EP0352160A1 - Omnidirectional antenna, particularly for the transmission of radio or television signals in the decimetric-wave range, and radiation system formed by an arrangement of these antennas - Google Patents

Abstract

This antenna, in particular intended for the transmission of radio or television signals in the decimetric-wave range, comprises: - a vertical, central support tube (100), - a plurality of identical radiating arrays (200), preferably three in number, uniformly distributed around the central tube and each formed from a vertical bifilar line (210, 210') supporting, coupling and supplying in a symmetrical manner, a plurality of horizontal dipoles (220, 220') uniformly distributed along this bifilar line, and - an equipower, equiphase power distribution system, preferably housed entirely inside the central support tube, identically and simultaneously supplying the three radiating arrays from a single coaxial supply line. The antenna lacks a reflector, this significantly reducing its weight and its wind resistance relative to conventional panel antennas generally used in this range. Advantageously, these antennas are superimposed and enclosed in a sealed, essentially cylindrical, self-supporting and superimposable radome. This antenna achieves an omnidirectional pattern at 0.9 dB between 460 and 860 MHz, and can radiate 5 to 7 kW per antenna element. <IMAGE>

Description

The present invention relates to an omnidirectional antenna.

This antenna is particularly applicable to the transmission of radio or television signals in the UHF band (so-called UHF band), where it will be seen that it provides particularly advantageous advantages.

The invention is however not limited to this application or to this frequency band, and could just as well be suitable for a very wide variety of different situations.

For broadcasting or television antennas, a radiating system must be available (with some exceptions) having the most omnidirectional diagram possible (by "omnidirectional diagram", we mean a diagram with no trough less than 3 dB over 360 °).

This system which must also have mechanical characteristics of compactness and lightness making it possible to place it at the top of a pylon while minimizing both the static load (self-weight of the radiating system) and dynamic (taken in the wind) supported by it- this.

Most often, an antenna system called "panel antenna" is used for this purpose, consisting of radiating elements each formed by a dipole placed in front of a reflector, the dipole being oriented vertically or horizontally depending on the polarization. desired.

Such a radiating element being a gain element, therefore directive, it is necessary to group four of them, arranged at 90 ° from one another, to obtain the desired omnidirectional diagram.

To increase the admissible power, a plurality of these radiating elements (generally two, four or eight radiating elements) are generally superimposed so as to form radiating panels, the reflector being most often common.

Each panel is supplied separately with the same phase and same power as all the others (unless you want to play on the shape of the diagram by introducing phase shifts or power variations) by means of a distributor assembly.

This configuration, although operating satisfactorily on a very large number of current transmitting stations, has a certain number of drawbacks.

First of all, it is necessary to provide, at the top of the pylon proper, a pylonet making it possible to arrange the superimposed radiating elements forming the four radiating faces of the configuration.

This pylonet must satisfy two conflicting conditions:
- firstly, it must be dimensioned sufficiently to allow the installation of the power supplies for each panel and the passage of a man in the center of the configuration to be able to ensure maintenance: we have seen that each radiating element was supplied by a clean supply coaxial and, since this coaxial must necessarily be placed behind the reflective panel of the radiating element so as not to disturb its operation, the bundle of coaxial cables must pass to the inside of the pylonet, which should therefore be of sufficient size (for faces with eight superimposed radiating elements, there are thus 32 coaxials to pass through this pylonet).
For this convenience of installation, and also for good mechanical rigidity, it is therefore desirable that the structure of the pylonet be as wide as possible.
- secondly, from the radio-electric point of view, the hollows in the diagram will increase as the phase centers of the radiating elements go away.
To obtain the most regular diagram that is, it is therefore desirable to bring the radiating elements of each group as close as possible, therefore to provide the smallest possible pylonet section (limited however by the minimum size of the reflectors).
To minimize the static and dynamic load mentioned above, it is also desirable to minimize the section of the pylonet, especially since the radiating assembly must be protected by a radome whose size, taking into account the dimension of the radiating elements, will have a very large wind-absorbing surface and will therefore stress all the more the pylon.

Another drawback of this type of antenna results from the complexity of its supply system (each of the radiating elements having to be supplied by a clean coaxial, as indicated above), which means that a large number of coaxial secondary power and junction boxes; the cost price of such an antenna system will therefore increase very quickly with the number of radiating elements.

In addition, losses will increase rapidly, both due to the multiplication of junction boxes and the lengthening of secondary supply coaxials; typically, for a radiating system with panels of eight superimposed radiating elements dimensioned for the band 470-860 MHz, the height of the pylonet, and therefore of the longest coaxials, is of the order of 12 meters, which creates losses not negligible in such a frequency range.

This typically leads to pylonet sections of the order of 0.8 x 0.8 m and radome diameters of the order of one meter for emissions in the 470-860 MHz band, a group of four panels with its radome having a specific mass of 350 to 400 kg and having a windward surface of the order of 1.3 m².

Another type of antenna suitable for the aforementioned use, although less used, is the so-called "super-turntable" antenna.

In this type of antenna, to obtain the desired omni-directional diagram, the principle of the rotating field is used, the radiating element then being made up of two plane "bat wings", vertical and perpendicular to each other, crossing their center and 90 ° out of phase with each other.

A high number of radiating elements are thus superimposed, each radiating element being supplied separately by means of a common distributor system, and the two dipoles formed by the "bat wings" of each radiating element being supplied in quadrature aperiodically by a 3 dB coupler.

This type of antenna, although it has a much smaller overall diameter than an antenna panel system due to the absence of a reflector, and thus makes it possible to significantly reduce the size of the pylonet, has a number of drawbacks, however:
- first of all, the need to carry out the aperiodic quadrature supply between the dipoles leads to the use of 3 dB couplers placed in the radiation field, the balancing charge of the coupler having to be sized according to the power at issue,
- then, the coaxial cables for supplying the dipoles are located in the radiation field of the antenna and therefore disturb the radiation of the latter by creating hollows in the diagram,
- in addition, for equal gain, the total height of the antenna is greater than that of an antenna with radiating panels, also correspondingly causing problems of compensation of the diagram in elevation in the case of antennas with a large number of radiating elements ,
- finally, the cost price is high due to the mechanical complexity, the presence of 3 dB couplers and the multiplication coaxial power cables.

As can therefore be seen, the two types of antennas used up to now for radio or television transmitters in the UHF range are not entirely satisfactory because they do not allow both mechanical performance to be achieved simultaneously ( compactness to limit wind resistance, reduced weight, structure simple to manufacture) than radio (omnidirectionality of the diagram, possibility of accepting a large power) desirable.

The present invention proposes to resolve these drawbacks by proposing a new type of antenna which, while having excellent radio-electric properties, is compact, light and achievable at low cost thanks to both its simple mechanical structure. (absence of pylonet, in particular) and the reduction to a minimum of coaxial cable connections.

To this end, according to the invention, the antenna comprises:
- a central, vertical support tube,
a plurality of identical radiating networks, regularly distributed around the central tube and each formed by a vertical two-wire line supporting, coupling and supplying symmetrically a plurality of horizontal dipoles, regularly distributed along this two-wire line, and
- an equiphase and power distribution system supplying the radiating networks identically and simultaneously from a single coaxial supply line.

According to a number of advantageous features:
- there are three radiating networks;
- each radiating network comprises four horizontal dipoles;
- the dipoles are of the shortened half-wave type, calculated on the central operating frequency of the antenna with a shortening coefficient of about 0.9, the distance between two consecutive superimposed dipoles is one shortened half-wave, calculated for the central operating frequency of the antenna with a shortening coefficient of approximately 0.85, and the distance from the dipoles to the central axis of the system is a quarter wave not shortened, calculated for the central frequency of antenna operation;
- the power distribution system is fully housed inside the central support tube;
- The two-wire line has a lower half and an upper half, each of these halves being excited at a point located halfway up by a coaxial line passing inside one of the conductors of the two-wire line, this line being itself connected to the power distribution system located in the central support tube approximately at the connection of the two halves of each two-wire line;
- Each branch of the dipole has a substantially circular curvature whose center of curvature is located approximately on the central axis of the antenna.

The invention also relates to a radiating system formed by a plurality of antennas of this type, these antennas being superimposed and supplied distinctly by own coaxials connected to a common distributor.

Preferably, each antenna is then enclosed in a sealed radome, substantially cylindrical, self-supporting and stackable.

• - la figure 1 est une vue perspective d'un ensemble rayonnant constituant l'antenne selon l'invention,
• - la figure 2 montre une pluralité de ces ensembles rayonnants, superposés et couplés de manière à accroître la puissance rayonnée, l'ensemble étant protégé par un radôme extérieur,
• - la figure 3 est une coupe selon la ligne III-III de la figure 2, montrant l'ensemble rayonnant de la présente invention vu de dessus,
• - la figure 4 est une vue en élévation, partiellement en coupe, de l'ensemble rayonnant de la figure 1,
• - la figure 5 est une vue de face, prise selon la direction V-V de la figure 4, de l'un des trois réseaux rayonnants que comporte cet ensemble rayonnant,
• - la figure 6 est une vue de dessus, en coupe selon la ligne VI-VI de la figure 4, montrant le détail de la première section du coaxial d'alimentation du réseau rayonnant,
• - la figure 7 est une vue de face, en coupe, correspondant à la région repérée VII sur la figure 5, montrant plus en détail l'une des deux branches constituant la seconde section de ce coaxial d'alimentation,
• - la figure 8 est un diagramme en azimut relevé pour une antenne selon l'invention, montrant le caractère omnidirectionnel du diagramme de celle-ci,
• - la figure 9 et le diagramme en site correspondant à cette même antenne, et
• - la figure 10 est une comparaison des diagrammes en azimut que l'on aurait pour deux antennes similaires dont l'une comporterait trois réseaux rayonnants à 120° (comme sur les figures 1 à 7), et l'autre quatre réseaux rayonnants à 90°.

Other characteristics and advantages of the invention will appear on reading the detailed description below, made with reference to the appended drawings in which:

• FIG. 1 is a perspective view of a radiating assembly constituting the antenna according to the invention,
• FIG. 2 shows a plurality of these radiating assemblies, superimposed and coupled so as to increase the radiated power, the assembly being protected by an external radome,
• FIG. 3 is a section along line III-III of FIG. 2, showing the radiating assembly of the present invention seen from above,
• FIG. 4 is an elevation view, partially in section, of the radiating assembly of FIG. 1,
• FIG. 5 is a front view, taken in the direction VV of FIG. 4, of one of the three radiating networks that this radiating assembly comprises,
• FIG. 6 is a top view, in section along line VI-VI of FIG. 4, showing the detail of the first section of the coaxial supply of the radiating array,
• FIG. 7 is a front view, in section, corresponding to the region marked VII in FIG. 5, showing in more detail one of the two branches constituting the second section of this supply coaxial,
• FIG. 8 is a diagram in azimuth noted for an antenna according to the invention, showing the omnidirectional nature of the diagram of the latter,
• FIG. 9 and the diagram in site corresponding to this same antenna, and
• - Figure 10 is a comparison of the azimuth diagrams that one would have for two similar antennas, one of which would include three radiating networks at 120 ° (as in Figures 1 to 7), and the other four radiating networks at 90 °.

The same reference numerals refer, in all the figures, to similar elements.

FIG. 1 shows the general structure of a radiating assembly constituting the antenna according to the invention: this essentially comprises a central support tube 100, vertical, provided in the lower part and in the upper part with fixing plates, respectively 110 and 120, allowing to assemble end to end several superimposed support tubes - and therefore a plurality of identical radiating assemblies - in order to increase the total radiated power.

Around this support tube are arranged at 120 ° from each other three identical radiating networks 200, each comprising a vertical two-wire line with two parallel conductors 210,210 ′ supporting a plurality of horizontal dipoles 220,220 ′ (four in the example shown) regularly distributed along this two-wire line.

The radio supply, which is brought into 310 by a coaxial cable arriving at the base of the antenna - therefore in an area which will not disturb the radiation pattern very much -, passes (while remaining in coaxial) inside the support tube 120, then is distributed (always coaxial) to each of the three radiating networks by passing inside a horizontal tube 340, mounted halfway up the central tube 100, which also ensures the mechanical maintenance of these radiating networks in combination with support arms 130,140 at the top and bottom.

As can be seen, the supply and distribution of radioelectric energy are entirely internal to the antenna structure, which eliminates any possibility of disturbance of the diagram due to the physical presence of supply lines in the field of radiation, as in the case of antennas of the prior art.

Typically, the antenna does not have a reflective panel.

To increase the radiated power, it is possible to superimpose (FIG. 2) a plurality of modules 10 each formed by a radiating assembly 11 similar to that illustrated in FIG. 1 and supplied by a coaxial 12 connected to a distributor in the lower part of the antenna, and a cylindrical protective radome 13. The assembly is placed at the top of a pylon 14, the upper module being closed by a cover 5 and possibly surmounted by a lightning rod (not shown), as is well known.

The radome 13 (Figures 2 and 3) is a reinforced polyester cylinder provided at each of its ends with flanges 16,17 intended for the assembly of the various superimposed modules, thus making it possible to produce a self-supporting radome, which greatly simplifies the mechanical production. The entire system is of course leak-proof.

Figures 4 to 6 describe in more detail the structure of the radiating assembly according to the invention, in particular of the supply of the three dipole networks.

The radioelectric power supply, connected at 310, is brought to mid-height of the central tube 100, inside of it, by a coaxial line 320 (the return conductor being formed by the very wall of the support tube ) comprising a plurality of sections 321 to 325 of increasing diameters forming a quarter-wave impedance transformer, and held centrally inside the support tube 100 by spacers 326,327.

The power is then distributed between the three radiating networks by equiphase and power distribution, always by a coaxial link.

The coaxial link 330,340 supplying each of the radiating networks consists (Figure 6) of a conductor 332, held inside a tube 341 by spacers 333, and one of the ends 331 of which is connected to the common line central 320.

This tube 341 constitutes both the return conductor of the coaxial line and a mechanical support connecting the radiating network to the central support tube; for this purpose, this tube 341 is provided at one of its ends with a connection 342 to a part 150 integral with the central tube, and at its other end with a part 343 supporting the two conductors 210, 210 ′ of the two-wire line , which extend on either side of this part 343 (Figure 7), and consist of hollow tubes of conductive material, for example fixed by brazing.

The core 332 of the coaxial line is then connected to a distributor element 334 which symmetrically supplies the upper branch and the lower branch of one of the conductors (in the drawings, the conductor 210 ′) of the two-wire line, l other conductor (conductor 210) being connected to the common ground.

For this, the core of the coaxial extends inside the conductor 210 to a point 339 located approximately halfway up each of the two upper and lower branches (this point 339, which will be the point of excitation of the two-wire line, is marked at P in Figures 4 and 5).

For this purpose, there is provided, between this point 339 and the distributor 334, a conductor, produced in two sections 335,336 of increasing diameters so as to act as an impedance transformer, these two sections being held inside the conductor 210 by spacers 337. The end of the supply line then crosses the conductor 210 at 211 to excite the conductor 210 ′ at 339 by means of a transverse connection piece 338.

Thus, as can be seen, the supply is entirely coaxial from the input connector 310 to the point of excitation P, this coaxial supply system being also entirely contained inside the support structure of the antenna (which therefore plays a dual mechanical and electrical role).

The two-wire line carries a plurality of dipoles 220, 220 ′ which will thus be supplied symmetrically and constitutes the radiating members proper of the antenna.

The 220,220 ′ dipoles used are of the shortened half-wave type, calculated on the central operating frequency of the antenna with a shortening coefficient of about 0.9.

The distance between two consecutive superimposed dipoles is one shortened half-wave, calculated for the central operating frequency of the antenna with a shortening coefficient of about 0.85.

The distance from the dipoles to the central axis of the system is a quarter wave, not shortened, calculated for the central operating frequency of the antenna.

The impedance reduced to the level of the excitation point P, that is to say of the connection of the two-wire lines supplying the dipoles is 50 Ω, the supply being effected by the coaxial lines for which a constant impedance is kept of 50 Ω thanks to the transformer system with quarter-wave lines exposed above.

It will be noted that the ends of the two-wire line correspond to intensity nodes, and can therefore be grounded directly by the spacers 130, 140, which also provide the mechanical maintenance of the assembly.

The assembly can be made of copper or copper alloy tube and assembled by brazing, which makes its mechanical construction particularly simple.

From the electrical point of view, the antenna thus formed therefore consists of four superimposed rings (such as those visible in FIG. 3), each formed by three dipoles placed horizontally at 120 ° from one another on three sides of an equilateral triangle, and supplied in phase with equal power.

Such a configuration provides, without the aid of any reflector, a quasi-omnidirectional diagram, as can be seen in the figure 8, which represents an azimuth diagram recorded for an antenna comprising an element such as that illustrated in FIGS. 4 to 7 and which we have just described, calculated for a central operating frequency of 520 MHz: as can be seen , the diagram is omnidirectional to within 0.9 dB.

Figure 9 shows the diagram of site diagram, the shape of which is quite suitable for a radio or television antenna.

With regard to electrical performance, it has been observed that the antenna makes it possible to radiate without damage a power of the order of 5 to 7 kW, this power can of course be multiplied by superimposing several identical radiating assemblies.

The impedance, as indicated above, is 50 Ω, the gain of 5 dB is the average standing wave ratio of 1.15.

With regard to mechanical performance, for antennas operating in the 460-860 MHz band, the radiating assemblies are enclosed in radomes of 0.54 m in diameter and 1.16 m high with a windward surface of 0 , 63 m² (to be compared to a wind catchment area of around 1.35 m² for an antenna operating in the same range, but produced from antenna panels, as described in the introduction to this description), and a complete module (radome plus radiant assemblies) with a mass of around 40 kg (compared to 375 kg in the case of a panel antenna).

The choice of grouping dipole arrays three in number is not limiting, but it is particularly advantageous.

Indeed, if we increase the number of networks, the adjacent ends of the dipoles of the same ring will be closer and closer together, which will increase their mutual coupling and accentuate the troughs in the diagram.

FIG. 10 illustrates this phenomenon: there is shown the diagram in azimuth D₄ raised for a system with 4 networks, to compare with the diagram D₃ for the system with three networks which is the subject of the present description: it can be seen that the maximum troughs are now at least 2 dB instead of 0.9 dB in the other case.

The solution to three networks is therefore the one which gives the most homogeneous diagram.

Finally, as a variant, the dipoles can be optimized by modifying their shape: instead of providing them rectilinear and forming the three sides of an equilateral triangle exinscribed to the circle passing through the centers of the three two-wire lines (configuration of FIG. 3), we can deform or bend the dipoles so as to bring them closer to the outline of this circle, or even make them match this outline (shape illustrated in broken lines on the figure 3).

This improvement makes it possible to reduce the radiation phase shifts between the different points of the dipole and thus to make the azimuth diagram even more omnidirectional.

Claims (11)

1. An omnidirectional antenna, in particular for the transmission of radio or television signals in the UHF band, characterized by:
- a central support tube (100), vertical,
- A plurality of identical radiating networks (200), regularly distributed around the central tube and each formed by a vertical two-wire line (210,210 ′) supporting, coupling and symmetrically supplying a plurality of horizontal dipoles (220,220 ′) regularly distributed over the along this two-wire line, and
- an Equiphase Equippower power distribution system (320) supplying the radiating networks identically and simultaneously from a single coaxial supply line. 2. The antenna of claim 1, in which the radiating networks (200) are three in number. 3. The antenna of one of claims 1 and 2, wherein each radiating network (200) comprises four horizontal dipoles (220,220 ′). 4. The antenna of one of claims 1 to 3, in which the dipoles (220,220 ′) are of the shortened half-wave type, calculated on the central operating frequency of the antenna with a shortening coefficient of 0, 9 approx. 5. The antenna of one of claims 1 to 4, in which the distance between two consecutive superimposed dipoles (220,220 ′) is a shortened half-wave, calculated for the central operating frequency of the antenna with a shortening coefficient of about 0.85. 6. The antenna of one of claims 1 to 5, in which the distance from the dipoles (220,220 ′) to the central axis of the system is a quarter wave not shortened, calculated for the central operating frequency of the antenna. 7. The antenna of one of claims 1 to 6, in which the power distribution system (320) is entirely housed inside the central support tube. 8. The antenna of one of claims 1 to 7, in which the two-wire line (210,210 ′) comprises a lower half and an upper half, each of these halves being excited at a point (P) located at mid-height by a coaxial line (335,336) passing inside one (210) of the conductors of the two-wire line, this line itself being connected to the power distribution system located in the central support tube approximately at the connection of the two halves of each two-wire line. 9. The antenna of one of claims 1 to 7, wherein each branch of the dipole has a substantially circular curvature whose center of curvature is located approximately on the central axis of the antenna. 10. A radiating system formed by a plurality of antennas (11) according to one of claims 1 to 9, these antennas being superimposed and supplied distinctly by own coaxials (12) connected to a common distributor. 11. The radiating system of claim 10, wherein each antenna (11) is enclosed in a radome (13) sealed, substantially cylindrical, self-supporting and stackable.

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