IES66918B2 - Antenna for portable radio communications equipment - Google Patents

Abstract

A retractable antenna for portable radio communications equipment has a radiating structure 10 comprising a helically loaded top section 12 and a linear bottom section 14. To improve the return loss bandwidth in the retracted position of the antenna the top section 12 includes a capacitive load 24 at the top end of the helix 12'. .

Description

This invention relates to a retractable antenna for portable radio communications equipment.

Portable radio communications equipment requires a compact and effective antenna to transmit and receive RF energy, and miniaturisation of electronic communications equipment is creating increased demand for ever smaller antennas. These small antennas must comply with system design requirements of radiation efficiency and good return loss over relatively wide bandwidths. Traditionally in portable radio equipment, antenna miniaturisation has been achieved by helical loading.

In general, while this achieves miniaturisation, it does result in a reduction of return loss bandwidth compared with full length antenna structures.

A typical retractable antenna consists of a radiating structure approximately a half wavelength in electrical length. The radiating conductors comprise a helically loaded top section and a linear bottom section which are joined to form a continuous radiator. RF energy is coupled to or from this radiating structure by means of a helical section connected to the RF stages of the communications equipment.

The helical coupler is approximately a quarter wavelength in electrical length. When the antenna is fully extended, the radiating’ structure is efficiently energised by the helical coupler and radiates as a halfwave dipole. In this extended position, the helical coupler couples to the free end of the linear radiator and typically achieves a 9% bandwidth for 10d3 return loss.

Sc When the antenna is fully retracted, the radiating structure is efficiently energised by the helical coupler and radiates as a quarterwave monopole with the ground plane provided by the communications equipment.

In this retracted position, the helical coupler couples to the helically loaded top section and typically achieves a 9% bandwidth for 5dB return loss. The linear bottom section, being within the ground plane provided by the communications equipment, does not radiate.

In the retracted position of the antenna it is desirable that the return loss be improved over the operating bandwidth. One possible solution to resolving this problem is by adjusting the electrical lengths of the helical and linear sections. However, using this technique any improvement in return loss bandwidth in the retracted position results in a disimprovement return loss bandwidth in the extended position.

It is therefore an object of the present invention to provide a retractable antenna in which the return loss bandwidth in the retracted position is improved.

Accordingly, the invention provides a retractable antenna for portable radio communications equipment, the antenna having a radiating structure comprising a helically loaded top section and a linear bottom section, the top section further including a conducting section providing a capacitive loading to the top section.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which; Figures 1, 1A and IB illustrate a retractable antenna of a type known in the prior art, and ‘e.

Figure 2 illustrates a retractable antenna according to the embodiment of the invention.

In the figures the same reference numerals have been used for the same or equivalent components.

The prior art antenna shown in figures 1, 1A and IB comprises a radiating structure 10 approximately a half wavelength in electrical length. The radiating conductors comprise a helically loaded top section 12 consisting essentially of a helix 12' and a linear bottom section 14 consisting essential of a linear conductor 14', the helix 12' and the conductor 14' being joined to form a continuous radiator. Both the linear bottom section 14 and the helically loaded top section of the radiating structure 10 are approximately a quarter wavelength in electrical length. The radiating structure 10 is covered with an electrically insulating material 16, e.g. a synthetic plastics material, the covering 16 including a hollow cylindrical housing 16’ at the top end to accommodate the helix 121 of the top section 12 and an extension 16 at the bottom end which projects beyond the lower tip 14 of the linear conductor 14' of the bottom section 14.

RF energy/· is coupled to or from the radiating structure by means of a helical coupler 18 which surrounds the radiating structure 10 and which is connected to the RF stages of the communications equipment. The helical coupler 18 is secured just inside the housing 20 of the communications equipment (the region to the right of the housing 20 in figure 1 is assumed to be inside the equipment), and the radiating structure 10 can slide through the helical coupler 18 between an extended position and a retracted position. The position of the helical coupler 18 relative to the radiating structure 10 when the structure 10 is fully extended Is indicated in figure 1A, and its position relative to the radiating structure 10 when the structure 10 is fully retracted is indicated in figure IB (figure IB also shows the internal structure of the helical coupler 18) . In the fully extended position, therefore, the top end of the coupler 18 is about level with the lower tip 14” of the linear conductor 14 ' , and in the fully retracted position the lower end of the helix 12' is received within the larger diameter helix 22 of the helical coupler 18. The extension 16 of the covering 16 serves to mechanically stabilise the radiating structure 10 within the housing 20 when the radiating structure 10 is fully extended. The helical coupler 18 is also approximately a quarter wavelength in electrical length.

When the antenna is fully extended, the radiating structure 10 is efficiently energised by the helical coupler 18 and radiates as a halfwave dipole. In this extended position, the helical coupler 18 couples to the lower tip 14 of the linear conductor 14 and typically achieves a 9% bandwidth for lOdB return loss.

When the antenna is fully retracted, the radiating structure 10 is efficiently energised by the helical coupler 18 and radiates as a quarterwave monopole with the ground plane provided by the communications equipment. In this retracted position, the helical coupler 18 couples to the helically loaded top section 12 and typically achieves a 2% bandwidth for 5dB return lOc$s. The linear bottom section 14, being within the ground plane provided by the communications equipment, does not radiate. As mentioned previously, in the retracted position of the antenna It is desirable that the return loss be improved over the operating bandwidth.

To this end the top portion of the helix 12’ within the top radiating section 12 has been replaced by a conducting section 24, figure 2, which acts as capacitive top loading to the radiating structure 10. In figure 2 the reduced length helix which remains after replacement of the top portion of the original helix 12' by the conducting section 24 is referenced 12. In the present embodiment the conducting section 24 is a metal (e.g. copper) cylinder which has substantially the same outside diameter as the helix 12” and is in electrical contact with the top end of the latter. The cylinder may be solid or tubular.

The capacitive loading provided by the conducting section 24 increases the RP current in the remaining lower portion 12 of the helix which improves the radiation efficiency. This leads to an improvement in the impedance match in the retracted position. This improvement does not adversely impact the impedance match in the extended position as improvement in the radiation efficiency of the top section of the total radiating structure will not degrade the impedance match in the extended position.

Advantageously the above construction can be used over the frequency range lOQMBz to 3GHz which includes the popular cellular frequency bands such as the 880-960MHz band. Approximate dimensions for an antenna constructed as above for operation in the 900MHz range are: Length of linear conductor 14' = 52mm, Axial length of helix 129 = 16mm, Length of cylinder 24 = 9mm, Outside diameter of cylinder 24 - 4mm, Outside diameter of helix 12’ = 4mm, giving an overall length of 77mm for the entire S radiating structure 10. The cylinder 24 may optionally have an axial bore through it, for example of 2mm diameter.

In general the conducting section 24 may be any conducting body which provides a primarily capacitive loading to the top section, as compared to the helix 12' which provides a primarily inductive loading, and should be chosen such that the capacitive loading provided by the conducting section 24 together with the reduced inductive loading provided by the helix 12” provides approximately the same resonant frequency of the top section 12 as the former helix 12'.

Further the conducting section 24 may be located elsewhere along the top section; for example it may replace a middle portion of the helix 129 such that two remaining helix portions 12 lie one on either side of the conducting section 24 along the axis of the top section 12. However, the conducting section 24 should not replace the portion of the helix 12 ’ which is received into the coupler 18 in the retracted position of the antenna.

The electrical length of the conducting section 24 may constitute from approximately 10% to 90% of the total electrical length of the top radiating section 12. Typically, replacing 30% to 50% of the axial length of the helix 12’ by a conducting section 24 results in a return loss improvement in the retracted position to 7dB over a 9% bandwidth.

Claims (5)

1. A retractable antenna for portable radio communications equipment, the antenna having a radiating v structure comprising a helically loaded top section and a linear bottom section, the top section further including a conducting section providing a capacitive loading to the top section.

2. An antenna as claimed in claim 1, wherein the electrical length of the conducting section constitutes from 10 to 90 percent of the total electrical length of the top section.

3. An antenna as claimed in claim 1 or 2, wherein the conducting section constitutes from 30 to 50 percent of the axial length of the top section.

4. An antenna as claimed in any preceding claim, wherein the capacitive load is a conductive cylinder of substantially the same outside diameter as the helix.

5. A retractable antenna for portable radio communications equipment, substantially as described with reference to figure 2 of the accompanying drawings.