US3564551A - Dipole antenna with electrically tuned ferrite sleeves - Google Patents

Abstract

A TUNABLE ANTENNA SYSTEM HAVING RADIATING ELEMENTS WITH MEANS HAVING A NORMALLY FIXED PERMEABILITY ASSOCIATED THEREWITH, A MEANS FOR PRODUCING A MAGNETIC FIELD OF VARYING STRENGTHS TO CHANGE THE PERMEABILITY OF THE MEANS HAVING THE NORMALLY FIXED PERMEABILITY AND MEANS FOR SELECTIVELY ADJUSTING THE STRENGTH OF THE MAGNETIC FIELD TO VARY THE INDUCTANCE EFFECT OFFERED TO THE RADIANT ENERGY.

Description

Feb. 16, 1971 H.A. MILLS E A!- DIPOLE ANTENNA WITH ELECTRICALLY TUNED FERRITE SLEEVES Filed Jan. 14, 1970 3 Sheets-Sheet 1 TRANSMITTER AND I OR RECEIVER :OSCILLATOR SUPPLY FIG.I

2 u O U 2 u 4 D SUPPLY OSCI L LATOR 2 HARRY MILLS EDWARD LIPSON TRANSMITTER NOEL R NELSON AND /OR RECEIVER Feb. 16, 1971 H. A. MILLS ETAL Filed Jan. 14, 1970 3 Sheets-Sheet 2 :OSClLATOR v D C 3 59 t SUPPLY m 58'.

TO TRANSMITTER AND OR RECEIVER 13.89 I400 I414 I428 l4.42 I455 WAVELENGTH 5 (f) 60 a a' 1.5 u) c v A0 2 '1' gm L 3 2 5 2 O I-I- [1 I 0 I 20 a 0 0 21.6 2L4 2L2 2L0 20.8 20.6

FREQUENCY INVENTOR 4 HARRY MILLS EDWARD LIPSON NOEL R.NELSON Feb. 16, 1971 Ls EI'AL 3,564,551

DIPOLE ANTENNA WITH ELECTRICALLY TUNED FERRITE SLEEVES Filed Jan 14, 1970 3 Sheets-Sheet 3 VARIABLE GMN g; 0 OSClLATOH :U

4 DIRECT 4 TRANSMITTER AND/ OR "0 CURRENT RECEIVER i t? 'SUPPLY 90) l 89 87 INVENTOR HARRY MILLS EDWARD LIPSON NOEL R. NELSON United States Patent G 3,564,551 DIPOLE ANTENNA WITH ELECTRICALLY TUNED FERRITE SLEEVES Harry A. Mills, 5904 32nd St. NW., Washington, D.C.

20015; Edward Lipson, 2105 Belvedere Blvd, Silver Spring, Md. 20902; and Noel R. Nelson, 12813 Camelia Drive, Wheaton, Md. 20906 Filed Jan. 14, 1970, Ser. No. 2,871 Int. Cl. H01q 9/16 US. Cl. 343-747 2 Claims ABSTRACT OF THE DISCLOSURE A tunable antenna system having radiating elements with means having a normally fixed permeability associated therewith, a means for producing a magnetic field of varying strengths to change the permeability of the means having the normally fixed permeability and means for selectively adjusting the strength of the magnetic field to vary the inductance effect offered to the radiant energy.

The present invention is directed to an antenna system capable of being electrically tuned over a Wide range of operating frequencies. It includes ferrite materials arranged in close relationship to the radiating elements of the antenna system and electrical means for magnetically biasing the ferrite materials to alter the permeability thereof and to thus increase the inductance that the ferrite offers to the radiated energy.

Certain prior art communication systems, as for example those for radio and television broadcast, operate at fixed frequencies. These have antenna systems which are constructed to be resonant at the operating frequency of the broadcast stations. Certain other prior art communication systems are operable at selectively different frequencies and require either a separate antenna for each operating frequency or an antenna that is capable of being tuned to resonance at the various operating frequencies. The latter is especially true where the operating frequencies to which the station is tunable is continuous over a wide range of frequencies.

Tunability of the antenna makes it possible to operate the station equipment at a frequency that provides the most favorable propagation conditions for communication between selected points. It also permits the operation of the station equipment at frequencies that will avoid interfering with other communication systems while at the same time maintaining a maximum radiating efficiency for all frequencies to which the antenna system is tuned.

Many prior art tunable antenna systems have had the disadvantage of being tunable over a comparatively narrow range of frequencies with an accompanying loss of radiating efficiency at the extremes of the tuning range. Also they have required cumbersome, complex and expensive mechanical systems for the actuation of the tuning means.

The present invention is directed to providing an antenna system that is capable of being electrically tuned over a comparatively larger range of frequencies while at the same time maintaining a substantially constant radiating efficiency; one that is less expensive to install, operate and maintain and one that eliminates the need for cumbersome complex mechanical equipment to effect the tuning of the antenna.

An object of the invention is to provide an antenna system that can be tuned by electrical means to widely different selected frequencies from a remote position.

Another object of the invention is to provide an antenna system capable of being operable over a Wide range of ice frequencies without the heretofore attending low radiating efficiencies at the extremes of the tuning range.

Another object is to provide an antenna system having multiple radiating elements with tuning means for simultaneously tuning each radiating element.

Still another object is to provide an antenna system capable of being simultaneously tuned with the station equipment to different operating frequencies to take advantage of better operating conditions offered by said different operating frequency.

Other objects of the invention will appear obvious from the disclosure in the specifications and drawings.

FIG. 1 of the drawing is a schematic view of a dipole antenna to Which the invention has been applied.

FIG. 2 is a schematic view of a vertical or whip type antenna to which the invention has been applied.

FIG. 3 is a schematic view of an antenna system comprising three separate antennas each operable in a range of frequencies of its own, having means for simultaneously tuning the separate antennas.

FIG. 4 is a chart disclosing the characteristics of the antenna shown in FIG. 1 as affected by the operation of the tuning means.

FIG. 5 is a schematic view of a beam type antenna having a single driven element and parasitic elements with means for tuning not only the driven element but also the parasitic elements as well.

The electrical length, as distinguished from the physical length, of the radiating elements of an antenna system may be varied by the introduction of a lumped inductance or capacitance. The effect of inductance is to increase the electrical length in terms of wavelengths relative to its physical length. The effect of capacitance in series with the antenna radiating elements is to shorten the electrical length relative to the physical length.

In the construction of antennas for operation on fixed frequencies, inductance means have been used on the one hand to foreshorten the length of the radiating elements and to initially adjust the antenna elements exactly to the desired operating frequency, or in other words, to obtain resonance in the antenna at the operating frequency. However, except in special types of antennas as for example those used on mobile structures, no subsequent tuning capability of the antenna elements is contemplated or secured to enable the station equipment to be operated at a different frequency than that at which the antenna is initially tuned to operate. The special types of antennas, above mentioned, have movable elements for adjusting the inductance offered by an induction coil connected in series with the antenna element. In these antennas the movable element makes contact with the coils of the lumped inductance along its length, to connect variable lengths of the coil in series with the antenna element. The movable element is either moved by hand, by manually operable means or through the use of powered mechanical means, either at the antenna or through re motely controlled means. For large fixed installations, the means for tuning the antenna can become cumbersome, complex and usually have only a narrow tuning capability.

Ferrite materials formed as sleeves have been heret0- fore used in the place of inductance coils, to initially introduce an inductance effect in the antenna structure. In each prior use instance, the object of the introduction of ferrite has been to obtain an antenna structure that has a physical dimension that is smaller and within reason and comparable to the space that is available for the antenna. In other words, using the ferrite sleeves makes it possible to use a shorter antenna element for a given operating frequency than would otherwise be possible without the sleeves and to space the radiating elements closer together. Once the sleeves are installed, to establish the proper electrical length and spacing of the radiating elements for the operating frequency of the station, the resonant frequency of he antenna is established and fixed and canot be varied except by changing the length of the sleeves.

Our invention also utilizes ferrite sleeves arranged about the radiating elements and obtains the foreshortened antenna structure and closer spacing of elements as in the prior art structures. However, to the ferrite sleeves has been added a means for magnetically biasing said ferrite sleeves, providing tunability of the antenna structure apart from structurally altering the antenna, whereby the electrical length of the antenna may at any time after the initial installation, be adjused to a different electrical length. I

It has not heretofore been recognized that the ferrite sleeves are especially suited to have their inductance effect altered by a magnetic field, which can be easily created by a biasing coil. About the sleeves we have placed biasing coils, through which a selectively adjustable constant current is made to flow to introduce in the ferrite sleeves a biasing magnetic field. The different magnetic biasing fields introduces changes in the permeability of the ferrite and thus changes the inductance effect of the ferrite that is offered to the radio frequency energy in the antenna radiating elements. Changes of the permeability of the ferrite sleeves permits a broad tuning range equally as broad as any that has heretofore been achieved by mechanical operable means and without the attending difficulties of the means producing mechanical operation.

For the purpose of discloser, the ferrite sleeves are shown as placed about the radiating elements to embrace about one half their physical length. The sleeves that are used have a composition that will give an initially low permeability when not biased, to provide for resonance in the radiating elements at the highest intended operating frequency. The biasing field is applied selectively in an amount or degree as to introduce an increase in the permeability of the ferrite sleeves to acquire an increase in inductance effect, whereby the antenna may be tuned to a lower operating frequency than that which is initially provided by the unbiased sleeves. The application of the biasing magnetic field operates to increase the permeability of the materials in the ferrite sleeves and thus increases the inductance effect produced by the ferrite sleeves, whereby the electrical length of the antenna is extended for operation at a lower resonant frequency than the initial frequency. This mode of operation is selected for purpose of disclosure only and is not intended to be limited thereto. There are certain ferrite or magnetic materials that may have their premeability decreased by an increasing magnetic field. It is possible through the selection of a material which has a decrease in permeability with increase in biasing field to start the tuning range from a lower operating frequency and to raise it to a higher tuned frequency by increasing the biasing field. Both modes of operation are possible within the broad concept of this invention.

For the purpose of producing a magnetic field, an exciting coil is placed around the ferrite materials on the antenna element and this coil is excited by a suitable direct current source to produce the necessary magnetic field in the ferrite materials. A direct current supply to the coil is made adjustable so that the strength of the magnetic field may be varied at Will to produce the de sired biasing force.

The permeability of magnetic material, in this instance the ferrite, is the ratio B/H of the flux density to magnetizing force and it depends on the composition of the ferrite. The initial permeability of the ferrite is nearly always much less than the permeability at some higher flux density. When ferrites are subject to a large direct current magnetization field as compared to a simultaneously applied superimposed A.C. magnetization field, there will be an increased inductance offered to the AC. current producing the AC. field. This is the incremental inductance and the correpsonding permeability of the ferrite is the increment permeability.

Incremental permeability and hence incremental inductance depend upon the magnitude of both the DC. and AC fields and upon the previous magnetic history of the materials. To eliminate the effect of the magnetic history on the incremental permeability the present structure also includes a degaussing means, for operation be tween each adjustment of the biasing current. When the materials are thoroughly demagnetized and thereafter are first magnetized, the relation between current in the winding and the flux is the usual B/H relation. If the magnetization is then successively reduced and reversed, the flux goes through the familiar hysteresis loop. When the alternating current is now superimposed on the materials through its field with the direct current field, the result is to cause the flux in the materials to go through a minor hysteresis loop, superimposed upon the usual hysteresis loop.

The incremental permeance of the materials hence the incremental inductance offered to the alternating current creating the alternating .field is proportional to the slope of the line joining the tips of the minor hysteresis loops.

The change in the incremental permeability by use of different biasing currents in an exciting coil is responsible for the change in the incremental inductance, responsible for change in the electrical length of the antenna elements and thus rendering the antena elements tunable over a wide range of frequencies.

We start out with a material that initially has a low permeability and a composition that dictates that the permeability will increase with an increase in the biasing field. With zero bias on the ferrite materials the antenna radiating elements with which the ferrite is associated have their shortest electrical length and resonate at their highest frequency. As the biasing current in the coils and biasing field is increased, the incremental permeability and inductance offered to the radio frequency energy in the radiating elements is increased. This has the effect of increasing the electrical length of the radiating elements of the antenna thus adjusting the radiating elements to a lower resonant frequency. Conversely, decreasing the biasing current from a maximum decreases the incremental permeability and inductance offered to the radio frequency energy. This has the effect of decreasing the electrical length of the radiating elements thus adjusting the radiating elements to a higher resonant frequency.

Reference is now made to FIG. 1 of the drawings for the disclosure of the first embodiment of the invention. In this figure is disclosed a dipole antenna having radiating element 1 in two sections supported by means, not disclosed, in insulated relation from each other. The sections of the radiating element '1 extend in opposite direction from each other in a common plane. About the adjacent ends of the element sections 1 are located sleeves 2 made of ferrite material. The sleeves 2 may be constructed in one length or may be constructed of a multiplicity of shorter sleeves with the combined length equal to the length of the sleeve desired. The length of the sleeves 2 is not more than one half the length of the antenna element sections. The shorter lengths of sleeves are preferable to a single length of sleeve, because it admits to flexing of the element under mechanical load without the attending possible damage to the sleeve. The ferrite sleeves 2 utilized in the construction of the invention were supplied by Ceramic Magnetic, Inc. of Fairfield, N.J., and are identified as type N-40. The ferrite sleeves are of sintered material having broad band characteristics. The sleeves 2 are embraced by biasing coils 3 throughout substantially their entire length. The coils 3 are connected together at their adjacent ends forming one continuous coil over the central portion of the antenna element 1. The remote ends of the coil 3 are connected to the radiating element 1 just beyond the remote ends of the ferrite sleeves 2. In the manner in which the sleeve biasing means are connected, the same transmission line that is used to connect the radio frequency energy to the antenna element is also used for the transmission of the biasing current to the biasing coils 3. If desired, the coils may be connected by a separate transmission means to isolate the biasing means from the radio frequency energy means.

As is customary in communication systems, the transmitter and/ or receiver 13 is connected through an isolating capacitor 14, to the inner conductor 11 and sheath 12 of the coaxial cable 10. The antenna end of the coaxial cable 10 is connected through isolating capacitors 9 to the balun 7 and from the balun 7 through the variable capacitors to the adjacent ends of the antenna element sections 1. The capacitors 5 together with the capacitor 6 shunting the balun 7 and the balun 7 operate in the usual fashion for the purpose of adjusting the impedance of the transmission line to the input impedance of the antenna.

For the purpose of biasing the coils 3, a direct current supply 17 having means for varying the output current therefrom is connected through the double-pole doublethrow switch 16 and a radio frequency choke 15 to the inner conductor 11 and the sheath 12 of the coaxial cable 10. The radio frequency choke 15 isolates the direct current supply 17 from the radio frequency energy from the transmitter 13. The upper end of the coaxial cable is connected through radio frequency chokes 8 directly to the adjacent ends of the antenna element sections and through the antenna element sections to the remote ends of the biasing coils. The chokes 8 permit the passage of biasing current to the coils 3 around the balun 7 and capacitors 9 and 5. In this manner the tuning means at the antenna is energized over the same transmission line as supplies the radio frequency energy to the antenna.

As previously stated, the ferrite materials after being biased at a certain field strength for a period of time, acquire a certain amount of residual magnetism, which operates against having the same adjustment of the DC. supply means always to produce the same biasing field. Between each adjustment of the biasing field to tune the antenna to a new resonant frequency, the ferrite sleeves are subjected to a degaussing, to bring the residual magnetism back to zero, prior to changing the biasing current supply may be calibrated in terms of resonant frethe same circuit as the direct current and comprise audio frequency alternating current supplied from a variable gain oscillator 18 through the double-pole double-throw switch 16. The radio frequency chokes 8 and offer very little impedance to the passage of the low frequency alternating current produced by the oscillator 18 and the capacitors 5, 9 and 14 isolate the radio frequency portions of the circuit from the degaussing currents.

The antenna 1 is tuned to resonant frequency of the transmitter 13. When the antenna is resonant, the inductive and capacitance reactance balance each other and the input resistance is at its minimum for the transmission of the maximum energy to the antenna. Either the direct current supply may be calibrated in terms of resonant frequency to permit preadjustment of the antenna or a l ad current indicating means or standing wave meter by an indication of lowest SWR will indicate when the antenna has been tuned to the resonant frequency for the opera tion. Such auxiliary equipment is usual with communication systems and is not therefore shown.

In FIG. 2, the invention is applied to a whip or vertical type antenna. This type is usually used in mobile systems. It includes a radiating element 20 having a capacity hat 21 on its extreme upper end and a ferrite sleeve 22 about its lower end. The ferrite sleeve 22 is embraced by an insulating material about which is positioned the biasing coil 23. The coil 23 is connected at its upper end to the radiating element 20 and at its lower end through a radio frequency choke 25 to the ground plane 28 and sheath 30 of the coaxial cable 29. The ground plane may be the metallic skin of the vehicle bearing the equipment, or the ground plane may be radial elements as are sometimes used with vertical antennas. The lower end of the radiating element 20 is connected through a variable capacitor 26 and fixed capacitor 27 to the inner conductor 31 of the coaxial cable 29, leading to the transmitter and/or receiver (not shown). Between the coaxial cable 29 and the transmitter and/or receiver is the usual isolating capacitor (not shown) to prevent direct current from flowing thereto. The capacitors 26 and 27 offer low impedance to the radio frequency energy and high impedance to direct current and audio frequency currents. Bypassing the capacitors 26 and 27 is a radio frequency choke 24 connecting the inner conductor 31 of the coaxial cable 29 to the lower end of the radiating element 20. Thus radio frequency energy may pass from the transmitter through the coaxial cable 29 through the capacitors 26 and 27 to the radiating element and biasing and degaussing currents may be supplied to the coil 23.

As in the prior embodiment, the lower or transmitter end of the coaxial cable is connected to a direct current biasing means 34, having means for adjusting the current output therefrom and to a degaussing means, such as the oscillator 33 having means for varying the gain thereof. The connection to the coaxial cable 29 is through the double-pole double-throw switch 32 and the radio frequency choke 35. The radio frequency choke 35 passes the direct current and the audio frequency currents but prevents the passage of radio frequency energy thus isolates the direct current supply and degaussing means from the radio frequency energy.

The embodiments of FIGS. 1 and 2 have substantially the same mode of operation. At all times, the antenna radiating elements are adjusted electrically to have an electrical length to produce resonance at the operating frequency. This is accomplished by adjusting the direct current supply to produce the optimum biasing current in the coils to produce resonance, either as indicated by a calibrated means on the direct current supply means, or, by adjusting the current input as the transmitter is feeding radio frequency energy to the antenna until the input meter shows a maximum indication, or, until the SWR meter shows a minimum indication. Once so adjusted, the radiating elements will be operating at resonance and at their maximum radiating efficiency commensurate with other things that control the radiating efficiency. When it appears that better operating conditions may be had at a different operating frequency, the station equipment is tuned to the different operating frequency. The double-pole double-throw switch is thrown to connect the degaussing means to the coils for the purpose of reducing the residual magnetism to zero. Thereafter, the direct current supply means is connected to the coils and the proper biasing current is supplied to the coils to produce the indication on the meters that show that a resonant condition has been reached.

FIG. 3 discloses a third embodiment of the invention wherein it is applied to a three antenna array. Each antenna is designed and constructed to be operable in a separate band of frequencies as determined by the physical length, the unbiased condition of the ferrite sleeves and their individual tuning range. The separate bands for which the antennas are initially tuned as determined by the structure, lie adjacent to one of the bands of the other antennas and the tuning range of the separate antennas are such that they overlap, whereby a continuous tuning range is provided approximately equal to three times the tuning range of a single antenna. Each of the separate antennas may be separately tuned through their own range of frequencies, or they may all be tuned simultaneously as in the disclosed structure. When resonance at the operating frequency of the transmitter and/ or receiver i not obtainable in one antenna, one of the other two antennas will resonate at the operating frequency.

As shown, the antennas each have different physical lengths 41, 44, and 47 which, without the tuning feature would be resonant at rather widely spaced frequencies, as for example somewhere near the upper end of the tuning range when the tuning feature is considered. Each of the elements 41, 44 and 47 are respectively provided with ferrite sleeves 43, 45 and 46 about which are respectively mounted exciting coils 48, 49 and 50. The coils on each antenna are designed to provide the proper bias for tuning the antenna through the range necessary to Obtain the desired overlapping of the tuning ranges. The coils on each antenna are connected at their adjacent ends to each other and at their remote ends to the radiating element of the antenna. Inasmuch as all radiating elements are driven, each antenna constructed in sections are connected together in parallel through the transmitter and/ or receiver as shown. Each antenna is provided with variable capacitor 51 connected to the adjacent ends of the radiating sections. Connected in series with the variable capacitors 51 is a fixed capacitor 52 which is in turn connected to a circuit that is connected with a coaxial cable 56.

Each series arrangement of capacitors 51 and 52 is bypassed by a radio frequency choke 53. The capacitors 51 and 52 offer low impedance to the radio frequency energy from the antenna end of the coaxial cable 56'. The radio frequency chokes offer high impedance to the radio frequency energy from the antenna end of the coaxial cable 56. The radio frequency chokes offer high impedance to the radio frequency energy but low impedance to direct current and audio frequency alternating current, thus, the coils are prevented from conducting radio frequency energy and a circuit is provided from the coaxial cable to the coils for their excitation by direct current or by a degaussing current.

The coaxial cable 56 is connected to the transmitter and/ or receiver through the conventional isolating capacitor to prevent the direct current and audio frequency currents from reaching the transmitter and/or receiver. The transmitter and/ or receiver and the isolating capacitor are not shown in this figure, it being substantially the same as in FIG. 1.

As shown, a variable gain oscillator 54 and a direct current supply 55 are connected through a double-pole double-throw switch 60 and a radio frequency choke 59 to the inner conductor 58 and sheath or outer conductor 57 of the coaxial cable 56. The radio frequency choke 59 offers high impedance to the radio frequency energy and thus isolates the means 54 and 55 from the radio frequency energy while at the same time permitting the passage of direct current from the direct current supply means and audio frequency alternating current from the oscillator to the coaxial cable 56.

As in the other embodiments of the invention, the oscillator is a variable gain type, having means for adjusting the amplitude of the alternating current to a value adequate for the degaussing of the ferrite material in the sleeves. Also, the direct current supply means is provided with means for manually adjusting the biasing current furnished thereby.

While the arrangement disclosed shows the single biasing means for the antenna connected in parallel and sup plied from a common source, it is within the scope of the invention to provide separate biasing means and degaussing means, or a separate control means for each antenna and separate circuits for connecting the biasing and degaussing means to the antennas.

The tuning ranges of the antennas overlap and an extended tuning range of approximately three times the tuning range of a single antenna is obtained. While the radio frequency energy is simultaneously connected to all the antennas, only the antenna that is tuned to resonance at the operating frequency of the transmitter and/ or receiver will resonate and produce a substantial signal output.

FIG. discloses a beam type antenna having a single 8 driven element and two parasitic elements. The driven element 76 is connected through capacitors 79 and 80 to the circuit 75 which in turn is connected to the inner conductor 94 and the sheath 93 of the coaxial cable 92 and through the isolating capacitor 91 to the transmitter and/ or receiver 90.

The driven element 76 and the parasitic elements 71 and 82 each have ferrite sleeves respectively '77, 72 and 93 embracing a substantial portion of their lengths. About the sleeves, insulatedly mounted thereon, are coils 78, 73 and 84 for producing a biasing magnetic field in the ferrite material of the sleeves. The biasing coils are shown connected together at their adjacent ends and at their remote ends to the radiating elements which in turn is connected through radio frequency chokes 81, 74 and 85 to the circuit 75 and through the coaxial cable 92 to the direct current supply 87 and the degaussing means 86. The connection as in the other embodiments is through a double-pole double-throw switch 88 and radio frequency choke 89.

The sections of the radiating element 71, here used as a reflector element, are connected together through a capacitor 71a which prevents the direct current from short circuiting the coil 73 and provides a path for the radio frequency energy in the element sections to pass from one to the other. Similarly, the sections of the radiating element 82, used as a director element, are connected together through capacitor 82a. The capacitors 71a and 82a enable the elements to operate as parasitic elements. The radio frequency energy can pass from one section to the other and back and the direct current and audio frequency energy is blocked. The sections of the driven element on the other hand are connected together through the transmitter and/or receiver. The parasitic elements receive their radio frequency energy by induction from the driven element. By reason of their different lengths and by reason of their spacing relative to the driven element the combined effect is to radiate more radio frequency energy in one direction than in the opposite direction as is usual in beam type antennas. As a consequence, more radiant energy is propagated in the direction from the reflector element towards the director than in the opposite direction. As a result, the signal in that direction will be enhanced compared with the signal in the opposite direction. With parasitic elements having fixed electrical lengths as with the driven element with fixed electrical lengths, there is only one frequency at which the elements are resonant. Thus for maximum output and maximum tuning range for the antenna, it is essential that the parasitic elements be tuned to the proper operating frequency with reference to the driven element for the parasitic elements to reinforce the signal in the desired direction. The application of tunable ferrite sleeves to the parasitic elements for tuning them to resonance at the proper frequency adds to the radiation efficiency and enables a beam antenna to be tuned in a manner similar to a single dipole antenna. Also, the presence of ferrite sleeves about the elements allows for a reduction of spacing between the elements and for a more compact antenna structure.

The embodiments of FIGS. 3 and 5 each have multiple radiating elements, and operate substantially the same in respect to the manner in which the antennas are tuned. As disclosed, the antennas operate differently to attain different results, one to expand the tuning range and the other to increase the signal strength in a given direction of propagation. The radiating elements are tuned in parallel from a common direct current supply. They may be connected in series for tuning without any change in effectiveness. The only difference is that the series arrangement would require a higher voltage direct current supply means.

As in the other embodiments, when it is desired to change the operating frequency, the transmitter and/0r receiver is tuned to the new frequency. The biasing coils are connected to the degaussing means for reducing the residual magnetism to zero by means of the double-pole double-throw switch. After the ferrite has been degaussed, the direct current source is adjusted to bring about a resonant condition in the radiating elements at the new operating frequency.

As in the other embodiments, the resonant condition is determined by the magnitude of the current supplied to the coils and by the intensity of the magnetic field produced thereby. The resonant condition can be recognized to have been reached when during the tuning operation the output from the transmitter reaches a maximum as indicated by the maximum indication of an output meter, or, by the minimum indication on a SWR meter.

The direct current supply may also be calibrated in terms of frequency so that after the degaussing operation, the direct current supply means may beadjusted to the proper operating frequency for resonance.

FIG. 4 discloses the characteristics'of the antenna shown in FIG. 1. On the chart are four curves A, B, C and D, representing respectively the biasing current, inductive reactance, standing wave ratio SWR and the antenna input resistance plotted against the frequency and electrical length of the antenna in wave lengths. In obtaining the data from which the curves are constructed, the antenna of the type shown in FIG. 1 was connected to a transmitter which had the capability of being tuned to different frequencies within a band of frequencies. The antenna was of the character shown with the ferrite sleeves and biasing means thereabout. It was elected to start at the high frequency end of the band at which, in this particular instance, was the frequency at which the antenna was resonant when there was no biasing current on the biasing coils. This frequency was as shown to be 21.7 megahertz (mHz.). The transmitter was successively tuned in steps of 0.1 mHz. from the starting frequency down to 20.2 mHz. At each step, the biasing current was adjusted at the direct current supply means until the maximum energy was flowing to the antenna as indicated by the minimum indication on a SWR meter. At each step the current, inductive reactance, SWR, and input resistance were measured. From the data thus procured, the curves were constructed showing the antenna characteristics. As mentioned, the curves were plotted against the frequency and the electrical length of the antenna. In a decreasing direction of the frequency the electrical length of the radiating elements increase. It shows a little more clearly the effect of tuning of the antenna by the biasing means.

The biasing current curve A discloses an increase was required up to the point where the operating frequency was about 20.9 mHz. and then there was a gradual but small reduction in the required biasing current, to effect the required tuning of the antenna. The small reduction in the biasing current is thought to come about by reason of the fact that between each adjustment of the frequency of operation of the transmitter, the ferrite material of the sleeves was not degaussed as is desired in the actual operation of the invention. As a consequence of the lack of degaussing, the ferrite material acquired increasing amount of magnetism which added to the flux produced by the current in the biasing coils. The total flux thus so increased produced the same increase in permeability and hence inductive effect as if the flux was produced entirely by the current in the coils. Normally, it is expected that when the ferrite material is degaussed between each adjustment of the operating frequency, the result would show a continual increase in biasing current through the whole range of tuning. As saturation is approached, to obtain the same increments of increase of the biasing field would require greater increments of increase in the biasing current. This is because the losses increase at saturation and above.

The inductive reactance B at first remained substantially constant as the operating frequency was varied from 21.7 mHz. to 21.6 mHz. and then dipped slightly before 10 gradually rising to its peak at a frequency of 20.9 mHz. The standing wave ratio, SWR, shown at C remained substantially constant throughout the tuning range, there being a slight decrease in the SWR about midpoint 20.7 mHz. of the tuning range. The input resistance shown at D shows an increase from 57 ohms at the frequency 21.7 mHz. to a peak of 61.0 ohms at a frequency 21.2 mHz. and a decrease to 48 ohms at the frequency of 20.2 mHz.

The erratic behavior of the inductive reactance curve also is attributed to the fact that the material was not degaussed before or between each adjustment of the operating frequency.

The curves were constructed for a portion only of the possible tuning range of the antenna and are not to be considered as limiting of the range of tuning.

As has already been indicated and is in part obvious from the foregoing disclosure, the invention has several important advantages.

One important advantage is that it eliminates the need for cumbersome, expensive and weighty mechanical tuning mechanisms thus requiring a less massive mounting structure. Secondly, it provides an easily controlled tuning means, one that is easily maintained, less apt to give trouble and one that will not be rendered inoperable through corrosion and icing conditions during frigid seasons of the year. Thirdly, the tuning range over which an antenna may be tuned is greatly increased with less loss of radiating efficiency. The relatively stable operating efiiciency over the tuning range is attributable to the broadening effect of the ferrites and coils upon the radiating elements.

Still another advantage is that because of lack of movable parts, the tuning of the antenna can be affected with greater ease and quicker than possible with mechanically operable tuning means. Other advantages are obvious from the disclosure.

What we represent to be our invention is set forth in the following claims:

1. A tunable antenna system comprising radiating means, ferrite means arranged about said radiating means to initially alter the electrical length of said radiating means relative to their physical lengths, field producing means surrounding said ferrite means for varying the effect of said ferrite means upon the electrical length of said radiating means and adjustable energy supply means connected to said field producing means for selectively varying the field to selectively vary the electrical length of said radiating means to be resonant at different operating frequencies, wherein said radiating means comprises three separate radiating elements, each divided into two sections, the sections of each radiating element extending in opposite directions from each other and all the sections of all the radiating elements having physical length to initially tune them to different resonant frequencies; wherein said ferrite means comprises sleeves embracing the adjacent ends of each section of the radiating elements over a substantial length thereof, for altering the electrical lengths of said radiating elements relative to their physical lengths; wherein said field producing means are coils surrounding said sleeves over their entire length and wherein said adjustable energy supply connected to said field producing means has a range of adjustment effective to vary the electrical lengths of each of said radiating means for resonance at a different operating frequency within bands of frequencies that slightly overlap each other to produce an extended range of tuning beyond that of a single radiating means.

2.. A tunable antenna system comprising radiating means, ferrite means arranged about said radiating means to initially alter the electrical length of said radiating means relative to their physical lengths, field producing means surrounding said ferrite means for varying the effect of said ferrite means upon the electrical length of said radiating means and adjustable energy supply means connected to said field producing means for selectively varying the field to selectively vary the electrical length of said radiating means to'be resonant at different operating frequencies, wherein said radiating means comprises three separate radiating elements, each divided into two sections, the sections of each element extending in opposite directions from each other and all sections lying 5 in a common plane, spaced transversely of each other at a distance less than one half wave length, wherein the ferrite means comprises of sleeves embracing the adja cent ends of each said section over a substantial portion of their lengths; wherein said field producing means 1 comprises coils surrounding said sleeves over their entire length and wherein said adjustable energy supply means is connected to each field producing means to vary the electrical lengths of said radiating means to produce resonance in each of said radiating elements and further 1 a 12 including means for' connecting radio frequency energy to one of said radiating elements.

1 References Cited UNITED STATES PATENTS 2,748,386 5/1956 Polydorotf 343-787 3,016,535 1/196-2 Hewitt 343-76 8 3,172,109 3/1965 Senrui 343-749 3,205,501 9/1965 Kuhn 343-778 3,302,208 1/1967 Hendrickson 343-787 3,339,205 8/1967 Smitka 343-701 ELI LIEB-ERMAN, Primary Examiner US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 5 4', 551 Dated February 16 1.971

Inventor) Harry A.I=-I1lls, Edward Lipson and Noel R.Ne1son It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, line 30, change "discloser" to --diso1osure--.

Line 50, correct the spelling; of --permeabil1t5 Column 5, line +5, cancel "supply may b e calihrated in terms of resonant fre-" and insert --on the coils. The degaussing current is suoplied over--.

Siqned and sealed this 22nd day of, June 1971.

(SEAL) .Attest: A

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLER, JR. Attesting Officer Commissioner of Patents FORM Po-wso (10-69) USCOMWDC U.5 GOVERNMENT PRINTING OFFiCE 2 199 O-JC-3l

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