an angle (180°- δ + γ) is formed between the first and second inner wall sections 210A, 210B. Both the first and second inner wall sections 210A, 210B are made from the absorbing material.

The upper part 200 further comprises an annular top portion 220 which partly covers the upper part 200. The top portion 220 may be formed as an integral part of the upper part 200 or may be detachable from the upper part 200. A radially inner end of an annular top surface 230 of the top portion 220 and the upper end of the second inner wall section 210B join at the circular line 55A defining the opening 55 of the cavity 50. Thus, the top portion 220 partly covers the cavity 50. Here and in the following, a radial direction on (a part of) the top portion 220 is understood as a direction that is projected on a direction perpendicular to the central axis when viewed from the above, i.e. along the central axis. The top surface 230 of the top portion 220 has a tapering configuration and is inclined towards the center of the shielding element 1. In other words, the radially inner end (radially inner circumference) of the top surface 230 is at a lower level of height with respect to the base part 100 than the radially outer end (radially outer circumference) of the top surface 230.

The top portion 220 further comprises an annular outer top surface 250 having a tapering configuration and being inclined towards a periphery of the shielding element 1. In other words, the radially inner end (radially inner circumference) of the outer top surface 250 is at a higher level of height with respect to the base part 100 than the radially outer end (radially outer circumference) of the outer top surface 250. The radially inner end of the outer top surface 250 and the radially outer end of the top surface 230 join at a circular line defining the highest part of the shielding structure 1 with respect to the base part 100.

In more detail, the top portion 220 is build up from a plurality of annular and concentric (i.e. their upper and lower faces are parallel to each other) absorbing elements 220A, 220B, 220C, 220D that are stacked along their thickness direction. Each of the annular absorbing elements is made from an absorbing material, e.g. Eccosorb ^® ANW77 by Emerson & Cuming. The lowermost of the annular absorbing elements is arranged such that the lower face thereof forms the second inner wall section 210B. The upper face of the uppermost of the annular absorbing elements forms the outer top surface 250.

The absorbing elements are arranged in pairs respectively on the upper and lower faces of annular metal plates (not shown in the figure) that are made e.g. from aluminum. Each annular metal plate has a smaller inner diameter than the respective absorbing elements arranged on the metal plate, so that a gap 240 is formed between the absorbing elements at the radially inner end faces of the absorbing elements. Neighboring assemblies comprising one metal plate and two absorbing elements each are arranged relative to each other so that a gap 240 is formed between the upper absorbing element of the lower assembly and the lower absorbing element of the upper assembly. Preferably, this latter gap 240 is larger than the gap 240 formed between absorbing elements arranged on the upper and lower faces of a given metal plate. Since the absorbing elements are concentric, the gaps 240 extend in parallel to the second inner wall section 210B. The assemblies of absorbing elements and metal plates may be arranged and fixed relative to each other by means of frame structure (not shown in the figure) that can be fixed to the outer wall portion 500. The radially inner end face 230A, 230B, 230C, 230D of each of the absorbing elements is slanted with respect to the upper and lower faces of the respective absorbing element, so that an acute angle is formed between the lower face and the radially inner end face and an obtuse angle is formed between the upper face and the radially inner end face of the respective absorbing element.

The absorbing elements are dimensioned and arranged such that the radially inner end faces of the absorbing elements form the top surface 230 of the top portion 220. Accordingly, the top surface 230 of the top portion 220 is inclined towards the center of the shielding structure 1 and interrupted by the concentric gaps 240 formed between neighboring absorbing elements. In other words, the radially inner end faces of the absorbing elements constitute a plurality of sub-surfaces into which the top surface 230 of the top portion 220 is divided by one or more concentric gaps 240.

In order to avoid moisture ingress e.g. through rain into the gaps 240, the top surface 230 of the top portion 220 may be covered by a non-metallic and non-absorbing material, such as a plastic cover.

Next, an effect of the above configuration of the top surface 230 of the top portion 220 will be explained. Seen from center of the shielding element 1, the top surface 230 comprises a plurality of sharp edges at which the lower faces and radially inner end faces 230A, 230B, 230C, 230D of the respective absorbing elements meet. A radio wave that is incident on the shielding element 1 from a direction facing the edges will be split at the first (innermost) edge into a bottom portion of the radio wave that is trapped and absorbed inside of the cavity 50 of the shielding structure 1, and an upper portion of the radio wave. The upper portion of the radio wave is successively trapped and diffracted at the further edges. Consequently, the respective radio wave is effectively absorbed or reflected away from the antenna and thus may not contribute to the background affecting reception quality of the desired signal.

Inside the cavity 50 a mounting means 300 (means for mounting the antenna) covered by the absorbing material is provided. The mounting means 300 can be a pole that is arranged perpendicular to the base part 100 at the horizontal center of the shielding element 1. The height of the mounting means 300 is dimensioned so that the mounting means 300 provides an antenna mounting position 400 at which the antenna can be mounted at a position that corresponds to the imaginary apex of the conical surface defined by the top surface 230 of the top portion 220. In other words, the mounting position 400 is provided at the intersection point of the central axis of the shielding structure 1 and a line that is obtained by extending a radial line on the top surface 230 towards the central axis. Thereby, radio waves arriving at the mounting position 400 from an angle of arrival with respect to the local horizon (elevation angle) below a threshold angle (first threshold angle) cannot reach the antenna provided at the mounting position 400. This threshold angle depends on the angle between the top surface 230 (i.e. a radial line thereon) and the horizontal plane.

As an alternative to a pole-like shape of the mounting means 300, a conical shape narrowing towards the upper end of the mounting means 300, a conical shape narrowing towards the base of the mounting means 300, or a hourglass-shape built up from two conical shapes, joined either at their respective narrower ends or respective wider ends can be employed.

To protect the shielding structure from moisture ingress, a coating made from a plastic material may be provided on the outer surface of the shielding structure 1. Preferably, the plastic material is the same as that used for sealing the polyurethane foam of the ANW77 absorbing material. The coating may cover the top surface 230, the outer wall portion 500, and/or the opening 55 of the cavity 50, depending on the particular environment in which the shielding structure 1 is employed. Further, the antenna mounting position 400 can be covered by a cap made from plastic material in order to protect the antenna from atmospheric exposure and mechanical damage. Fig. 2 illustrates a vertical cut along the central axis of a shielding structure for an antenna according to a second embodiment of the invention. Unless indicated otherwise, components of the shielding structure according to the second embodiment are identical to those of the shielding structure according to the first embodiment, and are referred to by identical reference signs.

According to the second embodiment, a plurality of absorbing

elements 110 having the shape of pyramids or cones (pyramidal absorbers) is provided on the base part 100 of the shielding structure 1. The base part 100 may additionally be covered by a layer of the absorbing material. The dimensions of the pyramidal absorbers 110, i.e. a diameter of a base thereof and an angle of inclination of a side thereof with respect to the base are chosen such that optimum absorbing properties of radio frequency electromagnetic waves are obtained. In particular, the dimensions of the pyramidal absorbers 110 are matched to an operating frequency of the antenna to be mounted at the mounting position provided by the means 300 for mounting the antenna.

Preferably, a height of the pyramidal absorbers 110 is in the range from Ι.Ολ to 2.0λ, wherein λ is a wavelength corresponding to an operating frequency of the antenna.

Apart from the provision of the above-described pyramidal absorbers 110, the shielding structure of the second embodiment is identical to that of the first embodiment. In further embodiments of the present invention, pyramidal absorbers 110 may be provided on the inner wall portion 210, in addition to the pyramidal absorbers 110 provided on the base part 100. In this case, the pyramidal absorbers 110 may be provided on the first inner wall section 210A only, the second inner wall section 210B only, or on both the first and second inner wall sections 210A, 210B. In yet further embodiments, pyramidal absorbers 110 may be provided on the side faces of the mounting means 300 for mounting the antenna, in addition to the pyramidal absorbers 110 provided on the base part 100 and/or the inner wall portion 210.

Fig. 3 illustrates the angular parameters of the shielding structure according to any of the above embodiments of the present invention.

In the following, an angle formed between a radially symmetric surface and a plane is understood as the angle of intersection between a radial line lying on the radially symmetric surface (or if applicable, an extension thereof) and the plane. An angle formed between two radially symmetric concentric surfaces is understood as the angle of intersection between two radial lines respectively on the two surfaces at the same azimuth angle (or, if applicable, extensions thereof).

As already indicated above, a first angle δ is formed between the first inner wall section 210A and the base part 100. The first angle δ is in the range from 60° to 90° . Preferably, the first angle δ is chosen to be 90 ° or close to 90 ° , i.e. in the range from 85 ° to 90 ° .

Further, a second angle γ is formed between an imaginary plane extending in parallel to the base part 100 and the second inner wall

section 210B. Accordingly, an angle (180°- δ + γ) is formed between the first inner wall section 210A and the second inner wall section 210B. The second angle γ is smaller than the first angle δ. Preferably, the second angle γ is in the range from 0° to 30° . It has shown to be of particular advantage if the second angle γ is chosen to be in the range from 18° to 22° . As an optimum value for the second angle γ, an angle substantially equal to 20° has been identified by the inventors. A third angle ε' = (90° - ε) is formed between the top surface 230 and an imaginary plane extending in parallel to the base part 100. Thus, the angle formed between the top surface 230 and the second inner wall section 210B is given by (γ + ε') = (90° - ε + γ). The third angle ε' is in the range from 2° to 10° . Preferably, the third angle ε' is in the range from 4° to 6° , wherein an angle substantially equal to 5° has been identified as an optimum value for the third angle ε'.

As follows from the above description of the geometric structure of the inventive shielding structure in connection with Fig. 1 and the angular parameters, the angle formed between the lower face and the radially inner end face of each of the absorbing elements of the top portion 220 of the upper part 200, or in other words, the angle formed between the top surface 230 and the second inner wall section 210B is equal to (γ + ε'). Consequently, the angle formed between the top surface 230 and the second inner wall section 210B is an acute angle and in particular is smaller than or equal to 40° . The angle formed between the upper face and the radially inner end face of each of the absorbing elements is equal to (180° - γ - ε'). The angle formed between the upper face of the uppermost absorbing element and the outer wall portion 500 is equal to (δ + γ).

Fig. 4 illustrates the dimensional parameters of the shielding structure according to any of the above embodiments of the present invention. In the following, dimensions of the shielding structure in absolute terms and in terms of wavelengths λ corresponding to an operation frequency of the antenna will be provided. The operation frequency of the antenna is understood as the center operating frequency or resonant frequency of the antenna. For example, for GNSS ground stations, one operating frequency is approximately 1.4 GHz and the corresponding wavelength λ is approximately 21cm. As is illustrated in Fig. 4, a radius of the cavity 50 directly above the base part 100 is denoted by r, so that a diameter d of the cavity directly above the base part 100 is equal to d = 2-r. Said diameter d = 2-r also corresponds to the maximum diameter of the cavity 50. An overall height of the cavity 50, from the upper surface of the base part 100 to the opening 55 of the cavity 50 is denoted by h. A radius of the circular line 55A defining the opening 55 of the cavity 50 is denoted by dist_c, so that the diameter of the opening 55 is equal to 2-dist_c, A height of a projection of the second inner wall section 210B on a vertical plane is denoted by h_up. Lastly, a thickness of the outer wall portion 500 is denoted by w.

In the inventive shielding structure as described above, a first ratio Rl between the radius dist_c of the opening 55 and the radius r of the cavity 50 or, correspondingly, a ratio between the diameter 2-dist_c of the opening 55 and the maximum diameter 2-r of the cavity 50, is in the range from 0.35 to 0.7, that is Rl = (dist_c/r), and 0.35 < Rl < 0.7. Preferably, the first ratio Rl is in the range from 0.48 to 0.6. As an optimum value for the first ratio Rl, a ratio substantially equal to 0.54 has been identified by the inventors. A second ratio R2 between the height h of the cavity 50 and the diameter d = 2-r of the cavity 50 is in the range from 0.22 to 0.45, that is, R2 = h/(2-r), and 0.22 < R2 < 0.45. Preferably, the second ratio R2 is in the range from 0.30 to 0.38. As an optimum value for the second ratio R2, a ratio substantially equal to 0.34 has been identified by the inventors.

Further, a third ratio R3 between the height h_up of a portion of the cavity 50 defined by the second inner wall portion 210B and the height h of the cavity 50 is in the range from 0.15 to 0.3, that is, R3 = (h_up/h), and

0.15 < R3 < 0.3. Preferably, the third ratio R3 is in the range from 0.19 to 0.24. As an optimum value for the second ratio R3, a ratio substantially equal to 0.22 has been identified by the inventors. ln absolute terms, the diameter d = 2-r is in the range from 9 λ to 15 λ, where λ is the operation frequency of the antenna. Preferably, the diameter d = 2-r is in the range from 11 λ to 13.5 λ. As an optimum value for diameter d = 2-r, a diameter substantially equal to 12.4 λ has been identified by the inventors.

Further, as already indicated above in connection with Fig. 1, in the above embodiments the thickness w is chosen to be equal to 3mm. However, depending on the particular requirements with regard to weight and mechanical stability of the shielding structure, alternative values of the thickness w between 2mm and 5mm can be chosen.

Next, an effect of the above choices of parameters δ, γ, ε', r, h, dist_c and h_up. illustrated in Fig. 3 and Fig. 4 or their respective ratios will be discussed. First of all, the inventive shielding structure described above is large compared to the wavelength λ corresponding to the operating frequency of the antenna. Accordingly, the far-field approximation is applicable, and e.g. a description of the electromagnetic field in terms of radio waves e.g. being reflected at surfaces of the shielding structure is well-defined.

Moreover, an electromagnetic back-reaction between the shielding structure and the antenna does not occur, that is the electromagnetic properties of the antenna are not affected by the presence of the shielding structure, and the electromagnetic properties of the shielding structure are not affected by the presence of an antenna mounted at the mounting position 400 of the shielding structure. This implies that the inventive shielding structure can be used to shield all antennas of a given operating frequency from interference and multipath, irrespective of the particular design of the antenna (as long as the antenna could be fit within a sphere with diameter up to 1.25λ). Thus, no re-design of already existing antennas that are subsequently equipped with the inventive shielding structure is necessary. On the other hand, the geometric structure of the inventive shielding structure has been optimized to achieve all the above advantageous effects at a minimum overall size and therefore, weight.

As has been described above in connection with Fig. 1, radio waves arriving at the mounting position 400 from an angle of arrival with respect to the local horizon below a threshold angle (first threshold angle) cannot reach the antenna provided at the mounting position 400, at least not without being attenuated. This threshold angle depends on the angle between the top surface 230 and the horizontal plane, i.e. on the third angle. Specifically, the threshold angle is roughly equal to the third angle ε', The present invention takes advantage of the fact that unwanted signals typically arrive at the antenna from low elevation angles, whereas the desired signals arrive typically from high elevation angles. This is particularly true in the case of GNSS base stations observing signals emitted by GNSS satellites that are orbiting earth. Therefore, by choosing an appropriate threshold angle, which is achieved by an appropriate choice of the third angle ε', the unwanted signals can be blocked from reaching the antenna. In other words, conceptually, the inventive shielding structure splits the world into two parts: an upper one with ε'≤ Θ < 90° and a lower one with - 90 °< θ≤ ε', where Θ is the angle of arrival with respect to the local horizon, i.e. Θ = 0° indicates horizontal arrival and Θ = 90° indicates arrival from the zenith.

As the inventors have further realized, if radio waves enter the cavity 50 that are not directly incident on the antenna, it needs to be ensured that these radio waves are not reflected (scattered) towards the antenna by the bounding surface of the cavity 50, at least not before the radio waves have been sufficiently attenuated through absorption at the absorbing material with which the bounding surface of the cavity 50 is covered. Each time the radio wave is reflected (scattered) at the absorbing material, the energy density carried by the radio wave is reduced by a given factor a. If the radio wave is reflected at the absorbing material n times before it is eventually incident on the antenna, its energy density is reduced by a factor of a ⁿ . The inventive shielding structure is configured to substantially prevent incidence of radio waves on the antenna that have undergone only a single reflection (n=l) at the absorbing material, that is with an energy density reduced by a factor of a ¹ . Further, the inventive shielding stricter is configured and has been optimized so as to suppress incidence of radio waves on the antenna having undergone only a small number of reflections at the absorbing material (small n) to the highest possible degree.

The shielding structure as described above has been devised so that incidence of radio waves having undergone only a single reflection at the absorbing material may be prevented. Specifically, with the above choices of parameters and/or ratios, a radio wave that is first incident on the second inner wall section 210B is scattered towards the first inner wall section 210A (at an opposite side of the cavity 50) or the base part 100, but not towards the antenna mounting position 400. Likewise, a radio wave that is first incident on the first inner wall section 210B is scattered towards the base part 100 but not towards the antenna mounting position 400.

Clearly, for reasonably chosen dimensional parameters of the shielding structure in accordance with the above ranges, the second angle γ can be chosen so that the above goal is achieved. On the other hand, for a reasonably chosen angle γ in accordance with the above range, dimensional parameters of the shielding structure can be chosen so that this goal is achieved.

The inventive shielding structure as described above may be composed of a number of modules, which are manufactured separately and which can be transported to the site of operation of the shielding structure separately. Thereby, transportation of the shielding structure, as well as maintenance of the shielding structure can be facilitated. A shielding structure having such modular

composition will now be described with reference to Fig. 5 through Fig. 7. Fig. 5 illustrates a top view (left panel) and a slant view (right panel) of the shielding structure according to the embodiments of the present invention. As can be seen in the left panel of Fig. 5, the top portion 220 of the upper part 200 is composed of four. modules 221, 223, 225, 227 (cf. Fig. 7) each representing a sector of 90 ⁰ of the top portion 220. As can be seen in the right panel of Fig. 5, also the remainder of the upper part 200 is composed of four modules each representing a sector of 90° of the remainder of the upper part 200. In particular, the outer wall portion 500 is comprised of four arc-shaped

elements 510, 520, 530, 540 (cf. Fig. 6) that are connected to each other e.g. by screws, rivets or bolts at flanges 520A provided at the vertical edges of the arc- shaped elements 510, 520, 530. The arc-shaped elements 510, 520, 530 are fixed to the base part 100 e.g. by screws, rivets or bolts at flanges 510B, 520B, 530B provided at the arc-shaped lower horizontal edges of the arc-shaped elements 510, 520, 530, 540. Each of the arc-shaped elements 510, 520, 530, 540 is covered by the absorbing material on its face facing towards the central axis of the shielding structure. As is further illustrated in Fig. 5, the base part 100 is shaped as a circular plate and extends in the radial direction beyond the outer wall portion 500. The base part 100 is provided with radial grooves 120A,..., 120D, 130A 130D respectively spaced apart by 45° of azimuth angle. Every other one 130A,..., 130D of the grooves extends up to the outer circumference of the base part 100, whereas the remaining grooves do not extend up to the outer circumference of the base part 100.

A cap 410 formed from a plastic material provided to surround the antenna mounting position 400 is shown in the right panel of Fig. 5.

Fig. 6 illustrates a top view (left panel) and a slant view (right panel) of one of the arc-shaped elements 510, 520, 530, 540 of the shielding structure according to the embodiments of the present invention. The arc-shaped element 510 comprises the lower flange 510B and two lateral flanges 510A. Further, the arc-shaped element 510 comprises an upper flange 510C provided at its arc-shaped upper horizontal edge for connecting the respective

module 221 of the upper part 220 to the arc-shaped element 510. The face of the arc-shaped element 510 facing toward the central axis of the shielding structure in it assembled state is covered by the absorbing material 515.

Fig. 7 illustrates a top view (left panel) of one of the modules 221, 223, 225, 227 of the top portion 220 of the upper part 200 of the shielding structure according to the embodiments of the present invention and a slant view (right panel) of the module of the top portion 220 in a state in which it is fixed to a respective one of the arc-shaped elements 510, 520, 530, 540 of the outer wall section 500.

The module 221 of the top portion 220 illustrated in the right panel of Fig. 7 comprises two metal plates 221A, 221B that are curved to have the shape of a 90° sector of a surface of a truncated cone. The metal plates 221A, 221B are provided with a homogeneous relative distance, i.e. the metal plates 221A, 221B extend in parallel to each other. The metal plates 221A, 221B are fixed to a metal frame 221C connecting the metal plates 221A, 221B. The metal frame 221C may be mounted on and fixed to the upper flange 510C (not shown in Fig. 7) of the arc-shaped element 510. Each of the metal plates 221A, 221B is covered by absorptive panels on both of its faces. Accordingly, the two metal plates 221A, 221B accommodate four absorptive panels 222A, 222B, 222C, 222D. The metal plates 221A, 221B preferably have equal thickness.

The absorptive panels 222A, 222B, 222C, 222D each are concentric and have opposing parallel faces (upper and lower faces) having the shape of a 90° sector of a surface of a truncated cone. The absorptive panels 222A, 222B, 222C, 222D have equal thickness and are, by being fixed to the metal plates 221A, 221B stacked along their thickness direction. The radially inner end face of each of the absorptive panels is slanted with respect to the coplanar upper and lower faces. In more detail, between the lower surface of each of the absorptive panels and the respective radially inner end face, an angle of (γ + ε') is formed. Further, the dimensions of the absorptive panels along their radial direction are chosen so that the radially inner end faces are aligned with each other and form a tapered surface tapering towards the center of the assembled shielding structure which is interrupted by the gaps 240.

As can be seen from Fig. 7, the width of the first and third gaps 240, counting from the center of the assembled shielding structure is determined by the thickness of the metal plates 221A, 221B, which are dimensioned to have a radial length shorter than that of the corresponding absorptive panels. The width of the second gap 240 is determined by the distance in which the metal plates 221A, 221B are held from each other by the metal frame 221C. Here, the width of the second gap 240 is chosen wider than the widths of the first and third gaps 240.

Features, components and specific details of the structures of the above- described embodiments may be exchanged or combined to form further embodiments optimized for the respective application. As far as those modifications are readily apparent for an expert skilled in the art, they shall be disclosed implicitly by the above description without specifying explicitly every possible combination, for the sake of conciseness of the present description.