HELICAL ANTENNA

Abstract

An 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 has a back end 14 and a front end 16 and the main axis defines a main beam direction. A transverse crosssectional area of the helix monotonously decreases from the back end 14 to the front end 16. The inter-turn spacing S.sub.1 . . . S.sub.n . . . monotonously decreases from the backend 14 to the front end 16. A feed-point 13 is provided at the back end 14.

Claims

1. 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.

2. The antenna as claimed in claim 1 wherein the turns are circular, each having a respective diameter and wherein the respective diameters of the turns decrease from the back end to the front end.

3. The antenna as claimed in claim 1 having an operational bandwidth extending between a first lower frequency and a second higher frequency and having a centre frequency, wherein the helix has a length which is at least two wavelengths of a signal at the centre frequency.

4. The antenna as claimed in claim 2 wherein the antenna comprises p turns comprising a 1.sup.st turn at the back end through to a p.sup.th turn at the front end and wherein a ratio between the diameter of the 1.sup.st turn with the largest diameter and the p.sup.th turn with the smallest diameter is larger than 1.2:1 and smaller than 3:1.

5. The antenna as claimed in claim 2 wherein a relationship defining the diameter of the turns and their inter-turn spacing is: $\tau =\frac{D_{n+1}}{D_n}=\frac{S_{n+1}}{S_n}$ where D.sub.n is the diameter of the n.sup.th turn, D.sub.n+1 is the diameter of the turn immediately adjacent turn n towards the front end and S.sub.n is the spacing between turns n and n+1.

6. The antenna as claimed in claim 2 wherein a relationship between the diameter of a turn n and its spacing from a next successive turn n+1 is given by: $\sigma =\frac{\mathrm{Sn}}{2\mathrm{Dn}}.$

7. The antenna as claimed in claim 4 wherein the diameter of the 1.sup.st turn is given by:
πD.sub.1=C.sub.1=K.sub.1λ.sub.max where: C.sub.1 is the circumference of the 1.sup.st turn; λ.sub.max is the wavelength of the first frequency of the frequency band; and K1 is a chosen truncation coefficient.

8. The antenna as claimed in claim 4 wherein the diameter of the p.sup.th turn is given by:
πD.sub.p=C.sub.p=K.sub.2λ.sub.min where: C.sub.p is the circumference of the p.sup.th turn; λ.sub.min is the wavelength of the second frequency of the frequency band; and K.sub.2 is also a truncation coefficient.

9. The antenna as claimed in claim 4 comprising a ground plane and a pillar mounted on the ground plane for supporting the helix and wherein the feed-point is provided between the ground plane and the 1.sup.st turn.

Description

BRIEF DESCRIPTION OF THE DIAGRAMS

[0025] The invention will now be described, by way of example only, with reference to the accompanying diagrams wherein:

[0026] FIG. 1 is a side elevation of an example embodiment of a helical antenna;

[0027] FIG. 2 is a perspective view of the antenna connected to a transceiver.

[0028] FIG. 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;

[0029] FIG. 4 is a graph of VSWR against frequency for the antennas referred to immediately above;

[0030] FIG. 5 show radiation patterns at 3000 MHz for the antennas; and

[0031] FIG. 6 show radiation patterns at 5000 MHZ for the antennas.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

[0032] An example embodiment of a unifiliar axial mode helical antenna is generally designated by the reference numeral 10 in the diagrams.

[0033] 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.

[0034] A transverse cross-sectional area of the helix monotonously decreases from the back end 14 to the front end 16. The inter-turn spacing S.sub.1 . . . S.sub.n . . . monotonously decreases from the backend 14 to the front end 16. A feed-point 13 (shown in FIG. 2) is provided at the back end 14.

[0035] The antenna 10 comprises a ground plane 18 and a pillar 20 for supporting the arrangement.

[0036] Each turn has a respective transverse cross-sectional area and an inter-turn spacing S.sub.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 D.sub.1, . . . D.sub.n, D.sub.n+1, . . . D.sub.p.

[0037] In this preferred embodiment, a relationship defining the diameter of the turns and their spacing is:

[00003]$\tau =\frac{D_{n+1}}{D_n}=\frac{S_{n+1}}{S_n}$

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

[0038] A relationship between the diameter of a turn n and its spacing from a next successive turn n+1 is given by:

[00004]$\sigma =\frac{\mathrm{Sn}}{2\mathrm{Dn}}$

[0039] In an example embodiment, it may be desired to cover a frequency band extending from f.sub.min to f.sub.max and having a centre frequency f.sub.c.

[0040] The diameter of the a 1.sup.st or largest turn at the back end 14 is chosen such that:

πD.sub.1=C.sub.1=K.sub.1λ.sub.max [0041] where: [0042] C.sub.1 is the circumference of the 1.sup.st turn; [0043] λ.sub.max is the wavelength associated with f.sub.min; and [0044] K1 is a chosen truncation coefficient.

[0045] Similarly, the diameter of the p.sup.th or smallest turn at the front end 16 is given by

πD.sub.p=C.sub.p=K.sub.2λ.sub.min [0046] where: [0047] C.sub.p is the circumference of the p.sup.th turn; [0048] λ.sub.min is the wavelength associated with f.sub.max; and [0049] K.sub.2 is also a truncation coefficient.

[0050] The antenna may be driven at feed-point 13. In FIG. 2, a transceiver 22 is provided connected to the feed-point. The antenna may be a transmitting and/or a receiving antenna.

[0051] In FIGS. 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 3000 MHz and d) radiation pattern at 5000 MHz, respectively.

[0052] The prior art antenna is 250 mm in length, the constant inter-turn spacing is 10 mm, the radius of the 1.sup.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 250 mm, the inter-turn spacing decreases logarithmically from 22 mm to 0.5 mm, the radius of the 1.sup.st turn is 15 mm and the radius of the last turn is 2.5 mm.

[0053] As can be seen in FIG. 3, the example embodiment of the antenna according to the invention has a far superior gain bandwidth extending from about 2200 MHz to 7000 MHz. The prior art antenna has a gain bandwidth of from about 2200 MHz to 4000 MHz. FIG. 4 illustrates superior VSWR over the band from 2200 MHz to 7000 MHz for the example embodiment of the antenna according to the invention. FIG. 5 illustrates the radiation patterns of both the antennas at 3000 MHz. FIG. 6 compares the radiation patterns at 5000 MHz 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.