EP0635899A1

EP0635899A1 - Microstrip array antenna

Info

Publication number: EP0635899A1
Application number: EP94111319A
Authority: EP
Inventors: Piermario Besso; Paolo Bielli; Davide Forigo
Original assignee: SIP SAS; SIP Societa Italiana per lEsercizio delle Telecomunicazioni SpA
Current assignee: SIP SAS; Telecom Italia SpA
Priority date: 1993-07-21
Filing date: 1994-07-20
Publication date: 1995-01-25
Also published as: CA2128454A1; ITTO930543A1; AU6342994A; ITTO930543A0; JPH0766626A; IT1260934B

Abstract

A microstrip array antenna consisting of a multilayer structure and of a carrier structure to which the connector and the multilayer structure are secured, made up of a first layer comprising the microstrip feed network, of a second layer made up of a grounded metal plane, periodically punched, and of a third layer comprising the array of radiating metal elements some of which are not fed and are placed along one or more perimeters around the array.

Description

The present invention relates to devices for microwave telecommunication systems and more particularly it concerns a microstrip array antenna.
Microstrip array antennas are microwave directive antennas, which can be advantageously used as antennas in telecommunication networks both for point-to-point and point-to-multipoint links, for services such as speech, data, video, and in the mobile radio network for base stations and for interconnection between the base stations themselves.
They are provided to be used for the radio links of the national telephone service in frequency bands around 19, 28 and 38 GHz, where the required bandwidths are of about 2 GHz.
These antennas are also specially suitable for use in the urban areas thanks to their planar structure enabling very reduced volumes and weights to be obtained. Their small thickness, just a few centimeters, makes them easy to camouflage and they can therefore be mounted without altering the characteristics of the structure where they are placed and this lets the specially critical environmental constraints in urban area be overcome.
Moreover, manufacturing costs are much lower than those of an equivalent reflector antenna.
The main purpose in designing microstrip array antennas is to obtain radiating characteristics which can be compared with those of reflector antennas having the same dimensions. More particularly, the most critical requirements are the reduction of the side lobes, the gain for the required bandwidth, the symmetry of the main lobe in E and H-planes and the reduction of return losses.
High gains are obtained by means of arrays made up of many radiating elements, even a thousand, fed by a proper microstrip network, the task of which is to equally share the power among all the elements.
A microstrip array antenna has been described in the paper "Wideband Aperture-coupled Microstrip Patch Array for Satellite TV Reception", by F. Rostan and alii, published in the Proceedings of the Eighth International Conference on "Antennas and Propagation", 30 March-2 April 1993, Edinburgh, UK. This antenna, designed for direct TV reception from satellite, was built in two experimental versions, one made up of an 8x8 element array and the other made up of a 16x16 element array. Measurements carried out on the second version of 16x16 elements, show a reflection coefficient lower than -20 dB only for a 3% bandwidth and a non perfectly symmetrical radiation lobe in E and H-planes. These performances are admissible for the reception of TV programmes, but they are not sufficient for telecommunication antennas.
A remedy to these drawbacks is the microstrip array antenna object of the present invention, which shows a symmetrical radiation pattern in E and H-planes, -20 dB return losses for a usable bandwidth equal to approximately 7% of central frequency and secondary lobes at 90ø reduced to -60 dB as compared to the main lobe. These are performances similar to those of current reflector antennas.
The special object of the present invention is a microstrip array antenna as described in the characterizing part of claim 1.
These and other characteristics of the present invention will be made clearer by the following description of a preferred embodiment of the same, given by way of non limiting example, and by the annexed drawings in which:

Figure 1 is a perspective view of the structure of an array antenna made up of 16x16 radiating elements;
Figure 2 is a section of the feed network;
Figure 3 is a diagram showing return loss trend as a function of frequency;
Figure 4 shows radiation pattern section in H-plane;
Figure 5 shows radiation pattern section in E-plane.

In Figure 1, 1 indicates the carrier structure made up of a metallic box, e.g. aluminium, provided with a bottom wall, normally having a square shape, and four side walls to which the antenna planar structure can be secured by means of screws. This latter is thus kept at an optimum distance from the metallic bottom, e.g. 1.25λ, where λ is the central band wavelength. The function of the carrier structure is to improve the antenna front-to-back ratio and to shield the feed network besides allowing to place a connector CO on one of the side walls and anchorage units on the bottom wall.
A layer 2 is a sheet made up of a dielectric material with a dielectric constant ε=2.2, on which the feed network is photoetched according to the printed circuit technique. This network is obtained by the periodical repetition of successive 3dB power splitters, ending in 16x16 matching and coupling circuits to the radiating elements. These circuits will be better described later.
A layer 3 is made up of a ground metallic plane, periodically punched by 16x16 rectangular slots, long λ/4, wide between λ/10 and λ/20 and spaced by λ/2 in both the orthogonal directions. The metallic plane extends beyond the outermost rows and the outermost columns by a length equal to at least 2λ, measured from the symmetry centre of the external slots.
A layer 4 is a sheet made up of a dielectric material with a dielectric constant ε=2.2, on which an array of square resonant metallic elements is photoetched. They form a 20x20 element array, the central part of which, made up of 16x16 elements, is fed by the feed network through the slots of layer 3.
The elements belonging to the remaining two external rows are intentionally not fed. For this reason, to these remaining rows there are neither slots in the layer below nor the matching and coupling circuits in the layer carrying the feed network.
The antenna is made operating by securing the multilayer structure, obtained by glueing the layers 2, 3 and 4 with one another and to the side walls of carrier structure 1 by means of screws inserted along the edges of the layers themselves and connecting the connector CO to the feed network.
The presence of the non-fed elements allows obtaining the mentioned performances and more particularly to obtain a return loss of -20 dB for a usable bandwidth equal to approximately 7% of the central frequency, while the usable bandwidth is only 3% without the non-fed elements.
The antenna can be made to offer a wider band, equal to 10% of the central frequency, at a -25dB return loss level, by overlapping to the multilayer structure another layer containing a radiating metallic element array similar to that already present on layer 4 (Figure 1).
Figure 2 shows two matching and coupling circuits connected to a 3 dB power splitter, indicated with DP. Each circuit is made up of a section of a transmission line LT, which carries the signal up to the centre of a rectangular slot AP, made in the overstanding layer 3, and a matching stub ST.
Figure 3 shows the amplitude AMP of the return loss in the 18 GHz band both for the antenna with non-fed elements, indicated as OL, and for an antenna without non-fed elements, indicated as NE, realized according to known technique for measurement purposes. In the first case the -20 dB band is of 1.2 GHz, and in the second case is only of 0.5 GHz.
Figure 4 shows the radiation pattern section in the H-plane and Figure 5 shows the radiation pattern section in E-plane. A good symmetry is evident in both planes.
It is clear that what described has been given only by way of non limiting example. Variations and modifications are possible without going out of the scope of the claims. For example there can be only one row or more than two rows of elements non fed externally to the array of fed elements, and the rows themselves may not be completed. Furthermore, in case of a non-square antenna, the non-fed elements would be located along one or more perimeters around the array of the fed elements and the metallic plane 3 (Figure 1) would extend below these non-fed elements.

Claims

A microstrip array antenna made up of:
- a first layer (2), made up of a dielectric sheet on which the microstrip feed network is made, obtained by a periodical repetition of successive 3dB power splitters ending in matching and coupling circuits;

- a second layer (3), made up of a ground metallic plane, periodically punched by rectangular slots;

- a third layer (4), made up of a dielectric sheet on which the radiating metallic element array is made, resonant at wavelength λ and fed by the feed network made on the first layer (2) through the slots carried out in the second layer (3);

- a carrier structure (1), made up of a metallic box provided with a bottom wall and side walls to which the layer succession (2, 3, 4) and a connector (CO) connected to the feed network are secured;
characterized in that non-fed resonant metallic elements are located along one or more perimeters round said radiating element array made on the third layer (4) and the metallic plane of second layer (3) extends below said non-fed resonant metallic elements.
A microstrip array antenna as in claim 1, characterized in that on the third layer (4) is placed a further layer containing an array of radiating metallic elements similar to that already present on the said third layer.