GB2238176A - Microwave radar transmitting antenna - Google Patents

Abstract

An antenna consists of a large number of individual transmitting elements arranged in a predetermined pattern within an area defined by a closed conic section. Each element comprises a feed section having a power amplifier arranged to operate at its maximum efficiency, and an antenna section. The area defined by the closed conic section is divided into at least two regions 30-33, each region being defined by a closed conic section of the same form as and coaxial with that defining the area e.g. circles. All the power amplifiers connected to antenna sections within one region have the same output power rating, which is different from that of the amplifiers connected to antenna sections within any other region. The result is a beam pattern with reduced side-lobes but optimum efficiency of power use. <IMAGE>

Description

MICROWAVE RADAR TRANSMITTING ANTENNA Modern microwave radar transmitting or receiving antennas frequently comprise a large number, perhaps several thousand, of individual elements each of which is required to act as a transmitting or receiving antenna as the mode of operation dictates. The element may comprise a physical dipole or an aperture in a conducting plate. In its simplest form in the transmitting mode power is fed from a common source to each individual element and in the receiving mode received signals are combined and applied to appropriate receiver circuits.

It is desirable that all the energy transmitted by a radar antenna should be concentrated into a beam of predetermined angular dimensions. In practice this is difficult to achieve with the result that side-lobes are producea into which some of the available power is directed.

Not only is this wasteful of energy but it may also interfere with the operation of the radar system as the sidelobe energy may be reflected from objects outside the main beam and give rise to spurious signals or "clutter". This degrades the performance of the radar system.

The intensity of the sidelobe energy may be reduced simply by reducing the amount of power applied to certain elements of the antenna. This involves modifying the usual feed network from the radar transmitter so that certain elements receive less power than others. This in turn involves dissipating a proportion of the power applied to the feed network before it reaches those elements whose power output is to be reduced. Clearly this is not only wasteful of power but also presents a cooling problem, since the unwanted energy is dissipated as heat.

It is an object of the invention to provide a microwave radar transmitting antenna in which these losses are avoided and which is therefore more efficient.

According to the present invention there is provided a microwave radar transmitting antenna which includes a plurality of individual transmitting elements arranged in a predetermined pattern within an area defined by a closed conic section, each element comprising a feed section having a power amplifier arranged to operate at its maximum efficiency and an antenna section, the area defined by the closed conic section being divided into at least two different regions each defined by a closed conic section of the same form as and coaxial with that defining the said area, and the plurality of power amplifiers being arranged such that all power amplifiers within any one region have the same output power as each other which is different from the output power of power amplifiers in each other region.

The invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic circuit diagram of part of a typical active microwave transmitting antenna; and Figure 2 shows a Taylor weighting function curve to illustrate the voltage profile across the centre of the array, with superimposed element distribution curves.

Referring more to Figure 1, this shows four only of a iarge number of active transmitting elements 10, shown schematically as dipoles. Each is connected to a power amplifier 11 by way of a variable phase shifter 12 used for beam steering purposes. The power amplifiers 11 are connected to a signal source such as a precision oscillator 13 through a corporate feed network comprising an arrangement of 3dB couplers, represented at 14. Such an arrangement is, in itself, well known. It is also known to control the radiated beam profile by reducing the power applied to certain of the dipoles by reducing the power output of the amplifier supplying those dipoles. It is necessary to use Class A amplifiers in an active radar transmitting antenna in order to avoid unacceptable distortion of the radar signal.However, the operation of Class A antennae at reduced power is essentially wasteful as they become inefficient and the wasted power is converted into heat, giving rise to a cooling problem as already mentioned.

The invention seeks to avoid this problem by using each amplifier at its maximum efficiency, hence reducing losses, but by using amplifiers having a number of different ratings. In addition, the distribution of amplifiers having these different ratings is determined in accordance with a suitable weighting curve such as the Taylor weighting function shown in Figure 2.

The function curve 20 is a standard curve used for weighting purposes and shows the voltage profile across the centre of the array. Superimposed on the curve are "boxes", the widths of which indicate the areas of each of a number of regions into which the antenna is divided. By way of example it is assumed that the antenna is circular and that it is divided into four concentric regions. The widths of the "boxes" therefore represent the diameters of the separate regions, assuming the overall radius of the antenna to be 1. The radii of the other regions are indicated on Figure 2. The height of the "boxes" represent the voltages applied to each element in that region, and hence the square root of the power output from each amplifier feeding a dipole within that region, again taking the maximum value of voltage to be 1.

Figure 3 is a plan view of the antenna showing the boundaries of the regions into which it is divided. Using the values shown above it will be seen that the power radiated by each dipole within the central region 30 is 1 unit, that for each dipole in the annular region 31 is 0.336 units, for region 32 the power is 0.112 units and for the outer region 33 the power per dipole is 0.037 units.

The areas of the various regions determine the number of dipoles in each region, assuming that the dipoles are evenly spaced. Taking the overall radius of the antenna to be 1 unit as stated above then the area of central region 30 is 0.454 units, that of region 31 is 0.793 units, that of region 32 is 0.918 units and that of the outer region 33 is 0.977 units.

Hence if, for the sake of example, the antenna has a total of 2000 dipoles, region 30 will contain 300, region 31 will contain 520, region 32 will contain 560 and region 33 will contain 620.

A simple comparison may now be made to show the effectiveness of the invention both in terms of power efficiency and reduced cooling load. A conventional transmitting array having 2000 elements each fed by a Class A amplifier which is 25X efficient and capable of a peak power output of 1 watt requires a total d.c. input power of 8000 watts. Due to the requirements for reducing sidelobes the rating of many of the amplifiers is reduced by suitable weighting so that the actual antenna output power may be as low as 518 watts. This results is an overall efficiency of only 6.5% and a cooling load of 7482 watts.

Consider now the example quoted above in describing the invention. There are 300 elements rated at 1 watt, 520 rated at 0.336 watts, 560 rated at 0.112 watts and 620 rated at 0.037 watts. The total radiated power is therefore 560 watts, 518 watts of which forms the main beam and 42 watts the error side lobes. However, because each power amplifier is working at its maximum efficiency of 252, the required input power is therefore only 2240 watts, giving an overall efficiency of 25% and a cooling load of only 1680 watts.

The determination of the relative dimensions of each "box" shown in Figure 2 is determined in the same way that the weighting of existing antenna is determined. However, the use of power amplifiers all operating at maximum efficiency leads to the advantages set out above.

Existing active antennae having weighted power distribution allow the power output of each separate element to be determined individually to suit particular sidelobe reduction requirements. In the arrangement of the invention it is necessary that the antenna shape is that of a closed conic section, that is a circle or an ellipse, and that each region into which the antenna is divided is of the same form, concentric with the outer boundary of the antenna. Thus if the outer boundary of the antenna is circular then each region is defined by a concentric circular boundary. If the outer boundary is elliptical then each region is defined by a similar ellipse having the same foci.

The maximum number of regions into which the antenna is divided is dependent upon the number of different amplifier ratings which it is reasonable to stock or supply. Four regions has been quoted in the above example but the minimum number is two.

The example described above has referred to dipoles as the radiating elements. However, the radiating element may take any of the usual forms. The supply of signals to the power amplifiers need not take the form of the corporate feed network shown in Figure 1. Each element of group of elements may have a separate signal source. Even if the feed network approach is used the signal source may be other than the oscillator shown in Figure 1.

Claims (4)

Claims

1. A microwave radar transmitting antenna which includes a plurality of individual transmitting elements arranged in a predetermined pattern within an area defined by a closed conic section, each element comprising a feed section having a power amplifier arranged to operate at its maximum efficiency and an antenna section, the area defined by the closed conic section being divided into at least two different regions each defined by a closed conic section of the same form as and coaxial with that defining the said area, and the plurality of power amplifiers being arranged such that all amplifiers within any one region have the same power output as each other which is different from the power output of power amplifier in each other region.

2. A transmitting antenna as claimed in Claim ' which comprises four regions.

3. A transmitting antenna as claimed in either of Claims 1 or 2 in which each region is defined by a circle.

4. A microwave radar transmitting antenna substantially as herein described with reference to the accompanying drawings.