US2291978A - Radio transmission system - Google Patents

Description

4, 1942- H. -MOURADIAN 2,291,978

RADIO TRANSIISS ION SYSTEM Filed June so, 194; s Sheets-Sheet 1 CARRIIER -BIAS -r0 0.4;

C souaca BAND PASS l emu PASS TO 2 TELEPHONE LINE OR RECT. TELEPHONE s'rmou 7 LOW mss HIGH {I PASS ,1

l I I FILTER INVE'N 1 OR.

Aug. 4, 1942. H. II'AOQURADIAN,

RADIO TRANSMISSION SYSTEI Filed June 30, 1941 3 Sheets-Sheet 2 INVEN IOR.

mun I PAss FILTER l 5 I6 :D 7 J T BAND .L PASS I F FILTER T0 o.c. TO INTERM. TO n.c. SOURCE FREQ.0SC.(PQ) SOURCE w FIG.3.

I To AUDIO Low AMPLIFIER PASS LAND FILTER LOUD SPEAKER w T0 ac. sourzcz FIG.4

IBAND BAND PASS .L I PASS N g o I I L I 22 T0 osc. TO 11c. T0 osc. T0 ac. FREQ.(F9) SOURCE FREQE H-] sougca FIG. 5 4 140 lfiumdi Aug. 4, 1942.

H. MOURADIAN RADIO TRANSMISSION SYSTEM Filed June 30, 1941 3 Sheets-Sheet 3 U 31 33 E on EH1 E5 32 34 LF. H.P. LR i 35mm. 55 F. F. TO SOURCE 0F F. f T0 CARRIER TO CARRIER 9 SUPPLY ,c c VAR. To FREQUENCY A FREQUENCY TELEPHONE (fo) LINE OR STATION LF. T- HP. LR Lg moo. E r2 D E; E2

I 43 45 v 5.9 GG E KK N LL 0 I F 44 46 49 am B P g 7 DEM. DEM F. DEM E BP 50 S E To T0 figg' 5OURCEOF SOURCE OF FRE UENC H CARRIER s CARRIER cs f S FREQUENCY FREQUENCY G BMP (Fg) (fmHg) F TO AUDIO 23g DEM L.R "AMPLIFIER DEM J 1-: AND LOUD SPEAKER 5.9

E &:To CARRIER c FREQUENCY (fz) FIG 7 #MMKA IN VENTOR.

soon-23,000

Patented Aug. 4, 1942 UNITED STATES PATENT OFFlCE 2,291,978 aanro TRANSMISSION SYSTEM Hughes Mouradlan, Philadelphia, Pa.

Application June 30, 1941, Serial No. 400,399

11 Claims.

This invention relates to a radio transmission and receiving system. The transmission system as disclosed fully hereunder has the following important advantages and characteristics:

1. It can be superimposed upon the presentday amplitude modulation transmitting and receiving systems without mutual interference between the two.

2. It affords a means for secrecy in transmission.

The advantage first mentioned is of the greatest importance in peace time activities. The advantage mentioned as in the second instance is of the greatest importance in war time activities.

Considering first the point first mentioned hereinabove, it is well known that the number of radio channels available for general use is strictly limited at the present time. The following table indicates the number of available radio channels for various frequency ranges as determined upon by international agreement:

Numbcr of available channels In the band between 6,000 and 23,000 kilocycles. where it is agreed that 624 stations can operate simultaneously, there are already 1,700 stations (other than amateur) in the world operating on these frequencies. In the broadcasting range where only 96 radio channels are available. it is possible today, with a -kilocycle separation between the frequencies of adjacent carriers, to operate several hundred broadcasting stations by locating the stations using the same frequency thousand miles or more apart. The above situation refers to'our North American continent. It is thus apparent that it is highly important to find some means of increasing the number of available channels, for broadcasting as well as for point-to-pointtransmission. The present invention discloses a system of radio transmission which allows, theoretically a least, multiplying by two (2) the number of radio channels available for communication.

The art, at the present time, uses the system of radio transmission using the so-called double side band system. This is universal in the broadcasting range. On point-to-point transmission some installations use single side band transmission, while a great majority of the pointto-point transmission systems still use the double side band system. In a restricted range of very high frequencies a system known as frequency modulation is also in some use. On all installations the modulating carrier is radiated with both. side bands or, in some cases, with one side band, the other side being suppressed at the transmitting station. The present system or systems en- Joy the advantage of simplicity, since they avoid the need for providing a local carrier supply at the receiving stations--a factor of some real economic importance.

When use is made of the system disclosed herein, it is possible to superimpose upon the present double side band system operating at a given carrier frequency, a second system operating at the same carrier frequency with no mutual interference between the two systems. Thus two channels can be made available where only one was available before. To effect this purpose, the following steps are taken:

1. Arrangement is made at the transmitting station to separate, by adequate filtering means, the two side bands into two different frequency channels.

2. A transposition is inserted in one of the two side bands. For universal application of the system described more fully hereunder, agreement must be reached that all transmitting stations should have,- let ussay, the upper side band always reversed in phase.

3. The carrier is suppressed at the transmitting station.

4. A separate carrier source, of variable frequency, is supplied at the receiving stations.

It is important that means be provided at the receiving end to set accurately the value of the carrier frequency, to avoid distortion. This can be done by the use of crystal type oscillators. Considerable progress has been made in the last decade in the development of sources of electric waves having great stability in frequency, irrespective of changes in ambient temperature, etc.

5. Arrangement is made at the receiving stations to reseparate by adequate filtering means the two received side bands into two distinct frequency channels.

6. A transposition is inserted in one of the two side bands, segregated as described.

If, by general agreement, the upper side band is reversed at transmitting stations, then the same upper side band is again reversed at the receiving stations, thus restoring normal relationship between side bands.

The above steps broadly cover the requirements of the system disclosed herein.

This invention will be clearly understood when read in connection with the attached drawings, in which Fig. 1 and Fig. 2 represent the transmitting system. Fig. 3 and Fig. 4 represent one form of receiving system, while Fig. 3 and Fig. 5 taken together, a second form of receiving system. Fig. 6 indicates in simplified diagrammatic form the combination of the transmitting set of Fig. 1 and Fig. 2 of the drawings with a second transmitting arrangement for the secret transmission of the variable frequency carrier itself to the distant receiving set or sets. Fig. 7 indicates also in simplified diagrammatic form, the combination of the receiving set of Fig. 3 and Fig. 4 of the drawings in the lower half of Fig. '7, with the receiving arrangement of the variable frequency carrier itself from the distant transmitting station and the manner of the use of said carrier in conjunction with element G of the receiving set, in the upper half of the same Fig. '7.

While none of the circuit arrangements required in conjunction with the various steps in transmitting and receiving as above recited are new, the combination of all the steps is such that the important objective of greatly increasing the number of radio channels available for general use is thereby accomplished for the first time. In a pending application, U. S. A. S. N. 314,960, I have described a similar system which, however, uses two carrier sources both at the transmitting and the receiving ends. The system described in the above cited application has, in addition to the properties claimed in the present application of non-interference with the presentday allocation of radio channels, the advantage of freedom from external and internal sources of noise. The equipment requirements of the system, in my said application S. N. 314,960, are, however, far more intricate and costly than those described in the present application. For general purposes and more particularly for broadcasting use, the arrangement disclosed herein is amply sufiicient to secure the greatly needed additional radio channels.

No detail explanations are required for an understanding of the operation of the conventional type radio broadcasting equipment of the amplitude modulation type which terminates in terminals I, 2, 5, 6. The essentially novel features of the arrangement consist in the use of twostep modulation for separating the two side bands produced by modulation. The first step in the modulation is illustrated on Fig. 1 and is carried out at a sufficiently low frequency to enable this separation conveniently and effectively. At the present stage of the development of the art, such effective separation can be made at frequencies as high as 60,000 to 80,000 cycles per second, requiring, however, the use of crystal filters for the purpose. Band pass filter A of Fig, 1 of the drawings allows the free transmission of frequencies included between f2 and (f2+10,000) cycles, where f2 represents the frequency of the first or intermediate step of modulation and a band of 10,000 cycles is considered adequate for the transmission of speech. Band pass filter B of the same figure of the drawings allows the transmission of the lower side band which includes the frequencies between f2 and (fz-10,000) cycles per second. It is important to note the transposition between terminals I, 2, 3, 4 which is inserted in The second step in the modulation which i 11-- lustrated on Fig. 2 of the drawings merely steps up the frequency by f0 cycles per second to the frequency assigned for normal operation of the two side bands to the broadcasting-or point-topoint transmitting station. The equation of the wave radiated by the antenna of Fig. 2 of the drawings is given by In the above formula, the following notation is used:

k=coeflicient of proportionality Cz amplitude of the carrier supply on Fig. 1 of the drawings.

Cs=amplitude of the carrier supply on Fig. 2 of the drawings S=amplitude of signal q=value of the frequency of the signal We also designate 2J2 by m, 2J0 by 110, and 01 represents the phase angle.

It is important to note that the transmitting system disclosed on Fig. 1 and Fig. 2 of the drawings is carrier-suppressed throughout, since the radiated wave as given by Formula 1 does not contain the carrier itself, but just the two side vbands, one of which has its phase reversed. The

formula indicates the upper side band as reversed. It is immaterial, as already noted, whether the upper or the lower side band isreversed, but only one must, of course, be so reversed. The suppression of the carrier C2 is obtained by the symmetrical feed of the carrier supply through the midpoint ofa coil in the input circuit of the modulator. The suppression of the second carrier of frequency in is obtained by the action of the high pass filter E1, which allows only the I flow of currents exceeding frequency cycles per second. The filter E2 suppresses all the higher harmonics of the modulation products which are allowed through by filter E1.

Fig. 3 and Fig. 4 taken together show the arrangement required at the receiving end. The equipment shown on Fig. 3 of the drawings includes, in addition to the receiving antenna, the radio frequency amplifier F and the intermediate frequency modulator G followed by band pass filters H and J. The modulator G is supplied with means for varying the frequency in of the intermediate carrier. The main demodulated waves in the plate circuit of element G of the drawings have the frequencies (f2+q), (fa-q), (2fo|-f2+q) and (Zfo-l-fz-q). The two first waves, representing the lower'side bands of the demodulation products are allowed to pass through by filters H and J. The third and fourth waves, representing the upper side bands, are suppressed by the same filters. The band pass filter H is designed for a free transmission band included between the frequencies f2 (f2+10,000) cycles per second. The band pass filter J is designed for a free transmission band and included between I: and (fa-10,000) cycles. The figures cited refer to a band of speech frequencies 10,000 cycles wide. In principle, instead of 10,000 cycles we could use 20,000 cycles, or still more generally the procedures herein indicated could be applied to systems of frequency modulation or television transmission.

The wave reaching terminals l3, It will be given by- The wave reaching terminals ll, IE will be given by- In the above Formulas 2 and 3, 1 represents a coeflicient of proportionality, C4 the amplitude of the carrier supplied to demodulator G.

We may now note that a transposition has been inserted between terminals i3, i4 and band pass filter H. The result of this transposition is to change the sign of the electric wave given by Formula 2 from to The equipment shown on Fig. 4 of the drawings completes the process of demodulation. The two side bands are amplified by intermediate frequency amplifier K, if amplification is necessary, and further demodulated by element L, which is supplied by a second source of carrier C at a fixed frequency of 1: cycles per second. The final result of the demodulation process as carried out by the equipment arrangement shown on Fig. 4 of the drawings is a speech wave which is given by the expression l: coefiicient of proportionality C'=amplitude of carrier C supplied to element It may now be shown that in a given territorial area, the radio transmission system illustrated on Fig. 1, Fig. 2, Fig. 3 and Fig. 4 of the drawings can be operated without mutual interference with the system at present in use, even though using the same wave length assignments.

Let us consider, first, the effect of the wave as given by Formula 1, radiated by the antenna of the transmitting system of Fig. 2 of the drawings of the present application. This wave, when received by a present-day receiver of the double side band amplitude modulation type tuned to (fo+f2) cycles per second, results in no effect in said receiveryas the two side bands included in that wave mutually cancel each other. The effect of an electric wave of the double slab band type as radiated by a present-day transmitting station operating at an assigned frequency of (Io-+42) cycles per second upon receiving sets of the type illustrated on Fig. 3 and Fig. 4 of the drawings is as follows:

in the wave of (fz+q) cycles per second reaching filter J.

(c) A wave of frequency I: cycles per second, due to the carrier itself of the double side band system, which is allowed through by both filters H and J but is canceled out in reaching terminals I9, 20 in view of the transposition ll, H, l5, l6.

It is thus seen that if a transmitting station is erected using the equipment shown on Fig. 1 and Fig. 2 of the drawings in close proximity to a transmitting station of the present-day type and both stations are operated at the same wave length or frequency, there will be no mutual interference between the two stations. Thus, in Washington, D. C.,-radio station WRC uses the assignment of 950 kilocycles. A second radio station, using the same assignment of 950 kilocycles could be erected in Washington, D. C., using the transmission system described in the present specifications and both systems operated free from mutual interference. This same situation holds true for the other two radio stations operating in the immediate vicinity of Washington, D. C., viz., stations WJSV and WOL operating at 1460 and 1230 kilocycles per second, respectively. In principle, therefore, it is possible to double the number of transmitting stations now available.

It may be pointed out here that the receiving stations of the type described in the present specifications can receive the programs of the present-day transmitting stations if a simple reversing key is installed which would reverse the transposition l3, l4, l5, is to normal line connections. This has not been illustrated on the drawings for the sake of simplicity, since the change suggested is quite obvious, The converse is not true, i. e., it is notpossible to receive the programs from radio transmitting stations constructed in accordance with Fig. 1 and Fig. 2 of the drawings at radio stations equipped with the present-day standard receivers of the double side band type.

It may be pointed out here that the suppression of the carrier at the transmitting station makes it possible to avoid the mutual interference between the transmission system disclosed herein and that in general use today. It is necessary today, as already pointed out, to space the high power transmitting stations using the same frequency and of the amplitude modulation type at distances of 1000 miles or more from each other. A very great deal of the difiiculty ex perlenced in mutual interference between such high power stations is due to the carrier interference or "whistle" between stations using the same frequency and this difiiculty is completely avoided in the case of the transmitting system herein disclosed in view of the suppression of the carrier. No new radio transmitting stations can be added to the present networks without this feature of carrier suppression. An insight into the importance of this feature may be obtained from standard set by the I. R. E. Standards Committee as to carrier interference to the effect that the permitted interference output power (due to the carrier whistle) should not exceed 1:1000 part of the desired modulation output power.

As noted above. thispossibility of doublin the number of broadcasting or transmitting stations is contingent upon the general use by the public of radio receiving sets of the type shown on Fig.

sive than present-day sets. An alternative arrangement, which would result in a smaller economic penalty, is to assign the additional radio channels made available to the mobile or pointto-point services of the nation. a

A second important advantage, as previously noted, of the system of radio transmission disclosed herein consists in the secrecy features. Many such systems have been proposed, some using double carrier operation with consequent complexity and greater waste of the ether available for general use. Another proposed system, actually in use over the transatlantic and transpacific routes inverts the various frequencies of speech. All of these systems, however, also radiate the carrier. Thus, the fact that radio transmission is under way is made immediately apparent. It is possible to get a bearing by special methods upon the location of these radio transmitting stations, even though the transmitted message may remain obscure at first. This is a serious disadvantage. Experimental testing, gives a means of re-inverting the inverted speech and thereby securing the message. The transmission system disclosed in Fig. 1, Fig. 2, Fig. 3

and Fig. 4 of the drawings has certain advantages over previously disclosed systems. One important aspect consists in the suppression of the carrier in the manner fully described hereinabove. It is difiicult in the absence of the carrier to secure a good bearing upon the location. of a. transmitting station. Enemy planes, flying at night, can get a fairly correct estimate of their own location above ground, when such ground has been blacked out, by getting bearings upon known radio transmitting stations and using such bearings in a system of triangulation. The shutting down of some radio stations to avoid this sion of the carrier and also on account of the fact that the two side bands are in opposite phase to each other. Hence, even if a radio receivin set of present-day design should be equipped with a local carrier supply, it would still' receive no signal. A transmitting station of the type illustrated on .Fig. 1 and Fig. 2 of the drawings can transmit signals to specially equipped receiving sets, giving a statement as to the movement of enemy planes without such planes being made aware of any measures taken against them. It appears unnecessary to multiply the special cases of use of the secrecy features of a radio transmission system such as outlined in the present specifications. Evidently, in time, countermeasures can always be taken against most types of secrecy systems once their secrecy features become known. However, even in such a case, by making the intermediate frequency f0 variable within limits and in arbitrary manner, in both the transmitting station and the receiving station arrangements as provided in the present specifications, the tapping of information by unwanted listeners could be made quite difilcult if not impossible. Two different methods have been worked out and are indicated hereunder. The first and simplest method consists in having the transmitting station transmit by pro-arranged code language the particular frequency In which will be next used and allowing time for the receiving stations to make the corresponding changes in the intermediate frequency of their own sets. When such changes are carried out in arbitrary manner and with sufiicient frequency,

it would be most difiicult for unwanted listeners to get any part of the secret information conveyed by radio. Of course, even such a procedure falls down should the code itself be stolen and become known. In any event, a fairly large measure of security can be obtained against the tapping of secret information by unwanted persons, in the manner described above.

A second method for further decreasing the chances of the tapping of secret information to a vanishing point is described hereunder and is illustrated by Fig. 1 and Fig. 2 of the drawings taken together as the transmitting station and by Fig. 3 and Fig. 5 of the drawings taken together to illustrate the arrangements required at the receiving station or stations. To efiect the above purpose, I have shown a method for carrying out the changes of intermediate frequency simultaneously and automatically at the transmitting and receiving stations. These changes are carried out under the control of the transmitting station but without requiring the collaboration of the receiving station attendant. The method just referred to, as first disclosed in these specifications consists in supplying the variable give-away has been reported in the press. It has intermediate frequency carrier supply to element G of the receiving equipment from the transmitting station instead of locally, as indicated on the drawings. This carrier supply is furnished to element G of Fig. 3 of the drawings from a special receiving set constructed in accordance with Fig. 3' and Fig. 5 of the drawings. While this special receiving set is arranged, in part, in accordance with Fig. 3 of the drawings, it is actually supplied with a local carrier, but its constituent elements are designed differently from those used in the first or normal receiving set. This special receiving set is thus used only to receive the variable frequency carrier supply from a second transmitting set at the transmitting station. This second transmitting set is constructed in accordance with Fig. 1 and Fig. 2 of the drawings, but its constituent elements are also designed differently from those of the first transmitting set. All of these special features are fully described hereinbelow.

In order to avoid detection of the transmission of the intermediate carrier supply and thus give unwanted listeners a chance to get a bearing upon the location of this second transmitting set, I propose to transmit the carrier in question in a wave train similar to that given by Formula 1, i; e., in the form of two phase opposed carrier side bands closely adjacent in frequency. Such a wave train under the conditions cited will produce no signal in any receiving set of the standard type. Even if unwanted persons had available receiving sets constructed in accordance with Fig. 3 and Fig. 4, no signal would be received since the final result obtained would be an incates the regular transmitting set of Fig. i and Fig. 2 of the drawings, also in simplified diagrammatic form.

cycles per second. On Fig. 6 of the draw-' ings this oscillator is designated as-To source of frequency f:-

(b) All other elements of the second or carriersupplying transmitting set are constructed in accordance with Fig. 1, special care being taken to use the same source of carrier supply for this set as for the regular transmitting set.

(c) No changes are required in the arrangement shown on Fig. 2 except for the choice of the cut-off points of filters EE1 and EEz, as will be noted below. It is necessary to use the same source of supply Ca for both transmitting sets.

It will be noted that, by assumption, carrier C: has a variable frequency. The variations of this carrier can be controlled manually by the transmitting station attendant or, if desired, an automatic arrangement following a definite frequency changing pattern can be used. These In the above expressions the notation used is the same as formerly, with the use of the following additional coefiicients:

G=Amplitude of the oscillator replacing the telephone station. h=Frequency of the oscillator G.

The frequency In is, as stated, made variable to secure additional secrecy. We might write lo=fm+fv where 'fm represents the minimum value and Iv the additional positive frequency variation of carrier Ca. As before, and 01 represent phase constants. I propose to eliminate waves A and B by suitable design of the wave filters EEJ. and Ella. To fix our ideas, we might take for the minimum value of jo=900,000 cycles per second, ,/a=50,000 cycles per second, and j =5,000 cycles per second. Under these assumptions, the minimum values of frequency of waves A, B, C, D are 955,000, 945,000, 845,000 and 855,000 cycles per second, respectively. The cut-off point of filter EE: (low-pass) would be 855,000+fv. If Jv is restricted to 10,000 or 15,000 cycles, as a maximum, per second, there will be sufilcient difference in frequency between the maximum value of wave D which must be transmitted and the lowest value of wave B which must not be allowed through to enable a practical design of filter EEz. A greater variation in the value of It can be obtained, if required, by simply increasing the value of the frequency J: at which the generator frequency f; is modulated. The expression for the cut-oi! point of low pass filter EE: is theresimplified diagrammatic form. On

fore (fn-i-fr-fa-i-fg). The high pass filter EEi is, in the present case, designed to allow free transmission for all frequencies in excess of the lowest useful frequency, i. e., (In-fa-fg) cycles per second and eliminate all spurious or undesired demodulation products.

The wave radiated by the antenna of the second transmitting set is composed of two parts of equal amplitude but phase opposed having the frequencies (fo-f2+fa) and (fa-fa-h), respec tively, when 10 is variable. The expression of the wave radiated by the antenna is given by-- The above formula gives the type of wave desired. While the value of the frequency 1; is to some extent arbitrary, it should be a fractionary part of I: and in addition should not be any greater than necessary in view of the congestion of the ether. In the broadcasting range (1. e., the low frequency range),- it might be advisable to restrict it to the 10-kilocycle band width new standard in that range.

The above wave train is received by a distinct and separate receiving set wired in accordance with Fig. 3 and Fig. 5 of the drawings as to detail, and in simplified diagrammatic form on the upper half of Fig. 7 of the drawings. The lower half of the same figure indicates the regular receiving set of Fig. 3 and Fig. 4 of the drawings in Fig. 7, the carrier supply to element GG is set at the minimum fixed frequency fm. This value is the minimum value of the frequency of the variable frequency carrier supply Ca at the transmitting end.

The effect of the demodulator GG is to step down the frequencies of both components of the wave expressed by Formula 5 by the amount of the frequency fm. Both of the upper side-bands of 1 the main products of the demodulation are eliminated by band pass filters HH and JJ. The two lower side bands will have the frequencies (fl-*fo-fg) and (la-fo-HU- The first one is transmitted through by filter JJ, the second one by filter HH. The cut-off points of these two filters must be set as follows:

Higher cut- Lower cutoil point of! point B cl filt H Ba nd tilts; EU IL}? fr fi f The expression of the wave reaching terminals i9, 20 of Fig. 5 of the drawings will be, hearing in mind the effect of the I6 upon the waves transmitted through filter H or the efiect of the transposition 43, 44, 5, 46 upon the waves transmitted through filter HH of the corresponding diagrammatic illustration of Fig. 7 of the drawings.

'ing four main waves willtransposition l3, i4, i5,

elimination of the first and the fourth of these waves without interfering with the second and the third, which we wish to retain undistorted. I find that I can solve this difficulty by assigning certain restrictions to the relative values of fv and jg. To show how this can be done we tabulate as follows:

Maximum Minimum value ol(f.) value of U.)

is wave n+- fzf.+2f| 2d and 3d waves fa fz-f. 4th wave fa2,f, fz-f.2f.

For the same reason, to avoid any component of the fourth wave from being transmitted through, we should have- The two inequalities therefore merge into a single one. In this limiting inequality, e is the percentage width which must be allowed, over and below the higher and the lower cut-off points,

- respectively. We thus note that, if the variation In of the intermediate frequency In of the carrier C2 is not greater, approximating, than twice the value of the frequency fg, then the purposes of the invention can be properly accomplished. If we do not wish iv to exceed kilocycles in the low frequency broadcasting range, then fg must be greater than 5 kilocycles. The band pass filter N of Fig. 5 of the drawings is therefore designed with the cut-off frequencies j2(1+e) and f2(e) -fv, as shown. A third modulation step is next carried out by means of element LL of Fig. 5 of the drawings which is supplied by a local carrier C5 with a frequency of (fm-Hz) cycles per second. The wave reaching this element after transmission through band pass filter N will have the expression where p is a coefficient of proportionality and 05 a phase constant. After demodulation by carrier C5 of element LL, the two resulting main waves will be proportional to, respectively, (a)

are also suppressed by this same filter O. The

wave obtaine'ddrom the final terminals of the second receiver set has the frequency (,fm+fv) or Io cycles per second. The second receiving set, wired as shown on Fig. 3 and Fig. 5 of the drawings and designed in the detail manner indicated hereinabove, thus acts as a source of carrier supply having a frequency which varies in strict accordance and almost simultaneously with the frequency of the carrier supply C3 at the transmitting end. This carrier supply is used, as already described, in conjunction with the first receiving set and is associated with element G of said receiving set in place of the carrier supply marked To intermediate frequency oscillator p" on said Fig. 3. To secure the specific secrecy features of the present invention we are using, as shown, two transmitting sets. The first set transmits the wave train given by Formula 4, the second set transmits the wave train given by Formula 5. Both of these wave trains, with the respective frequencies (fo+fz:q) and (fa-12th) consists of two terms phase opposed to each other and are thus inaudible to radio receiving sets of the double side band type. Furthermore, both wave trains are carrier suppressed and both wave trains are, in addition, of variable frequency. The location of the source of these wave trains is a matter of great difiiculty and the securing of any message transmitted in the manner described not possible, unless all of the detail ,secrecy features of the two-transmitting and receiving sets have been previously disclosed. An outstanding feature of the radio transmission system hereinabove described consists in the fact that it canuse any frequency now assigned to either fixed or mobile services without interfering with any of these. Furthermore, the system herein described can also be used for the transmission of pictures, television images, etc., thus openingan entirely new field of application of secret signaling for the defense forces of the nation. Itwillbenotedthat carrier supply sources C2 and C3 as shown on Fig. 6 of the drawings are common to both transmitting sets, and so is the antenna itself. This is an advantage from an economical point of view. Attention is also called to the fact that on Fig. 6, the first amplifier associated with the telephone line on Fig. l of the drawings and the intermediate amplifier C of Fig. 2 have no corresponding parts on Fig. 6. These have been omitted for the sake of simplifying the diagram of Fig. 6 since the omitted amplifiers are not necessary to illustrate the principle of operation of the combination of the two transmitting sets. It will also be noted that the radio amplifier F of Fig. 3 and radio amplifier K of Fig. 4 have no corresponding parts on Fig. '7 of the drawings. These also have been omitted since they add nothing to an understanding of the principle of .operation of the combination of the two receiving sets diagrammatically shown on Fig. '7 of the drawings. As a matter of fact, the effect of all amplifiers, whether in the transmitting or receiving sets have been indicated in Formulas 1 to 6 inclusive, as proportionality factors 10, 1 etc. Attention is also called to the fact that the use of a single antenna, both at the transmitting and at the receiving ends, is not a necessity but represents a feature of economy. On Fig. 7 of the drawings, four (4) different sources of alternating electric power are indicated having the fixed frequencies j 1:, fm, (f2+fm) respectively. It is possible, and in some cases advisable from an economic standpoint,

both

ings.

to obtain the last mentioned source having the frequency (fin-+12) by modulating the source of frequency In by the source of frequency I: through methods well known in the art. This procedure would reduce the number of independent sources of fixed frequency carrier required at the receiving station to three (3) instead of four (4), as indicated on Fig. 7 of the drawings.

If two-way transmission is desirable or required, such as in the case of parachute troops behind enemy lines, then the minimum number of independent sources of alternating current power required would be still three (3)v in number having the fixed frequencies In. h, and Im- The source of variable frequency carrier of the distant transmitting set (as opposed to the home transmitting set) of Fig. 6 of the drawings could certainly be supplied from some central source, be received on Fig. 7 of the drawings, and supplied to modulators D and DD of a number of transmitting sets, where two-way transmission is desired. This would further simplify the equipment requirements of such sets, a point which may prove of some importance. This arrangement has not been illustrated on the draw- It simply means that the output of the band pass filter 0 of Fig. 7 of the drawings would be used, at any given radio station, to supply variable frequency carrier not only to element G of Fig. 7 as shown but also to elements D and DD of Fig. 6 at that station. Thus one large single source of variable frequency carrier at a centralized transmitting set could be used to supply such required carrier at a number of auxiliary transmitting sets, thus lightening the equipment requirements of such sets.

I claim:

1. In a radio transmission system, a transmitting circuit comprising means for modulating a signal by a local carrier, means for segregating the two main side bands of the modulation products into two different frequency channels, means for reversing the phase of only one of the said side bands, means for suppressing the carrier and means for radiating both side bands with no accompanying carrier.

2. In a radio transmission system, a receiving circuit for the transmitting system specified in claim 1, comprising a receiving antenna, demodulating means, a carrier supply source for demodulating the received waves, means for retaining only the lower side bands of the demodulation products in combination with means for separating said lower side hands into two different frequency channels, means for reversing the phase of only one of said side bands in cooperative relation with the transmitting system specified in claim 1 for restoring the signal impulses in the two channels in proper phase relationship into a single reinforced signal impulse.

3. In a radio transmission system, a transmitting arrangement for supplying a carrier sup ply in the form of two phase opposed carrier suppressed side bands with arbitrarily varied frequency to distant points, comprising means for modulating a constant frequency signal with a local carrier which has its frequency arbitrarily varied, means for segregating the two main side bands of the modulation products into twodifferent frequency channels, means for reversing the phase of only one of said side bands, means for suppressing the carrier and means for'radiating both side bands with no accompanying carrier.

4. In a radio transmission system, a receiving arrangement for the transmitting circuit specified in claim 3, comprising a receiving antenna, demodulating means, a local carrier supply source for demodulating the received waves, means for retaining only the lower side bands of the demodulation products in combination with means for segregating said lower side bands into two different frequency channels, means for reversing the phase of only one of said side bands for restoring the normal phase relationship of said lower side bands, a second demodulator with a local oscillating source at the frequency of the constant frequency signal at the transmitting end, means for eliminating the constant frequency signal effect from said lower side bands, means for a third demodulation step raising the frequency of the retained wave train by a constant amount to the desired basic frequency for a transmitted source of variable frequency current supply.

5. In a radio transmission system, the combination of the transmitting arrangement of claim 1 with a second transmitting arrangement for supplying carrier to distant points comprising means for modulating a constant frequency signal with a local carrier which has its frequency arbitrarily varied, means for segregating the two main side bands of the modulation products into two different frequency channels, means for reversing the phase of only one of said side bands, means for suppressing the carrier and means for radiating both side bands with no accompanying carrier.

6. In a radio transmission system, the combination of the transmitting arrangement of claim 1 with a second transmitting arrangement for supplying carrier to distant points comprising means for modulating a constant frequency signal with a local carrier which has its frequency arbitrarily varied, means for segregating the two main side bands of the modulation products into two different frequency channels, means for reversing the phase of only one of said side bands, means for suppressing the carrier and means for radiating both side bands with no accompanying carrier, both transmitting arrangements of said combination having common sources of carrier supply. a 7. In a radio transmission system, the combination of the transmitting arrangement of claim 1 with a second transmitting arrangement for supplying carrier to distant points comprising means for modulating a constant frequency signal with a local carrier which has its frequency arbitrarily varied, means for segregating the two main side bands of the modulation products int two different frequency channels, means for reversing the phase of only one of said side bands, means for suppressing the carrier and means for radiating both side bands with no accompanying carrier, both transmitting arrangements of said combination having common sources of carrier supply and a common antenna.

8. In a radio transmission system, the combination of the receiving arrangement of claim 2 with a second receiving arrangement for receiving a variable frequency carrier supply in a wave train of two carrier-suppressed side bands, one of said bands having its phase reversed, representing the effect of modulation of a signal by a carrier at the distant transmitting station, comprising at said receiving station demodulating means, a local carrier supply for demodulating the received waves, means for retaining only the lower side bands of the demodulation products, means for reversing the phase of only one of said side bands, means for eliminating the signal frequency from said side bands and reconstituting the original variable frequency carrier supply as a modulating source for the receiving arrangement of claim 2, as stated.

9. In a radio transmission system, the combination of the receiving arrangementof claim 2 with a second receiving arrangement for receiving a variable frequency carrier supply in a wave train of two carrier-suppressed side bands, one of said bands having its phase reversed, representing the eilect of modulation of a signal by a carrier at the distant transmitting station, comprising at said receiving station demodulating means, a local carrier supply for demodulating the received waves, means for retaining only the lower side bands of the demodulation products, means for reversing the phase of only one of said side bands, means for eliminating the signal frequency from said side bands and reconstituting the original variable frequency carrier supply as a modulating source for the receiving arrangement-of claim 2, as stated, both :receiving arrangements of said combinations having a common antenna.

10. The method of secrecy transmission by radio which consists in generating at the transmitting end a wave train of two carrier-suppressed side bands, by modulating a signalling wave with a carrier wave, introducing a phase reversal into only one of said side bands, radiating both the reversed and the unreversed side bands with no accompanying carrier, demodulating at the receiving end said side bands by means of locally supplied carrier, introducing a phase reversal into one of said side bands and translating said two side bands into a single reinforced signal wave.

11. The method of secrecy"transmission of a variable frequency carrier supply between two distant points which consists in generating at the transmitting end a wave train of two carrier-suppressed side bands by modulating a signal with a carrier wave, introducing a phase reversal into only one of said side bands, radiating both the reversed and um'eversed side bands with no accompanying carrier, demodulating at the receiving end said side bands, reversing one of the two resulting side bands, eliminating the signal from said resulting side bands, thereby leaving only the original carrier frequency wave.

HUGHES MOURADIAN.

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