US2110852A - Radio direction finder - Google Patents

Description

March 15, 1938.

E. D. BLODGETT RADIO DIRECTION FINDER Filed Dec. 51, 1935 Patented Mar. 15, 1938 UNETED STATES PATENT QFFIQE RADIO DIRECTION FINDER tion of Delaware Application December 31, 1935, Serial No. 56,861

5 Claims.

My invention relates to radio direction finders. More specifically, my invention is a multi-frequency range radio direction finder in which a plurality of inductors are arranged for tuning with single variable capacitors. A suitable multi-pole, multi-position switch is used to simultaneously connect the several inductors of a frequency range to their respective tuning capacitors.

It has been the practice to tune the antenna of a radio direction findersystem with a variable inductor when the system is used for determining the sense of direction. The loop may be resonated by a variable capacitor and is rotated for determining the bearing with the antenna detuned. There are several drawbacks to the variable inductor tuning: one, it is. bulky; two, it is expensive; three, the inductor does not track with a variable capacitor; four, a multiplicity of variable inductors is required in a multi-range instrument; and live, the impedance characteristic .is not suited to the balance of the system.

I propose to overcome these difficulties and to gain certain advantages in performance by the novel means and arrangement I am about to disclose. One of the objects of my invention is embodied in the design of a multi-frequency range radio direction finder.

Another object is in an arrangement in which all of the tuning is effected by variable capacitors.

A further object is in the novel switching and detuning means which are embodied in my invention.

A still further object is in the design of a direction finder system in which the impedance characteristic of the antenna system is suitably related to the associated circuits and apparatus.

Additional objects will appear in the accompanying specification, claims and drawing to which reference is made for a proper understanding of my invention.

An antenna I is connected through a resistor 3 to ground. A trimmer capacitor 5 is connected across. the resistor 3. A small capacitor I couples the antenna to the movable contactor 9 of theswitch section II, and to the stator of tuning capacitor I3. The rotor of this capacitor is grounded.

Three inductors I5, I1, and I 9.01" suitable inductance values are connected to the fixed terminals of the switch section ii, and to the fixed terminals of a second switch section 2 I. The movable contact member 23 of this switch section 2| is connected to the lower left hand terminal of a double pole, double throw switch 25. The upper left hand and lower right hand terminal of the switch 25 are connected to ground.

Each of the three inductors I5, I! and I9 are tapped intermediate their ends. The taps of each inductor are connected through suitable variable resistors 21, 29, 3| to the fixed terminals of a third switch section 33. The movable contact element of this switch section 33 is connected to the upper right hand terminal of the double pole, double throw switch 25. Trimmer capacitors 35, 31, 39 are connected, respectively, between the upper terminal of each of the inductors l5, l1, l9 and ground.

The portions of the inductors between their upper ends and the taps are chosen so that each may be tuned throughout the required frequency range by the single variable capacitor I3. The

portion below the tap is chosen to have the required detuning effect when the receiver is operated on bearing balance as will be described below. The several resistors are of such values as will give the required broadness of tuning of the antenna circuit.

The upper movable blade of the double pole, double throw switch is connected to the primary M of a sense transformer 43. The primary winding is grounded and suitably shielded from its secondary by a shield 45. The lower movable blade of the double pole, double throw switch 25 is connected to the primary 4'! of a balance or bearing transformer 49. The primary is grounded and shielded from its secondary by a conductor 5!. The coupling between the primary 4! and the secondary is variable.

The loop circuit consists of the loop 53. The terminals of the loop are connected to terminals 55-57. One of these terminals 55 is connected to the serially connected loading coils 59, BI. The lower coil is connected to the secondary 53 of transformer 43. The remaining terminal of the secondary is connected to the variable capacitor and to the grid of radio frequency amplifier 91.

The remaining terminal of the loop circuit 51 is connected to the connected loading coils 61, 69. The lower coil 69is connected to the secondary II of the balance transformer 49. The upper terminal of this secondary II is connected to the remaining terminal of the variable tuning capacitor 65. A small trimmer capacitor I9 is connected between the tuning capacitor and ground to balance the grid-cathode capacitor of the radio frequency amplifier tube 91.

The several frequency ranges of the loop circuit are obtained by selectively varying the insuitable audio frequency amplifier I33.

ductance included in the loop circuit. One method of varying the inductance is to use both loading coils on each side of the loop circuit in series, using one and shorting the other load coil on each side of the loop circuit, or shorting both loading coils on each side of the loop circuit. The switching just described, is accomplished by four sections of three-way switches, l3, l5, ll, I9.

In ordinary practice, it is desirable to employ trimmer capacitors to line up the several tuning ranges of the loop circuit. One trimmer capacitor BI is connected across the loop circuit. Three additional trimmer capacitors 33, 85, ill are suitably connected by a pair of switch sections 89-4)! so that each frequency range is provided with a trimmer capacitor. It should be understood that the trimmer capacitors in the antenna, loop, and other circuits of this application are for alignment at the high frequency ends of each tuning range. The main variable tuning capacitor 65 of the loop circuit does not employ a grounded rotor.

Although numerous arrangements of amplifier and detector circuits may be connected to the loop circuit, I prefer a superheterodyne preceded by a radio frequency amplifier. The radio frequency stage is represented by the numeral $33. The control grid 95 of a radio frequency amplifier tube 9? is connected to one terminal of the variable tuning capacitor 65. The cathode S9 is connected through a self-biasing resistor IEll to ground. This resistor may be by-passed. The screen grid I03 is connected to a source of positive biasing potential. A suppressor grid iiit is shown connected to the cathode 99.

The anode Ill! of radio frequency amplifier tube 81' is connected through suitable coupling means to the first detector 869. The anode is also connected to a variable tuning capacitor III. The rotor of this capacitor is preferably grounded. The anode is also connected to the movable contact MB of a switch section IE5. The three fixed contacts of this switch section H5 are connected to three inductors H7, H9, IZI. The lower terminals of these inductors are connected to each other and to the movable contact of a switch section I23. The fixed contacts of this switch section I23 are connected to the inductors so that an inductor not in use, but whose natural frequency falls within the tuning range in use, may be short circuited to avoid undesirable coupling effects.

Each of these inductors may be shunted with a trimmer capacitor for alignment at the high frequency ends of their tuning ranges. The common terminal of these conductors is connected through a bypass capacitor I25 to ground and to a positive terminal of the 3 supply circuit.

The radio frequency amplifier 93 is coupled to the first detector I09, as explained above. The first detector I09 is coupled to an intermediate frequency amplifier I27 through suitable intermediate frequency transformers or the like. The output of the intermediate frequency amplifier I2? is impressed on the second detector I29. A local intermediate frequency oscillator Isl may be used for heterodyne r ception. The output of the second detector IZQ may be amplified by a The output of this amplifier is represented by the numeral I which may be telephone receivers, meters .or the like. The power supply for the heaters, biases, screen grids and anodes is represented by the numeral I31. The foregoing elements are so well known to those skilled in the art, and

not being a part of my invention per se, have not been shown.

The first detector IDS is supplied with local oscillatory currents which combine or mix with the incoming signal representing currents to form intermediate frequency currents. The local oscillations are generated in the conventional manner by a thermionic tube I39. The control grid MI of this tube is connected through grid leak resistor I53 to ground, and through grid capacitor M5 to the stator of a variable tuning capacitor I41. The rotor of this capacitor is grounded.

The stator of the tuning capacitor I ll is connected to the movable element of a switch section I 49. The fixed switch contacts are, respectively, connected through oscillator padding capacitors IEI, I53, I55 to the upper terminals of inductors I51, I59, Ifil. The padding capacitors I51, I59, and I6I may be shunted with trimmer capacitors i63, I64, and 35. The lower terminals of the three inductors I51, I59, IISI are connected to ground. Trimmer capacitors I56, I67, I68 are connected from the fixed contacts of switch section Hill to ground.

Taps intermediate the ends of the inductors I57, I59, lfil are connected to the fixed contacts of a switch section I69. The movable contact I'II of this switch section is connected to the cathode N3 of the oscillator tube I39. Two switch sections H5, II'I are suitably connected to the oscillator inductors so that an unused inductor, whose natural frequency of oscillation would fall within the range of the inductor in use, may be short circuited to avoid the deleterious eifects of coupling between circuits tuned to the same frequency.

The suppressor grid I'i9 of the oscillator tube 538 is connected to ground. The screen grid IBI is connected through resistor !83 to a suitable positive biasing potential. The anode I84 of the oscillator is connected to a screen grid electrode in the first detector we. The positive potential for the anode is derived through a resistor I85.

The several switch sections which are employed in the antenna, loops, radio frequency amplifier, and local oscillator circuits are connected by a common shaft I81. A single control knob I89 connected to the common shaft may be used to simultaneously operate all the switch sections. The rotors of the variable tuning capacitors in the antenna, loop, radio frequency, and local oscillator are likewise mounted on a common shaft IQI. A single control knob I93 connected to the shaft ISI simultaneously varies the several rotors for tuning to resonance,

Having described one embodiment of my invention, I shall now describe the operation. It should be understood that the frequency range of a direction finder is determined by the frequency of the transmitters whose direction is to be determined. The receiver frequency band having been determined for a particular transmitter, the double pole, double throw switch 25 is first turned to the left. In this position, the primary of the sense transformer 43 is grounded. The inductor of the required frequency range is connected with its maximum inductance in the antenna circuit. This detunes the antenna and eliminates the efiect of the sense direction through the ungrounded transformer 49.

The frequency knob I93 is varied until the receiver including the loop circuit, is tuned to resonance with the incoming signal. The loop circuit being symmetrically arranged, has a sensitween antenna and loop circuit currents.

tivity characteristic of a figure 8 with respect to the magnetic field radiated by the transmitter under observation. The loop is rotated about its vertical axis until a minimum or null signal response is obtained. The position of minimum response is that in which the plane of the loop is normal to the direction of the transmitter. The bearing of the transmitter is obviously on a line at right angles to the plane of the loop.

Thus having determined the bearing, it is now desirable to determine the sense of direction of the transmitter. The double pole, double throw switch 25 is now thrown toward the right. In this position, the primary of the bearing or balance transformer 49 is grounded. The antenna circuit is completed through the proper resistor to the tap on the inductor. The tap insures that the antenna circuit, already tuned by the capacitor is resonant to the required frequency. The resistor insures that the resonant point of the antenna is not too sharp for suitable tracking with the other tuned circuits, and that the proper amount of energy is fed to the loop circuit. The double throw, double pole switch connects the antenna circuit to the loop circuit through the sense transformer 43.

The loop may now be rotated about its vertical axis to indicate the sense of the direction of the transmitter. That is, the maximum signal will indicate that the transmitter lies along the previously determined bearing, and in the direction of the maximum response. The position of maximum response for a given installation may be determined with respect to a known side of the loop by calibration with respect to stations of known directions.

As previously indicated, the use of a variable inductor in the antenna circuit is not desirable because the tracking characteristic of a variable inductance and a circuit tuned with an ordinary variable capacitor are far from similar. With capacitors having similar rates of variation in the several tunable circuits, inductors of the properly related values may be used to insure substantially perfect tracking.

When the switches are in position for the balance or bearing indication, the antenna circuit is preferably tuned to a frequency about 15% lower than the incoming signal frequency. This detuning provides the required phase relation be- It should be understood that by appropriate changes the antenna inductors could be tuned to frequencies 15% higher instead of lower. These percentages are given merely by way of example as the exact percentage will depend upon the characteristics of the circuits employed.

When operating the sense indication, the resistors serve two purposes: one, broadening the resonant point; two, controlling the amplitude of the currents of the antenna circuit. Normally, the ratio of antenna energy to loop energy required for the bearing balance is greater at the high frequency end of a range of frequencies than at the low frequency end of the range.

Since the effective height of the loop increases with increasing frequency more rapidly than the effective height of the antenna, it is desirable to increase the transfer of energy from the antenna circuit to the loop circuit at the higher frequencies in a given band to maintain a substantially uniform ratio of balance energy throughout the band. By the proper choice of inductor and trimmer capacitor values, the detuning for resonance is made less at the high frequency end of the hand than at the low frequency end. This adjustment will give the required energy ratios.

By way of example, I have found a direction finder embodying the following frequency bands or ranges very useful in practice: A range 1500 to 540 kilocycles, B range 600 to 225 kilocycles, and C range 250 to 100 kilocycles. It should be understood that more or less frequency ranges may be employed. Likewise, a greater or lesser range of frequencies within a given band may be used.

I do not limit my invention to the precise switching means shown which is merely illustrative. The several inductors may be separately shielded, separately trimmed, and switched into or out of the circuits as a whole. Although it is usually more convenient to operate a single knob to switch to any of the frequency bands, separate switching in the several circuits may be employed.

Numerous modifications within the scope of my invention will occur to those skilled in the art. My invention is not to be limited to the precise embodiment shown and described, but is only limited as required by the prior art and appended claims.

I claim as my invention:

1. In a radio direction finder, a tunable loop circuit comprising a fixed loop inductor and variable capacitor, a tunable antenna circuit comprising an inductor and variable capacitor, a pair of transformers connecting said loop and antenna circuits, means for simultaneously varying said variable capacitors, means for changing the value of said inductor thereby detuning said antenna circuit and means for coupling through one of said transformers for determining the bearing of a transmitter by balancing the currents created by the radiation of said transmitter in said loop circuit, means for regulating the energy transferred from said antenna circuit and means for coupling through the other of said transformers for determining the sense of direction of said transmitter, and means for short circuiting the primary of the transformer which is not in use when either bearing or sense directions are being determined.

2. In a multi-frequency range radio direction finder, a tunable loop circuit comprising a fixed loop inductor, a variable capacitor, and a plwrality of loading inductors; means for selectively switching said loading inductors with respect to said loop circuit; tunable antenna circuit com.- prising a plurality of loading inductors and a variable capacitor; means for selectively switching said loading inductors with respect to said antenna circuit; a common control for said variable capacitors; a second common control for said switching means; a sense transformer having its secondary in said loop circuit; a balance transformer having its secondary in said loop circuit; means for detuning said antenna circuit during bearing determination; means for tuning said antenna circuit during sense of direction determination; means for substantially eliminating the effect of said sense transformer during bearing determination, and means for substantially eliminating the efiect of said balance transformer during sense of direction determination.

3. In a multi-frequency range radio direction finder, a tunable loop circuit comprising a fixed loop inductor, a variable capacitor, and a plurality of loading inductors; means for selectively switching said loading inductors with respect to said loop circuit; tunable antenna circuit comprising a plurality of loading inductors and a variable capacitor; means for selectively switching said loading inductors with respect to said antenna circuit; a common control for said variable capacitors; a second common control for said switching means; a sense transformer having its secondary in said loop circuit; a balance transformer having its secondary in said loop circuit; means for detuning said antenna circuit during bearing determination; means for tuning said antenna circuit during sense of direction determination; means for substantially eliminating the effect of said sense transformer during bearing determination, means for substantially eliminating the effect of said balance transformer during sense of direction determination, and means for detuning said antenna a smaller percentage of frequency at the higher frequencies than at the lower frequencies of the range.

4. In a device of the character of claim 2, means for broadening the tuning of said antenna circuit when said direction finder is operated to determine sense of direction.

5. In a device of the character of claim 3, means for broadening the tuning of said antenna circuit when said direction finder is operated to determine sense of direction.

EDWARD D. BLODGETT.

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