GB2207005A - Antenna - Google Patents

Abstract

An antenna is formed from a pair of cross dipoles 1A, 1B, 2A, 2B fed by two triplate feed systems 5, 6 sharing a common ground plane 10. <IMAGE>

Description

Antenna This invention relates to antennas of the type including a triplate fed cross dipole.

Cross dipoles are well known and are used to transmit and receive circularly and elliptically polarised radiation.

If it is desired to feed such a dipole from a triplate feed system the resulting assembly is complex, and thus expensive, to manufacture because each of the dipoles must have a triplate feed structure and because the two dipoles are generally physically different in order to allow their attachment to their respective triplate feeds.

This invention is intended to provide a triplate fed cross dipole at least partially overcoming these problems.

This invention provides an antenna comprising a pair of crossed dipoles connected to two triplate feeds sharing a common ground plane.

Figure 1 shows a top view of an antenna element constructed in accordance with the invention, Figure 2 shows a side view of the antenna element of Figure 1, Figure 3 shows an array made up of a plurality of antenna elements like that of Figure 1, Figure 4 shows a perspective view of one arm of the dipole of Figure 1, Figure 5 shows a cross section along the line x-x of Figure 2 and Figure 6 shows the internal feed arrangement of the triplates used in the dipole of Figure 1, identical parts having the same reference numerals throughout.

A first dipole 1 is formed by arms 1A and 1B. A second dipole 2 is formed by arms 2A and 2B. All four arms, 1A, 1B, 2A and 2B have a base section 3 which is perpendicular to the arm 1A, 1B, 2A and 2B.

The arms are each half a wavelength of the operating frequency of the system in height and are arranged to form two dipoles 1 and 2 having half wavelength notches as described in our co-pending patent application no.

8616576. The two dipoles 1 and 2 bisect one another at right angles forming a cross dipole 4.

Each of the dipoles 1 and 2 is fed with signals by one of the triplate feed structures 5 and 6 respectively.

Each of the triplate feed structures 5 and 6 has a central conductor, 7 and 8 respectively, and a pair of ground planes on either side of the respective central conductor. These ground planes are formed by earthed conductive plates 9 and 10 in the case of the triplates 5 and earthed conductive plates 10 and 11 in the case of the triplate 6. The plate 10 forms a common ground plane for both triplates 5 and 6.

A ground reflector is formed by a pair of conductive sheets 14 and 15 secured and electrically connected by bolts, not shown, to the edges of the plates 9 and 11 respectively.

The dipole arms 1A, 1B, 2A and 2B all have their base sections 3 secured and electrically connected by bolts, not shown, to the ground reflector 14 and 15.

The two dipoles 1 and 2 are linked to the central conductors 7 and 8 of the triplates 5 and 6 by lengths of co-axial line 12 and 13 respectively.

The co-axial line 12 has its earthed outer conductor 12A electrically connected to the dipole arm 1A and has its signal carrying core 12B electrically connected to the dipole arm 1B. Similarly the co-axial line 13 has its earthed outer conductor 13A electrically connected to the dipole arm 2A and has its signal carrying core 13B electrically connected to the dipole arm 2B.

The connections of the co-axial line 12 and 13 to the dipole arms 1A, 1B, 2A and 2B are all made approximately half way up the dipole arms.

Referring to Figure 6 the two triplate signal carriers 7 and 8 are both connected to a single signal supply lead 16 which in turn is connected to a signal generating and receiving system 17. The triplate 5 is arranged to provide a longer path to the cross dipole 4 than the triplate 6, this path difference being such that signals from triplate 5 are applied to the dipole 1 90" out of phase with the signals from triplate 6 applied to the dipole 2. As a result of this phase difference the cross dipole 4 will be able to transmit and receive circularly polarised radiation. A switch 18 will stop signals passing along the triplate 5 on receipt of a control signal on a line 19, this will allow only the dipole 2 to operate and will so allow the cross dipole 4 to transmit and receive linearly polarised radiation should this be considered necessary.

In an array of cross dipoles, as shown -in Figure 3, each conductive plate 9, 10, 11 would form a feed structure for a number of cross dipoles.

Although the triplate feed structure is described with reference to only one type of cross dipole it can be employed to feed any type of cross dipole.

Although the dipole shown is intended to transmit and receive circularly polarised radiation it could easily be arranged to transmit or receive eliptically polarised radiation by altering the phase.

Claims (4)

1. An antenna comprising a pair of cross dipoles connected to two triplate feeds sharing a common ground plane.

2. An antenna as claimed in claim 1 in which the arms of the cross dipoles are all identical.

3. An antenna as claimed in claim 2 in which each dipole is formed from two perpendicular portions.

4. An antenna substantially as shown in and described with reference to the accompanying figures.