US3528014A - Submarine communications antenna system - Google Patents

Description

Sept. 8, 1970 r. K. ALBEE SUBMARINE COMMUNICATIONS ANTENNA SYSTEM Filed June 10, 1966 RADIO 7 RECEIVER FIG?) M i M QM \& m z m m w E D.. P RE W W W mm .E\ NA W 17H IZN WK. m m W8 N MHWN 2 M v 2 m H MT\ UW M TM I T m ;l i m Wmmm m m w m 0 a m M M I E 4 m M /W m m L m F Tm /w Q m /W M t mm m mmmm m mwmw m m 25: z 5&8 @352 BY M,

FREQ. IN KHZ ATTORNEYS United States Patent Oihce 3,528,014 Patented Sept. 8., 1970 3,528,014 SUBMARINE COMMUNICATIONS ANTENNA SYSTEM Thomas K. Albee, Lisle, 11]., assignor to The Bunker- Ramo Corporation, a corporation of Delaware Filed June 10, 1966, Ser. No. 556,790 Int. Cl. H04]: 1/16 US. Cl. 325427 18 Claims ABSTRACT OF THE DISCLOSURE Method and apparatus for the cancellation of longitudinal non-Gaussian noise in a submarine antenna system and for increasing the Q of an antenna by compensating for the radiation resistance term of the inherent circuit losses by the coupling of a negative impedance means to the antenna circuit. The antenna system may provide a substantially constant bandwidth over a wide range of selected operating frequencies within the low frequency bands by the simultaneous tuning of the loop antenna system and the adjusting of the negative impedance means.

This invention relates generally to antenna systems and more particularly to methods and apparatus for increasing the signal-to-noise ratio in antenna systems of the type intended for use in the low frequency bands.

Radio communication for submarines is often conducted at operating signal frequencies within the LF. (low freqeuncy) and V.L.F. (very low frequency bands), referred to collectively herein as the low frequency bands, as underwater transmission of radio signals is particularly practical within these frequency bands. Submarine radio systems, for example, are usually operated at frequencies within a band of approximately three kilocycles to three hundred kilocycles, with operation at a frequency of twenty kilocycles being common.

Such low frequency radio communication systems generally utilize fixed or rotatable loop antennas which have been found to provide the best available performance characteristics in a Water environment with respect to signal-to-noise ratio, directivity patterns, and signal strength. A variable capacitive reactance is normally connected across the loop antenna as a tuning impedance to allow selective tuning of the antenna for operation at more than one frequency. Because submarine communication systems operate at relatively low frequencies, a loop antenna and variable capacitance having relatively large inductance and capacity with attendant large physical dimensions are generally required.

In the underwater communication systems heretofore known, and particularly in underwater V.L.F. radio receiving systems, the large impedances of the loop antenna and the variable tuning capacitance have introduced substantial component noise and inherent circuit losses which tend to decrease the sensitivity of the receiving system. This decrease in sensitivity is particularly troublesome in the low frequency bands because of the high level of atmospheric noise and man-made interference encountered there. Accordingly, it has long been desired to improve the signal-to-noise ratio of such underwater radio receiving systems.

In many instances an increase in the effective height of the antenna system will increase the amplitude of the input signal to the receiver, and therefore increase the receiver signal-to-noise ratio. However, it is usually difficult to substantially increase the effective height of the antenna system on a submarine, due to practical limitations.

The introduction of an ordinary pre-amplifier between the antenna system and the receiver input, although tending to increase the magnitude of the signal to the receiver, would also increase the magnitude of the noise present in the antenna circuit and could also introduce additional component noise originating in the pre-amplifier itself. Similarly, the use of conventional passive filter networks, although eliminating some of the noise from the receiver system, is not particularly effective in increasing the signal-to-noise ratio because of the substantial reduc tion in signal amplitude caused by the filter network.

Accordingly, it is an object of the present invention to provide a method of increasing the signal-to-noise ratio in tuned loop antenna systems of the type adapted for use in the low frequency bands.

It is an additional object of the present invention to provide an improved loop antenna system of the type adapted for use in the loW frequency bands, which antenna system presents a smaller operating bandwidth and a greater output signal amplitude than similar antenna systems heretofore known.

It is a further object of the present invention to obtain the aforementioned reduction of operating bandwidth and increase in output signal amplitude of tuned loop antenna systems by the insertion in such antenna systems of circuit means having relatively small physical size and weight.

A further object of the present invention is to use a negative impedance network to improve the sensitivity and selectivity of a low frequency tuned loop antenna system.

Briefly, the present invention contemplates the coupling of negative impedance circuit means with a tuned loop antenna system to effectively cancel a substantial portion of the inherent circuit losses in the antenna system. The resultant signal provided by the antenna system and the negative impedance means is fed to the active input stage of the receiver. Since the operating bandwith and the output signal amplitude of the tuned antenna system depend upon the magnitude of the inherent circuit losses in the system, a reduction in the magnitude of such circuit losses will provide a reduced bandwith and an increased output signal amplitude. The increased signal amplitude coupled with a reduced noise level produced by the reduction in bandwidth consequently serves to increase the signal'to-noise ratio of the antenna system. The present invention also optionally contemplates the provision of means for tuning the tuned antenna system to selected operating frequencies within the low frequency bands and simultaneously adjusting the magnitude of the negative impedance presented by the negative impedance circuit means to thereby maintain the operating bandwidth of the antenna system nearly constant over a wide range of selected operating frequencies.

The invention and its many advantages will be further understood by reference to the following detailed description illustrated in the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a tuned antenna system constructed in accordance with the teachings of the present invention;

FIG. 2 is the equivalent circuit diagram of the tuned antenna system shown in FIG. 1 of the drawings;

FIG. 3 is a graphic showing of the improvement in the quality factor or Q of a tuned antenna system obtained through the use of the present invention;

FIG. 4 is a graphic showing of the improvement in output signal amplitude of a tuned antenna system obtained through the use of the present invention; and

FIG. 5 is a schematic diagram of a tuned antenna system constituting an alternate embodiment of the invention.

Referring now to FIG. 1 of the drawings, there is shown an antenna system including a loop antenna which is connected to a complete radio receiver system 12 by means of leads 14-. Antenna 10 may take any of a number of conventional loop antenna configurations, but will generally comprise a large number of wire turns wound around a relatively large support in order to provide a sufiicient effective antenna height for signal reception in the low frequency bands. In submarine signalling applications, for example, antenna 10 will generally be located at a substantial distance from receiver 12, and therefore leads 14 may extend for lengths up to several hundred feet. At the wavelengths involved, however, such a line is electrically very short, although it may constitute a material resistance.

Connected in parallel with the loop antenna 10 is a variable capacitance 16 which may be selectively varied in magnitude in order to tune the inductive antenna 10 to receive a selected operating frequency within the low frequency bands the antenna is adapted to receive. Although shown as connected adjacent loop 10, capacitor 16 may be connected at other positions along leads 14. As previously mentioned, although frequencies within the band of three kilocycles to three hundred kilocycles are commonly used for underwater signalling, frequencies within a band of five kilocycles to one hundred kilocycles are of primary interest in the disclosed system. In practice, because of the large size of antenna 10, capacitor 16 usually comprises a plurality of small capacitors provided with a ganged switching arrangement to provide sufiicient capacitance for selective tuning of the antenna system. Again, because of the large size of antenna 10 and additionally, because of the required plurality of capacitors, large inherent circuit losses which are primarily resistive in nature are introduced into the system. These losses tend to attenuate the received signal and increase the bandwidth of the system, thereby adversely affecting the signal-tonoise ratio of the system output signals.

In accordance with the teachings of the present invention, a negative impedance circuit 18 is coupled with the loop 10 and capacitor 16 to supply the improved resultant signal to the radio receiver 12. As illustrated, the negative impedance circuit 18 is connected in parallel with the antenna system 10, 16 where it serves to effectively cancel a portion of the inherent circuit losses present in the antenna system. The negative impedance circuit may be coupled to the system adjacent the loop 10, the receiver 12, or as shown at an intermediate point along leads 14. In a manner hereinafter described in greater detail, the insertion of the negative impedance circuit causes a reduction of the operating bandwidth of the antenna system and an increase in the amplitude of the signal supplied to the input of radio receiver 12. Since the reduc tion in bandwidth of the system reduces the received noise level and compensates the increase in effective resistance at resonance in respect to thermal noise level of the loop and capacitor, and since the amplitude of the output signal is increased, the signal-to-noise ratio of the resultant signal available to radio receiver 12 is greatly increased.

With respect to negative impedance circuit 18, it may be observed that it has long been known to be possible to convert positive impedances into effective negative impedances by coupling the output of an amplifier back into its own input, and many circuits have been designed to obtain this result. While any one of a plurality of wellknown negative impedance circuits may be utilized in the present invention, a known type of transistor negative impedance circuit has been schematically illustrated in FIG. 1. As will be understood by those skilled in the art, circuit 18 is a push-pull type of negative impedance converter which employs cross-coupling feedback between two interconnected transistors 20 and 22. The two transistors and their associated circuit components are symmetrically disposed and commonly connected so that the output terminal of each transistor is coupled to the common connection of the other, thereby providing phase inverting feedback. The resulting magnitude of negative impedance presented by circuit 18 depends upon the portion of the voltage supplied across the circuit which is cross coupled between the transistors, and accordingly, circuit 18 includes several variable bias adjustments for adjustment of the negative impedance. For a detailed description of negative impedance circuit 18, reference may be had to the article by J. G. Linvill appearing in 41 I.R.E. Proceedings, 726-729, June 1953.

Additionally, a variable load impedance 24 is provided in circuit 18 to allow the magnitude of the negative impedance to be varied simultaneously with the tuning of the antenna circuit. To this end, a mechanical coupling 26 may be employed to interconnect the variable capacitance 16 with the impedance 24 in order to permit simultaneous adjustment of the two variables. As will become more apparent from the subsequent description, such a simultaneous adjustment will provide an equalized or substantially constant improved bandwidth for the antenna system as the tuned operating frequency of the antenna is changed. It will be understood, however, that the mechanical drive ratio between capacitance 16 and impedance 24 will usually be a complex function rather than a directly proportional relationship.

The operation of the present invention may best be understood by reference to FIG. 2 of the drawings, wherein the equivalent circuit diagram of the loop antenna system shown in FIG. 1 is illustrated. The loop antenna 10 in FIG. 2 is generally represented by an equivalent inductance L and the variable tuning capacitor 16 is represented at one tuned position by an equivalent capacitance C. The total equivalent series resistance R of the tuned antenna system is the resultant of the sum of all resistance losses in the circuit, such as the winding and cable resistance, the radiation resistance of the loop antenna, and the equivalent series resistance of the antenna core losses. As previously described, the loss resistance R is usually relatively large for tuned loop antenna systems adapted for use in the low frequency bands and thus normally tends to decrease the sensitivity of such systems.

The output impedance Z presented by a conventional loop antenna system when tuned to resonance at an operating frequency may be represented as:

RT (1) where X is the inductive reactance of the loop antenna and X is the capacitive reactance of the tuning capacitor.

The quality factor, or Q of the equivalent tuned antenna system shown in FIG. 2 is:

output impedance Z of the tuned antenna system will become:

It will be seen from an inspection of Equation 3 that if the absolute magnitude of the negative impedance Z is greater than the absolute magnitude of impedance Z the antenna system will be stable and Z will have a greater magnitude than Z Hence, if a negative impedance Z of a predetermined magnitude is inserted across a tuned antenna system according to the present invention, the effective resultant impedance of the tuned antenna system will be substantially increased from its normal magnitude.

Further, as may be seen from Equation 1, an increase in the output impedance Z of the tuned antenna system will result in a corresponding decrease in the effective total equivalent series loss resistance R of the tuned antenna circuit. Additionally, as may be seen in Equation 2, the quality factor Q of the antenna system will increase and the effective bandwidth Af of the system will decrease because of the reduction in magnitude of R Since the output signal, or open circuit output voltage, of the tuned antenna system is directly proportional to the magnitude of the Q in the system, an increase in Q will also increase the output signal amplitude presented by the tuned antenna system to the radio receiver 12. This increase in output signal amplitude coupled with the decrease in noise level brought about by the reduction in operating bandwidth of the system vastly improves the signal-to-noise ratio of the antenna system output.

As an example of the improvement provided by the present invention, consider a tuned antenna system as shown in FIGS. 1 and 2 of the drawings having the following values before the insertion of a negative impedance:

OhmS f =20K hertz Q=40 Therefore, from Equations 1 and 2:

R 1.5 ohms Af=500 hertz Z =2400 j60 ohms.

If a negative impedance -Z having a value of 2670+ 173.4 ohms is inserted across the tuned antenna in parallel with the variable capacitance, as illustrated in FIG. 2, then from Equation 3:

As this value of Z is now the effective resultant output impedance presented to the input of the receiver, the effective series loss resistance R and bandwidth Af of the antenna system are subtantially reduced to:

R =.15 ohm Af=50 hertz Similarly, from Equation 2, the Q of the circuit will be increased by a factor of to 400, thereby also effecting a corresponding increase in the open circuit resultant output voltage of the system. The above-described substantial improvements in the operating characteristics of the tuned antenna system are achieved without the insertion of significant additional noise. In practice, the effective tuned bandwidth of present loop antennas may be reduced by a factor of 40 which results in an improved db down signal-tonoise threshold sensitivity of 32 db. This figure is highly conservative because it does not take into account the further reduction in atmospheric noise level obtained by the reduced bandwidth.

FIG. 3 of the drawings illustrates the improvement in the quality factor or Q of a tuned antenna system obtained by the use of a negative impedance circuit as described herein. The readings were taken for a loop antenna with athwart winding and 227 feet of cable leading to the test equipment installation. The field was supplied by an overhead test wire energized at the signal voltage of 0.08 volt and arranged perpendicularly to the center line of the loop winding. FIG. 4 of the drawings shows the corresponding improvement in output voltage of the same antenna system with the same test parameters.

Although the improved antenna system of the present invention has thus far been described and illustrated with the negative impedance circuit connected in parallel circuit across the output of the tuned antenna, it should be pointed out that similar improvements in antenna system parameters may be obtained by inserting a suitably designed negative impedance circuit 18 in series circuit between capacitor 16' and radio receiver 12', as shown in FIG. 5 of the drawings. In this configuration, the receiver may preferably comprise as an initial stage an isolating amplifier so that its conventional active circuitry will not be subject to the negative impedance. In this case, however, since the effective terminal impedance Z of the system is:

Z2'=Z1' ZNI where Z is the equivalent impedance of the antenna 10' and tuning capacitor 16 and Z is the impedance of the negative impedance circuit 18', the absolute magnitude of the negative impedance Z must be less than the absolute magnitude of the equivalent impedance Z of the antenna circuit to avoid instability. The parallel circuit arrangement of FIG. 1 of the drawings is somewhat preferable to the series circuit arrangement of FIG. 5 because in the former arrangement the negative impedance is in parallel with the antenna terminals and therefore offers greater reliability and fail-safe operation.

Although not shown in the drawing, series negative impedance 18 could alternatively be placed between loop 10' and capacitor '16, in which case the latter would be directly connected to the conventional input circuitry of a receiver such as 12 of FIG. 1. As in the other embodiments disclosed, the selected negative impedance employed is of a value to compensate most but not all the resistive loss of the LC system of 10-16' at resonance. Consequently, the thermal noise developed in the tuned system, which establishes the value of the smallest detectable signal for the receiving system, is not degraded because, as Q is increased, the decrease in bandwidth compensates the increase in QX Although a preferred embodiment of the invention has been described herein, it is believed to be obvious that many changes could be made in the disclosed method and apparatus without departing from the scope of the invention. Accordingly, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An antenna system adapted for use in the low frequency bands comprising:

tuned antenna means adapted to receive radio signals in the low frequency bands, said tuned antenna means comprising: loop antenna means, capacitor means coupled across said loop antenna means for tuning the antenna system to a selected operating frequency within said low frequency bands, and antenna output means coupled to radio receiver circuit means, the operating bandwidth of said tuned antenna means and the amplitude of the output signal appearing at said output means being dependent upon the inherent circuit losses within said tuned antenna means; and

negative impedance circuit means coupled in series circuit with said antenna output means and said radio receiver circuit means for inserting a negative im pedance of an absolute magnitude less than the absolute magnitude of the equivalent impedance of said tuned antenna means without producing an unstable condition therein, to thereby reduce the operating bandwidth of the antenna system and increase the amplitude of the output signal therefrom.

2. An antenna system as claimed in claim 1 wherein said capacitor means and said negative impedance circuit means are adjustable to vary the selected operating frequency of the antenna system and wherein said antenna system further comprises:

means coupled to both said capacitor means and said negative impedance circuit means for simultaneously adjusting said capacitor means and said negative impedance means to provide a substantially constant bandwidth for said antenna system over a wide range of selected operating frequencies within said low frequency bands.

3. An antenna system as claimed in claim 1 wherein said negative impedance means includes a plural transistor balanced network whereby the longitudinal noise is cancelled.

4. An antenna system as claimed in claim 1 wherein said tuned antenna means is immersed in a conducting medium.

5. An antenna system as claimed in claim 4 wherein said negative impedance means includes a plural transistor balanced network whereby the longitudinal noise is cancelled and wherein the conducting medium is sea water.

6. An antenna system adapted for use in the low frequency bands comprising:

tuned antenna means adapted to receive radio signals in the low frequency bands, said tuned antenna means comprising: loop antenna means, capacitor means coupled across said loop antenna means for tuning the antenna system to a selected operating frequency within said low frequency bands, and antenna output means coupled to radio receiver circuit means, the operating bandwidth of said tuned antenna means and the amplitude of the output signal appearing at said output means being dependent upon the inherent circuit losses within said tuned antenna means; and

negative impedance circuit means coupled in parallel circuit with said tuned antenna means for inserting a negative impedance of an absolute magnitude greater than the absolute magnitude of the equivalent impedance of said tuned antenna means without producing an unstable condition therein, to thereby reduce the operating bandwidth of the antenna system and increase the amplitude of the output signal therefrom.

7. An antenna system as claimed in claim 6 wherein said capacitor means and said negative impedance circuit means are adjustable to vary the selected operating fre quency of the antenna system and wherein said antenna system further comprises:

means coupled to both said capacitor means and said negative impedance circuit means for simultaneously adjusting said capacitor means and said negative impedance means to provide a substantially constant bandwidth for said antenna system over a wide range of selected operating frequencies within said low frequency bands.

8. An antenna system as claimed in claim 6 wherein said negative impedance means includes a plural transistor balanced network whereby the longitudinal noise is cancelled.

9. An antenna system as claimed in claim 6 wherein said tuned antenna means is immersed in a conducting medium.

10. An antenna system as claimed in claim 9 wherein said negative impedance means includes a plural transistor balanced network whereby the longitudinal noise is cancelled, and wherein the conducting medium is sea water.

11. The method of increasing the output signal-to-noise ratio of tuned loop antenna systems of the type intended for use in the low frequency bands wherein the operating bandwidth and output signal amplitude of the antenna system depend upon the inherent circuit losses therein, comprising the steps of:

compensating for a substantial part, but less than all,

8 of said inherent circuit losses by the coupling in series circuit with the antenna system a balanced negative impedance means having an absolute magnitude less than the absolute magnitude of the equivalent impedance of the antenna system to thereby reduce the operating bandwidth of the system and increase the amplitude of the output signal therefrom, and feeding the resultant signal to radio circuit means to which said system is adapted to be coupled.

12. The method of claim 11 further comprising the step of tuning the said loop antenna system to selected operating frequencies within the said low frequency bands and simultaneously adjusting said negative impedance means to provide a substantially constant bandwidth over a wide range of selected operating frequencies within said low frequency bands.

13. The method of claim 11 wherein the antenna of the loop antenna system is immersed in a conducting medium.

14. The method of claim 13 further comprising the step of tuning the said loop antenna system to selected operating frequencies within the said low frequency bands and simultaneously adjusting said negative impedance means to provide a substantially constant bandwidth over a wide range of selected operating frequencies within said low frequency bands.

15. The method of increasing the output signal-to-noise ratio of tuned loop antenna systems of the type itended for use in the low frequency bands and wherein the operating bandwidth and output signal amplitude of the antenna system depend upon the inherent circuit losses therein, comprising the steps of:

compensating for a substantial part, but less than all,

of said inherent circuit losses by the coupling in parallel circuit with the said antenna system of negative impedance means having an absolute magnitude greater than the absolute magnitude of the equivalent impedance of the antenna system to thereby reduce the operating bandwidth of the system and increase the amplitude of the output signal therefrom, and

feeding the resultant signal-to-radio circuit means to which said system is adapted to be coupled.

16. The method of claim 15 further comprising the step of tuning the said loop antenna system to selected operating frequencies within the said low frequency bands and simultaneously adjusting said negative impedance means to provide a substantially constant bandwidth over a wide range of selected operating frequencies within said low frequency bands.

17. The method of claim 15 wherein the antenna of the loop antenna system is immersed in a conducting medium.

18. The method of claim 17 further comprising the step of tuning the said loop antenna system to selected operating frequencies within the said low frequency bands and simultaneously adjusting said negative impedance means to provide a substantially constant bandwidth over a wide range of selected operating frequencies within said low frequency bands.

OTHER REFERENCES K. Iizuka: IEEE-Transactions on Antennas and Propagation, VAP-l3 n. 1, January 1965, pp. 7-20.

KATHLEEN H. CLAFFY, Primary Examiner C. W. JIRAUCH, Assistant Examiner

Images