The Technical Field

The present invention relates to a helical antenna. More particularly, but not exclusively, the present invention relates to a multifilar helical antenna operating in the "back-fire scanning mode" suitable for satellite communications over a frequency range between 500MHz to 5GHz.

Background of the invention

A helical antenna may be used to generate substantially circularly polarised electromagnetic radiation. When the radius of a multifilar helix is much smaller than the pitch length (about half a wavelength) radiation is directed along the helical axis, opposite to the propagating direction of the wave giving rise to the radiation. A helix antenna operating in this mode is called a "back-fire helix antenna" . At high frequencies of operation, for a back-fire helical antenna having more than two turns, radiation is directed away from the helical axis at an angle, forming a conically shaped beam. The angle of the conical beam increases with increasing frequency. Since a multifilar, back-fire helical antenna can be designed with beam angles suitable for direct communication with satellites, it is well suited for land mobile satellite communications, or even GPS applications.

To reduce size and cost it is convenient to use the same antenna to transmit and receive. Transmit and

receive frequencies are, however, different. Therefore the beam directions of a scanning mode antenna will be different for the transmit and receive frequencies. In a typical multifilar back- fire helical antenna with a directivity of about lOdBi, a 5% change in frequency may cause more than a 10° change in beam direction. Thus, if the beam direction is optimised for reception the transmit beam will not be properly aligned.

In "A Linear Array Antenna Using Bifilar Helical Elements For Mobile Satellite Communications" Antennas and Propagation Society Symposium, digest volume 2 EP 1020-1023, June 1994, Y. Konishi et al address the problem of frequency scanning and propose using a linear array of low directivity helical elements. The array-factor of the radiation pattern overrides the frequency scanning effects of the individual elements to produce a stable radiation pattern. In all known prior art, however, the helix is fed at one end.

Disclosure of the Invention

It is an object of the present invention to reduce the frequency scanning of a back-fire scanning mode helical antenna or to at least provide the public with a useful choice.

According to one aspect of the invention there is provided a helical antenna configured to operate in a resonant mode at a desired operating frequency having one or more wire of the antenna fed at or near a point of voltage maxima along each wire at resonance.

The wires of the antenna are preferably fed at centrally positioned voltage maxima. The length of the antenna, preferably either short circuited or open circuited at both ends, is preferably an integer multiple of half the wavelength at which the antenna operates in a resonant mode. Preferably the multiple is an odd number for short circuited ends, or an even number for open circuited ends. Preferably the antenna comprises three or more wires.

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 : shows an end-fed quadrifilar helical antenna. The pitch length p is defined as the linear length of one turn of the helix, and r is the radius of the cylinder encompassing the helixes.

Figure la: shows the elevation pattern of a bottom- fed quadrifilar helix, 4.5 wavelengths long, excited at 0.969 times the resonant frequency.

Figure lb: shows the elevation pattern of a bottom- fed quadrifilar helix, 4.5 wavelengths long, excited at 1.032 times the resonant frequency.

Figure 2 : shows the phase distribution along the length of a multifilar helical antenna, excited at its resonance frequency, for a non-radiating wave.

Figure 3 : shows the phase distribution along the length of a multifilar helical antenna, excited at its resonance frequency, for a radiating wave.

Figure 4 : shows a typical voltage standing wave pattern along the length of the multifilar helical antenna.

Figure 5 : shows the phase distribution of a radiating wave along the length of a resonant centre-fed multifilar helix, excited at a frequency slightly lower than its resonance frequency.

Figure 6 : shows the elevation pattern of a centre- fed quadrifilar helix excited at a frequency of 0.969 times the resonant frequency.

Figure 7 : shows the elevation pattern of a centre- fed quadrifilar helix excited at a frequency of 1.032 times the resonant frequency.

Figure 8 : shows a quadrifilar helical antenna using an infinite balun according to a first aspect of the invention.

Figure 9 : shows a quadrifilar helical antenna using half-wave baluns and a hybrid power splitter according to a third aspect of the invention.

Figure 10: shows a trifilar helical antenna using a three-way power splitter and electrical

delay lines according to a further aspect of the invention.

Best mode for carrying out the invention

To facilitate understanding of the invention the operation of a multifilar helix will be described using the analogy of a transmission line. The quadrifilar helix shown in figure 1 can be viewed as two pairs of parallel wire transmission lines, each pair being twisted into bifilar helixes. Since the bifilar pairs are positioned on each others zero potential surface, they act as balanced, independent or isolated transmission lines.

When more than two interlaced helical wires are used, the transmission line will be nearly lossless for waves travelling in one direction along the line and leaky for waves travelling in the other direction. The energy leaking away causes the antenna to radiate. The reason for this phenomenon can be explained as follows: each lengthwise incremental section of the multifilar helix acts as a small circularly polarised antenna element. The phase of these incremental elements changes along the helical length due to two factors. Firstly, the phase of the wave exciting the elements changes as a function of position along the helical axis. Secondly, the geometrical rotation of each element around the helical axis changes the phase of the elemental circular polarisation as a function of position along the helical axis. These contributions to the phase gradient along the helical length will tend to cancel for a wave propagating in one direction, but will add up for a wave propagating in the opposite direction. The result is a steep phase gradient associated with the wave propagating in one direction along the

helix, and a small phase gradient associated with the wave travelling in the opposite direction.

For a steep phase gradient (see figure 2) the electromagnetic fields emanating from different parts of the helix will cancel some distance away from the antenna, and no radiation will occur. For a small phase gradient (see figure 3) , radiation will occur in a direction depending on the phase gradient.

If the transmission line is short circuited or open circuited at both ends and the total length is chosen to be an integer multiple of half wavelengths at the resonant frequency, the transmission line will be resonant (i.e: when excited a standing wave pattern will be produced along the line, as shown in figure 4) . Resonance can also be achieved when the line is open circuited at one end while short circuited at the other end, if the total length is equal to an odd number of quarter wavelengths. By feeding near a voltage maxima with a source operating at or near the resonant frequency, the transmission line will be excited to operate in a resonant mode .

At off-resonant frequencies, the phase gradient changes, and the beam direction will change for an end-fed helix, as illustrated by figures la and lb. In the resonant helix fed at a voltage maxima, a step occurs in the phase distribution at the feed point (see figure 5) . This phase step causes the average phase gradient of the radiating wave over the total length of the helix to be less dependent on the frequency. Thus frequency scanning will be reduced. The technique is most effective when the feed point is near the centre of the helix (in the longitudinal direction of the antenna) . It is therefore preferred that the multifilar helix, when short circuited/open

circuited at both ends, is an odd/even number of half wavelengths long in order to provide a voltage maxima at the centre of the antenna, where the feed point can be placed.

The reduction in frequency scanning can be illustrated by way of an example of a resonant centre-fed quadrifilar helix antenna, short circuited at both ends. If the total length is chosen to be 4.5 internal wavelengths long, and a pitch length of 0.553 free space wavelengths is combined with a radius of 0.031 free space wavelengths, the beam will be directed at an angle of about 45° from the helical axis. Using a moment method analysis, the elevation radiation patterns for the configuration are shown in figures 6 and 7 for frequencies below and above resonance respectively. The beam direction for frequencies above and below the resonant frequency fo can be inferred from figures 6 and 7 as follows:

Frequency 0.969fo 1.032fo Beam direction 44.3° 45.3°

Figures la and lb show that when the same antenna is fed at one end, while short circuited at the other end, the beam direction changes by over 8.6° over the same frequency range. Beam scanning is thus reduced to about 1° over a 6.3% band width using the centre- fed antenna of the invention. This reduction in scanning has been confirmed by practical experimentation.

The technique of the invention may be implemented in a variety of ways. It is preferred that the helical . antenna comprise three or more wires. Although monofilar and bifilar topologies exhibit one or more grating lobes in elevation, these grating lobes may be compensated by the use of reflectors etc. Thus

although it is preferred that the antenna comprise three or more helical wires there may be applications where monofilar or bifilar topologies may be used if suitable compensation is provided.

Referring firstly to figure 8 there is shown a quadrifilar helical antenna according to a first aspect of the invention. A radio frequency source signal is applied via a coaxial line 1 to a -3dB hybrid coupler 2. The hybrid coupler 2 provides an equal two way power split over a relatively wide frequency band, with a first output supplying a signal to coaxial line 3 that is 90° phase delayed with respect to the signal supplied to coaxial line 3a. A 50 ohm load 2a is connected to the isolation port . Power is delivered from the hybrid coupler 2 to the antenna via the two thin semi-rigid coaxial cables 3 and 3a, which also act as two adjacent helical wires of the helix. The other wires of the helix may consist of copper wires 4 and 5 with the same diameter as the coaxial cables 3 and 3a. At the centre of the helix, the centre conductors 6, 7 of coaxial cables 3, 3a are connected tc respective helical wires 4,5 diagonally across the cylindrical space defined by the helixes tc form an infinite bandwidth balun.

In this embodiment the antenna is an odd integer multiple of ~ (half the wavelength at which the antenna is resonant) long to ensure that a voltage maxima is present at the centre of the antenna. The copper wires 4 and 5 are both fed at the central voltage maxima at resonance. The wires are shorted at each end by conductive discs 8 and 9. The antenna operates in a resonant mode.

Feeding both copper conductors 4 and 5 at the central voltage maxima may pose difficulties from a

constructional point of view, especially when a larger number of wires are employed. In some embodiments it may therefore be desirable to feed copper conductors 4,5 at different voltage maxima (e.g. select the length of the antenna to have an even number of voltage maxima and feed copper conductors 4 and 5 at respective ones of the two central voltage maxima) . In some applications it may also be desirable to connect the transmit and receive feeds at different positions along the wires to provide additional compensation for beam tilt due to the different transmit and receive frequencies employed, thus producing substantially aligned transmit and receive beams.

For better isolation between receiver and transmitter, the antenna length can be chosen such that it will operate in different modes at these frequencies, i.e. the length at the transmit frequency being an integer number of half wavelengths longer or shorter than at the receive frequency. At the transmit frequency for example, the transmit feed may then be placed at a voltage maxima while the receive feed is at a voltage minima, and visa versa. In this way the two feeds will provide some degree of isolation between the receiver and transmitter.

The feed arrangement for this embodiment is relatively simple due to the infinite balun arrangement. This arrangement avoids the need to have feed lines passing along the longitudinal axis of the helix.

Referring now to figure 9 there is shown a quadrifilar helix according to a further aspect of the invention. As in the previous embodiment a -3dB hybrid coupler 11 provides an equal two way power

split to coaxial lines 12 and 13. The signal supplied to coaxial conductor 13 is phase delayed by 90° with respect to that supplied to coaxial line 12. The antenna comprises four copper wires 14, 15, 16 and 17 shorted at each end by conducting discs 18 and 19. The antenna is an odd multiple of half wavelengths long so that it operates in a resonant mode with a voltage maxima at the centre of each copper wire.

Coaxial lines 12 and 13 pass along the axis of the helical antenna to feed their respective copper wires at the centre of the antenna. Copper wire 14 is fed at its central voltage maxima directly from coaxial cable 12 and has a 0° delay. Copper wire 15 is fed at its central voltage maxima directly from coaxial line 13 and has a 90° delay (i.e. : the delay produced by the hybrid coupler 11) . Copper wire 16 is fed at its central voltage maxima via a half wavelength loop of the balun 20 from coaxial cable 12. This half wavelength loop of the balun introduces a 180° phase shift and so the feed signal supplied to copper wire

16 is 180° delayed with respect to that supplied to copper wire 14. Copper wire 17 is fed at its central voltage maxima via a half wavelength loop of the balun 21 from coaxial cable 13. The half wavelength loop of the balun introduces a 180° phase shift on top of the 90° phase shift produced by the hybrid coupler. Accordingly, the feed signal to copper wire

17 is 270° phase delayed with respect to that supplied to copper wire 14.

This arrangement has the advantage that the continuous copper wires give the antenna strength and stability.

Referring now to figure 10 a trifilar embodiment is shown. The antenna comprises three copper wires 27, 28 and 29 shorted at each end by conducting discs 30 and 31. Three way power splitter 23 divides an input signal into three equal signals supplied to coaxial cables 24, 25 and 26. The length of coaxial cable 24 is selected to produce a 0° relative phase delay. The length of coaxial cable 25 is selected to provide a 120° phase shift. The length of coaxial cable 26 is selected to produce a 240° relative phase shift.

Coaxial cables 24, 25 and 26 pass along the centre of the antenna to feed the copper wires at the centre of the antenna. Each coaxial cable 24, 25 and 26 is connected to the central voltage maxima of a respective copper wire 27, 28 and 29.

This embodiment has a relatively simple construction and may be easily adapted to an antenna having any required number of wires.

In the above embodiments the ends of the antenna wires are shorted and the length of the antenna is chosen to be an integer multiple of half wavelengths long (preferably an uneven multiple) to ensure that the antenna operates in a resonant mode. The antenna is operated at or near the resonant frequency of the antenna to ensure that the wires of the antenna are fed at voltage maxima. It will be appreciated that an antenna not having its ends shorted may be employed as long as the antenna is operating in a resonant mode. It is however, preferred that the ends of the antenna be shorted due to the ease of providing support to the wire ends.

Although it is preferred that the antenna is driven at a central voltage maxima along each copper wire it will be appreciated that the antenna may be driven at

any other voltage maxima if required for constructional or operational reasons. Different wires of the antenna may be driven at different voltage maxima. Further, the wires may be fed slightly to either side of the voltage maxima. In some embodiments transmit and receive feed lines 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 wires provided in the antenna may be selected for any particular application. The wires may be formed of a variety of conductive materials such as copper, silver plated brass or steel etc.

By more accurately aligning the transmit and receive beams a reduction in the required transmit power may be achieved, or an increase in the data rate may be facilitated. These improvements could mean considerable savings for the user of a communication system and/or allow a greater range of services to be provided.

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 or of the invention as defined in the claims.

Industrial Applicability

The antenna of the invention may find application in telecommunications applications, such as satellite communications.