T 1.869.659
`w.‘ anoaams
`Aug. 2.1932.
`METHOD OF MAINTAINING SEGRECY IN THE TRANSMISSION
`OF WIRELESS TELEGRAPHIG MESSAGES
`Filed NOV. 14. 1929
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`1
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`APPLE 1013
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`Patented Aug. 2, 1932
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`1,869,651
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`PATENT ~ OFFICE
`UNITED STATES"
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`WILLEM BROEBTJES, OF AMSTERDAM, NETHERLANDS
`METHOD OF MAINTAINING sncnncr IN rm: rmnsmssron or wmnmsss -rum
`one:-are mnssaens
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`Application filed November 14, 1929, Serial No. 40‘7,218,’and In Germany October 11, 1929.
`but also that one or more of said frequencia
`This invention relates to a method of main-
`may be left entirely out of use for a conside:
`ta1n1n , secrecy In the transmission of‘ wire-
`able time.
`less te egraphlc messages and to transmitting
`The_ working frequencies employed fc
`and receiving apparatus to be used in connec- V
`transmitting the signals may be 0 such per
`tion therewith.
`_
`odicity that they are sent out either direct, c
`The known methods of maintaining secrecy
`as modulations of one or more higher fn
`operate, in most cases, with codes or crypto-
`quencies. There are also intermediate form
`grams and with a periodically modified trans-
`in which the two methods may be employe
`mission frequency, which is received by means
`in combination.
`,
`' 10 of a receiving apparatus, the tuning of which
`If, high-frequency transmission be en
`‘is modified in synchronism, or a so-called
`played, a series of workin waves is used, i
`beam transmitter is employed. . ~ These meth-
`any convenient serial ortir.
`In a metho
`ods, however, do not guarantee the desired
`of this kind, secrecy is ensured by reason c
`security against interception of the messages
`the fact that an unauthorized receiver wk
`because a code can always be deciphered after
`at first, is tuned in to only a single frequent
`reception ‘and, in all circumstances a varying
`length, picks up only disconnected portiox
`transmission wave can be picked up by a.
`of the message.
`If he knows or discovei?
`receiver sensitive to a broad wave band;
`that a plurality of high-frequency waves.
`whilst beam transmitters transmit energy, not
`being used in sending, such waves have first 1
`only inthe desired direction, but also, in a
`be identified and the receiving apparatus wi
`sector which, although narrow, is outside this
`then have to be reconstructed so that the pol
`direction, and furthermore, in a few second-
`tions transmitted on these waves can be cc:
`ary directions.
`,
`~.
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`veyed to the same reproducing device (such :
`These difficulties are overcome by the pres-
`a telephone) where, taken together, they for:
`ent
`invention because the special method
`the complete message.
`‘
`adopted isnon-rigid, that is to say, its secret
`Before thishas been done _however, tl
`- variables can be modified during the trans-
`sending device-—by reason 0
`its non-rigi
`mission, so that even when the
`rinciple of
`character, which largely contributes to t]
`the method is known, great diflicu _ti‘es arisein
`maintenance of secrecy-—has already 101:
`attempts to intercept the messages in prac-
`had the opportunity by selecting another fr
`tice, the number of possible variations being
`quency, of again nullifying the artial su
`practically unlimited.
`The essential feature‘ of the invention re-
`cess attained y the intercepter.
`oreover
`the sender and receiver come to the nece
`sides in the fact that messages are trans-
`sary understanding beforehand, and desig
`mitted by means of a group of frequencies
`their apparatus accordingly, this change 4
`(working frequencies) known to the sender
`frequencies can be effected after a precedir
`and receiver alone, and alternated at will dur-
`warning’ signal, frequently and rapidly, :
`ing transmission of the messages. For ex-
`that the interception of the secret messag
`ample, five frequencies may be used, care be-
`-becomes practically impossible.
`ing taken at the transmitting station that the
`Another possibility consists in that the si;
`signals—i. e. the dots and dashes of the Morse
`nals may be produced by means of low-fr
`or any other a1pl1ab'et—are transmitted by
`quency oscillations superimposed, by modul
`these frequencies alternately. The alterna-
`tion, on a, carrier wave.
`In this case,- to
`tion may take place, both during the transmis-
`it will be very difficult to pick up the massa;
`5 sion' of the signals and between the signals,
`in the absence of knowledge of the group 1
`and be effected either mechanically or in any
`frequencies employed. This difficulty can, <
`convenient manner by hand. The term
`course, be increased, on the part of the sends
`“alternation” implies, not merely. that all" the
`by changing over, wholly or in part, to at
`frequencies of the selected group may be
`other group from time to time. The grot
`switched on in any convenient serial. order,
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`low-frequency oscillations can be superim-
`sed on one or more carrier Waves.
`If now, ,
`ese carrier waves be alternated during the
`Lnsmission (i. e. during or between the sig-
`ls), a combination of the two aforesaid pos-
`yilities can- be obtained, by means of which
`3 secrecy can -be increased still further. It
`self evident that this combination can also
`effected in such a way that the one portion
`the carrier Waves transmits the signals as
`termediate modulations, whereas the other
`rtion is simply interrupted in rhythm with
`e signals or is modified in intensity. The
`ties at which the change in the carrier waves
`affected is now also entirely a matter of con-
`nience.
`»
`,
`In all the solutions described, the tracing
`the key to the secret by any unauthorized
`rson is prevented by making it difiicult for
`11 to ascertain the scope of the group of
`nrking frequencies employed. This can be
`ne, for example by alternating only, three
`aquencies at first out of a. group, of, say
`e,_fre uencies whilst the transmission with
`ur an five frequencies is not resorted to
`til later. Moreover, although the group
`ected may contain, for example, seven fre-
`encies,
`the sender may transmit,
`in all
`ses, with only three frequencies, and may
`ry the trio within the group at conven-
`1ce._ Even the listener-in whohas recog-
`ied the, method is deceived in this case,
`zause he receives the impression that the
`mber of frequencies in the group is three, .
`d that a new group is employed by. the
`ider every time.
`’
`According to the invention, the mainte-
`nce of secrecy is still‘ further increased by
`oducing one or more what may be called
`en-circuit frequencies or spacing frequen-
`s, between the signals, and also varying
`id frequencies at will. Because they re-
`ver for which the message is intended does
`t pick up these open-circuit frequencies, the
`ider is free to choose their periodicity and
`'ial order at will. The unauthorized lis-
`1er-in, however,'who, in addition to the
`.~ g. 5) working «frequencies, detects a se-
`as of other frequencies, will have to as-
`rtain clearly, ‘inthe first place, which- of
`3 frequencies represent the secret message
`d which mustbe disregarded.
`In practice, such difliculties will thereby
`ise that a correct reception of the message
`unauthorized persons may be considered
`possible. '
`'
`The attempts of an unauthorized receiver
`find out the secretimay be nullified at any
`)ment, by the sender passing over to- an-
`1er group of working "frequencies, in the
`inner described above, or by wholly or par-
`ully alternating the working frequencies
`d open-circuit ‘frequencies (transmitting
`3 message by.means of what were origi-
`lly the open-circuit ‘frequencies, and in-
`
`terposing what were originally the working
`frequencies.
`‘
`It is also possible to hamper the solving
`of the secret ‘by interrupting the open-cir-
`cuit frequencies at will thus giving them the
`character of working frequencies. For in-
`stance, a dash can be transformed into one or
`more dots by suitable interruption.
`It will be evident that the frequencies, to-
`which, the term open-circuit frequencies has
`been applied above, by -reason of their being
`transmitted between the actual signals, can
`also be produced concurrently with the sig-
`nals, in which case they also assist in increas-
`ing the secrecy. They do not disturb the re-
`ceiver for whom-they are intended, since he
`does not pick up these open-circuit frequen-
`cies.
`In addition to the working frequen-
`cies, these open-circuit frequencies may also
`be of low or high periodicity. These two
`methods can also be combined, for example
`by alternating open-circuit
`frequencies of
`high’ and low periodicity, or the transmission
`by means of working frequencies of low peri-
`odicity, may be improved by theemployment
`of open-circuit frequencies of high periodic-
`ity or vice versa.
`The secrecy obtained by the employment of
`this invention is so complete that it is'un-
`necessary to employ codes or cryptograms
`in transmitting the letters or numerals of-
`which the message is composed an_d this not
`only facilitates reception by the receiver,
`b_ut also prevents mistakes, which are more
`difficult to clear up, in the case of crypto-
`grams than in that of ordinary script.
`.,
`As a. matter of principle, the variation of
`the working frequencies, can, as already men-
`tioned, be effected at any moment both during
`and between the signals, and the variations
`of the open-circuit frequency can be effected
`at any time.
`The variation of the‘ working frequencies
`during the signals can be visualized by as-
`suming, for. example, that the switch mecha-
`nism,for the working frequbncies (e. g. that
`for several tuned circuits) and the -sending
`key are connected in series.
`In such event,
`the switch mechanism——actuated by any con-
`venient means—varies the working frequen—.
`cies in any convenient serial order, only such.
`frequencies being sent out as occur during the
`closing of the key.
`If ‘open-circuit’ frequen-
`cies be employed in addition, another switch,
`mechanism in series with the key may be al-'
`lotted to said frequencies, so’ that such fre-
`quencies may be transmitted when the cir-
`cuit for the switch mechanism of the working’
`frequencies is broken;
`,
`,
`Such a variation of frequencies can, of
`course be effected not only in principlebut
`also as regards its practical embodiment, in
`various ways," according to the disposition
`of the senders’ control, the connection of the
`tuned circuits, thevposition of the key, &c. _-
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`If the change in the working frequencies
`is not to" take place‘ during the transmission
`"of the signals, the action of the correspondin
`switch mechanism must be independent 0
`the manipulation of the key; that is, it must ‘
`not operate unless the key is o ned.
`_
`In the-‘case of simple sen ing arrange-.
`T ments, ‘a circuit arrangement has been found
`highly advantageous 111 which, every time the
`sending key is depressed,,a certain working
`frequency is switched on which, if‘, desired,
`is replaced by any convenient open-circuit
`frequency on the key being released. This
`solutionrepresents a special modification of
`the methodin which the switchin over of the
`working frequencies is effected between the
`signals. The practical design will be ex-
`i plained later.
`-
`i The transmitting apparatus employed for
`carrying out the new method is "designed, ac-
`cording to the invention, so as to be capable
`of generating a number of frequencies which
`can be alternated at will durin the sending
`of themessage. _This may be e ected by pro-
`viding a tuned circuit of which theconstants
`~(such as self-induction," capacity, or both)
`are influenced by means of a switch; or sev-
`eral circuits may be provided, which can be
`switched on and oif alternatively as a whole.
`Accordingto the fundamental arrangement
`(one or more.
`of the generator‘ connection
`stages), such a circuit arrangement (i. e. a re-
`versible, .or several complete oscillation cir-
`‘ cuits)“wil1 be repeated once or several times
`m such a way that, in all circumstances the
`corresponding members will be influenced
`simultaneously. The manner in whichthe
`oscillations themselves are produced (the ac-
`tual generator connection) is immaterial to
`the principle of the invention and the present
`state of the art offers numerous arrangements
`suitable for this purpose.
`.
`"A suitable circuit arrangement, which has
`.-proved perfectly reliable in practice, and
`also combines great simplicity withease in
`manipulation, can be obtained by controlling
`a differential relay by means of the sending
`' key, so as to effect the progressive switching-
`on, by stages, of one or more disc switches in
`the tuned oscillation circuits, which effect the
`necesary switching operations simultaneous-
`ly. This ensures, in all circumtances, the cer-
`tainty that, on the key being depressed, or re-
`leased, the wave required for the nextperiod
`(working- or open-circuit period) ‘ will be
`switched .on. The closing of the contacts on
`the disc switches determines the serial order
`of the waves produced and the serial order can
`311; altered by employing interchangeable
`scs..
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`\
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`One form of the invention will be described
`with reference to the accompanying drawing
`which is a diagram of the aforesaid circuit
`' arrangement. The signals are sent out di-
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`rectly, as high frequency ._ oscillations, by
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`means of a group of six working frequencies
`and six open-circuit
`frequencies are em-
`ployed between the working frequencies.
`a
`The circuit arrangement consists of a trans-
`mitting valve Z, the anode circuit of which is
`tuned and is directly connected with the
`aerial A.
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`_
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`The anode current is supplied through a
`choke ‘coil P to the anode circuit which con-
`‘ sists of a coil L and three condensers I21, k2,
`and 7:23 and an unlimited number of different
`wave lengths can be generated by the aid of
`these condensers and the tappings 13-18 of
`the coil L. The action of the generatoris
`maintained by means of aacoil R; whichsis
`connected with the grid of the valve and is
`magnetically coupled with the anode coil L. .
`In‘ telegraphing, the sender Z is in con-
`' tinuous operation, and the alternation of the
`working and open-circuit waves is eflected by
`controlling the sending key S, the open-cir-
`cuit contact 20 of which is connected with the
`winding w’ and the working contact 21 with
`/w". These windings influence
`"the winding
`the armature D of an escapement actuating
`a toothed disc B, the‘ shaft of which is con-
`nected with the switch arm M.
`In the draw-
`ing, theikey S is represented in the open-
`circuit position, so that the winding w’ is‘
`energized and the armature D is turned clock-
`wise, on itsshaft. On connection being es-
`tablished with the working contact 21, the disc
`B will advance a step in known manner.
`In
`- all circumstances, the switch arm M is in con-
`tact with two oppositely disposed contacts
`- such as the contacts 1, 1’inthe positionshown;
`in the next position the ontacts 2, 2’, and so
`on. The circuit arrangements of the con-
`tacts are such that the movements of the
`switch arm cause an open-circuit wave and
`a working wave-to be generated alternative-
`ly.
`It follows from the drawing that, in con-
`junction with the condensers la’, Is‘-3, and I93,
`the tappings 13,’ 15 and 17 of the coil L deter-.
`»minewthevopen-circuit waves. On ~the,other
`hand,
`the tappings 14, 16, and 18 serve,
`through their connection with the condensers,
`for the generation of the working waves. Al-
`though a sufficient number of waves can be
`generated by the aid of the tappings alone,
`the condensers are provided in order that, by '
`adjusting their capacities, the total number
`of the waves can be further increased. Of
`course, the number of tappings and also con-
`'densers, is entirely a matter of convenience.
`The wires 1", r'‘, and 7'3, leading to the
`tappings for the open circuit waves are pro-
`- vided with interruptors 0‘, 02, and 03, which
`enable the "open-circuit waves to be interrupt-
`ed at convenience and given the character of
`working waves, during _the transmission of’
`the message. These interruptors can be set
`-out of action byshorteningthe switches 8’,
`3’, and 8“.
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`The serial order of the working and open— 1
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`ing-waves can be replaced by another at any
`moment.
`-
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`Moreover, -the unauthorized listener will
`be occupied for some considerable time in de-
`termining which of the frequencies are to be
`regarded as the workin
`frequencies, and
`Irequencies.
`- which as the open-circuit
`The changing over from the one group of
`working frequencies ‘to the other is not at-
`tended with any appreciable trouble, because
`the‘ variable members (condensers, variom-
`eters, &c.)‘ of the transmitter and receiver can
`be coupled and designed in sucha way that
`the same manual operations in both stations
`produce the same change effects.
`VVhat I claim is:
`1. Method for the wireless transmission of
`telegraphic messages by means of Morse or
`other code, wherein the dots and dashes of
`which the message is composed are trans-
`,mitted by means of a plurality of working
`frequencies which are interchanged at will
`during the transmission of the massage.
`2. Method for the wireless transmission of
`telegraphic messages as claimed, in claim 1, .
`wherein one or more spacing frequencies are
`used between the dots and dashes, which are
`interchanged at will.
`-3. Method for the wireless transmission of
`telegraphic messages as claimed’ in claim 1
`wherein one or more spacing frequencies are
`used between the dots and dashes, which are
`interchanged at Will, said spacing frequencies
`being interrupted at will during their trans-
`l'Il1SS10I1._
`'
`4. Method for the wireless transmission of
`-telegraphic messages as claimed in claim 1,
`,.wherein the group of Working frequencies em-
`ployed is replaced, from time to time, by an-
`other, during the transmission of the dots and
`dashes.
`'
`-
`5. Method for the wireless transmission of ‘
`telegraphic messages as claimed in claim 1, f-
`wherein one or more spacing frequencies are
`. used between the dots and dashes, which are
`interchanged at will, and wherein the alterna-
`tion of the working and spacing frequencies is
`effected by means of the sendin key.
`WILLEM BR ERTJ
`
`circuit frequencies during ‘the rotation of the
`switch armcan be varied at convenience by
`varying the connections between the contacts
`themselves, and between said contacts and
`the condensers and tapped coils. Care must,
`however,.be taken that the two kinds of waves
`alternate regularly.
`‘
`'
`The interruptors C‘, 02, and 03, interposed
`beween the coil L and the switch M serve to
`interchange the open—cireuit and working
`frequencies. If, for example the 01 be turned
`over, the wire 1"’ will be connected to the tap-
`. ping 14, and the wire w’ to the tapping 13.
`In this case, also, the open-circuit wave will
`be influenced by the interrupter.
`It is. self evident that the difierent fre- ,
`quencies can also be generated by means of
`quite differently connected and designed os-
`cillation circuits, the sole essential point be-
`ing that an open-circuit wave and a working
`wave are alternated by means of the switch‘
`arm M. There may also be provided between
`the aerial and the tuned anode or grid circuit
`of the transmitting valve, a tuned intermedi-
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`ate. circuit actuated by a corresponding
`‘ switch arm also mounted on theishaft of the
`pinion B, in which case it is advisable, to con-
`nect the condensers of the two circuits togeth-
`er, so that -during adjustment, equal changes
`may be made in the frequency groups.
`If the changing over of the working fre-
`quencies be effected at convenient moments,
`the manipulation of the key must be separate
`from that of the interrupting device. Free
`interruption of this kind can be eifected
`either by hand or automatically.
`In either
`case there is no relation at all between_the
`times at which the key and the interrupter
`perform their individual functions. If an
`automatic apparatus is used, the construction
`will be more compact, but is attended with
`the disadvantage that the serial order of the
`interruptions is ‘fixediin advance and will
`therefore be a‘ matter of constant repetition.
`This drawback is absent in the case of hand
`control.
`.
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`In all circumstances the receiving appara-
`tus employed for carrying out the new-method
`must be capable. of picking up several work-
`ing frequencies and transmitting their total
`effect to a single reproducing device (such as
`a writing apparatus) &c. Its design depends
`on the manner in which the signals are trans-
`mitted (high—frequency or low-frequency os-
`cillations, or both) and, in View of the re-
`ceiving circuits already known, can be em--
`bodied in various ways.
`By the employment of the open-circuit fre-
`quencies generated at will, the method ac-
`cording to the present invention assures the _
`requisite secrecy, even when the principle of
`the transmission is known. This result is due,
`in the first place to the fact that the method
`is non-rigid, and that the one .group-of»work-
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