INTRODUCTION AND BACKGROUND

This invention relates to an antenna, more particularly a helical antenna.

A helical antenna is an antenna comprising of one or more conducting wires wound in the form of a helix. One known family of helical antennas is the family of axial mode helices where the antenna diameter is more or less 1 wavelength at the frequency of operation and the helix is typically several wavelengths in length. Such antennas have a main axis, a front end and a back end and radiate in axial or end-fire mode with a main beam along the main axis.

Known embodiments of such helical antennas include a uniform diameter helical antenna comprising of a single (unifiliar) helical conductor which is fed at the back end of the antenna and radiates a main beam. Such helical antennas exhibit good gain dependent on the length of the helix, while bandwidth is typically limited to about 20% of a centre frequency of a frequency band of operation. Unifiliar back end fed helices with a tapering helix diameter, but constant inter-turn spacing along the length have also been described. These antennas achieve some marginal increase in bandwidth. Helices with a step change in both diameter and inter-turn spacing are also known, but performance across the operational frequency band is unsatisfactory. Uniform diameter helixes with both a taper in diameter and decrease in inter-turn spacing for the last few turns towards the front end are also known, but once again, give only a small improvement in antenna bandwidth. Bifiliar helical antennas comprising two helical conductors spaced 180 degrees are a different family of helical antennas in that the excitation is applied between the two helical conductors, typically at the front end of the antenna. These antennas often are tapered in diameter and the inter-turn spacing decreases. These antennas cover large bandwidths. They are often referred to as log-spiral or log conical spiral helices. These antennas hence achieve a bandwidth extension, but their gain, when configured as electrically long helices, are much lower than comparable back fed helical antennas. They are also more complex due to the two conductors and require a balanced feed-point at the front end of the antenna.

OBJECT OF THE INVENTION

It is an object of the present invention to provide an alternative helical antenna with which the applicant believes the above problems may at least be alleviated or which would provide a useful alternative for the known helical antennas.

SUMMARY OF THE INVENTION According to the invention there is provided a unifiliar axial mode helical antenna comprising:

- a single wire wound in a helix comprising a plurality of turns around a main axis with adjacent turns having an inter-turn spacing between them, the helix having a back end and a front end and the main axis defining a main beam direction, a transverse cross sectional area of the helix monotonously decreasing from the back end to the front end and the inter-turn spacing monotonously decreasing from the backend to the front end; and

- a feed-point at the back end.

The turns may be substantially circular, each having a respective diameter and wherein the respective diameters decrease from the back end to the front end.

The antenna may have a frequency band of operation or interest having a first lower frequency, a second higher frequency and a centre frequency, and the helix may have a length which is at least two wavelengths of a signal at the centre frequency.

The antenna may comprise p turns comprising a 1 ^st turn at the back end through to a p ^th turn at the front end. A ratio between the diameter of the 1 ^st turn at the back end with the largest diameter and the p ^th turn at the front end with the smallest diameter may be larger than 1 .2:1 and smaller than 3:1 . In one embodiment, a relationship defining the diameter of the turns and their inter-turn spacing is:

where D _n is the diameter of the n ^th turn, D _n+i is the diameter of the turn immediately adjacent turn n towards the front end 16 and S _n is the spacing between turns n and n+1 . S _n+i has a corresponding meaning.

A relationship between the diameter of a turn n and its spacing from a next successive turn n+1 is given by:

Sn

s =—

2 Dn

The diameter of the 1 ^st and largest turn at the back end may be chosen such that:

7G Di = Cl = Kl Amax where:

Ci is the circumference of the 1 ^st turn;

A _max is the wavelength of the lower frequency of the above frequency band; and K1 is a chosen truncation coefficient.

Similarly, the diameter of the p ^th or smallest turn at the front end is given by

7G Dp = Cp = K2 Amin where:

- Cp is the circumference of the p ^th turn;

- A _min is the wavelength of the higher frequency of the above frequency band; and

K ₂ is also a truncation coefficient.

The antenna may be driven at the feed-point at the back end between a ground plane and the largest or 1 ^st turn.

BRIEF DESCRIPTION OF THE DIAGRAMS

The invention will now be described, by way of example only, with reference to the accompanying diagrams wherein:

figure 1 is a side elevation of an example embodiment of a helical antenna;

figure 2 is a perspective view of the antenna connected to a transceiver.

figure 3 is a graph of gain against frequency for comparing performance of a prior art, constant inter-turn spacing (or fixed pitch) tapering antenna and an example embodiment of antenna according to the invention;

figure 4 is a graph of VSWR against frequency for the antennas referred to immediately above;

figure 5 show radiation patterns at 3000MHz for the antennas; and figure 6 show radiation patterns at 5000MHZ for the antennas.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

An example embodiment of a unifiliar axial mode helical antenna is generally designated by the reference numeral 10 in the diagrams.

The antenna 10 comprises a single wire wound in a helix 12 comprising a plurality of turns 1 , 2, 3, n, n+1 , p around a main axis 11 with immediately adjacent turns having an inter-turn spacing between them. The helix having a back end 14 and a front end 16 and the main axis defines a main beam direction.

A transverse cross-sectional area of the helix monotonously decreases from the back end 14 to the front end 16. The inter-turn spacing Si... S _n ... monotonously decreases from the backend 14 to the front end 16. A feed- point 13 (shown in figure 2) is provided at the back end 14. The antenna 10 comprises a ground plane 18 and a pillar 20 for supporting the arrangement.

Each turn has a respective transverse cross-sectional area and an inter-turn spacing S _n between a turn n and an immediately adjacent turn n+1 in a direction towards the front end 16. In a presently preferred embodiment, the turns are substantially circular, each having a respective diameter Di, ... D _n ,

Dn ₊₁ , Dp.

In this preferred embodiment, a relationship defining the diameter of the turns and their spacing is: where D _n is the diameter of the n ^th turn, D _n+i is the diameter of the turn immediately adjacent turn n towards the front end 16 and S _n is the spacing between turns n and n+1. S _n+i has a corresponding meaning.

A relationship between the diameter of a turn n and its spacing from a next successive turn n+1 is given by:

Sn

s = —

2 Dn

In an example embodiment, it may be desired to cover a frequency band extending from fmin to fmax and having a centre frequency f _c . The diameter of the 1 ^st or largest turn at the back end 14 is chosen such that:

7G Di Cl = Kl Amax where:

- Ci is the circumference of the 1 ^st turn;

- A _max is the wavelength associated with f _min ; and

K1 is a chosen truncation coefficient.

Similarly, the diameter of the p ^th or smallest turn at the front end 16 is given by

7G Dp = Cp = K2 Amin where:

- Cp is the circumference of the p ^th turn;

- A _min is the wavelength associated with f _max ; and - K ₂ is also a truncation coefficient.

The antenna may be driven at feed-point 13. In figure 2, a transceiver 22 is provided connected to the feed-point. The antenna may be a transmitting and/or a receiving antenna.

In figures 3 to 6 there are self-explanatory diagrams for comparing performance of a prior art, constant inter-turn spacing (or fixed pitch) tapering antenna and an example embodiment of an antenna according to the invention in terms of a) gain against frequency, b) VSWR against frequency c) radiation pattern at 3000MHz and d) radiation pattern at 5000MHz, respectively. The prior art antenna is 250mm in length, the constant inter-turn spacing is

10mm, the radius of the 1 ^st turn is 21 mm and the radius of the last turn (or turn at the front end) is 1 mm. The example embodiment of the antenna according to the invention has a length of 250mm, the inter-turn spacing decreases logarithmically from 22mm to 0.5mm, the radius of the 1 ^st turn is 15mm and the radius of the last turn is 2.5mm.

As can be seen in figure 3, the example embodiment of the antenna according to the invention has a far superior gain bandwidth extending from about 2200MHz to 7000MHz. The prior art antenna has a gain bandwidth of from about 2200MHz to 4000MHz. Figure 4 illustrates superior VSWR over the band from 2200MHz to 7000MHz for the example embodiment of the antenna according to the invention. Figure 5 illustrates the radiation patterns of both the antennas at 3000MHz. Figure 6 compares the radiation patterns at 5000MHz and illustrates a superior pattern for the example embodiment of the antenna according to the invention, especially along the main axis, where the prior art antenna exhibits severe degradation.