HIGH FREQUENCY QUAD ANTENNAE

Abstract: A high frequency quad antenna includes a full wavelength driven element, a director element spaced from one side of the driven element by a distance equivalent to between 0.12 and 0.50 of the wavelength at the operating frequency of the antenna, and a reflector element. The reflector element is spaced from the other side of the driven element by a distance which is greater than that between the driven element and the director element. The total conductor length of the director element is less than that of the driven element by between 5 and 15 % and the total conductor length of the reflector element is greater than that of the driven element by between 5 and 15 %.

Description: HIGH FREQUENCY QUAD ANTENNAE This invention relates to high frequency quad antennae and more especially, but not exclusively, to video surveillance and other systems which include transmitting and receiving quad antennae. Quad antennae are used for transmitting and receiving radio frequency (RF) signals in, for example, camera operated surveillance equipment in which video signals emitted by one or more inspection cameras are transmitted via such antennae to one or more monitors remote from the camera(s). Conventionally, the transmission distance between the transmitting and receiving antennae is limited, typically of the order of 120 metres for an output of lOMw with l λ stub antennae. It is an object of the invention to provide high frequency quad antennae capable of transmitting RF signals (including microwaves) over significantly greater transmission distances. Trials conducted with high frequency Quad antennae in accordance with the invention have achieved transmission distances of approximately four miles for an output power which approximates to lOMw, with line of sight - air to ground. According to the present invention in one aspect there is provided a high frequency quad antenna which includes a full wavelength driven element, a director element spaced from one side of the driven element by a distance equivalent to between 0.12 and 0.50 of the wavelength at the operating frequency of the antenna, and a reflector element spaced from the other side of the driven element by a distance which is greater than that between the driven element and the director element, the total conductor length of the director element being less than that of the driven element by between 5 and 15 % and the total conductor length of the reflector element being greater than that of the driven element by between 5 and 15% . By "high frequency" is meant an operating frequency not less than 50 Mhz. Typically frequencies of antennae in accordance with the invention are 1300 and 2400 Mhz. Each antenna element takes the form of a closed loop, typically in the shape of a square. Other known loop shapes may however be employed. The conductor for each element may take the form of copper wire of diameter between 1.5 mm and 2.5 mm. Alternatively, each conductor may comprise a board on which is printed a loop of conductive material, typically copper. In this embodiment the director and reflector board will each comprise one or more closed loops with the loop or loops of the driven element being connected to a power source through a co-axial cable. In this way each board is capable of operating at two or more selected frequencies. High frequency antennae in accordance with the invention comprise an array of a director element, a driven element and a reflector element aligned on a common axis whose sizes and spacing are as defined above. In a preferred embodiment the director, driven and reflector element each takes the form of a board printed with one or more conductive loops as described above, each such board being removably assembled, in a rack of a housing in one of a selected number of positions. The invention will now be described by way of example only with reference to the accompanying diagrammatic drawings in which :- Figures 1 and 2 are side views of high frequency antennae in accordance with the invention; Figures 3 and 4 are perspective views of the antennae shown in Figures 1 and 2 respectively; Figures 5 and 6 are respectively plan views of driven and director elements of an antenna in accordance with the invention; Figures 7, 8 and 9 are plan views of driven, director and reflector elements of an alternative antenna in accordance with the invention; Figure 10 is a view from above of a rack for receiving the elements illustrated in Figures 7 to 9; Figure 11 is a circuit diagram of a typical camera surveillance system which incorporates high frequency antennae in accordance with the invention; and Figures 12 and 13 are side and rear views respectively of a camera surveillance system incorporating high frequency antennae in accordance with the invention. As illustrated in Figures 1 to 4, high frequency antennae in accordance with the invention comprise a full wavelength driven element 1 positioned between a director element 2 and a reflector element 3. In the Figure 1 embodiment, the conductor for each element is freestanding copper wire typically of 1.5 mm diameter in the shape of a square. In the Figure 2 embodiment, each element takes the form of a circuit board printed with a loop of conductive material, e.g. copper. Plan views of the driven and director printed circuit elements 1 , 2 are shown in Figures 5 and 6. Other conventional conductor materials and shapes may, however, be employed. The reflector and director elements 2, 3 define closed loops and the ends of the driven element conductor are connected to a power source (not shown) via a co-axial cable 4. The elements 1 to 3 of the antenna shown in Figure 1 are supported within a non-conductive base 5 through which the coaxial cable 4 passes to make connection with the ends of the driven element 1. The elements of the antenna shown in Figures 7 to 10 are removably supported within a rack 6. The elements may be supported within the rack from below as shown in Figure 10 or along their sides, suitable racking elements being secured to the side walls of the rack housing. As will be seen from the drawings, the conductor length of the driven element 1 is greater than that of the director element 2 but less than that of the reflector element 3. The length (in metres) of the driven element is determined by the known formula f ^ hz) where f is the frequency at which the antenna is to be used. For an operating frequency of 2445.5 Mhz, the total length of the driven element will be approximately 123 mm. The reflector element 3 is spaced from the driven element 1 by a distance equal to between 0.12 and 0.50 of the wavelength of the operating frequency. Preferably the spacing equates to 0.25 of the wavelength. Typically, for an operating frequency of 2445 Mhz the spacing between the driven element 1 and the reflector element 3 will be of the order of 39mm. The director element 2 is always closer to the driven element 1 than the reflector element 3, the distance being between 0.115 and 0.45 of the operating wavelength, preferably 0.20 of the wavelength. Typically, the spacing between the director element 2 and the reflection element 3 will be of the order of 19mm. Additional director elements may be provided, these typically being spaced approximately 0.25 of a wavelength from one another. Typically, the spacing between the director elements will be of the order of 19mm between the first and second director elements, and 14mm between the second and third any an additional director element. All elements are positioned in -line with their axes generally co-axial. Thus, the spacings progressively decrease from the reflector element to the director element. The length of the director element 2 is approximately only 90% of that of the driven element 1 and the length of the reflector element 3 is typically 110% of the length of the driven element 1. Typically, the length of the director is 99.63mm. This arrangement provides a significant increase in gain for the antenna. As mentioned previously, in the embodiment illustrated in Figures 2 and 4, the driven, director and reflector elements comprise boards on which are printed conductive material which replicate the loop shapes of the three elements illustrated in Figures 1 and 3 of the drawings. In this embodiment, the boards can selectively be inserted in a selected one of several locations within the rack 6 which may be enclosed within a housing. The rack 6 is illustrated in Figure 10 and the driven, director and reflector circuit board elements 8, 9, 10 are shown in greater detail in Figures 7 to 9 respectively. Each circuit board element includes a mount 11 for location between neighbouring upstands 12 of the rack 6. The housing is typically produced from a plastics material and may be weather proofed. As mentioned previously, racking upstands 12 may be mounted on each side wall of the housing, the circuit boards then being supported along their side edges. As will be seen from Figure 7, the driven board 8 has printed on its surface two loops 14, 15 of conductive material, typically copper. The length of the inner loop 14 is that required to operate at one selected high frequency, typically 2400 Mhz, and the length of the outer loop 15 is that required to operate at another selected high frequency, typically 1300 Mhz. The required coaxial cable is wired to one or other of the frequency loops. The boards for the reflector and director elements similarly comprise inner and outer conductive loops (these being closed loops), the lengths of these loops being determined in the manner discussed above. In operation, two antennae are provided, one acting as a transmitting antennae and the other as a receiving antennae. Each antenna will be the same, the transmitting antenna operating, for example, to transmit video signals from a surveillance camera with the receiving antenna operating to collect these transmitted signals and to pass them to a surveillance monitor. With antennae in accordance with the invention, the distance between the antenna can be significantly increased, distances of the order of 4 miles being recorded. A schematic circuit diagram for a camera surveillance system is shown in Figure 11. In this figure, a mains or battery powered surveillance camera 16 sends video signals via a cable 17 to a transmitter 18. The transmitter in turn send high frequency signals via a coaxial cable to a transmitting antenna 19 in accordance with the invention. Remote from the antenna 19 is a receiving antenna 20, also in accordance with the invention. The antenna 20 is connected by a coaxial cable 21 to a receiver 22 and monitor 23. The camera surveillance apparatus illustrated in Figures 12 and 13 comprises a housing 25 (typically of aluminium) whose front face 26 is inclined and supports an electronic solar panel 27. Typically, the face 26 is inclined at 55° to the horizontal. The housing may, for example, be mounted on an item of street furniture (eg. a lamp post) or a building overlooking the area where surveillance is required. Fixings 28 are provided for this purpose. Secured to the underside of the housing is a miniature surveillance scanning camera 29. The camera is typically a closed circuit television board camera and is positioned within a transparent weatherproofed viewing dome 31 and powered by the solar panel 27 via wiring 32. Other sources of power may be provided. The camera 29 is connected via an ABS box 33 containing electronic circuitry, stand-by battery and transmitter, and associated coaxial cabling 34, to a Quad antenna 35 mounted through fixings 36 on the upper wall of the housing 25. The transmitter is typically a four-channel 2.4 Ghz transmitter capable of transmitting sound with vision. The features of this antenna 32 are as described above with reference to Figures 1 to 11. The antenna 32 transmits signals to a remote receiving Quad antenna to provide on a monitor at the remote station reproductions of the views captured by the camera. More than one surveillance device may be provided, each pairing data to the same or a separate receiving station. Importantly, the surveillance device is completely self-contained, easy to install and once in place requires virtually no maintenance except cleaning. There are no external co-axial transmission wires or power supply cables needed. The transmitted signal is typically in the 2.4Ghz or 1.394 Ghz bands (lOmw of output power in the 2.4Ghz band and 500 mw in the 1.394 Ghz band). Four channels are available in the 2.4 Ghz band. The Quad antenna enables the transmitted signal to be recovered up to 1 / 2 mile away in the 2.4 Ghz band and several miles in the 1.394 GHZ band (one channel) depending on terrain and antennae line of sight. During daylight hours the unit illustrated is run by electricity produced by the solar panel, which also charges the stand-by battery; during the night the system is powered by the battery. It is normally necessary for the unit to be installed as high as possible and away from cover so that the solar panel receives as much day and sunlight as possible to enable the battery to be recharged. The battery supplied should be capable of running the system for approximately 55 hours (without any light whatsoever) before needing to be recharged. Being mounted high also enables the transmitter range to be at its optimum. Applications for the device described are many and varied. The device has particular advantages in situations where CCTV systems are needed and where it would be virtually impossible or very costly to provide transmission wires or a mains power supply. The device is easy to install and may be set up literally in minutes. Up to four units can be used on 2.4Ghz band. Examples of potential usage include outlying buildings on farms, local authority surveillance of houses and road traffic, temporary installations at road works on motorways, building sites and airfields. It will be understood that the foregoing is merely exemplary of antennae in accordance with the invention and that modifications can readily be made thereto without departing from the true scope of the invention as set out in the appended claims.

Claims: CLAIMS 1. A high frequency quad antenna which includes a full wavelength driven element, a director element spaced from one side of the driven element by a distance equivalent to between 0.12 and 0.50 of the wavelength at the operating frequency of the antenna, and a reflector element spaced from the other side of the driven element by a distance which is greater than that between the driven element and the director element, the total conductor length of the director element being less than that of the driven element by between 5 and 15 % and the total conductor length of the reflector element being greater than that of the driven element by between 5 and 15% . 2. An antenna as claimed in claim 1 wherein each element takes the form of a closed loop. 3. An antenna as claimed in claim 2 wherein each element takes the form of a square. 4. An antenna as claimed in any one of the preceding claims wherein the conductor for each element takes the form of copper wire of diameter between 1.5mm and 2.5mm. 5. An antenna as claimed in any one of claims 1 to 3 wherein each conductor comprises a board on which is printed a loop of conductive material . 6. An antenna as claimed in claim 5 wherein the director and reflector board each comprises one or more closed loops with the loop or loops of the driven element being connected to a power source through a co-axial cable. 7. An antenna as claimed in any one of the preceding claims in which the director element, the driven element and the reflector element are aligned on a common axis. 8. An antenna as claimed in claim 5 wherein the director, driven and reflector element printed circuit boards are removably assembled in a rack of a housing in one of a selected number of positions. 9. A camera surveillance system including at least one high frequency antenna as claimed in any one of the preceding claims.

Inventors: SCOFIELD DEREK, SCOFIELD, DEREK
Applicants: SCOFIELD DEREK, SCOFIELD, DEREK
Application year: 1999
Main classification: H01Q7
IPC codes: H01Q7, H01Q19