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(11) | EP 2 237 372 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Dipole antenna for a CB band base station |
| (57) A dipole antenna (1) for a base station provided by means of a coaxial line that
comprising a first portion (2) that is connected to a second portion which constitutes
a choke coil (3) and to a third portion which constitutes a radiating element (6,
9).
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[0002] More specifically, the invention refers to a dipole antenna for a CB band base station which makes it possible to avoid using cumbersome radials and to eliminate the tuning coils that are usually necessary, thus obtaining a useful band of impedance that is much wider than that of known antennae, with the least possible hindrance.
[0003] The basic antennae commonly used are of the "ground plane", "quarter-wave", "half-wave dipole" or "five-eighths wave" type. Such antennae have a vertical pole of length that is proportional to the wavelength and, in some versions, radials. In particular, the "ground plane" type, well known in the literature, is constituted by a vertical pole of quarter-wavelength and by a plurality of radials, also of quarter-wavelength, which form an angle at 90° from the vertical pole (see figure 1). A second version is shown in figure 2 where the radials are bent downwardly until they form an angle that is on average comprised between 120° and 140° to improve the adaptation of the impedance to the classic 50 ohm. The aforementioned antennae are easy to manufacture, and can be supplied with power directly to the centre, but have cumbersome radials which must be inclined in order to obtain a correct impedance. The "half-wave" type shown in figure 3 is substantially a dipole antenna powered at its lower end by means of an impedance transformer. Manufacturing an antenna of this type does not require the use of radials but it does imply a careful construction of the impedance transformer which in any case limits its usage band.
[0004] The "five-eighths wave" type, with a signal gain that is slightly higher than those of the previous types, is constituted by a vertical pole, of length equal to five-eighths of the wavelength, and by a plurality of radials which are essential for correct functioning and, as with the half-wave type, an adequate impedance transformer which, in this case also, limits its usage band (see figure 4).
[0005] The aim of the present invention is to devise a dipole antenna of the type that makes it possible to avoid using cumbersome radials, and at the same time eliminating the tuning coils.
[0006] Within this aim, an object of the present invention is to devise a dipole antenna that makes it possible to obtain a useful band of impedance that is much wider than that of known antennae, with the least possible encumbrance.
[0007] Another object of the present invention is to devise a dipole antenna that is decoupled from the support structure, to avoid distortions in the radiation diagram.
[0008] Another object of the present invention is to devise a dipole antenna that is highly reliable, relatively simple to provide and at competitive cost.
[0009] This aim, as well as these and other objects which will become better apparent hereinafter, are achieved by a dipole antenna for a CB band base station, characterized in that it is provided by means of a coaxial line comprising a first portion that is connected to a second portion which constitutes a choke coil and to a third portion which constitutes a radiating element.
[0010] Further characteristics and advantages of the invention will become better apparent from the description of preferred, but not exclusive, embodiments of the dipole antenna according to the present invention, illustrated by way of a non-limiting example in the accompanying drawings wherein:
figures 1 to 4 show embodiments of conventional dipole antennae;
figure 5 shows a dipole antenna according to the invention in a first embodiment;
figure 6 shows the dipole antenna according to the present invention in a second embodiment;
figure 7 shows the dipole antenna according to the present invention in a third embodiment;
figure 8 shows the distribution of the currents on the radiator as a function of the electrical lengths as the frequency is varied.
[0011] With reference to the figures, the dipole antenna according to the present invention, globally indicated by the reference numeral 1, comprises, with reference to its first embodiment shown in figure 5, a coaxial feeder line 2, connected to a generator, not shown, such coaxial feeder line 2, with characteristic impedance of 50 ohm, continuing in a choke coil 3, made with coils that are distanced and arranged parallel to each other.
[0012] The coaxial feeder line 2, which thus continues in the choke coil 3, finally continues in a portion of coaxial line 4 that has a length that is equal to, for example, a quarter of the wavelength and self-impedance Zc. At the upper end of the portion 4 of coaxial line, the central conductor 5 of the coaxial line is electrically connected to a metal radiator 6 having a length that is equal to, for example, a quarter of the wavelength.
[0013] Therefore, the radiofrequency signal that comes from the generator travels inside the feeder line 2, continues into the coaxial cable forming the coils of the choke coil 3, and then continues into the portion of coaxial line 4 to arrive at the junction point with the radiator 6.
[0014] In this position, the current present in the central or internal conductor 5 continues along the radiator 6 while the current that was travelling on the inside of the shielding of the portion of coaxial line 4 is forced to continue on the outside of such shielding and to return downwards in the direction of the choke coil 3.
[0015] The current that travels through the radiator 6 will stop at the upper end of the radiator 6, while the external current of the portion of coaxial line 4 which returns downwards will be blocked by the choke coil 3, because a choke inductance is present which is connected in series with such current.
[0016] The choke coil 3 produces its choke effect on the currents beginning from a minimum frequency, to be identified according to necessity, and for all higher frequencies, and it is therefore intrinsically wide-band.
[0017] From the point of view of the antenna, the current that travels on the outside of the coaxial cable or portion of feeder line 4 is concordant with the current that travels through the radiator 6. We therefore have two parts radiating in phase, which overall form a single long radiating part, for example, half-wave. Another advantage of the antenna according to the invention is that, with the varying of the frequency, the currents on the radiator are always in phase for all lengths that are shorter than an entire wave, as shown in figure 8 (as in a classic dipole antenna, powered internally). Therefore, by means of suitable systems for impedance adaptation, this structure functions correctly even when its length is other than half-wave with signal gains proportional to its length.
[0018] Figure 6 shows the dipole antenna according to the invention in a second embodiment and in such figure the same reference numerals refer to identical elements.
[0019] The difference between the embodiment shown in figure 5 and the embodiment shown in figure 6 consists in that instead of having a radiator 6 located at the end of the portion of coaxial line 4, such radiator 6 is replaced by another coaxial line 7, arranged in such a way that the central conductor 5 of the portion of coaxial line 4 is connected with the outer part of the coaxial line 7, while the outside of the portion of coaxial line 4 is connected with the internal conductor 8 of the portion of coaxial line 7.
[0020] At the upper end, the central or internal conductor 8 of the portion of coaxial line 7 and the outer shielding of such portion 7 are electrically connected to each other. In this way, the outer shielding of the portion 7 of coaxial line performs such function of the radiator 6, while its internal conductor 8 defines a path and ensures the presence of a direct current short-circuit to protect the transmission apparatus from electrical discharges generated by atmospheric disturbances.
[0022] Moreover it is possible to use the global impedance of the antenna by suitably regulating the self-impedances Zc and Ze of the coaxial conductors.
[0023] In this way it is possible to obtain an antenna that has reduced encumbrance, since it is free from radials, as well as being decoupled from the support structure which in some cases could distort the radiation diagram, and which in addition is characterised in that it has a band that is wider by 250%-350% than those of conventional antennae, such as those shown in figures 1 to 4.
[0024] The sections of coaxial cable can conveniently be contained inside a dielectric tube, such as for example a fibreglass tube, which carries out the function both of mechanical support and of protection from atmospheric agents.
[0026] The difference between the third embodiment shown in figure 7 and the second embodiment shown in figure 6 consists in that a radiating element 9 is connected to the upper end of the coaxial line 7, at the internal conductor 8 of the coaxial line 7. The radiating element 9 can be compared to the radiator 6 in the first embodiment.
[0027] Therefore, substantially, the third embodiment of the invention is a combination of the first and second embodiments described previously.
[0028] The overall radiating part of the third embodiment of the invention is therefore composed of the internal conductor 8 and the radiator 9 and their overall lengths are, for example, quarter-wave, thus functioning like the upper portions of the embodiments in figures 5 and 6.
[0029] The length of the coaxial line formed by the coaxial line 7 and by the internal conductor 8 becomes usable as a new parameter for regulating and optimising the global impedance.
[0030] In practice it has been found that the dipole antenna according to the present invention fully achieves the intended aim and objects.
[0031] The antenna, thus conceived, is susceptible of numerous modifications and variations, all of which are within the scope of the appended claims.
[0032] In practice the materials employed, so long as they are compatible with the specific use, as well as the dimensions and the contingent shapes, may be any according to requirements and to the state of the art.
[0033] The disclosures in Italian Patent Application No. MI2009A000540 from which this application claims priority are incorporated herein by reference.
[0034] Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly, such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.
1. A dipole antenna (1) for a base station, characterized in that it is provided by means of a coaxial line comprising a first portion (2) that is
connected to a second portion that constitutes a choke coil (3) and to a third portion
that constitutes a radiating element (6, 9).
2. The dipole antenna (1) according to claim 1, characterized in that said second portion that constitutes said first coaxial coil (3) is formed by a plurality
of turns arranged parallel to each other so as to form said choke coil.
3. The dipole antenna (1) according to claim 1, characterized in that said third portion of coaxial line comprises a portion (4, 7) of coaxial line that
is connected to a radiator (6, 9).
4. The dipole antenna (1) according to one or more of the preceding claims, characterized in that said third portion of coaxial line comprises a portion (4) of coaxial line that is
connected to said choke coil (3) and in turn is connected to an additional portion
(7) of coaxial line, said additional portion (7) of coaxial line being connected by
means of its outer shielding to the central conductor (5) of said coaxial line and
by means of its internal conductor (8) to the outer shielding of said portion (4)
of coaxial line that is connected to said choke coil (3).
5. The dipole antenna (1) according to one or more of the preceding claims, characterized in that it comprises a radiating element (6, 9) that is connected to said third portion of
coaxial line, at said internal conductor of said third portion of coaxial line.
6. The dipole antenna (1) according to one or more of the preceding claims, characterized in that said coaxial line is inserted within a containment and protection tube.
REFERENCES CITED IN THE DESCRIPTION
This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.
Patent documents cited in the description