EP0540124B1

EP0540124B1 - Satellite antenna system

Info

Publication number: EP0540124B1
Application number: EP92203336A
Authority: EP
Inventors: Aurinsalo Jouko; Ismo Huhtinen; Arto Hujanen; Kalevi Laukkanen; Ilkka Marttila; Risto Mutanen; Ilkka Pitkänen; Tapio Salminen; Stefan Tallquist
Original assignee: VTT Technical Research Centre of Finland Ltd
Current assignee: VTT Technical Research Centre of Finland Ltd
Priority date: 1991-10-30
Filing date: 1992-10-29
Publication date: 1998-01-14
Anticipated expiration: 2012-10-29
Also published as: FI91028B; FI915134A0; EP0540124A1; FI915134A7; DE69224033D1; FI91028C; DE69224033T2

Description

The invention relates to a satellite antenna system defined in the preamble of claim 1.

Such an antenna system is known in itself from the publication "Electronics & Communications in Japan", Part 1 - Communications, Vol. 71, No. 4, 1988, New York (US), pages 117-124, T. Hori et al. "Switched-Element Spherical Array and Antenna for Wide Scan Angle".

This known steerable antenna system is suitable to be used for mobile communications.

Although this publication describes the principle of the set-up of a spherical array antenna with a wide scan angle it does not give any information about the practical structure of such an antenna system.

The invention aims to propose an antenna system of the above-mentioned kind which is economical in production costs and does suitable for mass production, can be fitted in a small space and retains the advantages obtained by the configuration as proposed in the above-mentioned publication.

These aims are achieved by the measures as described in the characterizing clause of claim 1.

Preferred embodiments are described in the subclaims. The configuration as proposed in claim 2 results in an excellent matching between the antenna element, its feed/reception circuit and the load.

The specific structure as described in claim 3 not only has the advantage of being stiff, but also provides a good isolation between neighbouring antenna elements. Furthermore, there is a stabilizing effect on the impedance of the feedpoint.

The measures of claims 4-6 result in that the beam of each active antenna unit, composed of two antenna elements, can be adjusted at, for instance, ± 7° with respect to the average normal direction of the antenna elements, so that flexible and reliable tracking of a satellite is possible. Moreover, with a relatively small number of antenna elements (for instance 12) in the circular configuration at least twice the number (thus 24) of beams can be produced, which cover the azimuth plane without any gaps.

When using the measure of claim 7 the elevation angles of all antenna elements are set to the same position in the azimuth plane, so that the elevation angles of all the beams of the antenna elements are correct with respect to the location of the satellite.

As for the further advantages of the invention, the following can be maintained: the satellite acquisition routines function rapidly after switching the system on. The system also takes into account a fairly large steering error of the radiator unit, with respect to the band width and gain loss of the antenna element. Moreover, an advantage of the system is that the signal to noise ratio is small. Further, the antenna system functions reliably irrespective of changes in short and long term signal levels. Yet another advantage of the antenna system is that it is rapidly recovered from disturbance situations. In addition, the antenna system causes a minimal amount of distortion to communication channels.

The invention is explained in more detail below, with reference to the appended drawings where

figure 1 is a top-view illustration of a satellite antenna system of the invention;

figure 2 gives a cross-section A - A of the satellite antenna system of figure 1;

figure 3a illustrates the antenna element seen from the top, and figure 3b the same antenna element seen from the side;

figure 4 is a block diagram of the main parts of the satellite antenna system of the invention;

figure 5 is a schematical illustration of the hybrid;

figure 6 is a schematical illustration of the phase shifter;

figure 7 is a layout of the power divider and phase shifter units;

figure 8 illustrates the beams; figure 8a illustrates the beams of the active pair of antenna elements, and

figure 8b illustrates the realizable beams of a radiator unit with 12 antenna elements; and

figure 9 is a block diagram of the steering unit, together with connected units and devices.

Figures 1 and 2 are schematical illustrations of a satellite antenna system of the invention. The antenna system comprises a radiator unit 1', with a number of identical antenna elements 1; 1¹ - 1¹² and their ground planes 9; 9¹ - 9¹², which are arranged on a disc-like base element 2. They are installed adjacently in a circular configuration, on the periphery of the base element 2. They are arranged at regular intervals from each other, so that they cover the whole circumference sector by sector. In this preferred embodiment, the radiator unit 1' is formed of twelve antenna elements 1¹ - 1¹² and twelve ground planes 9¹ - 9¹².

The antenna system also includes support members 3, whereby the elements 1¹ - 1¹² and their ground planes 9¹ - 9¹² of the radiator unit 1' are arranged at a suitable elevation angle α with respect to the base element 2. The support members 3 are for instance support bars, which are adjustable in length, either stepwise or continuously, manually or by means of a suitable actuator, in order to adjust the elevation angle α.

The antenna system also includes the control unit 4 and the switching unit 5. Among the antenna elements 1¹ - 1¹², to one and the same active antenna unit, for instance 6, there belong simultaneously two

antenna elements

1⁷, 1⁸, which are chosen by means of the control unit 4 and the switching unit 5 to receive circularly polarized electromagnetic radiation from a desired direction, and to transmit the same to essentially the same direction.

The satellite antenna system is provided with a radome 7 in order to protect the antenna elements 1¹ - 1¹² and other equipment pertaining to the antenna system.

The antenna elements 1; 1¹ - 1¹² are identical, discrete travelling-wave type air dielectric elements, as is illustrated in figures 3a and 3b. Each antenna element 1 is formed of a thin plate 8 made of some conductive material, advantageously metal such as copper or brass. The antenna element 1 includes a platelike part, i.e. curved part 8a, which has a standard width and is essentially circular in shape. This curved part 8a fills a 270° sector of the circle. The nominal electric length of the curved part 8a is near the employed wavelength. The curved part 8a is fitted at a standard distance h from the ground plane 9.

At both ends of the curved part 8a, there are provided narrowing

points

8b, 8c, advantageously having the shape of an isosceles triangle. The

points

8b, 8c are arranged, with respect to the plane of the curved part 8a, at an angle towards the ground plane 9. They are advantageously made of the same uniform plate material as the curved part 8a and bent thereof. In between the

points

8b, 8c there is a slot 10. The tips of the

points

8b, 8c are formed to be blunt, and are advantageously cut as straight blunt tips 8d, 8e, as is illustrated in figure 3a. In the vicinity of the straight-cut tips 8d, 8e, unsymmetrically with respect to the medium lines D-D, E-E of the

points

8b, 8c, i.e. at the sides of the blunt tips, there are arranged the

poles

11, 12. One pole serves as the feed pole, and the other as the load. By forming the

points

8b, 8c as blunt points, particularly as straight-cut blunt tips 8d, 8e, and by placing the

poles

11, 12 in an unsymmetrical fashion, there is achieved an optimal matching (roughly 50 ohm) in between the

antenna elements

14, 15 and the feed/reception circuit. The antenna element is symmetrical with respect to the straight line F-F running in the middle of the slot 10 and parallelly thereto. Depending on the employed direction of circular polarization, both

poles

11, 12 can serve either as feed or load poles.

Coupling pins lead from the

poles

11, 12 through the ground plane 9, electrically insulated therefrom, to the other side of the ground plane, where they are connected to the switching unit 5 and to the matched loads 15 (cf. figure 4). At the blunt tips of the

points

8b, 8c, such as the straight-cut tips 8d, 8e, the antenna element 1 is attached, by means of coupling pins, to the ground plane 9 serving as the base, but in such a fashion that an electrical connection is not created, i.e. an insulating plate or film is left in the coupling. Moreover, the antenna element 1 is supported, most advantageously in the middle of the curved part 8a, by an electrically insulating support 10a against the ground plane 9.

The radiation power of the antenna element 1; 1¹ - 1¹² can be adjusted by adjusting the width b of the curved part 8a of the plate 8, as well as its distance h from the ground plane 9. An optimal antenna gain is achieved, when roughly 90% of the power fed in the antenna element produces radiation and 10% is absorbed in the matched load.

The active antenna unit, for instance 6 in figure 1, is formed electrically by choosing two adjacent antenna elements, such as 1⁷, 1⁸ from among the antenna elements 1¹ - 1¹². Thus remarkable advantages are gained as compared to only one chosen antenna element. It has been found out that an optimal gain value and optimal width of the beam depend on the shape and size of the ground plane 9; 9¹ - 9¹². The direction of the main beam maximum is somewhat dependent on the used frequency, and deviates from the ground plane normal for about 5 - 15°. This angle deviation is also dependent on the shape of the ground plane. By means of two co-operating adjacent antenna elements 1¹ - 1¹², which elements are evenly spaced and fed parallelly with a suitable phase difference, the angular dependence of the main beam on the frequency can be practically eliminated.

The ground plane 9 is a trapezoid plane, the

edges

9a, 9b, 9c and 9d whereof are turned upwards, so that the ground plane forms a shallow trough (note: in figure 3a, the edges are turned to horizontal plane for illustrative purposes). The central width 1 of the ground plane 9 is of the same order as the total width of the plate 8 and the distance a between the antenna elements, and the length k of the ground plane in turn is of the order 1.5 - 2.0 x a. The depth s of the trough-like ground plane is of the order 0.1a. In a preferred embodiment the dimensions of the ground plane are: height k = 150 mm, width on the wide side 11 = 150 mm, width on the narrow side 12 = 90 mm, and depth h of the ground plane = 20 mm. The ground plane 9 is made of some suitable conductive material, such as aluminium.

Figure 4 is a block diagram of the satellite antenna system of the invention. The feed points 11; 11¹ - 11¹² of the antenna elements 1; 1¹ - 1¹² of the radiator unit 1' are connected to the two

switch groups

13, 14 of the switching unit 5, and respectively the second points 12; 12¹ - 12¹² are connected to the matched loads 15; 15¹ - 15¹². The antenna elements 1¹ - 1¹² are grouped so that

adjacent elements

1¹, 1²; 1², 1³; 1³, 1⁴, ... are connected to

different switch groups

13, 14. Thus the antenna elements 1¹ - 1¹² are connected alternately to the first switch group 13 or to the second switch group 14. In this case each

switch group

13, 14, has six outputs 13; 13¹ - 13⁶ and 14; 14¹ - 14⁶. By means of the control unit 4, the switch groups are controlled so that two

adjacent elements

1¹, 1²; 1², 1³; ... can always be chosen to function simultaneously as an active antenna unit.

The

inputs

16, 17 of the

switch groups

13, 14 are connected to the power divider and phase shifter unit 18. The power divider and phase shifter unit 18 comprises two phase shifters, i.e. the first phase shifter 19 and the second phase shifter 20, and a 180° hybrid 21. The first input 21a of the hybrid 21 is the input of the power divider and phase shifter unit 18, and it is connected to the detecting and measuring unit 29, as well as to the receiver-transmitter unit (not illustrated in the drawing). The second of the inputs of the hybrid 21 is grounded through the load 22. The

outputs

21c, 21d of the hybrid 21 are respectively connected to the input of the first phase shifter 19 and to the input of the second phase shifter 20.

The hybrid 21 is schematically illustrated in figure 5. The input and output ports are denoted with the same reference numbers as in figure 4. The input port 21a is a difference or D-port, and the input port 21b is a sum or S-port. When a signal is fed in through the D-port, from the

output ports

21c, 21d there are received output signals, which are in a 180° phase difference. When again a signal is fed from the S-port to the hybrid, the signals received from the output ports are in phase.

The phase shifters 19, 20 are realized by means of transmission cables and switch members, as is seen in figure 6. The

phase shifter

19, 20 includes two

parallel transmission cables

23a, 23b and 24a, 24b connected at one end to both the input and output port P1, P2, and

transmission cables

25a, 25b connected in between the ports. In addition, the

phase shifter

19, 20 comprises

switch members

28a and 28b installed at both ports P1, P2, at the ends of the

matching cables

26a, 26b; 27a, 27b. The

switch members

28a, 28b are realized by means of suitable diodes, and they can be switched to on and off positions. Both

switch members

28a, 28b are simultaneously in the same state, so that the

phase shifter

19, 20 is symmetrical in structure. The shifting properties of the

phase shifter

19, 20 from the port P1 to the port P2 or vice versa are thus similar. Such a loaded line

type phase shifter

19, 20 has small losses and a wide frequency band. Moreover, the phase shifter has good matching properties.

A preferred embodiment of the power divider and phase shifter unit 18 is illustrated as a layout in figure 7. The hybrid 21 and the

phase shifters

19, 20 are produced on the same substrate by using the microslip method.

In this embodiment, the

phase shifters

19, 20 are optimized to create a 33° phase shift. This means a roughly 14° (± 7° from straight middle line) angle difference for the beams obtained from the active antenna unit. By changing the states of the

phase shifters

19, 20, and particularly the states of their switch members 31, 32, the direction of the beam of the active antenna unit can be shifted in between the normal, i.e. middle direction, and extreme positions of the ground plane 9, as is explained below.

The phase shifters 19, 20 are utilized in steering the beam while performing the satellite tracking. By suitably manipulating the

switches

28a, 28b of the

phase shifter

19, 20, it is possible to move from the "right" beam to the "left" beam or to the middle beam "mid", which beams are illustrated in figure 8a. The switching from the "right" beam to the "left" beam of vice versa is realized so, that the states of both

switch members

28a, 28b of the

phase shifters

19, 20 are changed. Thus the phase shifters become mirror images as regards their properties. Correspondingly, if the state of only one switch member, either 28a or 28b, is changed, the beam is shifted from the middle beam "mid" either to the "right" or "left" beam. The width of the beams is somewhat affected by the elevation angle α, and also the employed reception and transmission band. The above explained power divider and phase shifter unit 18 is mainly designed for the frequency band 1,5 - 1,7 GHz.

An active antenna unit is formed of two

adjacent antenna elements

1¹, 1²; 1², 1³, ... For the active antenna unit, there can be arranged two beams as was explained above, the said beams deviating up to even 15°. By means of the radiator unit 1, a twofold number of beams is produced on the circle as compared to the number of the antenna elements 1¹ - 1¹². Thus 24 beams can be produced with 12 antenna elements, the said beams being spaced essentially evenly in a circular configuration in the azimuth plane. This is illustrated in figure 8b.

Figure 9 illustrates, in the form of a block diagram, the steering unit 30 together with the connector and peripheral devices. The steering unit 30 comprises a data processing unit 31a and a connected memory unit 31b. To the data processing unit 31a, there is further connected, by a suitable bus 32, a number of peripheral devices through the intermediation of the connector units, for instance the elevation angle detector 33 by intermediation of its connector unit 34, the A/D converter 35 of the detecting and measuring unit 29, the phase shifter switching unit 36 and the control 37 of the support members. The control unit 4 of the switching unit 5 also is connected to the bus 32 of the steering unit 30. In addition to this, the steering unit 30 includes a connector unit 38 for feeding information, such as programming and other information for the steering device, and a connector unit 39 for connecting the steering unit to external systems. Moreover, the steering unit 30 advantageously comprises a compass connector unit 40 in order to connect a compass 41 to the system.

The connector unit 34 of the elevation angle detector is connected to the elevation angle detector 33 measuring the elevation angle a of the antenna elements 1, and the said detector 33 is arranged in between the base element 3 and the antenna elements 1; 1¹ - 1¹² (cf. figure 2). The detecting and measuring unit 29 contains an intermediate frequency unit and a rf-detector, as well as a measuring unit for measuring the rf-level. This measuring signal is fed to the steering unit via the A/D converter 35. The switching unit 36 is connected to the

switch members

28a, 28b of the

phase shifters

19 and 20 of the power divider and phase shifter unit 18. In this case the support members 3 are provided with an actuator 42 (cf. figure 2), such as electric motor, in order to lengthen and shorten the support members 3. The control 37 of the support members is connected to the actuator 42 of the support members.

By means of the steering unit 30, the acquisition and tracking of the satellite is carried out as follows. When starting the satellite antenna system, the elevation angle a of the elements 1¹ - 1¹² of the radiator unit 1 is checked. If the elevation angle a does not correspond the location of the land terminal with respect to the latitude and the satellite, it is adjusted for instance at 10° intervals between 10 - 50°. The form and width of the beam is such, that the 10° adjusting steps are sufficient for a good antenna gain and signal to noise ratio. The correcting of the elevation angle α is carried out by adjusting the length of the support members 3 to be suitable by means of the actuator 42 of the support members, so that the desired elevation angle α is achieved. The information of the elevation angle α is sent to the steering unit 30 through the elevation angle detector 33.

After setting the elevation angle α, the acquisition of the satellite is started. The radio frequency level, i.e. rf-level of the satellite is measured by means of the detecting and measuring unit 29, by activating a pair of

antenna elements

1¹, 1²; 1², 1³; 1³, 1⁴; etc, one pair at a time, so that all of the antenna elements 1¹ - 1¹² are checked. The measured rf-level values, obtained from the said antenna units activated in succession, are recorded in the memory 31b. During this measurement, the beams of each pair of antenna elements are observed, both the right and the left beam, so that the azimuth plane will be scanned throughout by the beams illustrated in figure 8b.

When the first series of measurements is made, in the radiator unit 1' there is activated that pair of antenna elements which gave the maximum rf-level signal. Thereafter the system proceeds to tracking.

The tracking phase is based on the tracking of the rf-signal level by means of two beams, i.e. the left and the right beam, as was explaned above in connection with the devider and phase shifter unit. By following this procedure, it is attempted to keep the active antenna unit continuously electrically steered to the target satellite, in order to maintain the communication connection irrespective of the movements of the land terminal.

The steering unit can be provided with an electric compass 41 or other such detector, so that the turning of the vehicle can be observed, and the steering unit 30 can be effectively helped in maintaining the radiator unit steered to the satellite.

Claims

A satellite antenna system particularly for land mobile voice communications, comprising a radiator unit (1') which is steerable to the satellite in the azimuth plane, and is formed of a number of antenna elements (1; 1¹ - 1¹²) and their ground planes (9; 9¹ - 9¹²), a control unit (4) and a switching unit (5); the antenna elements (1¹ - 1¹²) of the radiator unit (1'), together with their ground planes (9¹ - 9¹²) being adjacently arranged in a circular configuration, so that among the said antenna elements (1¹ - 1¹²), to one active antenna unit (6) there belong simultaneously two adjacent antenna elements (1⁷, 1⁸), and the said two elements are chosen, by means of the control unit (4) and the switching unit (5), to receive circularly polarized electromagnetic radiation from a desired direction and to further transmit the same to essentially the same direction.
characterized in that each antenna element (1; 1¹ - 1¹²) is formed of a thin plate (8) made of conductive material, and that each antenna element (1; 1¹ - 1¹²) comprises a platelike, circular part, i.e. curved part (8a), which has a standard width (b) and is arranged at a standard distance (h) from the ground plane (9; 9¹ - 9¹²), and narrowing, pointed end parts' (8b, 8c), which are located at both ends of the curved part (8a) and arranged at an angle with respect to the plane of the curved part (8a) and towards the ground plane (9); with at the tips of these points (8b, 8c) poles (11, 12), of which the first is coupled to the feed/reception circuit and the second to the load (15).
The satellite antenna system of claim 1,
characterized in that the points (8b, 8c) of each antenna element are formed as blunt tips, (8d, 8e), and that the poles (11, 12) are located, in the vicinity of these blunt tips, an unsymmetrically with respect to the shape of the tips.
The satellite antenna system of claim 1-2, characterized in that the ground plane (9; 9¹ 9¹²) of each antenna element (1; 1¹ - 1¹²) is formed as a trough-like element, wherein the antenna element (1; 1¹ - 1¹²) is installed.
The satellite antenna system of claim 1-3, characterized in that the switching unit (5) comprises two switch groups (13, 14), whereby two adjacent antenna elements (1¹, 1²; 1², 1³; 1³, 1⁴; ...) can be chosen from a desired spot on the circular configuration.
The satellite antenna system of claim 4, characterized in that the antenna system comprises a power divider and phase shifter unit (18), provided with two phase shifters (19, 20) and a 180° hybrid (21), which power divider and phase shifter unit (18) is coupled to the switching unit (5).
The satellite antenna system of claim 5, characterized in that each phase shifter (19, 20) comprises two parallel transmission cables (23a, 23b; 24a, 24b) connected at one end to the input and output ports (P1, P2); transmission cables (25a, 25b) provided in betweeen the input and output ports (P1, P2); and switch members (28a, 28b), installed at the ends of the matched cables (26, 26b; 27a, 27b) connected to the input and output ports (P1, P2), in order to realize the phase shift.
The satellite antenna system of any of the preceding claims, characterized in that the antenna elements (1; 1¹ - 1¹²) are provided with suppbrt members (3), whereby the antenna elements are arranged on a base element (2) at a suitable elevation angle (a) with respect to the azimuth plane (B - B), and that the said support members (3) are advantageously adjustable members for adjusting the elevation angle (α).
The satellite antenna system of any of the preceding claims, characterized in that the antenna system is covered with a radome (7).
The satellite antenna system of any of the preceding claims, characterized in that the antenna system is provided with a steering unit (30) for the acquisition and tracking of the satellite, the said steering unit comprising a detecting and measuring unit (29) for measuring the rf-signal level; and that the said steering unit is connected to a control unit (4) and switching unit (5) for choosing two co-operating antenna elements (e.g. 6) among the antenna elements (1; 1¹ - 1¹²), which two chosen antenna elements give the best signal level in data communication with the satellite.