EP0337841A1 - Broadband transmitting antenna loop with asymmetric feed and array of a plurality of these loops - Google Patents

Abstract

This antenna, which is fed via a coaxial line (1), comprises:… - a radiating element in the form of a symmetric flat loop (4) closed up on itself and connected to the core (2) of the coaxial feed line at one point of the loop,… - a reflector plane (5) extending parallel to the plane of the loop and remote from it, and… - a symmetrising element in the form of a metal plate (6) arranged between the loop and the reflector and extending parallel to the plane of the loop and to that of the reflector, and of general form corresponding substantially to that of the part of the loop connected to the core of the coaxial line, this plate being connected to the earth (3) of the coaxial feed line. …<??>Array antenna formed of an ordered grouping of a plurality of antennas exhibiting this characteristic structure, and excited simultaneously, the loop and the plate (6) of each antenna extending above a common reflector plane (5). Application to broadband transmission antennas, in particular in the L band. …<IMAGE>…

Description

The present invention relates to a loop type antenna with asymmetrical feed.

We know the characteristics of loop antennas, especially their wide bandwidth and their relatively open diagram, which makes them used mainly in reception.
In most of the cases, the loop is used alone, that is to say without associated reflector, and it is supplied either by a two-wire line whose two conductors terminate at the two ends of the loop (which is not closed on itself), or by a coaxial line, the connection of the loop to the line then being made with the interposition of a balun housing.
The loop antennas currently used are ill-suited to use in transmission because of their ROS (Stationary Wave Report) penalizing for the radiated energy.
In addition, if the output of the transmitter as is generally the case is on a coaxial line, the use of the balun further increases the ROS; moreover, it allows suitable operation only in a relatively narrow frequency band (around the tuning frequency of the balun), which is incompatible with broadband transmission.

One of the aims of the present invention is to provide a loop type antenna which can be supplied by a coaxial line, in particular for use in transmission, ensuring excellent adaptation both with regard to the ROS and the radiation pattern. , and this over a relatively wide frequency band.

In addition, and as will be seen, the invention makes it possible to have a suitable radiating source which is simple to produce and of low cost, which optionally makes it possible to multiply the number of antennas so as to form a large array ( "slab" antenna), for large radiated powers.

To this end, according to the present invention, the antenna, which is supplied by at least one coaxial line, comprises:
- a radiating element in the form of a symmetrical flat loop closed on itself, connected to the core of the coaxial supply line at a point of the loop,
a reflective plane extending parallel to the plane of the loop and at a distance therefrom, and
- At least one balancing element in the form of a metal plate, arranged between the loop and the reflector and extending parallel to the plane of the loop and to that of the reflector, and of general shape corresponding substantially to that of the part of the loop connected to the core of the coaxial line, this plate being connected to the ground of the coaxial supply line.

In addition, according to a number of advantageous features:
- Said loop portion is a rectilinear segment, the largest dimension of the rectangular plate being substantially equal to or less than the length of this loop segment located opposite; preferably, the loop is then of substantially square shape;
- the length / width ratio of the rectangular plate is at least equal to 4;
the distance from the plane of the plate to the plane of the loop is less than one tenth of the wavelength;
the ratio between the plane of the plate and the plane of the loop / distance from the plane of the plate to the plane of the reflector is between approximately 1: 5 and 1:10;
the distance from the plane of the loop to the plane of the reflector is substantially equal to or less than half a wavelength;
- The loop is fed, selectively or simultaneously by a second coaxial line at a second feed point located substantially orthogonally on the loop relative to the first feed point.

The invention also relates to a network antenna formed by an ordered grouping of a plurality of antennas having the aforementioned characteristic structure, excited simultaneously, the loop and the plate of each antenna extending above a plane. common reflector.

• - la figure 1 est une vue schématique, en perspective, de l'antenne de la présente invention,
• - la figure 2 est une courbe montrant la variation du ROS sur une large plage de fréquences,
• - la figure 3 représente divers diagrammes de rayonnement relevés pour l'antenne de la figure 1,
• - la figure 4 est une vue en plan d'une variante de réalisation dans laquelle la boucle est circulaire,
• - la figure 5 est homologue de la figure 1, pour un mode de réalisation dans lequel la boucle est excitée séparément en deux points par deux coaxiaux différents, et
• - la figure 6 est homologue de la figure 4, dans le cas du mode de réalisation de la figure 5.

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

• FIG. 1 is a schematic perspective view of the antenna of the present invention,
• FIG. 2 is a curve showing the variation of the ROS over a wide frequency range,
• FIG. 3 represents various radiation patterns noted for the antenna of FIG. 1,
• FIG. 4 is a plan view of an alternative embodiment in which the loop is circular,
• FIG. 5 is homologous with FIG. 1, for an embodiment in which the loop is excited separately at two points by two different coaxials, and
• FIG. 6 is homologous to FIG. 4, in the case of the embodiment of FIG. 5.

In FIG. 1, the reference 1 designates the supply line, which is a coaxial line - therefore asymmetrical -, with a central conductor 2 forming the core of the cable and a peripheral conductor 3 forming mass and return conductor.
This supply line leads to an antenna which includes a loop 4, a plane reflector 5 and a balancing element 6 in the form of a plate; each of these three elements is plane, and the planes of the three elements are mutually parallel.
The loop 4 is preferably square in shape, and fully closed on itself. It is excited by the core 2 of the coaxial line, which is connected at a midpoint on one of the sides of the loop, so as to keep the symmetrical structure specific to loop antennas.
A plane reflector 5 is arranged below the antenna; its dimensions are not critical, this reflector ideally extending to infinity.
To ensure the transition between the symmetrical loop-reflector structure and the asymmetrical supply line, a balancing adapter element is provided, consisting of a metal plate 6 placed between the loop and the reflector, at a short distance from the loop, and connected to earth 3 of the supply line.
This plate 6 has the effect of extending the mass of the coaxial line in the form of a ground plane ensuring coupling with the loop, because this plate is located in the close vicinity of one of the sides of the loop; in Figure 1 there is shown by arrows the direction of the field lines between the loop 4 and the reflector 6, thus visually showing how the transition is made between the asymmetric supply and the symmetrical radiating element. Preferably, the plate 6 has the shape of an elongated rectangular plate extending parallel to one of the sides of the square formed by the loop 4, the length of the plate being of the same order as the length of this side, preferably slightly lower.

FIG. 1 shows the respective preferred dimensions of the different elements, all given as a function of the wavelength of the central frequency of the antenna operating band.

Figures 2 and 3 give the results of measurements made with an antenna having the characteristics and dimensions indicated in Figure 1.

Figure 2 shows the variation of the ROS according to the frequency on the band 1250 - 1450 MHz, and we can see that the ROS always remains lower than 1.5 on the whole band, which allows to use in a way very suitable the antenna of the present invention for broadband transmission (in the case of FIG. 2, the frequency varies over ± 7.5% around the central frequency).

FIG. 3 shows the respective radiation diagrams: in the plane containing the electric field (curve E), that is to say in the plane perpendicular to the loop and to the side supplied by the coaxial line (this will be, for example , from the azimuth diagram, the loop and the side supplied by the coaxial line being vertical); in the plane, orthogonal to the previous one, containing the magnetic field (curve H, which would be the diagram in site in the preceding hypothesis); and the diagram corresponding to a cross polarization (curve X). As can be seen, the gain of the antenna remains uniform over a relatively wide angular opening.

Note that the loop is not necessarily square or rectangular; alternatively, it may include a curvilinear part in the vicinity of the excitation point, or even be completely circular as shown in FIG. 4, the plate 6 then being curved to follow the contour of the curvilinear part around the point of excitation.

Furthermore, as illustrated in FIGS. 5 (square loop) and 6 (circular loop), loop 4 can be attacked by a second coaxial 1 ′ situated on the loop at 90 ° relative to the first coaxial 1. It is then provided , as for the first coaxial, a plate 6 ′ forming a balancing element.
The two coaxials can be used either selectively (one can then obtain, depending on the coaxial chosen for the excitation, a vertical or horizontal polarization), or simultaneously with phase shift of ¶ / 2 between the two supply signals of the loop (on can then get circular polarization)

Finally, it is possible to provide, above a common reflecting plane, a plurality of loop-plate assemblies arranged so as to form a network and supplied simultaneously (possibly with an appropriate phase shift) so as to be able to radiate more power. important and / or to modify the radiation diagram by playing on the relative phase shifts of excitation of the various elements of the network, as is well known in the art.

Claims (9)

1. A loop type antenna supplied by at least one coaxial line (1), characterized in that it comprises:
- a radiating element in the form of a loop (4) symmetrical planar closed on itself, connected to the core (2) of the coaxial supply line at a point of the loop,
a reflective plane (5) extending parallel to the plane of the loop and at a distance therefrom, and
- At least one balancing element in the form of a metal plate (6), disposed between the loop and the reflector and extending parallel to the plane of the loop and to that of the reflector, and of general shape corresponding substantially to that of the part of the loop connected to the core of the coaxial line, this plate being connected to the ground (3) of the coaxial supply line. 2. The antenna of claim 1, wherein said loop portion is a rectilinear segment, the largest dimension of the rectangular plate being substantially equal to or less than the length of this loop segment located opposite. 3. The antenna of claim 2, wherein the planar loop (4) is symmetrical in substantially square shape. 4. The antenna of one of the preceding claims, in which the length / width ratio of the plate (6) is at least equal to 4. 5. The antenna of one of the preceding claims, wherein the distance from the plane of the plate to the plane of the loop is less than one tenth of the wavelength. 6. The antenna of one of the preceding claims, in which the ratio distance from the plane of the plate to the plane of the loop / distance from the plane of the plate to the plane of the reflector is between approximately 1: 5 and 1:10 . 7. The antenna of one of the preceding claims, in which the distance from the plane of the loop to the plane of the reflector is substantially equal to or less than half a wavelength. 8. The antenna of one of the preceding claims, in which the loop is supplied, selectively or simultaneously, by a second coaxial line (1 ′) at a second feed point located substantially orthogonally on the loop relative to the first feed point. 9. An array antenna, characterized in that it is formed of an ordered grouping of a plurality of antennas according to one of the preceding claims, excited simultaneously, the loop and the plate of each antenna extending over- above a common reflecting plane.

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