Multi-frequency band antenna

[0002] The invention relates to a multiband antenna.

[0003] Most mobile communication is handled via the GSM 900 network, that is to say in the 900 MHz band. In addition, the GSM 1800 Standard has been established, inter alia, in Europe, in which Standard signals can be transmitted and received in an 1800 MHz band.

[0004] Such multiband base stations therefore require multiband antenna devices for transmitting and receiving different frequency bands, which normally have dipole structures, that is to say a dipole antenna device for transmitting and receiving the 900 MHz band range and a further dipole antenna device for transmitting and receiving the 1800 MHz band range.

[0005] In practice, therefore, multiband, or at least two-band, antenna devices have already been proposed, namely, for example, a dipole antenna device for transmitting the 900 MHz band and for transmitting the 1800 MHz band, with the two dipole antenna devices being arranged alongside one another. Two antennas are therefore required in each case for the at least two frequency band ranges which, in fact, since they are arranged physically alongside one another, interfere with one another and have an adverse effect on one another, since they shadow each other's polar diagram. It is thus no longer possible to achieve an omnidirectional polar diagram.

[0006] It has therefore also already been proposed for two corresponding antenna devices to be arranged one above the other for operation in two different frequency band ranges. This, of course, leads to a greater physical height and demands a larger amount of space. In addition, the omnidirectional polar diagram is in some circumstances also adversely affected, at least to a minor extent, since the connecting line leading to the higher antenna device has to be routed past the lower antenna device.

[0007] The object of the present invention, in contrast, is to provide an improved two-band or multiband antenna device.

[0008] According to the invention, this object is achieved by the features specified in claim 1. Advantageous refinements of the invention are specified in the dependent claims.

[0009] In comparison to the prior art, the present invention provides, in a surprising manner, a completely novel, extremely compact antenna device which can be operated in a two frequency band range. However, if required, this antenna device can also be extended as required for a multiband range covering more than two frequency bands.

[0010] Specifically, the invention provides for the dipole antenna device for the first frequency band and the dipole device for the at least second frequency band, which is offset from the former, to be formed coaxially with respect to one another and in the process, such that they are located interleaved in one another.

[0011] To this end, according to the invention, the dipole halves are preferably in the form of sleeves, with the sleeve diameters of the dipole halves differing from one another to such an extent that the sleeves are arranged one inside the other. The length of the dipole halves in this case depends on the frequency band range to be transmitted. Those dipole halves which are in the form of sleeves, are designed to have the shorter length and are required for the higher frequency band range are in this case located on the outside, with those dipole halves which are designed to be appropriately longer for the lower frequency band range being arranged inside these outer sleeves, with their length projecting beyond the outer dipole sleeves.

[0012] The outer and inner sleeves of the dipole halves are each electrically and mechanically connected at their inner ends to a short-circuiting point which is similar to a sleeve base, with the one dipole halves, which are interleaved in one another in the form of sleeves, making contact with an inner conductor, and the other dipole halves, which are interleaved in one another, making contact with the outer conductor.

[0013] The particular feature of this design principle is that, for example, the outermost dipole halves which are in the form of sleeves and are suitable for the higher frequency band range act as dipole radiating elements towards the outside, but act as a detuning sleeve towards the inside, so that those dipole halves which are in the form of sleeves and are provided for the low frequency band range cannot be identified for these radiating elements.

[0014] Those dipole halves which are in the form of sleeves, are provided for the lower frequency band range and, in contrast, are each designed to be longer act as radiating elements over their entire length outwards, without the blocking effect of the outer radiating element, which is in the form of a sleeve, having any effect for the higher frequency band range, but act as a detuning sleeve towards the inside, so that no surface waves can propagate onto the outer conductor.

[0015] If more than two frequencies or frequency bands are to be transmitted, the design principle can be extended appropriately, with the sleeves for the higher frequency each having a larger diameter in their shorter length extent, and the dipole halves, which are in the form of sleeves, for the lower frequency band range in each case being accommodated such that they are interleaved in one another.

[0016] This design principle also allows central feeding via a common connection or a common coaxial line, which is preferably used not only for feeding but is also used at the same time for mechanical robustness and holding the antenna. The coaxial vertical tube which is in the form of the outer conductor is in this case mechanically and electrically connected to the one dipole half at the appropriate feed point, that is to say at the short-circuiting point of this dipole half, with the inner conductor continuing slightly beyond the outer conductor, where it is electrically and mechanically attached to the short-circuiting points, which are similar to sleeve bases, of the other dipole halves. If the inner conductor has appropriate strength, there is no need for any further additional measures for robustness. Otherwise, additional measures which electrically have no effect but are used for robustness could be provided between the short-circuiting points, which are in the form of sleeves, of the mutually adjacent dipole halves. Apart from this, the entire antenna illustrated in the attached figure is accommodated in a protective tube, for example a tube composed of glass-fiber-reinforced plastic, which engages over the antenna arrangement, fitting it as accurately as possible, so that the inner conductor has to withstand and absorb only the weight of the upper dipole halves, since tilting loads and movements are absorbed by the protective tube.

[0017] It can also be seen from the figure that a further major advantage is that only a single coaxial cable connection is required for feeding the at least two or more frequency band ranges to the antenna device.

[0018] However, the dipole halves need not necessarily be in the form of tubular structures which are in the form of sleeves and are short-circuited at their feed points. These dipole halves, which are in the form of sleeves, may have circular or cylindrical cross sections, or may be provided with a polygonal or even oval cross section. They need not necessarily be in the form of closed tubes, either. Multi-element structures are also feasible, in which the dipole halves, which are similar to sleeves, are composed of a number of individual conductor sections or electrically conductive elements, or are broken down into these sections or elements, provided these sections or elements are short-circuited to one another at their respective feed end which is adjoined to the respective adjacent second dipole.

[0019] In particular, according to the invention, not only a single band but also a multi-frequency band antenna device is possible, which preferably comprises at least two antenna devices located one above the other, which can in turn transmit in at least two frequency band ranges each.

[0020] This can be achieved according to the invention in that the coaxial feed line arrangement is routed axially through that antenna device which is preferably in each case lower, and is continued to the next higher antenna device. In the feed line, the outer electrical conductors of the multiple coaxial feed lines are in each case used to feed the dipole halves of the lower antenna device while, in contrast, those conductors of the coaxial line (for example the inner conductor, which is generally in the form of a wire, and the innermost coaxial conductor surrounding it) which are inside the former are in each case used for electrically feeding that antenna device which is higher than the other and has the dipole halves provided there.

[0021] The design principle can be cascaded in a corresponding manner, so that three or more antenna devices can also be arranged one above the other.

[0022] This can preferably be achieved in a highly advantageous and effective manner by using a specific feed and output-coupling apparatus.

[0023] The invention will be explained in more detail in the following text with reference to exemplary embodiments. In the figures, in detail:

[0024] FIG. 1a: shows a schematic, axial longitudinal cross section of one exemplary embodiment of a two-band antenna (dipole structure);

[0025] FIG. 1b: shows a schematic axial longitudinal cross section through one exemplary embodiment of two two-band antennas arranged one above the other;

[0026] FIG. 2: shows a narrowband lightning protection device, which is known from the prior art, for a coaxial line;

[0027] FIG. 3: shows a detail of the schematic axial sectional illustration to explain the principle of a feed and output-coupling apparatus according to the invention for feeding a triax line for one frequency band;

[0028] FIG. 4: shows a development, according to the invention, of a multiband feed apparatus or output-coupling apparatus;

[0029] FIG. 5: shows a schematic cross-sectional illustration along the line V—V in FIG. 4;

[0030] FIG. 6: shows an exemplary embodiment modified from that in FIG. 4;

[0031] FIG. 7: shows an exemplary embodiment, once again modified from that in FIG. 4, of a multiband output-coupling apparatus for feeding three frequencies (three frequency bands), which are transmitted or received via two antenna devices;

[0032] FIG. 8: shows an exemplary embodiment, which is developed further with respect to that in FIG. 4, for feeding three antenna devices, which cover two frequency band ranges and are arranged one above the other, by means of a quadruple coaxial line; and

[0033] FIG. 9: shows an embodiment, which is comparable to that in FIG. 4, but with only a single inner conductor (for example as lightning protection for a two frequency band device).

[0034] A multiband antenna 1 as shown in FIG. 1acomprises a first antenna 3 with two dipole halves 3′ and 3″ which, in the illustrated exemplary embodiment, are formed from an electrically conductive cylindrical tube. The dipole half 3′ which is at the top in the figure is in this case in the form of a sleeve, that is to say it is closed in the form of a sleeve at its end 7′ adjacent to the second dipole half 3′.

[0035] The length of these dipole halves 3′ and 3″ depends on the frequency band range to be transmitted and, in the illustrated exemplary embodiment, is matched to transmission of the lower GSM band range, that is to say, in accordance with the GSM mobile radio standard, to transmission in the 900 MHz band.

[0036] A second antenna in the form of a dipole is provided for transmitting a second frequency band range, in the illustrated exemplary embodiment this being 1800 MHz, and the dipole halves 9′ and 9″ of this antenna are designed with a shorter length, corresponding to the higher frequency band range to be transmitted, and, in the illustrated exemplary embodiment, are only about half as long as the dipole halves 3′ and 3″ since the transmission frequency is twice as high.

[0037] These dipole halves 9′ and 9″ are likewise in the form of tubes or cylinders in the illustrated exemplary embodiment, but have a larger diameter than the diameter of the dipole halves 3′ and 3″, so that the dipole halves of the antenna 9 which has the shorter length are accommodated within the dipole halves 3′ and 3″ having the greater longitudinal extent, and can engage over them.

[0038] The dipole halves 3′ and 9′, together with 3″ and 9″, are jointly designed in the form of sleeves, are each located such that they are interleaved in one another and are each located at the mutually adjacent inner ends 7′ and 7″ of the dipole halves, and are in this way electrically connected to one another, forming a short-circuit 11′ or 11″, respectively.

[0039] The drawing also shows that the lower dipole halves 3″ and 9″ are fed via an outer conductor 15 of a coaxial feed line 17, with the inner conductor 19 being routed beyond the short-circuit 11″ at the end 7″ of the lower dipole half as far as the short-circuiting connections 11′, which are in the form of sleeves, of the upper dipole halves 3′ and 9′, where they are electrically and mechanically connected to the bases, which are in the form of sleeves, of these dipole halves 3′ and 9′.

[0040] In this embodiment, it is possible to feed both dipole antennas 3 and 9, which are arranged such that they are interleaved in one another, via a single coaxial connection 21.

[0041] The antenna operates in such a way that those dipole halves which are provided for the higher frequency band range have a shorter longitudinal extent acting as radiating elements towards the outside, while the inside of these dipole halves 9′ and 9″, which are in the form of sleeves, act as a detuning sleeve. This detuning-sleeve effect ensures that no surface waves can propagate onto the dipole halves of the second antenna, which have a greater longitudinal extent.

[0042] However, the detuning sleeve for the higher frequency of the outer dipole halves 9′, 9″ which are in the form of tubes or sleeves “cannot be identified” or is effective for the second antenna 3 with the dipole halves 3′, 3″ which extend over a greater length, so that these dipole halves also act as individual radiating elements towards the outside. The inside of the lower dipole half 3″, which is in the form of a sleeve, acts as a detuning sleeve, however. This detuning sleeve effect ensures that no surface waves can propagate on the outer conductor of a coaxial feed line.

[0043] This design results in an extremely compact antenna arrangement, which also has optimum omnidirectional radiation characteristic which has never been known in the past; and nevertheless has simplified feed via only a single, common connection.

[0044] However, in contrast to the illustrated exemplary embodiment, the dipole halves need not necessarily be in the form of tubes or sleeves. Instead of a round cross section for the dipole halves 3′ to 9″, polygonal (n-polygonal shaped) dipole halves, as well as other dipole halves whose shapes are not circular, for example being oval, are also feasible. Furthermore, structures for the dipole halves are also conceivable in which the circumferential outer surface is not necessarily closed, but is broken down into a number of individual elements which are curved in three dimensions or are even planar, provided these are electrically connected to one another at their mutually adjacent inner end 7′ or 7″, respectively, of the dipole halves at which the short-circuits 11′ or 11″, respectively, which are in the form of sleeves and have been mentioned above, are formed, and, at the same time, are designed such that the said blocking effect of the respective outer sleeve with respect to the inner sleeve is maintained, in order to ensure that no surface waves can propagate.

[0045] The dashed lines in the illustrated exemplary embodiment in the attached figure indicate that this design principle can be extended without any problems to other frequency band ranges. A dashed line in this case indicates that, for example, a further outer sleeve could also be provided for dipole halves 25′ and 25″ of a third antenna 25, which is designed for an even higher frequency and therefore has an even shorter longitudinal extent. These dipole halves 25′ and 25″ are also each short-circuited to the end of the other dipole half at their inner ends which point towards one another. The outside of these dipole halves 25′ and 25″ acts as a radiating element for this frequency, with the inside acting as detuning sleeves with respect to the next inner dipole halves. These detuning sleeves are, however, once again not effective for the dipole halves which are interleaved in one another.

[0046] In contrast to the exemplary embodiment shown in FIG. 1a, a dipole half which is not in the form of a sleeve or hollow cylinder, or the like, that is to say a dipole half in the form of a rod, for example, could also be used instead of the upper, innermost dipole half 3′, since this dipole half does not need to accommodate either a further dipole half or a feedline connection in its interior.

[0047] A multiband antenna as shown in FIG. 1bcomprises a first antenna device A whose design corresponds to that of the antenna device shown in FIG. 1a. The reference symbols used in FIG. 1aare just given the suffix letter “a” for the antenna device A in FIG. 1b.

[0048] The antenna device shown in FIG. 1b,however, also comprises a second multiband antenna device B, which is designed on the same principle, but for which the suffix letter “b” is used, rather than “a”, for the first multiband antenna device A for the reference symbols for this second antenna device B.

[0049] In this embodiment, it is possible to feed both the dipole antennas 3aand 9a,which are arranged interleaved in one another, via a single coaxial connection 21a,at which a coaxial connecting line 52 is connected to an outer conductor 51 and an inner conductor 53, and the feed line 17, which starts from this point, and has the outer conductor 15aand the inner conductor 19a.

[0050] In an antenna such as that shown in FIG. 1b,it is thus desirable to have the capability to feed the upper multiband antenna device A, for example, via a triple coaxial line 17, that is to say via the inner coaxial line 17awith the inner conductor 19aand the outer conductor 15a, and to feed the lower antenna device B via the outer coaxial line 17bwith the inner conductor 19band the outer conductor 15b. In this case, the central coaxial conductor thus has two functions, firstly, it is the outer conductor 15afor the upper antenna device A and, at the same time, it is the inner conductor 19bfor the lower antenna device B. Since, however, the outer conductor 15aof the inner coaxial line is connected to ground (for example by the coaxial connecting link 21a), and this outer conductor 15aof the inner coaxial cable 17aat the same time represents the inner conductor 19bof the outer coaxial cable 17b, this means that the inner and outer conductors 19b, 15bof the outer coaxial cable 17ball have the same potential, namely ground.

[0051] Additional technical measures are therefore required which allow a corresponding feed for operation of the upper and lower antenna devices A and B, respectively, and which also allow an inner conductor to be connected to the potential of the outer conductor.

[0052] A solution which is known from the prior art for a coaxial line 17 with an inner conductor 19 and an outer conductor 15 is shown in FIG. 2, which has a coaxial spur line SL at a connecting point 46, the coaxial outer conductor AL of which spur line SL is electrically connected to the outer conductor 15, while its inner conductor IL is connected to the inner conductor 19 of the coaxial line 17. At the end of the spur line, the outer conductor AL is short-circuited to the associated inner conductor IL via a short-circuit KS in the form of a sleeve, by which means the inner conductor 19 is thus connected to the outer conductor 15 of the coaxial line 17. This is done, for a specific frequency or a specific frequency band, in such a manner that the electrical length of the coaxial spur line LS corresponds to 1=λ/4, where λ is the wavelength of the relevant frequency, or of the relevant frequency band. However, this is only ever possible in a narrowband form for a specific frequency, and thus for a specific wavelength.

[0053] Should the antenna described in FIG. 1 and having an upper and a lower antenna device be operated in only one frequency band, then this can be achieved via a common multiple coaxial line with a feed apparatus or output-coupling apparatus according to the invention, as shown in FIG. 3.

[0054] The exemplary embodiment shown in FIG. 3 differs from FIG. 2, inter alia, in that the coaxial line 17 makes a right-angle bend at the connecting point 46, that is to say coming from above, it is not routed downwards, as shown in FIG. 2, but, as it continues, bends away to the left at the connecting point 46. In the exemplary embodiment shown in FIG. 3, the spur line which is shown in FIG. 2 is shown lying in an axial extension of the coaxial connecting line which runs vertically upward above the connecting point 46. A further difference is that the inner conductor 19 shown in FIG. 2 is replaced by a coaxial line 17ain FIG. 3.

[0055] An electrical connection for the inner conductor 19aand for the outer conductor 15aof the inner coaxial line 17afor feeding the upper antenna device A can now be produced via a coaxial cable 52 which leads to a coaxial connection 21aand has an inner conductor 53 and an outer conductor 51, with the outer coaxial line 17bbeing fed appropriately via a second feed line 42 with an inner conductor 43 and an outer conductor 41, via a coaxial connection 21band a coaxial intermediate line 62 with an inner conductor 63 and an outer conductor 61, for which purpose, finally, the inner conductor 63 of the second connecting line 42 is electrically connected to the inner conductor 19b, and the outer conductor 41 is connected to the outer conductor 15b, of the feed line 17b, at the connecting point 46. Thus, in the electrical sense, the intermediate line 62 represents the outer coaxial feed line 17bwith the inner conductor 19band the outer conductor 15b. If, as in this exemplary embodiment, the upper and lower antenna devices A and B, respectively, shown in FIG. 1 are operated in only one frequency band range, then they are fed at the connecting point 46 in such a way that the length 1 of the coaxial spur line SL and of the associated outer conductor AL corresponds to 1=λ/4 at the frequency under discussion. An open circuit is transformed at the connecting point 46 [lacuna] by the short-circuit KS, which is in the form of a sleeve, as a result of which the outer outer conductor 15bis electrically short-circuited to the inner outer conductor 15a.The corresponding antenna device can thus be fed for operation in one frequency band using the feed and output-coupling apparatus explained with reference to FIG. 3.

[0056] However, in contrast, if the antenna described in FIG. 1 is intended to be operated with two antenna devices A and B, arranged one above the other, in two frequency band ranges, then a feed apparatus or output-coupling apparatus as explained in FIG. 4 is required, and this will be described in the following text.

[0057] For the antenna device, shown in FIG. 1, for operation of, for example, two different frequency band ranges, two coaxial λ/4 lines, which are each short-circuited via a respective short-circuit KS1 or KS2, are interleaved, with the outer λ₁/4 line SL1 being used for matching for the higher frequency (for example for transmission of the 1800 MHz frequency band range, for example PCN), and the inner λ/4 line SL2 being used for matching for the lower frequency, for example for the 900 MHz band (for example GSM). In consequence, the outer conductor AL1 of the first spur line SL1 is short-circuited at the end of the spur line (with respect to the feedpoint 46) by means of a radial short-circuit KS1, that is to say a short-circuit in the form of a ring or sleeve, to the outer conductor AL2 of the coaxial spur line SL2, and the outer conductor AL2 of the spur line SL2 is in turn short-circuited via a further radial short-circuit KS2, that is to say a short-circuit in the form of a ring or sleeve, to the inner conductor 19bof the outer coaxial line. The inner outer conductor AL2 ends freely, adjacent to the connecting point 46.

[0058] Thus, according to the exemplary embodiment, the upper antenna device A is fed via a first coaxial cable connection 21a, with the inner conductor 53 merging into the inner conductor 19aand the outer conductor 51 of the connecting line 52 merging into the outer conductor 15aof the coaxial feed line 17afor the upper antenna device A.

[0059] The lower antenna device B is fed via a second coaxial cable connection 21band a downstream intermediate line 42 with an associated outer conductor 41 and an inner conductor 43, in such a way that the inner conductor 43 is electrically connected to the inner conductor 19bof the coaxial feed line 17, and the outer conductor 41 of the second coaxial cable connecting line is electrically connected to the outer conductor 15bof the triax line. In this case, the desired matching is carried out, as a function of the wavelength λ₁/4 and λ₂/4 with respect to the two frequency bands to be transmitted, at the lower end of the feed and output-coupling apparatus, by means of the spur lines SL1, SL2, which are interleaved in coaxial form and are each short-circuited at their end, with the first short-circuiting line KS1, which is in the form of a sleeve, being located approximately in the axial center with respect to the electrical length of the coaxial spur line SL2 and being matched to the frequency band ranges of 900 MHz and 1800 MHz, which are to be transmitted in this exemplary embodiment.

[0060] The two short-circuited λ/4 spur lines SL1 and SL2 which have been explained are thus connected in series such that the associated short-circuits KS1 and KS2 are each transformed to an open circuit at the connecting point 46 for the respective frequency band range.

[0061] FIG. 6 shows that the design principle of the series-connected short-circuiting lines KS1 and KS2 can also be implemented in the opposite sequence, namely if the λ₂/4 spur line SL2 (with the outer conductor AL2) for the lower frequency is arranged on the outside, and the λ₁/4 spur line SL1 (with the outer conductor AL1) for the higher frequency is arranged (concentrically) on the inside of the first spur line. However, the design complexity for this is somewhat greater.

[0062] In addition to the exemplary embodiments which have been explained above, a number of short-circuited λ/4 lines, for example three such lines, can also be interleaved in one another, thus feeding or providing output coupling for a number of frequency band ranges (for example three frequency bands).

[0063] FIG. 7 will be used only to explain the design principle for the situation in which it is intended to feed three frequency bands, which are offset with respect to one another, into a corresponding multiple coaxial feed line 17, for which purpose a third short-circuiting connection KS3 is provided for matching, with the assumption being made in this exemplary embodiment that the third short-circuit KS3 has a length λ₃/4 for the transmission of an even higher frequency band range.

[0064] An exemplary embodiment which is once again modified with respect to that shown in FIG. 4 for a feed apparatus or output-coupling apparatus is illustrated in FIG. 8, in which apparatus, for example, in addition to the exemplary embodiment shown in FIG. 1, three antenna devices which are arranged one above the other can be fed jointly via one multiple coaxial cable line 17, with these antenna devices operating in two frequency band ranges. This is done in cascade form via two feed and output-coupling apparatuses, as explained with reference to FIG. 4, each with appropriate matching between an outer outer conductor and an associated inner conductor which at the same time represents the outer conductor for the next inner inner conductor. In each of the envisaged stages, an outer conductor is connected by its associated inner conductor to a common potential in each case via the described feed apparatus or output-coupling apparatus 101 or 103, respectively, according to the invention. The exemplary embodiment in FIG. 8 shows how this method can also be extended to a number of stages by further outer conductors AL1, AL2 and short-circuits KS3, KS4.

[0065] FIG. 9 shows another feed and output-coupling apparatus for a single coaxial line 17, but provided with broadband lightning protection, in the illustrated exemplary embodiment for two frequency band ranges.

[0066] The function in this case corresponds to the exemplary embodiment shown in FIG. 4, with the difference being that only a single inner conductor 15 is provided instead of the inner coaxial conductor 17ashown in FIG. 4, so that this inner conductor is passed through so that it runs without any curvature in the axial direction, and the two interleaved spur lines SL1 and SL2, which are once again short-circuited at the end, branch off at right angles from this coaxial line 17. With regard to the design and method of operation, reference is otherwise made to the exemplary embodiment shown in FIG. 4 which, with regard to the outer coaxial conductor 17billustrated in FIG. 4 and the outer conductor 15band inner conductor 19b, can be transferred analogously to the exemplary embodiment shown in FIG. 9.