ANTENNA SYSTEM FOR WIDE BANDWIDTH SIGNALS

Abstract: A system for transmitting and receiving very wide instantaneous bandwidth signals. Synchronised short pulse generators are positioned at the feed location of each element (11a...11n) of a linear array of short caped monopole antennas to sequentially excite the antenna elements which are spaced in accordance with the sequential interval to provide a beam in the desired direction and space. Reflections of the radiated short pulses are received by a feed monopole (22) backed by a reflecting monopole (23) and coupled to a short pulse receiver through a duplexing switch.

Description: This invention pertains to the field of airborne antennas and more specifically to antennas mounted on a cylindrical metallic surface with limited extensions therefrom for radiating and receiving signals with very wide instantaneous bandwidths. Constraints on antenna system design are determined by the instantaneous bandwidth of the radiated signal, the desired spatial distribution of that signal, the desired relation and reception efficiency, and dimensional limitations for a particular application. For cw (carrier wave) or narrow band signals, design parameters to meet these objectives are generally well known. When the fractional bandwidth, however, becomes very wide (50% to 100%), only one or two cycles of the carrier exists within a radiation time interval and the narrow band approach to antenna design is no longer applicable. This wide signal bandwidth condition suggests quasi-optical or time delay techniques as effective antenna design procedures. Of the two techniques for designing antennas for wide bandwidth signals, the quasi-optical approach, wherein reflectors, lenses, and horns, though dispersive, are utilised to achieve the design objectives has found greater acceptance. In many applications, however, space constraints and the dispersive characteristics of the optical techniques establish the time delay technique as a more suitable approach. The present invention is defined in the appended claims and utilises time delay techniques to provide wide instantaneous bandwidth signal antennas mounted on a cylindrical metallic surface, such as the fuselage of a missile or small aircraft, that do not significantly extend above the surface. A wide instantaneous bandwidth signal radiation and reception system embodying the principles of the present invention may include a linear array of capped monopoles positioned on the surface of a cylinder in a parallel relationship with the cylinders axis. Each element in the array is energised by one of a plurality of pulse generators which are sequentially triggered with intervening time intervals corresponding to the antenna element spacing, thus establishing a radiation pattern with a peak in the direction of the sequential order. Radar reflections are received by an antenna comprised of a capped monopole coupled via a switch to a receiver internal to the cylinder and a second capped monopole positioned as a reflecting element. Isolation between the receiving monopole and the transmitting monopole array is achieved by diametrically positioning the receiving and transmitting elements on the cylindrical surface, splitting the cylindrical surface in a plane perpendicular to an axis in line with the receiving and transmitting elements, placing a non-conducting material between the two cylindrical sections so formed to substantially reduce surface currents, and decoupling the receiver from the receiving antenna during transmissions by maintaining the switch therebetween in the open position during this interval. The invention will be more clearly understood from the following description and from the accompanying drawings which illustrate, as non-limitive examples, preferred embodiments thereof, and in which:- Figure 1 is an illustration of transmitting and receiving antennas positioned on the surface of split cylinders having a non-conductive material positioned between the sections, Figure 2 is a schematic diagram of a pulse generator for use with an element in the transmitting array, Figure 3 is a schematic diagram of a sequential pulse generator useful for sequentially coupling energy to elements in the transmitting array, Figure 4 is a schematic diagram of a sequential pulse generator wherein common diode charging and discharging elements are employed, and Figure 5 is a block diagram, partially in schematic form, of the transmitting and receiving system. Antennas that may be employed with the system of the present invention are shown mounted on a cylindrical surface in Figure 1. An array 11 of antenna elements 11a.....11n, each of which may be a capped monopole that protrudes radially from the surface of a cylinder 12 along a line 13 that is substantially parallel to the cylinder axis 14, may be employed as a transmitting antenna by sequentially exciting the elements 11a through 11n, as will be discussed hereinafter. The caps 15a through 15n on each monopole 16a through 16n are of a diameter that is substantially equal to the height of the monopole, which may be for example less that ct r , where t r is the rise time of the pulse to be radiated from the antenna element and c is the speed of light. The spacing s between the elements 11a.....11n may be substantially equal to the distance travelled by a signal during the sequential interval between the excitation of adjacent elements. This arrangement forms a beam in the direction indicated by arrow 17 when the elements are excited in sequence from 11n to 11a. A receiving antenna positioned on the cylinder 12 diametrically from the transmitting antenna 11 may comprise an active element 22 and a reflecting element 23, shorted to the cylindrical surface 12, each of which may be a conical monopole. The active and reflecting elements 22, 23 are shaped to minimise impedance mismatches to the feeding transmission line. Capped conical monopoles with half cone angles of 47°, shown in Figure 1, at heights of approximately λ o /8 and spacing d of substantially λ o /4 ( λ o being the wavelength at the centre frequency in the band) provide a minimum mismatch to a 50 ohm transmission line when the cap diameter of the active element 22 is substantially equal to the element height, and the cap diameter of the reflecting element 23 is substantially equal to twice the element height. The half cone angle of 47° is not critical and deviations therefrom may be practiced without significantly altering the match to the feed line. It should be recognised by those skilled in the art that the capped monopole and capped conical monopoles are not limitative and that other antenna types, such as capped discones, may be employed. Additionally, it should be recognised that like elements may be employed for the transmitting array and receiving antenna. Capped monopoles and capped conical monopoles possess impedance characteristics that may be readily matched, respectively, to the impedances of the transmitter and receiver, each yet to be described, utilised in the preferred embodiment of the present invention. When each element in the transmitting antenna 11 is energised, currents are induced on the cylindrical surface 12 which is coupled to the receiving antenna 21. These currents may be of sufficient amplitude to damage the receiver coupled to the active element 22 and limit the minimum operational range of the radar system. These currents may be significantly reduced by longitudinally cutting the cylindrical surface into two equal parts and replacing the sections thereby removed with electrical absorbing or non-conductive material 24. Significant decoupling between the transmitting and receiving antennas may be accomplished in this manner. Additional decoupling in many applications may be required. This may be accomplished by coupling a switch between the active element of the receiving antenna and the receiver as will be described subsequently. A pulse generator suitable for driving the capped monopole elements of the transmitting array 11 is shown in Figure 2. An avalanche transistor 25, which may be of the type known in the art as 2N2369, is triggered by an external pulse coupled to the base 25a via a terminal 26 to provide a fast rising pulse of an amplitude of the order of 30 volts and duration of the order of 4ns. This pulse is coupled via an isolation delay line 27 and charging inductance 28 to charge a multiplicity of charge storage devices in which may be serially-coupled diodes 31 of the step recovery type (stack of diodes). The pulse coupled through the delay line 27 is also coupled via a capacitor 32 to over-voltage the collector 33a of a transistor 33, which may be of the type known in the art as MPSU04, causing it to avalanche at a time prdetermined by the bias voltages applied thereto. This produces a negative-going pulse of very high amplitude (of the order of 350 volts) which rapidly drains the stack of diodes 31 of the free charges stored therein by the charging pulse. When the free charges in the stack 31 are reduced to substantially zero, the voltage across the stack 31 "snaps" from a level of substantially zero to a level of the order of -200 volts over an interval of approximately 200ps and returns to a level of substantially 0 volts in a comparable time, thereby establishing a pulse of substantially 400ps duration at node 34, wherefrom it is coupled to a capped monopole 35. An increase in radiated power, over that achievable with the circuit of Figure 2, may be obtained by providing a linear array of elements sequentially fed, such that the pulses from each radiating element coalesce at the final excited element during its excitation interval to provide a radiated beam in a desired direction. Referring now to Figure 3, an avalanche transistor 36 triggered by a pulse coupled to its base from an input terminal 37 substantially simultaneously charges diode stacks 41 to 44 via isolation delay lines 41a to 44a and charging inductors 41b to 44b, as previously described. Pulses from the delay lines 41a through 44a are also coupled to over-voltage the collectors of transistors 41c to 44c which thereafter avalanche in a sequence determined by the bias voltages established by the setting of potentiometers 41d to 44d and a positive voltage source not shown. Pulses sequentially generated by the diode stacks 41 to 44 are coupled via lines 41e to 44e to the array elements 41f to 44f, respectively. Avalanche transistors are temperature sensitive, thereby subjecting antenna characteristics that depend upon the sequential operation of a multiplicity of avalanche transistors to performance variations that are functions of temperature. This temperature sensitivity may be minimised by discharging the diode stack through a common avalanche transistor. Referring to Figure 4, a charging avalanche transistor 51 and discharging avalanche transistor 52 are triggered with a time interval therebetween by a trigger generator 53 to couple charging and discharging pulses via delay lines 54 and 55, respectively, to a node 56. The charging and discharging signals, with the time interval therebetween, are coupled via delay lines 57a, 58a, 59a and 60a to diode stacks 57, 58, 59 and 60, respectively, in sequence, the sequential interval being determined by delay lines 57a-60a. Each of the diode stacks 57-60 charges and discharges in the manner previously described to sequentially excite the antenna elements 57b, 58b, 59b and 60b having a spacing therebetween corresponding to the sequential interval. This circuit charges all the diode stacks through a single transistor, and discharges all the diode stacks through another transistor, thus eliminating transistor temperature variations that may cause variations in the sequential interval and the width of the radiated pulses. It should be recognised that the single discharge transistor may be replaced with a stack of transistor when an amplitude is desired that cannot be achieved with the utilisation of a single transistor. Refer now to Figure 5, this shows a block diagram of the equipment coupled to the transmitting array 11 and receiving antenna 21 of Figure 1. The main limitation upon the minimum detectable range of the short pulse radar system is the direct coupled signal from the transmitter 73 to the receiver 74. A large amplitude signal, directly coupled from the transmitter to the receiver, may trigger the detector therein, even if its is biased off or the range gate is closed. As previously discussed, an absorber may be positioned between the two sections of the cylinder to reduce direct coupling due to surface currents. Further reduction of direct coupling may be obtained with a switch 75 positioned in the circuit between the receiving anenna 72 and the receiver 74. The switch 75 may be a SPST PIN diode controlled by a switch control generator 76. Trigger pulses may be coupled to the switch control generator 76 from a transmitter 73 and may be the same pulses used to trigger the charging pulse generator 51.

Claims: A short pulse antenna system characterised in that it comprises means (26) for applying trigger pulses; charging means (25) responsive to the trigger pulses for supplying charging pulses; pulse means (31) charged in response to pulses coupled thereto from the charging means; means (31,41-44) responsive to the charging means and coupled to the pulse means for providing discharge pulses to the pulse means at predetermined time intervals subsequent to the charging pulses to discharge the pulse means and establish a sequence of short pulses of predetermined duration, preselected rise time, and having the predetermined time intervals therebetween; and an array of antenna elements (11a..11n) responsive to pulses from the pulse means in corresponding sequence for providing radiated pulses, the antenna elements having spacings (S) therebetween equal to distances travelled by light propagating in free space for a time equal to the predetermined time interval. A system according to claim 1, characterised in that the array of antenna elements are capped monopoles (11a..11n) having a monopole height less than a distance travelled by light propagating in free space for a time equal to said preselected rise time, and a cap thereon of diameter equal to the monopole height. A system according to claims 1 or 2, characterised in that the pulse means (31) includes a plurality of charge storage devices, charges and discharged by said charging and discharging means to establish the short pulses. A system according to claim 3, characterised in that the charge storage devices include a plurality of serially-coupled step recovering diodes. A short pulse antenna system characterised in that it comprises means (53) for supplying a trigger pulse; charging means (51) responsive to the trigger pulse for supplying a charging pulse: discharging means (52) responsive to the trigger pulse for supplying a discharging pulse at a predetermined time after the charging pulse; and pulse means (57-60) coupled to the charging means and the discharging means for providing a sequence of short pulses having preselected rise time in response to the charging and discharging pulses, the sequence having predetermined time intervals between pulses; and a plurality of antenna elements (57b-60b) responsive to the short pulses in corresponding sequence for providing radiated pulses, the antenna elements having spacings (S) therebetween equal to the distances travelled by light propagating in free space for a time equal to the predetermined time intervals. A system according to claim 6, characterised in that the pulse means (57-60) includes: a plurality of charge storage devices, each charged and discharged, respectively, by the charging and discharging means to establish a short pulse having a preselected rise time; and a plurality of delay devices (57a-60a) for providing time delays differing from one another, coupled to the charging means, the discharging means, and correspondingly coupled to the plurality of charge storage devices such that the charging and discharging pulses are coupled to the charge storage devices with time intervals therebetween to provide said sequence of short pulses. A system according to claims 5 or 6, characterised in that the charging (51) and discharging (52) means comprise avalanche transistors. A system according to claims 6 or 7, characterised in that each charge storage device (57-60) includes a plurality of serially-coupled step recovery diodes. A system according to any of the preceding claims, characterised in that the antenna elements are capped monopoles (11a..11n). A system according to any of the preceding claims, characterised in that it further includes antenna means (22,23) for receiving reflections of the radiated pulses. A system according to claim 10, characterised in that the receiving antenna means includes a feed antenna element (22) and a reflector antenna element (23). A system according to claim 11, characterised in that the feed (22) and reflector (23) antenna elements are discone antennas. A system according to claim 11, characterised in that the feed and reflector antenna elements are capped monopoles. A system according to claim 11, characterised in that the feed (22) and reflector (23) antenna elements are conical monopole antennas. A system according to claim 11, characterised in that one of the feed (22) and reflector (23) antenna elements is a conical monopole, and one of the feed (22) and reflector (23) antenna elements is a discone antenna. A system according to any of the preceding claims, characterised in that it further includes means (74) for receiving short pulse signals; means (75) coupled between the receiver means (74) and the feed antenna element (22,72) for coupling and decoupling the receiver means to the feed element in response to a switching signal applied thereto; and control means (76) coupled to the switch means for providing the switching signal in response to the trigger pulses.

Inventors: LAMENSDORFT, DAVID, ROBBINS, KENNETH WARREN, ROSS, GERALD FRED
Applicants: UNISYS CORPORATION
Application year: 1987
Main classification: H01Q9
IPC codes: G01S7, G01S13, H01Q9, H01Q21, H03K3