EP0497196A2

EP0497196A2 - Antenna with multiple elements

Info

Publication number: EP0497196A2
Application number: EP92100968A
Authority: EP
Inventors: Shahrokh Ehsani
Original assignee: Vitroselenia SpA
Current assignee: Vitrociset SpA
Priority date: 1991-01-23
Filing date: 1992-01-22
Publication date: 1992-08-05
Also published as: ITRM910048A0; IT1244905B; EP0497196A3; ITRM910048A1

Abstract

Antenna with multiple elements capable to overcome the rapid variations in the power received in urban areas and inside buildings.

Said antenna consists essentially of radiation elements arranged at equal distance from each other , on a circumference having a diameter ranging from 1/4 to 1/3 of the minimum wave length in the used band, with a flat conductor, these radiation elements having a cylindrical or conical shape.

The advantage of the invention resides in the fact that it eliminates the quick variations in the power received in mobile radios. The invention relates to the technical field of microwaves and finds its most suitable application in the field of telecommunications in urban areas.

Description

Description of the Invention

The invention relates to an antenna to be used on vehicle roofs, in order to make possible telephone communications, high-volume data transmissions, etc. Its specific field of use relates to communications between mobile radios in urban areas, inside buildings, in broad-band transmissions.
Said invention solves from the technical point of view the inconvenience of rapid power variations in mobile radios. In order to overcome the spatial variations of received power, an antenna with multiple elements was proposed by Brennan (Proc. IRE, June 1959), with RAYLEIGH's probability density function, identical and independent in all points within a certain extension. In 1988 Cox proposed the use of directional diversity in order to overcome the phenomenon of rapid variations with the hypothesis of the RAYLEIGH scattering and the noncorrelation within adjacent directions.
From Fig. 1 can be seen that the power received in one spot is correlated with the power in neighboring spots and from other measurements results that "the function of frequency correlation" has an extension higher by a few tens of a MHz. The use of spatial diversity and its classification have been proposed in an general manner by Brennan in 1958 with RAYLEIGH's density probability function, independently and identically distributed, in order to overcome the decrease in received power. The statistics used were based on RAYLEIGH's function of independent and identical density, within a certain extension. A study which comes much closer to us is the one by Cox which refers to directional diversity. This too is based on the RAYLEIGH scattering and therein can be found the correlation point/direction to point/direction. Both these diagrams (Cox and Brennan) can not be applied in an environment with multiple reflections, neither for signal solvability nor for sets of broad- band antennas. The hypotheses of the functions identically but independently distributed make it relatively easy to calculate the convolutions in order to obtain the function of compound density, (characteristic component).
The pattern illustrated in Fig. 2 uses the expressed correlation of the received power in adjacent points. It shows the vectorial combination of the power in two points along the vector of the power gradient, distanced from each other (the two points) by a quarter wave length (lambda). Given the distance between the pickup elements within the quarter lambda = D/2, their combination differs from their phased sum (optimal combination) of 1.5dB. In the case of 6 elements the practical effect of their combination is equipotential to the optimal combination of the correspondingly phased vectors, and is therefore robust. The extension of function of the frequency correlation or rather of the correlation of received power allows a use of a determined type in order to overcome the rapid variations of received power. Since the spatial distance between the minimum and the maximum of received power is equal to a quarter wave length, the antenna with multiple elements ought to overcome the problems generated by the vicinity of radiation elements and preserve the transmission band.
The object of the antenna of the present patent application is precisely to overcome, by means of a composite in narrow space of the rapid power variations, independently from the probability density function of the power received in one point due to the distribution of RAYLEIGH, RICE or NAGAKAMI (scientists who have studied this problem).
From the point of view of utility, the invention solves an important problem of civil intervention. Such as can be the case of a nuclear incident or of a chemical incident, like the notorious one in Seveso. In urban areas, in buildings in general, the propagation of electromagnetic waves is subject to high variations in narrow spaces, which make communications by means of radiotelephone difficult and the transmission of data unreliable.
At this point, the intervention of the antenna of the present invention makes all connections possible. In fact this antenna system sums the power received by mutually distanced points so that when one radiation element receives the minimum power, another, or its opposite, receives the maximum power.
As known, the flat wave reflected by a flat surface is a plane whereupon the "poynting" vector is constant in form and direction. The wave found in urban areas and inside buildings is the composite result of a great number of reflected waves plus possibly a direct wave. Studying the "poynting" vector in an area of several tens of meters, it appears to have fast variations within short intervals (Fig. 1 IEEE, JSAC, page 45, January 1989).
Looking at Fig. 1, one can see in the diagram that the power received in one point is closely correlated with the power received in adjacent points.
This close correlation makes possible the formulation of a simple linear diagram consistent with taking into consideration, each in a somewhat limited manner, the vector of "poynting"/power gradient, the resultant of the compounding of field vectors originating from the reflection centers, which can be decomposed into two field vectors with the direction of the gradient (perpendicular) and the reverse for the computation of mutually opposite phases. Thus, in an otherwise rather long gradient, there are points where the two fields are in phase with respect to each other (maximum of power received) and points wherein the two fields are in opposition (Fig. 2).
The invention takes advantage of the known linear diagram according to which two points at a distance of a quarter wave length are in phase quadrature with respect to each other and the modules are equal to sin. or cos. Therefore, their combination is constant (Fig. 2). With three pairs, whichever the direction of the "poynting" vector might be, in one (whichever) point the output is not subject to quick variations.
Therefore, an antenna which is insensitive to quick power variations results.
As known, the propagation in urban areas and inside constructions/buildings is subject to two types of variations in the received power. One is the slow type of variations which has constant statistic characteristics (average value) within a distance of a few tens of meters. The other is the rapid type of variations which between two points at a distance of a quarter wave length from each other, are of the order of 30 - 40 dB.
The antenna with multiple elements, which is the object of the present invention, using the close correlation noted during rapid attenuations, eliminates the inconvenience thereby achieving high dependability in data transmission between mobile radios, comparable to the one provided by stationary radios.
Subsequently the invention is being described with reference to the enclosed drawing figures, in an illustrative, non-limitative manner.
Fig. 1, reproduced from IEEE JSAC January 1989, schematically illustrates the quick and slow variations of the signal received in an industrial building.
Fig. 2 shows diagramatically the power variations, expressed sinusoidally in the space of a quarter wave length in the direction of the power gradient.
Fig. 3 illustrates the configuration of the antenna with six multiple cylindrical radiation elements and a flat conductor
Fig. 4 illustrates the configuration with six conical radiation elements closed by a rhombus and a flat conductor.
It is to be noted that Figs. 3 and 4 have to be considered the significant ones.
Further, when proceeding with the description of the physical structure of the invention, reference will always be made to the enclosed figures, for the purpose of example, but in a non-limitative manner.
The antenna consists of three to six radiation elements, set at an equal distance around a circumference with a diameter of 1 quarter to 1 third of the minimal wave length in the used band, on flat conductor. In the free space, it behaves like a single radiation element.
The radiation elements set on this flat conductor can have various configurations:

a) cylindrical, with a rate of diameter/length of less than 1 tenth and with a cone in the zone cylinder - flat conductor (Fig. 3);
b) cone of rotation with the apex angle of 5° to 20° and a plug at 90° as in Fig. 4.

The antenna system in question set inside an urban area, functions as follows:
The radiation elements are reached by power (signals) which are different from each other (Fig. 1). These powers are conveyed towards a point which is at a distance of at least one wave length from the foot of the antenna with exponential bands with tapered sections
The mentioned power reaches the receiver free of quick fluctuations, allowing the self-propelled carrier to move freely. Translating the above into simple words, the signal becomes non-fluctuating, meaning that the transmission will be dependable.
In conclusion, the solution is obtained by using the correlation illustrated in Fig. 1 and is indicated in Fig. 2. From Fig. 1 also results that in the direction of the vector of poynting/power gradient, approximately two pickup elements arranged at a distance of a quarter wave length from each other have as a sum a constant output.
This diagram is true to measured reality and is based on the fact that all vectors are decomposed into two vectors in the direction of the gradient, where one increases in phase and the other decreases.

Claims

Antenna with multiple elements characterized by the fact that the radiation elements (Fig. 3) are arranged on a circumference with a diameter of one quarter wave length to one third wave length, to be preferably used on self-propelled carriers in urban areas or inside buildings.
Antenna with multiple elements as per claim 1, characterized by the fact that it applies the linear diagram according to which two points at a distance of a quarter wave length are in phase quadrature (opposition) with respect to each other and the modules are equal to sin. and cos., forming a constant combination therewith.