AU703819B2 - An adjustable helical antenna - Google Patents

Description

WO 96/19846 PCT/N,95/00136 -1- An adiustable helical antenna Field of the invention The present invention relates to an adjustable helical antenna. More particularly, but not exclusively, the present invention relates to a helical antenna, the wires of which may be adjusted to steer the beam of the antenna.

Background to the invention A helical antenna can be used to generate substantially circularly polarised electromagnetic radiation. When the radius of the helix is chosen to be much smaller than the pitch length, radiation is directed away from the helical axis, opposite to the direction of propagation of the wave giving rise to the radiation. A full discussion of helix antennas operating in "backfire scanning mode" is to be found in PCT/NZ95/00128, the disclosure of which is herein incorporated by reference.

The following discussion is directed towards quadrifilar antenna topologies. However, the adjustment mechanism described may be employed in monofilar, bifilar trifilar, and other helical antenna configurations.

Helical antennas are often used in mobile satellite communication systems. This type of antenna is particularly suitable in this application as such antennas can be constructed in the form of a relatively thin rod and mounted vertically on a vehicle or other mobile structure.

WO 96/19846 PCT/NZ95/00136 2 To direct the beam of an antenna of a desired satellite some form of beam steering is required to allow for azimuth variations in the transmission/ reception direction.

A number of beam steering techniques have been proposed in the art such as: The helix geometry may be twisted. Such mechanical adjustment may result in the desired beam steering depending on the reproducibility and degree of control available.

Antenna geometry may be altered by keeping the pitch or length constant. If the pitch remains constant the change in radius must be large.

If the length is constant, the change in radius is smaller. It is difficult to support the wires of the antenna while the radius changes.

Summary of the invention The present invention is concerned with a practical means of adjusting the geometry of a helical antenna which obviates the abovementioned difficulties.

It is an object of the present invention to provide a mechanism by which beam steering may be produced in helical antennas by means of uniform mechanical variation, or to at least provide the public with a useful choice.

According to one aspect of the invention there is provided a helical antenna including a means for varying the length and pitch of the antenna whilst keeping the antenna radius substantially constant.

WO 96/19846 PCT/NZ95/O0136 Preferably the antenna includes means for rotating one end of the wire or wires of the antenna relative to the other end as the length of the antenna is varied so as to maintain the radius substantially constant.

Preferably the wire or wires at the one end are attached to a first carriage and the wire or wires at the other end are attached to a second carriage and arranged so that variation of carriage separation causes relative rotation of the carriages in such a manner as to keep the antenna radius substantially constant.

Preferably the means for rotating includes a central member having threaded portions at each end, each threaded portions cooperating with a treated bore in each movable carriage to cause relative rotation of the carriages.

Preferably the antenna is fed at centrally located voltage maxima and the ratio the distance from the feed point to each end of the antenna remains substantially constant.

Preferably the antenna is a multifilar antenna, preferably having three or more wires.

There is further provided a method of adjusting a helical antenna comprising moving one end of a wire of a helical antenna towards or away from the other end of the wire to vary the length and pitch of the wire whilst relatively rotating the one end relative to the other end to maintain the radius of the antenna substantially constant.

WO 96/19846 PCT/NZ95/00136 4 Preferably the antenna is a multifilar antenna and each wire is simultaneously adjusted in the same manner.

Brief description of the drawings The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1: illustrates a cross-sectional view through a quadrifilar antenna incorporating the adjustment device.

Figure 2: illustrates a perspective detail of the construction indicated by Best mode for carrying out the invention The following description relates to a centrefed quadrifilar helical antenna. Such an antenna is described in PCT/NZ95/00128 which disclosure is hereby incorporated by reference. This patent discloses a helical antenna in which the wire or wires are fed at points of voltage maxima (preferably central voltage maxima) at the resonant frequency of operation.

It is to be understood that the adjustment device described herein is not restricted to such an antenna and the novel adjustment means may be adapted to other helical antenna constructions and topologies.

Beam steering of an antenna array such as that found in a multifilar helical antenna, viewed continuous array of circularly polarized antenna elements, may be achieved by changing the array WO 96119846 PCTNZ95/O0136 pattern factor. This is accomplished most simply by changing the relative phases of the element excitations.

The phase of a circularly polarised antenna element can be changed by simply rotating the element, the phase change being equal to the rotation angle.

Thus, beam steering in an array of infinitesimal quadrifilar helical sections can be accomplished by rotating each infinitesimal section around the central axis with respect to the previous section.

Referring to figure i, an exemplary embodiment of an adjustable helical antenna is shown. A radome 14 has threaded couplers 1la and 1lb attached at each end.

First and second moveable carriages 15a and 15b are threaded into the bore of the adaptors 1la and llb.

If the carriages are rotated about the axis of the bore 24a and 24b, they will move along the axis of the radome. Therefore, if the radome is rotated about its axis relative to the carriages, the carriages will move axially along the bores 24a and 24h The thread connecting coupler 1la and carriage is left handed and that connecting carriage and coupler llb is right handed. Therefore, as the radome/coupler assembly is rotated with respect to the carriages, the carriages 15a and 15b move towards or away from each other depending on the direction of rotation of the radome.

As the carriages move along the bore, they are guided along threads 12a and 12b which will rotate the carriages about the axis of the bore.

A central core member comprises a core tube 19, upper threaded rod 12a and lower threaded rod 12b. The core member is fixed in relation to the mounting 9- WO 96/19846 PCT/NZ95/00136 -6member 16. The upper and lower rods 12a and 12b have a coarse thread formed by twisting two rigid tubes together which are inserted through corresponding threaded bores 24a and 24b of carriages 15a and respectively. Both the upper and lower coarse threaded rods 12a, 12b have a right-handed pitch for an antenna with left-handed helical wires; or a lefthanded pitch for an antenna with right-handed helical wires. The pitch of treaded rod 12a is preferably the same as that of threaded rod 12b.

The radome assembly is secured to the mount by means of bearing 20, thereby allowing the radome to rotate about its axis with respect to the mount 16. With the inner core tube 19 and threaded rod construction held fixed with respect to the mount 16 it can be seen that, depending on the direction of the rotation of the radome 14, carriages 15a and 15b move together or apart under the action of the upper and lower fine threads while simultaneously rotating axially along the coarse threaded rods.

Thus, the antenna undergoes length variation via linear movement of the carriages simultaneously with pitch variation via carriage rotation about the axis (i.e resulting in twisting the helix). The degree of pitch change and length variation are related such that the helical radius is kept constant.

Helical radiating wires (filaments) 22a, 22b, 22c and 22d extend between carriages 15a and 15b. Only two wires 22a and 22b are shown for clarity. Segments 22c and 22d of the remaining two wires are indicated as the two dotted elements in figure 1.

The filaments form a helix with a radius defined generally by the inner diameter of the radome 14 and is therefore supported by the radome. The radius may ,I -I ~4 ra~ WO 96119846 PCT/NZ95/00136 7 be smaller than this diameter, down to the smallest which can be accommodated by the central core 19. In this example additional support means (not shown) is needed iunning along the length of the core tube 19 which maintain the filament spacing and relative orientation as the length and pitch are varied. Such modifications to the filament construction are within the purview of one skilled in the art.

In the part 4 -ular embodiment described herein, the filaments are shorted at each end and the antenna operates in a resonant mode. The antenna is fed at the centre by feedlines 23a, 23b, 23c and 23d. The feedlines 23a, 23b, 23c and 23d are routed to the centre of the antenna by means of a threaded rod 12b.

Figure 2 illustrates the detail of the core tube segment where the feeder wires 23a-d exit. When assembled the upper and lower sections are coupled producing a tube with feeder lines 23a-d protruding from it.

It is to be appreciated that the feeder network may be constructed in a number of ways and adapted to the mechanical constraints of the novel adjustment mechanism.

For example, the threaded rods 12a and 12b may be in the form of a continuous rod running the entire length of the antenna threaded over part or all of its length, and serve only to rotate the upper and lower carriages 15a and 15b upon rotation of the radome. In this construction, the feeder lines 23a, 23b, 23c and 23d may be connected to the centre of the antenna via wires internal to the rod and exit the base of the carriage 15b where it is connected to I ac-- ~C~ WO 96/19846 PCT/NZ95/00136 8 the base 16, via some mechanical means as is known in the art.

Referring again to figure 1, a power splitter 24 provides an equal two way power split to coaxial lines 17a and 17b. The signal supplied via coaxial conductor 17a is phase delayed by 900 with respect to that supplied via coaxial line 17b due to the greater length of line 17a. The antenna is an odd number of half wave lengths long so that it operates in a resonant mode with a voltage maxima at the centre of each copper wire. For clarity only two complete radiating wires 22a and 22b are shown. Segments 22c and 22d of the remaining two wires are shown by the dotted elements in figure 1.

Co-axial lines 17a and 17b are threaded along the axis of the helical antenna by way of the threaded rod 12b. Wire 22d is fed at its central voltage maxima directly from coaxial line 17b and has a 00 delay. Wire 22b is fed at its central voltage maxima directly from coaxial cable 17a and has a 90° delay the delay produced by the greater length of cable 17a than 17b). Wire 22c is fed at its central voltage maxima via a balun 18b from coaxial cable 17b. This balun introduces a 1800 phase shift and so the feed signal supplied to the wire 22c is 1800 delayed with respect to that supplied to wire 22b.

Wire 22a is fed at its central voltage maxima via a balun 18a from coaxial cable 17a. The balun introduces a 1800 phase shift on top of the 900 phase delay produced by cable 17a. Accordingly, the feed signal to the wire 22a is 270° phase delayed with respect to the signal supplied to wire 22d.

The feedline arrangement described in the above particular embodiment is adapted for a quadrifilar I~ I ~C WO 96/19846 PCT/NZ95/00136 9helical antenna construction. However, it is to be appreciated that similar feedline arrangements may be constructed for other multifilar antennas operating in resonant or non resonant modes. Such variations are described in PCT/NZ95/00128.

The method of attaching the radome to the mount 16 is shown in figure 1. Other constructions may be implemented wherein such constructions are within the scope of one skilled in the art. For example, the bearings 20 may be located inside the radome wherein they are connected to the mount 16.

As discussed in PCT/NZ95/00128, different filaments of the antenna may be driven at different voltage maxima. Further, the filaments may be fed slightly to either side of ,e voltage maxima. In some embodiments transmit and receive feedlines may be connected at different points along each wire to improve alignment of the transmit and receive beams, and/or to provide some isolation between the transmitter and receiver.

It is also to be appreciated that the number of filaments provided in the antenna may be selected for a particular application. The filaments may be formed of a variety of conductive materials such as copper, silver plated brass or steel.

The radome 14 may be constructed from a non conducting rigid material and, while shown in the present embodiment as a cylindrical tube, other cross-sectional shapes are envisaged. Such shapes would require a more complicated construction as well as modification to ensure that the helical filaments retain a constant radius along the length of the antenna. Such modifications are herein incorporated 19 WO 96/19846 PCT/NZ95/00136 10 where they constitute such variations that would be within the scope of one skilled in the art.

It is also envisaged that carriage rotation could be effected using stepper motors controlled electronically. Such a technique could provide for independent control of the carriages position and rotation and hence allow tapered helix topologies.

Such a method may employ microprocessor techniques to control the stepper motors.

Where in the foregoing description reference has been made to integers or components having known equivalents then such equivalents are herein incorporated as if individually set forth.

Although this invention has been described by way of example it is to be appreciated that improvements and/or modifications may be made thereto without departing from the scope of the invention as defined in the claims.

Industrial applicability The antenna of the invention may find application in mobile satellite communications.

Claims (11)

1. A helical antenna including means for varying the length and pitch of the antenna whilst keeping the antenna radius substantially con- t ant.

2. An antenna as claimed in claim 1 including means for rotating one end of a wire or wires of the antenna relative to the other end as the length of the antenna is varied so as to maintain the radius substantially constant.

3. An antenna as claimed in claim 2 wherein the wire or wires at the one end are attached to a first movable carriage and the wire or wires at the other end are attached to a second movable carriage and arranged so that variation of carriage S"0 separation causes relative rotation of the carriages in such a manner as to keep the antenna radius substantially constant.

4. An antenna as claimed in claim 3 wherein the means for rotating includes a central member having threaded portions at each end, each threaded portion cooperating with a threaded bore in each movable carriage to cause relative rotation of the carriages as the separation of the carriages is varied. 20

5. An antenna as claimed in claim 3 or claim 4 when dependent on claim 3, wherein the antenna is surrounded by a radome having threaded portions at either end thereof which engage with complementary threaded portions of said carriages in such a manner that rotation of said radome relative to said carriages varies the separation between said carriages. JRG:JA:#25298 February 1999) WO 96/19846 PCT/NZ95/00136 12

6. An antenna as claimed in any one of the preceding claims wherein each wire of the antenna is fed at a centrally located point of voltage maxima along the wire at resonance.

7. An antenna as claimed in claim 6 wherein the ratio of the distance from each centrally located point of voltage maxima to each end of each wire remains substantially constant.

8. An antenna as claimed in any one of the preceding claims wherein the antenna is a multifilar antenna.

9. An antenna as claimed in claim 8 wherein the antenna includes three or more wires.

A method of adjusting a helical antenna comprising moving one end of a wire of a helical antenna towards or away from the other end of the wire to vary the length and pitch of the wire whilst relatively rctating the one end relative to the other end to maintain the radius of the antenna substantially constant.

11. A method as claimed in claim 10 wherein the antenna is a multifilar antenna and each wire is simultaneously adjusted in the same manner.