GB2048571A - Circularly polarised antenna array - Google Patents

Abstract

A linear array of crossed dipole antennae is formed by shaping the edges of the ground planes 1, 2, of a strip-line system into a series of T shapes. The upright 6 of each T has three slots. The central slot 7 is of length lambda /4 and the other two slots, which are adjacent to it, are of length lambda /8. One of the dipoles of each antenna is formed by the arms 4, 5 of the T and the other dipole is formed by an upstanding rod 20 and a depending rod, which are fixed to arms 11, 12 defined between the long slot 7 and the two short slots. A feed-line 19, between the ground planes 1, 2 is attached to one element of the two dipoles of each antenna in the array. In an alternative arrangement described with reference to Figure 2 (not shown) each antenna of the array has two feed-lines attached to respective dipoles and arranged to feed them in phase quadrature. The use of two such feed-line is however not essential. <IMAGE>

Description

SPECIFICATION Circularly polarised antenna This invention relates to an antenna for radiating or receiving circularly polarised electromagnetic radiation, particularly, but not exclusively, of microwave wavelengths.

The invention provides an antenna for radiating or receiving circularly polarised electromagnetic radiation comprising: a balun divided into two parts by a first space, each part having a second space which divides that part into first and second arms which are respectively closer and farther from the first space; a first dipole, two elements of which are connected to respective first arms; and a second dipole, the two elements of which are connected to respective second arms.

In operation, a feed line supplies energy to one of the first and/or second arms and, providing the dimensions of the first and second spaces are correct, the antenna radiates circularly polarised electromagnetic radiation. Alternatively, if the antenna is being used as a receiver, it will be operative to receive circularly polarised radiation.

For the antenna to operate best the dimensions of the first space are preferably such that the conductive path between the elements of the first dipole is A12 multiplied by a first integer (preferably unity).

The symbol X represents a wavelength at which the antenna is designed to operate and will normally be twice the overall length k/2 of one of the dipoles. The second space is preferably such that the conductive paths between each element of the first dipole and an element of the second dipole is approximately k/4 multiplied by a second integer (also preferably unity).

The balun and the feed/line are preferably formed by a so called strip-line system. A strip-line system comprises an inner conductive strip sandwiched between two ground planes. Each ground plane is formed by a conductive sheet; or a layer of conductive material deposited on an insulating sheet. The inner strip is separated from the ground planes by a suitable dielectric, e.g. air. When a strip-line system is used in implementing the present invention, the balun, and possibly also one of the dipoles, is/are formed by etching or cutting the ground planes to an appropriate shape. The other dipole can be formed by rods or other longitudinal conductors secured conductively to the balun. In such a system the inner strip, which forms the feed line, will be connected by a conductive link to one of the arms of the balun.

One of the advantages of using a strip-line system is that a linear array of antenna can easily be manufactured as an integral unit. In such an arrangement the ground planes are shaped along adjacent edges thereof so as to form a plurality of baluns each connected to an associated crossed pair of dipoles.

Each ofthese baluns is associated with a separate feed line.

Two particular embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure I is a perspective view of an antenna constructed in accordance with the invention and forming part of an array of antenna; Figure 2 is a plan view of part of another antenna, also constructed in accordance with the invention and also forming part of an array; and Figure 3 is a plan view of three of the antenna of the array shown partly in Figure 1.

The antenna array of Figures 1 and 3 comprises a strip-line system having upper and lower ground planes in the form of metal sheets 1 and 2. These sheets 1 and 2 are spaced by spacers 3 so as to form an air gap. Instead of using air as the dielectric it would, in an alternative form of the invention, be possible to use a solid insulating material.

The plates 1 and 2 are cut along their edges to form a series of T shapes, one of which is illustrated in Figure 1 and three of which are illustrated in Figure 3. Each T shape has arms 4 and 5, the distance between the free end of these arms being B/2. Each T shape has an upright 6 which is slightlytapered.

Each upright 6 is divided by a central slot 7 of length h14 so as to form a balun having two symmetrical parts 8 and 9 on opposite sides of lye slot 7.

Each part, 8 and 9, of each balun is divided by a slot 10 of length k/8. This forms, on each balun, two inner limbs 11 and 12 and two outer limbs 13 and 14.

Adjacent inner limbs 12 belonging to respective ground planes 1 and 2, are interconnected by a conductive sleeve 15. They are also connected to a rod 16 which forms one element of a vertical dipole.

The parts 12, 15 and 16 are clamped together by a threaded bolt 17 which screws into the rod 15 and the head 18 of which is countersunk into the arm 12.

The inner limbs 11 are connected to each other, to a conductive strip 19 (which forms the feed-line) and to a second element 20 of the vertical dipole by conductive tubes 21. The parts 11, 19, 20 and 21 are clamped together by a bolt 22. The vertical distance between the free ends of the rods 16 and 20 is k/2.

In operation the slots 7 and 10 provide the correct path lengths to ensure that the currents in the individual elements of each dipole are balanced and to ensure that the currents in one dipole are 90" out of phase with the currents in the other dipole so that circularly polarised radiation is emitted or received bythe antenna. It should be explained here that the term "circularly polarised" is intended to cover situations where the power of the transmitted or received signal is different for the different dipoles.

Indeed in the embodiment shown in Figure 1 the vertical dipole 16, 20 can be expected to transmit or receive a signal of slightly higher powerthan that transmitted or received by the horizontal dipole 4, 5.

The manner in which the slots 7 and 10 act to produce the phase quadrature effect is not fully understood at present but the fact that they do perform this function has been proved experimentally.

Referring now to Figure 2, there is shown a slightly modified version of the antenna of Figure 1 in which the feed-line 22 is divided into two paths 23 and 24, these paths being connected respectively to the vertical and horizontal dipole elements. This en sures, with a greater degree of certainty than with the arrangement of Figure 1, phase quadrature between the two dipoles. In Figure 2, parts corres ponding to those indicated by reference numerals on Figure 1 and 3 are indicated with identical reference numerals with the suffix A.

Claims (12)

1. An antenna for radiating or receiving circularly polarised electromagnetic radiation comprising; a balun divided into two parts by a first space, each part having a second space which divides that part into first and second arms which are respectively closer and farther from the first space; a first dipole, the two elements of which are connected to respec tive first arms; and a second dipole, the two elements of which are connected to respective second arms.

2. An antenna according.to claim 1 in which the first space is such that the conductive path between the elements of the first dipole is k/2 multiplied by a first integer where k is a wavelength at which the antenna is designed to operate.

3. An antenna according to claim 2 in which said first integer is unity.

4. An antenna according to any preceding claim in which each second space is such that the conduc tive paths between each element of the first dipole at an element of the second dipole is approximately A/4 multiplied by a second integer where X is a wavelength at which the antenna is designed to operate.

5. An antenna according to claim 4 in which the second integer is unity.

6. An antenna according to any preceding claims including a feed-line connected to one of the first arms.

7. An antenna according to any preceding claims including a feed-line connected to one of the second arms.

8. An antenna according to any preceding claims in which the balun is formed by a conductive sheet or layer having slots which form the first and second spaces.

9. An antenna according to claim 8 when depen dant on claims 6 or 7 including two similar spaced conductive sheets or layers, the feed-line being formed by a conductive strip between the sheets or layers.

10. An antenna according to claim 8 or 9 in which the or each conductive sheet or layer is shaped to provide the elements of one dipole.

11. An antenna substantially as described with reference to Figure 1 of the accompanying drawings and substantially as illustrated therein.

12. An antenna substantially as described with reference to Figure 2 of the accompanying drawings and substantially as illustrated therein.