EP2676323A1

EP2676323A1 - An antenna assembly having vertically stacked antennas and a method of operating the antenna assembly

Info

Publication number: EP2676323A1
Application number: EP11703716.8A
Authority: EP
Inventors: Johan Granholm
Original assignee: Thrane and Thrane AS
Current assignee: Thrane and Thrane AS
Priority date: 2011-02-18
Filing date: 2011-02-18
Publication date: 2013-12-25
Also published as: WO2012110098A1

Abstract

An antenna assembly, and a method of operating it, comprising at least two antennas each adapted to communicate at at least a predetermined wavelength, the antennas being positioned along a predetermined direction and extending at least 0.1 times the wavelength along the predetermined direction. The assembly further comprises an electrically conducting element positioned between two of the antennas and having an area which, projected on to a plane perpendicular to the predetermined direction, covers between 0.07 times the predetermined wavelength squared and 0.7 times the predetermined wavelength squared. This conducting element acts to increase the gain/sensitivity along the direction, whereby the antenna(s) may be dimensioned to have larger gain at large angles to the direction, so that an overall gain increase is seen.

Description

AN ANTENNA ASSEMBLY HAVING VERTICALLY STACKED ANTENNAS AND A METHOD OF OPERATING THE ANTENNA ASSEMBLY

The present invention relates to an antenna assembly which may be implemented as a simple, compact and low cost vertically stacked antenna assembly having omnidirectional radiation pattern in the horizontal plane and a controllable radiation pattern in the vertical plane. The assembly uses two or more antennas which are fed individually.

An antenna assembly comprising two antennas may be seen in e.g. US4316194.

Maritime travellers to a large extent rely on satellite communications (SATCOM) for business, pleasure and, not the least: safety and emergency. While some satellites utilized for such communication systems are LEO or MEO (Low Earth Orbiting, Medium Earth Orbiting, respectively) and therefore constantly change location in the sky as seen from the position of the user at sea, other systems rely on GEO (Geostationary Earth Orbiting), where the satellite(s) remain on the same orbital position. In both systems an antenna is needed to transmit signals to and receive signals from the satellite. Depending on the user's geographical location at sea, and depending on the satellite position in space, the satellite may be seen from the user position at any azimuth angle, and at any elevation angle from approx. 5° above the horizon and up to zenith. If at sea, and the ship further exhibits some amount of roll, pitch and yaw, the antenna must remain able to perform even under such turbulent sea conditions. Assuming that the ship rolls up to e.g . 20°, and if the satellite is positioned 5° above the horizon, the antenna must be able to function from -15° under, to 90° over "its" horizon.

In many SATCOM systems the antennas must be circularly polarized, preferably with good axial ratio particularly at low elevation angles, to minimize the effect of multipath-fading which is most pronounced at low angles. Although circularly polarized antennas for LEO, MEO and/or GEO SATCOM systems and for fixed mounting exist with omni directional azimuth pattern, and with a very wide elevation radiation, such antennas inevitably have relatively low gain owing to the wide beam. The low gain of such omni directional antennas is, however, insufficient for modern high data-rate e.g. GEO SATCOM systems, and instead directional (and hence higher-gain) antennas are often used . Such antennas have a more narrow radiation beam that must constantly be kept pointing precisely towards the satellite, regardless of ship position and attitude. Antennas of this directive-beam type are typically constructed to have a fixed beam with reference to the antenna structure. Such fixed-beam antennas must therefore be mounted on a mechanically moving pedestal that continuously maintains the correct antenna-pointing towards the satellite. Such antennas, however, are subject to potential failure owing to the mechanical moving parts therein. It would therefore be both desirable and advantageous if a simple, compact and inexpensive antenna could be constructed for application in e.g . maritime SATCOM systems; an antenna having no mechanical, moving parts and where the ship's attitude need not be known, yet with sufficiently high gain for application with modern SATCOM systems. In a first aspect, the invention relates to an antenna assembly comprising at least two antennas each adapted to communicate at at least a predetermined wavelength, the antennas being positioned along a predetermined direction, wherein : the antennas each extend at least 0.1 times the wavelength along the predetermined direction and the assembly further comprises an electrically conducting element: o positioned between two of the antennas and o having an area which, projected on to a plane perpendicular to the predetermined direction, covers between 0.07 times the predetermined wavelength squared and 0.7 times the predetermined wavelength squared .

In the present context, two or more antennas are used. Usually, two antennas are used, but more antennas may be incorporated, such as a total of 3, 4, 5 or 6 antennas, in order to e.g . provide a larger gain and/or obtain a larger physical distance between the antennas for different purposes, as will be described further below. In this context, the wavelength may be the free space wavelength, i .e. the vacuum wavelength.

Naturally, communication may be performed on multiple wavelengths. In that situation, the predetermined wavelength may be one of the wavelengths, such as a lowest wavelength used for the communication. Presently, the preferred frequency is in the interval of between 100 M Hz and 10 GHz, i .e. a wavelength of between 3 m and 3 cm. For some applications, the wavelength is on the order of 10-25 cm, such as between 16 and 20 cm. Communication may be transmission and/or receipt of electromagnetic radiation with the preferred wavelength. The antennas are positioned along the predetermined direction or axis. Then, one antenna may be positioned further along the direction than another of the antennas. In a preferred situation, the predetermined direction goes through the antennas and preferably coincides with symmetry axes of the antennas, if such axes exist. When the antennas extend the at least 0.1 times the wavelength along the predetermined direction, the antennas may have a larger gain or sensitivity at wider angles away from a centre axis or symmetry axis of the antennas. Antennas with conductors primarily present in a single plane (i .e. having a low height) have a tendency of high gain in a direction perpendicular to that plane but low gain in that plane. Thus, this extent may facilitate communicating at directions perpendicular to the predetermined direction. In this respect, the part(s) of the antennas extending along the predetermined direction preferably is/are electrically conducting elements adapted to transmit and/or receive electromagnetic radiation.

The electrically conducting element may be what is also called a ground plane. This element is positioned between two of the antennas, whereby it will affect radiation emitted from one of the antennas toward the other.

Preferably, the electrically conducting element is positioned so that the predetermined direction/axis, when going through the antennas, also goes through the electrically conducting element. The element has an area with a size within the above interval when projected on to a plane perpendicular to the predetermined direction. This size may be seen to generate a "shadow" seen from one antenna toward the other. The element thus may have any shape and a wide variety of sizes or areas. The element may be solid (unbroken), or may have one or more openings in it. A preferred element is circular and extends perpendicular to the

predetermined direction. In this manner, the element is symmetrical around the direction.

Other elements may be oval and non-perpendicular to the direction. Naturally, other shapes, such as square, triangular, pentagons, hexagons, star shapes, crescent shapes, pie shapes or the like may be used.

Preferably, the conducting element has an area which, projected on to the plane, covers between 0.11 times the predetermined wavelength squared and 0.4 times the predetermined wavelength squared . 0.11 times the predetermined wavelength squared thus, for a wavelength of 18 cm, is 0.11 x 18 cm x 18 cm = 35,64 cm², which may be implemented as a circular disc with a diameter of 6,74cm .

In this context, the element is electrically conducting . Naturally, different metals, such as copper, silver, gold, aluminium or the like may be used for this element, as may other conducting materials, such as carbon/metal-loaded materials, plastics, or the like. Naturally, the element may be made solely of a conductive material or may be a layered product comprising a layer of a conductive material, such as a PCB.

It has been found that the function of this electrically conducting element is an increase of sensitivity along the predetermined direction. Thus, the antennas or at least one of the antennas may then be adapted to have less sensitivity along the predetermined direction but more sensitivity at directions at an angle to, such as 60-120° to the predetermined direction. Antennas having an extent along the predetermined direction may be made to have a larger gain/sensitivity at such large angles from the predetermined direction. In a particularly interesting embodiment, the antenna assembly is used on vessels and in communication with (receiving signals from and/or transmitting signals to) satellites which are positioned close the horizon. In this manner, sensitivity or efficiency in this

communication may be increased without loosing too much sensitivity along the

predetermined direction which, in that embodiment, usually is vertical, at least when the vessel does not roll, yaw or pitch.

In one embodiment, the assembly further comprises a circuit for converting a first signal into corresponding second electrical signals for each of the antennas, the converting means being adapted to provide a first and a second second signal each for transmission to one of the antennas, where the first second signal has a phase difference in relation to the second second signal . Preferably, the second signals are identical apart from the phase difference provided there between. These means thus may be a simple splitter dividing the first signal into a number of identical signals of which one is then provided with a phase difference.

It is known that altering the phase, in an antenna assembly of this type, will alter the sensitivity at angles to the predetermined direction. Thus, the receiver, such as a satellite, for which the emitted signals are intended, may be tracked and the phase shift adapted accordingly so that increased gain and thus signal strength is obtained toward the receiver. This adaptation may, for example, take into account a distance between the antennas along the predetermined direction. The tracking of the receiver may be performed on the basis of information of a direction of the predetermined direction in relation to a predetermined direction, such as vertical or the direction of gravity. Thus, pitch, yaw, roll of a ship may be known from other information sources. Also, this tracking may be performed using knowledge of a position of the assembly, such as using GPS equipment, and a position of the receiver, such as a satellite. Naturally, the tracking may also be performed on the basis of signals output by the receiver.

In that or another embodiment, the assembly further comprises a circuit for converting third electrical signals received from each of the antennas into a fourth signal, the converting means being adapted to alter a phase of one of the third signals and subsequently combine the third signals to generate the fourth signal . Thus, in a similar manner, the phase between the third signals may be adapted to a direction of e.g . a transmitter in order to increase the sensitivity in that direction. In this manner, signals or beams from other directions, such as reflected beams or multipath beams may be reduced. Again, other signal processing, such as a filtering, may be performed of the third signals, in addition to the phase difference, but usually this is not desired. In one embodiment, the assembly further comprises conductors for conducting signals to/from the antennas, the conductors extending at least substantially parallel to the predetermined direction, at least one of the conductors extending through one of the antennas and is connected to another of the antennas, the other of the antennas having a plurality of electrically conducting antenna elements, the at least one conductor and the plurality of conducting antenna elements being electrically connected to a feeding/splitting circuit provided at or on the electrically conducting element.

In this manner, different signals may be fed to the antenna, such as to different ones or different pairs of the conducting antenna elements.

Usually, pairs of antenna elements are provided, normally positioned opposite to each other, to form dipoles used for emitting and/or receiving signals. A dipole of the conducting antenna elements may be fed by one conductor. Alternatively, multiple dipoles may be fed by the same conductor.

Providing different dipoles, the radiation emitted or received may be polarized in different manners, which will be determined by the construction and the orientation in space of the dipole antennas and a phase difference by the radiation emitted by or received by the dipoles. A single dipole normally is fed a single signal which is directly fed to one of the two conducting antenna elements of the dipole and is phase shifted, 180°, before feeing to the other antenna element of the dipole. If only this dipole emits radiation, this radiation will be linearly polarized.

If two dipoles are used for emitting radiation (and those two dipoles are arranged having mutual orthogonal orientation in the same plane), the signals fed to these dipoles may be phase shifted to e.g . have circularly polarized radiation output.

If two dipoles are used, the circuit adapted to provide the phase shifts usually is called a quadrature hybrid, which receives one or more signals and generates the signals for the conductive antenna elements. A quadrature hybrid, being a four-port device, has its two ports connected to the input/output ports of the two orthogonal oriented dipoles, and the remaining two ports of the quadrature hybrid is being used to transmit and/or receive circular polarization signals of orthogonal polarization, designated RHCP and LHCP (right-hand circular polarization and left-hand circular polarization, respectively) . If only e.g . the RHCP signal is being used, the LHCP port may be terminated, and vice versa if the LHCP signal is only being used . The quadrature hybrid may receive only one signal if only one orientation of circularly polarized radiation is desired.

Then, usually, the feeding/splitting circuit will be splitting the signal into identical signals and then provide a phase shift to one or more of the generated signals. This dividing and/or phase shifting may be performed multiple times if desired.

In general, the flow of the signal may be reversed so that this circuit may be used also for receiving signals by the antennas and combining these as desired. Thus, e.g . all the above considerations relating to the emission of radiation are equally relevant for receiving radiation.

When this circuit is provided at or on the electrically conducting element (if e.g implementing the conductive element as a metallic ground plane of a double-sided PCB, where the other side of the PCB may be used for a microstrip feeding circuit, e.g . comprising a quadrature hybrid), this circuit may be positioned quite close to the antenna receiving the signals output of the circuit, whereby fewer conductors may be required for providing the signal(s) to that antenna/conducting element, as such conductors may extend through other antennas. Also, this circuit may be positioned at a position, at a conducting element, where it has no detrimental effect on the operation of the antennas neighbouring the conducting element.

In a particular embodiment, at least one of the antennas is a drooping dipole antenna.

Preferably, the drooping dipole antenna is a crossed drooping dipole antenna having four conducting antenna elements each being drooping and which are combined in two pairs of such elements, the two pairs being positioned perpendicular to each other.

In another embodiment, at least one of the antennas is a quadrifilar helix antenna .

Alternatively, a bifilar helix or a turnstile antenna could be used. Other circular polarization antenna elements known in the art may be used as well, which are therefore equally covered by the present invention!

The advantage of the drooping dipole antenna and/or the quadrifilar/bifilar helix antenna is that these extend along the predetermined direction and thus have a suitable gain/sensitivity at a wide range angles perpendicular to the axis of symmetry. The crossed drooping dipole antenna is well suited for generating/receiving both orientations of circularly polarized radiation (RHCP and LHCP), whereas the helix antennas are adapted to mainly receive one orientation of circularly polarized radiation.

In one embodiment, at least one of the antennas comprises at least 4 electrically conducting antenna elements positioned so that, from a centre axis parallel to the predetermined direction, two planes perpendicular to each other extend, between which the individual conducting antenna elements extend, opposite pairs of the antenna elements being connected to an individual circuit also connected to an electrical conductor for transmitting a signal to or from the pertaining pair of antenna elements.

Thus, four quadrants are defined, and the antenna elements each is positioned within a quadrant. If the antenna elements were allowed to extend over the quadrant of another element, as is the situation in helix antennas, this antenna's sensitivity toward one of the orientations of circularly polarized radiation will fall or reduce to zero.

Preferably, the antenna elements are flat elements each, or pair wise, extending within planes parallel to the predetermined direction and extending inside each quadrant. Thus, when the elements each stay within a single quadrant, the antenna will be able to receive both orientations of circularly polarized radiation.

When opposite pairs of the conducting antenna elements are connected to a circuit which also is connected to a conductor, this circuit may have the above function of

splitting/combining and phase shifting, as is known in the art. It is also known that providing two crossed dipoles, polarization diversity will become possible.

In one embodiment, the assembly further comprises an additional electrically conducting element positioned oppositely to one of the two of the antennas, where a distance between the electrically conducting element and the additional electrically conducting element exceeding 0.5 times the predetermined wavelength.

Naturally, this additional electrically conducting element could have an area which, projected on to a plane perpendicular to the predetermined direction, covers at between 0.07 times the predetermined wavelength squared and 7 times the predetermined wavelength squared, but this element could easily be much larger, as it preferably comprises or forms part of a cover for electronic circuits for feeding the antennas or receiving signals there from. In fact, this element could form part of a roof or top of e.g . a vehicle or vessel on which the assembly is mounted .

The distance between the two conducting elements is desired for a number or reasons. In one situation, each of the at least two antennas are connected to a conducting element, and the distance between the antennas may be adapted to e.g . reduce the effect of multipath radiation. Also, the distance should provide space/height for the antenna positioned between the two element and which may have any height above 0.1 times the wavelength, such as in the interval of 0.3-0.6 times the wavelength. Thus, this distance preferably is between 0.5 and 1 times the wavelength.

Another aspect of the invention relates to a method of operating an antenna assembly, such as that of the first aspect of the invention, comprising at least two antennas each adapted to communicate at at least a predetermined wavelength, the antennas being positioned along a predetermined direction wherein : - the antennas each extend at least 0.1 times the wavelength along the predetermined direction and the assembly further comprises an electrically conducting element: o positioned between two of the antennas and o having an area which, projected on to a plane perpendicular to the predetermined direction, covers between 0.07 times the predetermined wavelength squared and 0.7 times the predetermined wavelength squared, the method comprising : - receiving a signal from each antenna and generating an output signal from the received signals or generating, from an original signal, a second signal for each antenna, and feeding the second signals to the antennas.

As mentioned above, the assembly may be used for transmitting radiation and/or receiving radiation.

In one embodiment, the generating step comprises providing a first and a second second signal each for transmission to one of the antennas, where the first second signal has a phase difference in relation to the second second signal . As mentioned above, further signal processing may be performed. Also, the phase shift may be determined based on a desired direction, such as a direction at an angle to the predetermined direction.

In that or another embodiment, the receiving step comprises altering a phase of one of the received signals and subsequently combining the phase altered signal and the other received signal(s) to generate the output signal . As mentioned above, the sensitivity at angles to the predetermined direction of the assembly may be altered by altering the phase shift. In one embodiment, the method further comprises conducting signals to/from the antennas via conductors extending at least substantially parallel to the predetermined direction, at least one of the conductors extending through one of the antennas and is connected to another of the antennas, the other of the antennas having a plurality of electrically conducting antenna elements, the at least one conductor and the plurality of conducting antenna elements being electrically connected to a feeding/splitting circuit provided at or on the electrically conducting element. Thus the signals are fed to/from the circuit where they are split/altered (such as phase shifted) and fed to or received from the antenna elements.

When the circuit is positioned at or on the conducting element, is may have very little or no impact on the operation of the antennas, and fewer conductors may be required for feeding the antenna or transporting signals away from the antenna. In one embodiment, at least one of the antennas is a drooping dipole antenna, such as a crossed drooping dipole, which is able to output or receive both dual-linearly polarized radiation as well as dual-circularly polarized radiation of any polarization orientation.

In another embodiment, at least one of the antennas is a quadrifilar or bifilar helix antenna. In one embodiment, at least one of the antennas comprises at least 4 electrically conducting antenna elements positioned so that, from a centre axis parallel to the predetermined direction, two planes perpendicular to each other extend, between which the individual conducting antenna elements extend, opposite pairs of the antenna elements being connected to an individual circuit also connected to an electrical conductor for transmitting a signal to or from the pertaining pair of antenna elements. In this manner, the generating step may comprise generating signals causing the antenna to output dual-linearly polarized radiation or single- and/or dual-circularly polarized radiation.

Alternatively, the receiving step may comprise receiving radiation of any polarisation and outputting a corresponding output signal. In one embodiment, the assembly further comprises an additional electrically conducting element positioned oppositely to one of the two of the antennas, a distance between the electrically conducting element and the additional electrically conducting element exceeding 0.5 times the predetermined wavelength. Thus, part of the signal processing may be performed in circuitry covered by this additional element. In the following, preferred embodiments of the invention will be described with reference to the drawing, wherein : figure 1 illustrates an embodiment of an assembly according to the invention, figure 2 illustrates gain and peak gain of the assembly of figure 1 compared to a single antenna, figure 3 illustrates varying gain/sensitivity by varying a phase difference of signals to the antennas, figure 4 illustrates the effect of the size of the ground plane and the distance between the ground planes, figure 5 illustrates a ground plane on one side with a quadrature hybrid, and figure 6 illustrates the effect of multipath radiation and space diversity.

In figure 1, a circularly polarized antenna assembly 10 with no mechanical moving parts is disclosed. The assembly 10 has two crossed drooping dipole antennas 12 and 14 positioned along a direction D. Each antenna 12/14 has four U-shaped conductors, illustrated by 12' and 14', which are adapted to pick up or emit electromagnetic signals at a predetermined wavelength. Below the antenna 14 is a ground plane 18, and between the antennas 12 and 14 is a ground plane 16 implemented on a printed circuit board, PCB. As will be described further below, the ground plane 16 may incorporate a feeding network on the upper or lower side to feed the conductors 12'.

The lower antenna 14 is mounted on the ground plane 18, which may contain the feeding network for the lower antenna 14. Also, the lower structure 18 may contain parts of the SATCOM receiver and/or transmitter circuitry.

Then, the signals for the conductors 12' may be fed along the direction D between the conductors 14' so as to create as little disturbance in the operation of the conductors 14' and the antenna 14 as possible.

Each of the antennas 12 and 14 has a omnidirectional radiation pattern in the horizontal (i.e. azimuth) plane, and a pattern in the vertical (i.e. elevation) plane.

The overall radiation pattern of the assembly 10 can be electronically formed, or steered, by controlling the phase between the signals to/from the antenna 12 in relation to those from the antenna 14, from having a relatively high gain towards the horizon (perpendicular to the direction D) to having a relatively high gain towards zenith (along the direction D). It is noted that this controllability is obtained with no moving parts.

The operation of the ground plane 16 has been found to actually increase the gain or sensitivity of the assembly 10 along the direction D, whereby it is possible to design at least one of the antennas 12 and 14 to have a larger gain or sensitivity at directions at larger angles to the direction D, such as 60°-120° to the direction D, so that if the direction D is vertical, larger sensitivity/gain is obtained at or around horizontal.

The two antennas 12 and 14 may be identical, or may be different. Each antenna element is constructed to receive and transmit either dual-linearly or single- or dual-circularly polarized signals. The antennas may be implemented in a variety of ways; as e.g. as crossed, (drooping or not) dipoles, as turnstile antennas, as crossed, cavity-backed slots, as (drooping or not) mono-, bi- or quadrifilar helix antennas, as nested and segmented wire(s), as double- folded monopoles or as dielectric resonator antennas or as yet other embodiments known in the art. Each antenna may, or may not, further incorporate passive (parasitic) electric and/or dielectric elements, such as e.g . slots, monopoles, loops, patches, or other combinations vertical, horizontal or slanted metallic and/or dielectric structures etc. to achieve further, desirable characteristics of the radiation characteristics of the overall antenna . Such passive elements may furthermore incorporate switches to e.g. connect or disconnect them from other metallic parts of the antenna and hence allow for additional electronic control of the antenna performance.

If the upper and lower antennas 12/14 exhibit some variation in the azimuth and/or elevation plane radiation pattern performance, one of the antennas 12/14 may be rotated with respect to the common vertical axis to average out the effect of such asymmetry of the individual antenna radiation pattern. This ability will help improve the performance of the combined two-element antenna group.

Both antennas are being actively used . Consequently, means are desired to connect the transmit and/or the receive signals to both antennas 12/14. Typically, means for coupling the signal(s) to/from an antenna involve the use of one or more transmission line(s), which are capable of carrying the radio-signal frequencies of interest with relatively little loss. In this embodiment, the feeding line(s) to the upper antenna 12 pass(es) through or pass(es) by the lower antenna 14 in such a way, that both the performance of the lower antenna 14 and the performance of the combined two-antenna assembly 10 is not significantly deteriorated owing to this/these feeding line(s) . In the case e.g. of the lower antenna 14 being constructed as a crossed, (drooping or not) dipole pair, or a crossed, cavity-backed slot, or a (drooping or not) mono-, bi- or quadrifilar helical antenna, the feeding line(s) to the upper antenna may be implemented as a coaxial line, a microstrip line, a stripline, a coplanar waveguide, a pair of coupled (balanced) strips or wires, a slotline, or other transmission lines known in the art. The feeding line(s) to the upper antenna 12 may preferably pass through the centre of the lower antenna 14 and may, or may not, be partially or fully integrated in or with the lower antenna 14. If the feeding line(s) to the upper antenna 12 does not pass through the centre of the lower antenna 14, one or more metallic and/or dielectric impedance and/or radiation pattern compensation structures may be implemented to minimize the effect of the off-centre feeding line(s) to the upper antenna 12. The illustrated embodiment of the invention shown in figure 1 has a coaxial cable 20 connected to the upper antenna 12, this cable 20 passing centrally through the lower antenna 14 and down to the receiving/transmitting equipment which is contained in the metallic structure 18 below the lower antenna 14.

The upper and lower antennas 12/14 are being fed separately. Each antenna 12/14 may have one or two feeding ports. For antennas with integrated circular-polarization feeding networks, these ports are RHCP and/or LHCP (right-hand and left-hand circular polarization ports). The transmitting signals to and the receiving signals from both ports, or only to and from one port, of either or both antennas 12/14, may, or may not, be used. Unused port(s) may be terminated in arbitrary impedance. In the case of an antenna with inherent dual- linear polarization (having no integrated circular-polarization beam-forming network), the two feeding ports of such an element would be the orthogonal, dual-linear polarization. In the case of an antenna element with such dual-linear polarization, an external feeding network may be used to achieve circular polarization. Alternatively, for dual-linear polarization antenna(s) and in the reception case, the signal combining can be done by digital I/Q combining or by other digital signal techniques. If the upper antenna 12 is implemented as having two ports, and the signals to and from both ports are to be used, the signals to/from these may be carried down through the lower antenna 14, to the receiver/transmitter equipment 18, using two separate or integrated transmission lines 20. The antenna assembly 10 needs not transmit and receive using the same polarization, a further advantage supported by the present embodiment. Another characteristic of the embodiment of figure 1 is that the gain of the assembly 10 (in its peak direction) is significantly higher compared to the gain of an antenna having a fixed, continuous, near- or extended-hemispherical coverage. This is illustrated in figure 2 where the antenna gain versus elevation direction is plotted for the lower antenna 14, when the ground plane 16 and the upper antenna 12 have been removed, versus the peak-gain of the dual antenna assembly of figure 1. In the latter, the phase difference (delay) between the signals from/to the upper and lower elements is increased by 45° from 0° to -315°. In figure 2, the centermost bold (solid) curve shows the radiation pattern of a single prior-art element (having continuous hemispherical coverage) while the uppermost dashed curve shows the peak-gain of the scanned antenna assembly 10 of figure 1. Finally, the bold dot-dashed curve (lowest bolded curve) shows the gain increment of the assembly of figure 1 group over the gain of the single element. The latter, third curve is calculated as the difference between the two aforementioned curves. The many curves indicated with narrow print are the radiation patterns for the assembly when scanned to different peak-directions in elevation (scanned by varying the phase-difference between the two elements from 0° to -315° in steps of 45°). The left vertical axis illustrates the gain of single-element and two-element group (dBi), and the right vertical axis illustrates the two-element group gain-increase over that of the single element (dB).

It is seen that the gain-increment of the assembly ranges from an average 3 dB in the angular region between -135° < Θ < -50° while the gain-increase achieves a peak value of more than 6 dB in the region around the zenith direction (note that the gain-increase curve uses the right-hand Y-scale).

Because of this higher gain, the assembly 10 is useful for medium-speed SATCOM applications with satellite elevation from approx. 5° above the horizon and up to Zenith. This assembly is useful in both stationary, land-mobile, aeronautical as well as in maritime environments.

Figure 3 illustrates, in the left side, how antenna assemblies with no controllable sensitivity and with controllable sensitivity angle are operated. In the upper, left illustration, the same signal is fed to the two antennas 12/14, or the two signals from the antennas is simply added. In the lower illustration, a phase difference may be generated whereby the angular sensitivity or gain may be controlled.

In the right side of figure 3, two 3D-radiation pattern plots are shown for different phase differences. In the uppermost, right illustration, the assembly 10 has been controlled to have a high gain towards an approx. 45° elevation direction above the horizon (by having a 180° phase shift between the two antennas 12/14), and in the bottommost right illustration, the radiation pattern has been controlled to be approx. 25° over the horizon in the elevation plane by having a 112° phase delay from/to the upper antenna 12. Both antenna pattern simulations and measurements confirm the advantageous properties of the assembly 10.

As is well-known to those in the art, stacking or spacing of two identical e.g. dipole antennas will maximally yield a gain-increase of approx. 3 dB, if the antenna spacing is typically on the order of 0.9 to 1 λο (free-space wavelenght). In the example shown in figure 2, the spacing between the two vertically stacked antennas 12 and 14 is only on the order of 0.7 λο and yet from figure 2, it is observed that a peak-gain increase of the two-element group of up to approx 6 dB is achieved. The explanation for this high two-element antenna gain is owing to the incorporating the horizontally oriented, metallic so-called ground plane 16.

It has been noted that the assembly 10 can comprise the electrically conducting horizontally oriented ground plane 16 integrated directly in close proximity to the upper antenna 12, without this - in terms of its electromagnetic size: Relatively large - upper ground plane is having any adverse effect(s) on the performance of neither the lower antenna 14, the upper antenna 12, nor the combined assembly 10. Furthermore, another important characteristic of the assembly 10 is that the upper antenna 12 allows the simple implementation of a compact, high-performance and low-cost beam-forming network to be integrated directly with the upper ground plane 16. Such an integrated feeding- and/or impedance-matching network, e.g. implemented in microstrip or stripline technology, will significantly reduce the complexity and cost of the antenna 12 while at the same time maximizing the impedance and radiation bandwidth of the upper antenna 12, as compared to implementing the feeding network below the lowermost antenna 14 - owing to the short transmission line(s) between the input of the upper antenna 12 and the feeding and matching network.

The variation in the Zenith directivity versus the diameter of the upper ground plane 16, and versus the distance from the lower ground plane 18 to the upper ground plane 16 is shown in figure 4. To illustrate the effect of the upper ground plane 16 to the performance of the lower antenna 14, the upper antenna 12 was removed during the calculation of figure 4. In the left illustration of figure 4, it is seen that the Zenith directivity of the lower antenna 14 is around 2.4 dBi with no or a very small upper ground plane 16, increasing to more than 6.5 dBi for the upper ground plane 16 sized approx 0.5 λ₀. In this example, the distance to the upper ground plane 16 is on the order of 0.7 λ₀. In the right illustration of figure 4, it is seen that the distance between the ground plane 18 and the ground plane 16 allows for the Zenith- directivity of the lower antenna 14 to be varied from 4.5 dBi and up to more than 7 dBi. For the calculation of the right illustration of figure 4, the diameter of the upper ground plane 16 was on the order of 0.5 λ₀.

An example of a micro strip feeding layout implemented on the upper side of an element also having the ground plane element 16 is depicted in figure 5, which element simultaneously functions both as a ground plane for the upper antenna 12, and as a ground plane for a microstrip feeding network 26 also for the upper antenna 12. The conductors 12' contact the network 26 at openings or pads 22, and the input/output to the network 26 is at pads 24 and 24' which may be used for controlling the orientation of circularly polarized radiation (RCHP and LHCP, respectively). Then, this element may be a multilayer PCB having, at its upper side - except for a few holes - an unbroken copper-clad surface. The bottom side may be that illustrated in figure 5 which has the microstrip feeding and impedance matching network 26 used for creating the desired dual-circularization input/outputs signals, seen at left and designated "RHCP" and "LHCP".

Even though the present assembly is particularly interesting for use in receiving signals from e.g. satellites, it may also be used for transmitting signals. A simple manner of emitting a signal is to use only one of the antennas 12/14, but if both antennas 12/14 of the assembly 10 are used, higher directivity may be achieved, which is advantageous in many situations.

Especially in maritime SATCOM, as is illustrated in figure 6, the net-signal (from the low- elevation satellite 30) received at the antennas 12/14 is the vector-sum of the parts 32/34 received directly from the satellite 30 (through the direct path) and the signal part(s) 36/38 received from reflections e.g . from the sea surface 40. These two or more receiving signals will combine in amplitude and phase at the antenna assembly 10. The net effect owing to the direct, line-of-sight-path signals combining with the one or more reflections is that the received signal-strength will exhibit some fluctuation over time; this phenomena known as fading or multi-path effect. Such signal-strength variation is particularly pronounced for low- elevation mobile SATCOM . However, this inherent and unavoidably problem can to some extent be overcome by the present embodiment using two or more receiving antennas 12 and 14 implemented in e.g . a so-called space diversity system as shown in figure 6, where - in this example - the two antennas 12/14 are vertically separated at a distance of "h" and both antennas 12/14 are used to receive and/or transmit signals from the same satellite(s) .

Now with reference to figure 6, it is noted that, owing to the path-length difference between the direct-path signals 32 and 34 (see enlarged part in the circle), the direct signal 34 from the satellite 30 received by the lower antenna 14 will be slightly delayed compared to the direct signal 32 received by the upper antenna 12 (the direct-signal path difference is indicated by the arrow A) . Likewise, but in the opposite fashion, the reflected signal 36 received by the upper antenna 12 will be slightly delayed compared to the reflected signal 38 received by the lower antenna 14.

It is yet another desirably feature of the assembly that it does not need any attitude information about e.g . a vessel on which it is mounted, since it may automatically adjust the antenna pattern to best track the satellite using the GPS position (which is already available in the satellite receiver) and from the knowledge of the satellite orbital position (which is also known) and/or by using the maximum-ratio combining technique. This assembly thus may provide a substantially simpler and lower-cost, high-performance SATCOM system.

As earlier mentioned, and as is well known to those skilled in the art, a purely polarized, e.g. RHCP, signal with ideal axial-ratio and transmitted from a satellite towards a maritime user positioned e.g. at the sea surface, will be received by the user as a more or less elliptical polarized signal, owing to the reflections caused by the signal impinging upon and being reflected by the sea surface, and scattered towards the receiver antenna, where it will combine with the direct-path signal, as illustrated in figure 6. The net result from this is that the co-polarization signal will undergo some fluctuation in level and that there will be a significant amount of the cross-polarization signal (in this example the LHCP) at the antenna. This fading or multipath-effect will cause fluctuations in the RHCP-signal . If, however, the receiving antenna furthermore has the capability to receive both the co-polarized, RHCP, signal and also the cross-polarization, LHCP, signal, the net result is that the overall dual- polarization receiving system may be made even more immune towards the effect of fading . This technique is commonly known as polarization diversity and takes advantage of receiving the total amount of usable signal-power impinging onto the receiving antenna(s) . Since the invention also allows for dual-polarization antenna elements to be used, with one or both antenna elements having the dual-polarization receiving and/or transmitting capability, the invention is able to combine the effects of both space-diversity and polarization-diversity, which marks a further advantage.

Claims

1. An antenna assembly comprising at least two antennas each adapted to communicate at at least a predetermined wavelength, the antennas being positioned along a predetermined direction, wherein : the antennas each extend at least 0.1 times the wavelength along the predetermined direction and the assembly further comprises an electrically conducting element: o positioned between two of the antennas and o having an area which, projected on to a plane perpendicular to the predetermined direction, covers between 0.07 times the predetermined wavelength squared and 0.7 times the predetermined wavelength squared .

2. An antenna assembly according to claim 1, further comprising a circuit for converting a first signal into corresponding second electrical signals for each of the antennas, the converting means being adapted to provide a first and a second second signal each for transmission to one of the antennas, where the first second signal has a phase difference in relation to the second second signal .

3. An antenna assembly according to claim 1, further comprising a circuit for converting third electrical signals received from each of the antennas into a fourth signal, the converting means being adapted to alter a phase of one of the third signals and subsequently combine the third signals to generate the fourth signal .

4. An antenna assembly according to any of the preceding claims, further comprising conductors for conducting signals to/from the antennas, the conductors extending at least substantially parallel to the predetermined direction, at least one of the conductors extending through one of the antennas and is connected to another of the antennas, the other of the antennas having a plurality of electrically conducting antenna elements, the at least one conductor and the plurality of conducting antenna elements being electrically connected to a feeding/splitting circuit provided at or on the electrically conducting element.

5. An antenna assembly according to any of the preceding claims, wherein at least one of the antennas is a drooping dipole antenna.

6. An antenna assembly according to any of the preceding claims, wherein at least one of the antennas is a quadrifilar helix antenna .

7. An antenna assembly according to any of the preceding claims, wherein at least one of the antennas comprises at least 4 electrically conducting antenna elements positioned so that, from a centre axis parallel to the predetermined direction, two planes perpendicular to each other extend, between which the individual conducting antenna elements extend, opposite pairs of the antenna elements being connected to an individual circuit also connected to an electrical conductor for transmitting a signal to or from the pertaining pair of antenna elements.

8. An antenna assembly according to any of the preceding claims, further comprising an additional electrically conducting element positioned oppositely to one of the two of the antennas, a distance between the electrically conducting element and the additional electrically conducting element exceeding 0.5 times the predetermined wavelength.

9. A method of operating an antenna assembly comprising at least two antennas each adapted to communicate at at least a predetermined wavelength, the antennas being positioned along a predetermined direction wherein : the antennas each extend at least 0.1 times the wavelength along the predetermined direction and the assembly further comprises an electrically conducting element: o positioned between two of the antennas and o having an area which, projected on to a plane perpendicular to the predetermined direction, covers between 0.07 times the predetermined wavelength squared and 0.7 times the predetermined wavelength squared, the method comprising : receiving a signal from each antenna and generating an output signal from the received signals or generating, from an original signal, a second signal for each antenna, and feeding the second signals to the antennas.

10. A method according to claim 9, wherein the generating step comprises providing a first and a second second signal each for transmission to one of the antennas, where the first second signal has a phase difference in relation to the second second signal.

11. A method according to claim 9, wherein the receiving step comprises altering a phase of one of the received signals and subsequently combining the phase altered signal and the other received signal(s) to generate the output signal .

12. A method according to any of claims 9-11, further comprising conducting signals to/from the antennas via conductors extending at least substantially parallel to the predetermined direction, at least one of the conductors extending through one of the antennas and is connected to another of the antennas, the other of the antennas having a plurality of electrically conducting antenna elements, the at least one conductor and the plurality of conducting antenna elements being electrically connected to a feeding/splitting circuit provided at or on the electrically conducting element.

13. A method according to any of claims 9-12, wherein at least one of the antennas is a drooping dipole antenna.

14. A method according to any of claims 9-12, wherein at least one of the antennas is a quadrifilar helix antenna .

15. A method according to any of claims 9-14, wherein at least one of the antennas comprises at least 4 electrically conducting antenna elements positioned so that, from a centre axis parallel to the predetermined direction, two planes perpendicular to each other extend, between which the individual conducting antenna elements extend, opposite pairs of the antenna elements being connected to an individual circuit also connected to an electrical conductor for transmitting a signal to or from the pertaining pair of antenna elements.

16. A method according to any of claims 9-15, further comprising an additional electrically conducting element positioned oppositely to one of the two of the antennas, a distance between the electrically conducting element and the additional electrically conducting element exceeding 0.5 times the predetermined wavelength.