EP0980156A2

EP0980156A2 - Radio broadcasting system comprising an antenna system, which is constituted by at least a portion of the above-ground component of an electricity distribution network

Info

Publication number: EP0980156A2
Application number: EP99305586A
Authority: EP
Inventors: Paul Anthony Brown
Original assignee: Norweb PLC
Current assignee: Norweb PLC
Priority date: 1998-07-15
Filing date: 1999-07-14
Publication date: 2000-02-16
Also published as: GB9916555D0; GB2340310A; GB9815410D0; EP0980156A3

Abstract

According to the invention there is provided a broadcast system including a transmitter for transmitting communication signals which transmitter includes a transmitting antenna which is supported on or is constituted by an above-ground component of an electricity distribution and/or power transmission network.

In embodiments of the invention, a part of an existing EDN can take the place of a dedicated transmitting antenna, so avoiding the inconvenience and cost of finding a site for and installing a transmitting antenna.

Support towers (also known as pylons) which serve to support cables in an EDN are typically constructed as a metal space frame. A system embodying the invention can advantageously use such a tower, or part of such a tower, as a radiating element of the transmitting antenna.

Description

The present invention relates to a radio broadcasting system, and in particular to a radio broadcasting system which takes advantage of the existence of structural components of an electricity distribution and/or transmission network.
In the UK, it is conventional to describe a power network for 33kV and above as a "transmission network", and one for less than 33kV as a "distribution network". In this specification the term "electricity distribution and/or power transmission network" (abbreviated in this specification to 'EDN') is normally used, but general references to power networks and to transmission of signals are to be construed as applying to all such networks, unless the context indicates otherwise.
In order to provide a local radio broadcasting service, it is necessary to provide a local transmitter which includes a transmitting antenna. In cases where broadcasting is to take place in the medium and long wave bands (approximately 150 kHz to 1.65 MHZ), the transmitting antenna is of necessity quite large. This is a problem because it is often the case that a transmitter is sited close to a centre of population, but in such a place, a suitable secure site is likely to be costly to obtain. Indeed, planning regulations may not permit construction of a transmitting antenna at all. Furthermore, an additional installation is required to convey signals to be broadcast from their source (e.g. a studio) to the site of the transmitter.
It is an aim of the invention to provide a broadcast system which can be installed more easily and at less cost than a conventional broadcast system.
According to the invention there is provided a broadcast system including a transmitter for transmitting communication signals which transmitter includes a transmitting antenna which is supported on or is constituted by an above-ground component of an electricity distribution and/or power transmission network.
In embodiments of the invention, a part of an existing EDN can take the place of a dedicated transmitting antenna, so avoiding the inconvenience and cost of finding a site for and installing a transmitting antenna.
Support towers (also known as pylons) which serve to support cables in an EDN are typically constructed as a metal space frame. A system embodying the invention can advantageously use such a tower, or part of such a tower, as a radiating element of the transmitting antenna.
As will be understood by those skilled in the art, ground waves predominate in medium or long wave broadcast radio signals. Therefore, a good transmitting antenna can be formed by a vertical mast of height one quarter of the wavelength λ of the broadcast signal, or of an odd-integral multiple of that height. For a signal of frequency 0.5MHz, the value of λ/4 ≈ 150m, and for a signal of 1.5MHz, the value of λ/4 ≈ 50m. In the UK, the height of 132kV electric power cable towers is typically between 22 and 44 metres. This is apparently too small to be of use for transmission within this frequency range. However, the inventor has observed that EDNs typically include an earth conductor electrically connected between the tops of the support towers, and has realised that this earth conductor can be made to act as a capacity "hat" for the tower. The capacity hat loads the tower, with the result that it has the radio transmission capabilities of a comparatively higher tower.
In embodiments according to the last but one preceding paragraph, it will be recognised that a radiating element (the tower) is at ground potential. In order that the communication signals which are to be radiated by the tower are not carried to earth, the tower is tuned to place a current node for the communication signals at its base.
The present invention can particularly advantageously be used to transmit signals having a carrier frequency in the range of 150kHz to 1.65MHz. This covers long and medium wavelength bands commonly used for local radio transmission.
Typically, an EDN associated with an embodiment of this invention carries alternating current at a frequency of approximately 50Hz or 60Hz. In the UK, a suitable EDN may be a power transmission network operating at one of the following typical voltages: 132 kV, 275 kV or 400 kV. The EDN may have a single live phase, or may have multiple phases (most commonly 3 phases).
Signals may be conveyed from a signal source (for example, a studio) to the transmitter on a signal carrier, part of which signal carrier is supported on or is part of the EDN. An example of such a signal carrier is an optical fibre supported on a conductor of the EDN. Alternatively, they may be transmitted over a suitable radio link, typically in the microwave band. As a further alternative, signals might be conveyed to the transmitter over a conventional permanent telecommunication line.
Conveniently, the transmitter is located at a sub-station of the EDN. The sub-station can provide a secure location for apparatus associated with the transmitter, and typically a sub-station has a low-voltage supply of electricity and has telephone and other communications systems which can be used to support operation of the transmitter. Alternatively, the transmitter may be provided with a low-voltage supply of electricity from a local electricity distribution network.
Embodiments of the invention will now be described in detail, by way of example, with reference to the accompanying drawings, in which:
Figures 1 and 2 are somewhat schematic diagrams of, respectively, transmitters being first and second embodiments of the invention.
With reference first to Figure 1, a typical support tower 10 for an electricity transmission network includes several steel legs 12 each of which is approximately vertical. Various additional metal components are included to enhance the tower's strength (these have been omitted from the drawings in the interest of clarity). The whole tower thus forms an approximately vertical conductor. The tower 10 is supported on the ground on its legs 12. The legs 12 are not electrically isolated from the ground and the entire tower is therefore earthed at 16.
Current carrying conductors 20 are supported by the tower 10. In a typical tower, several side arms 22 (only one is shown in Figure 1) project from the tower 10 laterally. One or more insulators 24 are carried on each arm, the current carrying conductors 20 being suspended from the insulator or insulators, as the case may be. The tower 10 also carries an earth conductor 26 which is electrically bonded to the metal components of the tower 10. Normally, the earth conductor 26 is carried at the top of the tower 10.
In a electricity transmission network, there are many towers 10 as described above. The current carrying conductors 20 and the earth conductor 26 extend between successive towers 10.
It will be seen that the earth conductor 26 and the towers together form one system of interconnected electrical conductors, which is electrically isolated from the current carrying conductors 20.
Where a tower 10 is to be used as part of a transmitter for a radio broadcast system, a transmitter unit 18 is provided at the tower 10. The transmitter unit 18 is electrically connected to the tower 10 to feed radiofrequency signals to the tower such that the tower serves as a transmitting antenna for the transmitter. Signals to be transmitted by the transmitter can be conveyed to it through a dedicated communication line 30. Such a line is commonly installed at sub-stations within an EDN. Alternatively, signals to be transmitted may be conveyed to the transmitter unit 18 by an optical fibre link carried on the earth conductor 26.
In order for the tower 10 to operate successfully as a transmitting antenna, it is preferably tuned to match the signal that it is to transmit. To achieve this it must first be realised that the earth conductor 26 forms a capacitor with respect to the ground 14. The effect of this is that the tower is capacitively top loaded: the earth conductor 26 therefore has a function equivalent to that of a capacity hat, which will be familiar to those skilled in antenna design, which serves to reduce the height of the antenna required to transmit a signal of a given wavelength. Thus, with suitable impedance matching, the top-loaded tower can operate as a ground wave transmitting antenna.
A further consideration is that there should be a current node for the communication signals in the tower 10 where the legs 12 of the tower are supported on the ground 14. This obviates the need to insulate the tower from earth.
The wide variation in design and location of support towers for EDNs means that it is impossible to produce a detailed description of the exact steps which must be taken in order to match a tower to the signal which it is to transmit. However, those skilled in the art will recognise that such matching is possible by experiment using substantially conventional and routine techniques, including the use of suitable matching transformers and providing inductive loading on appropriate parts of the tower 10 and of the earth conductor 26. Such inductive loading can conveniently be achieved by securing magnetically permeable materials such as ferrite, at appropriate locations.
There is also a need to prevent communication signals being conducted to earth through another tower of the EDN. This can be achieved by blocking flow of the communication signals through the earth conductor 26 by creating a localised inductance within the earth conductor 26, for example, by securing a ferrite ring around it.
A second embodiment will now be described with reference to Figure 2. This embodiment is constructed on an EDN which comprises spaced towers 50 which support a plurality of conductors including an earth conductor 52. A transmitter unit 54 is located between two of the towers 50. An uplink conductor 56 extends from the transmitter unit 54 to the earth conductor 52.
In this embodiment, the uplink conductor 56 acts as a radiating element and the earth wire 42 serves as a capacitive hat for the uplink conductor 56. The towers 50 do not radiate significantly. It will be understood that the towers need not therefore be made of a conductive material; they can, for example, be wooden poles, such as are commonly used in electricity distribution systems in rural areas. However, this embodiment could equally be used in an electricity transmission network incorporating large metal towers.

Claims

A broadcast system including a transmitting antenna for transmitting communication signals which is constituted by at least a portion of an above-ground component of an EDN.
A broadcast system according to claim 1 in which a support tower for power distribution cables constitutes a radiating element of the transmitting antenna.
A broadcast system according to claim 1 or claim 2 in which an earth conductor of the EDN constitutes a radiating element of the antenna.
A broadcast system according to any preceding claim in which a radiating element of the antenna is connected to earth potential at a current node of the communication signals.
A broadcast system according to any preceding claim in which the communication signals have a carrier frequency in the range of 150kHz to 1.65MHz.
A broadcast system according to any preceding claim in which the EDN distributes alternating current at a frequency of approximately 50Hz or 60Hz.
A broadcast system according to any preceding claim in which the power distribution system operates at a voltage in excess of 7kV.
A broadcast system according to claim 7 in which the power distribution system operates at a voltage in excess of 33kV.
A broadcast system according to any preceding claim in which the communication signals are conveyed from a signal source to the transmitting antenna on a signal carrier, part of which signal carrier is supported on or is part of the EDN.
A broadcast system according to claim 9 in which the signal carrier includes an optical fibre supported on a conductor of the EDN.
A broadcast system according to any preceding claim in which the antenna is located at a sub-station of the EDN.