ANTENNA LOW-NOISE Q SPOILING CIRCUIT

Abstract: Input coupling circuit for detuning the Q of a high-Q ferrite rod antenna. A high impedence low-noise amplifier (4) is used to amplify the output signal from the LC resonant tank (L1, 1) of the rod antenna (M). A portion of the amplified signal is fed back directly into the magnetic circuit of the antenna tank. This negative feedback reduces the losses produced in the magnetic circuit due to the presence of nearby conductors. As a result of this reduction, an increase in the antenna sensitivity and a decrease in the cross-feed from other nearby antenna is produced.

Description: ANTENNA LOW-NOISE Q SPOILING CIRCUIT BACKGROUND OF THE INVENTION This invention relates to antenna input detecting cir¬ cuits. More particularly, it relates to a ferrite rod an- tenna input coupling circuit for detuning the Q of the an- tenna. Λ to achieve both a broadband frequency response and an increase in the antenna's sensitivity when positioned near conductive parts, i.e. metal supports, printed circuit copper lands, chassis, etc. It is known in the prior art that the Q of an anten¬ na's input resonant tank circuit may be reduced (spoiled) by applying a portion of the detected antenna signal as negative feedback into the resonant tank circuit. By re¬ ducing the 0 of the antenna resonant tank detecting circuit, a broadband frequency response is obtained. United States Patent No. 2,787,704 illustrates the use of negative feed¬ back to achieve a constant band-width input frequency re¬ sponse for a high-Q rod antenna. In this reference, the output from the rod antenna resonant tank circuit is buf- fered by a vacuum tube amplifier and a portion of the buf¬ fered signal applied through a feedback transformer into the resonant tank circuit. The secondary of the feedback transformer is connected in series with the tank inductor formed around the ferrite rod. Special tuning capacitors in the feedback circuit vary the feedback ratio of the O.V.PI 7 V.:-0 , > feedback signal according to the frequency detected by the antenna to achieve a constant bandwidth regardless of the center frequency to which the antenna is tuned. Some of the advantages of active Q-spoiling are also disclosed and discussed by this reference. That is, the sensitivity of the antenna is not diminished even though the Q of the antenna is reduced. (As understood by those skilled in the art, the antenna sensitivity or signal-to- noise ratio of its output signal, which is expressed in micro-volts per meter, refers to the amount of external magnetic field required to increase the antenna output signal by a factor 1.414 or 3db over the noise level when no external fields are present. ) This is true because the high input impedence of the active amplifier does not appear as a resistive load to the resonant tank circuit. Resistive loading of a resonant tank reduces the Q but increases the noise in the antenna output signal. Another advantage discussed by this ' reference relates to the broadband frequency response resulting from the detuning of a high-Q circuit. The negative feedback voltage in series with the resonant tank voltage reduces the Q of the tank to effectively open up the frequency band to detect more frequencies which occur near the resonant center fre¬ quency. This enables the antenna to effectively respond to several relatively separated frequencies. One of the main advantages of a ferrite rod antenna is that it may be contained physically in quite a small volume. Therefore, in receivers which utilize rod anten¬ nas, it is inevitable that these antennas will be placed near other components, antenna's and metal parts. The re¬ sultant degradation of performance in the antenna's pack¬ aged configuration compared with that obtained when the rod antenna is isolated may amount to a having of the Q- factor of the antenna. In addition, the sensitivity of the antenna (signal-to-noise ratio of the detected output signal) is likewise reduced. When more than one rod an¬ tenna is placed in the same physical area, such as when two antennas are placed adjacent and orthogonal to one another, a problem of cross-talk between the antennas is created. This cross-talk between antennas produces unde¬ sirable signal responses in both antennas. In order to avoid this cross-talk problem, prior art receivers have resorted to elaborate configurations for the antennas. One such technique involves the use of four high-perme¬ ability ferromagnetic rods arranged in a square with the resonant tank coils for opposite side rods interconnected to form a single effective rod antenna. For this tech¬ nique, the physical configuration of the ferrite rods is critical in order for the antenna to maintain its desired frequency response and, at the same time, to minimize the cross-talk. A long range navigation system which employs a plur¬ ality of transmitters transmitting on discrete frequencies, such as the Omega Navigational System, requires a signal receiver whose antenna is capable of detecting each of the very low frequency (VLF) signals that are transmitted. In such a receiver, two orthogonally positioned ferrite rod antennas are required in order to produce a detected sig¬ nal regardless of the orientation of the receiver. Accordingly, it would be desirable to provide a rod antenna input coupling circuit which enables the antenna to have a broadband frequency response while minimizing the cross-talk between adjacent rod antennas, and to have an increased sensitivity by reducing the antenna's inter¬ nal losses due to the presence of nearby conductors. SUMMARY OF THE INVENTION In accordance with this invention, an antenna input coupling circuit for detuning the Q of a high-Q rod anten¬ na resonant tank circuit is provided. Detuning of the Q of the antenna resonant tank by the present invention in¬ creases the antenna sensitivity by desensitizing the an- tenna to the presence of nearby conductors. A parallel inductor-capacitor resonant tank circuit is used to pro¬ duce an antenna detect signal in response to the magnetic field generated in the ferrite rod by the electro-mag¬ netic field detected by the antenna. A high input i pe- dence amplifier is used to amplify the antenna detect sig¬ nal. Associated with the ferrite material of the rod antenna is a second feedback inductor which is magnetic¬ ally coupled to the magnetic field present in the resonant tank inductor. A feedback resistor is connected between the output of the amplifier and the feedback inductor to apply a portion of the amplifier output signal back to the magnetic circuit of the resonant tank. The signal fed back is in a negative sense. The necessary phase inver¬ sion to achieve negative feedback may be accomplished by the arrangement of the turns of the feedback inductor around the ferrite rod material or it may be produced by inversion through the amplifier. BRIEF DESCRIPTION OF THE DRAWINGS For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accom¬ panying drawings, in which: Figure 1 is a circuit diagram of the preferred embodi¬ ment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to Figure 1 which is a circuit diagram of the preferred embodiment of the present invention, a high permeability ferrite rod M is shown magnetically coupled to inductors LI and L2. That is, the magnetic flux in rod M passes through both inductors. Both inductors LI and L2 consist of a number of turns wrapped around the ferrite rod M. Connected in parallel to inductor Ll is a parallel combination of capacitors Cl and C2. Capacitors Cl, C2 and inductor LI comprise the rod antenna LC resonant tank cir¬ cuit 3. The magnetic field produced in the ferrite mater¬ ial M by the electromagnetic field detected by the anten¬ na generates the antenna detect signal. The antenna detect signal is inputted to a negative feedback means consisting of amplifier 4 and coupling means 5. The nega¬ tive feedback means functions to produce the antenna out¬ put signal 2 ' by amplifying the antenna detect signal 1 and to feed a portion of the amplified signal back into the magnetic circuit of the resonant tank 3. For the preferred embodiment, amplifier 4 consists of a field effect transistor Ql coupled to an operational amplifier Al. The high-impedence low-noise field effect transistor Ql is used both to provide amplification and to present a high-impedence load to the antenna detect signal 1. The amplified signal from Ql is capacitively coupled through capacitor C3 to the inverting input of operational amplifier Al. The series circuit consisting of resistor R3, capacitor C4 and resistor R4 is connected to the non- inverting input of amplifier Al. This circuit provides an identical input impedence circuit to the non-inverting input as is present on the inverting input in order to balance the input currents to amplifier Al. Operational amplifier Al operates as an inverting amplifier whose closed loop gain is determined by the ratio of R5 to Rl. A voltage divider network consisting of resistors R6 and R7 is used to apply a portion of the amplified antenna detect signal 2 back into the field effect transistor amplifier circuit via capacitor C5. As a result, the closed loop gain of the amplifier 4 is controlled by the ratio of re¬ sistors R6 and R7. For the preferred embodiment, the close loop gain of amplifier is approximately 40db. Also connected to the output of amplifier 4 is a coup¬ ling means 5 consisting of a feedback resistor Rf and in- ductor L2. As previously mentioned, inductor L2 is coupled to the magnetic material M so that it responds to the same magnetic field as inductor LI. The turns of inductor L2 are applied to the ferrite rod M in the same sense as in¬ ductor LI. That is, the voltage applied to inductor L2 will produce a voltage on Ll that is in phase with the voltage on L2. Because of the inversion that is present in amplifier 4, the magnetic field produced by the current in L2 will be 180° out of phase with the antenna detect signal 1. The magnitude of feedback resistor Rf controls the amount of current applied to inductor L2 thereby con¬ trolling the amount of signal that is fed back into the antenna's magnetic circuit. While the preferred embodiment shows an inverting amplifier 4 to produce the necessary phase inversion to achieve negative feedback, it is obvious to a person of ordinary skill in the art that other ways are possible to achieve negative feedback, such as the use of a non- inverting amplifier 4 with a reversal in the sense of in¬ ductor.. L2 relative to inductor L1. Additionally, it is obvious that other ways of controlling the amount of sig¬ nal fed back other than by the use of a feedback resistor in series with inductor Ll are possible. As explained above, the sensitivity of an antenna is defined to be the amount of external magnetic field, ex¬ pressed in micro-volts/meter, needed to increase the an¬ tenna output signal by a factor of 3 db. This sensitivity may be measured by isolating the antenna from all external magnetic fields and nearby conductors and measuring the noise level present on the output of amplifier 4. Having measured this voltage, the antenna is subjected to a mag¬ netic field of the appropriate frequerfcy to produce an antenna output response signal. The level of magnetic field is increased until the measured voltage on the out¬ put of amplifier 4 increases by a factor of 3 db. At this point, the amount of required signal to produce the desired antenna output signal above the level of noise when the antenna was isolated is the measure of the antenna's sensi¬ tivity. If the antenna resonant tank is unloaded and the antenna is isolated as described above, the sensitivity of the antenna will be the same as when the antenna is iso¬ lated and there is Q spoiling (loaded) . However, if the antenna is placed in the proximity of electrical conductors, and the sensitivity measurement is made for both the unloaded and the Q spoiled condition according to the present invention, an improvement in the antenna sensitivity will be measured. That is, the losses in the ferrite rod due to the proximity of conductive parts is reduced when negative feedback of the amplified antenna detect signal 2 is fed back into the magnetic circuit of the rod antenna. Nearby conductors respond to the magnetic field of the rod antenna by producing eddy currents in their conductive surfaces. These eddy currents, in turn, represent magnetic losses in the magnetic circuit. The resultant degradation of performance compared with that obtained when the rod antenna is isolated may amount to a having, of the Q factor. Because the feedback signal of the present invention is applied to the magnetic circuit in a negative sense, the magnitude of the induced eddy cur- rents in nearby conductors is significantly reduced — as much as 30:1. The eddy current induced losses in the mag¬ netic material M appears in the antenna output signal as noise. Because the antenna is desensitized to the presence of the nearby conductors and to the presence of other near- by antennas, the noise level is reduced. Accordingly, the amount of external magnetic signal needed to produce the 3 db increase in the antenna's output signal above this noise is reduced. Hence, an improved sensitivity. It should be clear that the values of the various cir- cuit components depicted in the drawing and described above will vary in dependence upon the intended use. In a pre¬ sently preferred embodiment of a Q spoiled rod antenna used in a receiver to detect the radio signals in the Omega Navigational System, the table below sets out exemplary values which have been found satisfactory. TABLE OF EXEMPLARY VALUES Reference Designation Value M ferro-magnetic high perme¬ ability material Ql 2N 6550 Al 4136 by Raytheon Ll 1200 turns of #30 wire L2 2 turns Cl 1800 pf (C1+C2 C2 select for center nominal ; frequency 2100 pf) C3 2.2 micro-farads C4 2.2 micro-farads C5 10 micro-farads Rl 1.21 K R2 510 R3 1.21 K R4 18.7 K R5 42.2 K R6 1.21 K R7 20 ohms Rf 1.5 K (selected for desired Q) In describing the invention, reference has been made to its preferred embodiment. However, those skilled in the art and familiar with the disclosure of the invention may recognize additions, deletions, substitutions or other modifications which would fall within the purview of the invention as defined in the appended claims.

Claims: WHAT IS CLAIMED IS: 1. A rod antenna detuning circuit for reducing the un¬ loaded Q of the antenna LC resonant tank comprising: a negative feedback amplifier, for amplifying the resonant tank output signal and for negatively coupling ' a portion of the amplified tank sig¬ nal back into the magnetic circuit of the tank, the amplified signal comprising the antenna output signal. 2. The detuning circuit of claim 1 wherein said negative feedback amplifier comprises: (a) a low-noise field effect transistor amplifier, for providing a high impedence load to said tank circuit, and for amplifying the tank out- _ put signal without introducing any appreciable noise to the antenna output signal; (b) an operational amplifier associated with said transistor amplifier, said operational ampli¬ fier and said transistor amplifier cooperat- ing together to produce the antenna output signal; (c) an inductor magnetically coupled to the mag¬ netic circuit of the tank; and (d) a feedback coupler, for coupling the antenna output signal to said inductor, said coupler and said inductor cooperating to establish the amount of antenna output signal fed back. 3. The detuning circuit of claim 2 wherein said feedback coupler is a resistor. 4. A rod antenna input coupling circuit for detuning the of the antenna to increase the antenna sensitivity by desen tizing the antenna to nearby conductors, the coupling circu comprising: (a) a resonant tank circuit, for producing a detect signal in response to the magnetic field generated in said tank circuit by the electro-magnetic field detected by the antenna; and (b) a negative feedback means, for amplifying the detect signal to produce the antenna output signal and for negatively feeding back a portion of the antenna output signal into the magnetic field of said tank cir- cuit. 5. The detuning circuit of claim 4 wherein the resonant tank comprises: (a) a first inductor having a core of a magnetic conductive material which contains the mag- netic field produced by the electro-magnetic field detected by the antenna; and (b) a shunt capacitor, for shunting said first inductor to ground, said first inductor and said capacitor forming an LC resonant tank circuit. 6. The detuning circuit of claim 5 wherein the negative feedback means comprises: (a) an amplifier operably connected to said tank circuit, for amplifying the detect signal to produce the antenna output signal, said amplifier having an input impedence large enough to prevent resistive loading to said tank; and (b) coupling means for coupling a portion of the antenna output signal into the magnetic field of said tank circuit, said amplifier and said coupling means producing a negative feedback signal. 7. The detuning circuit of claim 6 wherein said amplifier comprises: (a) a low-noise field effect transistor amplifier, for providing amplification for the detect sig- nal without introducing any appreciable noise to the signal; and (b) an operational amplifier associated with said transistor amplifier, said operational ampli¬ fier and said transistor amplifier cooperating together to produce the antenna output signal. 8. The detuning circuit of claim 6 wherein said coupling means comprises: (a) a second inductor which is magnetically coupled to the magnetic field in said first inductor; and (b) a feedback resistor for coupling the antenna output signal to said second inductor, said resistor cooperating with said second inductor to establish the amount of output signal fed back. 9. In a rod antenna input circuit having, (a) an inductor-capacitor parallel resonant tank circuit having a magnetic circuit produced by the electro-magnetic field detected by the antenna, for tuning the antenna to cer¬ tain frequencies, the unloaded Q of the tank substantially greater than 1, (b) a high input impedence low-noise amplifier, for amplifying the output from said resonant tank without resistive loading to said tank, and (c) a negative feedback circuit for negatively coupling a portion of the amplified tank sig¬ nal as a series voltage with the tank output signal to reduce the Q of said tank and to obtain a broadband frequency response, an improvement in the coupling circuit comprising an improvement in said negative feedback circuit wherein: the negatively coupled feedback signal is coupled into the magnetic cir- cuit of said tank. 10. The improvement circuit of claim 9 wherein said nega¬ tive feedback circuit comprises: (a) a feedback inductor responsive to current produced by the amplified tank signal, for generating a magnetic field in the magentic circuit of said tank; and (b) a feedback resistor for coupling the ampli¬ fied tank signal to said feedback inductor, said feedback resistor and said inductor determining the amount of signal fed back. _c:-π_ - ''λ-- " ■" - ■•

Inventors: FOWLER C, PENROD B
Applicants: FOWLER C, PENROD B, TRACOR, TRACOR INC
Application year: 1979
Main classification: H03H11
IPC codes: H01Q7, H03H11, H04B1