US 3745567 A
Description (OCR text may contain errors)
Quinn et al.
tes Patent 1 1 July 10, 1973 WIDEBAND RADAR RECEIVER PROTECTIVE APPARATUS  Assignee': The United States of America as represented by the Secretary of the Air Force, Washington, D.C.
 Filed: Mar. 9, 1972  Appl. No.: 233,066
 U.S. Cl 343/5, 333/7, 333/13, 333/99 PL, 313/220  lnt. Cl. G015 9/00, 1-10lp 1/14, l-l0lj 17/16  Field of Search 313/220; 333/9, 13, 333/99 PL; 343/5 R  References Cited UNITED STATES PATENTS 2,922,131 l/l960 Braden 333/13 X 3,396,388 8/1968 Goldie 333/13 X 3,600,712 8/1971 Williamson 333/13 X Primary ExaminerBenjamin A. Borchelt Assistant ExaminerA. M. Psitos AttorneyHarry A. Herbert Jr. et a1.
 ABSTRACT A wideband radar receiver protective apparatus is pro vided in which a glass envelope in the shape of an elongated doughnut is utilized to provide a low pressure glow discharge in a coaxial configuration. The coaxial configuration consists of two separated sections of coaxial transmission line with the glass envelope interposed therebetween. The coaxial configuration interconnects a transmitter and the receiver to be protected. Generation of the glow plasma is controlled by means of a power source separate from the transmitter. The length of the glow region determines the amount of at tenuation. The glass envelope includes an inert gas in combination with a barium getter to preserve the purity of the gas almost indefinitely.
5 Claims, 2 Drawing Figures Pawn? SUF'FLY WIDEBAND RADAR RECEIVER PROTECTIVE APPARATUS BACKGROUND OF THE INVENTION This invention relates to a receiver protector apparatus, and more particularly to apparatus to attenuate a signal to protect radar receivers.
Conventional transmitter-output-pulse-fired duplexers suffer from a number of shortcomings. For example, spike leakage results in one limitation. Spike leakage is the high power energy passing through to the receiver during the formative time of the gas discharge. Furthermore, this formative time (e.g., 30 nanoseconds) precludes nanosecond pulses (e.g., l nanoseconds) because the transmitter pulse is shorter than the formative time. Also, bandwidth is narrow and is suitable only for long pulse narrow band radars. Fast recovery to the receive condition is commonly achieved by adding water vapor or other impurities to the discharge gas; this technique makes it more difficult to initiate and maintain the discharge and shortens the life of the device drastically. All of these problems are eliminated by'the present invention.
SUMMARY OF THE INVENTION A wideband receiver protective apparatus is provided to protect radar receivers from the high power pulses of the associated radar transmitter. It may also be used in other similar circumstances where it is desired to protect a vulnerable component from a de structive signal. The apparatus makes use of a low pressure glow discharge in a coaxialconfiguration to attenuate the potentially destructive signal to a safe power level. Generation of the glow plasma is controlled by means of a power source separate from the transmitter; thus the apparatus is much more versatile than the conventional transmitter-output-pulse-fired duplexers in common use. The length of the glow region determines the amount of attenuation; early models easily attained attenuation well in excess of 100 db. The bandwidth of the apparatus is the bandwidth of the coaxial line, i.e., extremely broadband, and for this reason it is ideally suited to the more sophisticated radars such as those employing pulse compression or nanosecond pulses. Recovery of the apparatus to the receive condition is accelerated by a clean-up voltage applied to the electrodes of the appartus. The latter eliminates the need for adding water vapor or other impurities to the discharge gas to achieve fast recovery. Use of an inert gas such as argon in combination with a barium getter is therefore possible thereby preserving the purity of the gas almost indefinitely (barium getters absorb nearly all gases except the inert gases) and resulting in easier faster firing and apparatus lifetime several times that of similar apparatus using water vapor.
The apparatus of this invention includes a discharge tube in the form of a galss envelope mounted between two separated sections of coaxial transmission line. The glass envelope which contains the aforementioned gas is formed of two glass cylinders of different diameters, one positioned inside the other, and the ends of the cylinders being jointed to each other at their common ends to form a type of elongated doughnut. Operation with the gas in its dormant state achieves RF transmission. In the protective state, the gas is energized which sets up a lossy barrier thereby causing attenuation of the signal. The inside surface of the large glass cylinder and the outside surface of the smaller glass cylinder are aluminized and electrical connection made thereto. The aluminizing does not extend to the end regions of the glass envelope to provide windows" for passage of RF energy. The aluminized outer and inner glass cylinders in conjunction with the inner and outer conduc tors of the coaxial lines are utilized as blocking capacitors. In the protective state, an appropriate voltage is applied to the aluminized areas by way of their electrical connections to provide a uniform glow discharge thereby presenting a very lossy medium to the RF entering one of the windows."
The apparatus of the present invention may be used with triggerable transmitters. The receiver protective apparatus is simply triggered into the protective state prior to the arrival of the transmitter pulse. However, a radar system is provided that has the added advantage that it is useable with self-firing (nontriggerable) transmitters as well.
DESCRIPTION OF THE DRAWINGS FIG. 1 shows a side view of the wideband receiver protective apparatus of the present invention; and
FIG. 2 shows a radar system including the wideband receiver protective apparatus.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Now referring in detail to FIG. 1, the apparatus illustrated is essentially a means for creating and containing a glow discharge in between two separated coaxial transmission lines 22 and 23. While there are several ways to construct the structure the method fianlly chosen obviated the need for glass to metal seals at the two ends where RF windows are required.
Glass envelope 10 has the shape of an elongated doughnut and was formed from two equal length glass cylinders of the proper diameters. Small diameter cylinder 11 was positioned inside larger cylinder 12 and the ends were bent" toward each other and sealed to form RF windows 13 and 14. Barium getter section 15 is attached to the main body of glass envelope 10. The getter is operated in the conventional manner. All of this construction may be performed by a skilled glass technician using a glass lathe and special torches and tools. Note that the inside surface of larger glass cylinder 12 and the outside surface of small glass cylinder 11 were aluminized to form aluminum metal surfaces 16 and 17, respectively. Electrical feedthrough terminals 18 and 19 were provided, prior to the glass forming, to provide electrical contacts to aluminum surfaces 16 and 17, respectively. Further note that the aluminizing does not extend into the end regions so as to include RF windows" 13 and 14 but extends only to end regions 20 and 21, respectively. When the structure is complete, it is evacuated and backfrlled with argon or other appropriate gas to an approximate pressure of one mm mercury. The exact pressure is then optimized for maximum attenuation.
Now referring again to two separated coaxial transmission lines 22 and 23 which are connected by glass envelope 10. One end of glass envelope 10 extends partially by a pressure fit into coaxial transmission line 22 and the other end extends partially by a pressure fit into coaxial transmission line 23. The main body of glass envelope 10 is not encompassed by either one of the coaxial transmission lines and is dotted to indicate that the relative length is usually much greater,
It is noted that coaxial transmission line 22 is comprised of inner conductor 22a and outer conductor 22b. Coaxial transmission line 23 is comprised of inner conductor 23a and outer conductor 23b. Blocking capacitor 24 is comprised of a portion of inner conductor 22a, a portion of glass envelope 10, and a portion of aluminized surface 17. Blocking capacitor 25 is comprised of a portion of outer conductor 22b, a portion of glass envelope 10, and a portion of aluminized surface 16. Blocking capacitor 26 is comprised of a portion of inner conductor 23a, a portion of glass envelope 10, and a portion of aluminized surface 17. Blocking capacitor 27 is comprised of a portion of outer conductor 23b, a portion of glass envelope 10, and a portion of aluminized surface 16. Thus two blocking capacitors are provided for each coaxial line.
In the nonprotective operation of this apparatus, an RF signal enters at conventional dielectric coaxial line 22. RF window 13 passes the RF with little loss or reflection. Immediately to the right of RF window 13 is a blocking capacitor section comprised of two blocking capacitors 24 and 25, one associated with inner conductor 22a and the other with outer conductor 22b, respectively. In each case, the capacitor is formed by the glass cylinder as dielectric sandwiched between the coaxial line cylinder and the aluminized surface on the glass. With little loss or reflection, the RF signal passes through the blocking capacitor into the aluminized glass coaxial line. The RF signal propagates down the aluminized glass coaxial line to blocking capacitors 26 and 27 which are exact duplicates of blocking capacitors 24 and 25 respectively, passing through to conventional air dielectric coaxial line 23. Both reflection and loss due to the entire structure is very low for the nonprotective state.
In the protective state, it is desired to isolate conventional coaxial line 23 from signals entering from conventional coaxial lne 22. To accomplish isolation, an appropriate voltage is selectively applied to terminals 18 and 19 by power supply 28 through a predetermined valued current limiting resistor 29. The purpose of resistor 29 is to maintain a well behaved glow discharge in the volume between the inner and outer aluminized surfaces. This glow discharges presents a very lossy medium to the RF signal, entering from coaxial line 22, thereby reducing the signal power typically by 50 db or more. Thus, in the protective state the apparatus has low reflection but great loss, the energy being absorbed in the glow discharge plasma.
When used with triggerable transmitters, the apparatus shown in FIG. 1 is simply triggered into the protective state prior to the arrival of the transmitted pulse. However, the system illustrated in FIG. 2 has the added advantage in that it is useable with self-firing (nontriggerable) transmitters as well.
Now referring in detail to the radar system shown in FIG. 2, receiver protective device 30 represents the aluminized glass envelope shown in FIG. 1 in association with coaxial lines 22 and 23. Coaxial lines 22 and 23 of FIG. 1 are represented as coaxial lines 32 and 33, respectively, in FIG. 2. In this embodiment, receiver protective apparatus 30 is normally on, i.e., conventional vacuum tube or solid state switches 40 and 41 are normally closed. Thus, an RF pulse emanating from transmitter 31 travels through low loss (typically 3 db) attenuator 34, splits at junction 35 with half of the energy going to antenna 36 and half being absorbed in receiver protective apparatus 30. It is noted power supply 46 and resistor 47 are comparable to power supply 28 and current limiting resistor 29 respectively of FIG. 1. However, in leaving transmitter 31 a small trigger signal is picked up conventionally from transmission line 37 by probe 38 and used to trigger pulser 39. Pulser output is a pulse of length equal to the desired radar listening time. This pulse splits, half going as pulse 44, to shut off electronic switch 40. At this point in time, due to the natural delay in pulser 39 and associated transmission lines, the RF pulse has long since left antenna 36. Electronic switch 41, still being a closed switch, allows a reduced clean up voltage with amplitude determined by resistor 42 to exist across the receiver protective apparatus 30 until half of the pulser pulse delayed by delay 43, shown as pulse 45, reaches electronic switch 41. Once electronic switch 41 is fully off the noise generated by the clean up current disappears. At the end of te pulse, electronic switches 40 and 41 go into conduction automatically and the cycle is ready -to repeat. During the listening time, signals reflected from targets enter antenna 36 and split at junction 35, half of the energy going directly to receiver 48 through receiver protective apparatus 30, now in the low loss non-protective state. The other half of the energy passes through attenuator 34 to transmitter 31 and is wasted. Any energy reflecting from transmitter 31 must again pass through attenuator 34 and again split at junction 35. Thus, assuming 3 db for the attenuator, this nuisance signal will be at least 9 db below the direct signal to the receiver.
It is noted that power supply 46 has two terminals 46a and 46b, and each of electronic switches 40 and 41 has two ends and a control input. For example, switch 40 has ends 40a and 40b and control input 400, and switch 41 has ends 41a and 41b and control input 410.
It is emphasized that because of the separate control feature, the apparatus may be placed in the protective state before the output pulse arrives. Still further, the use ofa barium getter to maintain the purity of an inert discharge gas such as argon is possible because water vapor or other impurities are not required to accelerate recovery as in conventional duplexers. The barium getter results in faster formative time, lower driving re quirements and increases the life manyfold.
It is claimed: I
1. A receiver protective apparatus being comprised of an annular emission chamber formed from first and second glass cylinders ofa predetermined equal length, said first glass cylinder being of a predetermined larger diameter than said second, said second being positioned inside said first with the common ends bent to wards each other and sealed to form first and second means for passage of RF signals therethrough, a first aluminized surface applied to the inner surface of said first glass cylinder excluding said first and second passage means, a second aluminized surface applied to the outside surface of said second glass cylinder excluding said first and second passage means, an inert gas at a predetermined pressure contained in said annular chamber, a barium getter integrated with said annular chamber, receiver means, first and second coaxial transmission lines physically separated from each other, said first coaxial transmission line receiving RF signals, said second coaxial transmission line feeding RF signals to said receiver, said annular emission chamber interconnecting said first and second coaxial transmission lines with a portion of said annular emission chamber including said first passage means being inserted into said first coaxial transmission line and a portion of said annular emission chamber also including said second passage means being inserted into said second coaxial transmission line, current limiting means, and power supply means to selectively apply a preselected voltage through said current limiting means to said first and second aluminized surfaces to provide a glow discharge therebetween thus operating to provide a preselected attenuation of the RF signals being fed to said reciever.
2'; A receiver protective apparatus *as' described in claim 1 wherein said inert gas is comprised of argon.
3. A receiver protective apparatus as described in claim ll wherein said current limiting means is comprised of a resistor.
41. A radar system including a receiver protective apparatus being comprised of transmitter means providing RF output pulses to be transmitted towards targets of interest, antenna means directing said RF output pulses towards said targets, a first coaxial transmision line connected to said antenna means, receiver means, a second coaxial transmission line connected to said receiver, said first and second coaxial transmission line being physically separated, an annular emission chamber formed from first and second galss cylinders of a predetermined equal length, said first glass cylinder being of a predetermined larger diameter than said second, said second being positioned inside said first with the common ends bent towards each other and sealed to form first and second passage means for RF signals therethrough, a first aluminized surface applied to the inner surface of said first glass cylinder exclusing said first and second passage means, a second aluminized surface applied to the outer surface of said second glass cylinder excluding said first and second passage means, an inert gas at a predetermined pressure contained in said annular emission chamber, a barium getter integrated with said annular emission chamber, said annular emission chamber electrically interconnecting said first and second coaxial transmission line at said physical separation therebetween with a portion of said annular emission chamber including said first passage means inserted in said first coaxial transmission line and with a portion of said annular chamber including said second passage means inserted in said second coaxial transmission line, power supply voltage means having first and second terminals, said first terminal being connected to said first aluminized surface, first and second switches normally closed, each of said switches having first and second ends and a control input, said second terminal being connected to said first ends of said first and seond switches, current limiting means connected between said second end of said first electronic switch and said second aluminized surface to provide a glow discharge between said first and second aluminized surfaces during the closed period of said first electronic switch, resistor means connected from said second end of said second electronic switch and said second end of said first electronic switch to provide a reduced voltage to exist across said annular emission chamber during the open period of said first electronic switch and the closed period of said second switch, means to couple a trigger pulse from said RF output pulses, pulse means providing a control pulse of predetermined length in response to receipt of said trigger pulse, said predetermined length being determined by the listening period of said radar system, said control input of said first electronic switch receiving said control pulse and operating the open said first electronic switch, and means for a predetermined delay passing said control pulse to said control input of said second electronic switch also for opening thereof, said first and second switches automatically closing at the end of said control pulse and the delayed control pulse.
5. A radar system as defined in claim 4 further including a preselected attenuator interposed between said transmitter means and said antenna means.