GB2140974A - Microstrip planar feed lattice - Google Patents

Abstract

A microstrip planar feed lattice feeds microwave energy to radiating elements in a two dimensional array. The lattice is formed of equal length microstrip line segments (21, 22) arranged in equi-angular axial symmetry about intersection points. The line segments may form equilateral triangles, squares or regular hexagons. Radiating elements may be formed by making some line segments thinner than others, or by providing stubs. <IMAGE>

Description

SPECIFICATION Microstrip planar feed lattice The present invention relates generally to microwave antennae and in particular to antennae made up of two dimensional arrays of radiating elements.

Linear microstrip arrays are known. See for example "Microstrip antennas and arrays, Part 1 Fundamental Action and Limitations", by J. R. James and G. J.

Wilson, in IEE J. "Microwave optics and acoustics, number 5, September 1977, pages 165 to 174; "Linearly Polarised Microstrip Antennas", by A. G.

Derneryd, in IEEE Trans. Antenna Propagation, AP24, November 1976, pages 846 to 851.

It has also been proposed to make a two dimensional array of radiating elements using microstrip radiators. However, hitherto such two dimensional arrays have typically been made up of a parallel set of linear arrays with a corporate feeding network provided to supply microwave energy to the various linear arrays. Such corporate feed networks provide significant problems due to spurious radiation in the network, large antenna size, low efficiency and manufacturing complexity. Radiation from the feed network itself can significantly degrade the performance of the entire array.

According to the present invention, there is provided a microstrip planar feed lattice for feeding microwave energy to radiating elements in a two dimensional array, the lattice being formed of equal length microstrip line segments arranged in equiangular axial symmetry about intersection points.

A lattice of this form can be made which can provide a resonant feed to the various radiating elements of a two dimensional array with substantially zero radiation at the various intersection points on the lattice itself. It will be understood that a microstrip line intersection point with equi-angular axial symmetry does not radiate if all the line segments meeting at the intersection point have the same width. Various symmetrical lattice structures can therefore be formed which can distribute microwave energy throughout the extent of the lattice with substantially no radiation at intersection points of the lattice, provided the length of the microstrip line segments is chosen to be equal to a whole number of half guide wavelengths for the microwave frequency in question.In this context, of course, the guide wavelength is the wavelength of microwave energy at the frequency in question when conducted along the microstrip line.

In various forms of the invention, the line segments may form equi-lateral triangles, squares or regular hexagons.

A planar array antenna may be formed comprising the feed lattice mentioned above together with radiating elements formed to radiate microwave energy in the lattice at selected points forming said two dimensional array, and coupling means for coupling to the lattice microwave energy at a frequency such that the length of said microstrip line segments is nag/2, where Xg is the line wavelength of said energy along the microstrip lines and n is an integer.

In one arrangement of the antenna, said microstrip line segments forming the lattice may have equal widths and the radiating elements may then be formed by additional microstrip line stubs. The stubs may be connected to the lattice at at least a selection of intersection points. However alternatively, the stubs may be connected to the lattice at points midway along selected said microstrip line segments, the frequency of the microwave energy then being such that the length of the segments is Xg.

In another arrangement, said radiating elements are each formed by providing one of the microstrip line segments at an intersection point of the lattice of a different width to the other said segments at the intersection point. This latter arrangement has the advantage that the coupling coefficient of microwave energy from the lattice to free space by radiation at each of the intersection points can be adjusted by selecting the width of the microstrip line segment. The coupling may be reduced to zero if the microstrip line segment is made exactly the same width as the other segment at the intersection point.

This may be useful in shaping the coupling of radiation across a two dimensional array antenna for example for reducing side lobes.

Examples of the present invention will now be described by reference to the accompanying drawings in which: Figures 1, 2 and 3 illustrate three arrangements of symmetrical feed lattice embodying the present invention; Figures 4 - 7 illustrate four different planar array antennae employing a square feed lattice; Figures 8 and 9 illustrate planar array antenna employing a triangular feed lattice; Figures 10, 11 and 12 illustrate planar array antenna employing a hexagonal feed lattice; Figure 13 is a plan view of a planar array antenna formed in microstrip and Figure 14 is a sectional view illustrating a microwave connection between co-axial feeder cable and the array antenna of Figure 13.

Figure 1 illustrates a microstrip planar feed lattice in which equal length microstrip line segments 10 are arranged to form squares a shown. All the internal intersection points 11 of the lattice are symmetrical in the sense of being formed by microstrip line segments arranged in equi-angular axial symmetry about the intersection point. If the widths of the microstrip line segments making up the lattice are all the same about each internal intersection point 11, no radiation takes place from the intersection points 11 when the lattice is energised with microwave energy. It can be seen that if the frequency of the microwave energy fed to the lattice is chosen such that the length of the microstrip line segments 10 is equal to no912, the RF voltage fed to the lattice at one of the intersection points 11 is repeated at all the other intersections.The lattice is resonant at the particular microwave frequency.

In the square lattice of Figure 1, radiation can take place at intersections 12 between line segments around the boundary of the lattice, where the junctions are asymmetrical. However the radiation from the boundary junctions may be limited if the length of the line segments is X,/2, so that radiation from alternate junctions 12 is out of phase. Net radiation from the boundary junctions should be low because the spacing of adjacent junctions is small (less than half the free space wavelength of the energy).

Figure 2 illustrates the triangular lattice arrangement in which internal points 13 are formed by the junction of six axially symmetrically distributed microstrip line segments 14. With this arrangement it is necessary to make the length of the line segments 14 equal to nAg, in order to avoid a polarity contradiction around any given triangle. In this arrangement radiation at consecutive asymmetrical junctions around the boundary can be of similar phase.

Figure 3 illustrates the hexagonal lattice arrangement in which microstrip line segment lengths may be made Xg/2. Radiation from the asymmetrical junction points along the upper and lower boundaries (in Figure 3 as shown) indicate good cancellation for vertically polarised radiation components but horizontally polarised components are in phase so that radiation may be expected. The vertical boundaries in Figure 3 have horizontally polarised radiation which is in phase.

Reverting now to Figures 4, 5, 6 and 7, various planar resonant array antennae are illustrated employing the square lattice feed of Figure 1. In Figure 4, radiation is provided at each of the internal intersection points 11 by forming alternate horizontal extending microstrip line segments 15 of slightly greater width than the remaining interconnecting line segments 16 and 17. As a result, radiation will take place from each of the internal intersections 11 and can be seen to be in phase at all intersections 11 for a line segment length of At/2. For this array, radiation around the boundary is minimal since adjacent radiating points radiate at opposite phase.

Radiation at the internal intersections 11 can be made as small as required by modifying the size of the thicker lines 15. It can be seen that radiation from each internal intersection 11 can be reduced to zero in the limit since these intersections are symmetrical.

By selecting various line widths across the two dimensional array of Figure 4, the radiation across the array can be tapered in amplitude e.g. reduced from a maximum at the centre of the array to a minimum at the boundaries of the array. Such tapering may be employed to reduce side lobes in the resultant array antenna.

It will be appreciated that the input conductance of the resultant array antenna to microwave energy is the sum of the individual radiator conductances.

Instead of making the lines 15 wider than the other line segments of the lattice, the lines 15may be made narrower. Figure 5 illustrates a limiting form of this embodiment of the invention in which the lines 15 have been made so narrow as to disappear altogether.

Figure 6 illustrates an array antenna in which half wave resonators 18 are connected to each of the intersection points of the array, the width of the line segments of the array itself being constant throughout the array so that only the resonators 18 produce radiation.

Figure 7 illustrates a further example using halfwave resonators connected mid way between horizontal line segments of the lattice, where the line segment length of the lattice is made equal to Xg.

The triangular feed lattice arrangement of Figure 2 may be used to feed halfwave resonators connected to the lattice either at the intersection points of the lattice as shown in Figure 8 or halfway along the horizontal line segments as shown in Figure 9.

Figure 10 illustrates the use of the hexagonal lattice arrangement in Figure 3 with radiators formed by thicker line segments 19. As an alternative, Figure 11 shows the hexagonal lattice arrangement in which radiating elements are provided by half wavelength resonators 20 connected at selected intersection points of the lattice. In each of these arrangement, radiation polarisation is Ehorizontal and boundary radiation will also be essentially E horizontal.

Figure 12 illustrates another form of hexagonal lattice radiator employing half wavelength resonating stubs.

Figure 13 illustrates in more detail a resonant array antenna employing the hexagonal lattice arrangement particularly shown in Figure 10. Horizontal line segments 21 are made slightly thicker than the slanting line segments 22 so that radiation takes place at each of the internal junctions of the lattice.

The lattice illustrated is formed by a copper conducting layer provided on an insulating substrate preferably of a low loss material. The substrate thickness is about 1/32". The length of the line segments in the honeycomb structure of the lattice was selected to correspond to Xg/2 at 13.325 GHz.

The width of the sloping line segments 22 as shown in Figure 1 should be minimised to reduce spurious radiation from the array boundary. However, there is a practical minimum width of 0.7 mm which corresponds to 100 ohms impedance for the substrate used. The radiation conductance for each equi-angled symmetrical junction in the array is determined by the width of the horizontal lines. The sum of the individual conductances should be made about 20 ms for a good match to 50 ohms. In the present example, the width of the horizontal line segments 21 was made the same over the entire array so that amplitude of radiation from each of the junctions should be constant over the array, except at boundary edges.

Figure 14 illustrates the method of connection of a coaxial feeder to the array of Figure 13 to energise the array with appropriate microwave energy. A threaded bush 23 is bonded to the ground plane 24 of the substrate 25 and a coaxial conductor 26 is connected by means of a standard coaxial receptacle 27 screwed into the bush 23. Connection between the outer of the receptacle 27 is made directly by pressure contact against the ground plane 24. The core 28 of the coaxial conductor is fed through a hole drilled in the substrate 25 at an internal intersection of the lattice of the array, and the core is then soldered to the microstrip line at the junction. It is important that the connector bush 23 is accurately concentric with the hole drilled through the substrate 25.

The horizontal.line segments 21 of the antenna were made to be 1.3 mm.

In testing the array antenna made as above, the voltage standing wave ratio was found to reduce to a very low value (less than 1.25) at a frequency of 12.78GHz. This discrepancy of resonant frequency from the design frequency (13.325GHz) is believed to be due to effects at the junctions of the lattice.

The measured gain of the antenna was about 18.2 dBi.

In the above description, the planar array antenna has been primarily described, for clarity, in the radiating or transmitting mode. It would be appreciated that the antenna is also suitable for reception of microwave energy.

Claims (9)

1. A microstrip planar feed lattice for feeding microwave energy to radiating elements in a two dimensional array, the lattice being formed of equal length microstrip line segments arranged in equiangular axial symmetry about intersection points.

2. Afeed lattice as claimed in claim 1 wherein the line segments form equi-lateral triangles.

3. A feed lattice as claimed in claim 1 wherein the line segment form squares.

4. A feed lattice as claimed in claim 1 wherein the line segments form regular hexagons.

5. A planar array antenna comprising a feed lattice as claimed in any preceding claim, radiating elements formed to radiate microwave energy in the lattice at selected points forming said two dimensional array, and coupling means for coupling to the lattice microwave energy at a frequency such that the length of said microstrip line segments is no912, where Xg is the line wavelength of said energy along the microstrip lines and n is an integer.

6. An antenna as claimed in claim 5 wherein said microstrip line segments forming the lattice have equal widths and the radiating elements are formed by additional microstrip line stubs.

7. An antenna as claimed in claim 6 wherein the stubs are connected to the lattice at at least a selection of said intersection points.

8. An antenna as claimed in claim 6 wherein the stubs are connected to the lattice at points mid-way along selected said microstrip line segments, the frequency of the microwave energy being such that the length of the segments is Ag

9. An antenna as claimed in claim 5 wherein said radiating elements are each formed by providing one of the mirostrip line segments at an intersection point of the lattice of a different width to the other said segments at the intersection point.