GB2237686A - Antenna alignment - Google Patents

Abstract

A satellite antenna 4 is aligned in accordance with indications given by an indicator 38 which is driven by a signal superimposed on an antenna cable 8 by a remotely-located receiver 10. The receiver responds to error checking codes in the received signal for determining the degree of misalignment and deriving therefrom the signal for driving the indicator. <IMAGE>

Description

ANTENNA ORIENTATION INDICATOR This invention relates to antennas, and particularly but not exclusively to antennas for receiving microwave signals broadcast by satellite.

It is well known that precise alignment of the antenna is essential for good signal reception in satellite broadcasting systems. Equipment is available to achieve this, but it is expensive and often requires the use of skill to operate it. It is becoming increasingly important to be able to supply antennas which can be both purchased and set-up inexpensively. One proposal for achieving this is described in our co-pending British Patent Application No. 8827773.6, wherein there is described a structure incorporating an indicator mounted in proximity to the antenna and providing an indication of whether or not the antenna is correctly oriented in response to a signal from a circuit provided to convert the antenna output signal into a lower-frequency signal for supply to an antenna cable.This arrangement, however, although very useful in certain circumstances can provide misleading results if there is more than one signal source, as the indicator may respond to signals received from a source other than that to which the antenna is to be aligned, or may respond to a combination of signals from different sources.

The present invention provides an alternative arrangement which, in the preferred embodiment, enables a single unskilled person to align an antenna by using equipment which costs a small fraction of the cost of the antenna itself. The preferred embodiment achieves more accurate alignment than the arrangement of the co-pending patent application described above, and is not subject to error due to the reception of signals from more than one source.

According to one aspect of the invention there is provided a signal receiving system comprising an antenna, receiving apparatus located remotely from the antenna and connected thereto via an antenna cable or lead, and an indicator in proximity to the antenna for providing an indication of whether or not the antenna is correctly aligned in response to a signal transmitted along the antenna cable from the receiving apparatus. Other aspects of the invention are set out in the accompanying claims.

The invention is particularly useful when applied to systems for reception of broadcast signals which include redundancy codes for error detection or error correction. For example, such codes are included in television broadcasts transmitted using the DMAC standard. It is known in such systems for the receiving apparatus to derive a value, referred to herein as the bit error rate or BER, indicative of the number of errors in the digital data received by the apparatus. In the preferred embodiment of the present invention, this value is converted into a signal representative of antenna misalignment, which signal is then transmitted along the antenna cable toward the antenna where it operates the indicator.

In some systems, such as the DMAC system, the transmissions also include a code indicative of the identity of the source of transmission. In the preferred embodiment, the indicator signal is provided in dependence upon whether this identification code matches a predetermined code, so that the person aligning the antenna will not incorrectly align it to the wrong source.

An arrangement embodying the invention will now be described by way of example with reference to the accompanying drawing, the single figure, Figure 1, of which shows in schematic form a signal receiving system of the present invention.

The system 2 comprises a satellite signal receiving antenna 4 coupled to frequency-converting circuitry 6. The circuitry 6 converts the antenna signal of about 12 GHz into an intermediate frequency of about 1 GHz for supply to an antenna cable or lead 8 which delivers the signal to a receiver 10. The circuitry 6 is mounted in a housing which is fixed either to the antenna 4 or to a support carrying the antenna 4. The purpose of the circuitry 6 is to convert the antenna signal into a frequency which can be transmitted along a relatively inexpensive and fairly long antenna cable 8 without significant degradation. The circuitry 6 is often referred to as the low-noise block or LNB.

The receiver 10 includes an RF tuner 12 responsive to the intermediate frequency signals from the antenna cable 8. The demodulated signals are delivered to a standard DMAC decoder 14, which provides signal outputs on a number of lines such as that shown at 16 and which also extracts from the signals codes which can be read by a microprocessor 18.

The receiver 10 also includes a power supply 20 coupled via a choke 24 to the antenna cable 8. The power supply applies a constant DC voltage, of e.g. 17 volts, to the antenna cable 8, which is used to power the LNB 6. The choke 24 prevents any significant loss of the intermediate frequency signal supplied by the LNB 6 which might otherwise be caused by the coupling of the cable to the power supply 20.

The receiver 10 as described so far is of a known form. A further description of the other receiver functions will be omitted as they are not relevant to the present invention.

The only added hardware components in the receiver 10 are a diode 22, a switch 26 and a coupling circuit 28. The switch 26 is operable by a user to put the microprocessor 18 into a "set-up" mode, the purpose of which will be explained below. If desired, the switch 26 could be omitted, and its function could be adopted by one or more of the other normally-provided switches (not shown) of the receiver, or of a remote control unit therefor, or the function could instead be provided by the operating program of the microprocessor 18. The coupling circuit 28 is used to provide pulses from an output line 30 of the microprocessor 18 to the antenna cable 8 via the choke 24. The circuit 28 may simply comprise a diode 32 and capacitor 34 connected in series.The choke 24 performs the additional function of isolating the intermediate frequency of the broadcast signal from the pulses, as well as from the d.c. of the supply 20. The diode 22 isolates the supply 20 from the pulses applied via coupling circuit 28.

At the antenna end of the cable 8 there is provided an indicator circuit 36. This includes a speaker 38 connected to the outer conductor of the co-axial cable 8 and, via a radio-frequency choke 40 and capacitor 42, to the central conductor 44 of the co-axial cable. The components 40 and 42 block both the DC signal applied by power supply 20 to the cable and the intermediate frequency signal applied by LNB 6 to the cable. However, the pulses applied by microprocessor 18 and coupling circuit 28 to the cable are passed by these components and drive the speaker 38 to provide an audible frequency. The pulses may for example have a frequency of about 500 Hz to 2 KHz, and a voltage of about 2 to 3 volts. The indicator circuit 36 preferably additionally comprises d.c.

stabilisation components for connecting the cable to the LNB 6. In particular, a resistor 46 coupled in parallel to a capacitor 48 are connected between the central conductor 44 and a node 50 to which the LNB is connected. A series resonant circuit comprising a radio-frequency choke 52 and a d.c. smoothing capacitor 54 are connected between the node 50 and the outer conductor of the cable 8.

Although the indicator 38 in the present embodiment is a speaker, it could alternatively be a meter, or possibly an LED or other visual indicator capable of providing an indication which can be controlled by the manner in which pulses are applied by the microprocessor 18 to the output line 30. For example, varying the frequency of the pulses would vary the tone of the audible signal emitted by a speaker, or vary the flashing rate of an LED. Varying the mark-space ratio of the pulses would vary the average voltage level applied to the indicator, thereby producing a variable level indication on a meter or a variable intensity if the frequency is high and an LED is used.

The operation of the embodiment in setting up an antenna is as follows. First, the user operates the switch 26 to put the microprocessor 18 into the set-up mode. At that time, if the aerial is severely misaligned, no signal will be picked up, and therefore no codes will be provided by the decoder 14. The user must therefore first align the antenna approximately to the signal source of interest. This could be achieved either using a compass or the like, or a coarse orientation indicator may be provided. For example, the circuit 6 may be provided with an indicator in accordance with the proposals set out in co-pending U.K. Patent Application No. 8827773.6.For this purpose, the same indicator 38 may be used for responding to the signal strength indicated by the frequency-conversion circuit 6 and for responding to the indication signal formed by the pulses applied by the microprocessor 18.

Once the antenna 4 is approximately aligned, and a weak signal is being picked up and decoded by the decoder 14, the microprocessor 18 checks the identity of the signal source by determining whether the channel identification number decoded by DMAC decoder 14 matches a predetermined code stored by that receiver 10. Assuming that it does match, the microprocessor then reads the bit error rate BER decoded by the decoder 14. In response to the BER, the microprocessor then applies to output line 30 pulses having a fixed width, but a repetition period which is inversely proportional (but which may alternatively be proportional) to the detected BER.

Thus, for large values of the BER, the repetition rate is low so the mark-space ratio and the frequency of the pulse train are high. An indication of the frequency or mark-space ratio is provided by the indicator 38. The user then adjusts the antenna orientation in very small amounts, until the indicator 38 indicates that the lowest possible frequency or mark-space ratio has been achieved. The antenna is then correctly oriented, and the switch 26 can be operated to terminate the set-up mode.

It will be understood that the orientation of the antenna can take place while the operator is in proximity to the antenna and the indicator 38, and remote from the receiver 10.

Once the setting-up has been achieved, the indicator circuit 36 is preferably removed.

Preferably, the indicator circuit 36 is in the form of a unit which is detachably connectable at points 56 to the system between the antenna end of the cable 8 and the LNB 6.

In an alternative embodiment, the indication circuit 36 forms part of the LNB 6, and is mounted in the same housing.

A switch such as that shown at 58 could be provided for selectively disconnecting the indicator circuit from the signal path and thus rendering the indication circuit inoperable.

Claims (18)

CLAIMS:

1. A system for reception of broadcast signals, the system comprising an antenna, receiving apparatus located remotely from the antenna and coupled thereto by an antenna lead, and an indicator mounted in proximity to the antenna and responsive to an indication signal supplied by the receiving apparatus via the antenna lead for providing an indication dependent on the orientation of the antenna.

2. A system as claimed in claim 1, wherein the receiving apparatus is capable of deriving from a received signal redundancy codes for error detection and/or error correction.

3. A system as claimed in claim 2, wherein the receiving apparatus is arranged to place on the antenna lead an indication signal determined by the detected error indicated by the redundancy codes.

4. A system as claimed in claim 2 or 3, wherein the receiving apparatus is arranged to derive from a broadcast signal a code indicative of the source of the signal.

5. A system as claimed in claim 4, wherein the receiving apparatus is arranged selectively to apply said indication signal to the antenna lead in dependence upon the source indicated by the derived code.

6. A system as claimed in any preceding claim, including means enabling the user to place the receiving apparatus into a set-up mode, the receiving apparatus being able to apply said indication signal to the antenna lead only when it is in the set-up mode.

7. A system as claimed in any preceding claim, wherein the indication signal comprises pulses having a frequency or a mark-space ratio dependent upon the degree of antenna misalignment detected by the receiving apparatus.

8. A system as claimed in any preceding claim, including a frequency-conversion circuit located in proximity to the antenna and arranged to convert the frequency of the signal from the antenna to a lower-frequency signal for supply to the antenna lead.

9. A system as claimed in claim 8, wherein the receiving apparatus is arranged to supply to the antenna lead a supply voltage for powering the frequency-conversion circuit.

10. A system as claimed in claim 8 or claim 9, wherein the indicator is detachably attached to terminals located in the signal path between the antenna end of the lead and the frequency-conversion circuit.

11. A system as claimed in claim 8 or claim 9, wherein the indicator forms part of the frequency-conversion circuit, switch means being provided for selectively disabling the indicator.

12. A system as claimed in any preceding claim, wherein the antenna is arranged for reception of satellite broadcasts.

13. Receiving apparatus for a system as claimed in any preceding claim, the receiving apparatus being capable of detecting misalignment of the antenna and of superimposing on the antenna lead an indication signal indicative of misalignment for driving an indicator located in proximity to the antenna.

14. A frequency-converting circuit for a system as claimed in claim 11, the circuit having means for converting the frequency of a signal from the antenna to a lower-frequency signal for application to the antenna lead, and an indicator responsive to a signal superimposed on the antenna lead by the receiving apparatus for providing an indication of antenna alignment.

15. A circuit as claimed in claim 14, including switch means for selectively disabling the indicator.

16. A method of aligning an antenna, the method comprising positioning the antenna approximately in alignment with a signal source, and adjusting the orientation of the antenna in accordance with indications provided by an indicator driven by an indication signal superimposed on an antenna lead coupling the antenna to a remotely-located receiving apparatus which receives a broadcast signal from the antenna and derives the indication signal therefrom.

17. A signal receiving system substantially as herein described with reference to the accompanying drawing.

18. A method of aligning an antenna, the method being substantially as herein described with reference to the accompanying drawing.