EP0814537A1 - Method and radiating device below the horizon having a high ratio front/backward radiation - Google Patents

Abstract

The method involves selecting the position where the field produced by the antennae is a minimum and adjusting the phase of each antenna so the value of the field reduces at a predetermined position to the front of the antennae. The transmitter signal is fed to a variable power distributor (6) which adjusts the signal amplitude to each antenna to compensate for the difference in the signal due to the difference in height and give a minimum radio field in the given region downward from the antennae. Phase correctors (7,8) compensate for the reflected waves to provide a zero field in a chosen region. The power distributor is a quarter-wavelength volume conductor comprising a fixed line and two moving lines excited by a moving line connection while the phase correction uses an adjustable line. The height above the ground of the upper antenna may be 34m with a spacing between the antennae of 2m.

Description

The invention relates to a method and a device for reducing the rear radiation of a transmitting antenna while lowering this weakened radiation towards a selected area located below the horizon. It applies more particularly to aerials consisting of at least two elementary antennas whose phases are opposite in pairs on the rear of said aerial.

Such a system mainly allows a linear broadcast or not of a plurality of transmitters operating on the same frequency, synchronized or not. This is particularly the case for a chain of isofrequency transmitters along a motorway, the weak radiation from the antennas on their rear considerably reducing the length of the interference zone with the previous transmitter.

In French patent application n ° 88 15254, the applicant describes a method and a device in which there are, along the route, transmitters, or retransmitters, hereinafter called relays, placed between transmitting and transmitting antennas. reception, and set on the same frequency, said relays being directional and two consecutive relays being directed approximately in the same direction. According to this request, directional groupings of elementary antennas staggered in space are used, horizontally by λ / 4 and vertically by about 2 m, and supplied in such a way that the emitted fields are added in the direction of the emission and subtract to cancel in the opposite direction.

Thanks to this arrangement of antennas, the system makes it possible to reduce the interference zone between relays to a few hundred meters.

However, the cancellation of the backward fields is only really effective on the horizon. In the case where the receiving antenna is located at a height other than that of the transmitter, the rear radiation from each of the antennas, located at a different level, arrives at a different angle to the receiving antenna in the area d interference with the radiation from the previous transmitter, zone of almost equal field for two consecutive relays. In particular, the receiving antennas placed on vehicles are located at a much lower height than those of the antennas of the transmitter. The radiation arrives at a certain angle, certainly small but whose effects are not negligible in a system where one wants to obtain a zero field. To this is added the radiation reflected by the ground which does not arrive at the same angle at the receiving antenna. This reflected radiation is theoretically 180 ° out of phase with respect to direct radiation. However, due to the Brewster effect, the angle of reflection is not absolutely equal to the angle of incidence and the phase shift is not exactly 180 °. Despite this, we can neglect the Brewster effect later.

As a result of these unavoidable conditions, the effect of canceling the radiation of the array of antennas towards the rear is disturbed. For example, the upper antenna being located 34 m away, and the lower antenna 2 m below, there is a difference of a few dB between the two radiations in the interference zone. It is therefore necessary to compensate for these undesirable effects so that a minimum of energy reaches the interference zone.

The object of the invention is therefore to reduce the rear radiation of a group of transmitting antennas while lowering this attenuation towards a selected area below the horizon.

• on sélectionne l'endroit où le champ résultant doit être minimal en ajustant les phases de chacune des antennes élémentaires de telle façon que la valeur minimale du champ soit obtenue dans une zone déterminée à l'arrière du groupe d'antennes et vue de celui-ci sous un angle déterminé. Les antennes étant situées à des hauteurs différentes, les niveaux reçus sur la route de chacune d'entre elles sont différents. Ce phénomène est dû à ce que les zones de réflexion n'étant pas situées au même endroit, la sommation du trajet direct et du trajet réfléchi pour chaque antenne donne une valeur différente. On égalise les amplitudes des antennes élémentaires en répartissant la puissance de façon que le champ reçu résultant soit minimal dans la zone choisie.

This object is achieved according to the invention, in that:

• the location where the resulting field must be minimal is selected by adjusting the phases of each of the elementary antennas so that the minimum value of the field is obtained in a determined area at the rear of the group of antennas and seen from it. ci from a determined angle. The antennas being located at different heights, the levels received on the road of each of them are different. This phenomenon is due to the fact that the reflection zones are not located in the same place, the summation of the direct path and the reflected path for each antenna gives a different value. The amplitudes of the elementary antennas are equalized by distributing the power so that the resulting received field is minimal in the chosen area.

According to the invention, the vertical interval between the elementary antennas of a pair is related to the wavelength and the intrinsic structure of the antenna. According to a particular form of the invention, this interval is approximately 2 m.

The device for implementing the invention is characterized in that it includes a variable power distributor making it possible to adjust the amplitude of the field radiated by each of the elementary antennas so as to obtain identical resulting fields in a determined area at the rear of the group of antennas and a phase correction device on each of the elementary antennas in order to obtain perfect phase opposition.

According to a particular form of the invention, the power distributor consists of a conductive volume of length equal to λ / 4, of a fixed line and of two mobile lines moving relative to the previous one so as to increase the level of the low antenna and reduce that of the high antenna. To avoid having rotating contact, the mobile lines are supplied by serial lines open at their ends (immaterial short-circuit between the start of the serial line and that of the mobile line).

The phase correction device consists of an adjustable line

• La figure 1 représente schématiquement la propagation du rayonnement des antennes élémentaires d'un système aérien d'antennes vers l'arrière de celui-ci ;
• La figure 2 représente, dans un repaire X Y, l'addition vectorielle des champs avec un champ résultant non minimalisé ;
• La figure 3 représente schématiquement le montage d'alimentation des antennes élémentaires selon l'invention ;
• La figure 4 représente schématiquement le répartiteur de puissance variable utilisé dans un exemple de réalisation de l'invention.

The invention will be better understood by means of a nonlimiting exemplary embodiment described below and shown in the accompanying drawing in the case of a radio transmitter of a chain of isofrequency transmitters located on the edge of a highway. .

• FIG. 1 schematically represents the propagation of the radiation from the elementary antennas of an aerial system of antennas towards the rear thereof;
• FIG. 2 represents, in an XY coordinate system, the vector addition of the fields with a non-minimized resulting field;
• FIG. 3 schematically represents the supply arrangement of the elementary antennas according to the invention;
• FIG. 4 schematically represents the variable power distributor used in an exemplary embodiment of the invention.

As seen in Figure 1, the elementary antennas 1 and 2 of the aerial radiation system 3 of a radio transmitter are arranged one above the other and offset horizontally.

According to the embodiment, the height H1 of the upper antenna 2 is 34 m. and the lower antenna 1 is located below at an interval ΔH of approximately 2 m. The receiving antenna 4 of a vehicle traveling on the highway culminates at a height H2 of approximately 2 m. Each antenna radiates a V field. Due to the horizontal offset of λ / 4 of the antennas, the field V1 of the antenna 1 and the field V2 of the antenna 2 are added towards the front, so that one has in front of the transmitter a resulting field equal to V1 + V2, and are entrenched backwards, so that the resulting field behind the transmitter is V2 - V1. When the amplitude of the fields received from the antennas is the same, there is behind the aerial a field resulting zero at the height of the transmitting antennas: V2 - V1 = 0, provided that the conditions are those of free space . On the other hand, at the height H2 of the receiving antenna 4, the radiation of each antenna makes an angle with the horizontal and the Fresnel ellipsoid is more or less masked.

Due to the difference in height ΔH of the two elementary antennas, these angles are different and the resulting field V2 - V1 will not be zero.

On the other hand, the radiation is reflected on the road. One neglects the Brewster effect which can however be corrected by the invention. For the same reasons as above, the reflection of the two radiations is not done in the same place and the reflected fields, therefore not having the same amplitude, therefore do not cancel each other out. The receiving antenna also picks up the resulting reflected field and this disturbs reception.

In FIG. 2, we see the composition of the field vectors in the case of an uncorrected system.

The receiving antenna 4 receives from the transmitting antenna 1 a direct field V1 (X1, Y1) and a reflected field VR1 (X2, Y2); from antenna 2 a direct field V2 (X3, Y3) and a reflected field VR2 (X4, Y4). Y1 and Y3 are neighbors of 0.

The value of the resulting field is equal to: $$\text{R =}\sqrt{{\text{<figure-callout id="2" label="ΣX" filenames="imgf0001.png,imgf0002.png" state="{{state}}">ΣX</figure-callout>}}^{\text{2}}{\text{+ <figure-callout id="2" label="ΣY" filenames="imgf0001.png,imgf0002.png" state="{{state}}">ΣY</figure-callout>}}^{\text{2}}}$$

The object of the invention is to lower the value of R.

The device according to the invention is shown in FIG. 3.

The signal from the transmitter 5 is transmitted to the variable power distributor 6 which adjusts the amplitude of the signals to the elementary antennas to compensate for the difference in levels received by the receiver from each of the antennas following their difference in altitude so as to obtain a minimum field R in a given area behind the antennas.

As shown very schematically in Figure 4, this power distributor consists of a cylinder having a central line coupled to two lines with variable coupling by rotation. The mobile lines are excited by an open serial line connection to make a contactless rotating joint. The rotation of the two lines with variable coupling around the central line allows to distribute the power

Each of the adjusted outputs of the distributor is connected to an adjustable phase correction line 7 for the signal from antenna 1 and 8 for the signal from antenna 2.

Thus, in a given zone, the phase differences originating from the different paths of the radiation, direct and reflected, and variations in the center of phase from one antenna to the other, are compensated by the adjustable lines 7 and 8. By a adequate setting, we can therefore choose the zone where these phase differences are compensated. Since the amplitudes of the signals have been adjusted in order to obtain a zero field in a given area, it is possible to create a zero field in a chosen area. In the exemplary embodiment, this zone is the zone of interference between the rear radiation of the transmitter and the front radiation of the preceding transmitter in an isofrequency linear broadcasting chain.

Claims (6)

Method for reducing the rear fields of a group of directional antennas comprising at least one pair of elementary antennas offset between them vertically by a certain interval and horizontally by λ / 4, characterized in that: - the place where the resulting field must be minimal is selected by adjusting the phases of each of the elementary antennas in such a way that the value of the field is reduced in a determined area at the rear of the group of antennas and viewed from it here under a determined angle, under the horizon, - the amplitudes of the signals received from the elementary antennas are equalized by distributing the power so that the resulting radiated field is minimal in the chosen area. Method according to claim 1, characterized in that the vertical interval between two elementary antennas of a pair is approximately 2 m. Device for radiating a radio emission, comprising an aerial antenna system consisting of at least a pair of elementary antennas offset vertically by a certain height and horizontally by λ / 4, characterized in that it further comprises a variable power distributor to adjust the amplitude of the fields received from each of the elementary antennas and a phase correction device on each of the elementary antennas, so as to obtain a zero resulting field in a determined area behind the group of antennas and, seen from it, from a determined angle. Device according to claim 3, characterized in that the power distributor consists of a conductive volume of length equal to λ / 4, of a fixed line and of two mobile lines, excited by an open line connection, moving relative to the previous one so as to increase the level of the low antenna and reduce that of the high antenna. Device according to claim 3, characterized in that the phase correction device consists of an adjustable line. Application of the method according to claim 1 to a chain of isofrequency transmitters along a traffic lane.

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