GB2122456A - Method of an apparatus for duplex communications - Google Patents

Abstract

Method and apparatus for a duplex communication system wherein a local station transmits a modulated signal at a nominal, fixed frequency (f1) to a remote station which transmits, simultaneously, to the local station, a modulated signal at a nominal, fixed frequency (f2), which frequency (f2) is displaced from frequency (f1), by a frequency (f2-f1) being the intermediate frequency of the receivers at both the first and second stations. A portion of the transmitter output at each station is fed to the receiver of that station to act as a local oscillator for the i.f. stage.

Description

SPECIFICATION Method of and apparatus for duplex communications This invention relates generally to duplex com munications systems and more particularly to a method'of implementing duplex communications within a single band of radio frequencies.

A duplexcommunications system is defined as a communications system in which the functions of transmission and reception can be carried on simultaneously. Principal among the known systems are the radiotelephone and the cordless telephone.

Therearetwo basictypes of radio telephone systems, thetmobiletelephone service (MTS) and the improved mobile telephone system (IMTS). In each system, the intended radio channel of communication must be selected priorto instigation of that communication. In MTS the user must manually switch between radio channels until a clear (unused - at - the moment) channel is found.The lMTS provides an automatic scannersystem which automatically searches the available radio channels until a clear one is found and then locks onto it Both systems, however, still suffer from the inconvenience ofthe push -to -talk mode of operation since the transmit and receive frequencies are usually separated by a fixed frequency range so as to provide two - way communications.

Users must press a microphone switch to transmit and then release itto receive. This mode of operation can be confusing since it usually means that each com municating userwill have to say "over" or some such code word when that communicator is ready to listen tothe other party speak. One callercannot interrupt the other in such a call because, bythe very nature of thesystem,thespeakeris not listening when he is speaking.

While the radio telephone is an independentsystem, the cordless telephone, by contrast, is simply a means of patching into an existing conventional, hard-wired telephonesystem. That is, radiotransmis- sions are used ta send a caller's audio signal to the remote unit from the base unit and also to send the audio signal from the operator ofthe remote unitto the basestatian and then to the central telephone system. Thus atelephone conversation may be carried on which is indistinguishable from the same conversation being conduted over a conventional hard-wired system. Voice transmission and reception maythus be accomplished concurrently.This is in conlrastto the radio telephone ortransceiver in which transmission and reception may not be accomplished concurrently.

The implementations ofthecordlesstelephone range from answer-only cordless handset to cordless devices which perform all the functions of a hardwired telephone instrument. True cordless telephones work in conjunction with base stations connected to standard telephone lines. Maximum operating ranges mayvary from about 50 feetto more than 500 feet.

Most operate on the same general pinciples, wherein both base units and remote handsets each contain both transmitters and receivers, and whereby the base unit patches the telephone line into a transmitter which transmits to a receiver contained in the remote handset. The operator with the remote handset transmits back to a receiver in the base unit which then routes this received audio information into the hardwired telephone line.

The cordless telephone system generally consists of two units comprising the base unit or station attached directly to the telephone line through a mating connector, and a wireless remote unit. In presently conventional models, the base unittransmitsaudio information from the telephone line to which it is directly attached at a low frequency of about 1.7 MHz.

The base unit also contains a receiver which is operated atabout49.8 MHz in the radio spectrum. The base unit is generally equipped with an antenna that is used for receiving transmissions from the portable unit atthis radio frequency. It is necessary to have the base unit connected directly into the AC line in the building sothatit may have operating current. There is, however, another very important use made by the base unit ofthe wiring in the building in which it is located. The output of the 1.7 MHz base unit transmitting system is generally split through a transformer on each side ofthe AC line and is connected thereto through a blocking capacitor.The building electrical system then becomes a very complicated antenna system fortransmission of the 1.7MHz transmission frequency of the base unit. The remote unit has its transmission near49.8 MHz, as has been noted, and usually contains a telescoping antenna within its case fortransmitting purposes only. For reception of the base signals, the remote unit usually has a loop stick antenna located within its case. The ferrite loop antenna consists of a ferrite core wound with many turns of very small diameter wire which forms a resonance circuit at about 1.7MHz and serves as the receiving antenna for transmissions from the base station.The reason for using a frequency near 49 MHz for transmission on the remote unit and a frequency of about 1.7 MHzfortransmission from the base unit is that if the frequency of the transmitter is very close to that ofthe receiver in a single unit, then the close proximity of the two antennas involved will cause much ofthe transmitted signal to be fed into the front end ofthe receiver located nearby and may cause overload and even damage to circuit components.

Thus the separation of about 47 to 50 MHz is adequate to prevent such interference. In this presently conventional system, the base unit has a retractablewhip antenna that is used onlyforthe purpose of receiving transmissions from the remote unit and uses the building wiring as antenna for its own transmissions; whilethe remote unit has a retractable whip antenna used onlyfortransmitting at its higher transmission frequency, information to the base station and contains also a ferrite loop antenna tuned to the approximately 1.7 MHz transmission frequencyfrom the base station. The frequency separation here is necessary, as has been observed, in order to prevent a transmitted signal from being fed into the fronted of a closely located receiver. Ferrite loop antennas, as employed in presently conventional cordless telephones, in addition to being inefficient, also suffer from extreme directivity.

Itis interesting to notethatthe approximately 50 MHztransmission utilized by the remote unit will travel a greater distance to be received by the base unitthan the 1.7 MHztransmission will travel from the base unit. There are several factors involved, one being thatthe lowerfrequency has more of a tendency to be absorbed by wiring systems, radio receivers in the home, and many other metallic objects and electronic devices than does the higher frequency transmission. It is also true that the ferrite loop stick antenna is much less efficient than a suitably tuned whip antenna would be.It is also true thattransmission distance from the base unitwill be determined to a large extent by the size and configuration ofthe electrical system within the building; the amount of steel and other metals used in the building construction; and whether or notthe wiring in the building has been run through some sort of metal conduit. If conduit has been used to enclose the wiring, itwill be nearly impossible to get any transmission range from the base unit because the antenna (the building wiring system) is now completely encased in a grounded metal shell.

Many different devices have been employed in the attemptsto solve the problems presented. Most have either presented new problems or only partially solved the problems presented, or both. Most of the devices tried have thus met special needs as presented by specific problems and have therefore served narrow purposes. These prior art devices, among other disadvantages, have either caused unacceptable attenuation of signals, or unacceptable distortion of these same signals and thus have been unreliable and unpredictable in operation under continued use, and have been very expensive and complicated to manufacture.

Itwould thus be a great advantage to the artto provide a duplex communication system that is implemented within a single radio frequency band.

Another great advantage would be to provide concurrent transmission and reception without the need to cut offtransmission from either location in orderto receive, orto cut off reception from either location in ordertotransmit.

It is therefore an object of the present invention to provide a duplex communication system that is implemented within a single radio frequency band.

The present invention is a method of achieving duplex radio communications between at least first and second radio stations in a communications system in which each station has a transmitter circuit, a receiver circuit, and an input-output circuit, operating each station in boththetransmit mode andthe receive mode concurrently on different but closely spaced carrierfrequencies, which includes the steps of: generating in the transmitter circuit of said first station a transmitter output signal and applying it to the input-output circuitthereof so as to transmit radio signals from said first radio station at a first carrier frequency; generating in the transmitter circuit of said second station a transmitter output signal and ap plying itto the input-output circuit thereof so as to transmit radio signals from said second radio station at a second carrierfrequency differing from said first carrierfrequency by an intermediate frequency; receiving said radio signals from said first radio station at said second radio station; receiving said radio signals from said second radio station at said first radio station; intermodulating at said first radio station a predetermined fraction of said transmitter outputsignal generated at said first radio station with the radio signals received atsaidfirst radio station from said second radio station so as to produce a first signal at said intermediate frequency; intermodu lating at said second radio station a predetermined fraction of said transmitter output signal generated at said second radio station with the radio signals received at said second radio station from said first radio station so asto produce a second signal atsaid intermediate frequency; and then demodulating and utilizing said first and second signals at said intermediate frequency, at the respective stations.

The present invention is also a communications station for use in a duplex communications system, said station being characterized bytheabilityto transmit and to receive concurrently on differentbut closely spaced carrierfrequencies, said station comprising, in combination: a transmitter circuit; a receiver circuit including a demodulator; an inputoutput circuit; and an intermodulatorcircuit coupling said transmitter circuit, said receiver circuit and said input-output circuit; said intermodulator circuit comprising: (a) means for passing a transmitter output signal from said transmitter circuitto said inputoutput circuit; (b) means for generating an image signal which is nearly equal in magnitude and substantially opposite in phase to said transmitter putput signal; (c) means for summing said transmitter output signal and said image signal so as to produce a greatly weakened version of said output signal which in effect isa local oscillatorsignal; (d) means for applying said local oscillator signal to said demodulator; and (e) means for passing a received signal from said input-outputcircuitto said demodulator; the difference between the transmitted and received carrierfrequencies providing an intermediatefre quencyto which said demodulator is tuned, and said effective local oscillator signal having an energy level which is comparable to that of the received signal whereby efficient demodulation is achieved.

According to the present invention, a duplex communications system is provided in which full duplex operation on closely spaced frequencies is achieved withoutthe use offilters. Additionally, a portion of transmitter energy may be utilized in the implementation ofafirst local oscillator at each station location so asto develop an intermediate frequency.

According to embodiments of the present invention, at each ofthe stations a portion ofthe local transmittersignal, comparable in magnitudeto the signal received from the remote transmitter and differing infrequencyfrom that remote transmitted signal bythe intermediate frequency, is applied to the local converter circuit along with the received signal from the remote transmitter. Thus, an intermediate frequency signal, modulated with the intelligence transmitted from the remote station, is derived. This signal is then processed so as to recover the transmit- ted information.

In keeping with this concept, according to the illustrated embodiments ofthe present invention, at each ofthe stations, there is generated concurrently with the transmitter output signal, a cancellation or image signal which is nearly equal in magnitude and substantially opposite in phase. The transmitter output signal and the cancellation or image signal are applied concurrently to the associated receiver unit so as to thereby supply a greatly weakened version of the transmitter output signal to the receiver circuit, thus effectively providing a local oscillator signal. Thus in the receiver circuit, the received incoming signal and the weakened transmitter output signal are of comparable magnitudes, thereby making it possible to produce a heterodyne action for recovering the intelligence, an audio signal, from the incoming signal.

Embodiments ofthe present invention will now be described, by way of example, with reference to the accompanying drawings, wherein like reference characters referto like partsthroughoutand in which: Fig. lisa high level block diagram of a conventional transmitter; Fig. 2 is a high level block diagram of a conventional receiver; Fig. 3 is a high level block diagram of conventional cordless telephone base and remote units; Fig. 4 is an idealized radiation pattern from a dipole antenna; Fig. 5 is a block diagram of a duplex communication system according to the present invention; Fig. 6 is a schematic diagram of a simple transmitter circuit; Fig. 7 is a schematic diagram of a simple receiver front-end circuit;; Fig. 8 is a schematic diagram of a part of a simple transmitter circuit showing two secondarytransfor- mercoupling windings helpful in developing an important embodiment of the present invention; Fig. 9 is a schematic diagram of the transformer circuit of Fig. 8, showing both secondarytransformer windings connected to the antenna while leaving one winding undeterminated; Fig. 10 is a schematic diagram of combined simple transmitter circuit and receiver front-end circuits, both coupled to the antenna according to a first embodiment ofthe present invention; Fig. 11 is a schematic diagram showing implementation of a second embodiment ofthe invention; Fig. 12 is a schematic diagram showing implementation of a third embodiment ofthe invention;; Fig. 13 is a block diagram of one station in accordance with the invention, helpful in further explaining and amplifying the novel advantages of the invention; Fig. 14 is a high level block diagram of one station in the duplex communication system ofthe invention; Fig. 15is another high level blockdiagram of one station emphasizing the first embodiment ofthe duplex communication system of the invention; Fig. 16 is a general conceptual configuration of an implementation ofthe invention; and Fig. 17 is a general conceptual configuration of an application of the invention to cordless telephones.

Although specific embodiments of the invention will now be described with reference to the drawings, it should be understood that such embodiments are by way of example only and merely illustrative of but a small number ofthe many possible specific embodiments which can represent applications of the principles of the invention. Various changes and modifications, obvious two one skilled in the art to which the invention pertains are deemed to be within the spirit, scope and contemplation ofthe invention as further defined in the appended claims.

Referring to Fig. 1 with greater particularity, the general operation of a conventional transmitter may be examined. A radio frequency oscillator 50 provides its input into a modulator and power amplifier 52. At the same time an audio inputfunction denoted bythe numeral 53 is furnished to an audio amplifier system 54 and also applied to modulator and power amplifier 52. The combined output of oscillator 50 and audio amplifier 54 developed in modulator and power amplifier 52 is then furnished to transmitting antenna 10 to be radiated into space.

In Fig. 2, receiving antenna 10 intercepts some of the radiated signal from a conventional transmitter as described with respect to Fig. 1. This very small signal is applied to the front-end circuits of the conventional receiver shown in Fig. 2 as radio frequency amplifier 55. Radio frequency amplier 55 furnishes its output signal to mixer 56 which also receives input from local oscillator so asto develop an intermediatefrequen- cy. This intermediate frequency signal is furnished to intermediate frequency amplifier 58 for amplification.

Signal detection for recovery of the received intelligence is accomplished in detector59which receives the amplified intermediate frequency signal from the intermediate frequency amplifier 58, and the detected signal is furnished to audio frequency amplifier 60.

This audio frequency signal is now amplified and fed to output transducer 61 shown here as a loudspeaker.

In the foregoing, the functions oftransrnission of audio information and reception of audio information have been carried out independently of each other.

Various problems arise however, when an attempt is made to combine these two functions in close physical proximity to each other, and also when an attempt is made to use the same antenna both for transmission and for reception.

With reference to Fig. 3, there is illustrated a duplex communications system in the specific form of a conventional cordless telephone system, however, having many disadvantages that have been successfully attacked and solved by the present invention.

All true cordless telephones normally work in conjunction with a standard telephone system. The cordless telephone, as an example of a duplex communications system, must not be confused with the radio telephone, although basic principles are the same. Present cordless telephones merely provide apparatus, method and meansfortapping into an existing hard-wired telephone system. As illustrated, a base unit81 comprises a base receiver 72 and a base transmitter 73, both connected to the building telephone system by hard-wired telephone connecting wires 94 into receptacle 71. This base unit is also connected to the building electric power system denoted by the numeral 74 by hard-wire electrical connecting wires 95. The wires 95, by means of suitable blocking capacitors, not specifically shown, are also employed to connect the base unittransmitter to the building's wiring system which will then act as antenna forthe transmitter's transmission frequency, normally about 1.7MHz. Base station 81 also has a whip antenna 75, however, this antenna has no purpose otherthan reception of signal transmissions from remote portable unit 80 at its normal transmits sion frequency of about 49.9 MHz.

Remote portable unit 80 comprises a transmitter 77 and a receiver 78. Because of the frequency difference between transmission frequencies in the base unit and the portable unit, the remote portable unit also has two antennas, remote receiving antenna 79 for receiving the 1.7MHz transmission from the base unit 81 and remote transmitting antenna 76fortransmitting the 49.9MHz remote transmission signal to the base station. It is to be noted that base station 81 and remote portable unit 80 have antennas 75 and 76, respectively, that are conventional whip antennas.

The conventional whip antenna exhibits an omnidirectional radiation pattern. Reference to Fig. 4 shows the radiation pattern 83 of dipole antenna 82.

Herethetwo elements forming the dipole are shown in vertical orientation so that the radiation pattern obtained is doughnut shaped, that is, a torus. In general, awhip antenna comprises the top dipole in physical form and a bottom dipole provided as a reflected image from a ground plane. If we slice horizontally through the centre of the torus 83 as shown at dotted line 83a, we will see the ideal radiation pattern of a whip antenna thus illustrating its omnidirectivity. One of the problems attacked and solved bythe present invention is the problem ofthe antennas. In the present invention instead of using separate antennas fortransmission and reception, the same whip antenna is utilizedforthe performance of both functions simultaneously.

In orderto setforth more clearly howthe duplex communications system ofthe present invention differs from otherfull duplex communications systems in operating on closely spaced frequencies, using a portion ofthe local transmitter signal energy as a first local oscillator, and in not using filters, a statement of problems involved and conventional attempted solutions may be helpful.

The performance of a full duplex communications system depends in large measure on the ability to attenuate the effect upon the local receiver, ofthe local transmitter power delivered to the antenna. In some existing systems, a switching function is performed to preventthetransmittersignal energy from entering the front end of the local receiver; thus, during the time of effectivity of this switching function, the local receiver is inoperable and thus no true duplex communications system is achieved.

When operating frequencies of transmitter and receiver are widely different, for example, 1.7 MHz and 49.9 MHzfor a frequency separation of 48.2 MHz, the problems involved are fairly simple and straightfor- ward. However, when the involved frequencies are within one ortwo per cent or less of each other, the problems become much more complex, particularly where the same antenna isto be used for both transmitting and receiving functions. The primary problem remains the high transmitter signal energy present at the front end of the receiver.This high transmitter energy present atthe inputto the local receiver must be reduced to an acceptable level, that is, a level which will not cause desensitization or overload damage in the radio frequency amplifier, converter or intermediatefrequency strip. In most receivers, the acceptable level is at most a very few millivolts, and hence a small fraction ofthetransmitted signal.

The standard solution to the problem of transmitter interference in the receiver has been to use filters in the receiver front end to allow passage ofthe desired received signal, while at the same time reducing the transmitter power, and in the low power stages ofthe receiver,to reducethe associated base band noise which would also reduce the receiver sensitivity.

In the duplex radio telephone communications system ofthe present invention as shown in Fig. 5, station 62 may be designated the local station in the system and 62' the remote station. Station 62 has an audio signal inputto audio frequency amplifier 63. The output of audio amplifier 63 is coupled to radio frequency amplifier 65. Radio frequency oscillator/ modulator 64 additionallyfurnishes input signal to radio frequency amplifier 65. The output of radio frequency amplifier 65 raises the signal level of the modulated radiofrequencysignal to that level desired before application to antenna 10. The output of radio frequencyamplifier65 is also coupled to intermodula tor 66 which, in turn, is also coupled to antenna 10.

Antnna 10 acts in both the transmit and receive modes. Intermodulator66, in addition to furnishing the signal from radio frequency amplifier 65 to antenna 10, also provides an intermediate frequency signal at its internal output 220 as a result of intermodulation between the signal from radio frequency amplifier 65 and the signals received at antenna 10 transmitted from remote station 62'. This intermediate frequency signal is fed to intermediate frequency amplifier 67 whose output signal is coupled to demodulator 68. The output of demodulator 68, the recovered intelligence broadcastfrom station 62', is coupled to audio frequency amplifier 69 for amplification priorto being suppled to a transducer, for example a loudspeaker.

In station 62', the remote station, audio signals may be received at the inputterminal to audio frequency amplifier 63'. The amplified signal from audio frequency amplifier 63' is coupled to radio frequency amplifier 65'. A radio frequency carrier signal is furnished by radio frequency oscillator/modulator 64' at a frequency which differs from the carrier frequency of radio frequency oscillator 64 buy a predetermined amount, the intermediate frequency. The output signal of radio frequency amplifier 65', as before, raises the level ofthe modulated radio frequency signal to that level desired at antenna 10' for broadcast.

At this point it should be evident that the unprimed and primed numerals refer to like elements in local and remote stations where, by our convention, the local station bears the unprimed numerals.

Signals received at antenna 10 of local station 62 from remote station 62' are intermodulated with a controlled portion ofthe signal from radio frequency amplifier65to produce, at outputterminal 220 of intermodulator66, an intermediate frequency signal which is amplified in intermediate frequency amplifier 67 and then passed on to demodulator 68 for recovery of the original intelligence transmitted from station 62'.

Contemplated operation of this system of the invention is explained as follows. Avoice signal is applied to audio frequency amplifier 63 which, after amplification modulates a radio frequency carrier signal. After amplification in radio frequency amplifier 65, the amplified, modulated signal is then applied to antenna 10through intermodulator 66 and a mod ulated signal is broadcast at a nominal carrier frequency of, for example, 49.830 M Hz.

In station 62', similarfunctions are being performed exceptthatthe source ofthe audio input to audio frequency amplifier 63' may be the voice of the operatorofthat remote station. The radio frequency oscillator carrier signal in the remote station has a frequency of, forexample, 49.860 MHzsothatthe signal transmitted from antenna 10' is a radio frequencysignal, nominally at 49.860MHz, but modulated bythe audio signal derived from the remote operator's voice.

At station 62, the incoming signal from station 62' at 49.860 MHz is mixed in with a small fraction ofthe 49.830 MHz signal from radio frequency amplifier 65, thus producing an intermodulation product at 30 KHz.

This is the intermediate frequency signal which appears at terminal 220 of intermodulator66. In termediate frequency amplifier 67 is tuned to 30 KHz and appropriately amplifies the modulated signal received from terminal 220. The intermediate frequency signal thus derived may be demodulated in dernodulator.68 and the resulting audio frequency signal amplified in audio frequency amplifier 69 from which it is applied to an audio transducer, for example, a loudspeaker.

At station 62', which in this case we have designated the remote station, the incoming signal broadcast from station 62 at 49.830MHz is is received at antenna 10' and is fed into intermodulator 66', where it is mixed with a fraction ofthe signal from radio frequencyamplifier65' which signal is noinally at 49.860 MHz and the intermodulation product of 30 KHz appears atterminal 220' of intermodulator 66' from whence it passes to intermediate frequency amplifier 67'. Again, the intermediate frequency signal appear- ing atterminal 220' is a 30 KHz signal modulated with the audio signal derived from the station 62 operator's voice.That modulated intermediate frequency signal is amplified by a 30KHz amplifier in an intermediate frequency amplifier 67' and is fed to demodulator 68 from which an audio signal is extracted and applied to audio frequency amplifier 69' and thence to a transducer as before. Because system intermediate frequency amplifiers operate on the difference frequency, it is apparent that a second remote station, such as station 70, could use station 62' if the second station had its nominal frequency set at49.890 MHz.

It is instructive in developing the presently preferred embodiment ofthe invention, to examine in Figs. 6 and 7 conventional transmitter and receiver input circuits, respectively. The conventional transmitter output transformer 14 has a tuned primary 13 which is connected to supply voltage 15 and to the collector 18 of the output device, transistor 17. The secondary 12, usually a fewturns, is then connected between ground and the antenna circuit as shown in Fig. 6.

Antenna 10 is connected through antenna tuning circuit 11 to transformer secondary 12. The transfor mertuning capacitor is represented bythe numeral 16. Signal source21 furnishesinputsignaltothebase 20 transistor 17 whose emitter 19 is shown as grounded. Transmitter primary winding 13 and tuning capacitor 14 are connected in parallel between the supply voltage terminal 15 and collector 18 of transistor 17. This circuit allows efficient transfer of power to the antenna. If general circuitconfigura- tion is used as a receiver input circuit, it will transfer energy from the antenna to the radio frequency amplifier 34 as shown in Fig. 7.Received signal energy incidentatantenna 10 will betransferred by antenna tuning circuit 11 into transformer 24 by means of primary 22. Tuned secondary comprising the coil 23 and capacitor 26 transfers signal energy into the base 30 of transistor 27. The emitter 29 of transistor 27 is shown as grounded. The collector 28 oftransistor 27 transfers signal energy into tuned circuit 31 comprising coil 32 and capacitor 33. Tuned circuit 31 is connected to supply voltage by means ofterminal 25.

Referring now to Fig. 8, we knowfrom elementary transformertheorythat if the outputtransformer 14 has two identical secondarywindings, 12 and 12a, the voltages induced across the two secondaries will be very nearly equal. If we now connect one of the two identical secondarywindings 12 between the antenna circuit and ground, and connect the remaining secondarywinding 12a, in-phase to the antenna circuit leaving one end unterminated, we will have the configuration of Fig. 9. And importantly, we have the standard transmitter circuit of Fig. 6 with no change in performance. If we now measurethetransmitter energy present atthe unterminated end of the remaining secondary winding 1 2a and ground, we find that it is nearly zero.It will not be exactly zero because of smal I differences between the two secondaries and between their degrees of coupling to the primary winding. We have thus accomplished a direct connection to the antenna circu it with virtually no transmitter energy present. By terminating the unter- minated end of secondary winding 1 2ato a tank circuit tuned to the receiver frequency, it is possible to optimize the transmitter signal energy level necessary for a converter stage as is shown in Fig. 10. The bipolar junction transistor arrangement shown, however, is not intended to limit or in anyway detract from the generality of the invention as taught and described.

In Fig. 10, thetransrnitter signal generated in signal generator 21 is furnished to the base 20 oftransistor40 whoseemitter 19 is shown grounded. Collector 18 provides its signal energy to transmitter output primary 13 tuned by capacitor 1 6,to transmitter transformer 14.Transformer output secondary 12 then furnishes this transmitter power to antenna 10 by way of antenna tuning circuit 11. Secondarywinding 1 2a develops substantiallythe same signal as winding 12, but since the two windings are connected in back to - back relationship, there is only very low transmit tersignalenergypresentatthebottom ofthewinding 12a and thus very lowtransmittersignal energy will be transferred to tank cicuit 37 of converter 84 by coupling coil 36so asto produce a greatlyweakened version of said transmitter output signal which in effect is a local oscillatorsignal.

When a signal is received by antenna 10, it will proceed through winding 1 2a and be linked by coupling coil 36to tankcircuit37 along with the very low transmitter signal energy already present. As conventional, converter 84 comprises tank circuit 37, itself comprising tuning capacitor 39 with coil 38.

Output signal from tank circuit37 isfurnished to the base 44 of a second transistor 41 whose emitter 43 is grounded. The collector 42 oftransistor41 is in turn connected to transformer47, comprising tuned primary45,tuning capacitor49 and secondary46. Supply voltage isfurnished by terminal 48. Converter 84 thus develops an intermediate frequency signal representing the difference frequency between the transmitter frequency and the frequency ofthe received signal, while atthe same time preventing an unacceptable signal level from the transmitter being incident at the receiver front end. Again, the bipolarjunction transistor arrangement shown is not intended as a limitation on the invention.

Fig. 11 illustrates a second embodiment which provides for cancellation ofthetransmittersignal in the input receiver circuit. Thetransmitter signal energy developed in the tank circuit 180 is first directed through an antenna loading coil 181 before being applied to tap 187 ofthe input transformer 182.

Inputtransformer 182 has two winding portions, 183 and 184. Firstwinding portion 183 is connected atone end to tap 187 and at its other end to antenna 10.

Second winding portion 184 is also connected at one end to tap 187 and at its other end to resonant circuit 185. A portion of the transmitter signal energy thus flows in one direction in inputtransformer 182 toward the antenna 10 by means offirstwinding portion 183 while another portion flows in the opposite direction in input transformer 182 toward resonant circuit 185 by means of second winding portion 184. A near null signal is thus induced in the secondary 186 of input transformer 182 as a result of the incidence ofthe transmitter signal energy, however, a small portion of transmittersignal energyofan order of magnitude suitable for conversion is realized.

When an incoming signal is incident at antenna 10, it travels through input transformer 182. A smaller portion travels through antenna loading coil 181 and the transmitter winding, however, because of resonant circuit 185, the amount of received signal energy in the antenna loading coil 181 path will be significantly less than the amount developed across the receiver input windings 183 and 184. The antenna loading coil is normally placed between the source of the transmitter signal energy and the antenna.In this embodiment ofthe invention, however, the antenna loading coil has been placed between the cancellation windings, 183 and 184 of input transformer 182, and thetransmitterwinding. Such placement has been effected in orderto prevent a shunt capacitance from affecting the near null transmitter signal induced in the seconda ry 186. The difference in currentflowis always common to both windings, since the common source of currentthrough the antenna loading coil is common to both windings.Thus, transmitter signal energy is nearly nullified in the receiver tank circuit 186 and received signal energy is induced into winding 186 ofthe receivertank cirnuit, being the sum ofthesignalsdevelopedthroughwindings 183 and 184 so that signal mixing takes place in the converter field effect transistor 188. The field effect transistor configuation is not intended to limitthe invention to that or any otherspeciflc method of performing signal conversion.

Fig. 12 presentsanotherembodiment of the invention which permitsthe use of a predetermined portion ofthetransmittersignal ate fixed nominal frequency to beat with the incoming signal from a commumicating station to produce the desired intermediate frequency signal. In Fig. 12 a modulated signal at a frequency, again for example, of 49.860 MHzgener- ated as at 111 is applied to terminal 96 for application to bipolartransistor 97 at its base 98. An amplified signal at a nominal frequencyof49.860 MHz appears at collector99 of transistor 97 and is supplied to coil tap 100 on tank coil 102, which is shunted bytuning capacitor 104. Operating voltage for transistor 97 is applied through terminal 101.Terminal 101 is bypassed to ground through by-pass condenser 103. The tank circuit comprising tank coil 102 and tuning condenser 104 has been designed to present a high irnpedanceatthe nominaF frequency of 49.860 MHz.

The tank circuit comprising tank coil 102 and condenser 104 is link-coupled by means of a low-impedance link 105to a second transformer 112 through a first primary 114. Transformer 112 also has a second primary winding 116 which is connected at one end 106 to ground at the other end 107 th roug h an antenna tuning coil 108 to antenna 10. Secondary winding 109 oftransformer 112 is shunted by a condenser 110 to tune itto the nominal frequency of 49.860 MHz. The Q ofthe tank circuit 130 comprising condenser 110 and coil 109 is such that its half-width is about KHz.

Because ofthis band width, tank circuit 130 accommodates both the transmitted signal generated at oscilla tor111 art a nominal frequency of 49.860 MHz and the signal received at antenna 10which has a nominal frequency, again for example, of 49.830 MHz. The impedance looking intotankcircuit 130 is high,for example, 100,000 ohms. Terminal 132 of tank circuit 130 is coupled to gate 134 of field effect transistor 136, which hasa high input impedance atgate 134.

Terminal 131 oftank circuit 130 is coupled to terminal 118 oftankcircuit 120 which comprises a tank coil 122 shunted by a tuning condenser 123, the combination being tuned to a nominal 49.860 MHz. Again,tank circuit 120, liketank circuit 130, has a high impedance at the nominalfrequency of oscillator I11.Tank coil 122formsthe secondarywinding of atransformer 125, the primary 124 of which is in the circuitfrom emitter 113 of bipolartransistor97to ground. The emitter current at the frequency of oscillator 111 is 180 degrees out of phase with the collector current at the frequency of oscillator 111. The relationship of the windings oftankcircuit 120 and tank circuit 130 is such thatthesignal atthefrequencyof oscillator 111 appearing at terminal 118 is 180 degrees out of phase with the signal atthe same frequency appearing at terminal 131 oftank circuit 130. Thus, between gate 134 connected to terminal 132 of tank circuit 130, and ground, the signal approaches zero through the cancellation effectjust described. To enhance the effective level ofthe signal received by antenna 10 at the nominal frequency of 49.830 MHz, a crystal or magnetostrictive filter 15 may be coupled between terminal 131 oftank circuit 130 and ground.Device 115 may have a pass-band of only 3 KHz and it is tuned to the frequency ofthe signal received in antenna 10 namely, for example, 49.830. Thus the signal received from antenna 10 is enhanced as it is applied to gate 134 of field effecttransistor mixer 136, whereas a signal from oscillator 111 is reduced to level of the order of 1 0-3 volts by reason ofthe cancellation effects between tank circuits 130 and 120. This reduction of the signal from oscillator 111 before it is utilized to heterodyne with the signal received in mixer 136 is necessary to get an acceptable mixer conversion factor. It is well established that if the ratio of two signals being mixed is too great, the efficiency in the mixer is greatly reduced. Therefore, the present cancellation method is necessary and desirableto achieve efficient mixing and reasonable conversion gain.The outputfrom drain 135 offield effect transistor 136 is coupled through a carrierfrequency trap 126 to a tank circuit 127 tuned to the desired intermediate frequency, in this case, KHz, by a pair of capacitors 128 and 129. The reactance of capacitor 128 is lowerthan that of capacitor 129 atthe operating frequency. Output to the intermediate frequency amplifying transistor is taken from terminal 119.

Terminal 117 is by-passed to ground for radio frequency purposes by by-pass condenser 121.

Source 133 of field effect transistor 136 is coupled to ground through a biasing network 137.

Directing particular attention now to a fourth embodiment of the invention shown in Fig. 13, a modulated signal may be received at antenna 10, for example, at a nominal frequency of 49.860 MHz and applied b means of a lead 138 to antenna coupler 139.

Antenna coupler 139 additionally receives signal input of a modulated signal at a nominal frequency of, for example, 49.830 MHz by means of a signal lead 140 from transmitter power amplifier 141. Amplifier 141 additionallyfurnishes its signal on lead 142 to phase-shifter circuit 143. The resulting signal, shifted in phase by 180 degrees isfurnished by signal lead 144 to capacitor 145. This shift in phase emphasized by the legend in block 143 of Fig. 13. At point 158 of Fig. 13, both the in-phase transmitter signal, in our example at nominal frequency of 49.830 MHz and the received signal, in our example at nominal frequency 49.860 MHz are present and are thus applied by means of signal lead 159 to capacitor 146.At the same time, the transmittersignal inourexample,alsoata nominal frequency 49.830 MHz but shifted in phase by 180 degrees, is applied by means of signal lead 144to capacitor 145. Obviously the two signals in addition to being 180 degrees out - of- phase, also are of somewhat different magnitudes, thus summing point 147, thejunction of capacitors 145 and 146, will see a signal that contains, in addition to the received signal, a local transmitter signal that has been attenuated by a factor dependent on the relative strengths of in-phase and out - of- phase local transmitter signal applied.

Referring to Fig. 14, a high level block diagram shows transmitter receivericonverter 96 providing output to intermediate frequency amplifier 85. The amplified intermediate frequency from intermediate frequency amplifier85, as conventional, is applied to some form of detector86. Detector 86 thus develops an audio frequency signal which is applied to audio frequency amplifier 87 whose output in turn is applied to outputtransducer 88, mostcommonlya louds peaker.

Fig. 15 emphasizes the importance of coupling coil 36 of Fig. 10 in providing tank circuit 37, comprising capacitor 39 and coil 38, with both a received signal from antenna 10 and a portion ofthetransmitter signal from transmitter power amplifier 90. Transmitter amplifier 90 will provide its output signal energy into outpuVinputtank and antenna coupling circuit and transmitter phase cancellation circuit 89. A signal to be transmitted will then be applied to antenna 10 with a small portion being bled off, so to speak, by coupling coil 36.A signal received by antenna 10 will also be transmitted to coupling coil 36 and applied to tank circuit 37 there to be applied to converter 84 where an intermediatefrequency resulting from the frequency of the local transmitter and the received signal will be developed. Again, as before, this intermediate frequency signal is applied to intermediate frequency amplifier 85 and thence to detector 86 and audio frequency amplifier 87 and finally to output transducer 88.

An example of the full duplex communications system is given in Fig. 16wherethetwotransmitter frequencies involved have again been chosen as 49.830MHz and 49.860MHz. The firsttransmitter/ receiver station 91 comprises antenna 10, a transmitter operating at 49.830MHz and a receiver. The second transmitter/receiver station 92 comprises an identical antenna 10, a transmitter operating at 49.860 MHz and a receiver. Transmissions from either station to the other will develop an intermediate frequency of 30 KHz in their respective receivers. In neither case will a local receiver be swamped bythe signal from its local transmitter. This is so because ofthe unique circuit configuration of the invention.

Fig. 17 illustrates a cordless telephone application of the invention. The base transmitter/receiver station 93 will comprise a base receiver and a base transmitter operating at, for example, 49.860 MHz. Receiver and transmitter will be hard-wired by means of hard-wire telephone connecting wires 94to the hard-wire telephone system within the building by means of receptacle 71. This base transrnitter/receiverwill also be wired to the building electrical system 74 by means of hard-wire electrical connecting wires 95. It is to be notedthatthe building electrical system is not to be used as in the prior art as transmission antenna for this basestation.This has been madeunnecessaryby reason of the higher frequency transmitting signal contemplated in the present invention vis-a-vis the lowerfrequency known in the prior art. Remote transmitter/receiver 91 may have the same configuration as one of the receivers illustrated in Fig. 16. What is needed is a different transmitter frequency from the transmitterfrequency of base station 93. Here a transmitterfrequency of49.830 MHz has been illus trated so as to develop an intermediate frequency of 30 KHz between signals. As noted above, an additional remote station employing a transmitting frequency of 49.890 MHz may additionally be implemented if desired.

Thus, there has been described a duplex com munications system thatwill permitthe simultaneous functions of signal transmission and signal reception.

Great improvements in convenience of communication, ease of operation and economy have been provided through the novel advantages ofthe invention.

Claims (25)

1. A method of achieving duplex radio communications between at least first and second radio stations in a communications system in which each station has a transmitter circuit, a receiver circuit, and an input-output circuit, operating each station in both the transmit mode and the receive mode concurrently on different but closely spaced carrierfrequencies, which includes the steps of:: generating in the transmitter circuit of said first station a transmitter output signal and applying itto the input-output circuit thereof so as to transmit radio signals from said first radio station at a first carrier frequency; generating in thetransmitter circuit of said second station a transmitter output signal and applying it to the input-output circuit thereof so as to transmit radio signals from said second radio station at a second carrier frequency differing from said first carrier frequency by an intermediate frequency; receiving said radio signals from said first radio station at said second radio station; receiving said radio signals from said second radio station at said first radio station;; intermodulating at said first radio station a predetermined fraction of said transmitter output signal generated at said first radio station with the radio signals received atsaidfirst radio station from said second radio station so as to produce a first signal at said intermediate frequency; intermodulating at said second ratio station a predetermined fraction of said transmitter output signal generated at said second radio station with the radio signals received at said second radio station from said first radio station so as to produce a second signal at said intermediate frequency; and then demodulating and utilizing said first and second signals at said intermediate frequency, atthe respective stations.

2. A method as claimed in claim 1, wherein the step of intermodulating at said first radio station a predetermined fraction of said transmitter output signal generated at said first radio station with the radio signals received at said first radio station from said second radio station so asto produce a first signal at said intermediatefrequency includes the steps of:: selecting a transmitter transformer having a primarywinding and having first and second substantially identical secondarywindings; applying thetransmitter output signal generated in the transmitter circuit of said first station to said primary winding; connecting one end of said first secondary winding to said input-output circuit and the other end to a common circuit potential point; selecting a coupling coil having two ends; connecting one end of said second secondary winding to said input-outputcircuitso that it is in-phase with said first secondary winding; connecting the other end of said second secondary winding to one end of said coupling coil;; connecting the other end of said coupling coil to said common circuit potential point so that said coupling coil will contain a predetermined fraction of said transmitter outputsignal generated in the trans mitter circuit of said first station, producing in effect a local oscillator signal, and the radio signal received from said second radio station; coupling said coupling coil so as to develop an intermediate frequency.

3. A method as claimed in claim 1, wherein the step of intermodulating at said first radio station a predetermined fraction of said transmitter output signal generated at said first radio station with the radio signals received at said first radio station from said second radio station so as to produce a first signal at said intermediate frequency includes the steps of:: generating afirstradiofrequencycarriersignal of a predetermined frequency; applying said carrier signal to afirsttunedtank circuit; coupling said firsttuned tankcircuitto a linking coil; selecting a second tuned tank circuit; coupling said linking coil both to said second tuned tank circuit and to said input-outputcircuit; generating a second radio frequency carrier signal of said predetermined frequency but of opposite phase to said first carrier signal; and applying said second carrier signal to said second tank circuit so that said second tank circuit will contain a composite signal comprising a predetermined fraction of said transmitter output signal generated in thetransmittercircuitofsaidfirststation, producing in effect a local oscillator signal, and the radio signal received from said second radio station.

4. A method as claimed in claim 1, wherein the step of intermodulating at said first radio station a predetermined fraction of said transmitter output signal generated at said first radio station with the radio signals received at said first radio station from said second radio station so asto produce a first signal at said intermediate frequency including the steps of:: generating a first radio frequencycarriersignal of a predetermined frequency; selecting afirsttuned tank circuit; coupiing said firsttuned tankcircuitto said input- output circuit; applying said firstcarrier signal to said first tuned tank circuit; coupling said firsttuned tank circuit to a first summing capacitor; generatingasecond radiofrequencycarriersignal of said predetermined frequency but of opposite phase to saidfirstcarriersignal; selecting a second tuned tank circuit; applying said second carrier signal to said second tuned tank circuit; coupling said second tuned tank circuit to a second summing capacitor;; coupling said first and second summing capacitors together at a summing point so that said first carrier signal and said second carrier signal, of opposite phase to said first carrier signal, substantially cancel each other so as to sum to a very low signal value producing in effect a local oscillatorsignal; selecting a third tuned tank circuit; and applying said summing point to said third tuned tank circuit so that said third tank circuit will contain a composite signal comprising a predetermined fraction of said transmitter output signal generated in the transmitter circuit ofsaid first station, producing in effect a local oscillator signal, and the radio signal received from said second radio station.

5. A method of operating a communications system having a transmitter circuit, a receiver circuit, and an input-output circuit, in both the transmit mode and the receive mode concurrently on different but closely spaced carrierfrequencies, comprising the steps of: generating in the transmitter circuit a transmitter output signal and applying itto the input-output circuit; concurrently with the generation of said transmitter output signal, generating an image signal which is nearly equal in magnitude and substantially opposite in phase; summing said transmitter output signal and said image signal so as to produce a greatlyweakened version of said output signal which in effect is a local oscillator signal; applying said local oscillator signal to the receiver circuit;; as an incoming signal is received bythe inputoutput circuit, directing a substantial portion of its energy to the receiver circuit so that the local oscillator signal and the incoming signal presented to the receiver circuit are of comparable magnitudes; and then utilizing the receiver circuit to produce a heterodyne action between the incoming and local oscillator signals so as to recoverthe intel I igence carried by the incoming signal.

6. A method as claimed in claim 5, wherein the stepofsumrningsaidtransmitteroutputsignaland said image signal so asto produce a greatly weakened version of said output signal which in effect is a local oscillator signal includes the steps of: selecting a transmitter transformer having a primarywinding and having first and second substantially identical secondarywindings; applying the transmitter output signal generated in the transmitter circuit of said first station to said primary winding; connecting one end of said first secondary winding to said input-output circuit and the other end to a common circuit potential point; selecting a coupling coil having two ends; connecting one end of said second secondary windings to said input-output circuit so that it is in-phase with said first secondary winding;; connecting the other end of said second secondary winding to one end of said coupling coil; and connecting the other end of said coupling coil to said common circuit potential point so that said coupling coil will contain a predetermined fraction of said transmitter output signal generated in the transmitter circuit of said first station, producing in effect a local oscillator signal, and the radio signal received from said second radio station.

7. A method as claimed in claim 5, wherein the step of summing summing said transmitter output signal and said image signal so as to produce a greatly weakened version of said output signal which in effect is a local oscillator signal includes the steps of: generating a first radio frequency carrier signal of a predetermined frequency; applying said carrier signal to a first tuned tank circuit; couplingsaidfirsttunedtankcircuittoa linking coil; selecting a second tuned tank circuit; coupling said linking coil both to said second tuned tank circuit and to said input-output circuit; generating a second radio frequency carrier signal of said predetermined frequency but of opposite phaseto said first carrier signal; and applying said second carrier signal to said second tank circuit so that said second tank circuitwill contain a composite signal comprising a predetermined fraction of said transmitter output signal generated in the transmitter circuit of said first station, producing in effect a local oscillator signal, and the radio signal received from said second radio station.

8. A method as claimed in claim 5, wherein the step of summing said transmitter output signal and said image signal so asto produce a greatlyweakened version of said output signal which in effect is a local oscillator signal includes the steps of: generatiing a first radio frequency carrier signal of a predetermined frequency; selecting a first tuned tank circuit; coupling said first tuned tank circuit to said inputoutput circuit; applying said first carrier signal to said first tuned tank circuit; coupling said firsttuned tankcircuitto a first summing capacitor; generating a second radio frequency carrier signal of said predetermined frequency but of opposite phase to said first carrier signal; selecting a second tuned tank circuit; applying said second carrier signal to said second tuned tank circuit; ; coupling said second tuned tank circuit to a second summing capacitor; coupling said first and second summing capacitors together at a summing point so that said first carrier signal and second carrier signal, of opposite phase to said first carrier signal, substantially cancel each other so asto sum to a very low signal value producing in effect a local oscillatorsignal; selecting a third tuned tank circuit; and applying said summing point to said third tuned tank circuit so that said third tank circuit will contain a composite signal comprising a predetermined fraction of said transmitter output signal generated in the transmitter circuit of said first station, producing in effect a local oscillator signal, and the radio signal received from said second radio station.

9. A communications station for use in a duplex communications system, said station being characte rized bytheabilitytotransmitandto receive concurrently on different but closely spaced carrier frequencies, said station comprising, in combination: a transmitter circuit; a receiver circuit including a demodulator; an input-output circuit; and an intermodulator circuit coupling said transmitter circuit, said receiver circuit, and said input-output circuit; said intermodulator circuit comprising: (a) means for passing a transmitter output signal from said transmitter circuit to said input-output circuit; (b) means for generating an image signal which is nearly equal in magnitude and substantially opposite in phase to said transmitter output signal; (c) meansforsumming said transmitter output signal and said image signal so as to produce a greatly weakened version of said output signal which in effect is a local oscillator signal; (d) meansforapplying said local oscillator signal to said demodulator; and (e) means for passing a received signal from said input-output circuit to said demodulator; the difference between the transmitted and received carrierfrequencies providing an intermediate fre quencytowhich said demodulator is tuned, and said effective local oscillator signal having an energy level which is comparable to that ofthe received signal whereby efficient demodulation is achieved.

10. Acommunications station as claimed in claim 9, wherein said intermodulator circuit includes a transformer having a primary winding and two substantially identical secondarywindings; the transmitter output signal being coupled to said primary winding, one of said secondarywindings being coupled to said input-output circuit, said two secondarywindings being connected in a series loop circuit; said series loop circuit also including means for developing a summation or difference signal from said two secondarywindings and for applying same to said demodulator circuit.

11. A communications station as claimed in claim 10, wherein said last-named means includes a winding which has a small number ofturns compared to secondarywindings.

12. A duplex communication system including: local and remote radio stations, each having a transmitter which generates radio signals at a carrier frequency, a demodulator which receives and processes intermediate frequency signals, an antenna which both receives and transmits radio signals, and an intermodulator coupled between said transmitter, said demodulator and said antenna; said intermodulator in each of said local and remote stations serving to communicate said transmitter radio signals to its associated antenna; said intermodulatorin each of said local and remote radio stations providing to said demodulator in each station the product of a predetermined small fraction of said radio signal energy generated by its associated transmitter and substantially all of the radio signal energy received by said antenna in the respective radio station;; each said demodulator having an intermediate frequencyamplifiercoupledto its respectiveintermodulator and operating at an intermediate frequency common to all of said demodulators in said system; said carrierfrequency of said radiosignalsfrom said transmitter in said local radio station differing from the carrier frequency of said radio signals from said transmitter of said remote radio station by a frequency equal to said intermediate frequency; and utilizing means coupled to respective demodulators for utiliizing the signals therefrom.

13. A system as claimed in claim 12, wherein said local transmitter is coupled to its antenna by means of a transmittertransformer, said transmittertransformercomprising: a transmitter output primary; afirsttransformersecondary; and a second transformer secondary; an antenna tuning circuit coupling said first transformer secondary and said antenna so as to provide transmitter output signal energy to said antenna; and a coupling coil coupling said second transformer secondary to said intermodulatorso asto provide a small portion ofthe local transmitter output signal energy and substantially all of the radio signal energy received by said antenna from said remote station to said intermodulator; whereby an intermediate frequency signal is derived.

14. Asystem as claimed in claim 12 orclaim 13, wherein said remote transmitter is coupled to its antenna by means of a transmittertransformer, said transmitter transformer comprising: a transmitter output primary; a first transformer secondary; a second transformer secondary; an antenna tuning circuit coupling said first trans forrnersecondaryandsaid antenna so asto provide transmitteroutputsignal energy to said antenna; and a coupling coil coupling said second transformer secondaryto said intermodulator so as to provide a small portion of the transmitter output signal energy and substantially all ofthe radio signal energy received by said antenna from said local station to said intermodulator; whereby an intermediate frequency signal is derived.

15. A system as claimed in claim 12, wherein said intermodulator includes a coupler responsiveto signals received in said antenna and also to radio frequency signals generated in said associated transmitterso asto produce a signal comprising a received radio frequency signal combined with a radiofrequency signal from said associated transmitter; and means for combining said signals so as to produce an intermediate frequency.

16. A system as claimed in claim 15,wherein said coupler comprises: a coupling coil connected to said antenna so as to be energized by signal energy received by said antenna; and a tank circuit inductively coupled to said coupling coil so that signal energy applied to said coupling coil will be induced into said tank ci rcuit while atthe same time signal energy received in said coupling coil from said transmitter will also be induced into said tank circuit.

17. Asystem as claimed in claim 12,wherein said intermodulator includes a coupler responsive to signals received in said antenna and also to radio frequency signals generated in said associated transmitting portion so as to produce a composite signal comprising a received radiofrequencysignal and a radio frequency signal from said transmitter portion; generating means for generating a radio frequency signal of like frequency with that from said transmitter portion but of opposite phase; summing means for combining said composite signal and said radio frequency signal of like frequency but of opposite phase so as to produce a resultant composite signal having relative signal component levels such that efficient frequency conversion is assured; and frequency conversion means responsive to said resultant composite signal.

18. Asystem as claimed in claim 17, wherein said coupler comprises: a linking coil connected to said antenna so as to be energized by signal energy received by said antenna; and a tank circuit inductively coupledto said linking coil so that signal energy applied to said tank circuitwill be induced into said linking coil while at the same time signal energy received in said linking coil from said antenna will be induced into said tank circuit.

19. A system as claimed in claim 18, wherein said summing means comprises: a first resonant circuit coupled to said generating means; a first summing capacitor coupled to said resonant circuit for receiving a radio frequency signal therefrom; a second summing capacitorcoupledto said tank circuit for receiving said composite signals therefrom; said first and second summing capacitors being electrically connected to form a summing point; and a second resonant circuit connected to said summing point and responsive to any signals therefrom to provide a composite signal having relative signal component levels such that efficient frequency conversion is assured.

20. A radio communications station comprising: a transmitter that generates radio signal energy at a firstcarrierfrequency; an antenna that both receives and transmits radio signal energy, responsive to said radio signals generated by said transmitter; an intermediatefrequencyarnplifierwhich receives and amplifies modulated intermediate frequency signals derived from radio signals received by said antenna;; an intermodulatorthat couples said antenna both to said transmitter and to said intermediate frequency amplifier so as to provide the product of a predetermined small fraction of said radio signal energy generated by said transmitter, providing in effect, a local oscillator, with substantially all of the radio signal energy received by said antenna at a second carrierfrequencytosaid intermediate frequency amplifier; a demodulator responsive to output signal from said intermediate frequency amplifier to demodulate said output signal; utilizing means coupled to said demodulator for utilizing the signals therefrom.

21. A radio communications station as claimed in claim 20, wherein said intermodulator includes: atransrnittertransforrnerthathasa primary winding and two substantially identical secondarywindings connected to said antenna; a coupling coil connected to one of said secondary windings so that said coupling coil contains a prede terminedfraction of said transmitter radio signal energy along with the radio signal energy received by said antenna at said second carrierfrequency.

22. A radio communications station as claimed in claim 20,wheren said intermodulator includes: a firsttank circuit responsive to said radio signal energy generated in said transmitter; a linking coil linking said firsttank circuit to said antenna; a second tank circuit also linked to said linking coil; a third tank circuit responsiveto the radio signal generated in said transmitter but of opposite phase, connected to said second tank circuit so as to sum said out-of-phase signal components to produce a greatly weakened version of said transmitter signal which in effect is a local oscillatorsignal; whereby said second tank circuit contains said in effect local oscillator along with the radio signal energy received by said antenna at a second carrier frequency.

23. A radio communications station as claimed in claim 20, wherein said intermodulator includes: a first tank circuit responsiveto said radio signal energy generated in said transmitter; a first summing capacitor connected to said first tank circuit; a second tank circuit responsive to the radio signal generated in said transmitter but of opposite phase; a second summing capacitor connected to said second tank circuit and to said first summing capacitor; whereby the junction of said first and second summing capacitors sums said out-of-phase signal componentsto produce a greatlyweakenedversionof said transmitter signal which in effect is a local oscillatorsignal.

24. A method of achieving duplex radio communications, substantially as hereinbefore described with reference to, and as shown in, Figs. 5 to 17 of the accompanying drawings.

25. Acommunicationsstationforuseina duplex communication system substantially as hereinbefore described with reference to Figs. 5 to 17 of the accompanying drawings.