WO2025021304A1

WO2025021304A1 - Feeding network, antenna and mobile communication base station

Info

Publication number: WO2025021304A1
Application number: PCT/EP2023/070824
Authority: WO
Inventors: Andreas Vollmer
Original assignee: Telefonaktiebolaget LM Ericsson AB
Current assignee: Telefonaktiebolaget LM Ericsson AB
Priority date: 2023-07-27
Filing date: 2023-07-27
Publication date: 2025-01-30
Anticipated expiration: 2026-01-27

Abstract

A feeding network (20) for an antenna (14) has at least one lower level node (22), a plurality of upper level nodes (24), and a plurality of loads (26). The at least one lower level node (22) is electrically connected in parallel to a plurality of the upper level nodes (24) by lower level signal lines (28). The upper level nodes (24) are electrically connected in parallel to a plurality of the loads (26) by upper level signal lines (30). The upper level nodes (24), the respective connected loads (26) and the corresponding upper level signal lines (30) form subnetworks (32). The lengths of the upper level signal lines (30) of one of the subnetworks (32) differ from the lengths of the upper level signal lines (30) of at least one other of the subnetworks (32). Further, an antenna (14) and a mobile communication base station (10) are shown.

Description

Feeding network, antenna and mobile communication base station

Technical Field

The invention relates to a feeding network for an antenna, an antenna, and a mobile communication base station.

Background

Antennas with radiators forming an antenna array are known.

Figure 1 shows a feeding network 1 with four radiators 2 forming an array in an antenna as known in the art. In the shown example, one lower level node 3 and two upper level nodes 4 are shown.

Of the four radiators 2, in each case two are connected to each of the upper level nodes 4. The transmission lines to connect the radiators 2 to the upper level nodes 4 do not have the same lengths. Instead, the transmission line for one of the radiators 2 of one pair is longer by a shift length then the other transmission line connecting the other radiator 2 of the pair to the respective upper level node 4.

The upper level nodes 4 are connected to the lower level node 3, wherein the transmission line to one of the upper level nodes 4 from the lower level node 3 is longer than the other one of the signal lines to the other upper level node 4 by two shift lengths.

In case of perfectly impedance matched radiators 2, the beam emitted by the array of radiators 2 would have a very prominent main lobe and small side lobes, wherein the beam direction would be tilted with respect to the direction of the array by an angle depending on the shift length used.

For example, CN11063996A and WO 2013/003700A2 show antennas with such feeding networks. It is, however, not always feasible to use perfectly impedance matched radiators. For example, radiators may comprise transparency structures or are optimized to increase the transparency for electromagnetic waves in a frequency band different from their own design frequency, leading to poorly matched radiators.

In these cases, at each junction, e.g. at each upper level node 4, reflection occurs and coupling occurs from one signal line into another signal line increasing the magnitude of the side lobes and/or the direction of the main beam. Larger side lobes deteriorate the directionality of the beam and, in particular, introduce a frequency dependence of the directionality.

Summary

Thus, it is an objective to provide an antenna having a high directionality with little farfield pattern variations over frequency even with less impedance matched radiators and/or with less isolated nodes.

For this purpose, in one aspect, a feeding network for an antenna is provided. The feeding network comprises at least one lower level node, a plurality of upper level nodes, and a plurality of loads, wherein the at least one lower level node is electrically connected in parallel to a plurality of the upper level nodes by lower level signal lines. The upper level nodes are electrically connected in parallel to a plurality of the loads by upper level signal lines, wherein the upper level nodes, the respective connected loads and the corresponding upper level signal lines form subnetworks. The lengths of the upper level signal lines of one of the subnetworks differ from the lengths of the upper level signal lines of at least one other of the subnetworks.

The inventors have realized that deviations from an ideal beamforming pattern are reduced when combining at least two subnetworks with the same center frequency phase shift between radiators but with different transmission line lengths from the upper level node to the radiators. In particular, the main beam direction and the sidelobe level is more stable over frequency for directional radiators that have high impedance variance over frequency and that are excited with different phases for beam steering or full 3 -dimensional beam forming.

For example, in an embodiment with radiators connected to the subnetworks, it is achievable in this way that at least two uncombined subnetworks generate, for at least one working frequency, a deviation from the farfield downtilt or main beam pointing direction average over all working frequencies bigger than 0.5°, 1°, 1.5°, 3°, 5° and the at least two subnetworks combined together generate a deviation from the downtilt or main beam pointing direction average over all working frequencies lower than 0.5°, 1°, 1.5°, 3°, 5° for all working frequencies.

In particular, each load is connected in parallel to the respective upper level node, and each upper level node is connected in parallel to the lower level node. In parallel means within this disclosure that the upper level node or loads are parallel to one another.

The electrical connections are in particular galvanic connections.

In an embodiment, the loads are not impedance matched with the corresponding upper level node and/or the corresponding upper level signal line, providing a feeding network having a high quality beam even for unmatched loads and/or less isolated nodes.

For example, the impedance differs by more than a factor of 1.25, 1.5, 2.0, 2.5, 3.0 or 4.0 for at least one working frequency and/or the real part of the impedance differs by more than 50 Ohm, 75 Ohm, 100 Ohm, 150 Ohm, 200 Ohm or 300 Ohm for at least one working frequency at the upper level node compared to one of the upper level signal lines and/or compared to the lower level signal line. The impedance difference at a node is understood as the difference of the impedances, obtained by removing the node and looking at the node point separately in direction of each signal line connected to the node. This way, the frequency dependence of the beams emitted by individual subnetworks are displaced further with respect to one another.

For precise tuning of phase shifts, each upper level signal line of one of the subnetworks may correspond to an upper level signal line of the other subnetworks, wherein the length of the corresponding upper level signal lines of two, in particular of more than two, more particularly of all of the subnetworks differ from one another.

In an aspect, the length of two, in particular of more than two, more particularly of all of the upper level signal lines within the same subnetwork differ from one another, providing a predefined phase shift at the loads.

In order to achieve a desired phase shift across the subnetworks, the length of two, in particular of more than two, more particularly of all of the lower level signal lines may differ from one another.

In an aspect, the longest ones of the upper signal lines of at least two subnetworks differ in length by more than 0.125, particularly by more than 0.175, more preferably by more than 0.25 of a wavelength at the highest working frequency, and the farfield from radiators fed with the at least two combined subnetworks differs in the downtilt or in the main beam pointing direction average over all working frequencies less than 0.5°, 1°, 1.5°, 3°, 5°.

In an embodiment, the feeding network comprises N upper level nodes, N being an integer greater than 1, each of the upper level nodes being electrically connected to M loads, M being an integer greater than 1, by a separate one of the upper level signal lines, wherein each upper level node forms a subnetwork with the M connected loads and the corresponding upper level signal lines yielding N subnetworks. This way, feeding networks of arbitrary size are achievable.

For example, M upper level signal lines are present in each subnetwork.

The M loads are in particular different for each upper level node, i.e. the feeding network comprises N x M loads. For example, each load is connected to only one of the N upper level nodes and/or is part of only one subnetwork.

Further, in an embodiment each subnetwork comprises only one upper level node.

In an aspect, a first signal line of the upper level signal lines of one of the subnetworks has a length being a basic length, providing the minimum length of an upper level signal line.

An mth signal line (l<m<M) of the upper level signal lines of one of the subnetworks may have a length being the basic length plus (m-1) times a phase shift length for shifting the phase of a signal at the loads of the same subnetwork, yielding a predefined phase shift across the loads.

In order to further improve beam quality, the phase shift length may be the same for all of the subnetworks.

For further eliminating the effect of mismatched impedances and coupling from one signal line into another at nodes, in particular a frequency dependence of the beam direction, the basic lengths of the upper level signal lines of two, in particular more of than two, more particularly of all of the subnetworks may differ from one another.

For example, the basic length of a first subnetwork of the subnetworks is an upper default length, in particular wherein the basis length of a nth subnetwork (l<n<N) of the subnetworks is the upper default length plus n-1 times a variation length, providing - for each subnetwork - a different response over frequency for reflected signals due to impedance mismatches.

In an embodiment, a first lower level signal line of the lower level signal lines has a length being a lower default length, and a nth lower level signal line of the lower level signal lines has a length being the lower default length plus M*(n-1) times a phase shift length minus n-1 times a variation length, providing signals to the subnetworks with a predefined phase shift that takes into account the phase shift introduced by the different basic lengths of the subnetworks.

For example, for the mth load of the nth subnetwork, the total length of the corresponding lower level signal line and corresponding upper level signal line is the sum of an upper default length, a lower default length and ((n- l)*m)+m times a phase shift length. This way, the relative phase shift between each pair of neighboring loads is exactly the phase shift length.

In an aspect, the subnetworks, lower level signal lines, upper level signal lines, upper level nodes and/or loads are ordered with respect to their spatial relation to one another, in particular the spatial relation of the corresponding load.

In an embodiment, the loads are electromagnetic radiators providing an antenna array.

For example, the upper level nodes are junctions, splitters and/or phase shifters, in particular arc radial phase shifters, linear phase shifter, T-Splitters, N-way junctions (with N being an integer greater than 2), or Wilkinson splitters, making use of reliable and cost-efficient components, in particular wherein an isolation between two ports of the respective upper level node is lower than 20 dB, 15 dB, 12 dB, 9 dB, 6 dB or 3 dB. In an aspect, the at least one lower level node is a junction, a splitter and/or a phase shifter, in particular an arc radial phase shifter, a linear phase shifter, a T-Splitter, an N-way junction (with N being an integer greater than 2), or a Wilkinson splitter.

For example, every lower level node is a structure or device with good isolated output ports and every upper level node is a structure or device with poorly isolated output ports.

In an embodiment, the at least one lower level node is a radio and the lower level signal lines have the same physical length, wherein the difference in length of the lower level signal lines is provided by the radio configured to feed signals to the lower level signal lines with a phase shift corresponding to an artificial difference in physical length.

The artificial difference in physical length is in particular, as described above with respect to the different length of the lower level signal lines, M*(n-1) times the phase shift length for shifting the phase of a signal at the loads of the same subnetwork minus n-1 times the variation length.

The 2-dimensional linear array with equidistant radiator spacing and similar subnetworks illustrates the basic principle.

It is conceivable that also 3 -dimensional arrays with non-equidistant radiator spacing and with an unequal number of radiators in every subnetwork can be designed in similar way. For 3 -dimensional arrays and beamforming, the downtilt angle may be replaced by the main beam pointing direction.

Non-uniform amplitude tapering may result in realization examples with different upper level line widths in the different subnetworks. Other embodiments, for example arrays with unequal radiator spacing or with length compensation of phase shifter structures, may require different phase shifts between the subarrays. It is also conceivable that the lower level node may be load for a subnetwork as described above at a lower level of the feeding network.

A further aspect relates to an antenna comprising an array of radiators and a feeding network as described above.

The radiators may in particular be the loads of the feeding network.

Another aspect relates to a mobile communication base station, the base station having at least one antenna as described above.

The features and advantages described with respect to the radiator also apply to the antenna and/or the mobile communication base station and vice versa.

Brief Description of the Drawings

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:

Fig. 1 shows a schematic of part of a feeding network of a prior-art antenna,

Fig. 2 shows a mobile communication base station according to an embodiment of the invention with an antenna according to an embodiment of the invention,

Fig. 3 shows a schematic of part of a generalized feeding network according to an embodiment of the invention,

Fig. 4 shows a schematic of part of a simplified feeding network according to an embodiment of the invention,

Figs. 5-8 show schematics of a first, second, third and fourth subnetwork of a simplified feeding network according to an embodiment of the invention, Fig. 9 shows a graph illustrating the beam direction of the individual beams emitted from the first, second, third and fourth subnetwork of Figures 5 to 8, respectively,

Fig. 10 shows a schematic of part of a feeding network according to a second embodiment of the invention,

Fig. 11 shows a schematic of part of a feeding network according to a third embodiment of the invention, and

Fig. 12 shows a schematic of part of a feeding network according to a fourth embodiment of the invention.

Detailed Description

Figure 2 shows an embodiment of a mobile communication base station 10 and a user device 12.

The mobile communication base station 10 has a plurality of antennas 14 for providing speech and data connections to user devices. Mobile communication base stations 10 are also referred to as mobile communication cell sites.

The mobile communication base station 10 may be an access network node of a radio access network of a telecommunication network, or any other similar 3rd Generation Partnership Project (3GPP) access nodes or non-3GPP access points.

Moreover, as will be appreciated by those of skill in the art, an access a network node is not necessarily limited to an implementation in which a radio portion and a baseband portion are supplied and integrated by a single vendor. Thus, it will be understood that network nodes include disaggregated implementations or portions thereof. For example, in some embodiments, the mobile communication base station 10 is an Open-RAN (ORAN) network node. An ORAN network node is a node in the telecommunication network that supports an ORAN specification (e.g., a specification published by the O-RAN Alliance, or any similar organization) and may operate alone or together with other nodes to implement one or more functionalities of any node in the telecommunication network, including one or more network nodes and/or core network nodes.

Examples of an ORAN network node include an open radio unit (O-RU), an open distributed unit (O-DU), and an open central unit (O-CU).

The antenna 14 of the mobile communication base station 10 is a multiband antenna to provide speech and data connections in various frequency bands.

The user device 12 has an antenna 16 and may be a mobile phone, a laptop computer, or the like. The antenna 16 of the user device 12 is also a multiband antenna allowing a speech and/or data connection to the mobile communication base station 10 and/or to a communication satellite.

The antenna 14 of the mobile communication base station 10 has a plurality of electromagnetic radiators 18 and a feeding network 20 for transmitting a signal to and from the radiators 18 (Figure 3).

The radiators 18 form an array designed for emitting and receiving electromagnetic wave in a first frequency band. Thus, the first radiators 18 are designed to transmit and receive electromagnetic waves in the first frequency band.

The radiators 18 are arranged in the antenna 14 in a predefined spatial relation, in particular a column forming the array or part of an array. Within the column, the radiators 18 are arranged equidistantly from one another. Within the antenna 14, the column extends vertically so that the radiators 18 are arranged one above the other.

Figure 3 shows a generalized part of a schematic of a feeding network 20 for an antenna according to the invention.

The feeding network 20 shown in Figure 3 may be only a part of an even larger feeding network, in particular a feeding network having more than the two levels shown.

The feeding network 20 comprises a lower level node 22, a plurality of N upper level nodes 24 and a plurality of M x N loads 26 (N and M being integers greater than one) as well as a plurality of N lower level signal lines 28 and a plurality of M x N upper level signal lines 30.

For example, the loads 26 are the radiators 18 of the antenna 14. In the following, only for differentiation and ease of understanding, the loads 26 will also be referred to as radiators 18 without limitation to generality.

It is also conceivable that the loads 26 may be other components of a feeding network or an antenna, e.g. other nodes 22, 24. For example, a lower level node 22 may be a load 26 of a subnetwork at an even more lower level of the feeding network 20.

The upper level nodes 24 are junctions, splitters and/or phase shifters, for example arc radial phase shifters, linear phase shifters, T- Splitters, N-way junctions with N=3,4,5,.. . , or Wilkinson splitters.

The lower level node 22 may also be a junction, beam splitter and/or a phase shifter, for example an arc radial phase shifter, a linear phase shifter, a T- Splitter, an N-way junction with N=3,4,5,.. . , or a Wilkinson splitter.

For example, every lower level node 22 is a structure or device with good isolated output ports and every upper level node 24 is a structure or device with poorly isolated output ports, e.g. with an isolation lower than 20 dB, 15 dB, 12 dB, 9 dB, 6 dB or 3 dB.

The upper level nodes 24 are electrically, in particular galvanically connected to the lower level nodes 22 in parallel, each by one of the lower level signal lines 28.

The lengths of the lower level signal lines 28 differ from one another, in particular pairwise, as indicated by the U-shaped delay section.

The loads 26 are grouped in groups of M loads 26, each group forming a subarray of the array of the antenna 14.

The loads 26 of each group are electrically, in particular galvanically, connected to one of the upper level nodes 24, each load 26 by one of the upper level signal lines 30.

Thus, each upper level node 24 is electrically connected to M loads 26, wherein each load 26 is only connected to one of the upper level nodes 24.

The loads 26 are not impedance matched with the upper level node 24 and/or the upper level signal line 30.

For example, the impedance differs by more than a factor of 1.25, 1.5, 2.0, 2.5, 3.0 or 4.0 for at least one working frequency and/or the real part of the impedance differs by more than 50 Ohm, 75 Ohm, 100 Ohm, 150 Ohm, 200 Ohm or 300 Ohm for at least one working frequency at the upper level nodes 24 compared to one of the upper level signal lines 30 and/or compared to the lower level signal line 28 connected to the respective node.

This way, N subnetworks 32 are formed each comprising one of the upper level nodes 24, the M loads 26 connected to the respective upper level node 24 and the M corresponding upper level signal lines 30 electrically connecting the M loads 26 to the respective upper level node 24. Each subnetwork 32 comprises only one upper level node 24.

The level of the feeding network 20 comprising the subnetworks 32 is referred to the upper level of the feeding network 20 within this disclosure, wherein the level comprising the lower level nodes 22, lower level signal lines 28 and upper level nodes 24 are referred to as the lower level of the feeding network 20.

The structure of the subnetworks 32 is explained with more detail with respect to Figure 4, showing a feeding network 20 having two upper level nodes 24 (N=2) and four loads 26 (M=4).

The order and numbering of the radiators 18 and with that the order and numbering of the corresponding nodes 22, 24, signal lines 28, 30 and subnetwork 32 depends on the spatial relation of the radiators 18. For example, within this disclosure, the subnetwork 32 comprising the uppermost radiator 18 is referred to as the first subnetwork 32.

Within the first subnetwork 32, the uppermost radiator 18 and the corresponding signal lines 28, 30 and nodes 22, 24 are referred to as the first radiator, first signal lines or first nodes, respectively.

The second radiator, nodes and signal lines will be the ones below the first radiator, signal lines and nodes, respectively, and so forth.

All of the subnetworks 32 are designed in generally the same fashion, in particular each of the upper level signal lines 30 has a corresponding upper level signal line 30 in each different one of the subnetworks 32.

As such, for example, each subnetwork 32 has a first radiator and a first upper level signal line 30 so that the upper level signal lines 30 with the same order number, i.e. first upper level signal lines 30, second upper level signal lines 30, etc., correspond to one another. Within each subnetwork 32, the length of the upper level signal lines 30 differ from one another, in particular upper level signal lines 30 with a lower order number are shorter than upper level signal lines 30 with a higher order number.

The upper level signal lines 30 are longer by a shift length Ls compared to the neighboring upper level signal line 30 with a lower order number.

For example, the first upper level signal line 30 of the nth subnetwork 32 (1 < n < N) has a length being a basic length LB.H. In particular, the basic length LB,II is specific for each subnetwork 32, wherein n denotes the order number of the respective subnetwork 32.

The second upper level signal line 30 of this nth subnetwork 32 has a length being the basic length LB,II plus the phase shift length Ls, as depicted in Figure 4.

A third upper level signal line 30 would have a length being the basic length LB,II plus two times the phase shift length Ls, and so forth.

In general, the mth upper level signal line 30 of the nth subnetwork 32 would have a length being the basic length LB.I, of the respective subnetwork 32 plus m-1 times the phase shift length Ls (1 < m < M).

Even though the basic length LB,_nis specific for each subnetwork 32, the phase shift length Ls is the same for all of the subnetworks 32.

Due to the differences in length of the upper level signal lines 30 being a multiple of the phase shift length Ls, a phase shift of the signals emitted from the radiators 18 is achieved. The phase shift leads to the beam generated by the antenna array having a tilt downwards, for example a tilt of 96° with respect to the vertical direction in which the radiators 18 are aligned. The subnetworks 32 are almost identical to one another except for the basic length LB, _n that is applied.

A comparison of the first subnetwork 32 with the second subnetwork 32 of Figure 4 shows that the upper level signal lines 30 that correspond to each other differ from one another in length, even though they comprise the same number of delay lines (having the phase shift length Ls).

The basic length LB,I of the first subnetwork 32 is a length referred to as upper default length LUD within this disclosure.

The first upper level signal line 30 of the first subnetwork 32 differs in length from the first upper level signal line 30 of the second subnetwork 32 by a variation length Lv.

Likewise, the second upper level signal line 30 of the first subnetwork 32 differs in length from the second upper level signal line 30 of the second subnetwork 32 by the variation length Lv.

Similarly to the length difference within the same subnetwork 32, the length of an upper level signal line 30 of a specific subnetwork 32 is longer by a variation length Lv than the corresponding upper level signal line 30 of the subnetwork 32 having the previous order number.

In other words, the basic length LB.I, of the nth subnetwork 32 is the upper default length LUD plus (n-1) times the variation length Lv (1 < n < N).

For example, the longest ones of the upper level signal lines 30 of at least two subnetworks 32 differ in length by more than 0.125, particularly by more than 0.175, more preferably by more than 0.25 of a wavelength at the highest working frequency.

In the example of Figure 4, the first upper level signal line 30 of the first subnetwork has a length being the upper default length LUD (N=l, M=l), the second upper level signal line 30 of the first subnetwork 32 has a length being the upper default length LUD plus 1 time the phase shift length Ls (N=l, M=l), the first upper level signal line 30 of the second subnetwork 32 has a length being the upper default length LUD plus 1 time the variation length Lv (N=2, M=l), and the second upper level signal line 30 of the second subnetwork 32 has a length being the upper default length LUD plus 1 time the variation length Lv plus 1 time the phase shift length Ls (N=2, M=2).

In fact, all of the upper level signal lines 30 have a length different from the lengths of all other ones of the upper level signal lines 30.

Turning now to the lower level of the feeding network 20, the lower level signal lines 28 have different lengths, in particular all lower level signal lines 28 have an individual length.

The first lower level signal line 28 has a length being a lower default length LLD.

The difference in length between two neighboring lower level signal lines 28, i.e. having consecutive order numbers, is determined by the number N of the radiators 18 in each subnetwork 32 and its order number.

The nth lower level signal line 28 has a length being the lower default length LLD plus M times n-1 times the phase shift length Ls minus (n-1) times the variation length Lv, i.e. LLD + (M x Ls - Lv) (n-1).

As such, the length between each radiator 18, i.e. each load 26, and the lower level node 22 is given by the load's 26 order number within its subnetwork 32 and the order number of its subnetwork 32. In particular, the length differs by the phase shift length Ls for neighboring radiators.

The mth radiator 18 of the nth subnetwork 32 is connected to the lower level node 22 by a signal line of length being the upper default length LUD plus the lower default length LLD plus (n-1 x M) + (m-1) times the phase shift length Ls.

It is to be understood that the length of any signal line within the upper level nodes 24 is regarded as part of the upper level signal line 30 (i.e. the upper default length LUD) or the lower level signal line 28 (i.e. the lower default length LLD). The upper level nodes 24 are not regarded to have a signal line of any length.

This way, a constant difference in signal propagation time between neighboring radiators 18 (corresponding to the phase shift length Ls) is achieved while the lengths between the radiators and the upper level nodes 24 differ for each subnetwork 32.

This leads to different dependencies on the frequency of the beam direction, as will be explained with respect to Figures 5 to 9.

Figures 5 to 8 show subnetworks 32 as previously discussed. However, the shown radiators 18 are dual polarized radiators so that each radiator 18 is fed by two sets of upper level signal lines 30, upper level nodes 24, lower level signal lines 28 and lower level nodes 22 being identical to one another.

Figures 5 and 6 show the first and second subnetwork 32 corresponding to the first and second subnetwork 32 shown in Figure 4, respectively. Figures 7 and 8 show the respective third and fourth subnetwork 32 of the same feeding network 20.

Thus, Figures 5 to 8 show the first four subnetworks 32 of a feeding network 20 having four subnetworks 32 with two radiators 18 each (N=4, M=2).

Figure 9 shows a graph illustrating the beam direction over frequency, more precisely the downtilt in degrees with respect to the vertical, for the radiators 18 of each of the four subnetworks 32, i.e. for each of the subarrays. The beam direction of the first subnetwork 32 (Fig. 5) is shown in a solid line, the one of the second subnetwork 32 (Fig. 6) is shown in a dashed line, the one of the third subnetwork 32 (Fig. 7) is shown in a dashed dotted line, and the beam direction of the fourth subnetwork (Fig. 8) is shown in a dotted line.

For example, in Figure 8 the upper level signal lines 30 of the subnetwork 32 are about 3 x Lv (thus 0.25 wavelengths) longer than in Figure 5, considering for the wavelength the effective relative permittivity of a microstrip transmission line on substrate.

As can be seen, the frequency dependence of the downtilt for each of the subnetworks 32 is qualitatively the same having peaks roughly around 0.75 GHz and 9.25 GHz. However, the frequency dependencies, e.g. the peaks, are shifted with respect to one another. For example, the peak of the beam direction of the first subnetwork 32 at about 0.73 GHz is complemented by a minimum of the beam direction of the fourth subnetwork 32.

Thus, the sum of the beams of the four subnetworks 32, being the total beam of the antenna 14, show a uniform direction over frequency with only little variation, as the rather strong frequency dependence of each of the subnetworks cancel out.

This way, an antenna 14 with a total beam having a uniform direction over frequency is achieved even for radiators 18 that are not impedance matched and/or for signal lines 30 with poor isolation at nodes 24.

Figures 10 to 12 show further embodiments of an antenna 14 according to the invention, more precisely the feeding network 20.

The further embodiments substantially correspond to the first embodiment so that the same and functionally the same components and lengths are labeled with the same reference signs and only the differences are discussed in the following. In the second embodiment of Figure 10, the lower level node 22 is a radio 34 of the antenna 14. Thus, the lower level signal lines 28 extend from the radio 34 to the upper level nodes 24.

In the second embodiment, all of the lower level signal lines 28 have the same physical lengths as the radio 34 is capable of providing signals to the various lower level signal lines 28 with different phases. It is also conceivable that the lower level signal lines 28 has different lengths as described with respect to the first embodiment.

The signals are fed to the lower level signal lines 28 having phase shifts corresponding to the phase shift that would occur if their physical lengths differed as discussed with respect to the first embodiment.

This way, also for active antennas a high directionality and high beam stability over frequency of the beam can be achieved with poorly impedance matched radiators 18 and poorly isolated signal lines 30 at nodes 24.

In the third embodiment of Figure 11, the feeding network 20 comprises three loads, i.e. radiators 18, for each subnetwork 32 (M=3). The delay line of the phase shift length Ls in the upper level signal lines 30 has been omitted for reasons of clarity.

The feeding network 20 has, just as in the first embodiment, two upper level nodes 22 (N=2).

The fourth embodiment shown in Figure 12 is similar to the third embodiment of Figure 11 in that respect that three radiators 18 are provided for each subnetwork 32 (M=3) and two upper level nodes 24 are present (N=2).

In the embodiment shown in Figure 12, however, the first and second upper level signal lines 30 of each subnetwork 32 share a common line portion. This way, a more compact design with respect to the embodiment of Figure 11 is achieved.

Even though the point at which the first and second upper level signal lines 30 split into separate lines may be regarded as another node or junction, the effect of the invention still applies to such an arrangement.

Claims

1. Feeding network for an antenna (14) comprising at least one lower level node (22), a plurality of upper level nodes (24), and a plurality of loads (26), wherein the at least one lower level node (22) is electrically connected in parallel to a plurality of the upper level nodes (24) by lower level signal lines (28), wherein the upper level nodes (24) are electrically connected in parallel to a plurality of the loads (26) by upper level signal lines (30), wherein the upper level nodes (24), the respective connected loads (26) and the corresponding upper level signal lines (30) form subnetworks (32), wherein the lengths of the upper level signal lines (30) of one of the subnetworks (32) differ from the lengths of the upper level signal lines (30) of at least one other of the subnetworks (32).

2. Feeding network according to claim 1, characterized in that the loads (26) are not impedance matched with the corresponding upper level node (24) and/or the corresponding upper level signal line (30), in particular wherein the impedance differs by more than a factor of 1.25, 1.5, 2.0, 2.5, 3.0 or 4.0 for at least one working frequency and/or the real part of the impedance differs by more than 50 Ohm, 75 Ohm, 100 Ohm, 150 Ohm, 200 Ohm or 300 Ohm for at least one working frequency at the upper level node (24) compared to one of the upper level signal lines (30) and/or compared to the lower level signal line (28).

3. Feeding network according to claim 1 or 2, characterized in that each upper level signal line (30) of one of the subnetworks (32) corresponds to an upper level signal line (30) of the other subnetworks (32), wherein the length of the corresponding upper level signal lines (30) of two, in particular of more than two, more particularly of all of the subnetworks (32) differ from one another.

4. Feeding network according to any of the preceding claims, characterized in that the length of two, in particular of more than two, more particularly of all of the upper level signal lines (30) within the same subnetwork (32) differ from one another.

5. Feeding network according to any of the preceding claims, characterized in that the length of two, in particular of more than two, more particularly of all of the lower level signal lines (28) differ from one another.

6. Feeding network according to any of the preceding claims, characterized in that the longest ones of the upper level signal lines (30) of at least two subnetworks (32) differ in length by more than 0.125, particularly by more than 0.175, more preferably by more than 0.25 of a wavelength at the highest working frequency.

7. Feeding network according to any of the preceding claims, characterized in that the feeding network comprises N upper level nodes (24), N being an integer greater than 1, each of the upper level nodes (24) being electrically connected to M loads (26), M being an integer greater than 1, by a separate one of the upper level signal lines (30), wherein each upper level node (24) forms a subnetwork with the M connected loads (26) and the corresponding upper level signal lines (30) yielding N subnetworks (32).

8. Feeding network according to claim 7, characterized in that a first upper level signal line (30) of the upper level signal lines (30) of one of the subnetworks (32) has a length being a basic length (LB,II).

9. Feeding network according to claim 7 or 8, characterized in that an mth upper level signal line (30) of the upper level signal lines (30) of one of the subnetworks (32) has a length being the basic length (LB R) plus m-1 times a phase shift length (Ls) for shifting the phase of a signal at the loads (26) of the same subnetwork (32).

10. Feeding network according to claim 9, characterized in that the phase shift length (Ls) is the same for all of the subnetworks (32).

11. Feeding network according to any of the claims 8 to 10, characterized in that the basic lengths (LB.H) of the upper level signal lines (30) of two, in particular more of than two, more particularly of all of the subnetworks (32) differ from one another.

12. Feeding network according to claim 11, characterized in that the basic length (LB,I) of a first subnetwork (32) of the subnetworks (32) is an upper default length (LUD), in particular wherein the basis length (LB.R) of a nth subnetwork (32) of the subnetworks (32) is the upper default length (LUD) plus n-1 times a variation length (Lv).

13. Feeding network according to any of the claims 7 to 12, characterized in that a first lower level signal line (28) of the lower level signal lines (28) has a length being a lower default length (LLD), and a nth lower level signal line (28) of the lower level signal lines (28) has a length being the lower default length (LLD) plus M*(n-1) times a phase shift length (Ls) minus n-1 times a variation length (Lv).

14. Feeding network according to any of the claims 7 to 13, characterized in that for the mth load of the nth subnetwork (32), the total length of the corresponding lower level signal line (28) and corresponding upper level signal line (30) is the sum of an upper default length (LUD), a lower default length (LLD) and ((n-l)*m)+m times a phase shift length (Ls).

15. Feeding network according to any of the claims 7 to 14, characterized in that the subnetworks (32), lower level signal lines (28), upper level signal lines (30), upper level nodes (24) and/or loads (26) are ordered with respect to their spatial relation to one another, in particular the spatial relation of the corresponding load (26).

16. Feeding network according to any of the preceding claims, characterized in that the loads (26) are electromagnetic radiators (18).

17. Feeding network according to any of the preceding claims, characterized in that the upper level nodes (24) are junctions, splitters and/or phase shifters, in particular arc radial phase shifters, linear phase shifter, T- Splitters, N-way junctions with N=3,4,5,... or Wilkinson splitters, in particular wherein an isolation between two ports of the upper level node (24) is lower than 20 dB, 15 dB, 12 dB, 9 dB, 6 dB or 3 dB.

18. Feeding network according to any of the preceding claims, characterized in that the at least one lower level node (22) is a junction, a splitter and/or a phase shifter, in particular an arc radial phase shifter, a linear phase shifter, a T-Splitter, an N-way junction with N=3,4,5,.. . or a Wilkinson splitter.

19. Feeding network according to any of the claims 1 to 17, characterized in that the at least one lower level node (22) is a radio (34) and the lower level signal lines (28) have the same physical length, wherein the difference in length of the lower level signal lines (28) is provided by the radio (34) configured to feed signals to the lower level signal lines (28) with a phase shift corresponding to a difference in physical length.

20. Antenna comprising an array of radiators (18) and a feeding network (20) according to any of the claims 1 to 19, in particular wherein the radiators (18) are the loads (26) of the feeding network (20).

21. Mobile communication base station having at least one antenna (14) according to claim 20.