Downed-aircraft Radio Locator-beacon Employing Plural Loop Antennas

Abstract

A compact dual-frequency specially-modulated radio transmitter and combined antenna assembly are mounted in an aircraft. Operation is initiated either automatically in the event of a crash or manually by a remote control, with the remote control system incorporated in a manner such that severing or shorting of its connections in a crash will not prevent automatic operation. Overall size and airdrag are minimized by sandwiching the principal transmitter components between a ground plane and a loop-shaped antenna parallel thereto.

Description

Among the numerous more-detailed improvements are the construction of interstage shield pockets formed in part by the antenna assembly, provision for utilizing both an internal battery and, if available, the aircraft battery, a constant-current oscillator which yields high frequency stability, a simplified harmonic-generating and signal-splitting system, direct coupling of the final amplifiers to the respective antennas and a modulator that produces a distress signal swept repeatedly through a range of audio signals at a sub-audible rate of repetition. Other features reside in a power control system that includes delayed actuation for the remote control to guard against false actuation, correlated arming of a firing device and discharge of the delay circuit to insure operation over a wide-range of battery voltage, similarly delayed remote turn-off control and integration into this same system of a shock-actuated or impact switch that initiates automatic operation which, in turn, can be overridden by the remote control in the event, for example, of an unusually severe landing shock that does not impair further operation of the aircraft.

BACKGROUND

The application describes a downed-aircraft radio-locator beacon. It also discloses a number of structural and circuitry features advantageously of use in various related radio apparatus.

With drastically increased numbers of private and commercial aircraft now in use, increasing attention is being given to the problems associated with locating downed aircraft. When an aircraft unintentionally returns to the ground, whether by way of a safe but forced landing, or by a crash, a first concern is for the safety of the persons involved. Besides the frequent necessity of prompt medical attention, most incidents occur in poor weather often involving snow and cold. There are U.S. Coast Guard records indicating that survival prospects deteriorate considerably unless rescue is made within twelve hours after an accident.

Other records indicate that when flight plans are not filed, as often is the case in private flying, the occupants of about only one in every six aircraft are rescued in spite of search efforts. It has been reported that, in 1964, 11 percent of lost aircraft remained missing without a trace of the wreckage. On the other hand, of the lost aircraft that were found, 28 percent had survivors. In 1966, according to reports, there were 171 lost civilian aircraft with a total of 379 persons aboard. Of that number, 111 persons were found alive, 268 perished and 31 of the aircraft were not found.

Such records clearly support the feeling that there is sufficient likelihood of saving lives to warrant the mounting of expensive rescue missions. At the same time, however, these rescue missions too often involve extreme risk on the part of the personnel taking part. In supporting a bill adopted by the California Legislature that requires civil aircraft registered in the State to be equipped with radio-locator beacons, Calif. Sen. Lew Sherman is quoted as saying, "For the first half of this year, there were 42 search missions involving over 2,000 individual flights, 3,000 hours of flying time, and almost 5,000 people." Not only are many of those people endangering their own lives, but it is estimated that search costs can run as high as four-hundred-thousand dollars.

The foregoing demonstrates the desirability of the inclusion in all aircraft of equipment capable of signalling the existence of distress and assisting in locating the distressed aircraft. Emergency radio apparatus is indicated for achieving this purpose, because at least most U.S. military planes and many commercial aircraft carry radio receivers that are automatically activated by distress signals transmitted on an assigned emergency frequency or frequencies. Consequently, it is not surprising that several manufacturers have announced the present or coming availability of emergency radio equipment to be used on such frequencies in the event of an incident.

While downed-aircraft-locator radio equipment thus is highly desirable, its incorporation into the aircraft poses what can be serious problems. For other reasons, aircraft typically have become more and more loaded with different auxiliary or safety apparatus to the point where the weight of and the space occupied by such apparatus has become a distinct limitation. Added weight can also be of difficulty with respect to the affect upon a plane's center of gravity within a given installation. Particularly for the private aircraft owner, the additional costs of such apparatus have become of serious concern. Also of interest with respect to aircraft performance is the additional drag imposed by antennas and the possibility of damage to such antennas while the aircraft is on the ground and also by objects such as birds, while flying.

A further deficiency in certain distress radio apparatus already on the market is the necessity of actuation and control by the pilot or other operator. In a crash situation, of course, the potential operator may be unconscious or otherwise physically unable to manipulate the equipment. Moreover, the distress apparatus must in itself be capable of withstanding the impact of the crash and of initiating operation without dependence upon the aircraft's electrical system which may be rendered ineffective as a result of the incident. Furthermore, the apparatus must be capable of maintaining operation under conditions of extremes in temperature and of wide variations in available power supply potential, while yet retaining its capability of transmitting its signals at the assigned frequencies to which search monitoring receivers are set to respond.

SUMMARY

It is, accordingly, a general object of the present invention to provide a new and improved downed-aircraft radio-locator beacon which overcomes or satisfies the aforenoted deficiencies and difficulties.

It is another object of the present invention to provide various structural assemblies and electronic circuitry which lend themselves beneficially to the attainment of the primary objective.

It is a related object of the present invention to provide such structure and circuitry which also find advantageous utility in similar control and communications environments.

In accordance with the disclosure, a downed-aircraft-locator beacon includes a stable-frequency oscillator which feeds an amplifier for developing an output signal. That signal is pulse modulated with a distress signal swept repeatedly through a range of audio signals at a sub-audible rate of repetition. A remote control permits actuating or deactuating the beacon upon selective manual operation only continuously for a predetermined minimum time interval to guard against false actuation. Also included is an impact switch that automatically actuates the beacon in response to a force thereon representing a crash. The modulated signal is fed directly to a loop-shaped antenna spaced from and parallel to a ground plane, with the signal developing apparatus sandwiched between that loop and the ground plane.

Other features include a compact nested arrangement of two such antennas, an interstage shielding structure completed by a portion of the antenna assembly, arrangement of the remote control system so that there is no interference with automatic impact operation even though the remote control or its connections may become severed or shorted as a result of a crash, and the inclusion of an oscillator for initially generating the resultant signals with a high degree of frequency stability notwithstanding variations in temperature or power source potential. Additional features include simplified signal distribution arrangements within the apparatus, circuitry for developing the special modulation characteristics, circuitry for permitting use of the aircraft's own battery, when available, as well as a self-contained internal battery and a control system coordinating impact switch operation with that of isolated "off" or "on" remote control.

The inventive features which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood, however, by reference to the following description taken in conjunction with the accompanying drawings in the several figures of which like reference numerals identify like elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side-elevational view of an aircraft incorporating a downed-aircraft radio-locator beacon located as herein described;

FIG. 2 is a fragmentary cross-sectional view of the beacon depicted in FIG. 1;

FIG. 3 is a view taken along the line 3--3 in FIG. 2;

FIG. 4 is a view taken along the line 4--4 in FIG. 2;

FIG. 4a is a longitudinal-cross-sectional view of a component included in the apparatus of FIG. 4;

FIG. 5 is a block diagram of a downed-aircraft radio-locator beacon of the kind installed on the aircraft shown in FIG. 1;

FIG. 6 is a schematic diagram of a portion of the circuitry included in the beacon;

FIG. 7 is a schematic diagram of another portion of the circuitry.

FIG. 8 is a schematic diagram of a detailed portion of the circuitry included in the diagram of FIG. 7;

FIG. 9 is another schematic diagram of a control circuit used in conjunction with the other circuits depicted;

FIG. 10 is a schematic diagram of the remote control portions of the circuitry illustrated in FIG. 5; and

FIGS. 11a, 11b, 11c and 11d illustrate wave forms of signals present in the circuit of FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates a typical private aircraft 15. The aircraft includes an elongated fuselage 16 with a tail assembly 17 at the rear end of the fuselage and a pilot station 18 towards the front end. In this case, the wings 19 are disposed below and somewhat to the rear of the pilot's station. Included in the upper portion 20 of the rearward one-third of fuselage 16 is a coded-signal radio transmitter or downed-aircraft-locator beacon 21. That location is selected because of the discovery, in the course of a study of crash records, that such portion of the aircraft, in the rear one-third but forward of the tail assembly, is most likely to survive a crash intact. Consequently, beacon 21 when in that position is least likely to be directly struck or torn apart in the unfortunate event of a crash.

Referring to FIGS. 2-5, beacon 21 includes a metallic chassis or enclosure 23 within which are disposed an oscillator 24, a buffer-doubler 25, a pair of drivers 26 and 27 and a pair of final amplifiers 28 and 29. Also within enclosure 23 are a power control system 30, an impact switch 31 and a modulator 32. With reference to FIG. 4, it will be seen that the interior of enclosure 23 is divided by various walls so as to form a plurality of individual pockets that shieldingly surround different stages or portions of the overall circuitry. That is, the principal components of oscillator 24 and buffer-doubler 25 are disposed around a pocket 34, driver 26 is primarily disposed within a pocket 35, and an active element in driver 27 is disposed in an aperture between pocket 34 and pocket 36 that houses final amplifier 29. Final amplifier 28 is principally disposed within a pocket 37. Situated within a large remaining pocket 38 is a printed-circuit board 39 on the underside of which (as viewed) are affixed most of the components which make up both power control 30 and modulator 32, those units being interconnected with the other units by means of a series of feed-through capacitors 40, 41, 42 and 43. Impact switch 31 is nestled against the internal wall of pocket 38 adjacent to the printed circuit board. As shown, each of the different stages and other portions of the circuitry have their individual component elements grouped together in their respective different pockets. The intent in FIG. 4 is merely to illustrate the internal element separation of these stages and circuit portions, while the details of their interconnections are depicted more fully in the schematic diagrams of FIGS. 6--9. Together, the circuitry constitutes radio apparatus in the form of a transmitter capable of broadcasting a pair of output signals on respective different frequencies with both being pulse modulated by a distress signal swept repeatedly through a range of audio signals at a sub-audible rate of repetition.

For radiating the two output signals, beacon 21 includes a pair of antennas 45 and 46 (FIGS. 2 and 3) each of which is composed of a loop-shaped radiator spaced from and parallel to an associated ground plane. Moreover, the different loops and ground planes together with enclosure 34 are nested together concentrically with enclosure 23 sandwiched between one of the ground planes and its associated loop. In more detail, a conductive sheet 48 is disposed immediately over the skin 49 of aircraft 15. Enclosure 23 is affixed directly to sheet 48 and carries a substrate 50 of fiberglass or other durable insulating material. Disposed circumferentially around the lower periphery of substrate 50 is a loop 51 of a conductive material such as copper. Loop 51 is continuous except for a comparatively narrow gap 52. One side of gap 52 is electrically connected to conductive sheet 48 by a metallic post or stud 53 secured to substrate 50 by a screw 54. A conductive lead 55 directly connects the collector of a transistor 56 in final amplifier 28 to a tap 57 spaced a short distance from post 53 so as to match a comparatively low-impedance point on antenna-loop 51 to the similarly low impedance presented by the output terminals of transistor 56. From another tap 59 on loop 51 intermediate its ends is a connection to one side of a capacitor 60 the other side of which is conductively affixed to the ground plane defined by sheet 48. For convenience, capacitor 60 in this case is composed of the parallel combination of a fixed capacitor 61 and an adjustable trimmer capacitor 62.

In operation, antenna 46 functions generally in accordance with the principles disclosed in U.S. Letters Pat. No. 3,151,328 -- BOYER (RE 26,196) and U.S. Pat. No. 3,247,515 -- BOYER and as described by J. M. Boyer in an article entitled "Hula-hoop Antennas: A Coming Trend?" which appeared in the Jan. 11,1963 issue of Electronics at pages 44-46. Considering the plane defined by loop 51 and its associated sheet 48 to be horizontal, the pattern of radiation from the antenna is omnidirectional in the horizontal plane with an intensity which exhibits a peak amplitude at a comparatively low angle relative to the horizontal plane. Correspondingly, the radiation pattern exhibits minimum intensity of radiation in the vertical direction so that it is said to have a "cone of silence" about a vertical axis with the apex of the cone appearing at the antenna. Thus, a search aircraft attempting to "home in" on the signal radiated by antenna 46 can be aided in its search by flying cross patterns and noting when, as exhibited by fading of the received signals, it passes into and out of the aforementioned cone of silence. As compared with those patents, antenna 46 affords additional flexibility in that, instead of being connected between the ground plane and the side of gap 52 opposite stud 53, capacitor 60 is tapped to loop 51 at a point intermediate its ends. This permits utilization of a larger value of capacitor 60 so that a greater range of adjustment in that value may conveniently be obtained.

When initially aligning the apparatus, capacitor 60 is adjusted to tune the antenna to resonance at its assigned frequency. In this particular case, antenna 46 is constructed and tuned to resonate at 121.5 Megahertz, the frequency of a standard civilian distress channel. At that frequency, loop 51 has an electrical length corresponding to one-fourth wavelength. It will thus be seen that the antenna exhibits a radiation pattern very much like a conventional quarter-wave vertical antenna. However, that result is achieved with a much more compact structure and that structure is capable of being physically formed with rugged components capable of withstanding the stresses developed under severe impact conditions. Another significant feature of the antenna, in combination with the radio apparatus contained within the antenna structure itself, is the direct single-wire connection 55 from loop 51 to transistor 56; this eliminates any need for the use of matching networks to couple the final amplifier to the antenna.

Spaced inside and lying in the plane of loop 51 is a conductive layer 64 that defines a ground plane for antenna 45. In turn, spaced from and parallel to layer 64 is a second and smaller conductive loop 65, loop 65 and layer 64 together forming antenna 45 and operating in the same manner as previously described with respect to antenna 46 except at a much higher frequency because of the reduced diameter and peripheral length of loop 65. As shown, layer 64 is affixed to the upper surface of substrate 50, while loop 65 is carried around the lower periphery of another substrate 66 held in place by a screw 67 threaded into a stud affixed to the center of substrate 50. As in the case of antenna 46, one side of a gap 68 in loop 65 is grounded to layer 64 by a stud 69 into which a screw 70 is threaded. The collector of a transistor 71 is directly connected by a short lead 72 to a tap 73 spaced from stud 69 by a distance to present an impedance matching that presented at the output of transistor 71. Also as before, a trimmer capacitor 75 is connected between a tap 76 on loop 65 and a ground plane defined by layer 64. Capacitor 75 is adjusted to tune antenna 45 to resonance at its assigned frequency which in this case is 243 Megahertz, the frequency of a standard military distress channel. It is on that channel that equipment already carried by many present-day aircraft is designed to be automatically activated by a received signal. Loop 65 has an electrical length corresponding to a quarter-wave length at this frequency. Again, antenna 45 exhibits a radiation pattern which is omnidirectional in the horizontal and includes a cone of silence in the vertical.

Substrate 50 is affixed as a cover to the external sidewalls of enclosure 23 by means of a series of screws 78 threaded into the corresponding fillets 79. Conductive sheet 48 is similarly secured to the bottom of enclosure 23. Screws 78 contact the peripheral portion of layer 64 so that the latter is conductively connected to the metallic walls of enclosure 23 and also to conductive sheet 48. With substrate 50 secured in place, conductive layer 64 serves not only as a ground plane for antenna 45 but also closely overlies the openings in each of pockets 34-38 so as to shieldingly complete the closure of those pockets and thus enhance the isolation between the different stages and other portions of the overall circuitry. Preferably, the shielding effectiveness is further improved by forming another conductive layer on the underside of substrate 50 immediately below layer 64; that layer, therefore, lies in direct physical contact with the upper edge surface of the walls defining and segmenting enclosure 23. In either event, an opening is formed through substrate 50 as well as through conductive layer 64 and the other underlying conductive layer, if included, and transistor 71 that projects upwardly from pocket 36 is disposed in that opening. Thus, the layout of the different pockets formed in enclosure 23 is such as to align transistor 71 with the desired position of tap 73 on loop 65. Similarly, pocket 37 is located so that the opening from that pocket through the wall of enclosure 23, in which transistor 56 is disposed, is in alignment with tap 57 on loop 51. Additional openings, such as illustrated at 80 in substrate 66, may be formed at different places in either of the substrates either for weight-reduction or stress-relief purposes.

Affixed to and beneath conductive sheet 48 is a housing 82 forming a compartment for an internal battery-type power supply. In a typical embodiment, housing 82 is a size sufficient to accept a quantity of eight D-Size alkaline-earth cells which are connected in series to constitute a nominally 12-volt source of power that exhibits a long shelf-life capability.

While the installation of beacon 21 on the aircraft fuselage only requires the formation of a comparatively small hole in skin 49 to accommodate interconnecting electrical leads and a few still-smaller holes to accept threaded fasteners securing compartment 82 through the skin to sheet 48, the beacon assembly further includes a plate 83 of a material stiffer than that of either compartment 82 or sheet 48. In this way, the rigidity of the entire assembly is enhanced.

An insulative and fire-resistant but radio-wave transmissive dome 85 (e.g. fiber glass, polycarbonate or other plastic), fastened to sheet 48 around its periphery by screws 86, forms a housing overlying antennas 45 and 46 as well as enclosure 23. Because of the extremely compact formation of the entire apparatus, housing 85 projects only a very short distance beyond the normal exterior surface of the fuselage so as to present only a minimal air drag during operation of the aircraft. Furthermore, the interior space within dome 85, as well as the spaces within enclosure 23 and compartment 82, are completely filled with a deliquefied foam-type insulating and cushioning material 87. Consequently, the whole unit exhibits a high degree of shock resistance, while at the same time being highly resistant to both moisture and fire. Though constituting two complete antenna systems and transmitters together with a self-contained power source, a prototype unit, including cabling, and an extra or remote control, weighs only 41/2 pounds. The transmitter has a diameter of 71/2 inches and a height of only 2 inches. Yet, depending somewhat upon terrain and altitude of the search aircraft receiver, the operating range is of the order of 200 miles.

FIG. 4a best depicts the details of impact switch 31. Slideable within a sleeve 89 of non-magnetic material is a magnet 90 urged toward one end of the sleeve by a coiled spring 91. One end sleeve 89 is fixedly closed by a cap 92, while another cap 93 is threaded upon the other sleeve end and abuts spring 91; turning cap 93 relative to sleeve 89 permits adjustment of the spring pressure against magnet 90. A reed-type electric switch 94 is mounted upon sleeve 89 in a position such that its movable contact 95 of magnetic material is pulled against a mating contact 96 upon movement of magnet 90 toward switch 94. Beacon 21 is installed in the aircraft so that sleeve 89 is oriented fore and aft with respect to the fuselage and with magnet 90 being toward the rear of the aircraft. Upon a crash of the aircraft, magnet 90 is propelled forwardly against the spring pressure and toward switch 94. The position of cap 93 is adjusted so that the strength of spring 91 together with the mass of magnet 90 results in actuation of switch 94 when a force approximately equivalent to 7 times the force of gravity is applied to the unit. It has been found that the force developed during a crash exceeds that value. Normal, uneven rough, operation of the aircraft does not effect false operation. Of course, the entire impact switch assembly is securely mounted in place within enclosure 23. For this purpose, it has been found satisfactory to utilize an epoxy resin for directly cementing sleeve 89 to the bottom and the adjacent wall of enclosure 23.

Illustrating the overall beacon system is the block diagram of FIG. 5. Oscillator 24 develops a signal of very-high frequency stability at 121.5 Megahertz. That signal is fed to buffer-doubler stage 25 in the output portion of which both the fundamental 121.5 Megahertz and a harmonic 243 Megahertz signals are produced. The fundamental signal is fed through a driver stage 26 to final amplifier 28 which is coupled to antenna 46. Simultaneously, the harmonic signal is supplied through driver stage 27 to final amplifier 29 which is coupled to antenna 45. Power control circuit 30 serves to supply electric power upon either automatic or manual command to oscillator 24, buffer-doubler 25 and modulator 32. The latter, in turn, is coupled both to drivers 26, 27 and to final amplifiers 28 and 29. The modulator develops a specially-coded modulation generally of pulse waveform so that the final amplifiers and driver stages are gated to an on condition in response to the application of each pulse supplied by modulator 32.

Power control 30 is always connected so as to derive electric energy from the internal batteries in compartment 82. The power control circuitry is also arranges so that, when available, power is simultaneously derived from the aircraft's own battery system 97. Without the availability of power from aircraft battery 97, internal battery 82 has sufficient capacity to operate beacon 21 for approximately 100 hours at an outside temperature of 70.degree. Fahrenheit and about 60 hours at an outside temperature of 0.degree. Fahrenheit. When aircraft battery 97 is not destroyed or disconnected as a result of a crash, its availability extends the operating time by hundreds of hours.

Finally, the overall system includes a remote control 98 coupled to power control 30. The remote control is usually installed in the pilot compartment so as to permit manual operation to activate power control 30 either for test purposes or in the case of a forced landing not amounting to a crash. Remote control 98 also permits power control 30 to be deactivated manually whenever desired such as at the conclusion of the test.

As shown in FIG. 10, the separate remote control circuitry is exceedingly simple. It includes an "on" push button 100 operative upon manual depression to close a switch 101 and an "off" push button 102 manually depressable to close a switch 103. As a redundancy backing-up switches 101 and 103, each of them is parallelled by a second switch, 104 and 105 respectively, simultaneously operated by the respective push buttons. The positive terminal of a small long-life battery 106, in this particular case having a potential of 9 volts, is connected to one side of all four switches 101 and 103-105. The other terminals of switches 101 and 104 are connected to a lead 107 which is enclosed within a flexible shield 108 that is connected to ground and connects to terminal 109 of power control 30 as shown in FIG. 9 and labeled "on." The other terminals of switches 103 and 105 are similarly connected to a lead 111 enclosed within a grounded shield 112 and connected to a terminal 113 of power control 30 that terminal in FIG. 9 being labeled "off." The negative terminal of battery 106 is returned to ground. Thus, remote control 98 serves as a separate and individual power source the energy from which can be selectively applied either to terminals 109 or 113 of power control 30 as desired. Preferably, push buttons 100 and 102 are mounted on a small panel area within easy reach of the pilot. To help guard against inadvertant depression of either push button accidentally by the pilot or as a result of an object moved toward the push buttons in the case of a crash, each of the buttons preferably is physically disposed within a cup-shaped element 114. The lips of elements 114 extend beyond the normal or rest position of the respective push buttons so that they may be operated only upon the insertion within the cup of the end of a finger.

Turning next to FIG. 6, oscillator 24 includes an NPN transistor 116 having a collector 117, a base 118 and an emitter 119. An inductor 120 is coupled between base 118 and emitter 119 by way of capacitor 40 and an emitter-bias resistor 121. An adjustable capacitor 122 is coupled in parallel with inductor 120 and is tuned to resonate therewith at the fundamental frequency of 121.5 Megahertz. A peizoelectric crystal 123 is connected between emitter 119 and a tap on inductor 120. Crystal 123 serves as a filter having a narrow passband centered at the 121.5 Megahertz frequency. A load resistor 124 is coupled between collector 117 and emitter 119 by way of capacitor 41 and resistor 121.

With the end of resistor 121 opposite emitter 119 being connected to ground, to which the negative terminals of both batteries 82 and 97 are connected, oscillator 24 is energized by applying a positive battery potential from terminal 125 of power control 30 (FIG. 9) to the junction of resistors 124 and 135 by way of feed-through 41. To that end, terminal 125 is connected to collector 117 through resistor 124 and to base 118 through a limiting resistor 126 and inductor 120. To assist in obtaining optimum frequency stability, oscillator 24 is operated in a constant-current condition. This is assured by returning the point between resistor 126 and inductor 120 to ground through a pair of series-diodes 128 and 129 that exhibit a constant potential drop throughout a wide range of potential variation in the battery power source. While a single diode of the proper characteristics would suffice, for convenience a pair of silicon diodes are employed so as to achieve a constant voltage drop of 1.2 volts.

In operation, oscillator 24 regenerates by virtue of positive feedback at the desired frequency through filter 123. That feedback potential is applied across the lower portion of inductor 120 which serves as an autotransformer to induce a voltage gain in the feedback signal applied to base 118. As employed in practice, filter 123 is a quartz crystal active at a submultiple of the desired 121.5 Megahertz and exhibiting active response at either the fifth or seventh overtone of that submultiple. Contributing further to the excellent frequency stability of oscillator 24, emitter resistor 121 has a low value of resistance, typically 47 ohms, comparable to the impedance presented by emitter 119. At the same time, load resistor 124 similarly has a value, again typically 47 ohms, that is comparable to the low impedance presented by collector 117. One result of these impedance relationships is that there is minimal reflection of impedance from buffer-doubler 25 into oscillator 24 by way of collector 117; the arrangement functions much like the well-known electron-coupled oscillator.

In practice, oscillator 24 had been found to exhibit a deviation from desired center frequency of less than 5 Kilohertz under conditions of temperature variation all the way between minus 60.degree. and 100.degree. Centigrade in the 121.5 Megahertz region. This corresponds to a frequency tolerance of .+-. 0.005 percent. That same frequency tolerance is exhibited with a simultaneous variation in power supply battery potential from 12 volts to 6 volts. 154 Buffer-doubler stage 25 includes a PNP transistor 130 having an emitter 131, a collector 132 and a base 133. Load 124 is coupled between base 133 and emitter 131 by way of an emitter-bias resistor 135 paralleled by a by-pass capacitor 136. Its load circuit includes an inductor 137 series-connected with an adjustable capacitor 138 connected between collector 132 and ground and an adjustable capacitor 139 paralleled by an inductor 140 with both also connected between collector 132 and ground. A capacitor 141 is connected from a point intermediate inductor 137 and capacitor 138 to the base 143 of an NPN transistor 144 in driver stage 26 and which also has a collector 145 and an emitter 146. Connected from base 143 to ground is a radio-frequency choke 147. A tap on inductor 140 is connected to the emitter 148 of a transistor 149 that has a grounded base 150 and a collector 151.

For operation, bias resistor 135 is selected to have a value such that transistor 130 operates as a Class B amplifier and very near to operation in the Class C region. As a result, the signals amplified by transistor 130 include a harmonic frequency of 243 Megahertz. Capacitor 139 is tuned to be in parallel resonance with inductor 140 and the input reactance of transistor 149 at the higher 243 Megahertz frequency. At the same time, capacitor 138 is tuned, together with the fixed reactance of the input network including transistor 144, to be in series resonance with inductor 137 so that the fundamental-frequency signal at 121.5 Megahertz is fed to driver stage 26. Thus, the output circuitry of stage 25 serves as a passive network to separate and distribute signals of the two desired output frequencies. Stage 25 also serves as a buffer between oscillator 24 and the driver stages in order to prevent undesired reaction upon the oscillator by subsequent modulation of driver stages 26 and 27.

In the output circuit of driver transistor 143 is an inductor 152 coupled between collector 145 and ground by way of capacitor 43. In parallel with inductor 152 is an adjustable capacitor 153 also connected between collector 145 and ground to which emitter 146 is also connected. A tap on inductor 152 is connected to the base 154 of output transistor 56 the emitter 156 of which is returned to ground also through capacitor 43. Capacitor 153 is tuned, together with the input reactance of transistor 154, to resonate with inductor 152 at the 121.5 Megahertz fundamental frequency. Consequently, the fundamental-frequency signal is amplified by transistor 143 which in turn drives transistor 56 in final amplifier 28. As already described in connection with FIGS. 2 and 5, collector 157 of transistor 56 is directly connected and impedance matched to tap 57 on antenna 46 which radiates the finally amplified fundamental-frequency signal.

Similarly, driver stage 27 includes an inductor 158 coupled between collector 151 and ground by way of capacitor 42. A tap on inductor 158 is connected to the base 159 of transistor 71 which has its emitter 160 also returned to ground through capacitor 42 and a collector 1.sup.61 directly connected and impedance matched to tap 73 on harmonic-frequency antenna 45. An adjustable capacitor 162 connected between collector 151 and ground, together with the input reactance of transistor 159, is tuned to be in parallel resonance with inductor 158 at the harmonic frequency. Energization for both driver stage 27 and final amplifier 29 is achieved by the application of a positive potential through feed-through 42 from a terminal 164 of modulator 32 in FIG. 7. Similarly, terminal 164 is connected to driver stage 26 and final amplifier 28 through feed-through 43. Finally, the diagram of FIG. 6 includes an indication of a plurality of test points 165 for convenience of the technician servicing or adjusting the apparatus.

With reference to FIG. 7, modulator 32 has a series of stages including a sweep-repetition generator 168, a sweep-frequency generator 169, a flip-flop or bistable multivibrator 170 and several additional stages. From terminal 125 of power control 30 (FIG. 9), a positive potential is applied to a bus 171, while a negative bus 172 is connected to ground.

Sweep-repetition generator 168 includes a uni-junction transistor 174 having its first base 175 connected to ground and its second base 176 connected to positive bus 171. A filter capacitor 177 is connected between busses 171 and 172. A charging network includes a resistor 178 in series with a capacitor 179 between bus 171 and ground. Emitter 180 of transistor 174 is connected to a point intermediate resistor 178 and capacitor 179 from which point a signal also is sent to the base 181 of an NPN transistor 182 which serves as an isolation stage.

In operation, generator 168 develops a series of successive sawtooth pulses 184 as shown in FIG. 11a and having a repetition rate of between 2 and 4 times per second. The sawtooth pulses are developed upon capacitor 179 which is charged through resistor 178 until a potential level is reached on emitter 180 to render transistor 174 conductive. At that time, the charge on capacitor 179 is conducted to ground with the cycle then repeating a transistor 174 is thereby rendered nonconductive and capacitor 179 again begins to charge.

Isolation transistor 182 has a collector 183 connected to bus 171 and an an emitter 184 connected to the second base 185 of another uni-junction transistor 186 the other base 187 of which is connected to ground through resistor 188. Completing sweep-frequency generator 169 is a charging network composed of a resistor 190 in series with a capacitor 191 both connected between bus 171 and ground and with a point intermediate the resistor and capacitor being connected to an emitter 192 of transistor 186.

While sawtooth pulses 184 developed by sweep-repetition generator 168 have a constant repetition rate and duration, sweep-frequency generator 169 develops sets of successive sawtooth pulsations 194 as shown in FIG. 11b. The sets of pulsations 194 repeat at the repetition rate of pulses 184 and also progressively change in duration within each set. Ignoring resistor 188 and assuming for a moment that base 185 is connected directly to bus 171, the operation of generator 169 would be very much like that of generator 168. That is, capacitor 191 charges through resistor 190 until the potential on emitter 192 is sufficient to render transistor 186 conductive. At that time, the charge on capacitor 191 would be conducted to ground, transistor 186 would again become nonconductive and the cycle would repeat.

However, the firing potential of emitter 192 is a function of the potential applied to base 185 of transistor 186, and that base potential is repeatedly changing in proportion to the instantaneous changes in amplitude of pulses 184 from generator 168. Consequently, the period of time during each charge cycle of capacitor 191, before emitter 192 fires, is progressively changing throughout the duration of each of pulses 184. Accordingly, FIG. 11b represents the potential on emitter 192. In practice, pulsations 194 change, during the duration of each pulse 184, between a repetition rate between about 2,500 and 500 Hertz. It may be observed in passing that pulsations 194 in FIG. 11b are idealistically drawn and are shown to reach the like amplitudes. Although it does not affect operation in practice, pulsations 194 do individually increase in amplitude progressively during each set of pulsations because of the increased time permitted for capacitor 191 to charge before again firing emitter 192.

The value of resistor 188 is small compared to the reactance of charging capacitor 192 so that, when transistor 186 is rendered conductive, the resulting short time constant circuit composed of resistor 188 and capacitor 191 permits rapid discharge of the energy stored in capacitor 191. Consequently, a series of spike-shaped pulses are developed across resistor 188, one such pulse appearing each time capacitor 191 is discharged. As shown in FIG. 11c, the corresponding series of spike pulses 195 similarly occur in sets repeating at the repetition rate of pulses 184 and progressively changing in interpulse spacing within each set. Spike pulses 195 are then fed successively through a series of three DC-coupled amplifiers from the output of which they appear in amplified form across a resistor 196.

Thus, the point intermediate resistor 188 and base 187 is connected to the base 197 of an NPN transistor 198 that has its emitter 199 grounded and its collector 200 connected to bus 171 through a resistor 102. The point intermediate collector 200 and resistor 102 is connected to the base 202 of a PNP transistor 203 the emitter 204 of which is connected to bus 171 while its collector 205 is connected to ground through a resistor 206. Finally, the point intermediate resistor 206 and collector 205 is connected to the base 207 of an NPN transistor 208 having its collector 209 connected to bus 171 and its emitter 210 returned to ground through resistor 196. Like in the remainder of power control 30 as well as in the circuitry of FIG. 6, each succeeding transistor, in progressing through a series of direct-connected transistor stages, is of a different polarity-type (NPN or PNP) than the one proceeding or following it. This, of course, enables direct-current energization of the transistors with only a two-terminal source of direct-current power.

Spike pulses 195 are fed from the top of resistor 196 through a limiting resistor 212 to the input terminal 213 of multivibrator 170. The equivalent circuit of multivibrator 170 utilizing discrete components shown in FIG. 8, and typical values of all of the components therein together with a detailed description of its operation will be found beginning at page 50 of the Mar., 1966 issue of "Electronics World" as well as in standard reference works and handbooks under the standardized designation "JK Flip-Flop." By itself, the JK Flip-Flop is a fully integrated, monolithic circuit which is the equivalent of 15 discrete transistors combined with 17 discrete resistors. It is capable of counting by two, automatically steering its own input to the proper side on every count and acting as a shift register or memory element. By manufacturer's definition, JK Flip-Flops are available from Fairchild Semiconductor under type numbers UL 923 and 9923. While, as utilized in the present embodiment, several of the capabilities of the JK Flip-Flop are ignored, it is presently preferred because of its low-cost availability as a plug-in unit in a size smaller than an ordinary garden-variety green pea.

Nevertheless, any known device may be substituted which serves as a binary divider. That is, for each one of spike pulses 195, multivibrator 170 develops an output signal which alternately is first "on" and then "off" as shown by the series of square pulses 215 in FIG. 11d. The binary divider function of multivibrator 170 will be noted by observing that it requires two successive ones of spike pulses 195 to define any one of pulses 215. Consequently, square pulses 215, being alternately initiated and terminated by successive ones of spike pulses 195, occur in sets repeating at the repetition rate of sawtooth pulses 184 and progressively changing in duration throughout each set. By reason of the divider action, the repetition rate or frequency of square pulses 215 changes repeatedly between 1,250 and 250 Hertz. That is, square pulses 215 vary in frequency at an audio-frequency rate, while the sets of audio variations repeat at a sub-audible rate.

Because the JK Flip-Flop is available as a small plug-in unit, the receptacle schematically indicated in FIG. 7 is physically provided as a series of holes in printed board 39 to each of which the different interconnections are conventionally laid down. Completing the necessary connections in FIG. 7, terminals 214, 215 and 216 are connected to ground, terminal 217 is connected to B+ bus 171 through a limiting resistor 218 and the output from multivibrator 170 is taken from a terminal 219 and fed to the base 220 of a buffer and inverting transistor 221. To eliminate possible transient response, a filter capacitor 222 is connected between terminal 217 and ground.

Referring again to FIG. 8, and utilizing conventional integrated-circuit terminology, it will be observed that spike pulses 195 are applied to toggel input terminal 213, while square pulses 215 are derived from "zero" output terminal 219. "Set" input terminal 214 and "reset" input 216 are returned to ground, rendering those two possible functions inoperative. "Preset" input terminal 224 and "one" output 225 remain unconnected.

The isolation stage, including NPN transistor 221, serves to isolate multivibrator 170 from any switching transients developed during the action of the succeeding modulation PNP transistor 227 which operates as a saturated switch to alternately apply and remove the positive potential on bus 171 to and from driver stages 26, 27 and final amplifier stages 28, 29 by way of terminal 164. Thus, transistor 221 has its emitter 228 returned to ground through a bias resistor 229 and its collector 230 connected through a limiting resistor 231 to the base 232 of modulation transistor 227. A decoupling resistor 233 is connected between bus 171 and the point intermediate resistor 231 and base 232. Emitter 234 of transistor 227 is connected to bus 171, while its collector 235 is connected directly to modulation output terminal 164.

Transistor 221 is arranged to operate also as an inverter of square pulses 215 so that modulation transistor 227 becomes conductive on the application to its base 232 of each negative-going one of the square pulses. Consequently, the signals radiated from antennas 45 and 46 occur in bursts of energy varying in frequency of repetition rate over an audio-frequency range in sets of bursts that repeat at the sub-audible repetition rate of sawtooth pulses 184.

Turning finally to power control system 30 in FIG. 9, the supply of a positive potential from battery 82 to the radio-frequency and modulation apparatus of FIGS. 6 and 7 is under the control of a pulse-operated valve 240 capable of responding to a gating pulse to become and continue being conductive for the purpose of connecting battery 82 to terminal 125. When conductive, valve 240 is subject to being rendered nonconductive in response to the occurrence of a different electrical pulse in order to disconnect battery 82 from modulator 125 and thereby de-energize the load represented by the radio-frequency and modulation circuitry.

The positive terminal of aircraft battery 97 is connected to the positive terminal of battery 82 through a diode 241 poled to permit current flow to valve 240 only when the potential of external battery 97 is higher than that of internal battery 82. Thus, the aircraft's own battery power supply may be used, when available and appropriately charged, in order to greatly extend the period of time during which beacon 21 remains operative after being activated. At the same time, isolating diode 241 blocks the occurrence of any drain from battery 82 toward external battery 97 should the latter for any reason become substantially discharged or a short be developed in the lead extending from diode 241, which is inside the beacon, to that external power source.

To obtain the characteristics mentioned, valve 240 is a silicon-controlled rectifier (SCR) which exhibits a thyratron characteristic and has an anode 242 connected to terminal 82, a cathode 243 connected to power-supply terminal 125 and a gate electrode 244. One of contacts 95 and 96 in impact switch 31 is connected to terminal 82 while the other is connected through a limiting resistor 245 to gate 244. Gate 244 is also returned to its cathode 243 through a resistor 246, and a filter capacitor 247 is connected between cathode 244 and ground. Accordingly, automatic activation of beacon 21 is achieved simply by the closure of impact switch 31 to fire SCR 240 which, even though the contacts in switch 31 subsequently open, remains conductive.

Thus, both the automatic initiation and continuance of operation of beacon 21 is powered by energy from internal battery source 82 and/or external battery source 97. Remote and manual initiation of beacon operation, however, requires energy from an additional and separate power source which in this case is battery 106 of remote control 98 (FIG. 10). It will be recalled that depression of push button 100 causes a positive potential with respect to ground to appear on terminal 109, while depression of "off" push button 102 similarly causes a potential to appear at terminal 113. Depression of button 100, then, results in the supply of energy from battery 106 by way of terminal 109 to a timing or firing stage 250 which, after storage of that supplied energy for a predetermined minimum time interval, feeds an electrical-energy pulse over a blocking capacitor 251 to gate 244 of valve 240 in order to actuate the valve and thereby initiate operation of beacon 21.

In more detail, firing stage 250 includes a charging resistor 252, an energy storage device in the form of a charging capacitor 253 in series with resistor 252 between ground and a lead connected through a current-limiting resistor 254 to terminal 100. The gate electrode 255 of a uni-junction transistor 256 is connected to a point intermediate resistor 252 and capacitor 253. The first base 257 of transistor 256 is returned to ground through a load resistor 258, while second base 259 is connected to the supply side of resistor 252 through a diode 260 polarized to pass current from terminal 109 only in a direction toward base 259. Connected between base 259 and ground is an arming capacitor 262. A discharge path for capacitor 253 is created from the point intermediate resistor 252 and capacitor 253 through a diode 263 and a resistor 264 to ground. While diode 262 electrically shunts resistor 252, it is poled to permit current flow only in the direction from capacitor 253 toward resistor 254.

In operation, the depression of push button 100 enables flow of current through diode 260 into arming capacitor 262 which has a sufficiently large value to exhibit a long-time constant and thereby maintains an arming potential on base 259 relative to base 257. At the same time, current from terminal 109 through resistor 252 charges capacitor 253, the rise in potential on capacitor 253 occurring with a sawtooth wave form in the same manner as described with respect to generators 168 and 169 in FIG. 7. Like in those generators, the potential on capacitor 253 applied to gate 255 continues to increase until, in this case preferably until a time interval of between 4 and 5 seconds has elapsed, transistor 256 becomes conductive. The resultant dumping of energy from capacitor 253 through base 257 and resistor 258 results, like in generator 169, in the development of a spike pulse across resistor 258. It is that pulse which is then fed to gate 244 of value 240, whereupon the latter becomes conductive and the radio apparatus is activated.

By delaying the development of the firing pulse on resistor 258 for several seconds following the depression of push button 100, inadvertant false actuation of beacon 21 is guarded against, Should the push button be depressed for a lesser time interval and then released, the energy stored on capacitor 253 thereupon discharges through diode 263 and resistor 264 so that the firing circuit is automatically restored to the condition in which it will demand continued depression of push button 100 for the total designed delay interval before actuating valve 240.

Another firing stage 266 similarly responds to depression of push button 102 to develop a pulse across a resistor 267 that is utilized to render valve 240 nonconductive and, hence, disconnect the supply of power from the radio apparatus. To that end, current from terminal 113 is connected through a limiting resistor 268 and a charging resistor 269 into storage or charging capacitor 270. After a similar delay interval of 4 to 5 seconds, the potential on capacitor 270 causes a gate 271, of a uni-junction transistor 272, to establish a conductive path to ground through a first base 273 of that transistor and load resistor 267. Also as before, current from terminal 113 initially assures operative arming of transistor 272 by means of rapid current flow through a diode 274 into arming capacitor 262 which is connected between the second base 275 of transistor 272 and ground. Additionally the same as in firing stage 250, a diode 277 shunts resistor 269 and establishes a discharge path from capacitor 270 through a discharge resistor 278 to ground.

Capacitor 262 thus serves in common to arm both transistors 256 and 272. Moreover, the value of capacitor 262 is sufficiently high that its discharge rate, through bases 275 and 259 and the back resistances of diodes 260 and 274 together with the resistances of any other leakage paths, is correspondingly sufficiently slower than the discharge rates of either capacitor 253 or capacitor 270, through their respective resistors 264 and 278, that transistors 256 and 272 remain in an armed condition over a substantial range of potential variation in battery 106. Even should the potential of battery 106 become lower than normal, the arming potential maintained on bases 259 and 275 is kept sufficiently high that a lowered potential on gates 255 or 271 is not permitted to cause pre-mature firing of transistors 256 or 272 as otherwise could occur with a reduced potential available for bases 259 and 272.

Completing the circuitry for extinguishing operation of value 240 is response to depression of push button 102 is a similar valve 278 in the form of a silicon-controlled rectifier having an anode 279 connected to internal battery terminal 82 and a cathode 280 returned to ground through a resistor 281 in series with another resistor 282. The point between resistor 267 and the base 273 is connected through a blocking capacitor 283 to the gate electrode 284 of SCR 278. Connected directly between cathodes 243 and 280 is an extinguishing capacitor 285. Finally, the side of capacitor 283 opposite gate 284 is returned to ground through a resistor 285a, a similar resistor 286 returning the corresponding side of capacitor 251 to ground.

When valve 240 is fired, either by impact switch 31 or firing stage 250, its continued operation thereafter maintains a charge across capacitor 285 polarized to negative in the direction away from cathode 243 by reason of the conductive path through valve 240 to source 82 and the ground return on the other side through resistors 281 and 282. Upon subsequent depression of push button 102 and the thereafter delayed development of the spike pulse across resistor 267, the application of that pulse to gate 284 momentarily renders valve 278 conductive. Consequently, cathode 280 is connected to the positive potential of battery 82, and, by reason of the ground return from cathode 280 through resistors 281 and 282, the total instantaneous potential on cathode 243 of valve 240 (with respect to ground) becomes twice the battery supply voltage that appears in the other and normal direction between anode 242 and cathode 243. The result of so imposing a net potential across valve 240, in a direction which renders its cathode 243 positive relative to its anode 242, is to deactivate valve 240 so that it once again becomes nonconductive but yet ready again to energize beacon 21 upon the next occurrence of an energy pulse either from impact switch 31 or firing stage 250. At the same time, resistor 282, connected between ground and gate 284 as well as to cathode 280 by way of resistor 281, is assigned a sufficiently high value of resistance that SCR or valve 278 ceases to be conductive immediately upon cessation of the spike pulse, developed across resistor 267, and the discharge of capacitor 285.

It will be observed that the arrangement of power control 30 and remote control 28 is such that continued operation of beacon 21 in response to actuation by impact switch 31 is assured regardless of disruptive effects upon remote control 28 or its interconnecting leads. As noted earlier, each of those leads 107 and 111 are individually ensheathed within respective grounded sleeves 108 and 112. As a result of either or both of leads 107 and 111 may become grounded or entirely disconnected. Similarly, the lead between the push button switches and the battery or the battery itself may become grounded or disconnected. In any of those events, however, there is absolutely no affect upon power control 30. Consequently, the integrity of automatic crash activation of beacon 21 is retained regardless of damage to the remote control system included to enable manual operation by the pilot or other operator.

Where desired, various refinements and modifications are available. For example, an audio monitor of the distress signal may be provided in the pilot compartment. Instead of being semi-permanently installed, beacon 21 may be mounted upon an eject mechanism operable to propel the beacon away from the aircraft in case of a crash into a body of water. In that case, the beacon may be affixed on its underside to a mass of buoyant material and that mass may be formed about the internal batteries to constitute an enclosure therefor.

It will be evident that the many different advantages present in the disclosed downed-aircraft-locator beacon stem from the incorporation of a number of distinct but related features. In addition to other points of merit already mentioned, a successfully operated beacon constructed as above described develops a power output, at full battery voltage, of about 500 milliwatts peak at each of the two assigned frequencies. This has been found to permit the desired omnidirectional range of transmission of up to 200 miles. Since the small batteries required are available at the present time with a shelf-life of perhaps two years with a retention of 90 percent or original capacity, their low-cost replacement at each annual inspection of the aircraft assures continued reliability. Being of very rugged, compact construction, composed of fire-resistant material, such as fiberglass in the formation of substrates 50 and 66, dome 85, compartment 82 and foam 87, and capable of automatic operation within its self-contained assembly, as well as reason of its location in a portion of the aircraft least likely to be subject to injury, the beacon affords a highly maximized degree of enduring operability. The coordination of the two different antenna assemblies and the physical inclusion therewithin of the radio apparatus not only results in a minimum overall space requirement but contributes to the complete self-containment wherein no reliance is placed upon the integrity of cables and leads disposed through the different parts of the aircraft.

In addition to the overall attributes of the beacon, beneficial enhancement of performance is secured by careful attention to and selection of the nature of the different system components. These include the extremely high stability available from the oscillator, the simplified yet effective frequency-doubling and separating circuitry, the highly efficient mode of coupling between the final amplifiers and their respective antennas, the modulation techniques employed and the safeguarding functions of the power control. At the same time, it is recognized that these individual features find advantageous utility in other but related environments.

While a particular embodiment of the present invention has been shown and described, it is apparent that changes and modifications may be made therein without departing from the invention in its broader aspects. The aim of the appended claims, therefore, is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

Claims

I claim:

1. In a radio apparatus translating signals on at least a pair of mutually-different frequencies, an antenna assembly comprising:

a conductive sheet defining a first ground plane;

a substrate of insulating material spaced from and parallel to said conductive sheet;

a first conductive loop affixed to said substrate and together with said conductive sheet constituting an antenna responsive at one of said frequencies;

a conductive layer, defining a second ground plane, affixed to said substrate and spaced inside said first loop;

and a second conductive loop spaced from and parallel to said conductive layer and together therewith constituting another antenna responsive at the other of said frequencies.

2. Apparatus as defined in claim 1 in which said second conductive loop is disposed on the side of said substrate opposite said conductive sheet.

3. Apparatus as defined in claim 1 which further includes a second substrate of insulating material to which said second loop is affixed.

4. Apparatus as defined in claim 1 in which at least one of said conductive loops includes a gap with said one loop at one side of said gap being conductively connected to its associated ground plane, a capacitor coupled between said associated ground plane and a point on said one loop intermediate its ends for resonating said one loop at its respective frequency of response, and means tapped on said one loop at a point spaced from said gap for coupling the respective one said signals between said radio apparatus and said one loop.

5. Apparatus as defined in claim 4 in which said radio apparatus is disposed between said conductive sheet and said substrate and said coupling means extends through said substrate and said conductive layer from said second loop to said radio apparatus.

6. Apparatus as defined in claim 1 in which said radio apparatus is disposed between said conductive sheet and said substrate.

7. Apparatus as defined in claim 6 in which said radio apparatus includes a plurality of individual pockets of shielding material respectively housing different stages of said apparatus, said substrate closely over-lies the openings in said pockets and conductive material affixed to said substrate shieldingly closes said pockets.

8. Apparatus as defined in claim 6 in which an insulative dome affixed to said conductive sheet forms a housing overlying said loops and said substrate, and the interior space within said housing and said radio apparatus are filled with a deliquefied foam of insulating and cushioning material.

9. Apparatus as defined in claim 6 which further includes a compartments, in which batteries are disposed for energizing said radio apparatus, affixed to the side of said conductive sheet opposite said substrate.

10. Apparatus as defined in claim 9 which further includes a plate disposed between and of a material stiffer than said compartment and said conductive sheet.

11. Apparatus as defined in claim 6 in which said radio apparatus has a plurality of pockets respectively housing different stages of said apparatus including separate pockets for selective stages individually responsive at respective ones of said frequencies, which includes means for coupling said selective stages individually to corresponding taps on respective ones of said first and second loops, and in which the pockets housing said selective stages are individually disposed in at least approximate alignment with respective ones of said taps.

12. Apparatus as defined in claim 11 in which the one of said selective stages coupled to said first loop is disposed adjacent to an exterior wall housing said radio apparatus.

13. Apparatus as defined in claim 6 in which said radio apparatus is contained within an enclosure and the side walls of said enclosure, said conductive sheet, said substrate and said loops all are nested together with mutual concentricity.

14. In a radio apparatus translating signals on at least a pair of mutually-different frequencies, an antenna assembly comprising:

a conductive sheet defining a first ground plane;

a first conductive loop spaced from and parallel to said sheet and together therewith constituting an antenna responsive at one of said frequencies;

a conductive layer, defining a second ground plane, disposed in the plane of and spaced inside said first loop

and a second conductive loop spaced from and parallel to said conductive layer and together therewith constituting another antenna responsive at the other of said frequencies.