GB2305298A - Process for the production of helically wound antenna with a moulded sheath - Google Patents

Abstract

In the production of a sheathed helical antenna by injection moulding a sheath of plastics material around a helical antenna element, the antenna element is formed with a conically tapering helix (21, Fig. 3 not shown) and is located within a mould cavity (2) around a mould core having a section (17, Fig. 2 not shown) that tapers conically at substantially the same cone angle as the helical antenna, so that the conical portion of the mould core serves to maintain the helices of the antenna at the required pitch during injection of molten material into the mould cavity.

Description

PROCESS FOR THE PRODUCTION OF HELICALLY WOUND ANTENNA WITH A MOULDED SHEATH This invention concerns improvements in and relating to the production of helical antennas of the kind provided with a moulded sheath, usually of resilient synthetic plastic material.

Such antennas are commonly used in the field of mobile communications, for example on mobile radio or telephone handsets.

Whilst such antennas are compact and durable, they can suffer from the disadvantage that, as a result of the manufacturing process employed, the pitch of the coils of the antennas may vary beyond acceptable tolerances, causing a corresponding variation in the tuned resonance of the antenna and its performance in a given frequency band.

Various means have hitherto been proposed in the manufacture of such antennas for maintaining a required configuration of the helical antenna during the process of application of the external sheath, for example by injection moulding, but such means have lead to increased complexity in the manufacturing process.

It is further known to provide a helical antenna of tapering configuration, although the reason for providing such a taper has been unrelated to the manufacturing process and has been determined, for example, by stylistic or performance related criteria.

It is accordingly an object of the present invention to provide an improved process for the formation of a sheathed helical antenna in which antennas of consistent quality can be produced in a simple manner.

In accordance with the invention there is provided a process for the production of a sheathed helical antenna wherein an antenna of helically formed wire is located around a mould core within a mould cavity and a sheath of resilient material is formed around the helical wire antenna by injection of molten synthetic plastics material into the mould cavity, the mould core being at least partially of conically tapering configuration, the helical antenna being of corresponding conically tapered configuration in order to fit closely around the mould core, and wherein the molten material to form the sheath is injected into the mould cavity in such a manner that the helical antenna remains in close contact with the mould core, the conical portion of the latter serving to maintain the helices of the antenna conforming thereto at a required pitch.

In accordance with the invention it has been found that a mould core of conical configuration is effective to retain the required pitch of the coils of a spring wire antenna having a taper that corresponds closely with that of the former.

The invention is applicable to a helical antenna of any desired cone angle and it has been found in practice that even a relatively low cone angle, for example between 1.5 and 2", is nevertheless effective in maintaining the desired pitch of the spring and preventing displacement of the helices thereof under the applied pressure of injection moulded material.

Advantageously, the molten material is injected into the mould cavity in the region of the narrower diameter of the conical portion of the mould core so that molten material flows towards the wider end of the mould core.

Thus the flow of molten material serves to tighten the engagement between the helical spring and the mould core, thereby improving the conformation of the spring to the core.

The taper of the mould core additionally improves the subsequent release of the moulded product from the injection mould.

The invention is illustrated by way of example in the accompanying drawings, in which Figure 1 is a fragmentary plan view of one half of an injection mould, showing the configuration of a mould cavity, Figure 2 is a partial side elevation of a mould core for engagement within the mould cavity of Fig. 1 viewed in a direction radial to the longitudinal axis thereof, and Figure 3 is a similar view of a helically wound spring antenna adapted to conform to and be supported by the mould core of Fig. 2.

Referring to Fig. 1 of the drawings there is shown a fragmentary plan view of a mould block 1 forming one half of an injection mould, the view showing the parting face of the mould block and a mould cavity therein.

As will be understood by one skilled in the art the mould block 1 is identical to another half of the mould, each containing one half of a mould cavity 2 that is symmetrical about a central longitudinal axis 3. Each half of the mould block also contains one half of a passage 4 for the injection of molten material, the passage communicating with the mould cavity 2 via a gate 5 in conventional manner. Although only a part of the mould block 1 is shown, it will be appreciated that each block may contain two identical cavities 2, each of which is fed from the passage 4 via a gate 5.

Each mould cavity 2 comprises a section 6 of cylindrical configuration open to one end 7 of the mould block and an adjoining section 8 that merges with the cylindrical section 6 and tapers conically along the longitudinal axis 3 towards a rounded end 9 connecting with the gate 5. The junction between the cylindrical section 7 and the conical section 8 is indicated at 10.

Referring to Fig. 2 there is shown a mould core to be supported within the mould cavity illustrated in Fig. 1.

The mould core is also symmetrical about a central longitudinal axis 11 and comprises a cylindrical section 12 the external diameter of which corresponds exactly with the internal diameter of the section 6 of the mould cavity. The cylindrical section 12 terminates in a shoulder 13 that, in use, is arranged to register with the junction 10 of the mould cavity. The other end of the cylindrical section 12 is connected to a portion, not shown, by means of which the mould core is located relatively to the mould block 1 and is moved relatively thereto during the injection moulding process, in known manner, for removal of the injection moulded product.

The mould core further comprises a cylindrical section 14 adjoining the shoulder 13 and of smaller diameter than the section 12 and a further cylindrical section 15 connected to the section 14 by means of a radiused shoulder 16. A conically tapered section 17 of the mould core has a larger diameter end that is of slightly smaller diameter than the shoulder 15 and is connected to the latter by an obliquely angled step 18. The smaller diameter end 19 of the section 17 terminates in a radiused portion as shown.

Referring to Fig. 3 of the drawings, there is illustrated an antenna comprising a helically wound spring formed, for example of spring steel wire. As shown, the antenna comprises a tightly wound section 20 formed from five closely wound turns of wire all having the same external diameter, and a section 21 that is helically wound with an open pitch and a tapering configuration. At the narrow end 22 of the antenna a free end of the wire is angled inwardly towards a central axis 23 of the spring for a purpose to be described below.

As shown in chain dotted lines 24 the internal circumference of the section 21 of the wire spring conforms closely to a frustum of a cone having the same dimensions as the section 17 of the mould core shown in Fig. 2. Furthermore, the internal diameter of the closely wound section 20 corresponds closely to the external diameter of the section 15 of the mould core, whereas the external diameter thereof corresponds closely to the diameter of the section 14. The axial distance between the section 20 and the inturned portion at the end 22 of the wire spring is equal to or slightly less than the axial length of the section 17 of the mould core.

In use, a wire spring as shown in Fig. 3 is located on a mould core as shown in Fig. 2 with the cylindrical section 20 located over the cylindrical section 15 of the mould core, the section 21 in close contact with the section 17 and the inturned end of the wire at the narrow end 22 of the spring in contact with the radiused portion 19 of the mould core. The mould core is inserted in the mould cavity 2 with the shoulder 13 in register with the junction 10 and the two halves of the mould are closed so that the cylindrical section 6 of the cavity is fully closed by the section 12 of the mould core. Molten material is injected through the passage 4 and the gate 5 into the section 8 of the mould cavity and thus forms a sheath surrounding the exterior of the spring antenna.

It will be appreciated that the sheath moulded in this manner extends beyond the outer circumference of the spring, having a wall thickness defined by the difference between the external dimensions of the spring and the internal dimensions of the cavity section 8. It will thus be seen that this wall thickness corresponds approximately to the difference between the radius of sections 14 and 12 of the mould core.

In practice it has been found that a sheathed antenna manufactured in this manner can be produced with lower variation in dimensional tolerances and consequent variation in tuning, as a result of the fact that the tapered wire spring is caused to conform closely to the section 17 of the mould core during the injection moulding process.

The invention is not limited to the particular embodiment illustrated or to any given angle, pitch or other dimension of the spring antenna.

In one example, however, the wire spring of Fig. 3 might be formed of spring steel wire of lmm diameter, the formed spring having a length of approximately 40mm and a tapering cone angle of 1.7". The pitch of the tapering helices of the section 21 of the spring may typically be between 5 and 6mm whereas the external diameter of the section 20 of the spring might be approximately 7.5mm and the largest internal diameter of the mould cavity 8 might be approximately lOmm, the cone angle of the section 8 of the mould cavity being approximately 1.5 . It will be appreciated that these dimensions are given for the purposes of illustration only and that the invention is not limited thereto. Further modifications may also be made to the illustrated embodiment without departing from the scope of the invention. For example the end 22 of the spring may terminate as a helix without the inturned end referred to, in which case this end of the spring is located solely by the conical section 17 of the mould core and does not engage the end 19.

Claims (4)

Claims:

1. A process for the production of a sheathed helical antenna wherein an antenna of helically formed wire is located around a mould core within a mould cavity and a sheath of resilient material is formed around the helical wire antenna by injection of molten synthetic plastics material into the mould cavity, the mould core being at least partially of conically tapering configuration, the helical antenna being of corresponding conically tapered configuration in order to fit closely around the mould core, and wherein the molten material to form the sheath is injected into the mould cavity in such a manner that the helical antenna remains in close contact with the mould core, the conical portion of the latter serving to maintain the helices of the antenna conforming thereto at a required pitch.

2. A process according to Claim 1, wherein the molten material is injected into the mould cavity in the region of the narrower diameter of the conical portion of the mould core so that molten material flows towards the wider end of the mould core.

3. A method according to Claim 1 or 2 substantially as described herein with reference to the accompanying drawings.

4. A helical antenna when formed by the process of any one of Claims 1 to 3.