GB2221577A - Blade antenna - Google Patents

Abstract

A blade antenna for use mounted on the fuselage of an aircraft is derived from the known patch antenna. The invention combines two antenna elements each comprising a 'live' sheet 9, 11 and an equal ground plane 13, 15 pointing in opposite directions, mounted side by side and fed by a stripline network 21, 23, 25 which maintains the necessary phase relation between the feeds. The ground planes of the feed network 19 and of the antenna elements 13, 15 may be combined (Fig. 3). <IMAGE>

Description

Blade Antenna This invention relates to blade antennas, that is, antennas whose general form is blade or fin like and suitable for use, on aircraft say, where aerodynamic considerations are important.

Microstrip patch antennas, such as shown in Figures 1(a) and 1(b) are used on missiles, for example, where they are fixed flat on the surface and conforming to it. The patch antenna as shown, consists of a conductor layer 1 mounted on a substrate 3 which is backed by a ground plane conductor 5. The ground plane conductor 5 is necessarily of much greater extent than the layer 1 to reduce as far as possible the effect of the inherent un-balance of the antenna. A number of such patch antennas are used singly at different positions around the fuselage to provide coverage in different planes and directions. They essentially give broadside coverage in planes through the missile axis.

The length (L) of the patch is nominally one quarter-wavelength at the operational frequency and since one end of the patch is short-circuited to the ground plane by plated-through holes 7 (for example), the patch provides a radiating open-circuit at the other end, as shown. The electric field E is horizontally polarized when the patch antenna is oriented as shown in Figure 1(b) with the dimension L horizontal and the dimension W vertical. A patch antenna such as that of Figure 1 provides a directional characteristic, in the plane of the paper relative to Figure l(a).

This characteristic is substantially broadside to the patch antenna.

Two such patch antennas arranged back to back unfortunately provide, not an omni-directional characteristic but one having two distinct nulls.

An object of the present invention is to provide a plane-polarized blade antenna operable at radar frequencies and being omni-directional in one plane.

According to the present invention, a blade antenna having a polarization plane to which the blade antenna is generally perpendicular comprises two microwave transmission line sections each having a termination end and a radiating end, the line sections being directed in opposite directions in the polarization plane and each line section comprising a conductive sheet, a ground plane of comparable extent and an intervening substrate, the two line sections at least partially overlying each other, the two conductive sheets being fed in parallel and substantially in phase at their radiating ends, and the arrangement being such as to provide an omnidirectional response characteristic in the polarization plane.

The termination ends may be constituted by short circuits and the length of the line sections be such, in conjunction with the short circuits, as to provide open circuits at the radiating ends at the operating frequency. Alternatively, the termination ends may be constituted by open circuits and the length of the line sections be such, in conjunction with the open circuits, as to provide open circuits at the radiating ends at the operating frequency.

The antenna preferably includes a feed network which comprises a first stripline which divides symmetrically into two second striplines, the second striplines being connected to the conductive sheets respectively, the feed network further including a ground plane for the striplines which extends to form a ground plane for the conductive sheets. The feed network may include a further substrate sandwiched between the first and second line sections, the ground plane for the stripline extending around both sides of the further substrate to provide the ground plane for each of the two line sections. The substrate for one of the line sections may extend to provide a substrate for the striplines. The striplines may be co-planar with one of the conductive sheets, one of the second striplines being continuous with the one conductive sheet.

A blade antenna in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, of which: Figures 1(a) and 1(b) are side and plan views of a microwave patch antenna of known form; Figures 2(a) and 2(b) are perspective view and end elevation respectively, in diagrammatic form, of a blade antenna according to the invention; Figure 3(a) is a perspective view of a modification of the device of Figure 2; Figure 3(b) is a diagrammatic plan view of the antenna of Figure 3(a) illustrating the essential balanced current flow of the antenna; Figure 4 is a diagram of the antenna of Figure 2 or 3 mounted on a surface simulating an aircraft fuselage; Figure 5 shows the return-loss/frequency characteristic of the antenna transmission coupling; Figure 6 shows the antenna beam characteristic in elevation;; Figure 7 shows a similar characteristic in azimuth; and Figure 8 is a perspective view of a further modification of the antenna of Figure 2.

Referring to the drawings, Figure 1 shows a known form of patch antenna as explained above. When mounted on an aircraft for azimuthal viewing the azimuth range is limited to less than a semi-circle, the characteristic being substantially broadside to the antenna in the plane of the drawing in Figure 1. This is commonly inadequate for detection of enemy radars for example. If two such patch antennas are placed back to back in an attempt to overcome this disadvantage two nulls are created, so defeating the object.

The development of an omni-directional blade antenna is illustrated by Figure 2.

Figure 2(a) shows two microstrip elements each of which is derived from a patch antenna such as in Figure 1. These microstrip elements comprise a conductive sheet, i.e. a metal layer, 9 and 11, overlying a ground plane, 13 and 15, of comparable extent, (and in this example of substantially equal extent) unlike that in Figure 1.

Each of these microstrip elements is in effect a section of transmission line extending horizontally and of length a (nominal) quarter wavelength at the operational frequency of interest. The transmission line section is terminated at one end by a short circuit 10, 12 so providing an open circuit at the other, radiating, end. The two antenna sections thus formed are placed side by side, overlying each other but facing in opposite directions. The feed arrangement consists of a 100 ohm transmission line 21 providing a first stripline in conjunction with a ground plane 19, and second striplines 23 and 25 branching from the first and of 50 ohm impedance. These striplines 23 and 25 are so formed as to lie opposite the radiating ends respectively of the two antenna sections.Connections are then made from the striplines to the antenna conductors 9 and 11 by copper wires 29 and 31, the ground plane conductor 19 being relieved at 27 to avoid interference.

The ground plane 19 extends from a position underlying the striplines 21, 23 and 25 to the region between the antenna sections, the ground plane 19 (having a portion 19(a) extending down alongside ground plane 15 and a portion 19(b) extending up again alongside ground plane 13. The various ground planes are then connected by wire conductors 33.

The two antenna sections are in fact formed on substrates 35 and 37 e.g. of material RT Duriod 5870 shown in Figure 2(b) but omitted from 2(a) for clarity. The striplines 21, 23 and 25 and the ground plane 19 are also mounted on a similar substrate.

The antenna of Figure 2 is designed to operate at 2.8 GHz and has the following dimensions. The length of each transimssion line section (9, 11), nominally one quarter-wavelength, is 17 millimetres and the width (i.e. the height in Figure 2) is 41.5 millimetres. The substrate is 1.57 millimetres thick with a dielectric constant of 2.33.

The disparity between the actual line length (17 mm) and the theoretical quarter-wavelength is due to edge effects and can be predicted.

In operation the blade antenna of Figure 2 is mounted from its upper end 41 from the surface of an aircraft fuselage for example, the 1blade1 lying in line with the slip stream. Thus, in Figure 2(b) the main aircraft axis is perpendicular to the paper. A standard stripline coaxial line coupling is fitted to the stripline 21 for connection to a receiver mounted in the fuselage.

Figure 3(a) shows a more compact form of the blade antenna in which the ground planes 13 and 17 are removed and the conductor sheets 9 and 11 closed up to the ground plane portions 19(b) and 19(a) respectively. The short circuiting is in this case provided by 'plated-through-holes' 43 replacing the bridging conductors 10 and 12 of Figure 2(a).

Figure 3(b) shows a diagrammatic plan view of the antenna of Figure 3(a) and illustrates the essentially balanced nature of the construction. The strip-line feeds 23 and 25 at opposing points produce current paths 24 and 26 which complement each other and produce a continuity of path somewhat analoguous to that of a loop aerial. This effect is best achieved by the wholly side-by-side arrangement of Figures 2 and 3 but will be partially apparent in a construction in which the two line sections are relatively displaced, one up in Figure 3(b) and the other down, for example.

In the single patch antenna of Figure 1, a current path around the back of the ground plane is prevented by the comparatively great extent of the ground plane. In the embodiments of the invention being described such problems are overcome by balancing two antennas each against the other, the current paths of each interacting with the other to provide the balance and the resulting omni-directional characteristic without the nulls that would arise from merely placing two patch antennas back to back (i.e.

short-circuited edge to short-circuited edge). Thus it will be seen that the effects of the two antenna sub-units are not merely additive but are essentially interactive. A basically unbalanced feed to the antenna -coaxial cable and stripline - is converted to a balanced antenna without the use of a balun or like transformer and purely by the inherent design of the combined sub-unit structure.

In a further modification shown in Figure 8, the substrate 35 is removed, together with the return part (19(b) of the ground plane.

The conductor sheet 9 is then mounted directly on the feed network substrate 39 and the ground plane 19 is limited to one side only of the substrate 39. The stripline 23 and the conductor 9 can then be formed integrally as parts of the same metal layer.

The embodiments described above have used transmission line in the form of antenna sections having a nominal length of one-quarter wavelength with a short circuited termination. In a modification of the design the section length is made a nominal half-wavelength and the termination (opposite to the radiating or feed-end) is left open circuit. The antenna response is then similar but the gain is improved to some extent. Clearly, the useful line lengths are cyclic and any combination of line length and termination can be chosen which will give a satisfactory radiation pattern.

Referring now to Figures 5, 6 and 7, Figure 5 shows, for the antenna of Figure 2, the return loss characteristic with frequency, for both free space and for the simulated operational conditions in which the antenna is mounted on a large conductive sheet as shown in Figure 4, the sheet being approximately 1300 millimetres by 900 by 6.

In Figure 4 the elevation plane is considered to be the semicircle lying transversely through the blade antenna, the elevation angle 6 being deemed to be + 90" in the horizontal direction and zero degrees in the vertical direction.

The azimuth angle ss extends from -180" in a direction aligned with the antenna blade to +1800 after a full horizontal circle. Thus in operation on an aircraft employed for ground target surveillance, the metal plate 45, or its equivalent conductive fuselage surface, would be mounted horizontally, i.e. on top or preferably underneath the aircraft, with the 0 -180 azimuth (ss) axis along the fuselage.

It may be seen from Figure 5 that, in both the free space and the simulated conditions of Figure 4, the return loss over a band 4% of the operating frequency is better than -10 dB.

Figure 6 shows the peak beam amplitude at an 'elevation' of 90 , i.e. in the horizontal direction - see Figure 4, for the free space situation, but shifted downwards (i.e. away from the metal sheet/fuselage) by 300 for the operational situation. This is in fact a desirable effect for an aircraft flying horizontally and searching for ground radars.

Figure 7 shows the uniformity of the radiation pattern throughout the complete azimuth range -180 to +1800, both for the free space and operational situation.

The antenna of the invention can be seen to provide an efficient omni-directional response while maintaining the necessary slimness of a blade antenna. It will be appreciated that the dimensions of the illustrated devices have been exaggerated for clarity.

Claims (8)

1. A blade antenna having a polarization plane to which the blade antenna is generally perpendicular, the antenna comprising two microwave transmission line sections each having a termination end and a radiating end, the line sections being directed in opposite directions in said operative plane and each line section comprising a conductive sheet, a ground plane of comparable extent and an intervening substrate, the two line sections at least partially overlying each other, the two conductive sheets being fed in parallel and substantially in phase at their radiating ends, and the arrangement being such as to provide an omnidirectional response characteristic in said polarization plane.

2. A blade antenna according to Claim 1, wherein said termination ends are constituted by short circuits and the length of said line sections is such, in conjunction with said short circuits, as to provide open circuits at said radiating ends at the operating frequency.

3. A blade antenna according to Claim 1, wherein said termination ends are constituted by open circuits and the length of said line sections is such, in conjunction with said open circuits, as to provide open circuits at said radiating ends at the operating frequency.

4. A blade antenna according to any preceding claim including a feed network which comprises a first stripline which divides symmetrically into two second striplines, said second striplines being connected to said conductive sheets respectively, said feed network further including a ground plane for said striplines which extends to form a ground plane for said conductive sheets.

5. A blade antenna according to Claim 4, wherein said feed network includes a further substrate sandwiched between said first and second line sections, said ground plane for said stripline extending around both sides of said further substrate to provide the ground plane for each of said two line sections.

6. A blade antenna according to Claim 4, wherein the substrate for one of said line sections extends to provide a substrate for said striplines.

7. A blade antenna according to Claim 6, wherein said striplines are co-planar with one of said conductive sheets and one of said second striplines is continuous with said one conductive sheet.

8. A blade antenna substantially as hereinbefore described with reference to Figures 2, 3 or 8 of the accompanying drawings.