GB2067842A - Microstrip Antenna - Google Patents

Abstract

A broadband microstrip antenna comprises at least two sheet radiators (3, 6) having the same resonant frequency and having their radiating edges (4, 7) spaced parallel to each other to effect capacitative coupling between them. Only one of the radiators (3) has a feed connection (9) thereto. In one form their opposite edges (5, 8) are spaced g/4 from their respective radiating edges (4, 7) and shorted to the ground-plane. In another form the fed radiator has two radiating edges spaced g/2 apart and radiators coupled to each such edge; in a modification of this form giving circular polarisation, the fed radiator has two further radiating edges spaced g/2 apart and normal to the first two edges and a quadrature feed, radiators being coupled to all four edges. <IMAGE>

Description

SPECIFICATION Improvements in or Relating to Microstrip Antennas This invention relates to microstrip antennas.

In known forms of microstrip antennas, a microstrip board comprising a dielectric substrate has a conductive ground-plane on one face and one or more conducting sheet radiators or its other face, the or each radiator being coupled to a feeding arrangement such as a conductive feedline on the same face as the radiator(s).

The present invention is concerned with those forms of microstrip antenna which comprise at least two such radiators and in which the radiation is from one or more substantially straight edges of each radiator, which edges can be regarded as analogous to radiating slots. The invention therefore principally concerns such radiators which are substantially square or rectangular in plan. In the present invention the radiators are fed in a manner which gives increased bandwidth relative to comparable kinds of microstrip antennas.

According to the present invention a microstrip antenna comprises: a microstrip board comprising a dielectric substrate having at least two conducting sheet radiators of the same resonant frequency on a first face thereof and a conductive ground-plane on its second face; each said radiator including at least one substantially straight radiating edge and a said edge of a first of said radiators being spaced at least approximately parallel to a said edge of a second of said radiators to provide capacitative coupling between the two edges; and a feed connection to the first only of said radiators so that the second radiator is fed only by said capacitative coupling to said first radiator.

The above-defined arrangement, in which the second radiator is fed parasitically from the first radiator, is to be contrasted with existing practice in which adjacent radiators are fed independently and separately.

The antenna may comprise more than two radiators, eg the first radiator may have a plurality of radiating edges each capacitatively coupled to a respective radiating edge of a said second radiator.

The radiators themselves may be of known forms. For example they may comprise sheet radiators whereof two opposite radiating edges are spaced apart by a half-wavelength in the dielectric material, or radiators whereof a single radiating edge is spaced a quarter-wavelength from an edge which is shorted to the groundplane. Antennas according to the present invention may comprise a mixture of different forms of radiators.

The feed connection may likewise be of a known kind, for example a conductive feed-line on said first face of the substrate connected to the first sheet radiator. Alternatively the feed connection may be made through the substrate to a suitable point on the radiator. The position of connection to the radiator is determined in the usual way by matching considerations.

A particular form of antenna according to the invention, which approximates to a single slot and thus gives a broad E-plane polar diagam, and also has good bandwidth, comprises two radiators each having a single radiating edge spaced a quarter-wavelength from an edge which is shorted to the ground-plane, their radiating edges being aligned and capacitatively coupled as aforesaid and a feed connection being made to one only of the radiators.

To enable the nature of the present invention to be more readily understood, attention is directed, by way of example, to the accompanying drawings wherein: Fig. 1 is a perspective diagram of one form of antenna embodying the present invention.

Fig. 2 is an equivalent circuit diagram of the antenna of Fig. 1.

Fig. 3 is a circuit diagram reduced from Fig. 2 for one resonant mode of the antenna.

Fig. 4 is a circuit diagram reduced from Fig. 2 for a second resonant mode of the antenna.

Fig. 5 is a perspective diagram of two other forms of antenna embodying the present invention.

In Fig. 1 a section of microstrip board comprises a dielectric substrate 1 having a conductive ground-plane (not shown) on its under face 2. On its upper face is a first conducting sheet radiator 3 having an open-circuit radiating edge 4. Its opposite edge 5 is spaced A/4 (in the dielectric material) therefrom and is shorted to the ground-plane through the substrate 1. As shown, the edge 5 is shorted continuously along its length, but this is not essential; shorting can be effected by a series of pins or wires spaced along the edge.A radiator of this kind is known per se; see for example Sanford C G and Klein L, "Recent developments in the design of conformal microstrip phased arrays", IEE Conf on Maritime and Aeronautical Satellite Communications and Navigation, 7-9 March 1978, IEE Conf Publn No l60,pp 105-108.

Spaced from and parallel to edge 5 is the corresponding edge 7 of a second such radiator 6 whose opposite edge 8 is likewise shorted to the ground-plane. Radiator 3 is fed in a known manner at point 9 by a wire connection which extends through the substrate, suitably the central conductor of a coaxial cable whose sheath is connected to the ground-plane. There is no feed connection to radiator 6, which is energised solely by the capacitative coupling which exists between edges 4 and 7. Radiator 3 may alternatively be fed by a conducting feed-line extending over its shorted edge 5 as shown in Fig 3 of the above paper by Sanford and Klein, or connected to one or other of its edges 10.

As is well known, the radiating edge 4 of radiator 3 can be regarded as a magnetic current source, the current magnitude N being mathematically equal to the voltage V between the edge and the ground-plane. This current, and the capacitatively induced similar current at edge 7, are represented by the arrows M in Fig. 1. In Fig. 1 the currents are shown in phase, corresponding to the voltages V between the respective radiator edges and the ground-plane being in anti-phase, which is the radiating mode as explained hereafter.

Each radiator 3, 6 has a field distribution like that of a l/4 transmission line short-circuited at one end and having a resistance representing the radiation due to the magnetic current source at its other end. The equivalent circuit of the two capacitatively coupled radiators, referred to one open-circuit edge as the input point, is shown in Fig. 2, where L and C are the lumped inductance and capacitance of each corresponding transmission line, C' is the coupling capacitance between the edges 4 and 7, and R, represents the radiation. (The equivalent circuit for each single radiator 3 or 6 in isolation consists of a single said L and C connected in parallel with the same value of R, as for the coupled circuit).

The circuit of Fig. 2 can support both an evenmode resonance (ie the V's for the two radiators in phase) and an odd-mode resonance (ie the V's for the two radiators in antiphase). The circuit of Fig. 2 reduces to that of Fig. 3 in the even-mode case and to that of Fig. 4 in the odd-mode case.

Only in the odd-mode case (Fig. 4) is there radiation from the antenna, because the currents N are then additive, whereas in the even-mode case (Fig. 2) these currents are opposite in phase and cancel out. In the odd-mode (radiative) case the resonant frequency fr is

in the even-mode (non-radiative) case the resonant frequency f, is

Effectively the coupling capacitance increases the tuned circuit capacitance and so reduces the resonant frequency.

The bandwidth of a parallel tuned circuit such as each of the two shown in Fig. 4 is proportional to wL/R, (w=2nf,). The ratio of the bandwidth B of the arrangement of Fig. 1 to that of a single radiator 3 or 6 is therefore WL wL =2:1 Rrl2 Rr Rr Another useful comparison is with a known single square or rectangular sheet radiator in which both of two opposite open-circuit edges are radiating edges and are spaced a halfwavelength (in the dielectric material) apart.

Radiators of this kind are described for example by Howell, J. Q, "Microstrip antennas", IEEE Trans, Vol AP-23, No. 1, pp 90-93, Jan 1975; and by Derneryd, A G, "Linearly polarised microstrip arrays", IEEE Trans, Vol AP-24, No 6, pp 846-851, Nov 1976. The feed in this instance is by feed-line to either radiating edge or through the substrate to a point spaced from one edge as in Fig. 1. The equivalent circuit in this case is similar to that for a single radiator in Fig. 1; except that L is halved and C doubled; R, is halved to take account of the existence of two current sources (ie two radiating edges) but an additional' sealing factor at (a divisor) is required to allow for the effect of mutual coupling between the sources.The value of t is typically about 1.2 for a substrate dielectric of relative permittivity about 2.5. The value of wL/R in this case is therefore wL12 = 1.2wL Rr/(2x1.2) Rr and the relative bandwidth ratios become: BFig 1 single twin-edge radiator Bsingle Fig 1 radiator =2:1.2:1 The antenna of Fig. 1 therefore in principle has a bandwidth superior to either of the two prior forms described.

In the present invention the bandwidth increase arises largely from the mutual coupling between the radiating edges of the sheet radiators when closely spaced. The present antennas can therefore take various forms and two such variants are shown in Figs. 5 and 6, in which the substrate is omitted for simplicity.

Fig. 5 shows a conventional sheet radiator 22 of the kind described in the immediately preceding paragraph but one, having two radiating edges 11 and 12 spaced A/2 apart. It is fed through the substrate at point 13. In accordance with the present invention two radiators 14 and 1 5, each similar to one of the radiators in Fig. 1, have their open-circuit edges spaced from and capacitatively coupled to the edges 11 and 1 2. The three-radiator antenna thus formed has the known radiation pattern of the radiator 22 alone but approximately double its bandwidth. This antenna can be modified to give circular polarisation, assuming radiator 22 is square, by adding two further radiators 1 6 and 17, similar to radiators 14 and 1 5. at the remaining free edges, and feeding point 18 in quadrature with point 13.

By way of example, one embodiment of the antenna of Fig. 1 has the following dimensions and characteristics: Substrate material: Fluorglas (PTFE impregnated and coated glass fabric).

Substrate relative permittivity: 2.50 Substrate thickness: 3.18 mm Radiator material: copper Radiator 3,6, dimensions: 64 mmx30 mm Edges 4,7, length and spacing: 64 mmxl.5 mm Odd-modefr: 1266 MHz Bandwidth at odd-mode fr: 57 MHz Bandwidth of single radiator 3 or 6 at fr of single radiator: 36 MHz Q 1449 MHz) The bandwidth ratio for the above embodiment is seen to be about 1.6:1 rather than 2:1 as predicted This is due to the dependence of the radiation resistance Rr on frequency, which was not taken into account in the foregoing simplified theoretical analysis. (Rr is proportional to 1/fr2 where fr is the resonant frequency.) It will be seen from the above embodiment that for individual radiators having the same dimensions, fur for the single radiator is substantially higher than for the two coupled radiators (1449 MHz against 1266 MHz). This is because the coupling capacitance C' reduces f,.

Thus a truer comparison is obtained by increasing the A/4 dimension of the single radiator to reduce its fr to that of the coupled pair, when a measured bandwidth ratio greater than 2:1 is obtained (approx 2.40:1). A corresponding effect is obtained with the three-radiator antenna of Figure 5, where a measured bandwidth ratio of approx 2.12:1 is obtained relative to the single radiator 22, for the same fr- It will be appreciated that, although described in relation to their use as transmitting antennas, the present antennas can, as normal, also be used for receiving, and the present invention includes such receiving antennas.

Claims (6)

Claims

1. A microstrip antenna comprising: a microstrip board comprising a dielectric substrate having at least two conducting sheet radiators of the same resonant frequency on a first face thereof and a conductive ground-plane on its second face; each said radiator including at least one substantially straight radiating edge and a said edge of a first of said radiators being spaced at least approximately parallel to a said edge of a second of said radiators to provide capacitative coupling between the two edges; and a feed connection to the first only of said radiators so that the second radiator is fed only by said capacitative coupling to said first radiator.

2. An antenna as claimed in claim 1 wherein the first of said radiators has a plurality of radiating edges and a respective said second radiator is capacitatively coupled to each of a plurality of the radiating edges of said first of said radiators.

3. An antenna as claimed in claim 1 comprising two said radiators, each radiator having a single said radiating edge spaced a quarter-wavelength at the operating frequency from an edge thereof which is shorted to the ground-plane.

4. An antenna as claimed in claim 2 wherein said first of said radiators has two radiating edges spaced a half-wavelength apart at the operating frequency and wherein a respective said second radiator is capacitatively coupled to each of said two edges, each said second radiator having a single said radiating edge spaced a quarterwavelength at the operating frequency from an edge thereof which is shorted to the groundplane.

5. An antenna as claimed in claim 4 for producing circular polarisation wherein said first of said radiators has two further radiating edges spaced a half-wavelength apart at the operating frequency and substantially normal to said firstmentioned two radiating edges, and wherein a further respective said second radiator is capacitatively coupled to each said two further edges, each further second radiator having a single said radiating edge spaced a quarterwavelength at the operating frequency from an edge thereof which is shorted to the groundplane, two feed connections being made to the first of said radiators for feeding it in quadrature.

6. A microstrip antenna substantially as hereinbefore described with reference to the accompanying drawings.