Abstract:
A transmit antenna for a surface wave radar system, including a linear array of active monopole antenna elements for transmitting signals in respective frequency ranges. The relative spacings and the relative heights of successive elements along the array have logarithmic relationships. The transmit antenna includes impedance matching circuits for the active monopole antenna elements, and switches for selecting one of the active antenna elements to transmit a signal in a corresponding frequency range while grounding the remaining active antenna elements.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a transmit antenna, and in particular to an antenna for transmitting signals for high frequency surface wave radar.  
       BACKGROUND  
       [0002]     High frequency surface wave radar (HFSWR) systems have been developed to overcome the line-of-sight limitation of microwave radar systems. HFSWR exploits a phenomenon known as a Norton wave propagation, whereby a vertically polarised electromagnetic signal propagates efficiently as a surface wave along a conducting surface. HFSWR systems operate from coastal installations, with the ocean providing the conducting surface. The transmitted signal follows the curved ocean surface, and an HFSWR system can detect objects beyond the visible horizon, with a range of the order of 300 km.  
         [0003]     As shown in  FIG. 1 , a surface wave radar system includes a transmitter  12 , and a receiver  14 . The transmitter  12  includes transmitter electronics  18  and a transmitting antenna  16 . The transmitting antenna  16  is a directional broadband antenna, such as a log-periodic antenna array, capable of generating a substantial surface wave and a relatively insubstantial overhead skywave. The transmitting antenna  16  transmits high frequency (5-10 MHz) electromagnetic surface wave signals from a shoreline  26  across the ocean surface. The transmitted signals are reflected from objects such as a ship  28 , and reflected surface wave signals are received by the receiver  14 .  
         [0004]     The receiver  14  includes a data processing system  24  and a broadside array  20  of vertically polarised antenna doublets. The broadside array  20  is oriented approximately perpendicular to a principal receiving direction  25  for reflected surface wave signals, and, in this case, is approximately parallel to the shore  26 . The receiver  14  can also include an endfire array  22  of vertically polarised monopole antenna elements, oriented perpendicular and adjacent to the broadside array  20  to form a two-dimensional (2-D) receiving antenna array.  
         [0005]     A standard log-periodic antenna array is suitable for the directional transmission of vertically polarised signals over a wide bandwidth and beam width. However, it is often necessary to transport the antenna to various locations. Log-periodic antenna arrays designed to transmit signals in the appropriate frequency range (5-10 MHz) are large and expensive structures that require considerable effort for disassembly, transportation, site preparation and reassembly. It is desired, therefore, to provide a transmit antenna that alleviates one or more of these difficulties, or at least provides a useful alternative to existing transmit antennas.  
       SUMMARY OF THE INVENTION  
       [0006]     In accordance with the present invention, there is provided a transmit antenna for a surface wave radar system, including:  
         [0007]     a linear array of active monopole antenna elements for transmitting signals in respective frequency ranges, the relative spacings and the relative heights of successive elements along the array having substantially logarithmic relationships;  
         [0008]     impedance matching circuits for the active monopole antenna elements; and  
         [0009]     switch means for selecting one of the active antenna elements to transmit a signal in a corresponding frequency range while grounding the remaining active antenna elements. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     A preferred embodiment of the present invention is hereinafter described, by way of example only, with reference to the accompanying drawings, wherein:  
         [0011]      FIG. 1  is a schematic diagram of a surface wave radar system;  
         [0012]     FIGS.  2  to  4  are schematic diagrams of a preferred embodiment of a transmit antenna of the radar system;  
         [0013]      FIG. 5  is a graph of voltage standing wave ratio (VSWR) as a function of frequency for each impedance matched antenna element of the transmit antenna; and  
         [0014]     FIGS.  6  to  11  are graphs of the simulated and measured radiation patterns from each antenna element with impedance matching, at frequencies of 5.1, 6.1, 7.1, 8.1 9.1, and 10.2 MHz, respectively. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]     As shown in  FIG. 2 , a transmit antenna  16  includes a linear array of four active base-fed monopole antenna elements  30  to  36  and two passive elements  38 ,  40 , at respective ends of the array. Each of the active monopole elements  30  to  36  transmits signals in a unique portion of the antenna&#39;s 5-10 MHz frequency transmission range, as shown in Table 1.  
                                                     TABLE 1                                       frequency           height   position   spacing   range       element   (m)   (m)   (m)   (MHz)                                passive   16.00   0   —   —       reflector (40)       Element 1 (30)   12.78   13.94   13.94     5.0-5.7       Element 2 (32)   10.75   25.66   11.72     5.7-7.1       Element 3 (34)   9.04   35.52   9.86      7.1-8.15       Element 4 (36)   7.60   43.81   8.29     8.15-10.0       passive director   6.39   50.78   6.97   —       (38)                  
 
         [0016]     The tallest passive element  40  is a sixteen metre wind-up lattice mast and acts as a reflector at the low frequency end of the antenna&#39;s operating frequency range. The other passive element  38  is shorter and acts as a director at the high frequency end of the antenna&#39;s operating frequency range. Thus the maximum transmit signal intensity is directed along the array direction  41  leading away from the reflector passive element  40  toward the director passive element  38 , and accordingly the transmit antenna  16  is oriented so that this direction  41  points towards the potential objects of interest; i.e., in the arrangement of  FIG. 1 , pointing away from the shoreline  26  towards the ocean. The six elements  30  to  40  have logarithmic relationships in height and position within the antenna array, as can be determined from the data of Table 1. That is, the height h of the n th  (passive or active) antenna element can be represented as log(h)=a−bn, where a and b are real numbers. Similarly, the spacing s of the n th  antenna element from the (n−1) th  antenna (passive or active) element can be represented as log(s)=c−dn, where c and d are real numbers. The specific values for the heights and positions are determined using standard antenna design software such as numerical electromagnetic code (NEC) and SPICE.  
         [0017]     A grounded radial wire counterpoise under each antenna element reduces ground losses and stabilises the impedance of each antenna element under varying ground conditions. The configurations of the counterpoises are shown in a plan view of the transmit antenna  16  in  FIG. 3 . Viewed from above, the nine wires of each counterpoise extend radially from one side of the base of the corresponding antenna element, with each adjacent pair of wires being separated by an angle φ=22.5°. Thus the wires of each counterpoise form a semicircular region oriented towards the high frequency end of the antenna array. Eight different wire lengths are used to form the counterpoises, referred to as wires A to H, as shown in Table 2. The selection and arrangement of wires A to H in each counterpoise is provided in  FIG. 3 .  
                                         TABLE 2                                   Wire   Length (m)                                        A   13.9           B   17           C   20           D   16           E   11.27           F   16.25           G   12.45           H   9.16           I   13.27           J   10.16           K   7.846           L   11.4           M   8.73           N   6.52           O   9.54           P   7.3                      
 
         [0018]     As shown in  FIG. 4 , the antenna includes interface modules  42  to  48  for interfacing the respective monopole elements  30  to  36  to the transmitter electronics  18 . Each of the interface modules  42  to  48  includes a respective two or three element LC impedance matching network  50  to  56  and a standard high-power latching radio-frequency (RF) power relay or switch  58 . The impedance matching networks  50  to  56  each include a respective capacitor  60  to  66  and inductor  68  to  74  in parallel with the transmitter signal; the second (second lowest frequency) network  42  and the fourth (highest frequency) network  46  also include an additional inductor  76 ,  78  in series with the signal.  
         [0019]     The RF switches  58  allow each antenna element to be independently connected to the transmitter electronics  18  via the coaxial cable  76 , or shorted to ground potential. When a signal of a particular frequency is transmitted, the antenna element whose allotted frequency range includes that frequency is connected to the transmitter electronics  18 , and the three remaining antenna elements are shorted to ground. This switching is performed by remotely controlling the switches  58  to  64  by sending appropriate signals on control cables  80 . Specifically, a 24-volt gate pulse signal sent to one of the RE switches  58  to  64  on that switch&#39;s control cable activates the RF switch to connect the coaxial cable  76  to the corresponding interface module (e.g., the second interface module  44 ), and thereby to the corresponding antenna element (e.g., the second antenna element  32 ). The other antenna elements (e.g., the first, third and fourth antenna elements  30 ,  34 ,  36 ) are shorted to ground and act as additional reflectors or directors.  
         [0020]     Table 3 provides details of the values of capacitance and inductance for each of the active antenna element matching networks  50  to  56 .  
                                   TABLE 3                                       parallel   series   parallel               capacitance   inductance   inductance           element   (pF)   (μH)   (μH)                           passive   —   —   —           reflector (40)           Element 1 (30)   2578   —   0.339           Element 2 (32)   1557.4   0.574   0.438           Element 3 (34)   1276.8   —   0.330           Element 4 (36)   1339.5   0.400   0.244           passive director   —   —   —           (38)                      
 
         [0021]     As shown in  FIG. 5 , the voltage standing wave ratios (VSWRs)  601  to  604  for the four antenna elements  30  to  36  are predominantly between 1.2:1 and 1.4:1 over the entire operating frequency range of the antenna  16 .  
         [0022]     FIGS.  6  to  11  are graphs of the measured  701  and simulated  702  azimuthal radiation patterns for the antenna array at frequencies of 5.1, 6.1, 7.1, 8.1 9.1, and 10.2 MHz, respectively. An azimuthal angle of 0 degrees corresponds to the direction  41  leading from the low frequency end of the antenna array towards the high frequency end of the antenna array, and it can be seen that the maximum gain is obtained in this direction. The simulated radiation patterns  702  include antenna and mismatch losses and appear to match the measured patterns closely. Some discrepancies are apparent at the higher frequencies, probably due to factors such as adjacent buildings and structures that affect surface-wave attenuation.  
         [0023]     The antenna  16  is designed to have a limited frequency range of 5-10 MHz, but the high-frequency characteristics of the array can be extended by adding one or more active elements to the high-frequency end of the array. This slightly increases the gain of the fourth antenna element  36  without significantly affecting its impedance.  
         [0024]     In comparison with a standard log-periodic antenna array, the logarithmic monopole antenna array  16  has a lower gain and broader azimuth and elevation radiation patterns. However, the cost of manufacture is greatly reduced, and the logarithmic monopole antenna is readily transportable.  
         [0025]     Many modifications will be apparent to those skilled in the art without departing from the scope of the present invention as herein described with reference to the accompanying drawings.