US2572211A - Radio direction finder having sense antenna - Google Patents

Description

Oct. 23, 1951 J. G. SPEER 2,572,211

RADIo DIRECTION FINDER HAVING sENsE ANTENNA Filed Jan. 12, 1948 Jose/ob G. Sloeer ATTORNEYS.

Patented Oct. 23, 1951 RADIO DIRECTION FINDER HAVING SENSE ANTENNA Joseph G. Speer, Merriam, Kans., assigner of onehalf to H. G. Koenig, Merriam, Kans.

Application January 12, 1948, Serial No. 1,775

12 Claims.

This invention relates to the field of radio and particularly to direction finders, the primary object being to provide a device of such character as forms the 'subject matter of my co-pending application, Serial No. 707,696, led November 4, 1946, now Patent No. 2,490,660, granted December 6, 1949, this being a continuation in part that Vis inherent in the device forming the subject matter of said earlier led application.

Another important object of this invention is the provision of a radio direction finder having a sense antenna wherein means is provided to improve operating accuracy, making possible more correct bearings on the maximum, thereby permitting continuous aural monitoring and minimizing interference from static and undesired radio signals.

Another important object of this invention is the provision of a radio direction finder including amplifier stages, having new linear ampliflcation characteristics, in both the laudio and rate of change channels, thereby maintaining the audio output and the triggering impulses approximately uniform regardless of the amplitude of the received radio signal.

A further object of this invention is to provide a radio direction finder having a voltage limiter in the rate of change channel that limits the triggering impulses to just above the value required for operation, which together with pulse time delay, renders the direction finder practically immune to bursts of static.

A still further object of this invention is the provision of a radio direction finder having a limit type of automatic volume control that is ineffective, throughout the change in signal level required for proper functioning of the direction nder, but is effective at signal level changes in excess of these requirements.

Additional features and objects will be set forth and made clear during the course or" the following specification, reference being had to the accompanying drawing showing a diagrammatical view of an electric circuit forming a part 2 of a radio direction iinderhaving a sense antenna made in accordance with my present invention, and showing schematically the loop antenna, sense antenna, motor, motor shafts, gears, radio receiver and associated parts thereof.

In the drawing a directional antenna, constituting normally an electrostatically shielded loop, is broadly designated by the numeral I0 and the numeral I2 designates a non-directional or sense antenna.

Thewindings of the loop I0 are connected to a pair of collector rings I4 and I6 and from the rings by means of contact brushes (not shown) to a pair of wires i8 and 20. 'I'he received signal is conducted over these wires to a radio receiver tuner amplifier 22 where it is amplified to the level required for detection. No details of these receiver elements are shown as they may be quite conventional.

The amplified radio signal is conducted through a transformer 24, which is tuned by a pair of condensers 26 and 28, to a diode 32 of a vacuum tube 30. The radio frequency signal is rectified by the diode 32 and a resultant direct current voltage appears across a resistor 34, together with a superimposed audio frequency signal if the radio signal was modulated. The radio frequency component is greatly attenuated by a condenser 36 whose value is so chosen as to cause inappreciable attenuation of the audio frequency component. These voltages are conducted through a resistor 38 to the grid 39 of vacuum tube 30. A condenser 40 contributes almost complete radio frequency attenuation and large audio frequency attenuation.

'Ihe negative direct current voltage applied to the grid 39 of the vacuum tube 30 increases its grid bias and hence reduces its plate current. The positive cathode potential as produced by the plate current flowing through a pair of resistors 52 and 54 consequently decreases as the incoming signal increases.

The audio frequency voltages appearing across the resistor 34 are conducted through a resistor 42 and a condenser 48 to the grid 49 of a vacuum tube 56. The condenser 48 blocks the direct current voltage from the grid 49. The resistor 42, a resistor 44 and a condenser 45 provide additional, almost complete attenuation of the radio frequency component and some uniform audio frequency attenuation.

The grid 49 of vacuum tube 56 is positively biased by reason of the voltage drop across the resistor 54. Therefore the cathode of tube 56 should be also positive and this is accomplished by means of a resistor 58.

The voltage drop across the resistor 54, and hence the Agrid bias of the vacuum tube 56, changes with signal level. If the values of the resistors 54 and 58 are suitably chosen, the change in amplication of the vacuum tube 56 will vary approximately inversely as the signal level. By this means the audible output may be held approximately constant.

The audio signal as amplified by the vacuum tube 56 is conducted from its plate over a wire 60 and through suitable coupling means (not here shown) to subsequent conventional audio amplication as required for aural indications.

The voltage at the cathode of the vacuum tube 30, which rises and falls as the signal decreases and increases respectively, is impressed upon the grid 61 of a vacuum tube 68 over a wire 66.

The cathode vof the vacuum tube 68 must be more positive than the grid 61 throughout the operating range of signal levels. This positive bias may be conveniently secured from a usual storage battery 64 which in most aircraft is 12 volts or more. Noise and ripple may be removed by a choke 72 and a condenser 14.

For the vacuum tube 6B, one of suitable characteristics should be chosen, and suitably loaded by a resistor 'l0 of proper value. This may be accomplished so that, to a fair approximation, the absolute voltage change at the plate of tube 68 is proportional to the percentage voltage change at the gridY |51.l Thus the output voltage change is independent of signal level.

Now as the signal rises and falls the voltage at the plate of the vacuum tube 68 rises and falls. This rise or fall of plate voltage causes a charging current to flow through a condenser 16 and a resistor 'I8 to ground. A voltage is therefore, produced across the resistor 78 that is proportional to the rate of rise or fall.

If the signal and Vplate voltage of the vacuum tube 68 are rising, the voltage across the resistor 18 is positive. Thisv causesno significant action. The loop antenna I is presumably rotating towards a position of maximum response.

After the loop |0 rotates beyond the point of maximum response, the signal begins to fall. The percentage rate of fall is proportional to the product of the rotative speed of loop antenna |0 and the tangent of its angle of departure from the point of maximum response.

Ihis produces a negative voltage across the resistor 18. Assuming a constant loop rotative speed, the magnitude of this voltage isY proportional to the tangent of the angular departure of loop |0 from maximum and, as we have seen, is nearly independent of the strength of the received signal.

This negative voltage is conducted through a resistor 90 to the grid 9| of a vacuum tube 96 causing its plate current to decrease, and its plate voltage, because of the decreased drop in a resistor 98 to increase.

After these voltages attain a certain value, subject to a slight delay in time because of a pair of condensers 92 and |00, the first trigger tubes consisting of a pair of vacuum tubes |02 and |04 are tripped in the following manner:

The vacuum tube |04 is normally operated with its plate current'saturated. Its grid is at cathode potential and is only prevented from going substantially positive, by virtue of the limiting action of a, pair of resistors ||2 and I4. This current flowing through a resistor |06 biases the cathode of the vacuum tube |02 to a point considerably more positive than its grid, thus holding its plate current extinguished.

The plate of the vacuum tube 96 is coupled through a resistor to the grid of the vacuum tube |02. Therefore, the rising potential at the plate of the vacuum tube 96 is carried to the grid of the vacuum tube |02 so that it soon enters the conducting region at which point the plate current ilowing through a resistor |08 causes the plate potential of the vacuum tube |02 to fall. As this action continues, this plate voltage soon falls to the point where the potential distribution across the resistors ||2 and ||4 is such as to bring the grid potential of the vacuum tube |04 below its cathode potential. This causes a decrease in its plate current and hence a decrease in drop across the resistor |06.

A resistor 0 is made very much smaller than the resistor |08. Also because of the amplification of the vacuum tube |02 its grid changes potential less than that of the vacuum tube |04. Both of these effects cause the decreasingl current of the vacuum tube |04 to over-ride the increasing current of the vacuum tube decreases and the cathode of the Vacuum tube |02 becomes less positive. But the grid 'potential has been made more positive by the tripping signal so it is evident that the' eiects just described effectively increase the exciting voltage once the tripping potentialhas been reached.

In fact, the currents promptly surge so that the plate current of the vacuum tubes |02 and |04 reach saturation and extinction respectively. This surge is accentuated by the charging current through the condenser |00 so that the action is practically vinstantaneous regardless of the rate at which the tripping voltagev rises. This new condition of equilibrium continues as long as the tripping voltage is maintained. If the rate of change of the signal should decrease or cease this tripping voltage will fall and at a definite release value surges will take place in a reversed manner and the circuit will restore to normal. Y

This releasing voltage is somewhat lower in magnitude than the tripping voltage. This 'is an advantage as it prevents' momentary surges from causing multiple trips.

The instant the vacuum tubes |02 and |04 are tripped and prior to their release, a second trigger circuit is actuated which controls the direction of rotation of the loop antenna |0.

This trigger circuit consists of a pair of Vacuum tubes ||6 and H3 in the respective plate circuits of which are a pair of fields |20 and |22 for an actuating motor |24 for antenna |01 The armature of the motor |24 is continuously supplied with power so that the motor |24 runs continuously in one direction or the other dependingV upon which of the two field windings |20 or |22 are receiving current.

This second trigger circuit, like the first is unstable, unless the plate current of one of the two tubes IIS or ||8 is extinguished. In this condition the circuit is stable until, being tripped by the rst trigger circuit, the current in the conducting'vacuum tube ||6 or ||8 is extinguished and the current rises to normal in the vacuum tube ||E or ||8 that was extinguished. This switches the elds |20 and |22 of the rotating motor |24 causing it and the loop I0 to reverse direction of rotation.

|02.V Therefore, the net drop across the resistor |06 The grid of vacuum tube [I6 is held at a denite potential by the voltage divider consisting of a pair of' resistors |3 and |132. An identical voltage divider consisting of a pair ofv resistors Iil and ISl determines the grid potential of the vacuum tube II8. Note that in each case the voltage dividers extend from a ground to the plate of the opposite vacuum tube I I5 or I I8,

The plate voltage of the extinguished vacuum tube I|6 or IIB is much higher than that ofthe one carrying normal plate current. This is because of the absence of voltage drop across the motor eld or |22 through which plate currents are drawn. Therefore, each tube IIG and IIB maintains a suitable grid potential for the opposite tube. Each grid potential is positive but that of the extinguished tube is substantially less so. The cat-bodes are, therefore, positively biased by a common cathode. resistor |38 so that the relative grid potential of the extinguished vacuum tube is substantially negative and well below the extinction point. The relative grid bias of the conducting vacuum tube is also negative but only by a moderate amount so that its plate current is responsive to any change introduced at the grid.

The grids of the vacuum tubes I I8 and H8' are connected through a pair of condensers 26 and |28 to the plate of the Vacuum tube l. This plate voltage drops suddenly when the first trigger circuit is tripped. This negative pulse is impressed on the grids of the vacuum tubes H and IIB. This, directly,L causes no signicant change in the plate current of the extinguished vacuum tube. The plate current of the other vacuum tube is however, suddenly dropped towards extinction. This causes a reduction of current through the resistor |38 and hence a reduction in cathodepotential. Likewise, the fall in plate current of the con ducting vacuum tube causes a rise in its plate voltff r age. This rise, reduced in magnitude, is transmitted to the grid of extinguished vacuum tube where it overcomes the negative pulse potential as received from the rst trigger circuit. Plate current now begins to flow producing a plate voltage change that transmitted to the other grid. accentuates the change.

The conducting condition of the two vacuum tubes I I6 and I I8 is now reversed. Likewise there has been an interchange of current between the motor fields |253 and |22. This causes the motor |24 to stop and reverse. This motor 24 causes the loop It to rotate through suitable reduction gears such as |40 and |42. Consequently the rotation of loop ID also reverses` direction and now moves towards the point of maximum response with a resulting rise in signal strength,

This rise acting through the rate of change circuits causes the first trigger circuit to release, if it has not already done so. This produces a positive pulse through the condensers I 25 and |28 to the second trigger circuit. This produces no significant action and the pulse promptly decays.

The loop antenna I Il continues to rotate, crosses the maximum to the region of falling response. When it reaches the point where the rate of fall is of the designed magnitude, the trip circuits again function and again reverse the direction of rotation of loop I.

Thus it is evident that the loop will continue to oscillate to equal angular displacements each side of the point of maximum response. The actual. bearing is of course its average position.

It is desirable that the relatively large loop oscillations be reduced in magnitude as indicated by an azimuthv indicator F46. To accomplish thisy a hunting integrator may be interposed in shafting |41 between the loop II] and the azimuth indicator |46..

Such an integrator forms the subject matter of a. pending application for U. S. Letters Patent, Serial No. 707.730 led November 4, 1946,k now Patent No. 2,447,171. This is symbolically shown as. indicated by the numeral" |44.

In the above description several important features have been omitted as not being absolutely essential to the operation of the direction finder. These features do contribute materially to its operation in a really practical and satisfactory manner.

Such are. a pair of rectiers, indicated by the numerals 8l). and 82. The conducting direction is in each case as indicated by the arrow. These. could be. thermionic. diodes but are more. oon;- veniently of the germanium crystalA type.

By means of a voltage divider consisting of resistors d4', 86 and 38, the rectier 82 is biased negativelyI and the rectifier 8E! is biased positively with respect to the resistor '58,. This places a positive bias on the grid of they vacuum tubey 96 so. a resistor 94 is incorporated producing a positive cathode potential slightly greater than that o the grid.

As previously explained a definite negative volt.- age is required across the resistor 'I8 to trip the circuit. The bleeder current and the resistor 8B should be so proportioned that the drop across the resistor BIS is slightly inexcess of this voltage value.

The. rectifier 82 has little effect on voltages up to the trip value as the back resistance. is very high. voltages appreciably above the trip value however, cause conduction in the rectier 82 S0. that these excess voltages are substantially absorbed or clipped.

this marmerV a| noise or static surge is limited and of' short duration will not trip the circuit due to the delay imposed by charging the condenser 92 through the resistor 90 and the con denser Ittth-rough its associated resistors 98 andv III! together with plate resi-stance of tubes 96 and IM., Suitable time constants for these circuits are approximately .0.5 second. The rectifier dii is positivelyy biased in a similar manner and serves to clip positive voltage surges in the same way'. The resisto-r 8.6 isv of the same magnitude as or' slightly less than the resistor` 88..

It is desirable that all audio modulation and other fluctuations. be removed from the rectified carrier before transmittal. to the rate of change circuits. Now static noise bursts. may cause a rapid change in carrier strength. Such changes should be accommodated with little delay sothat they may be effectively clipped and delayed as previously explained.

If long time constant. filters are usedv the eilectiveness of the clipping is. greatly reduced. For these reasons the resistor 38 and the condenser 40 should have a time constant of about .01 second. A resistor |50 and a condenser |48 should have a time constant aboutv the same.

A resistor |54 and a condenser |52 which govern the screen voltage of the pentode 30, should have a time constant of about .02 second. Now the screen contributes only about 20% of the current so the additional delay will not cause any great direct change in the current rise or fall. But the plate and screen currents are but little affected by the plate voltage and greatly aieoted by the screen voltage. The effect is such as to resist change in current.

Therefore the effect of the relatively slow time constant lter on the screen is to provide a delay in the restraint imposed by its voltage change. This results in a somewhat higher initial change in total current which reaches its ultimate value without the excessive tapering off that is usually characteristic of resistance capacity filters.

It was previously explained how the vacuum tubes 30, 56 and 68 functioned so as to be independent of signal level variations. Now this is true only throughout the limited operational range of the tubes in question and would by no means accommodate the signal strength ratio encountered in practice, which may be many thousandfold.

The need for some sort of volume control is evident. A simple manual volume control would serve but its inconvenience would indicate the desirability of an automatic volume control.

Now the conventional volume control tends to prevent change in the rectified carrier, which is precisely the effect required in this direction finder. Such a volume control would require excessive time delay in the automatic volume control circuits and excessive trip circuit sensitivity.,V This is undesirable for several obvious reasons.

The automatic volume control employed, operates to reduce or increase the amplification to any desired degree or at any desired speed.` Over -a limited range it is however, almost completely ineffective. It is within this range that all of the action previously explained takes place.

Two rectiers are employed acting between an automatic volume control buss |66 and the cathode of vacuum tube 30. A rst rectifier |56 is conductive towards, and a second rectifier |58 is conductive away, from the cathode of vacuum tube 30.

At the initial no signal condition the cathode of vacuum tube 30 reaches a positive potential somewhat greater than that of the cathodes of the vacuum tubes being controlled. A bleeder current may be drawn from the plate supply source. to the cathode bias point through a pair of resistors 50 and 5|. These resistors should be so proportioned that, at their junction, the positive potential will equal the positive potential of the cathode of vacuum tube 30 at the no signal condition.

` A resistor 6| should have about four times the resistance of a resistor |60. Now, as the signal increases, the cathode potential of vacuum tube 30 decreases in positive potential. The positive cathode potential of the rectier |56 likewise decreases but the decrease will be one-fifth smaller in magnitude.

As the incoming signal continues to increase. the cathode potential-of the rectifier |56 will eventually Vdrop below that of the automatic volume control bus |66. When this takes place, the conductivity of the rectifier |56 is such, that the AVC bus |66 will then have its positive potential reduced. This rapidly d-ecreases the amplification of the receiver until a balance is reached.

If the signal subsequently decreases, the cathode potentials of vacuum tube 30 and the rectier |56 rise. No immediate change occurs in potential of the AVC bus |66, as the rectifier |56 is now non-conductive. If the change continues the cathode potential of the vacuum tube Y 30 soon exceeds the potential of the AVC bus |66. At this point the rectifier |58 becomes conductive and the potential of Ythe AVC bus |66 rises with the potential of the cathode of the vacuum tube 30. This rapidly increases the amplification of the vacuum tube receiver until a balance is reached. i

This will permit a change in signal strength of any desired magnitude without automatic volume control action. If the resistors |60 and 6|l are proportioned in the ratio o f 1 to 4 a 20% drop in signal strength is required to start AVC action. This should be adeguate to accommodate the direction finding functions.

Amplification changes may be made at any desired speed by controlling the charging rate of a condenser |64. If desired these speeds may differ for increases and decreases in amplification.

The condenser |64 is charged through a resistor |62 for increases in amplification and discharged through the parallel combination of the resistors |60 and |6| for decreases in amplif-lcation.

It is evident that, throughout all of the volume control action just described, and for all operating conditions, the grids of all vacuum tubes beings controlled are at positive potential from ground.

It is necessary, however, that the receiver tubes being controlled operate at all times with their grids negative with respect to their cathodes. It is, therefore, necessary to bias these cathodes positively to the same degree as the initial automatic volume control bias.

This may be conveniently done by connecting all the cathode returns of the receiver tubes being controlled to a Wire |68 which is connected to the same positive bias source that was used for the cathode of the vacuum tube 68.

An additional desirable precaution would be to limit the maximum positive excursion, of the automatic volume control bus, to the cathode potential of the receiver tubes. This may be done by providing a diode Yrectier within one of the receiver tubes. YThis diode is connected between its cathode and the automatic volume control bus. This idode is non-conductive when the bus is less positive than the cathode.

Satisfactory loop rotative speeds have been found to be between 30 to 60 per second. Trip- Y ping may be readily accomplished at 30 or more` from points of maximum response.

The foregoing description has described how automatic direction finding may be accomplished using only a single antenna.

Due to the well known directional characteristics of a loop antenna two bearings apart are always found. Under the conditions just described this 180 ambiguity is likewise experienced in this direction finder.

This uncertainty may be resolved by employing a sense antenna. In this case, as in the case of previously used direction finders, the phase of the signal received by the Vloop antenna signal is compared to that received by the sense antenna. Now the phase of the sense antenna signal is constant While that of the loop antenna shifts 180 as it rotates across the point of no response.

For several reasons it was found undesirable to receive the sense signal continuously. The sense signal is only applied momentarily following each tripping action.

The non-directional sense antenna |2 is connected to the grid of a vacuum tube |10. The grid is biased to ground Athrough a pair of resistors 200 and |90. The resistor |90.is electively Icy-passed by a condenser |88 so that the effective termination consists of the resistor 299. This resistance 209 mustbe large compared to the reactance of the antenna and associated wiring, at the lowest frequency in the operating range. This is in order to maintain proper phase relations One-half megohm is a suitable value for this resistance.

Suitable supply voltages are established for the plate, screen and cathode by means of resistors |18, |80, |82 and |84. These resistors are of such value that the positive cathode bias will nearly extinguish the plate current. A pair of condensers |12 and |14 effectively by-pass these resistors at radio frequencies.

The signal output from the vacuum tube |10 is transmitted through a transformer |16 to the input of the receiver in parallel with the loop antenna -|0.

This transformer |15 should have a very close coupling between its windings. The secondary inductance should be several times the inductance of the loop antenna l so as not to unduly affect the transmission from the latter. The primary, connected to the plate of the vacuum tube |10, will `have a very much lower inductance. This inductance .should be just high enough to transmit .sense signals of desired strength. Any transmission in excess of this is undesirable. The magnitude of this signal need not exceed 20% of the maximum signal of loop antenna I0.

It is evident that normally no sense sig-nal is transmitted as the vacuum tube is normally extinguished. Following each tripping action a pulse of short duration is transmitted in the following manner:

vThe cathodes of the vacuum tubes |02 and |04 are connected through a condenser |98 to the, pulse shaping network consisting of the condenser |88, the resistor |90 and a resistor |92 and a pair of reetifiers |94 and |96. These rectiers 19,4 and |96 may be .of the germanium crystal 'type .and their directions of conductivity are as indicated 'by the arrow. f f

VWhen tripping action occurs-a negative pulse is transmitted from the cathodes of the vacuum tubes |02 and |04 through the condenser |98 and is promptly absorbed by the rectier |96. No significant action takes place in the circuits of the sense antenna |0. The loop rotating motor |24 slows down and then reverses. Shortly before or after this motor reversal the trigger circuit releases.

his .sends a positive pulse through the condenser |.98, rectifier |94 and the resistor v|92 to the condenser |88 which is shunted by the high resistance |90. This charges the condenser |88 `positively and through the resistor 200 places a positive voltage on the grid of the vacuum tube '|10 thus rendering it conductive and permitting a sensev signal to be transmitted to the .receiver .and associated direction finding circuits.

The sense signal must not be injected into the receiver 22 abruptly. If this is done the limiter .and time delay circuits would prevent its transmittal to the tripping circuit.

j For this reason, the current charging condenser |88 Tis limited by the resistor |92 in series with the vrectifier -|94. A time constant greater than that of the resistor 90 and the condenser 92 is then obtained.

` 'During this charging period the amplication 'of the vacuum tube |10 is continuously increased,

The amplitude and l0 rate of this injected sense signal must be such as to be capable of actuating the tripping circuits. After the condenser |88 receives its maximum charge, it begins its discharge through the resistor |90 only the rectifier |94 being non-conduct; ing for currents in this direction. The discharge resistor .|90 is made muchl larger than the chag= ing resistor so that the rate of change .of the falling sense antenna signal will be too small to aotuate the tripping circuits.

Now this momentary .injection of lsense signal may act vin either of. two ways depending upon which of the two .ambiguous bearings the loop antenna is responding.

In the rst way, the injected sense signal will be in phase with the loop antenna signal anda rise in combined :signal will result. This causes no significant action. In this case the loop antenna i0 'is oscillating about the proper bearing.

In the second way, the injected sense Asignal will be out of phase with the loop antenna signal. This indicates that the loop antenna :l0 is oscillating about the wrong bearing. In this case a fall in the combined signal will result due to their difference in phase. This causes the tripping circuits to be reactuated and the loop rotation, after but a .momentary hesitation, continues towards the point of minimum response.

'I'he first trigger circuit is held tripped by the continuously falling signal and usually releases only after the point of minimum response is crossed. In such a case .no further significant action will take place until the loop antenna I0v rotates beyond the opposite point of maximum response and .again enters a region of falling sig.

nal response. Subsequent tripping and sensing actions will then continue .about the proper point of maximum response.

A somewhat different action may result if the automatic volume control action is diierent. In this case after the retripping due to the .sense pulse the trigger circuit is initially held in the tripped position as before. However, the continuously falling signal must shortly .cause the `automatic volume control to .begin functioning. If the automatic volume control functions rapidly enough, .it may increase the amplincation almost as rapidly as the signal falls. Insuch a case the trigger circuit will release which in itself Vwill 4cause no significant action. However, as rotation .continues towards the point of minimum .responseJ the rate of signal change rapidly increases and may again exceed the speed of .action of the automatic volume control so that another tripping action will ensue. .In such a case .the .sense cycle is repeated and, after but a momentary hesitation, loop rotation is continued as before. It is thus evident that in either case proper lsense is assured.

When severe precipitation static exists, excessive noise is picked up by the sense antenna .|.2 that will only affect slightly a shielded loop antenna. Under vthis condition the sense -antenna circuit may be disabled. This will stillggive continuous :automatic direction ii-nding 'but rwith ambiguity. This ,may bev accomplished by operation of va switch |86. This grounds the sense .antenna l2 and adds -a resistor 202 in the cathode bias circuit of the vacuum tube |70. The resistor 202 should be large enough to hold the vacuum tube |10 Ycompletely extinguished dur-ing sense pulses.

All plate voltages may besecured from :a power source 62 which may be a battery, dynamo-tor or .any other convenient source.-

VThe laments of all vacuum tubes may secure power from the source 64 which will usually be a storage battery but need not be restricted to such. The wiring of the filaments is not illustrated on the drawing as such methods are well known.

The several positive biases previously mentioned may be secured from the source 64 but are not restricted to such a source as they could be obtained from the source B2 or some other source without in any way aiecting theV operation.

' Many obvious variations in the circuit described are possible without affecting its principles of operation. Among these are the following.

Any of. the reotiers may be thermionic diodes or other suitable types. I'he rectier 32 may be separated from the vacuum tube 3U.

Multigrid tubes may replace triodes and vice versa.

Selsyn motors may be used in place of indicator M6 for remote indication of azimuth. Deviation compensation may be associated with azimuth indication.

The loop rotating motor |24 may be controlled 'by a relay. Saturable core reactors controlling an alternating current motor could also be used.

Having thus described the invention, what is claimed as new and desired to be secured by Letters Patent is:

l. In a radio direction finder of the kind described, structure adapted to receive and transmit radio signals, said structure being rotatable,

whereby the amplitude of the transmitted signals changes as the structure rotates; automatic means for rotating said structure in either of two directions; an electric circuit connected with said structure for receiving signals therefrom; circuit elements operably connected with said electric circuit and responsive to the rate of change in amplitude in said signals for controlling said automatic means; and filtering means for rendering said elements less responsive to bursts of static received by said electric circuit.

2. In a radio direction finder of the kind described, structure adapted to receive and transmit radio signals, said structure being rotatable, whereby the amplitude of the transmitted signals changes as the structure rotates; automatic means for rotating said structure in either of two directions; an electric circuit connected with said vstructure for receiving signals therefrom; circuit elements operably connected with said electric `circuit and responsive to the rate of change in amplitude in said signals for controlling said automatic means; and lter means interposed between said electric circuit and said circuit elements for delaying the application of changes in -signal amplitude to said elements, whereby to reduce elects of bursts of static received by said electric circuit upon said elements.

3. In a radio direction finder of the kind described, structure adapted to receive and transmit radio signals, said structure being rotatable, -whereby the amplitude f the transmitted signals -changes as the structure rotates; automatic .means for rotating said structure in either of two directions; an electric circuit connected with said structure for receiving signals therefrom; 'circuit elements for controlling said automatic means; means interposed between said electric lcircuit and said circuit elements for applying voltage to said elements only during changes in amplitude of said signals; and lter elements for delaying the application of said voltage to said 'i elements.

4. In a radio direction finder of the kind described, structure adapted to receive VandV transmit radio signals, said structure being rotatable, whereby the amplitude of the transmitted signals changes as the structure rotates; automatic means for rotating said structure in either of two directions; an electric circuit connected with said structure for receiving signals therefrom; circuit elements for controlling said automatic means; means interposed between said electric circuit and said circuit elements for applying voltage to said elements only during changes in amplitude of said signals; lter elements for delaying the application of said voltage to said elements; and means for limiting the said 'voltage applied to said iilter elements.

5. In a radio direction finder of the kind described, structure adapted to receive and'transmit radio signals, said structure being rotatable, whereby the amplitude of the transmitted signals changes as the structure rotates; automatic means for rotating said structure in either of two directions; an electric circuit connected with said structure for receiving signals therefrom; circuit elements operably connected with said electric circuit and responsive to the rate of change in amplitude in said signals for controlling said automatic means; and an automatic volume control interposed in said electric circuit and having parts operable to control ampliiica-` tion of said signals outside a predetermined limited range.

6*. In a radio direction finder of Ythe kind described, structure adapted to receive and transmit radio signals, said structure being rotatable,`

cuit elements operably connected with said electric circuit and responsive to the rate of. change in amplitude in said signals for controlling said automatic means; and an automaticvolume control interposed in said electric circuit and having parts operable to control amplification of said signals outside a predetermined limited range, said parts including a pair of rectiers arranged to decrease and increase the amplification by said electric circuit as the magnitude of said signals increase and decrease respectively. '7. A radio direction nder comprising rotatable structure adapted to receive and transmit radio signals; means connected to said structure for receiving signals therefrom, said kmeans including an amplifier tube and means to rectify the carrier frequency voltage of said signals; automatic means for rotating said Vstructure in either of two directions to produce a change in amplitude of the incoming signals; circuit elements responsive to the rate of change in ampli- -as the magnitude of the signals increases .and

decreases respectively.

8. In a radio direction iinder of the kind described, rotatable structure adapted to receive and transmit radio signals; automatic apparatus for rotating said structure in either of two di.- rections to produce a change in amplitude of the incoming signals; a sense antenna; an electric circuit connected with said structure and said antenna for receiving signals therefrom; circuit elements operably connected with said electric circuit and responsive to the rate of change in amplitude of said signals for rendering said automatic apparatus operable to normally oscillate said structure about either of two possible positions where the magnitude of the signal received by said structure is at maximum; and means for applying the signals emanating from said sense antenna to the electric circuit to control said circuit elements only while said structure tend to oscillate about one of said positions.

9. A radio direction iinder of the character described comprising rotatable structure adapted to receive and transmit radio signals; automatic apparatus having reversible parts for rotating said structure in either of two directions to produce a change in amplitude of the incoming signals; an electric circuit connected with said structure for receiving signals therefrom; circuit elements operably connected with said electric circuit and responsive to the rate of change in amplitude of said signals for periodically reversing said automatic apparatus; and means for rendering said automatic apparatus operable to oscillate the structure about only one of two possible positions where the magnitude of the signal received by said structure is at maximum.

10. A radio direction iinder of the kind described comprising rotatable structure adapted to receive and transmit radio signals; automatic apparatus having reversible parts for rotating said structure in either of two directions to produce a change in amplitude of the incoming signals; a non-directional sense antenna; an electric circuit connected with said structure and said antenna for receiving signals therefrom; circuit elements operably connected with said electric circuit and responsive to the rate of change in amplitude of said signals for periodically reversing said automatic apparatus; and means including said sense antenna for rendering said automatic apparatus operable to oscillate the structure about only one of two possible positions where the magnitude of the signal received by said structure is at maximum, said means having parts for directing signals from said sense antenna to said electric circuit only momentarily at each reversal of said automatic apparatus.

11. A radio direction finder of the kind described comprising a rotatable structure adapted to receive and transmit radio signals; automatic apparatus having reversible parts for rotating said structure in either of two directions to produce a change in amplitude of the incoming signals; a non-directional sense antenna; an electric circuit connected with said structure and said antenna for receiving signals therefrom; circuit elements operably connected with said electric circuit and responsive to the rate of change in amplitude of said signals for periodically reversing said automatic apparatus; and means including said sense antenna for rendering said automatic apparatus operable to oscillate the structure about only one of two possible positions Where the magnitude of the signal received' -duce a change in amplitude of the incoming signals; a non-directional sense antenna; an electric circuit connected with said structure and said antenna for receiving signals therefrom;

circuit elements operably connected with said electric circuit and responsive to the rate of change in amplitude of said signals for periodically reversing said automatic apparatus; and means including said sense antenna for rendering said automatic apparatus operable to oscillate the structure about only one of two possible positions where the magnitude of the signal received by said structure is at maximum, said means having parts operable to control said circuit elements only when the signals received by said sense antenna are incorrectly phased with the signals received by said structure.

JOSEPH G. SPEER.

REFERENCES CITED The following references are of record in the fue of this patent:

UNITED STATES PATENTS Number Name Date Re. 23,066 Moseley Dec. 21, 1948 2,257,757 Moseley Oct. 7, 1941 2,423,337 Moseley July 1, 1947 2,455,939 Meredith Dec. 14, 1948 2,495,591 Meredith Jan. 24, 1950

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