GB2308504A - Phase compensation in a patch antenna - Google Patents

Abstract

A multi-element patch antenna having three antenna elements (20,21,22), substantially symmetrical connections (23,24,25) between elements and a centre-point (10), which is directly connected to the centre of a coaxial feed line. The connections between the elements and the centre-point are of differing lengths in order to compensate for phase shift in the received signal. Also disclosed is a method to prevent tangling of the feed cable when the antenna is rotated.

Description

PATCH ANTENNA Field of the Invention This invention relates to the field of patch antennas, for example those suitable for use in survey applications, or receiving satellite-transmitted signals.

Background to the Invention It is desirable for rotary antennas, such as those used to transmit/receive signals from satellites to be relatively compact and easily installed, particularly for applications where the equipment is to be used in a confined space on a ship.

Of those antennas known to the applicant, even the smallest is still relatively heavy, bulky and of complex construction and thus is prone to wear (which in turn can degrade reliability).

Known systems also tend to be relatively complex; for example having up to four movement-controlling stepper motors and a "wrap/unwrap" coaxial feed line as described below.

Generally, a coaxial feed line is needed between the antenna dish itself and the amplifier and other processing equipment which may or may not be in close proximity to the dish. In order to permit 360" rotation, with a "fixed" coaxial line in place, a "wrap/unwrap" system is conventionally used.

In such a system as the dish rotates, say clockwise, the co-ax coils or "wraps" up inside a specially designed housing. Once the co-ax is fully coiled the dish hits an end-stop and cannot rotate clockwise any further. If the dish needs to be positioned a few degrees, for example, further clockwise than the end-stop will allow, the dish must rotate in an anti-clockwise direction so as to "unwrap" the co-ax, continuing on in an anti-clockwise direction to "wrap" the co-ax the other way (eventually reaching another end-stop). In this way, the full 360" can be covered.

This type of "wrap/unwrap" system has numerous disadvantages. Most obviously, the arrangement can mean that a desired movement of only a few degrees requires the dish to rotate almost 3600 to reach it because of the endstop arrangement. This excessive movement causes wear on the moving parts of the system and loss of signal during the unwrap process. In addition the constantly "wrapped" or "unwrapped" co-ax is prone to tangling and wear. The housing and necessarily long co-ax increase the weight, bulkiness and expense of the equipment.

The patch antenna layout itself is also complex (for example having up to nine elements). Necessarily, the connections to the elements are lengthy so as to make best use of the limited space available on the dish. The longer the connections, the greater likelihood of significant signal losses.

The demodulator and control unit are typically remote from the antenna dish itself (for example on a ship they may be below deck). This necessitatcs the use of long connections (co-ax and the power/data line) between dish and demodulator/control unit.

It is an object of the present invention to provide a lightweight patch antenna of low complexity and good performance.

Brief Description of the Drawings In the drawings: Figure 1 is a schematic representation of the antenna system; Figure 2 is a schematic representation of the layout of the patch antenna elements.

Description of the Preferred Embodiments The patch antenna of this invention may be used in conjunction with the apparatus described in our co-pending patent application (which has the same filing date as this application) entitled Antenna Mounting.

Referring now to the drawings of this present invention, a support 1 is mounted on turn-table 2 as shown in Figure 1. The turn-table 2 is driven by a stepper motor 3 and drive belt 4 so that it revolves about axis X-X.

A co-ax line 5 is connected to low noise amplifier 6 (LNA); in use, this connection is stationary with respect to the revolving turn-table. Bearing 7 is fitted with a length of metal pipe 8. Pipe 8 and bearing tube 13 are preferably of a diameter great enough to allow the connector (not shown) which connects the co-ax 5 to LNA 6 to easily pass through.

Support 1 is also provided with a bearing 9, through which co-ax line 5 passes.

The end 10 of co-ax line 5 is directly attached to the surface of lightweight patch antenna dish 11 (for example by soldering). This eliminates the need for an extra connector thus reducing potential losses.

The elevation angle of antenna dish 11 can be manually adjusted by pivoting the top portion of support 1 about pivot 12 or, optionally, could be remotely controlled while tracking the signal source using a stepper motor arrangement (not shown).

In use, turn-table 2 is driven by stepper motor 3 via drive belt 4 to rotate about axis X-X which causes support 1 and consequently dish 11 to also rotate, thus allowing the correct azimuth for dish 11 to be selected.

During this rotation, co-ax line 5 does not rotate (about its longitudinal axis) because one end of it is fixed (with respect to the turn-table 2) to LNA 6.

Owing to the low mass of antenna dish 11 (and its direct connection to co-ax 5), the dish 11 is forced to turn i.e rotate about axis Y-Y preventing the twisting or tangling of co-ax 5. This effect occurs automatically, i.e as a result of the torque of the flexible connection between antenna dish 11 and pipe 8 and does not need to be powered in any way (other than by the stepper motor 3 which drives turn-table 2).

Since the co-ax 5 does not coil up as in the "wrap/unwrap" prior art, the turntable can (theoretically, at least) rotate indefinitely in one direction without encountering the end-stop problem outlined above. The apparatus as a whole is very much simpler, lighter (and hence cheaper) than conventional apparatus and is less prone to wear. Wear could be further reduced by introducing a gradual acceleration/deceleration as the turn-table starts and stops moving, or changes direction.

The feed to dish 11 need not necessarily be exactly at the centre-point indicated by lead line 10 and axis Y-Y. Alternatively, a hole (not shown) could be provided in bearing tube 13 so that the feed could be taken to an off-centre position such as that indicated (for example) at Z.

Since there is a straightforward connection between LNA 6 and dish 11, the coax line 5 can be completely removed and replaced very easily, especially since pipe 8 is of sufficient diameter to allow the co-ax/LNA connector (not shown) to pass through. This means that the turn-table assembly does not need to be dismantled in order to remove the co-ax line 5.

Turning now to Figure 2, dish 11 is made from lightweight material, of low dielectric loss and low dielectric constant, sandwiched between an aluminium ground plane and a thin layer of PCB material. The three antenna elements 20, 21, 22 are arranged with relatively short and almost symmetrical connections 23, 24, 25 to centre-point 10 which is directly connected to the centre of the coaxial feed line (not shown). The differing lengths of connectors 23, 24, 25 compensate for the phase shift of the received signal between each of the three elements, ensuring that they reach cenlrc-point 10 in phase. Because of the right-hand circular polarisation, the signal at element 21 is delayed by one-third of a wave-length with respect to element 20 and the signal at element 22 is delayed by two-thirds of a wave-length with respect to element 20.

Any losses suffered between the low noise amplifier 6 and antenna dish 11 are extremely significant. It is thus desirable to minimise the number of connectors etc between the two. The system of the present invention thus has as direct a connection as possible between the two with other circuitry (filters etc) being located after the low noise amplifier 6.

The problems associated with having the demodulator and control unit remote from the antenna are reduced by having the demodulator and control circuitry combined and integrated into the antenna unit. This reduces losses and improves compatibility (since normally the control unit and demodulator would be bought from different suppliers, both separately from the antenna itself).

Control of the antenna is thus improved as feedback between the antenna and control unit is facilitated and more reliable. Using this integrated system, much more information is available as communication is improved between the various parts of the system. A single integrated unit has the advantage for the user of a simple and easy installation and increased reliability.

Claims (6)

1. A multi-element patch antenna having three antenna elements, substantially symmetrical connections between elements and a centre point, said centre-point being directly connected to the centre of a coaxial feed line.

2. A multi-element patch antenna having three antenna elements substantially symmetrical connections between element and a centre point, said centre-point being directly connected to the centre of a coaxial feed line, the substantially symmetrical connections being of differing lengths, the selected lengths being suitable to compensate for the phase shift of the received signal, in use, between each of the three elements, ensuring that they reach the centre-point in phase.

3. A multi-element patch antenna comprising a dish having layers of materials selected from the group comprising lightweight packing material of low dielectric loss and low dielectric constant, aluminium and PCB material.

4. A multi-element patch antenna having connections between each element and a centre-point of differing lengths such that the received signal, in use, at a first element is delayed by a one-third of a wavelength with respect to a second element and the received signal at a third element is delayed by two-thirds of a wavelength with respect to said second element.

5. A multi-element patch antenna substantially as described herein with reference to and as illustrated by any appropriate combination of the accompanying drawings.

6. A satellite receiver including a multi-element patch antenna substantially as described in any of the preceding claims, a demodulator and control unit combined and integrated therewith to provide an integrated receiver unit.