US2492138A - Mechanical modulator for radio beacons utilizing two tones - Google Patents

Description

Dec. 27, 1949 c. w. EARP MECHANICAL MODULATOR FOB- RADIO BEACONS UTILIZING TWO TONES 3 Sheets-Sheet 1 Filed Dec. 12, 1944 e Sill/n4 t+ esmmt Dec. 27, 1949 c. w. EARP 2,492,138

MECHANICAL MODULATOR FOR RADIO BEACONS UTILIZING TWO TONES Filed Dec. 12, 1944 3 Sheets-Sheet 2 I 1949 c. w. EARP 2,492,1'3

MECHANICAL MODULATOR FOR RADIO BEACONS UTILIZING TWO TUNES Fild Dec. 12, 1944 s Sheets-Sheet 3 q 24- 45-3 QNVENTOR. B 1slHMRLEs WILLIAM EARP A TTORNEY CAL MODULATOR F018 RADIO 2-. BEACUNS UTE-G TWO TQNES Charles William Earp, London, England, assignor, by mosne assignments, to International Standard Electric Corporation, New York, N. Y., a

corporation of Delaware Application December 12, 1944, Serial No. 567,835 in Great Britain February 23, 1940 Section 1, Public Law sec, August s, 1946 Patent expires February 23, 1960 rangements are preferably mechanical, as a very high degree of balance between the several radiations is necessary, this condition making valve modulating systems unsatisfactory.

In some of these systems, it is desirable to radiate no carrier wave, or to radiate a single sideband from a particular aerial. In particular, in systems for radio landing where off-set radiation patterns are used, it is most advantageous to radiate no carrier wave, and still better to radiate single sideband from at least one aerial.

It is the object of the present invention to provide arrangements whereby double sidebands without carrier, or single sideband may be produced by mechanical modulation. The stability of mechanical modulation devices depends solely upon mechanical accuracy.

A particular advantage of the mechanical modulators constructed according to this invention is that they are specially suitable for modulation of very high frequencies, where filtering methods are not practical A valuable application for the single sideband producer may be found in radio telegraph systems, where it is desired to radiate a multi-channel single sideband system, or to key marker and spacer waves on slightly different. frequencies.

According to the broad aspect of the invention a modulator for the production of a single or double sideband corresponding to the modulation product of a high (carrier) frequency modulated by a low frequency, comprises two mechanical parts electrically associated with each other so that the electrical condition of one is dependent upon theelectrical condition of the other which is excited by electrical energy of the said highfrequency, said two parts being also relatively movable with respect to each other at a relative speed such that the frequency of the cycle of variations of the electrical conditions of said one.

a balanced modulator for the production of two frequencies corresponding to the sidebands resulting from modulation of a high frequency wave by a low frequency is characterised in that the modulation is effected by the rotation of one or more mechanical parts.

According to another aspect of the invention, a modulator for the production of a single frequency sideband corresponding to one of the bands resulting from the modulation of a high frequency by a low frequency is characterised in that the modulation is efiected by the rotation of one or more mechanical parts.

ing simultaneously two sidebands;

Figs. 2a and 2b show arrangements for obtaining a single sideband;

Figs. 3a and 3b show a modified arrangement of Fig. 2b incorporated in an unsymmetrical and a symmetrical system respectively;

Fig. 4 shows a modified arrangement of Fig. 311 for the case when the wavelength of the high frequency is rather large to meet practical applications of the invention; and

Fig. 5 shows an arrangement adapted for use with polyphase supply of high frequency.

The principle of operation of the devices according to the invention is the rotation of a collector in a radiogoniometer. In Fig. 1a, a fixed coil F having two parts symmetrically located on opposite sides of a diameter is fed with radio frequency ,f1, and a coil S located between the two parts of F rotates at frequency is in the field of F. The output from the rotating coil S contains frequencies (f1+,f2) and (f1f2), but not h or f2. If the coupling between the two coils F, S is proportional to the cosine of the angle between them, then (f1+f2).and (f1'fz) are the only frequencies obtained. If the coupling does not follow this law, then sidebands corresponding to harmonics of is will also appear in the output of coil S. r

Fig. 1b shows a modification of Fig. lain which capacitative coupling is used instead of inductive coupling between stator and rotor, and which is more practical for very high frequencies. This arrangement also produces a double-sideband output, like any balanced modulator.

The mechanical construction of this capacity or double condenser arrangement may take many forms. Fundamentally it is a condenser with two independent stators SI, 82, and two rotors RI, R2 which are mechanically coupled to rotate together and each cooperates or "meshes" alternately with the two stators to form a variable condenser therewith. If the two condensers thus formed between rotors and statorsv at all times are well matched, then no carrier wave will be. transmitted therethrough. By shaping of the vanes in known manner, the law of variation of coupling with angular position of the rotors can be made to follow a cosine law, when a simple double-sideband will be produced, (11+j2) and (11-h).

If the two pairs of stators of two double-sideband generators are fed by carrier waves of the same frequency but different phase, for example, e sin out and sin (w1t+a) giving the eiiect of a rotating field, and if the rotors are coupled together mechanically at an angle of sand totated at a speed of n revolutions per second. (21m=wz) in the same direction as the rotation of the field; and if the outputs from the rotors are connected in parallel to give a single output, this output equals:

valves used to provide polyphase currents will cause imperfect suppression of the unwanted sideband, but this will be of no importance in the application of the modulator to radio beacon systems.

Figs. 3a and 3!: show modified forms of the single sideband generator, for respectively unsymmetrical and symmetrical inputs and outputs. In both of these cases, a transmission line is formed into a loop P having a length equal to m wavel nsths of the frequency f1 carried by the line, and the output is coupled to the linethrough rotating collector or collectors R, ,RiRz preferably of the capacity type, i. e. arranged so as to form a capacity with the line as shown. The collectors rotate at frequency is. Since a single revolution e sin w1i Bin wi s (w1t-isin (whoos (w m- 1+ 1) which expression contains a single frequency Preferably the angle a should be E and the total output for the case under consideration is then 2 cos (w1+wa) t. For any other angle, the total output is smaller.

A similar result is obtained if the rotors are rotated in the opposite direction to that of the rotating field efiect, giving a frequency It is not necessary, in practice, however, for obtaining a single sideband to use two ganged double-sideband generators. The same principle of operation can be used to provide the single sideband generators as shown diagrammatically in Figs. 2a and 2b, which are respectively in efiect the radio goniometer commonly used for radio direction finding purposes (Fig. 2a), and the equivalent capacity goniometer (Fig. 2b) In these goniometers, one field coil Fl (Fig. 2a), or one pair of stators Si, 82 (Fig. 2b) is fed with a carrier e sin wit, and the other field coil F2 or pair of stators S3, S4 is fed with a carrier as collectors.

of the collectors adds to or subtracts from the input wave m cycles, the output will be h-i-mfz, or fi-flif: according to direction of rotation of the In practice this system is not workable without special precautions, for the open ended transmission line to which the collectors are capacitatively coupled will produce virtual short-circuits across the line at odd multiples of quarter wavelengths from the open end. This may be avoided by a resistance termination of the line, but such a remedy is rather wasteful with regard to power.

One method for preventing the unwanted effect of the open line is illustrated in Figs. 6a, 6b, and

6c. These figures are perspective diagrams of a second or "dummy" collector. In Fig. 6a, the loop P corresponding to the loop P in Fig. 3a, is in "one plane together with its rotating collector R .for picking up the output. The loop Q is in another plane together with its collector D and the inductance coil 13 and the ground connection. The loop Q is an open line connected in parallel with the loop P at the point H. The method is to arrange a second or dummy collector D for the case of Fig. 3a or second or dummy pair of collectors R, or R1Rz.

collectors D1, D1 (see Fig. 6b) for the case of .Fig. 3b, which terminate the line in a very low impedance at a point one quarter wavelength nearer to the open end of the line than the output Fig. 6b is a perspective view of an arrangement of dummy collectors for the type of modulator illustrated'in Fig. 3b. The

The field coils or pairs of stators are arranged at n reva/seo.

,to provide an output at frequency according to the sense of rotation.

One suitable method for feeding the ii-phase loop P1 together with the collector R1 and the associated output conductor lie in one plane. The

loop P: and its associated collector R2 and its associated output conductor lie in another plane. The collectors P1 and P; correspond to those illustratedin Fig. 3b. The loop Q1 and its collector D1 and the coil II associated with D1 lie in another (0 plane and the loop Q3, its associated collector 1)),

and its ted inductance coil i3 lie in the fourth plane. Q1, Q1, D1 and D2 represent the ant disclosed in this paragraph. The loop Q1 is connected to the loop P1 at I 4.and the loop Q: is similarly connected to the loop Dz.

acca ss are. The hne between the points In and i1 is the projection of the dummy collector D in Fig. 6a and the dummy collector D1 in Fig. 6b on to the plane of the coil P or P1. The distance from 9 to ill measured on the circumference is equal to a quarter of a wave length of the high frequency If This will have the effect of giving the "dead" end of the transmission linea very high terminat ing impedance, 'and n'o power will be lost. The impedance at the position of the output collectors is always the characteristic impedance of the transmission line. The low impedance or dummy collectors are preferably of similar type to the output collectors, and are arranged virtually to short-circuit the transmission by terminating them with an inductance [3 which tunes out the capacity of the collector coupling. This dummy" collector must not of course short circuit the input transmission line when the output collector is on the last quarter-wavelength portion of the line loop. In practice, the dummy" collectors are arranged to rotate around conductors Q or Q1, Q2 in parallel with the loop conductors P or P1, P2, and the portion of the parallel conductors ll, representing the first portion of the loops covered by the collectors are omitted. Conductor loops Q or Q1,Qz are connected to the loops P or P1, P2 at the points It. These points are one quarter of a wave length distance from the points l at which the line is attached to the conductor loops P or P1, P2. To operate correctly it is necessary that the distance It to It is negligible with respect to a quarter wave length or that it is a full wave length.

When the modulation frequencies required are too high for rotation of collectors at the fundamental, a transmission line loop of several wavelengths may be used. If the total size becomes too large, it is not necessary to collect output from all points of the.line as the latter may be looped out of the circle, returning to suitable electrodes El, etc. at intervals of quarter-wavelengths as is shown in Fig. 4 for an unsymmetrical system.

-In this case correct matching of the output is maintained if a very low impedance auxiliary or "dummy collector D '(see Fig. '6c) is arranged to rotate with they output collectorsbut always one electrode, i. e. one quarter-wavelength of transmission line nearer to the open end, i. e.'

4 open end of the transmission line. As in the previous case described this dummy collector must not short-circuit the input line in position of electrode No. 1 when the output collector is on the last electrode: in practice it is arranged to rotate round a setv of parallel electrodes I8, I9, 26, 2 I, 22, 23 and '24 in which set the first electrode is omitted, i. e. the electrode that would correspond to electrode i in distance from the open end of the transmission line is omitted. Connection is made between the loop P and the loop Q from the electrode 2. The loop Q and its dummy collector D-lie in a different plane from that in which the loop P lies and to make this obvious the loop Q has been drawn in a broken line although it is a real part of the circuit. In this arrangement as in the previous one described the dummy collectors are arranged to" short circuit vertically the transmission by terminating them with an inductance which tunes out the capacity of the collector coupling.

still another modification is shown on Fig. 5 for an unsymmetrical system in which two carrier inputs from Li, L2 are used and, are of the same frequency Fl, but in phase quadrature. The ilnes Ll, L2 from each source contact with alternate -electrodes Ei, etc. The electrodes of .each set .Ei-S-S etc. or E2-d-6 etc. are connectedeither by'very shortleads; orbyleads of half wavelengths. In the arrangement shown in Fig. 5 dummy or short-circuiting'collectors are required and arranged as described in connection with Figs. 6a, and 6c, and the sideband frequency produced corresponds to double the speed of rotation of the collector. It will be observed that the electrical distance between two successive electrodes is virtually one quarterwavelength.

It will be understood from the examples described that many variations of the single sideband generatorare possible without departing from the scope of the invention. The essential principle is the rotation of mechanical parts at the frequency of the required sideband or at a sub-multiple of the required frequency when a polyphase current is employed.

The single sideband generators are particularly applicable to and provide a simplification in radio landing systems. For example, if a carrier wave of frequency fl is radiated omnidirectionally in the horizontal plane, but with high horizontal directivity in a vertical plane, and if sidebands f2 and fa are radiated on aerials of different directivity in the horizontal plane, then a receiver tuned to the total transmission will have three frequencies fi-f2, fa-fa, Irv-fr, in its output. Any particular predetermined relative values of the amplitudes of these three frequencies defines a definite line in space. v

What is claimed is:

l. A modulator for the production of a single side band corresponding to the modulation product of a high frequency wave modulated by a low frequency and for the elimination of the carrier wave, comprising a source of high frequency voltage, a first transmission line, a first conductor that is one of the pair of conductors of said first transmission line and is formed into an open loop whose circumferential length is an integral number of wave lengths of said high frequency, means for applying voltage from said source to said first conductor, a second transmission line, a second conductor that is one of the pair of conductors of said second transmission line, means for capacitatively coupling said second conductor to said first conductor, means controlled by the low frequency to rotate said second conductor around said loop so that the frequency of the high frequency voltage transferred by the capacitative coupling to the said second conductor is either greater than or less than the said high frequency by an amount equal-to the said low frequency.

2. A modulator'according to claim 1 in which the first transmission line is a symmetrical line, the other conductor of the said first transmission line is formed into an open loop whose circumferential length is an integral number of wave lengths of the high frequency, and a rotating

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