US3395351A - System for shifting resonant frequency of an antenna - Google Patents

Description

July 30, 1968 KEYING SIGNAL SOURCE TRIGGER CONTROL l8 KEYING souhbe C2 3:

L. F. DEISE ET AL SYSTEM FOR SHIFTING RESONANT FREQUENCY OF AN ANTENNA Filed NOV. 9, 1967 INVERTER SIGNAL SOURCE To TRI GGER Tb CONTROL FIG.|.

CONTROL CURRENT SOURCE LUMJ INVERTER 2 Sheets-Sheet l SOURCE l I P I P Ll r c -z+z RF SOURCE l and Henry A. Musk.

ATTORNEY July 30, 1968 5 55 ET AL SYSTEM FOR SHIFTING RESONANT FREQUENCY OF AN ANTENNA 2 Sheets-Sheet 2 Filed Nov. 9, 1967 TIZ SOUIIRILI;

INVERTER FIG.5.

L m0 GD m 7 I R 2 O 3 C B 7 T Mu I N R U G U I 0 K55 D.C. SUPPLY 6 G n In E m T JQ I I FIG? '3 TIME FIG.8.

TIME

ABSTRACT OF THE DISCLOSURE The present disclosure relates to a system for controlling the resonant frequency of the antenna of a highpower, low-frequency transmitter in which a saturable reactor is utilized in the resonant circuit of the antenna. The inductance of the saturable reactor is controlled to effect the frequency shift in the transmitter, with the inductance being controlled by changing the current level supplied to the saturable reactor. The current level supplied to the saturable reactor is controlled through controlled switching devices wherein selected groups of the devices are placed in a selected conductive state to provide a first current level; to increase the current level to a second level; to maintain the second level; and to reduce the current level to the first level when desired.

Cross reference to related application This is a continuation-in-part of application Ser. No. 502,223, filed Oct. 22, 1965, now abandoned.

Background of the invention The present invention relates to circuitry for controlling the inductance of a circuit, and more particularly to circuitry for rapidly varying the resonant frequency of the antenna of high power radio transmitters.

For very long range, highly reliable communications, high power, very low frequency transmitters are advantageously utilized, especially in military applications. The antenna systems for such high power, very low frequency transmitters typically have a quite narrow bandwidth, which restricts the rate of transmitting intelligence to very low speeds. The rate of transmitting intelligence can be increased, however, by using frequency shift keying and shifting the resonant frequency of the antenna system in synchronism with the keying frequency of the transmitter. The resonant frequency shift of the antenna system may be accomplished by connecting a saturable reactor in the antenna resonant circuit and varying the inductance of the saturable reactor. This will in turn vary the resonant frequency of the antenna.

Because of the high power requirements of very low frequency transmitters, it becomes quite difiicult to provide the necessary change in inductance of the saturable reactor in the short time period necessary without the excessive dissipation of power in the control circuitry. For example, in a teletype system transmitting 60 words per minute in a 500 kilowatt very low frequency transmitter, to obtain a 100 cycle frequency shift, it would be necessary to provide a to 1 increase of control current, e.g., from 25 amperes to 125 amperes in approximately 2 milliseconds. To bring about a 5 to 1 change of current in the required time through the use of a resistor placed in series with the control winding of the saturable reactor, it would require over 100 kilowatts of control power.

Vacuum tube circuitry is taught in US. Patent No. 2,853,634, issued Sept. 23, 1958, by L. F. Deise, assigned to the same assignee as the present invention, which re- United States Patent 0 3,395,351 Patented July 30, 1968 Summary of the invention It is, therefore, an object of the present invention to provide a new and improved system for rapidly changing the inductance of a circuit.

It is a further object to provide a new and improved system for shifting the resonant frequency of a tuned circuit while requiring relatively little control power.

It is a further object to provide a new and improved system for shifting the resonant frequency of an antenna of a high power transmitter which utilizes relatively little power and is highly reliable.

It is a further object to provide a new and improved control system utilizing a saturable reactor wherein the control current is controlled to vary the inductance thereof and, in turn, the resonant frequency of a high power antenna system.

Broadly, the present invention provides a new and improved system for controlling the resonant frequency of an antenna of a high power transmitter in which a saturable reactor is included in the resonant circuit of the antenna, with the inductance of the saturable reactor being controlled through the selective switching of controlled switching devices which permit: the necessary transfer of control current to the control winding of the saturable reactor to bring about the desired resonant frequency shift.

Brief description of the drawings These and other objects and advantages of the present invention will become more apparent when considered in view of the following specification and drawings in which:

FIGURE 1 is a schematic diagram of one embodiment of the present invention;

FIG. 2 is a waveform diagram of the control current as developed in the circuit of FIG. 1;

FIG. 3 is a waveform diagram showing curves A, B and C which aid in explaining the operation of the circuit of FIG. 1;

FIG. 4 is a schematic diagram of another embodiment of the present invention;

FIG. 5 is a schematic diagram of still another embodiment of the present invention;

FIG. 6 is a waveform diagram of the control current as developed in the circuit of FIG. 5;

FIG. 7 is a waveform diagram of the voltage developed across the charging capacitor C3 of FIG. 5; and

FIG. 8 is a waveform diagram including curves A, B, C, D and E which are used in explaining the operation of the circuit of FIG. 5.

Description of the preferred embodiments Referring to FIG. 1, a circuit is shown capable of shifting the resonant frequency of an antenna of a very low frequency high power transmitting system. The shifting of the resonant frequency of the antenna is done in synchronism with a keying signal which is used to supply modulation to the transmission of information from the transmitter. For example, a telegraph code might be used with dashes of a first time period being transmitted at a given frequency, dots being transmitted of a shorter time duration at the same frequency as the dashes, and the time period between dots and dashes being transmitted at the shifted frequency from the dot and dash transmitted frequency.

,II 1 FIG. 1, an antenna A1 is shown schematically in? cluding an antenna load coil L1 and the capacitance of the antenna, shown, symbolically as a capacitor C1, connected across the antenna to ground. Radio frequency (RF) energy is supplied to the antenna A1 from an input coil L2, which is connected to an RF source 1 which supplies a constant voltage at R)? frequencies, for example, 18 to 40 kc. in the present low frequency transmitter. The input coil L2 is coupled to a coil L3 which is connected as part of the antenna load coil to ground.

A saturable reactor SR is provided in order to control the inductance of the antenna A1 and thus its resonant frequency. The saturable reactor SR includes a pair of saturable magnetic core members 2 and 4, an input control winding L4 and a pair of output windings L5 and L6 connected in series. A capacitor C4 is connected across the control winding L4 in order to keep RF power from the antenna out of the control circuit. The free end of the winding L5 is connected to a tap 6 on the antenna load coil L1 and the free end of the winding L6 is grounded. The saturable reactor SR is thus connected in parallel with a portion of the antenna load coil L1. By controlling the inductance presented by the saturable reactor SR, it will control the inductance of the antenna A1 and therefore the resonant frequency of the antenna A1.

As is well known, the inductance provided by a saturable reactor may be controlled by the application of different levels of energizing current to its input control winding. That is, if a high current large enough to drive the saturable reactor into its saturated state is applied to the control winding, a relatively low value of inductance will be supplied. However, if a relatively low value of current which will keep the saturable reactor in its unsaturated state is applied to the control Winding, the saturable reactor will provide a relatively high value of inductance.

In FIG. 2, a waveform diagram of a control current versus time is shown wherein the current level 1 typifies the unsaturated low current case with a current level of, for example, 25 amperes, and the current I typifies the saturated high current state with a current level of, for example, 125 amperes. A substantially trapezoidal current waveform is provided with the current increasing from 1 to I during the times t to t and then remaining substantially constant at the current 1 until the time t and then decreasing between the times t and L to the current level I The control circuitry necessary to effect the different control currents will now be described. A DC source 8 is provided to supply the input power for the control circuitry. Connected across the output terminals of the DC source 8 is an energy storage capacitor C2. The DC source 8 serves to drive an inverter 10 which operates to convert the DC output of the source 8 to an alternating one, for example, a square wave output having a frequency of 2,000 to 10,000 cycles per second. The alternating square Wave output of the inverter 10 is supplied to a winding W which forms the primary winding of a transformer TRl. The transformer TR1 includes three center tapped secondary windings W1, W2 and W3. The windings W1, W2 and W3 each have a different number of turns to define different turns ratios with winding W0 as will be discussed below. The ends of the windings W1, W2 and W3 are connected, respectively, to the anode electrodes of a pair of silicon controlled rectifiers (SCRs) S1 and S2, S3 and S4, and S and S6. The cathodes of the SCR pair S1 and S2, the SCR pair S3 and S4, and the SCR pair S5 and S6 are commonly connected through a lead 12 to the top end of the control winding L4. The center taps of the windings W2 and W3 are connected through a lead 14 to the bottom end of the control winding L4, while the center tap of the winding W1 is connected through a resistor R1 to the lead 14. One end of a resistor R2 is connected to the center tap of the Winding W1, with its other end being connected to the anode of a diode D1. The cathode of the diode D1 is connected to the lead 12.

The gate electrodes of the SCR pair S1 and S2 are commonly connected to an output terminal Ta of a trigger control circuit 16. The gate electrodes of the SCR pair S3 and S4 are commonly connected to an output terminal Tb of the trigger control circuit 16. The gate electrodes of the SCR pair S5 and S6 are commonly connected to an output terminal Tc of the trigger control 16. From the output terminals Ta, Tb and Tc are supplied the pulse wave forms as shown in the respective curves A, B and C of FIG. 3. The application of a positive polarity signal to the gate electrodes of a given SCR will render the device conductive to permit the passage of current from anode to cathode if the anode has a positive polarity voltage with respect to the cathode. Thus, in the connection as shown with the cathode and gate electrodes being respectively commonly connected, the SCR pairs will act as full wave rectifiers to translate the alternating signals from the inverter 10 therethrough to the control winding L4 of the saturable reactor SR whenever a given pair of SCRs are gated on.

The trigger control circuit 16 may be of any standard design to provide pulse output waveforms as shown in FIG. 3 in a given time sequence thereof and may comprise well known logic circuits capable of providing such pulse output waveforms. The trigger control 16 is responsive to a keying signal source 18 which has its output connected to the input of the trigger control 16 to provide a keying signal 20 thereto, which may have a square waveform for example. The keying pulses supplied by the keying signal source 18 are supplied to the trigger control 16 in synchronism with the transmitter keying frequency so that the shift of the resonant frequency of the antenna will occur in synchronism with the change in transmitter frequency.

Referring now to FIGS. 1, 2 and 3, the operation of FIG. 1 is such that at a time prior to the time t at the beginning of the frequency shift cycle, a pulse P1 is applied from the terminal Tc of the trigger control 16 to the gate electrodes of the SCRs S5 and S6 which permits the passage of current therethrough from the inverter 10, through the transformer TR1, via the primary winding W0 and the secondary winding W3 connected to the anodes of the SCRs S5 and S6. The turns ratio W0; W3 of the windings W0 and W3 is selected so that a sufficient output voltage is developed across the winding W3 to maintain the current level supplied to the control winding L4 at the 1 level as shown in FIG. 2.

The rectified output of the SCRs S5 and S6 are supplied to the control winding L4 of the saturable reactor SR. With the controlled switches S5 and S6 conductive, the control current is held at a level I as shown in FIG. 2. At the time t the control rectifier pair S1 and S2 is turned on by the application of a pulse P2 from the terminal Ta of the trigger control 16 to the gate electrodes thereof, while the pulse P1 is removed from the gate electrodes of the control rectifiers S5 and S6 which become nonconductive. The pulse P2 lasts until the time t with the current building up from the value I to the value I at the time t The energy required for increasing the current to the value I is supplied from the inverter 10 and the transformer T=R1 via the winding W0 and W1. The number of turns for the winding W1 is selected to define a turns ratio W0, W1 so that a sufficiently large voltage is developed across the winding W1 to cause the rated buildup of current from the value 1 to the value I is the relatively short period of time t t as shown in FIG. '2. The required high instantaneous energy is supplied by the capacitor C2 connected across the DC power supply 8 to permit the increase of current as shown.

At the time t the pulse P2 is removed from the gate electrodes of the control rectifiers S1 and S2 so that these devices are turned off, while a pulse P3 is applied to the gate electrodes of the SCRs S3 and S4 to render them conductive. The control current remains constant at the current value I with the rectifier pairS3 and S4 conductive so that the current I is supplied to the control winding L4 until the time t If the rectifier pair S1 and S2 were not turned ofi at the time t the control current would tend to increase as shown along the dotted line. However, the turning off of the rectifier pair S1 and S2 permits the current to be maintained at the current level I with the rectifier pair S3 and S4 being conductive. The resistor R1 will, however, limit the current to a safe value if the control devices S1 and S2 fail to block. The number of turns on the winding W2 is selected so that the turns ratio W; W2 supplied the proper voltage across the winding W2 to maintain the current level I through the control windings L4 with the controlled rectifiers S3 and S4 conductive.

At the time t;,, a short time duration pulse P is applied to the gate electrodes of the control rectifier pair S1 and S2. The duration of the pulse P4 is made short enough that only a single half cycle of the supply frequency will pass therethrough. At the reversal of the next half cycle, pulse P4 will have been removed which will then permit the rectifierpair S1 and S2 to return to its nonconductive state. Also at the time t the pulse P3 is removed from the rectifier pair S3 and S4 which turns olf this pair of devices. The gating on the rectifier pair S1 and S2 for onehalf cycle is done in order to permit the control rectifier pair S3 and S4 to turn oil at the time t which may otherwise not occur due to the high current passage therethrough. With all three rectifier pairs S1-S2, S3-S4, and S5486 nonconductive, the control current will decay through the diode D1 and the resistors R1 and R2, as shown in FIG. 2, to reach a current level I at the time 1 at the end of the frequency shift cycle.

At the time t the controlled rectifiers S5 and S6 are gated on by a pulse P5 supplied by the trigger control 16 at its output terminal Tc, which turns on the devices S5 and S6 to hold the control current at the level I If the control rectifiers S5 and S6 were not gated on at the time 12;, the control current would decrease toward zero on the dotted curve as shown. However, the current is held at a value I which is desired control current for supplying a predetermined inductance and resonant frequency for the antenna A1. The frequency shift cycle would repeat itself in response to keying signals from the keying signal source 18 to efiect the change in control current again from the level I to the level I The control power may be calculated in a typical system assuming the following circuit parameters:

Inductance of the saturable reactor Also, assume that a 60 cycle per minute teletype is being utilized, with control current being switched from one level to the other and back again at a rate of 25 times per second so that each switching must be accomplished in approximately 2 milliseconds. If the current shift from 25 amperes to 125 amperes is accomplished in 1.7 milliseconds and the decay from 125 amperes to 25 amperes is accomplished in 2.3 milliseconds, the average power from the control rectifiers S1 and S2 at the maximum switching rate would be approximately 5,600 watts. There would be instantaneous peaks somewhat over 200 kilowatts, but these peaks of power are supplied from the capacitor C2, which will maintain a reasonably constant drain on the power supply. Since the actual inductance of the saturable reactor changes with control current, the

average power required would be somewhat less than the value indicated which was calculated for a constant inductance of 20 millihenries. If the control rectifier pairs 83-84 and S5-S6 operate into .2 ohm loads, the average power required therefrom would be approximately 1,625 watts, making a total average power for the three rectifier pair to be 7,225 watts. This average power requirement is substantially less than 10% of the power required if resistances alone were utilized to accomplish the change in control current levels.

FIG. 4 shows a circuit which may be advantageous in certain applications in which a separate winding L7 is provided on the saturable reactor SR to which is applied the control current I from a source 24 which is coupled thereto through a blocking inductor L6. By the use of the separate winding L7, to supply the current I the rectifier pair 8,; and S may be eliminated from the circuit of FIG. 1 as well as reducing the power requirement from the rectifier pair S3 and S4. The circuit of FIG. 4 is otherwise substantially similar to that of FIG. 1.

During the time period that the current I level is de sired to provide a predetermined resonant frequency for the antenna Al, the current I is supplied to the Winding L7. At these times, no current is supplied to the control winding L4 since both the rectifier pairs S1 and S2 and S3 and S4 are in their nonconductive states. At the time it is desired to change the current level from I to 1 the rectifier pair S1 and S2 is switched on by the application of a gating pulse to the gate electrodes thereof. The current then in the control winding L4 increases from a zero level toward a level I2I1. 'With the above design criteria this current level will be reached in approximately 2 milliseconds. The turns ratio W0, W1 of the transformer TR1 in FIG. 4 is so selected that a suflicient voltage is developed across the Winding W1 to increase the current level to I2/I1 in the control winding L in the desired period of time. At this time, the control rectifier pair S3 and S4 is switched on and the control rectifiers S1 and S2 are rendered nonconductive. Thus, the conduction of current through the rectifiers S3 and S4 will hold the current level in the control winding L4 to a value I2 I1- The turns ratio W0, W2 of the transformer TR1 of FIG. 4 is selected so that the voltage developed across the winding W2 is suflicient to provide the current I -I through the control winding W The total control current through the saturable reactor SR is then 1 with I being supplied to the controd winding L7 and the current Ig-I being supplied to the control Winding L4. The control current can be returned to the I level by the removal of a gating pulse from the gate electrodes of the rectifier pair S3 and S4 so that this pair will be nonconductive with the current through the winding L4 decaying through the diode D1 and the resistors R1 and R2 to zero current. The I level is then supplied to the other control winding L7 from the source 24.

Referring now to FIG. 5, another embodiment of the present invention is shown which eliminates the resistors R1 and R2 and the attendant losses therein and also reduces the power requirements of the inverter 10. In the circuit of FIG. 5, stored energy is transferred from a storage element, a capacitor C3, to the inductance of the saturable reactance control winding L4, with losses being made up by controlled rectifier pairs. The circuit of FIG. 5 includes similar components as FIG. 1 for the antenna Al, the saturable reactor SR, the DC power source 8, the inverter 10, the RF source 1 and the keying signal source 18. A high voltage DC power supply 30 is however also provided to charge the capacitor C3 to the polarity as shown. A controlled rectifier device S7 is connected at its anode electrode to the high voltage DC power supply 30' and its cathode is connected through a coil L7 to the positive side of the capacitor C3. The gate electrode of the controlled rectifier S7 is connected to a trigger control circuit 32 at an output terminal T7. The trigger control circuit is of such a design to provide the pulse output at 7 its various output terminals as shown in FIG. 8, the output from the terminal T7 being shown in the curve D thereof. The negative end of the capacitor C3 is returned to the high voltage DC power supply 30.

The storage capacitor C3 is connected across a bridge circuit including silicon controlled rectifiers S8, S9, S10 and S11. The gate electrodes of the control rectifiers S8 and S9 are connected to output terminals T8 and T9, respectively, of the trigger control 32. The gate electrodes of the controlled rectifiers S10 and S11 are connected to output terminals T10 and T11, respectively of the trigger control 32. The trigger control signals provided at the terminals T 8 and T9 are shown in curve A of FIG. 8, and the signals provided at terminals T10 and T11 are shown in curve B. The cathodes of the control rectifiers S9 and S11 are commonly connected to the lead 12 which is connected to the control winding L4 of the saturable reactor SR. The anodes of the control rectifiers S8 and S10 are commonly connected to the other end of the control winding L4. The cathodes of the control rectifiers S8 and S10 are connected, respectively, to the anodes of the SCRs S11 and S9, to complete the bridge circuit.

Rectifier pairs S12-S13 and S14S15 are provided and are connected similarly to the rectifier pairs S3-S4 and 55-86 of FIG. 1. The rectifier pairs S14S15 and S12- S13 have their cathode electrodes commonly connected to the lead 12 with anode electrodes connected across windings W4 and W5, respectively, which are the secondary windings of a transformer TR2, which has primary windings W6 and W7 that are supplied from the inverter 10. The center taps of the secondary windings W4 and W are connected to the bottom end of the control winding L4 of the saturable reactor SR.

Assume initially that the capacitor C3 has been charged from the high voltage DC power supply 30 to a predetermined potential E as shown in FIG. 7, at a time prior to the time t Assume also that a pair of controlled rectifiers S12 and S13 are keyed on at a time prior to the time t to permit the control current to reach a current level 1 The gate electrodes of the rectifier pair S12S13 are connected to an output terminal T12 of the trigger control 32, with the cathode electrodes thereof being connected to the lead 12, and the anode electrodes connected across a secondary winding W5 of a transformer TR2 which is supplied from an output winding W7 of the inverter 10. The turns ratio W7, W5 is selected so that a sufiicient voltage is developed across the winding W5 in response to the energization of the winding W7 by the inverter to supply the current 1 to the control winding L4 via the SCR pair S12S13. The current level prior to the time t of I is shown in FIG. 6.

At the beginning of the resonant frequency shift cycle at the time t gate pulses are removed from the gate electrodes of the control rectifiers S12 and S13, as shown in curve E, and are applied to the rectifier pair 58-39, as

shown in curve A. The SCRs S8 and S9 are thus turned on, and the control rectifiers 12 and 13 are turned off. The charge on the capacitor C3 will cause the current through the control winding L4 to increase from 1 to I; by the time t as shown in FIG. 6. The voltage across the capacitor C3 is shown in FIG. 7 as the voltage E and will decrease to zero by the time t The voltage E is so selected to cause the rapid increase of current from I to I in the time period t -z In order to insure the blocking of the control rectifiers S14 and S15 at the time t when increasing the control current, the charge voltage E of the capacitor voltage can be made to go slightly negative as shown in FIG. 7. Alternately, resistors could be inserted between the center taps of the windings W4 and W5 and the bottom end of the control winding L4 in order to discharge the capacitor C3 to zero. However, additional power losses will result from this latter method.

At the time t gating pulses are removed from the gate electrodes of the controlled switches S8 and S9, curve A of FIG. 8, and are applied to the gate electrodes of the controlled switches S14 and S15 from the output terminal T14 of the trigger control 32, as shown in curve C of FIG. 8. The turning on of the control rectifiers S14 and S15 holds the control current at the current level I during the time period t to t as shown in FIG. 6. The turns ratio W6, W4 of the transformer TR1 is so selected that a sufficient voltage is developed across the winding W4 to maintain the current level through the control winding L4 at the level I with the SCR pair S14S15 being supplied by the winding W4.,During the time period t to t the resonant frequency of the antenna A1 is at its shifted value. At the time t;;, gate pulses are removed from the controlled rectifier pair S14 and S15, with these rectifiers turning off, while at the same time, a gate pulse is applied to the controlled rectifiers S10 and S11 to turn them on. Turning off the controlled rectifiers S14 and S15 will cause the control current to decrease from the current level I to the value I at the time L The turning on of the SCRs S10 and S11 permits the capacitor C3 to begin to recharge toward the voltage level E1. However, as shown in FIG. 7, at the time t the capacitor C3 has not reached the value E due to the internal losses in the circuit.

The controlled rectifiers S12 and S13 are keyed on by a gate pulse being supplied to the gate electrodes from the output terminal T12 of the trigger control 32 at the it; which will hold the control current to the value I At the time t the SCR S7 is gated on by a pulse being supplied thereto from the output terminal T7 of the trigger control 32. Turning on of the rectifier S7 connects the high voltage DC power supply 30- to the capacitor C3, which causes the capacitor C3 to charge to the voltage level E1 by the time t when the gate pulse is removed from the SCR S7. The controlled rectifier S7 will turn off when the current therethrough decreases to substantially zero when the capacitor C3 has charged to approximately the potential of the high voltage DC power supply 30.

With the capacitor C3 charged to the desired voltage level E the losses sustained in the circuit operation over the switching cycle have been compensated for, and the circuit is ready for the next frequency shift cycle of operation in the manner as described above. The losses that must be supplied by the high voltage power supply 30 would be the order of to 200 watts for keying speeds corresponding to 60 words per minute teletype, assuming a saturable reactor control inductance of 20 millihenries, a capacitance of 80 microfarads for the capacitor C3, a control current shift from 25 amperes to amperes in a time period of two milliseconds, and a voltage of approximately 2,000 volts for the voltage E It can thus be seen that a substantial decrease in the control power required is provided in the circuit of FIG. 5.

Although the present invention has been described with a certain degree of particularity, it should be understood that the present disclosure has been made only by way of example and that numerous changes in the details of circuitry and the combination and arrangement of elements and components can be resorted to without departing from the spirit and the scope of the present invention.

We claim as our invention:

1. In a system for shifting the resonant frequency of an antenna, the combination of: a saturable reactor including a control winding and an output winding, said output winding operatively connected to said antenna; supply means for supplying alternating current; a plurality of groups of controlled switching devices operatively coupling said supply means and said control winding to supply direct current to said control winding; and control means for selectively controlling the conductive state of said controlled switching devices to maintain control current at a first level through said control winding, to increase the current therethrough to a second level by rendering conductive one of said groups of devices so that said saturable reactor will present a different inductance to said antenna than presented when at its first current level, to maintain the current through said control winding at the second level by rendering conductive another of said groups of devices and nonconductive said one group of devices, and to decrease the current through said control winding to the first level by rendering nonconductive said one and said another groups of devices.

2. The combination of claim 1 wherein: said controlled switching devices comprise first, second and third groups of controlled rectifier devices operatively coupling said means and said control winding to supply direct current thereto; and wherein said control means selectively controls the conductive states of said groups of controlled rectifier devices to maintain current at a first level through said control winding by rendering conductive said first group of devices and nonconductive said second and third groups, to increase the current through said control winding to a second level by rendering conductive said second group of devices and nonconductive said first and third groups so that said saturable reactor will present a different inductance to said antenna than presented when at its first current level, to maintain the current through said control winding at the second level by rendering conductive said third group of devices and nonconductive said first and second groups, and to decrease the current through said control winding to the first level by rendering nonconductive all of said groups of devices.

3. The combination of claim 2 further including: discharge means operatively connected to said control winding; and wherein said control means selectively controls the conductive state of said controlled rectifier devices, to decrease the current through said control winding to the first level by rendering all of said groups of devices nonconductvie so that said control current will discharge through said discharge circuit.

4. The combination of claim 1 wherein said: saturable reactor includes two control windings; and direct supply means for supplying current at a first level to one of said control windings; and with said control means ren dering nonconductive said one and said another groups of said controlled switching devices permitting said first level current to be applied from said direct supply means to said saturable reactor.

5. The combination of claim 4 further including: high voltage supply means for supplying a direct voltage; and energy storage means including a capacitor to be charged by said high voltage supply means, with said controlled rectifier devices operatively connecting said high voltage supply means and said energy storage means; said energy storage means operatively connected to said control windings; and with said control means controlling the conductive state of said controlled switching devices to recharge said capacitor from said high voltage supply means in order to replenish any energy loss during the shift cycle between current levels in the system.

6. The combination of claim 1 including: a plurality of groups of controlled rectifier devices, a first and a second of said groups of devices operatively coupling said supply means and said control windings to sup-ply direct current thereto; high voltage supply means for supplying a direct voltage; energy storage means including a capacitor to be charged by said high voltage supply means; switching means operatively connecting said high voltage supply means and said energy storage means; said energy storage means operatively connected to said control windings; a third group of devices operatively connected to said energy storage means; said control means selectively controlling the conductive state of said groups of controlled rectifier devices to maintain current selectively at a first level through said control windings by rendering conductive said first group of devices and nonconductive said other groups, to transfer energy from said energy storage means to said control winding to increase the current therethrough to a second level by rendering conductive selected ones of the devices of said third group and nonconductive said other groups so that said saturable reactor will provide a dilferent inductance to said antenna than presented when at its first current level, to maintain the current through said control winding at the second level by rendering conductive said second group of devices and nonconductive said other groups, and to decrease the current to said control winding to the first level by rendering conductive selected of the devices of said third group; said control means further rendering conductive said switching means to recharge said energy storage means from said high voltage supply means in order to replenish any energy loss during the shift cycle between current levels in the system.

References Cited UNITED STATES PATENTS 1,768,433 6/1930 Alexanderson 325I73 X 2,883,524 4/1959 Diese et al. 325-473 X 3,255,414 6/1966 Kawalek et al. 325-173 3,319,168 5/1967 Olson 325-172 ROBERT L. GRIFFIN, Primary Examiner. B. V. SAFOUREK, Assistant Examiner.

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