GB2130799A - Structural member for radar apparatus - Google Patents

Abstract

A strut 120 used in forming a framework for a geodesic radome or other radar apparatus has a constant cross-section and longitudinally disposed wires arranged to match the member dielectrically to free space at the radar frequency of the apparatus. The dielectric match ensures that the member 120 is substantially transparent to radiation of the relevant frequency, and avoids the problem of a radar antenna being masked by the strut members. The framework so formed may be covered by a plastics skin to form a radome. The struts may have rectangular, L-shaped or annular cross-sections. <IMAGE>

Description

SPECIFICATION Structural member for radar apparatus This invention relates to a structural member for radar apparatus such as a radome or an antenna structure.

Space frame radomes are employed for environmental protection of large ground radar antennas. Such a radome typically consists of a geodesic space-frame of frame members or struts over which a skin membrane is stretched. The structural material of the frame members or struts may be metal or plastics material, the latter giving improved performance at the lower microwave frequencies. However, the dielectric effect of struts of plastics material is to produce discontinuities in the phase and amplitude of a radar signal, the strut appearing as a capacitative shunt susceptance to the incident radiation.The reflective and adverse phase effects of such discontinuities may become serious for antennas mounted near the surface of a radome, where radar waves from the antennas experience oblique angles of incidence and encounter comparatively small apertures at the radome frame giving rise to the known "Venetian Blind" effect. Similar difficulties are experienced in the provision of focal supports for large Cassegrain objective radar reflectors.

It is know to employ inductive wire grids in attempting to match out the capacitative effects of plastics materials employed in comparatively small radomes, such as aircraft radomes.

However, this approach is difficult and cumbersome to employ in large-scale applications such as space-frame radomes or structural members for large radar antennas.

It is an object of the present invention to provide an alternative means for matching out the dielectric effects of structural members of radar apparatus.

The present invention provides an elongate structural member for radar apparatus, the structural member having a constant cross-section, being composed of plastics material incorporating longitudinally disposed wires arranged such that the member is substantially dielectrically matched at the radar frequency of the apparatus.

The structural member of the invention may conveniently be tested for dielectric matching by insertion of a short length thereof in a waveguide, and by testing for its effect on wave propagation.

The arrangement of wires incorporated in the structural member may then be varied to identify a substantially dielectrically matched cross-section.

This has the advantage that any length of structural member is dielectrically matched if the cross-section is both matched and constant.

Moreover, structures such as radomes of considerable size may be constructed from such structural members without appreciable effects on transmission of radar waves therethrough.

Conveniently, the structural member of the invention is formed of glass-reinforced polyester resin (GRP). In one embodiment, the member is of rectangular cross-section and incorporates either two or three wires. Alternatively, the structural member may be of L-section, or square or circular annular section incorporating six or eight wires.

The longitudinally disposed wires may be continuous or discontinuous, ie periodically broken.

A structural member of the invention may be used in the construction of large space-frame radomes or in focal supports for Cassegrainobjective antennas.

In order that the invention might be more fully understood, embodiments thereof will now be described, by way of an example only, with reference to the accompanying drawings, in which: FIGURE 1; is a perspective view of a rectangular structural member of the invention, FIGURE 2: schematically shows a large geodesic space-frame radome, FIGURE 3: schematicaliy illustrates bridge equipment for measuring a radome transmission coefficient, FIGURES 4 and 5; illustrate model radome positioning relative to the Figure 3 bridge equipment for the purposes of testing radome signal transmission.

FIGURES 6 to 9: are graphs of signal transmission against relative position of signal transmitter horn to model radome centre, and FIGURES 10 to 13: are perspective views of alternative forms of structural members of the invention.

Referring to Figures 1 and 2, a dielectric structural member or strut 10 for a geodesic radome is composed of E-glass reinforced polyester resin (GRP). The strut 10 is of constant rectangular cross-section, having longer and shorter cross-sectional edges 11 and 1 2 respectively 4 and 0.8 inches (10.16 and 2.03 cm) in length. The strut 10 incorporates two longitudinally disposed dielectric matching wires 13 and 14 each located centrally of a respective half of the strut cross-section, ie the centre of each wire 13 or 14 is 1.0 inch (2.54 cm) from a shorter edge 12 and 0.4 inch (1.02 cm) from either longer edge 11.Tests in a wave-guide operated at the appropriate radar frequency indicate that the capacitative shunt susceptance of the strut dielectric is substantially matched by the equal and opposite inductive susceptance of the wires 13 and 14. In Figure 2, there is shown a known form of geodesic space-frame radome 20 constructed of struts 21. The radome 20 incorporates a stretched skin membrane (not shown) over the struts 1.

Referring now to Figure 3, there is shown a known form of free space bridge indicated generally by 30 and arranged for measurement of radome transmission coefficients for radar signals.

The bridge 30 has a variable frequency I-band power source 31 connected via a power splitter 32 to a transmitter horn 33 and a 10 ft (3.05 m) coaxial air line 34 acting as a comparison reference. Power from the transmitter horn 33 flows via a 10 ft (3.05 m) air space 35 to a receiver horn 36. The air line 34 and receiver horn 36 are connected to a frequency converter 37 and thence to a network analyser 38. The horns 32 and 35 are square and of 4 inch (10.2cm) diameter. An X-Y plotter 39 has an X input connected to the power source 31 and a Y input connected to the network analyser 38. The plotter 39 indicated the effective impedance between the transmitter and receiver horns 33 and 36 as a function of signal frequency.Tests employing the bridge 30 are performed by placing a test structure over the transmitter horn 33.

The bridge 30 was employed to test the design of the strut 10 when incorporated in a radome of the kind shown in Figure 2. Since such radomes are large of the order of tens of feet (N10 m or larger) in linear dimensions, an accurate 1/10th scale space frame radome was constructed. This model radome or "matched radome" correspondingly incorporated 1/10th scale struts; ie the linear dimensions of the matched radome struts were 1/10th of those described with reference to Figure 1, but were otherwise of equivalent design and incorporated 0.006 inch (0.014 cm) diameter wires. An exact 1/10th scale model of a space frame radome gives the same performance at 10 GHz as a full scale radome operating at 1 GHz.A second model radome was also constructed for comparison purposes, the second being indentical with the first in all respects except that it did not contain dielectric matching wires, and is hereinafter called the "unmatched radome." The model radomes were 30 ft (0.91 m) in height and 6 ft (1.83 m) in diameter. Both the matched and unmatched model radomes were tested with the aid of the bridge 30 of Figure 3 operated in the region of 10 GHz (8 to 11 GHz) to indicate the performances of similarfull-scale radomes in the 1 GHz region.

Referring now also to Figures 4 and 5, there are illustrated model radome/transmitter horn relative positions employed for test purposes.

Tests were performed with each model radome piaced over the transmitter horn 33, ths top 40 of the respective model radome 41 being at one of three vertical levels, 42, 43 or 44, above the centre 45 of the transmitter horn 33. The levels 42, 43 and 44 were respectively 4, 8 and 16 inches (10.2, 20.3 and 40.6 cm) above the centre 45. As indicated in Figure 5, the radome position was adjustable horizontally perpendicular to the line joining the transmitter and receiver horns 33 and 36. Solid and chain-line squares 50 and 51 in Figure 5 indicate central and displaced positions of the transmitter horn 33 within a radome viewed along the line joining the horns.

Using vertically polarized radiation, the signal intensity transmitted from the transmitter horn to the receiver horn was measured using the bridge 30 over the frequency range 8 to 11 GHz for various positions of the matched and unmatched radomes. Test results are indicated graphically in Figures 6 to 9. In Figure 6, relative signal amplitude (dB) is plotted against frequency (GHz) to give two shaded areas 60 and 61 for the matched and unmatched radomes respectively.

Each of the areas 60 and 61 indicates the maximum variation in detected signal intensity obtained by moving the respective radome horizontally as shown in Figure 5, the top of the radome being at vertical level 43 in Figure 4. The chain lines 63 and 64 indicate the detected signal in each case in the absence of a radome. It can be seen from Figure 6 that the signal variation with position for the matched radome is virtually constant over the frequency range at 0.5 to 0.6 dB, whereas the variation for the unmatched radome is 1.1 to 1.5 dB -- considerably greater.

Moreover, the maximum reduction in signal as compared to the respective free space level is 0.5 dB for the matched radome as compared to 1.3 dB for the unmatched radome. Accordingly signal intensity is considerably less affected by and less sensitive to the position of the matched radome as compared to the unmatched radome.

Referring now to Figures 7, 8 and 9, Figure 7 shows plots of relative signal intensity against transmitter horn distance or displacement relative to the radome centre, the radome top 40 being at level 42 above the transmitter horn 33.

The direction of displacement is as shown in Figure 5, ie the respective radome is moved perpendicular to the line joining the centres of the transmitter and receiver horns 33 and 36.

Measurements were performed at three frequencies 8.9, 9.9 and 10.5 GHz, and the solid and chain line graphs 71a to 71 c and 72a to 72c respectively indicate the effect of position of the matched and unmatched radomes on signal intensity received by the receiver horn 36. Figures 8 and 9 show similar graphs for model radomes at vertical levels 43 and 44.

Figures 7 and 8 show that the unmatched radome gives rise to peak transmission losses of 1.6 dB and 1.3 dB at vertical levels 42 and 43 respectively, in which the radome top was 4 and 8 inches (10.2 and 20.3 cm) above the transmitter horn 33. In contrast, the losses for the matched radome were below 0.5 dB in all but one case where the value touched 0.6 dB - curve 71 b. Wire matching of the matched model struts was accordingly effective in achieving a considerable reduction in transmission losses as compared to the unmatched radome, and this improvement was produced over a wide range of frequencies and radar wave angles of incidence at the radome or transmitter horn position relative to the radome top.

Referring now to Figures 10, 11, 12, and 13, there are shown alternative forms of matched struts suitable for incorporation in radomes, focal supports for Cassegrain antennas or other structural features associated with radar antennas.

Figures 10 to 1 3 respectively show a rectangular section strut 100 incorporating three matching wires 101, and L-section strut 110 incorporating six matching wires 111, an annular square section strut 120 incorporating six matching wires 121 and an annular circular section strut 130 incorporating eight matching wires 131. In individual cases, the strut section would be chosen on the basis of required mechanical strength and ease of manufacture. The position and number of matching wires incorporated in the strut would then be chosen on the basis of the performance of a test length of strut in a wave guide. An advantage of a constant cross-section strut containing matching wires is that, if wave guide tests indicate that a short test length is adequately matched, any length of such a strut will be so matched. Moreover, it is a simple matter to perform tests on scaled-down models at correspondingly scaled-up frequencies, as hereinbefore described.

A structural member which is perfectly dielectrically matched would necessarily produce no effect on a radar signal. This ideal would not generally be achievable in practice. However, as hereinbefore described it is possible to achieve a substantial degree of dielectric matching by selection on the basis of empirical wave guide trials of structural member or strut sections incorporating matching wires. For practical purposes, such selection is capable of providing substantial improvement in strut performance, or of substantially dielectrically matching a given cross-section by appropriate choice of matching wires in accordance with the invention.

Claims (7)

1. An elongate structural member for radar apparatus, the structural member having a constant cross-section, being composed of plastics material incorporating longitudinally disposed wires arranged such that the member is substantially dielectrically matched to free space at the radar frequency of the apparatus.

2. A structural member according to Claim 1 wherein the plastics material is glass-reinforced polyester resin.

3. A structural member according to Claim 1 or 2 having a rectangular, L-section, or square or circular annular cross-section.

4. A structural member according to Claim 1,2 or 3 incorporating longitudinally discontinuous wires.

5. A structural member according to any preceding claim incoporated in a radome structure.

6. A structural member according to any one of Claims 1 to 4 incorporated in a radar antenna focal support.

7. A structural member substantially as herein described with reference to and as illustrated in Figure 1,10,11, or 13.