GB2191344A - Microstrip rotman lens - Google Patents

Abstract

A Rotman lens consists of two flat parallel metal plates with convexly curved ends. At one end there is a number of radio frequency launchers, e.g. waveguides, whose axes are normal to the curve. At the other end there are radio frequency pick ups, coupled in use by coaxial cable to the elements of a linear array of antenna elements. If the launchers are successively pulsed, the antenna elements emit a narrow scanning beam. To improve this arrangement as constructed in microstrip, the ends of the launchers A1-A7 are cut at angles so that they emit beams into the lens at angles to the axes of the launchers. This is stated to improve scanning characteristics. <IMAGE>

Description

SPECIFICATION Microstrip Rotman Lens The present invention relates to a microstrip flatplate Rotman lens arrangement, such as may be used in a scanning radar antenna arrangement.

The basic principles of Rotman lens are described in a paper "Wide Angle Microwave Lens for Line Source Application", by W. Rotman and R. F.

Turner, IEEE Trans. Ap-1 1, pp 623-32, 1963. From this it will be seen that a Rotman lens consists of two flat parallel plates with convexly curved ends. At one of these ends there are a number of radio signal launchers, usually waveguide horns, whose axes are shown set normal to the curvature of the end. At the other end of the parallel plane structure there are a number of radio frequency probes, i.e. pick-up aerials, which are coupled by coaxial cables to respective radiators of a linear array of radiators.

The parallel plate region between the two curved regions can be relatively short so that the result is rather like a cross-section of a bi-convex lens. If the launchers are successively pulse-wise energised, a narrow scanning beam is emitted from the linear array of radiators.

An object of the present invention is to produce an improved Rotman lens arrangement, and an improved scanning antenna arrangement using such a lens arrangement.

According to the present invention, there is provided a radio-frequency microstrip lens arrangement, which includes two flat plates of electrically-conductive material located parallel to each other and spaced apart by a distance less than half the wavelength of the radio frequencies to be dealt with, wherein two sides of the parallel structure are curved outwards so that in plan view the structure resembles a convex lens or a barrel, wherein at one of the curved sides there is located a number of radio frequency launchers for launching radio signals into the region between the plates, wherein at the other of the curved sides there is located a number of radio frequency pick-ups, wherein the launchers or the pick-ups are so arranged that for each said launcher or pick-up its beam is launched or picked up at an angle to its axis, the said angles being such as to produce the desired scanning characteristics for the arrangement, and wherein when the arrangement is in use the radio frequency launchers may be pulsed successively to cause the radio beams to scan across the pick-ups.

An antenna assembly can be produced by the use of an arrangement as set out in the preceding characteristics with a linear array of radio frequency antenna elements each of which is coupled by a coaxial cable orthe like to one of the probes of the lens arrangement.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which Figure 1 is a representation of a practical form of a Rotman lens; Figures 2(a) and 2(b) represent two Rotman lens designs, usable in embodiments of the invention; Figure 3 shows a form of waveguide launcher with a cut angle; Figure 4 shows schematically an experiment to test a simulated microstrip launcher; Figure 5 shows schematically a test lens used to test different cut angles for the microstrip launcher; Figure 6 shows a preferred configuration for a Rotman lens arrangement embodying the invention; Figure 7 shows multiple beam patterns (array factor) predicted using two-dimensional aperture theory; Figure 8 shows the array factors calculated using the measured amplitudes and phases from the microstrip lens, at the design frequency.

When a Rotman lens is used in conjunction with a linear array of antenna elements, as indicated in the above-mentioned paper, with each of the probes of the lens arrangement coupled by a coaxial cable to one of the antenna elements, the array emits simultaneous multiple beam. Figure 1 shows a practical microstrip lens. In Figure 1 we have a number of beam ports along the "focal arc" formed by the bottom curved line, a parallel plate region of height less than Am/2, and a number of array ports at the top curved line, each to be connected when in use to an antenna element via microstrip and coaxial line. The region inside the parallel plates is filled with dielectric material, e.g. with ear=10.5. The electrical line lengths are a lens design variable.In such an arrangement, the angular displacement of a beam port from the lens centre line creates a phase slope across the antenna array, and hence steers the beam. An exactly linear phase slope is only available at focal points on the focal arc, with small phase aberrations for other beam port positions. If the lens line lengths are all equal, only two foci are available, while if the line lengths can vary as in the Rotman lens three foci are available. The phase aberrations are reduced for a three-focus lens, which enables a greater number of beam widths of scan to be attained. In fact, the microstrip lens of Figure 1, which is a Ruze lens, is a two-focus lens scanning a 1 6-element linear array two +30 .

The amplitude performance of microstrip Ruze and Rotman lenses can be analysed using two dimensionai aperture antenna theory. A microstrip lens port has a two-dimensional radiation pattern which approximates to that of a uniformlyilluminated aperture.

Figures 2(a) and 2(b) show two Rotman lens designs, with lens variables g =(a)1 1/cos a and (b)=1. The parameter g, as explained in the Rotman and Turner paper, determines the curvature of the focal arc, and can be chosen by the lens designer.

For a Ruze lens, such as shown in Figure 1, the focal arc and the array port contour can at least approximate to circular arcs each centred at the opposite lens contour. The microstrip port apertures follow the lens contours, so that the radiation pattern peaks point to the centres of the opposite contours. This is desirable for non-symmetrical array amplitude tapers, and for near minimum insertion loss.

For a Rotman lens, see Figures 2(a) and 2(b), either the beam port contour (the left-hand curve) or the array port contour (the right-hand curve) is strongly curved, and the port is required to be at an angle to the normal to the contour. In a waveguide fed lens the flared waveguide ports can be staggered along the lens contour to point where desired, but this cannot be done for a microstrip lens port, where there is no waveguide side wall.

Forthe lens arrangement to be produced, we need a microstrip port, viewed as a two-dimensional aperture antenna, such that its two-dimensional radiation pattern peak is deflected from the normal to the aperture. An open-ended waveguide radiator can have a cut angle end, see Figure 3, so that the beam from that radiator is deflected to a position between the aperture normal and the direction of the waveguide sides, i.e. the waveguide axis. The waveguide phase velocity differs from that of a TEM wave, which explains the beam direction. In a microstrip line, a quasi-TEM mode propagates, suggesting that a cut-angle microstrip port might produce a deflection from the aperture normal to the direction defined by the "guide" sides.

Figure 4 shows schematically an experiment to test the above idea, in which an open-ended waveguide is used as a convenient launcher to excite the air-spaced "microstrip" line. The angle of cut is defined by the relation between the waveguide axis and the parallel plate edge. A circular array of open-ended waveguides is used to determine the beam pointing direction, which follows the "guide" sides.

In Figure 4 we see a coaxial cable feeding a waveguide 2 arranged to launch the radio signal-usually microwave--into the region between top and bottom metal plates 3 and 4. The waveguide ends in a metal sheet 5, which represents a microstrip port, and whose end can be angled as desired for testing beam direction. The bottom plate is extended as shown at 6.

In a microstrip lens, the port has to be flared from a thin 50 ohm line to an aperture of about 0.5 to 1.0 Am for an array port. Figure 5 is a view of a test lens constructed to test different cut angles for the microstrip ports, as well as ports which are (a) flared and then angled after a straight section, (b) flared and angled immediately after. In this figure the dark area represents the region inside the lens arrangement, and seven launchers It to 17 are shown with different angles of cut as indicated. There are also nine outlet ports O to Og and some dummy ports connected to the other sides of the structure.

The potential advantages of (b) above are (I) a shorter transition and (II) there are less problems with mutual coupling for near parallel ports. The two port types give very similar results, so type (b) was adopted for the Rotman lens arrangement according to the invention. The deflection angles of the beams were found to be within 5 of what was expected, for angles of cut of up to 30 .

Figure 6 shows a preferred form of Rotman lens for producing four beams from a seven-element array, the elements of which are coupled respectively to seven array ports Al to A7. Note that in this figure the region inside the lens arrangement is represented by the lighter colour. The seven array ports are designed as set out above. The 50 ohm line lengths are unequal due to (a) the Rotman "W" differential (see the Rotman and Turner paper) and (b) the variation in electrical lengths of the array ports with position. There are apparently eleven beam ports, but these are fed in sets of four, with a shift of two ports between centres. Thus we feed ports 1+2+3+4, then 3+4+5+6, then 5+6+7+8, and so on. The eleventh port is a dummy port to maintain approximate symmetry. The doubled use of ports entails a 3 dB loss in an "overlap network" of hybrid junctions, which is used to create beams with low sidelobes and high cross-overs.

Figure 7 shows the multiple beam patterns (array factors) predicted using two-dimensional aperture theory, and Figure 8 shows the array factors calculated using the measured amplitudes and phrases from the microstrip lens at the design frequency. The poorer side-lobes for one beam in the particular example are due to a manufacturing error which caused a skew amplitude distribution for the beam. This error is overcome in the production version. Swept frequency results indicate that a good performance is maintained over a 25% bandwidth. The lens insertion loss, ignoring the overlap network, is 1.5 to 2.0 dB.

Claims (4)

1.A A radio-frequency microstrip lens arrangement, which includes two flat plates of electricallyconductive material located parallel to each other and spaced apart by a distance less than half the wavelength of the radio frequencies to be dealt with, wherein two sides of the parallel structure are curved outwards so that in plan view the structure resembles a convex lens or a barrel, wherein at one of the curved sides there is located a number of radio frequency launchers for launching radio signals into the region between the plates, wherein at the other of the curved sides there is located a number of radio frequency pick-ups, wherein the launchers or the pick-ups are so arranged that for each said launcher or pick-up its beam is launched or picked up at an angle to its axis, the said angles being such as to produce the desired scanning characteristics for the arrangement, and wherein when the arrangement is in use the radio frequency launchers may be pulsed successively to cause the radio beams to scan across the pick-ups.

2. A radio-frequency microstrip lens arrangement, which includes two flat plates of electrically conductive material located parallel to each other and spaced apart by a distance less than half the wavelength of the radio frequencies to be dealt with, wherein two sides of the parallel structure are curved outwards so that in plan view the structure resembles a convex lens or a barrel, wherein at one of the curved sides there is located a number of microstrip transitions which form radio frequency launchers (or pick-ups) for launching radio signals into the region between the flat plates (or picking up such signals therefrom), wherein the launching aperture (or receiving aperture) of each of the microstrip transitions is cut at an angle such that the radio frequency signal launched therefrom (or picked up therefrom) is at a defined angle to the aperture normal, wherein the angles to which the various apertures are cut are varied to give the lens arrangement the desired scanning characteristics, wherein at the other of the curved sides there is located a number of radio frequency pick-ups (or launchers), and wherein when the arrangement is in use the radio frequency launchers may be pulsed successively to cause the radio beams to scan across the pick-ups.

3. A radio frequency microstrip lens arrangement, substantially as described with reference to the accompanying drawings.

4. An antenna assembly which includes a lens arrangement as claimed in Claim 1, 2 or 3, and a linear array of radio frequency radiating antenna elements each coupled by a coaxial cable or the like zo one of the said pick-ups.