Abstract:
A HF radar uses the same antenna array ( 414, 424, 424′, 424 ′) for both transmission (TX) and reception (RX). Each radiating element of the array may be driven by its own local transmitter and may have its own local receiver, both being connected to a central processor via fiber optic cables conveying digital data to the local transmitter relating to element energization in the TX mode and data representing signals received by the radiating elements in the RX mode. Each antenna element may have its own TX/RX unit, or a single TX/RX unit may serve two or more radiating elements. Each radiating element may comprise a skeletal pyramidal dipole mounted at ground level.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to HF radar. 
     It particularly relates to RF radar installations consisting of arrays of receive/transmit modules and to modules for use in such installations. 
     As depicted in FIG. 1, whereas microwave radar is generally limited to line-of-sight surveillance. HF radar allows “over-the-horizon” surveillance to be made. 
     It is the practice in HF radar to provide different antennas for transmission and reception. FIGS. 2 and 3 depict an example of a prior art HF radar consisting of a transmitting antenna  20 , receiving, antenna  30 , RX/OPS cabin  32 , TX/OPS cabin  22  and generator  34 . 
     In order to provide the necessary directivity and radiation efficiency, the prior art HF radar utilises a transmitter antenna consisting of a conventional phased array. To obtain a satisfactory efficiency, the prior art has utilised relatively tall antennas so that the radiating elements are as high as possible. This has required the use of substantial bracing and/or substantial concrete foundations for each antenna element. To avoid excessive leakage of transmitted signal into the front end of the receiver, a discrete receiver antenna  30  is provided some distance from the transmit antenna array. The individual antenna elements of each array have also had to be disposed in accurate spatial relationship with each other. While this prior art arrangement has been found to be satisfactory in terms of its effectiveness as a radar, it does require the availability of a level site of substantial area and linear extent and the accurate spacing of the individual elements of the transmit antenna in particular. While such an arrangement may well be satisfactory for a permanent installation, it is not easily implementable as a mobile or temporary transportable installation which can be set up quickly on an unprepared site, or on poor, relatively weak, ground such as marshland or beaches. The present invention seeks to provide an arrangement in which the disadvantages of the prior art are at least ameliorated. 
     SUMMARY OF THE INVENTION 
     A first aspect of the invention provides a transmit/receive assembly for a HI radar apparatus, the transmit/receive assembly comprising a power transmitter for generating radio frequency signals, a receiver for receiving radio frequency signals and a duplexer arranged to couple the output of the power transmitter and the input of the radio receiver to a common port connection to a transmit/receive antenna. 
     A second aspect of the invention provides a transmit/receive antenna assembly comprising a transmit/receive assembly in accordance with the first aspect of the invention and an antenna. 
     A third aspect of the invention provides a HF radar comprising such transmit/receive assemblies or transmit/receive antenna assemblies. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will now be described by way of non-limiting example only, with reference to the drawings in which 
     FIG. 1 illustrates typical vertical radar range characteristics of microwave and HF radars, 
     FIG. 2 shows a perspective view of a prior art HF radar installation, 
     FIG. 3 shows a plan view of FIG. 2, 
     FIG. 4 shows a block diagram of an embodiment of the invention, 
     FIG. 5 shows a block diagram of one of the antenna modules of FIG. 4 in accordance with the invention, 
     FIG. 6 shows a block diagram of a further embodiment of the invention, 
     FIG.7 shows a block diagram of one of the antenna modules of FIG. 6 in accordance with the invention, 
     FIG. 8 shows a block diagram of a still further embodiment of the invention, 
     FIG. 9 shows a block diagram of one of the antenna modules of FIG. 8 in accordance with the invention, 
     FIG. 10 shows a block diagram of a yet further embodiment of the invention, 
     FIG. 11 shows a block diagram of one of the antenna modules of FIG. 10 in accordance with the invention, 
     FIG. 12 shows a perspective view of a HF radar installation in accordance with the invention, 
     FIG. 13 shows a plan view of one of the antennas of FIG. 12 on an enlarged scale. 
     FIG. 14 shows an antenna for use with the invention, and 
     FIG. 15 shows an antenna module in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The HF radar installation shown schematically in FIG. 4 consists of a control module  400  and a plurality of antenna modules, only three of which  410 ,  420 ,  430  are shown. A plurality of transmit control lines  414 ,  424 ,  426  convey information from control module  400  to the antenna modules  412 ,  422 ,  432 . A plurality of received data lines  418 ,  428 ,  438  convey information from the antenna modules  412 ,  422 ,  432  to the control unit  400 . Each antenna unit consists of a respective antenna  414 ,  424 ,  434  and a respective transmitter/receiver module  412 ,  422 ,  432 . All modules are identical. 
     One of the modules  412  will now be described with reference to FIG. 5. A receive/transmit module  410  has a HF transmitter  520  and a HF receiver  530 . The transmitter  520  is coupled to a first port of a duplexer  510  via line  524 . The receiver  530  is coupled to a second port of duplexer  520  via line  514 . A third port of duplexer  520  is coupled to antenna  414  via line  512 . Transmit control line  416  which in the present embodiment is a fibre optic cable, is coupled to transmitter  520  via a fibre optic interface  540  and line  544 . A fibre optic received data line  418  is coupled to receiver  530  via fibre optic line driver  550  and line  534 . 
     Operation of the embodiment will now be described. During a transmit phase the control module  400  sends instruction on lines  416 ,  426 ,  436  to the transmitters  520  of each module causing the transmitters of the modules to generate output signals in appropriate phased relationship with each other. These signals are fed to antennas  414 ,  424 ,  434  so that the signals emitted by the antennas coherently combine to generate a transmitted radio wave of desired properties in accordance with known principles of phased array antennas. During a receiving phase the transmitters are inhibited and signals received by antennas  414 ,  424 ,  434  are passed to their respective receivers  530  via respective duplexers  510 . Information about the received signals is converted to digital form and conveyed to the control unit  400  via cables  418 ,  428 ,  438 . The received information is processed to extract radar information therefrom in known manner. The extraction of such information is not part of the present invention and will not be described further. 
     The above described embodiment requires two links between the control unit and each antenna assembly. In a modification, not shown, this can be reduced to a single link by using multiplexers to provide two-way traffic via a single cable in known manner. 
     In general the transmitted signals will be regularly spaced signals of constant or predetermined duration and will have a fixed or predetermined relationship to each other. 
     The radar assembly shown in FIGS. 6 and 7 utilise this property to eliminate the need for a discrete communication link between the control unit and the transmitters. 
     Items having the same functions and properties as in the previous embodiment have identical numbers and will not be described further. 
     Each transmit/receive unit  612 ,  622 ,  632  has a respective antenna  414 ,  424 ,  434  and is coupled to a control unit  600  via respective fibre optic cables  418 ,  428 ,  438 . 
     Each unit  610  has a duplexer  510 , and transmitter  520 . A receiver  550  provides an output on line  534  to line driver  550  as in the previous embodiment. Receiver  550  also has a second output  552  responsive to a received signal from a previous transmission phase. This is passed via line  552  to synchroniser  560  which issues commands to transmitter  520  on line  562  instructing it when the next transmission cycle is to commence. This allows the transmitters to synchronise themselves to each other automatically. 
     Operation is otherwise as for the previously-described embodiment. 
     The previously described arrangements are robust in that each antenna unit is independently connected to the control unit and breakage or function of one connecting line only affects that antenna unit. However it does require a considerable amount of cabling especially where an array having a larger amount of elements is used. 
     The arrangement shown in FIGS. 8 and 9, while not as robust, allows considerable economy in cabling to be attained. In this embodiment the antenna units  812 ,  822 ,  832  and control users are “daisy-chained” together via cables  862 ,  864 ,  866 ,  868 ,  879 ,  872 . Each modules  812 ,  822 ,  832  has a serial data bus interface  902 ,  904  for extracting data from the control unit and conveying it to the transmitters  520  and for supplying information from receivers  530  to the control unit  800 . These interfaces  902 ,  904  operate in conventional manner and will not be described further. 
     The arrangement shown in FIGS. 10 and 11 corresponds with the array of FIGS. 7 and 8 but daisy-chained in the manner described above in connection with FIGS. 8 and 9. Each antenna unit  1012 ,  1022 ,  1032  has a single interface unit  1014  to allow received information to be arranged to the control unit  1000 . Operation is otherwise as described with reference to FIGS. 7 and 8 and will not be described further. 
     The above-described embodiments omitted the source of power of the modules in the interest of clarity. For a permanent installation having a source of mains supply each module car have its own mains-powered power supply. For field use or temporary installations, each module could have its own relatively small power source eg. a motor/generator set. Alternatively one or more relatively large generator sets could be used, power being supplied to the modules by conventional power cables. 
     It may be convenient to physically locate two or more modules in a common enclosure so as to reduce the number of cables between the various system components. 
     For example, only a single power cable is then required to provide power to all the modules physically located in the common enclosure and the digital output signals from the receivers in the common enclosure can be multiplexed on a single fibre optic cable in known manner. 
     Alternatively a single amplifier may be used to supply RF energy to all antennas coupled to the module, the RF input lines  524  of each coupler  510  being coupled in parallel to the single RF source output in known manner instead of having their own individual RF source  520  etc. 
     FIGS. 12 schematically depicts part of a coastal installation in which each transmit/receive module  1210  supplies two antennas  414 ,  424 . The array of antennas  414 ,  424 ;  414 ′,  424 ′ is located on beach material  1202  adjacent a body of water  1204 . As shown in more detail in FIG. 13, because dry beach material generally has poor electrical conductivity. Each antenna is placed on a respective propagation mat consisting of grid  1300  of electrically conductive material such as metal wire. Each grid  1300  is electrically connected to the body of water by one or more respective electric cables  1302 . For the purposes of illustration, and in the interest of clarity, only one cable is shown in the figure, but additional cables may be provided in electrical parallel, possibly running in different directions and terminating at different locations in the body of water, according to system requirements. 
     The provision of such propagation mats, while not essential to the invention, has been found advantageous with certain types of antenna on ground having poor electrical conductivity, but is not normally necessary on soils of good electrical conductivity. 
     Control module  1200  contains signal processing circuitry and displays to process and display information received by the antennas. 
     Each module  1210  is physically located between its respective antennas  414 ,  424 . Power for the modules is obtained from generator modules  1250 , only one of which is shown for clarity. The cabling between generator modules transmit/receive modules and the control module  1200  has been omitted for clarity. 
     FIG. 14 shows a perspective view of an antenna suitable for use with the invention. The antenna  1400  is a centre-fed dipole antenna having a lower radiating element  1402  and an upper radiating element  1404 . This antenna is described in UK patent GB 2302990B to which reference should be made for further details. 
     FIG. 15 shows a perspective view of an antenna module consisting of an antenna  1400  of the type illustrated in FIG. 14 and a transmit/receive module  412 . The bracing of the antenna has been omitted for clarity. In this embodiment a transmit/receive module  412  is physically located within the lower radiating element  1502  of the antenna. This has the advantage of only requiring a relatively short length of cable  512  between the transmit/receiver module  412  and the antenna feed point, with an attendant reduction in signal loss which would otherwise occur were the antenna fed from a central transmitter via a long length of cable. In field use, the weight of the transmit/receive module can be utilised to stabilise the antenna, for example by guy ropes or halyards (not shown) extending from the module to suitable anchor points on the antenna or, if the antenna and the transmit/receive module  412  both rest on a platform or skeletal platform, by virtue of its weight. Connections to and from the module are routed under the lower edge of the lower element  1402  of the antenna and therefore do not interfere with the antenna&#39;s electrical performance. 
     However, it is not essential for the transmit/receive module to be physically located within the lower radiating element and it may equally well be located elsewhere, such as between adjacent antennas as in the FIG. 12 embodiment, or beneath the antenna. 
     A number of modifications are possible within the scope of the invention. 
     While the type of antenna element illustrated in FIG. 14 has been found to give good results the invention is not limited thereto and any other type of antenna element capable of being used for both transmission and reception can be employed. 
     While the described data links comprise fibre optic cables, other transmission media such as conventional coaxial cables or twisted pair transmission lines carrying conventional electrical signals may be employed. 
     While grouping a number of units together in one relatively loose enclosure can result in economy of manufacture and can reduce the number of interconnections between enclosures, providing each antenna with its own smaller module means that each module is relatively light. This can be important in a transportable installation intended for temporary or field use where modules have to be positioned. Providing a number of discrete modules also makes the system more robust as damage to one module only affects one element of tire array.