BACKGROUND ART In our co-pending International Application PCT/GB98/00680 there is described an antenna having a driven element extending between a feed point and an attachment point to a top loaded element characterised in that the driven element is longer than the distance between the feed point and the attachment point. Typically the driven element is configured to follow a meandering path between the feed point and the attachment point. One example of a meandering path comprises the driven element being at least in part in the form of straight sections joined to give a zigzag configuration.

DISCLOSURE OF INVENTION According to a first aspect of the present invention there is provided an antenna of generally symmetrical form and having a longitudinal axis comprising: a feed point member located at one end of the antenna; an attachment point means at the other end of the antenna to the one end and spaced axially from the one end; an insulator member extending the axial length of the antenna between the feed point member and the attachment point means; a duct extending through the insulator member from the one end to the other end to the one end; and a reactive element of electrical conducting material extending through the duct to define an electrically conducting path linking the feed point member to the attachment point means.

According to a first preferred version of the first aspect of the present invention the attachment point means provides for the attachment of antenna, or at least the reactive element or an extension thereof, to an external component such as a circuit board to provide a ground plane for the antenna.

According to a second preferred version of the first aspect of the present the attachment point means is a member similar in form to the feed point means.

According to a third preferred version of the first aspect of the present invention or any preceding preferred version thereof the reactive element provides that the conducting path length to be longer than a direct distance along the duct between the feed point member and the attachment point means. Typically the reactive element at least in part follows a meandering path along the duct between the feed point member and the attachment point means. Alternatively the reactive element is, at least in part, in the form of a spiral having an axis co-axial with, or parallel to, the longitudinal axis.

According to a fourth preferred version of the first aspect of the present invention or any preceding preferred version thereof the feed point member is provided with a projection for engagement with a wall of the duct to locate the feed point member securely in the duct.

According to a fifth preferred version of the first aspect of the present invention or any preceding preferred version thereof the attachment point means is a member provided with a projection for engagement with a wall of the duct to locate the feed point member securely in the duct.

According to a sixth preferred version of the first aspect of the present invention the antenna of the fourth preferred version has a feed point member which further serves to locate the reactive element, or a part thereof, securely in the duct by means of the projection.

According to a seventh preferred version of the first aspect of the present invention in an antenna according to the fifth preferred version the attachment point member further serves to locate the reactive element, or a part thereof, securely in the duct by means of the projection.

According to an eighth preferred version of the first aspect of the present invention the antenna incorporates circuitry involving inductive and/or capacitative components such as for filtering signals received by or transmitted by way of the reactive element.

According to a second aspect of the present invention there is provided a unit, such as a mobile radio, equipped with an antenna according to the first aspect or any preceding preferred version thereof BRIEF DESCRIPTION OF DRAWINGS An exemplary embodiment of the invention will now be described with reference to the accompanying drawings of an antenna of which: Figure 1 is an external side elevation; Figure 2 is a plan view of Figure 1 in the direction of arrow II ; and Figure 3 is a side sectioned view on section III-III of Figure 2.

MODE FOR CARRYING OUT THE INVENTION The drawings variously show an antenna 11 of small overall envelope size in the form of a drum with a longitudinal axis A.

Upper end 12 of the antenna 11 comprises a coated brass first disc 13 serving as a feed point member for the antenna 11. The first disc 13 has a lower side 14 (as shown in Figure 3) with a central region in the form of a mushroom shaped projection 15. The mushroom shaped projection 15 has a top section 16 whose outside diameter D serves to secure 15 the first disc 13 relative to the rest of the antenna 11 as will be described hereafter.

Lower end 17 of the antenna 11 comprises a coated brass second disc 18 serving as an attachment point member for the antenna 11. The second disc 18 has an upper side 19 (as shown in Figure 3) with a central region in the form of a mushroom shaped projection 20. The mushroom shaped projection 20 has a top section 21 whose outside diameter D serves to secure the second disc 18 relative to the rest of the antenna 11 as will be described hereafter An insulator 23 of nylon extends between the upper end 12 and lower endl7. The insulator 23 is provided with a central duct 24 extending between the first disc 13 and second disc 18. The duct 24 has an internal diameter d which is less than that of external diameter D of projections 15,20. This provides that, on assembling the antenna 11, the first disc 13 and the second disc 18 can be readily driven into self aligning engagement with their respective ends of the duct 24 by way of, respectively, mushroom shaped projections 15,20.

Reactive element 25 extends along the wall of duct 24 between, and is in good electrical contact with, the first disc 13 and second disc 18. The reactive element 25 is of copper and in this case in the form of a helical coil disposed symmetrically about longitudinal axis A of the antenna 11. By being coiled the reactive element 25 provides an electrical path through the reactive element of greater length than the direct distance along the duct between the first disc 13 and the second disc 18. Effectively the reactive element 25 serves to define an electrically conducting path that meanders between the feed point 13 and the attachment point 18.

The mushroom shaped projections 15,20 in addition to providing for ready engagement with the wall of their respective end of duct 24 but also provides for the ready establishment of good electrical contact between their respective discs and the corresponding ends of the reactive element 25.

INDUSTRIAL APPLICABILITY The exemplary embodiment exemplifies a readily manufactured antenna for use with communication equipment such as a mobile telephone. The invention provides for embodiment in a range of different forms and variants Broadly the antenna 11 comprises two conducting discs 13,18 separated by an insulator 23 and linked by a reactive element 25. The size of the discs, the dielectric constant of the insulator 23, and the dimensions and form of the reactive element 25 all effect the resonant frequency of the antenna and its overall performance. In addition the proposed antenna configuration represents a small envelope for a given antenna 20 operating frequency. In the exemplary embodiment the end discs 13,18 are shown as circular in plan with a distinctive section. They are not limited to this for other applications. They can be of any thickness and section and any plan view besides circular such as such as square, 25 rectangular or strip form.

The exemplary embodiment provides for a reactive element to be in the form of a spiral which serves to define a conductive length for the reactive element which is greater than the shortest distance between the feed point member and the attachment member. However in an alternative embodiment the reactive member can provide for the conductive length to be similar to the shortest difference.

The exemplary embodiment shows an antenna having a first disc 13 serving as a feed point member and a second disc 18 serving as an attachment point member. In an alternative embodiment the attachment point member is in the from of a fitting whereby the antenna can be soldered or otherwise attached to a circuit board or other component whereby wireless signals to or from the reactive element are handled.

However the attachment point means provides for the reactive element to be attached to working circuitry the requirement for a good working plane for the reactive element is readily met in the present invention. In yet another embodiment the antenna of the present invention can be used to incorporate circuitry involving inductive and/or capacitative components such as for filtering signals received by or transmitted by way of the reactive element.