Magnetic antenna with time variations of core permeability

[57] ABSTRACT

An antenna system using a magnetic core provided with coils wound thereon such as to cause time variations of the permeability of the core itself is disclosed. With the time variations of the core permeability the inductances of the antenna coil and output coil wound on the core is changed for parametric amplification of the reception signal to obtain amplified antenna output. Also, by using a plurality of such antenna systems an array antenna having desired and controllable directional characteristics are obtained.

Claims, 14 Drawing Figures

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PATENTED Μ 31'973

3,750,180

SHEET 1 OF 6

PRIOR ART FIG. I

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PATENIELM311973

3,750,180

SHEET 2 OF 6

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PATENTED Μ 311973

3,750,180

SHEET 3 OF β

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PATENTED *31 BZ3

3,750.180

SHEET 9 OF 6

Power transfer gain G_f2

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PATENTEE Jilt 31197.1

3,750.180

SHEET 5 OF 6

FIG .8

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54

PATENT£DJUL31t973

3,750.180

SHEET 6 OF 6

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F I G . 13

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3,750,180

MAGNETIC ANTENNA WITH TIME VARIATIONS OF CORE PERMEABILITY

This invention relates to antenna systems using a magnetic core.

An object of the invention is to provide an antenna system having a magnetic core provided with coils wound thereon such as to cause time variations of the core permeability so as to cause variations of the inductances of the antenna coil and output coil wound on the core for parametric amplification of the reception signal, to thereby obtain amplified antenna output.

Another object of the invention is to provide an antenna system having a permanent magnet by which the d-c biasing of the core is readily achieved.

A further object of the invention is to provide an array antenna, which consists of a plurality of the aforementioned antenna systems used as antenna elements, and whose directional characteristics can be desirably controlled through the adjustment of the amplitude and phase of parametric pumping sources for the individual antenna elements.

The specification will now proceed with reference to the accompanying drawing, in which:

FIG. 1 is a pictorial representation of a prior-art antenna system having a magnetic core;

FIG. 2 is a graph showing inductances and mutual inductance versus d-c bias current in the same prior-art antenna system;

FIG. 3 shows an equivalent circuit for an embodiment of the antenna system according to the invention;

FIG. 4 is a perspective representation of the construction of the same antenna system according to the invention;

FIG. S is a view showing a magnetic field set up in the ³⁵magnetic core of the antenna system of FIG. 4 by current flowing in a pumping coil;

FIG. 6 is a perspective representation of an experimental arrangement, showing a connection of a signal source and a load to the antenna system of FIG. 4;

FIG. 7 is a graph showing power transfer gain plotted against internal resistance R_o, with the load resistance R₍ used as a parameter, in the arrangement of FIG. 7;

FIGS. 8 to 11 are perspective views showing some other embodiments of the antenna system according to the invention;

FIG. 12 is a perspective view showing the embodiment of FIG. 8 provided with coils;

FIG. 13 is a perspective view showing the embodiment of FIG. 10 provided with coils; and

FIG. 14 is a pictorial representation of an array antenna embodying the invention.

Referring now to FIG. 1, which typically shows a prior-art antenna system having a magnetic core, reference numeral 1 desginates a magnetic core consisting of two separate sections provided with an antenna coil 2 wound thereon. The two core sections 1 are coupled together through a third core section 3, on which an output coil 4 is wound. A further core 5 is provided in association with the core sections 1 and 3. A pumping coil 6 is wound on the core 5.

The magnetic flux produced in the core 5 by the current flowing in the pumping coil 6 penetrates part of the core sections 1 and 3 extending between the two parts of the antenna coil 2.

FIG. 2 shows the inductances La and Li of the antenna coil 2 and output coil 4 and mutual inductance

M between these two coils, which are plotted against corresponding values of d-c bias current flowing in the pumping coil 6. As is seen from the Figure, with variations in the d-c bias current the inductance La of the antenna coil 2 hardly changes, while the inductance Li of the output coil 4 and mutual inductance M appreciably change. By setting the d-c bias current to an appropriate value so as to obtain an optimum coupling between the antenna coil 2 and the output coil 4, the mu• tual inductance can be varied with frequency of the pumping current, which is supplied to coil 6.

In the prior-art antenna system of FIG. 1, the mutual inductance between the antenna coil 2 and output coil 4 is caused to vary at the pumping frequency, resulting in parametric amplification of a signal received by the antenna to obtain an amplified antenna output.

The variation of the mutual inducatance at this time is attributable to changes in the permeability of the magnetic core 3. Accordingly, in order to obtain large changes of the mutual inductance with small pumping current it is necessary to select a reluctance of the core 3 sufficiently high compared to the reluctance of the core 1 to obtain saturation of the core in the operation. However, with the above construction, where the core 1 is divided into two separate sections, the antenna efficiency is very low. Also, with such an arrangement of cores 1 and 5 as shown in FIG. 1, considerably large pumping power and d-c bias power are required to activate these cores 1 and 5. Further, leakage flux is considerably great.

According to the invention, the above drawbacks inherent in the conventional antenna system are overcome by providing improvements in the core construction and method of winding of the coils, so that the gain is extremely increased.

The efficiency of the conventional antenna system is low because it chiefly utilizes mutual inductance between the antenna coil and output coil which varies depending upon the pumping causing saturation of the magnetic core and also because it uses a magnetic core divided into two sections. In accordance with the invention, the magnetic core and the coils are so constructed and arranged that time variations of the permeability of the core itself may be caused, and on the basis of this variation the mutual inductance between the antenna coil and output coil is made variable to obtain a large amplification degree. Also, the winding of the pumping coil are arranged perpendicular to the winding of the other coils so that the coupling between the antenna coil circuit and the output coil circuit is made by the time-variable mutual inductance alone, thus eliminating the otherwise possible deterioration of the efficiency.

The operational principles underlying the invention will first be discussed in connection with FIG. 3, which shows an equivalent circuit for the antenna system according to the invention. In the Figure, L(t) represents an equivalent inductance accounting for variations in ₆₀ the inductances of the antenna coil! and output coil due to time variations of the permeability of the magnetic core. The circuit on the left hand side of the inducatance L(t) (hereinafter referred to as signal circuit) consists of an antenna coil tuning reactance Χ_υ Here, ₆₅ V, and R_o respectively represent the terminal voltage induced across the antenna coil and the resistance across the antenna coil terminals at the time of resonance. The circuit on the right hand side of the indue40

3,750,180 tance L(t) (hereinafter referred to as output circuit) consists of an output coil tuning reactance X₂ and a load R|. In this equivalent circuit, R, represents loss resistance in the coils and core. The Q of the aforementioned left hand side circuit (i.e., signal circuit) is 5 sufficiently high. Besides, this circuit is not directly coupled by any circuit element but it is coupled by the above inductance L(t).

The inductance L(t) changing at a pumping angular frequency of ω_ρ is generally given as 10

L(t) = L_o + 2Li cos Upt (i) equation 6 means that negative input resistance can be realized by the inductance changing with time and also that oscillation is possible. From equation 6 the condition for the oscillation is given as « = (ω,·ω₂·Ε₁²)/(Λ_Τα Λ_τ() -= 1 (7)

When the output circuit is tuned to ω₃, the power transfer gain G_ta may be similarly obtained as

Gta = 4 (ω₃/ω,) · (R_a/R_T(I) · (R|/R_i() · [a/(l + a)²] (8)

When signal current i,(t) at an angular frequency ω, and pumping current i_p(t) at a pumping angular frequency ω_ρ flow into the inducatance L(t), the voltage V(t) appearing at the terminal of the L(t) can be written as

V(t) = (d/dt) [L(t){i,(t) + i_p(t))J (2)

By setting (3) and i_v = Ipe^{] u} s'+ lp*e ~^J s' from equations 2 and 3 we can obtain the following relations among voltages and currents:

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ja_aL_a_

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(4) where ♦ represents conjugate complex, ω₂ = ω_ρ — ω„ and ω₃ = ω_ρ + L,.

When the tuning reactances Xj = —l/(w,Ci) and X_t= — l/(w₂C₂) of the respective signal and output circuits in FIG. 3 are adjusted such that these circuits are tuned to respective angular frequencies ω, and ω₂, the relations among the voltages and currents in these circuits are from equation 4 rV. Ί rj, η •-^«*4 — ja_tLi R.-j(a,L₀---Δ L.A*J

L \ «aCj/J (6)

In this case, the maximum gain is ω₃/ω,.

FIG. 4 shows an example of the construction of the antenna system, whose operation is based on the above principles. Referring to the figure, numeral 7 designates a magnetic core formed with two spaced open²⁰ ings or apertures 8 and 8'. Numeral 9 designates an antenna coil, which is partly wound round the whole body of the core 7, and some of whose turns 10 pass through the aperture 8 and 8'. Numeral 11 designates an output coil, which is wound such that all its turns pass through the apertures 8 and 8'. Numeral 12 designates a pumping coil, which is wound on part of the core between the apertures 8 and 8' such that its winding are perpendicular to the winding of the antenna and output coils ₃₀ 9 and 11.

FIG. 5 shows the magnetic field produced in the core 7 of the above construction by the current in the pumping coil 12. Since parts of the core 7 around the apertures 8 and 8' are magnetized, the permeability of these 35 parts is changed. Thus, the inductances of the antenna coil 9 and output coil 11 wound on the core parts around the apertures 8 and 8' change at the pumping frequency. The core 7 should be formed with at least two apertures as the above ones 8 and 8'.

Whether or not the antenna system of FIG. 4 actually operates in conformity to the afore-described principles has been investigated on an experimental circuit as shown in FIG. 6. In this circuit, the magnetic core and the state of winding of the individual coils are the same as for the construction of FIG. 4, so they are not described any further. Connected between the terminals of the antenna coil 9 is a signal source 13 at a frequency of 1 MHz with an internal resistance R,. Connected across the output coil 11 is a load resistor R₍ in series ⁵⁰ with a tuning capacitor C₂.

The above experimental circuit was put under parametric pumping at a pumping frequency of 4 MHz for measuring the power transfer gain for output frequency ω₂« 3 MHz. The power transfer gain G₍₁ in this case is ⁵⁵ from equation 6

Then, using equation 5 the power transfer gain G_n at resonance (ω, “ 1 /( VZ0C1) and ω₂ “ 1 /( VL₀C₂)) can be obtained as

G« *= 4 · (ω₂/ω,) (Λ./Λ_Γβ) · (Λ₍/Λ_Γ() · [«/(1 - α)²] (6) where a = (ω,·ω₂·£ι²)/(Λτ₀7?τι). and R_Ta and R_Ti are total series resistances of the respective signal and output circuits. The negative sign of the denominator in

G_n- 12 · ΙΛ,ΛΛ, + Λ,)]· [Λ//(Λ| +/?,)]· [α/( 1 -α)²] («)

FIG. 7 shows results of measurements of the power conversion gain G« which is plotted against the internal resistance R_e with the load resistance R_t used as a parameter. As is shown, the smaller the internal resistance R_g the greater is the gain, which is apparently due to the negative resistance. The oscillation takes place at a certain internal resistance value. Table 1 below lists values of the internal resistance R_g at which the os

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6 cillation sets in for certain values of the load resistance R₍.

TABLE 1

Load resistance R; in ohms 0 10 20

Internal resistance R„ in ohms for the oscillation to set in 9 5.5 2.5

From Table 1 above and equation 7 for the oscillation condition, the values of ω₈ω₂£ι² and the equivalent resistance R, for the loss are determined as ω,ω₂ί₁²= 1,310 and

R, = 15 (ohms).

Dashed curves in FIG. 7 show results of calculation of the power transfer gain G₍₂ from the above values and using equation 9. It will be seen that the measured values shown by the solid curves well agree with the calculated values, and this can well account for the fact that the above embodiment of the antenna system according to the invention operates in conformity to the principles discussed earlier.

Thus, in the antenna system according to the invention the antenna coil and the output coil provide the function of the inductance varying with time, and through this function parametric amplification of the received signal may be obtained to obtain amplified antenna output.

Also, since a completely closed magnetic loop is formed, the leakage flux is very small and the driving power and d-c bias power is reduced.

Now some other embodiments which use permanent magnets for the d-c bias will be described.

In the embodiments shown in FIGS. 8 and 9, permanent magnets are provided in close contact with the magnetic core, which is the same as that 7 shown in FIG. 4. In the embodiments of FIGS. 10 and 11, permanent magnets are provided at a spacing from the magnetic core 7.

In the FIG. 8 embodiment, two permanent magnets 50 are fixed in close contact to the opposite sides of part of the core between the apertures 8 and 8'.

FIG. 12 shows the core 7 of FIG. 8, which is provided with individual coils.

In the FIG. 9 embodiment, permanent magnets are fixed in close contact to opposite sides of portions of the core on both upper and lower sides of each of the apertures 8 and 8'. The coils are wound similarity to the embodiment of FIG. 12. More particularly, the antenna coil 9 is wound partly on the whole body of the core, and some of its winding is passed through the apertures so that it is wound on permanent magnets 51. The pumping coil 12 is wound on part of the core between the apertures 8 and 8'. The output coil 11 is wound such that all its winding pass through the apertures and are wound on the permanent magnets fixed to core portions on one side of each of the apertures 8 and 8'.

In the construction of the FIG. 8 or FIG. 9 embodiment, where the permanent magnets 50 or 51 are fixed in close contact to the core 7, the coils are partly wound on the permanent magnets. This is acceptable if the permanent magnets 50 or 51 have sufficiently high resistivity, for instance about 10^s ohm-cm, as of ferrite magnets. However, with metal magnets of low resistivity winding the coils on the magnets undesirably results in large core loss.

The FIG. 10 embodiment uses channel-shaped magnets 53, which are fixed to opposite sides of the core 7 5 with their legs 52 made of a non-magnetic material in close contact with the core. With this construction, it is possible to adjust the bias field set up by the permanent magnets 53 in the core 7 by varying the thickness of the non-magnetic material. In this embodiment, the 10 pumping coil 12 is wound directly on the core portion between the apertures 8 and 8' on the inner side of each of the permanent magnets 53. In this case, either metal magnet or ferrite magnet may be used for the permanent magnets 53.

The FIG. 11 embodiment also uses channel-shaped permanent magnets 54 each applied with a nonmagnetic material 55 on the inner recess side. These magnets 54 are fitted on the respective upper and lower edges of the core 7. In this case, it is of course possible 20 to adjust the bias field produced by the permanent magnets 54 in the core 7 by varying the thickness of the non-magnetic material layer 55. In this embodiment, the coils are wound similarly to the preceding embodiments.

²⁵ In the above embodiments of FIGS. 8 to 13, in which the bias field is produced by the permanent magnets in the core, no d-c bias source is required, so that the associated circuit construction can be simplified. Also, there is no possibility of resulting in deviations from ³⁰ resonance due to bias source fluctuations, so that steady operation can be ensured. Further, the adjustment of the bias field produced by the permanent magnets in the core may be easily done.

The antenna system described earlier in connection ³⁵ with FIG. 4 may be used as antenna element to construct array antennas. Such array antennas can provide useful advantages over the prior-art array antennas.

In the prior-art array antenna, to realize the desired directional characteristics either phase shifter and at^u tenuator inserted between the associated array element and transmitter or between array element and receiver are appropriately adjusted or a suitable lumped constant circuit or distributed constant circuit is provided.

Such phase shifters and attenuators or lumped cons³ tant circuits or distributed constant circuits are adjusted either electrically or mechanically to provide the desired directional characteristics of the array antenna.

These phase shifters and attenuators or lumped cons50 ^{tant circuits or} distributed constant circuits are required to give rise to small energy loss due to their insertion. However, with the usually employed ferrite phase shifters and diode phase shifters energy loss of 1 to 2 dB results. Also, with only a single phase shifter the ₅₅ variable phase range is limited, and where broader phase ranges should be covered it is necessary to use several phase shifter units in combination with inevitably accompanying increase of the energy loss. Therefore, the number of phase shifters inserted in combina6θ tion is limited due to the increase in energy loss.

Also, in case of inserting lumped constant circuits or distributed constant circuits, the amplitude and phase of the current in the antenna elements cannot be varied independently, so that the adjustment of the directional _6J characteristics encounters extreme complications to the disadvantage.

FIG. 14 shows an array antenna according to the invention. This array antenna consists of two array ele3,750,180 merits 101 and 102, each of which is the same as the antenna system shown in FIG. 4.

In each of the elements, the permeability of the magnetic core 7 is varied by the pumping current i_p = Z_p. cos(a»_pr + 0_P) at pumping frequency ω_ρ flowing in the pumping coil 12. This variation causes changes of the inductances of the antenna coil 9 and output coil 11 at the pumping frequency. In other words, the antenna coil 9 and output coil act as variable inductance element. This variable inductance L(t) is given as

L(t) = L_o + ILtCositopt + &lt;?„)

Through this variable inductance L(t) power transfer between signal received by the element and pumping signal is effected, that is, the received signal is subjected to parametric amplification. Thus, a high antenna output can be obtained from the output coil 11.

Now, assuming that a received signal current , flows in the afore-mentioned variable inductance element with inductance L(t) the voltage e induced across the variable inductance element is = [ j_a.L^^as^l +j (_ω„+ω.) (»p+“.)t+«p) (10) ^{2 2} ' (16)

The Li^l&gt; and L/³’ for the respective antenna elements 101 and 102 are respectively proportional to the amplitude of the parametric pumping current in these elements 101 and 102. The 0P^(,) and βρ^&lt;2) are identical with the phases of the parametric pumping sources of the antenna elements 101 and 102 respectively. Thus, 10 by suitably adjusting the amplitude and phase of the parametric pumping sources of the individual antenna elements 101 and 102 the desired directional characteristics are obtained. Also, the directional characteristics are controllable by varying the amplitude and 15 phase of the parametric pumping source.

Since the above antenna elements provide high gain and since they are directly coupled to the transmitter or receiver, it is possible to obtain an antenna array of extremely high efficiency. Such high efficiency antenna ²⁰ array may of course be similarly constructed by using antenna elements shown in FIGS. 12 and 13.

As has been described, according to the invention it is possible to obtain a high efficiency array antenna capable of providing amplified antenna output. Also, ac²⁵ cording to the invention the d-c biasing can be readily achieved.