WO2014020272A1

WO2014020272A1 - Method for the electromagnetic decoupling of an antenna and the supporting pole thereof, and corresponding supporting pole

Info

Publication number: WO2014020272A1
Application number: PCT/FR2013/051824
Authority: WO
Inventors: Sébastien PALUD
Original assignee: Tdf
Priority date: 2012-07-31
Filing date: 2013-07-29
Publication date: 2014-02-06
Also published as: EP2880712B1; EP2880712A1; FR2994341B1; FR2994341A1

Abstract

The invention relates to a method for the electromagnetic decoupling of a substantially vertical support pole (1) made of en electrically conductive material and an antenna (2) which emits, in a frequency band centred on a central frequency, waves having an at least partially vertical polarisation, and which is supported by said supporting pole (1), characterised in that said antenna (2) is mounted on said pole (1) by a connection arm (4) and in that at least two electromagnetic decoupling devices (3) are arranged on the supporting pole (1) between the antenna (2) and the pole (1), including at least one decoupling element (17) conducting electromagnetic waves having a wavelength that is one fifth to one third of the wavelength of the waves emitted by the antenna (2) at the central frequency, at least a first device (3) being arranged on the pole (1) above said connection arm (4) and at least a second device (3) being arranged on the pole (1) below said connection arm (4), so as to neutralise the currents induced in the pole (1) by the waves emitted by said antenna (2).

Description

Electromagnetic decoupling process between an antenna and its support mast and corresponding support mast

The invention relates to a decoupling method between an antenna and its support mast and a corresponding support mast.

Wave transmission systems for broadcast generally include an antenna transmitting horizontal polarization waves.

However, some systems, such as broadcasting or private radio, for example for police or firefighters, include antennas transmitting vertical polarization waves.

Such an antenna is generally mounted on a vertical support mast, made of an electrically conductive material, connected to the Earth and arranged to allow the passage of lightning to the Earth when it reaches the mast.

When the antenna emits waves omnidirectionally, the electromagnetic radiation waves emitted towards the mast induces currents on the mast, thus creating a coupling between the antenna and the mast.

Such inductive coupling between the antenna and the mast may cause parasitic radiation capable of deforming the antenna radiation, especially towards the mast, and reducing the efficiency of the antenna in this direction and in the surroundings, which has a disadvantage.

The object of the present invention is to remedy this drawback and to improve the efficiency of an antenna which is arranged to emit at least partially vertical polarization waves and which is carried by a vertical antenna support mast, made of an electrically conductive material. The invention is intended in particular to allow the electromagnetic decoupling between the antenna and its support mast.

To this end, the invention relates to a method of electromagnetic decoupling between a substantially vertical support mast, made of an electrically conductive material, and an antenna emitting, in a frequency band centered on a central frequency, polarization waves at least partially vertical and wide by said support mast, characterized in that said antenna is mounted on said mast by a connecting arm and in that there is arranged on the support mast, between the antenna and the mast, at least two electromagnetic decoupling devices comprising at least one electrically conductive decoupling element of length between one fifth and one third of the wavelength of the waves emitted by the antenna at the central frequency, at least one first device being disposed on the mast above said connecting arm and at least a second device being disposed on the mast below said connecting arm, so as to neutralize the currents induced in the mast by the waves emitted by said antenna.

By the term "neutralize" it is meant that the majority of currents induced by the antenna in the support mast are blocked so as to obtain a quasi-constant omnidirectional radiation of the antenna. The term "quasi-constant" means that the radiation deviation is less than 2 dB in all directions.

Thus, for example, while a dipole, mounted on a support pole and working in vertical polarization, sees its radiation decrease by several decibels in the direction of said mast, the method according to the invention allows substantially equal radiation in all directions , within plus or minus 1 dB.

The antenna is mounted on the mast by a connecting arm, at least a first decoupling device being disposed on the mast above said linkage arm and at least one second decoupling device being placed on the mast below said linkage connecting arm. This makes it possible both to neutralize the currents induced by the part of the antenna situated above the link arm and the currents induced by the part of the antenna located below the link arm, for example when the Antenna is a half-wave dipole arranged vertically and connected at its center to the mast by the link arm.

An antenna is generally configured to work on a frequency band [,], that is to say to emit waves at frequencies between a low frequency fi and a high frequency f ₂ , for example itself comprised in the VHF or UHF bands. Such a band is generally centered on a central frequency f ₀ , for example, f ₀ = (f ₂ - fi) / 2. In a particular embodiment of the method according to the invention, the frequency band can be reduced to a single frequency. Preferably, at least one decoupling device is disposed coaxially with the mast.

Preferably, the decoupling element has a length of between one quarter and one third, preferably approximately three tenths, of the wavelength of the waves emitted by the antenna at the central frequency.

Advantageously, the decoupling element is at least partially parallel to the mast.

By largely preventing the passage of radiofrequency currents, induced by the waves emitted along the antenna support mast by said antenna, the arrangement of such a decoupling device makes it possible to prevent these from decreasing the current. level of radiation from the antenna towards the antenna support mast. In other words, the arrangement of such a device on the mast allows the omnidirectionality of an antenna working with waves whose component is vertical, preferably whose main component is vertical, avoiding that the coupling of the The antenna and the support mast do not reduce the antenna radiation in the mast direction in the direction of the antenna towards the mast.

Preferably, at least one decoupling device is arranged along the mast at least over the height of the antenna. The invention also relates to a substantially vertical support mast, made of an electrically conductive material and comprising at least one antenna transmitting, in a frequency band centered on a central frequency, at least partially vertical polarization waves, characterized in that that the antenna is mounted on the mast by a connecting arm and in that it comprises at least two electromagnetic decoupling devices arranged between the antenna and the support mast, at least one first device being placed on the mast above the mast; above said link arm and at least one second device being disposed on the mast below said link arm, and comprising at least one electrically conductive decoupling element of length between one fifth and one third of the wavelength of the waves transmitted by the antenna at the central frequency, so as to neutralize the currents induced in the mast by the waves emitted by the the antenna. A decoupling device according to the invention can in particular make it possible to decouple independent antenna systems fixed on the mast.

Preferably, at least one decoupling device is coaxial with the mast.

Advantageously, at least one decoupling element is at least partially parallel to the mast.

Advantageously, at least one decoupling element comprises a portion of length between one-fourth and one-eighth of the wavelength of the waves emitted by the antenna at the central frequency, preferably extending parallel to the mast, so that to allow efficient operation of the broadband antenna.

Preferably, the mast comprises a plurality of decoupling elements, advantageously at least eight, preferably at least twelve, in the form of equilateral metal rods bent L-shaped and each comprising a first portion arranged vertically, in parallel at the mast, and a second portion disposed horizontally, perpendicular to the mast and connecting it to the lower end of the first portion so as to form a short-circuit line.

The upper end of the first portion is then in open circuit so as to allow on the mast, at the right of said upper end, an electrical break of the mast.

According to one aspect of the invention, when the decoupling device is a cage of substantially cylindrical shape as defined above, the diameter D of the cage is such that the impedance at the upper end of the first portion of the rods Ζ ₀ = η / 2π * In (D / d) is greater than 100 Ω, where d is the diameter of the mast so as to effectively reduce the propagation of currents induced on this support over a wide relative band of frequencies, greater than 20%. When the antenna is working with waves in a frequency band [f _{1 #} f ₂ ] centered around a central frequency f ₀ , the term "relative frequency band" refers to the ratio (f ₂ - fi) / fo, often expressed percentage.

When the mast comprises a plurality of cages, the vertical misalignment of the cages with respect to the mast modifies little the performance of the decoupling device. Nevertheless, the same orientation is preferentially maintained on all the cages to allow a regular electric break of the mast. By the term "break electrical "means here the neutralization of the currents induced by the antenna in the support mast.

Preferably, when the mast comprises a plurality of decoupling devices, the sum of the height of a device and the spacing between said device and a device disposed consecutively on the mast is less than a quarter of the wavelength at the high frequency of the frequency band in which the antenna operates. Therefore, when two decoupling devices are arranged so spaced along the mast, the portion of the mast at the right of said sum forms a section electrically insulating the portion of the mast above said section of the portion of the mast located below said section. Such an isolation section greatly attenuates the parasitic radiation created by the currents induced by the radiation of the antenna. It goes without saying that the more decoupling devices are available, spaced apart from each other by said sum of lengths, the more electrically insulating sections are created on the mat at least in line with the antenna, and the effects of said parasitic radiation are attenuated, even deleted. Thus, for example, a number of decoupling cages as defined above and placed at least over the height of the portion of the mast that is at right angles to the antenna makes it possible to limit the variations of radiation from the antenna to the antenna. or at least 1 dB at 360 °.

More preferably, the cage or cages are arranged over a height h greater than or equal to 1.5 × L, where L represents the largest dimension of the antenna in the vertical direction.

Advantageously, when the antenna is a dipole comprising a half-wave doublet, the number of cages is greater than or equal to four, preferably on a mast length of between half the wavelength and the length of the antenna. wave at the high frequency f ₂ of operation of the antenna.

Preferably, the antenna is omnidirectional.

Advantageously, the omnidirectional antenna is a dipole, for example of the half-wave dipole type, configured to emit substantially vertical polarization waves.

When the antenna is a half-wave doublet, it is preferably arranged vertically and substantially parallel to the mast. In an alternative embodiment, the omnidirectional antenna is a monopole, for example of the ground plane antenna type. Such a monopole comprises a ground plane, for example perpendicular to the monopole.

The antenna may also be in the form of a double dipole, for example positioned on the upper part of the mast or a double V-dipole generating elliptical polarization waves.

Preferably, the mast comprises at least one metal tube, for example whose section is less than or equal to one-tenth of the wavelength corresponding to the central frequency emitted by the antenna. This allows the antenna to work effectively in FM band.

According to one characteristic of the invention, the mast is in the form of a single metal tube.

Preferably, the decoupling device (s) are arranged along the mast in line with the antenna at least over the height thereof.

The invention also relates to an electromagnetic decoupling device between a support mast, as defined above, substantially vertical, made of an electrically conductive material, and an antenna emitting, in a frequency band centered on a central frequency, at least partially vertical polarization waves carried by said support mast, characterized in that it is configured to be disposed between the antenna and the support mast and in that it comprises at least one electrically decoupling element conductor of length between one fifth and one third of the wavelength of the waves emitted by the antenna at the central frequency, so as to neutralize the currents induced in the mast by the waves emitted by said antenna.

Such a device allows a significant reduction of the contact currents and risks of intermodulation on the carrier mast.

According to one aspect of the invention, the decoupling device comprises a plurality of decoupling elements, advantageously at least eight, preferably at least twelve, in the form of metal rods, preferably made of a metal rod. metal or a metal alloy of high conductivity, for example greater than 1.10 ⁶ S / m.

Preferably, the rods are equidistributed.

Advantageously, the rods are bent L-shaped and each comprise a first portion, configured to be disposed vertically, parallel to the mast, and a second portion, configured to be disposed horizontally, perpendicular to the mast and connecting it to the lower end of the first portion so as to form a short-circuit line.

In one embodiment of the device according to the invention, the decoupling device comprises an electrically conductive ring connecting the upper ends of the first portion of each of the rods so as to form a substantially cylindrical structure or cage configured to be mounted coaxially. on the support mast. Such a structure is electrically conductive and may be, for example, metallic.

The device may further comprise a second electrically conductive ring of substantially the same diameter as the first ring connecting the rods to each other at their bends.

The diameter D of a cage can be variable and depends on the diameter d of the mast. Such a device may allow the antenna to work with waves over a wide frequency band, for example wide enough to cover the FM band.

This is particularly the case when the number of rods is greater than or equal to twelve, for which the device operates on a relative frequency band greater than 20%.

The height of a cage can also be a function of its diameter D. Thus, when the cage is wider and lower, the decoupling device will be less selective in frequency. To work with a relative frequency band reduced to a few percent, the diameter D can be such that Ζ ₀ = η / 2π * In (D / d) <100 Ω, but in this case, for the device to be effective, the length of the first portion of the rods, that is to say the height of the cages, should preferably be between λ / 4 and λ / 5 at the central frequency f ₀ of operation. Advantageously, the total length of the rods, that is to say of the first portion and the second portion, is between λ / 4.1 and λ / 3, preferably approximately equal to 0.3 x λ, at the central frequency f ₀ of operation, for example for a half-wave dipole situated at a distance of λ / 4 from the mast.

Other features and advantages of the invention will become apparent from the following description given with reference to the appended figures given by way of non-limiting examples and in which identical references are given to similar objects.

Figure 1 illustrates a radiation pattern of an antenna mounted on a vertical mast of the prior art.

Figure 2 is a schematic sectional representation of a mast portion comprising a first embodiment of the decoupling device according to the invention.

Figure 3 is a schematic sectional representation of a mast portion comprising a second embodiment of the decoupling device according to the invention.

Figure 4 is a perspective view of a mast portion comprising a half-wave doublet and a third embodiment of the decoupling device according to the invention.

FIG. 5 is a perspective view of a mast portion comprising an antenna, comprising two V-shaped metal elements generating a predominantly vertical elliptical polarization, and the third embodiment of the decoupling device according to the invention of FIG. figure 4.

Figure 6a is a partial perspective view of a particular embodiment of a mast according to the invention comprising a ground plane antenna.

Figure 6b is a partial perspective view of a particular embodiment of a mast according to the invention comprising a ground plane antenna.

Figure 6c is a partial perspective view of a particular embodiment of a mast according to the invention comprising a ground plane antenna.

Figure 6d is a partial perspective view of a particular embodiment of a mast according to the invention comprising a ground plane antenna. Figure 6e is a partial perspective view of a particular embodiment of a mast according to the invention comprising a ground plane antenna.

Figure 7 illustrates a particular form of decoupling device according to the invention of substantially frustoconical shape.

Figure 8 illustrates a particular form of decoupling device according to the invention comprising decoupling elements of complex shape.

FIG. 9 illustrates a particular form of decoupling device according to the invention comprising elements made of dielectric material.

FIG. 10 illustrates a first radiation diagram of an antenna mounted on a vertical mast with and without the decoupling device according to the invention.

FIG. 11 illustrates a second radiation diagram of an antenna mounted on a vertical mast with and without the decoupling device according to the invention.

FIG. 1 represents a radiation diagram of an antenna mounted on a vertical mast which does not comprise a decoupling device according to the invention.

The antenna used is a dipole, emitting vertical polarization waves, positioned on the radiation pattern at zero degrees and zero dB. The mast is a vertically disposed metal tube whose position corresponds to the center of the radiation pattern.

Thus, in the absence of decoupling device according to the invention, it is found that the radiation of the antenna is attenuated by about 7 dB in the direction 180 °, that is to say in the direction of the mast , in the direction of the antenna towards the mast.

Figures 2 to 9 show a support pole 1, made of an electrically conductive material, substantially vertical and on which are mounted respectively different embodiments of the decoupling device 3 according to the invention.

The mast 1 is in the form of a cylindrical tube, of diameter d, made of an electrically conductive material, for example of metal or a metal alloy, preferably of high conductivity, for example greater than 1.10 ⁶ S / m. . The mast 1 could also be in the form of a post, for example of rectangular section, and / or comprise a plurality of support elements, for example a plurality of metal tubes, without this being limited to the scope. of the present invention.

The support mast 1 comprises an antenna 2, shown in FIGS. 4 to 9, mounted on the mast 1 by a connecting arm 4 to the right of the decoupling device 3. The antenna 2 is configured to transmit in a frequency band [f ₁ , f ₂ ], that is to say to emit waves at frequencies between a low frequency fi and a high frequency f ₂ , for example itself included in the VHF or UHF bands. This band is centered on a central frequency f ₀ , for example f ₀ = (f ₂ - fi) / 2. It goes without saying that the frequency band can be reduced to a single transmission frequency f ₀ .

The diameter of the mast 1 is advantageously of small size, preferably less than one-tenth of the wavelength λ ₀ associated with the central frequency f ₀ . For example, for an antenna transmitting in FM band, that is to say for frequencies between 87.5 and 108 MHz, the diameter of the mast 1 is preferably less than 340 mm, for example between 60 and 125 mm.

FIG. 2 illustrates a first embodiment of a decoupling device 3 of a mast according to the invention in the form of a tubular portion 5 made of an electrically conductive material.

The tubular portion 5 is open at its upper end and closed at its lower end by a contact ring 6 perpendicular to the mast 1 made of an electrically conductive material and extending over the surface between the mast 1 and the tubular portion 5 of to electrically connect the tubular portion 5 to the mast 1. The tubular portion 5 thus surrounds the tube 1 so as to form a coaxial guide.

The sum of the length of the tubular portion 5 and the radial dimension of the ring, between the tubular portion 5 and the mast 1, is of length approximately equal to three tenths of the wavelength λ ₀ of the waves emitted by the antenna (not shown) at the center frequency f ₀ .

At the central frequency f ₀ , to which the wavelength λ ₀ is associated, the impedance of the coaxial guide formed by the assembly of the mast 1 and the tubular portion 5 is theoretically infinite at point B since it is located at a quarter-wave of a short-circuit located at point A. The mast 1 is thus electrically cut at the plane orthogonal to the section of the tube and passing through the point B.

Such a decoupling device operates more particularly on cylindrical metal tubes of small section, that is to say whose diameter is less than one tenth of the wavelength associated with the central frequency f ₀ , and present, by its operating principle, a frequency selectivity. Indeed, on a frequency band f ₀ ± Af, the input impedance of the decoupling device in the plane B will be: Z _B = j Z ₀ cotan (π / 2 * Af / f ₀ ), where Z ₀ is the characteristic impedance of the decoupling device such that Ζ ₀ = η / 2π * In (D / d), η being the characteristic complex impedance of the vacuum, ie = 120 n, where D is the diameter of the decoupling device and d is diameter of the conductive tube.

The frequency selectivity, and thus the efficiency of the blockage of currents induced by waves at frequencies within large bandwidths, is therefore related to the characteristic impedance of the decoupling device which must be as high as possible .

FIG. 3 illustrates a second embodiment of a decoupling device 3 of a mast according to the invention in the form of a decoupling element 7 made of an electrically conductive material of length approximately equal to a quarter of the wavelength λ _0/4 of the waves emitted by the antenna (not shown) at the center frequency f ₀ .

The decoupling element 7 comprises a first portion 9 disposed parallel to the mast 1 and a second portion 11 connecting the first portion 9 to the mast 1 and disposed perpendicularly thereto. The sum of the lengths of the first and the second portion being approximately equal to a quarter of the wavelength λ _0/4 of the waves emitted by the antenna (not shown) at the center frequency f ₀ .

The decoupling element 7 thus behaves as a short-circuited asymmetric two-wire line where the two conductors, that is to say the mast 1 and the decoupling element 7, are of different cross-sections. Insofar as a section of half-wave line folded on itself does not radiate, we have in B 'an infinite impedance to f ₀ if the path ΒΆΆΒ is equal to one quarter of the wavelength λ ₀ associated with this frequency f

This type of device, less complex than the previous one illustrated in FIG. 2, thus has greater installation flexibility and is more easily adaptable to a mast 1 of larger section, for example of a diameter greater than 125 mm in diameter. FM band. However, such a decoupling device has a greater frequency selectivity than that of the decoupling device illustrated in FIG. 2. The almost single frequency characteristic of the induced electrical blocking can be substantially improved by widening the width of the constituent metal strips of the device. decoupling.

Figures 4 and 5 each illustrate a mast 1 comprising an antenna 2 and four decoupling devices 3 according to a third embodiment.

The antenna 2, illustrated in FIG. 4, is in the form of a half-wave doublet dipole disposed vertically, parallel to the mast 1, and configured to emit polarized waves substantially vertically in an omnidirectional manner.

The antenna 2, illustrated in FIG. 5, is in the form of two V-shaped metal elements 2b, generating elliptical polarization waves, each mounted on the link arm 4 and whose planes are substantially orthogonal to one another. .

In both embodiments of the decoupling device illustrated in Figures 4 and 5, the antenna 2 is mounted on the mast 1 by a connecting arm 4 perpendicular to the mast 1.

Each decoupling device 3 is in the form of a substantially cylindrical metal cage, of diameter D, comprising twelve decoupling elements in the form of metal rods 17, electrically conductive, bent L-shaped and evenly distributed around it of the mast 1.

The cages 3 are mounted superimposed one above the other along the mast 1 and coaxially with it. The distance between the rings 20 of two cages 3 arranged consecutively is less than a quarter of the wavelength λ ₂ corresponding to the high frequency f ₂ of the frequency band [f ₁ , f ₂ ] in which the antenna 2 is working.

Each rod 17 comprises a first portion 17a disposed vertically, parallel to the mast 1, and a second portion 17b, disposed horizontally, perpendicular to the mast 1 and connecting the lower end of the first portion 17a to the mast 1. The first portion 17a and the second portion 17b are interconnected at a bend 18.

The sum of the lengths of the first portion 17a and the second portion 17b is approximately equal to three tenths of the wavelength λ ₀ of the waves emitted by the antenna 2 at the center frequency f ₀ .

The rods 17 are connected together at their respective upper ends 19 by a metal ring 20 allowing an electrical connection between said rods 17 and reinforcing the structure of the decoupling device 3.

In both embodiments of the decoupling device illustrated in FIGS. 4 and 5, a second metal ring (not shown) may be arranged to connect the rods 17 at their respective bends 18 so as to create a second electrical junction between the rods 17 and to further strengthen the structure of the decoupling device 3.

The device 3, illustrated by FIGS. 4 and 5, thus makes it possible to decouple the antenna 2 and the mast 1 on a relative band of frequencies greater than 20%.

The number of metal cages 3 is evaluated so as to restrict the metal support sections contributing to the radiation of the antenna, that is to say the mast portions 1 located between two decoupling devices 3, to length sections less than λ _0/4 . Similarly, the metal cages 3 are arranged over a height h greater than or equal to the largest dimension h of the antenna 2 in the vertical direction, for example H = 1.5 × L, so as to block the currents induced by the antenna 2 on at least the height L thereof.

For a half-wave dipole, as shown in FIG. 4, the number of decoupling devices 3 is preferably greater than or equal to four over a metal support length of between λ _0/2 and λ ₀ depending on the height h. retained metal cages, for example between λ _0/4 and λ _0/8 at the center frequency f ₀ . Thus, the decoupling device 3 according to the invention can thus be configured to decouple antennas of the double dipole type, such as the half-wave doublet illustrated in FIG. 4, but also more generally any grouping of antennas or aerials of FIG. type dipoles or the like emitting polarization waves at least substantially vertical. The decoupling device 3 according to the invention can thus be configured to decouple V-dipole type antennas as shown in FIG. 5.

Furthermore, a decoupling device 3 of a mast can also be configured to decouple antennas 2 of the "ground plane" type. As illustrated by FIGS. 6a to 6e, such an antenna 2 comprises a monopole 22 of the quarter-wave type placed on a discretized ground plane 24 consisting of a limited to very limited number of metal rods 26, for example four. In general, the ground plane 24 constituted by these rods 26 is not flat but inclined downwards, as illustrated by FIGS. 6a to 6e, so as to reduce the main radiation to a site close to 0 ° (line horizon).

When such an antenna 2 is positioned to the right of the mast 1 by a link arm

4, a configuration of the decoupling device as shown in Figures 4 and 5 can be realized.

On the other hand, when the antenna is positioned at the top, that is to say above the support mast 1, as in the case of the monopole 22 illustrated in FIGS. 6a to 6e, the decoupling device does not allow to optimize the azimuthal diagram of the antenna but its diagram in site by blocking the currents induced on the mast and which make vary the angle of appearance of the lateral lobes of radiation.

In such a case, the following two configurations of the mast 1, the antenna 2 and the decoupling device 3 can be used:

in a first configuration, illustrated by FIGS. 6a and 6b, the metal cages

3 being independent of the ground plane 24 of the antenna 2 and comprising a small number of metal rods 26, a decoupling device 3 (Figure 6a), preferably two decoupling devices 3 (Figures 6b) are arranged under the antenna at the upper end of the support mast 1;

in a second configuration, illustrated in FIGS. 6c to 6e, in which the ground plane 24 constitutes the short-circuited part, that is to say the ring 20, of the cage metal 3 closest to the antenna 2 (Figure 6c) or the direct support of the latter (Figure 6d). For the mast illustrated in Figure 6c, according to the length of the metal rods 26 constituting the ground plane 24 of the antenna 2, an extension of the latter by a second decoupling device 3 positioned just below and slightly smaller diameter to that of the first device 3 can be realized to allow efficient operation over a wider relative frequency band, as shown in Figure 6d. Finally, a second metal cage 3 can be added to the mast 1 shown in Figure 6d to improve the efficiency of the device, as shown in Figure 6e.

In another particular embodiment of a device for decoupling a mast according to the invention, illustrated in FIG. 7, the decoupling devices 3 may be in the form of cages of substantially frustoconical shape. The cages 3 illustrated in FIG. 7 comprise a closed side wall surrounding the mast 1, the ends of the cage 3 being open. However, it goes without saying that the side wall 30 could be perforated or discretized. The solution can of course be discretized in the same manner as described above, the number of metal elements being preferably greater than twelve to ensure optimum performance. In another particular embodiment of a decoupling device of a mast according to the invention, illustrated in FIG. 8, the decoupling devices 3 may be in the form of cages formed of metal decoupling elements 32 of FIG. complex shape whose length is between one fifth and one third of the wavelength of the waves emitted by the antenna at the central frequency so as to neutralize the currents induced in the mast by the waves emitted by said antenna. The cage 3 here comprises a second ring 33 connecting the elements of complex shapes at their lower ends so as to form an electrical junction between them. In another particular embodiment of the decoupling device according to the invention, illustrated in FIG. 9, the cages 3 further comprise one (or more) element 34 made of a dielectric material completely filling the inside of the cage 3. The use of such a dielectric material reduces the dimensions of the decoupling device 3, in particular in the direction of the height h of the cage. The reduction is carried out on the dimension of each metal cage but not on the spacing between the cages 3. This solution makes it possible to reduce the height of cages of the order of ε _Γ ° ^'5 , where ε, - is the relative permittivity inside the cage, but may be more selective in frequency. In order to maintain the widest possible relative band operation, the diameter D of the cage must be such that Ζ ₀ = η / 2π * In (D / d) / _{r r} ^0'5 > 100 Ω.

Figures 10 and 11 illustrate two radiation patterns of an antenna 2 supported by a mast 1 in a frequency band centered respectively at frequencies 88 and 108 MHz. In these figures 10 and 11, the solid line curve illustrates the radiation of an antenna 2 supported by a mast 1 comprising a decoupling device 3 according to the invention and the curve in dotted lines illustrates the radiation of a supported antenna 2 by a mast 1 not including a decoupling device according to the invention.

Claims

CLAIMS A method of electromagnetic decoupling between a substantially vertical support mast (1) made of an electrically conductive material and an antenna (2) transmitting in a frequency band [f _{1 #} f ₂ ] centered on a center frequency (f ₀ ), polarization waves at least partially vertical and carried by said support mast (1), characterized in that said antenna (2) is mounted on said mast (1) by a connecting arm (4) and in that at least two electromagnetic decoupling devices (3) comprising at least one decoupling element (5, 7, 17) are arranged on the support mast (1), between the antenna (2) and the mast (1). electrically conductive of length between one fifth and one third of the wavelength (λ ₀ ) of the waves emitted by the antenna (2) at the central frequency (f ₀ ), at least one first device (3) being arranged on the mast (1) above said link arm (4) and at least one second device (3) being disposed on the mast (1) below said link arm (4), so as to neutralize the currents induced in the mast (1) by the waves emitted by said antenna (2).

2. Method according to claim 1, wherein at least one decoupling device (3) is disposed coaxially with the mast (1).

3. Method according to one of the preceding claims, wherein at least one decoupling device (3) is disposed along the mast (1) at least on the height (L) of the antenna (2).

4. Support mast (1), substantially vertical, made of an electrically conductive material and comprising at least one antenna (2) emitting, in a frequency band [f, f ₂ ] centered on a central frequency (f ₀ ), at least partly vertical polarization waves, characterized in that the antenna (2) is mounted on the mast (1) by a connecting arm (4) and in that it comprises at least two decoupling devices (3 electromagnetically arranged between the antenna (2) and the support mast (1) at least one first device (3) being arranged on the mast (1) above said linkage arm (4) and at least one second device (3) being disposed on the mast (1) below of said connecting arm (4), and comprising at least one electrically conductive decoupling element (5, 7, 17) of length between one fifth and one third of the wavelength (λ ₀ ) of the waves emitted by the antenna (2) at the central frequency (f ₀ ), so as to neutralize the currents induced in the mast (1) by the waves emitted by said antenna (2).

5. Mast according to claim 4, wherein at least one decoupling element (5, 7, 17) extends parallel to the mast (1) over a length approximately equal to one quarter of the wavelength (λ ₀ ) of waves transmitted by the antenna (2) at the central frequency (f ₀ ).

6. Mast according to one of the preceding claims 4 and 5, comprising a plurality of decoupling elements in the form of rods (17) Equipped L-shaped bent and each comprising a first portion (17a) arranged vertically. , parallel to the mast (1), and a second portion (17b) disposed horizontally, perpendicular to the mast (1) and connecting it to the lower end of the first portion (17a) so as to form a short line. circuit.

7. Mast according to claim 6, characterized in that at least one device (3) further comprises an electrically conductive ring (20) connecting the upper ends of the first portion (17a) of each of the rods so as to form a structure or cage of substantially cylindrical shape configured to be mounted coaxially on the support mast (1).

8. Mast according to claim 7, characterized in that at least one device (3) further comprises a second electrically conductive ring substantially of the same diameter as the first ring (20) connecting the rods (17) to each other at their elbows (18).

boy Wut. Mast according to one of the preceding claims 4 to 8, said mast (1) comprising a plurality of decoupling devices (3), and wherein the sum of the height of a device (3) and the spacing between said device (3) and a device (3) arranged consecutively on the mast (1) is less than a quarter of the wavelength (λ ₂ ) at the high frequency (f ₂ ) of the frequency band [f _{1 #} f ₂ ] in which the antenna (2) operates . Io. Mast according to any one of the preceding claims 4 to 9, in which the antenna (2) is omnidirectional and is in the form of a half-wave doublet (2a) arranged vertically and substantially parallel to the mast (1) configured to emit substantially vertical polarization waves.