EP0792528B1 - Half-wave dipole antenna - Google Patents

Description

The present invention relates to an antenna half-wave dipole type directive particularly intended for transmission or reception in the range of frequency of approximately 88 to 108 MHz, known as the FM band.

Currently, the known antennas used for radio transmission or reception in the FM band are generally poor guidelines. For broadcast the same program in broadcast areas neighboring, several antennas are necessary. The radiation patterns of neighboring antennas partially overlap. Each antenna is associated at a frequency different from the associated frequencies to neighboring antennas to avoid interference undesirable.

In a single frequency broadcast network, known as SFN (Single Frequency Network), all antennas transmit with the same frequency. The radiation patterns of these antennas must be set very precisely so that the areas of scattering associated with these antennas are adjacent but do not overlap so that it does not happen no interference between neighboring broadcast areas. For the frequency range considered, the antennas in an SFN network are very large. Often, for example when the antennas are arranged the one behind the other, like along an axis motorway, a large reflector is attached behind each antenna to avoid destructive interference between the field electromagnetic radiated by the antenna and that radiated by the previous antenna. The dimensions typical of these antennas are of the order of several meters. These dimensions increase the cost of manufacture of antennas, and complicate their installation and maintenance. Finally installed, these antennas degrade the landscape and present a strong wind which causes damage in storm event.

The present invention aims to provide an antenna small footprint directive and whose shape radiation pattern is easily adaptable under the operating conditions of a network of single frequency broadcast.

To this end, an antenna comprising a source radiant powered and a non-radiant source powered, is characterized in that the source powered is a half-wave dipole extending along a longitudinal axis and the unpowered source includes a first elongated conductive element extending along a secant longitudinal axis with the longitudinal axis of the half-wave dipole, and a first insulation means connecting a first end of the elongate element substantially in the center of the dipole half wave, one second end of the first elongated element and a first end of the half-wave dipole being isolated.

According to a first preferred embodiment, the directivity of the antenna is better controlled when the unpowered source, called parasitic, comprises, in in addition, a second elongated conductive element and a second insulating means. The second slender element extends along a longitudinal axis intersecting with the axis longitudinal of the first elongated element and with the longitudinal axis of the half-wave dipole. The first insulation means extends from a first end fixed at the center of the half-wave dipole towards a second end to which the first end of the first elongated element and a first end of the second elongated element are fixed. A second end of the second elongated element and a second end of the half-wave dipole are isolated by the second insulating means. More specifically, the first and second elongated elements form acute angles, preferably equal, with the axis longitudinal of the half-wave dipole. When such antenna is symmetrical about the axis along of which the insulation means extends, the antenna can radiate along this axis, mainly in one direction, either symmetrically or asymmetrical in both directions along the axis.

According to a second preferred embodiment, the unpowered source further includes third and fourth elongated elements conductors, a second means of insulation, and third and fourth insulating means. The third and fourth elongated elements extend respectively along secant longitudinal axes between them and with the longitudinal axis of the half-wave dipole. The second isolation means extends from a first end fixed at the center of the half-wave dipole towards a second end to which first ends of third and fourth elongated elements are fixed. Of second ends of third and fourth elongated elements and the first and second ends of the half-wave dipole are respectively isolated by the third and fourth insulating means. Advantageously, the first and second insulation means extend the along an axis of symmetry which is perpendicular to the longitudinal axis of the half-wave dipole and relative to which the first and third elongated elements are respectively symmetrical of the second and fourth elongated elements. The directivity of the antenna can be more accentuated in the second achievement compared to the first achievement, thanks to the third and fourth elongated elements.

The first end of a slender element can be linked to a reference potential such as the earth through, among other things, a reactance additional, preferably variable in order to adjust the characteristics of the antenna diagram by example based on other broadcast areas surrounding, or to periodically select predetermined radiation patterns of the antenna.

When the antenna includes the first and second elongated elements, the first ends of the first and second elongated elements can be linked together through at least one reactance additional, preferably variable. In addition, when the antenna includes the third and fourth elongated elements, the first ends of these can be linked together so analogous through additional reactance.

• la figure 1 représente schématiquement des zones de diffusion d'un réseau de diffusion à fréquence unique ;
• la figure 2 est une vue de dessus horizontale d'une antenne selon une première réalisation de l'invention ; et
• la figure 3 est une vue de dessus horizontale d'une antenne selon une seconde réalisation de l'invention.

Other characteristics and advantages of the present invention will appear more clearly on reading the following description of several preferred embodiments of the invention with reference to the corresponding appended drawings in which:

• FIG. 1 schematically represents broadcasting areas of a single frequency broadcasting network;
• Figure 2 is a horizontal top view of an antenna according to a first embodiment of the invention; and
• Figure 3 is a horizontal top view of an antenna according to a second embodiment of the invention.

Referring to Figure 1, a network of single frequency broadcasting, called SFN network (Single Frequency Network), includes broadcast areas contiguous, also called diffusion cells, arranged linearly to cover an axis AR motorway.

By way of example, in FIG. 1, a portion of the motorway axis AR is covered by four contiguous broadcasting zones ZD _i , ZD _{i + 1} , ZD _{i + 2} and ZD _{i + 3} comprising transmitting antennas A _i , A _{i + 1} , A _{i + 1} and A _{i + 3} , respectively.

The broadcasting network transmits with a frequency carrier common to all broadcasting areas. The carrier frequency is in the band of VHF, and more particularly in the FM frequency band from approximately 88 to 108 MHz. The broadcast areas are perfectly defined for minimize interference between zones neighbors.

Referring to Figure 2, the antenna A _i according to a first embodiment of the invention mainly comprises two radiating sources. A first radiating source is supplied and constitutes a pilot 1. The second radiating source is non-supplied and constitutes a parasite 2. The antenna A _i has a generally substantially triangular shape and is symmetrical with respect to a plane perpendicular to the plane of the figure 2 and having for trace the axis PP in FIG. 2.

Pilot 1 is a half-wave dipole extending along a longitudinal axis D-D perpendicular to the axis P-P and includes two identical metal masts 11 and 12 aligned along the axis D-D. Mast 11a transversely an isosceles triangular section or equilateral and is formed by three cylindrical rods metallic 111, 112 and 113 parallel to the D-D axis. Alternatively, the cross section can be circular, square or polygonal, being still openwork. Two rods 111 and 112 are only visible in Figure 2. The three rods 111, 112 and 113 are joined by a lattice of spacers 114. The structure of the mast 11 is rigid, while being light and with little wind resistance. The mast 12 is identical to the mast 11 and includes three rods 121, 122 and 123 secured by a lattice of spacers 124.

First ends 115 and 125 of the masts 11 and 12 close to the P-P axis are rigidly fixed to a first end 31 of an insulation base 3 which mechanically holds, while insulating electrically, the different parts connected to him. The foot 3 is elongated and centered on the axis P-P. The foot 3 is thus perpendicular to the masts 11 and 12 and is fixed thereto substantially in the center half-wave dipole 1. Second ends 116 and 126 of masts 11 and 12 form the ends of the dipole 1. Foot 3 includes a central core cylindrical 32 in dielectric material and a sheath protection 33 made of plastic. The extremities 115 and 125 of masts 11 and 12 are embedded in the protective sheath 33 so as to be isolated electrically. Alternatively, the foot has a section rectangular or is conical.

At a second end 34 of the foot 3 is fixed the parasite 2 comprising first and second shrouds electrically identical conductors 21 and 22 extending in the plane of Figure 2 along axes H1-H1 and H2-H2, respectively. The H1-H1 and H2-H2 axes are intersecting with each other and intersecting with the axis P-P at the end of the foot 34. Each of the shrouds 21, 22 is formed by a slender element such as a rod cylindrical metal with a first end 211, 221 is embedded in the protective sheath 33 of the foot 3 at the end 34 and a second end 212, 222 is fixed to an insulating element 41, 42. The element insulator 41, 42 is a stretched wire nylon type synthetic connecting the second guy end 212, 222 at second end 116, 126 of the mast 11, 12. The parasite 2 thus has the shape of a vee, the point of which is on the axis P-P and the branches are directed towards the ends 116 and 126 of the pilot 1. The ends 212 and 222 of the branches of the vee are separated from the ends 116 and 126 of pilot 1. As a variant, the shrouds are replaced by elongated metal blades or elongated cages of metal wires.

The masts 11 and 12, the shrouds 21 and 22 and the leg 3 can be removable from each other to others.

In order to fix the ideas, the foot 3 has a length of approximately 40 to 55 cm and a diameter of the order of approximately 5 to 10 cm. Each of the masts 11 and 12 has a length of approximately 70 to 90 cm, typically a quarter of a wavelength λ / 4 = 75 cm for an emission frequency equal to 100 MHz. The sides of the triangular section of the masts 11 and 12 are each about 3 to 4 cm long. The shrouds 21 and 22 have a substantially equal length, that is to say somewhat less or greater, to the quarter wavelength, that is to say approximately 60 to 80 cm, and a diameter of 22 mm, and the insulating wires 41 and 42 have a length of 10 to 20 cm and a diameter of 0.2 mm. The angle between a stay cable and the foot is approximately 60 °, that is to say the angle between a stay cable and the DD dipole axis is an acute angle of approximately 30 °. The foot 3 has a length between 35 and 40 cm and a diameter between 60 and 80 mm. The lengths of the masts, foot and shrouds as well as the angle between the shrouds and the foot, or even the relative position of each of the shrouds and of the dipole, are interdependent and define the shape of the radiation diagram, the gain and the directivity of the antenna for a given transmission frequency of the antenna A _i .

The antenna A _i is supported, for example by the end of the foot 31, by a support (not shown) of the foot arranged on the ground so that the masts 11 and 12 and the foot 3 are located in a horizontal plane as shown in top view in Figure 2, or are located in a vertical plane, depending on the desired diffusion area contour.

The masts 11 and 12 are each supplied with an emission signal by two respective supply terminals 117 and 127 embedded in the sheath 33 at the end 31 of the foot 3. The terminals 117 and 127 are thus protected from the unfavorable influence rain or frost on the electrical characteristics of the antenna A _i . Terminals 117 and 127 are respectively supplied by the inner conductors of two coaxial cables 51 and 52 of the same length connected to the outputs of a balun 5. The balun distributes the power of a transmission signal SE in FM band transmitted by a source 7 installed at the base of the support, through a coaxial antenna cable 70 winding in the support. The external conductors of the coaxial cables are connected to a reference potential, such as earth, by means of a metal plate 53 fixed to the end 31 of the foot 3.

Alternatively, signal symmetrization of emission SE in symmetrical signals feeding masts 11 and 12 is integrated in the first end 31 of foot 3. For example, the coaxial cable 70 is directly connected to a first coaxial section a balun having two coaxial sections, elongated, parallel and identical. Firsts ends of the outer conductors of the two coaxial sections are interconnected by a short circuit, one of these first ends being connected to the outer conductor of cable 70. second ends of the inner conductors of coaxial sections are connected to the mast terminals 117 and 127.

The shrouds 21 and 22 have a reactance intrinsic XI21 and XI22, depending in particular on their length. According to a first variant, the ends 211 and 221 shrouds are connected in series to the level of the end 34 of the foot 3. According to a second variant illustrated in Figure 2, two reactors additional adaptations XS21 and XS22 are inserted in series between the ends 211 and 221 of each guy at the second end 34 of the foot 3. The total reactance of the stay cable 21 is then XT21 = XI21 + XS21 and the total reactance of the stay 22 is XT22 = XI22 + XS22. Reactances XS21 and XS22 have a common terminal connected to a metal plate 23, similar to the plate 53, and fixed to the end 34 of the foot 3. According to a simplified variant, the two reactances are replaced by a single reactance connected between the ends 211 and 221 of guy lines 21 and 22. In all cases, pilot 1 is electrically isolated from the parasite 2.

According to another alternative embodiment, the core 32 of foot 3 is conductive, for example metallic, the sheath 33 being insulating. The plaques 23 and 53 at both ends 31 and 34 are then brought to the same reference potential, or mass common. Consequently the ends 211 and 221 of the shrouds are brought to the reference potential.

The total reactances XT21 and XT22 have equal values in the majority of applications so that the maximum of radiation is directed along the axis PP of foot 3 perpendicular to the half-wave dipole 1. More generally, the values of the reactances directly influence on the antenna radiation. For inductive guy lines, acting as a reflector, ie XT21> 0 and XT22> 0, the antenna A _i radiates mainly in the parasitic direction 2 towards pilot 1 substantially along the axis PP, that is to say from the top down in FIG. 2. In practice, for a given emission frequency, the intrinsic reactance XI21, XI22 increases when the length of the stay 21, 22 increases and the additional reactance XS21, XS22 increases with the value of the inductance inserted between the shroud 21, 22 and the foot 3.

Conversely, for capacitive guy lines, acting as director, ie XT21 <0 and XT22 <0, the antenna A _i radiates in the pilot direction 1 towards parasite 2. The reactance XT21, XT22 becomes more capacitive when the length of the guy line 21 , 22 decreases or when a capacitor of higher capacity as additional reactance is inserted between the stay 21, 22 and the stand 3, for a given emission frequency.

By choosing variable additional reactors XS21 and XS22, it is possible either to modify the radiation pattern of the antenna A _i for a given emission frequency, or to modify the emission frequency then to adjust the radiation pattern of the antenna A _i , for example so that the antenna is very directive, or else substantially bidirectional along the axis PP. The more the substantially equal variable total reactances XT21 and XT22 are high inductances or low capacitances, and therefore vary from a zero value corresponding to a short circuit, to a very high value corresponding to an open circuit, the less the antenna is directive, the radiation patterns in the two opposite directions along the foot axis PP are asymmetrical, and the more the antenna is bidirectional.

This variation in the directivity and the gain of the antenna A _i can be used to impose for example a broadcast of the transmission signal SE during a first period, for example during the day, in a bidirectional manner, that is to say - say almost omnidirectional, and for a second period, for example at night, in a directive manner. If the total reactances XT21 and XT21, or more precisely the variable additional reactances XS21 and XS22 are adjusted more and more differently, the directivity of the antenna A _i is modified with respect to the foot axis PP. The variable reactors XS21 and XS22 can be controlled by remote-controlled gearmotors from the base of the antenna support.

With reference to FIG. 3, a second embodiment of an antenna Aa _i according to the invention has a general diamond shape and is symmetrical with respect to a trace plane Pa-Pa perpendicular to FIG. 3. Only the main differences of the antenna Aa _i with respect to the previous embodiment A _i are described. The antenna Aa _i comprises a pilot analogous to the pilot 1, a first parasite 2a analogous to the parasite 2, a second parasite 6a analogous to the first parasite 2a and placed symmetrically with the latter relative to the longitudinal axis Da-Da of the pilot 1a, and a foot 3a. The pilot 1a is supplied in the same way as the pilot 1 by a transmission signal SEa transmitted by an FM source 7a through a balun 5a analogous to the balun 5.

Foot 3a is substantially twice as long that foot 3 and extends on either side of the pilot along the Pa-Pa axis.

The second parasite 6a comprises two guy lines 61a and 62a fixed between one end 34a of the foot 3a and two insulating wires 43a and 44a respectively fixed at the ends 116a and 126a of the pilot 1a. The third and fourth shrouds 61a and 62a extend along axes H61a-H61a and H62a-H62a, respectively. The axes H61a-H61a and H62a-H62a are intersecting with each other and intersecting with the Da-Da axis of the pilot 1a, and are preferably coplanar with the axes H1a-H1a and H2a-H2a of the first and second shrouds 21a and 22a.

The first ends 211a, 221a, 611a and 621a of the four shrouds 21a, 22a, 61a and 62a are connected two by two in series at the ends of foot 3a, as illustrated in FIG. 3, or by through an additional reactance of variable preference, such as reactance XS21, XS22, adding to the intrinsic reactance of each of the wire.

The lengths of the shrouds, the inclinations of the shrouds with respect to foot 3a and the values of the additional reactances condition the shape of the radiation diagram and therefore the directivity and the gain of the antenna Aa _i , which can be more directive or bidirectional than the 'antenna A _i .

The antenna Aa _i is not necessarily symmetrical with respect to the axis Da-Da, if the antenna must radiate asymmetrically with respect to the axis Da-Da of the pilot dipole 1a. In particular, according to a variant, the foot 3a has different lengths on either side of the pilot 1a. According to another variant, the lengths of the stays 61a and 62a, a priori equal to each other, are different from the length of the stays 21a and 22a.

Claims (8)

1. An antenna (A_i) comprising a supplied radiating source (1) and a non-supplied radiating source (2), characterised in that the supplied source is a half-wave dipole (1) extending along a longitudinal axis (D-D) and the non-supplied source comprises a first elongate conducting element (21) extending along a longitudinal axis (H1-H1) intersecting the longitudinal axis of the half-wave dipole (1) and a first insulating means (3) connecting a first end (211) of the elongate element substantially to the centre (115, 225) of the half-wave dipole, a second end (212) of the first elongate element (21) and a first end (116) of the half-wave dipole (1) being insulated.
2. An antenna according to claim 1, characterised in that the non-supplied source also comprises a second elongate conducting element (22) and a second insulating means (42), the second elongate element (22) extending along a longitudinal axis (H2-H2) intersecting the longitudinal axis (H1-H1) of the first longitudinal element (21) and also intersecting the longitudinal axis (D-D) of the half-wave dipole (1), the first insulating means (3) extending from a first end (31) secured to the centre (115, 125) of the half-wave dipole to a second end (34) to which the first end (211) of the first elongate element (21) and a first end (221) of the second elongate element (22) are secured, a second end (222) of the second elongate element (22) and a second end (126) of the half-wave dipole (1) being insulated.
3. An antenna according to claim 2, characterised in that the first and the second elongate elements (21, 22) are at acute angles, preferably equal, to the longitudinal axis (D-D) of the half-wave dipole (1).
4. An antenna (Aa_i) according to claim 2, characterised in that the non-supplied source (2a) also comprises third and fourth elongate conducting elements (61a, 62a) and a second insulating means (3a), the third and fourth elongate elements (61a, 62a) extending respectively along longitudinal axes (H61a-H61a, H62a-H62a) which intersect one another and intersect the longitudinal axis (Da-Da) of the half-wave dipole (1a), the second insulating means (3a) extending from a first end (31a) secured to the centre of the half-wave dipole to a second end (34a) to which the first ends (611a, 621a) of the third and fourth elongate elements are secured, and second ends (612a, 622a) of the third and fourth elongate elements and the first and second ends (116a, 126a) of the half-wave dipole are respectively insulated.
5. An antenna according to claim 4, wherein the first and second insulating means (3a) extend along an axis of symmetry (P-P) which is perpendicular to the longitudinal axis (D-D) of the half-wave dipole (1a) and with respect to which the first and third elongate elements (21a, 61a) are respectively symmetrical with the second and fourth longitudinal elements (22a, 62a).
6. An antenna according to any of claims 1 to 5, characterised in that the first end (211, 221; 211a, 221a, 611a, 621a) of an elongate element (21, 22; 21a, 22a, 61a, 62a) is connected to a reference potential, inter alia via a preferably variable additional reactance (XS21, XS22).
7. An antenna according to any of claims 2 to 5, characterised in that the first ends (211, 221; 211a, 221a, 611a, 621a) of the first and second elongate elements (21, 22; 21a, 22a; 61a, 62a) are connected together via at least one preferably variable additional reactance (XS21, XS22).
8. An antenna according to any of claims 1 to 7, wherein the half-wave dipole (1; 1a) comprises two metal masts (11, 12), an elongate element (21, 22; 21a, 22a, 61a, 62a) comprises a metal rod or strip, the second end (212, 222; 612a, 622a) of a second elongate element and the respective end (116, 126; 116a, 126a) of the half-wave dipole (1; 1a) are insulated by a stretched wire (41, 42; 41a, 42a, 43a, 44a) and the insulating means (3, 3a) has a cylindrical overall shape comprising a central core (32) of dielectric material and a sheath (33) of plastic material connected to the near ends (115, 225) of the masts and the first end (211, 212; 211a, 221a, 611a, 621a) of an elongate element.

Images