JPH046123B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a compact resonant circuit that can be used as a one-turn loop antenna in the HF or VHF region.

(Prior Art) There are various types of body coils and head coils used for high-frequency magnetic fields in nuclear magnetic resonance imaging systems, but a birdcage-type RF coil shown in FIG. 6 is used. The figure shows what is called a high-pass type of birdcage-type RF coils. In the figure, 1 is an RF coil that accommodates a subject and applies or detects a high-frequency magnetic field, and the circular surfaces of loop elements 2 and 3 are arranged in parallel and connected by a segment 4. 3
A plurality of capacitors 5 are inserted into.

The wantar coil, which has the characteristic of not generating an electric field around it, was created by modifying a known antenna or resonator as shown in FIG. 7 in order to achieve tuning. In the figure, figure A is a half-wavelength dipole antenna used for VHF transmission and reception. When this is successively bent from the tip as shown in the figure, both ends approach each other, the capacitance between the tips increases, and the resonant frequency decreases. Since the resonant frequency is lower with the same length, if the resonant frequency is made the same, the overall dimensions of the antenna will be reduced and the size will be reduced. In Figure C, a lumped constant capacitor _C1 is inserted in place of the distributed capacitance to make the capacitor even smaller. Figure D is a circular version of the oval in Figure C.

In the diagram E, a capacitor is inserted in two parts, and if the capacitance is chosen as C ₂ = 2C ₁ , they are connected in series, so the series capacitance is C ₁ .
Diagrams D and E are electrically equivalent for the purpose of obtaining resonance. Figure F shows the birdcage shape shown in Figure 6.
For use in the RF coil, e.g. capacitor 8
In the example of dividing and distributing individuals, we choose C ₃ = 8C ₁ to obtain a loop equivalent to the second diagram. As C is divided and distributed in this way, the RF applied to each C and generated in the inductance formed by the conductor between C and C increases.
The voltage decreases, and although a magnetic field is generated as a whole, the extent to which it emits an electric field to the outside becomes smaller. Therefore, the electric field applied to the test object housed in the birdcage-shaped RF coil in question is reduced, and the loss due to the electric field is reduced as a whole, so Q increases.

(Problems to be Solved by the Invention) By the way, as shown in the coil shown in FIG. For example, in this case
A capacitor with a capacity eight times larger than _C1 (several hundred PF) is required, and the Q of that capacitor becomes a problem. Furthermore, the capacitors used for this purpose need to be small, large in capacity, and highly accurate, so they are difficult to obtain, and even if they can be obtained, they are expensive.

The present invention has been made in view of the above points, and its purpose is to realize a one-turn loop resonant circuit that can be used in a loop antenna that is much smaller than the wavelength of the resonant frequency without using a lumped constant capacitor. It is in.

(Means for Solving the Problems) The present invention, which solves the above-mentioned problems, forms a loop using a dielectric material with a large relative dielectric constant, and calculates the loop capacitance and the inductance of the loop. It is characterized in that it is used in a state where it resonates.

(Function) Since a simple one-turn loop resonant circuit is formed using a material with a large dielectric constant, it can be used as an antenna that is very small compared to the wavelength, and as a one-turn loop resonant circuit that does not apply an external electric field.

(Example) Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 1 is an external view of one embodiment of the present invention. In the figure, 10 is a ring made of dielectric material, and is a one-turn loop resonant circuit in which capacitance C and inductance L are evenly distributed on the circumference, and the resonance point is in the HF or VHF region.

Next, in this example, the above-mentioned birdcage type
When creating a loop antenna equivalent to an RF coil, calculate the relative dielectric constant of the material to obtain a distributed capacitance equivalent to that of an inserted capacitor. In a birdcage-type RF coil as shown in Figure 6, a capacitor 5 with a capacitance of 800 pF is connected to the loop element 2.
Consider the loop elements 2 on one side, which are distributed at intervals of 45 degrees on the circumference. The combined value of eight series-connected capacitors 5 is 800pF÷8=100pF,
The one-turn loop resonant circuit 10 shown in FIG. 1 may have a combined distributed capacitance of 100 pF.

The diameter of the loop element 2 is usually about 40 to 50 cm, and the distributed capacitance when the loop element 2 is cut and stretched is calculated. FIG. 2 is an explanatory diagram for calculating relative permittivity using a cut and expanded element. In the figure, if the length is l, the diameter is d, and the cross-sectional area of the cut element is S, then the capacitance C is expressed by the following equation.

C=ε ₀・εγ・(S/l) …(1) Here, ε ₀ ; Permittivity in vacuum (8.854pF/m) εγ : Relative permittivity Now, use a ceramic rod with a diameter of 1 inch to create a loop with a diameter of 50 cm. When making an antenna, S=π×(2.54/2) ² =5.06cm ² l=πD=157cm (D=50cm) ∴S/l=0.0322cm=3.22×10 ^-4 m Therefore, assuming C=100pF ( From formula 1), C = εγ×8.854×3.22×10 ^-4 = εγ×2.851×10 ^-3 = 100 ∴εγ=100/2.851×10 ^-3 = 35075 In other words, a circle with a diameter of 50 cm is formed using a material with a diameter of 1 inch. To make a one-turn coil with a distributed capacitance of 100 pF, a dielectric material with a relative permittivity of about 35,000 can be used.

The dielectric constant is 10,000 or more, or 35,000 or
Considering the possibility of creating a dielectric material of about 40,000, it is possible to make use of the boundary layer of SrTiO ₃ -based dielectric ceramics or semiconductor ceramics, as long as the Q is in the tens of tens and the poor temperature coefficient is tolerated. It can be realized because it exists as .

Next, FIG. 3 shows several examples of methods for transmitting and receiving high frequency power to and from the one-turn loop resonant circuit. In the figure, the same parts as in FIG. 1 are given the same reference numerals. Figures A to E are diagrams showing different power supply methods. In the figure, 21 is an electrode made by wrapping an insulating tape and then wrapping a conductor band around it. A cable is attached to this electrode 21. In the figure, 22 is an electrode made by drilling a hole in the one-turn loop resonant circuit 10 and inserting a conductor through an insulator. A cable is attached to this electrode 22. In Fig. 3C, reference numeral 23 denotes a coupling coil provided separately, which is used to magnetically couple with the one-turn loop resonant circuit 10 to transmit and receive high-frequency power. This coupling coil may be placed inside the ring of the other one-turn loop resonant circuit 10 shown in this figure or on the side on the axis. Figures D and E show the case where the one-turn loop resonant circuit 10 is cut, and in Figure D, a current transformer 24 made of a toroidal core and having a transformation ratio of 1:n is inserted and coupled to the cut space. 25 is a conducting wire connecting the cut portions. Reference numeral 26 in Fig. E is a conductive wire connected to the cut portion and drawn out, which is connected to an external matching transformer 27 and coupled to the loop antenna 10. Various other coupling methods can be considered.

As mentioned above, a one-turn loop resonant circuit with a dielectric constant of tens of thousands has a low Q and a bad temperature coefficient, but if used in a loop antenna for shortwave broadcast reception, it will have a moderately low Q and will resonate appropriately. It becomes a broadband antenna with high performance and is convenient and can be put to practical use. Furthermore, even if Q is 10 or less in the VHF region, preferable results can still be obtained for applications that cover the entire FM band, for example.

The following methods can be considered for finely adjusting the resonant frequency of this one-turn loop resonant circuit.

(1) The temperature of the resonant circuit itself is changed by taking advantage of the large temperature coefficient of εγ of the material used.

(2) Cut off part of the ring as shown in Figure 4. In the figure, 30 is a scraped portion.
While monitoring the resonance characteristics, we will continue to remove the material. In this case, the more the ring capacitance is reduced, the more the ring capacitance is reduced, but the self-inductance of the ring is hardly reduced, so the more the ring is reduced, the higher the resonance frequency is.

(3) As shown in FIG. 5, conductor short bars 31 are pasted here and there so that they are electrically dense. The length and number of shot bars 31 are changed while monitoring the resonance characteristics. In this case, the shot bar shunts the current in each local area, and when the circulating capacitance is viewed in series, it has the effect that a portion of the circulating capacitance is shot at the local area, so the overall circulating capacitance increases, and on the other hand, Since the self-inductance of the ring hardly changes, the resonant frequency decreases as the number of shot bars increases and as their length increases.

(4) As shown in FIG. 8, when the conductor 28 is brought close to the ring, or when the conductor piece is placed inside it or close to it on the axis, the self-inductance of the ring decreases and the resonant frequency increases.

As explained above, according to the present invention and its embodiments, it is possible to obtain a resonant circuit as a one-turn self-resonant magnetic coupling means that has a low resonant frequency in terms of dimensions, has a simple structure, and can be used as a small loop antenna. . If dielectric ceramics are developed and materials with high Q and large dielectric constant become available, they can be used for a variety of applications, not just RF coils for nuclear magnetic resonance imaging systems.

(Effects of the Invention) As explained in detail above, according to the present invention, the one-turn antenna has an extremely simple structure, does not require the use of a capacitor, and can be used as a loop antenna that is extremely small compared to the wavelength of the resonant frequency. It is possible to obtain a loop resonant circuit, which has a great practical effect.

[Brief explanation of drawings]

Fig. 1 is an external view of one embodiment of the present invention, Fig. 2 is an explanatory diagram for calculating the dielectric constant, Fig. 3 is a diagram of a method of transmitting and receiving high frequency power to the resonant circuit of the embodiment, and Fig. 4 is Fig. 5 is a diagram of another method of resonant frequency fine adjustment, Fig. 6 is a diagram of the RF coil of a nuclear magnetic resonance imaging device, and Fig. 7 is a diagram of the RF coil of the nuclear magnetic resonance imaging device.
FIG. 8, which is an explanatory diagram of the process of obtaining a coil, is a diagram of another method of finely adjusting the resonance frequency. 1...RF coil of nuclear magnetic resonance imaging device, 2,
3...Loop element, 4...Segment, 5...Capacitor, 10...One-turn loop resonant circuit.

Claims (1)

[Claims] 1 A ring-shaped structure (loop) is formed using a dielectric material with a large relative permittivity, and it is used near the resonance point between the circulating capacitance inside the ring and the self-inductance formed by the ring. Features a one-turn loop resonant circuit.