JPH0720962Y2 - Loop antenna circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to a loop antenna circuit used in a pager or the like.

(Prior Art) FIG. 2 is an external view showing the mounting of a conventional loop antenna with a built-in device used in a pager or the like, and FIG. 3 is a circuit diagram thereof. 2 and 3, 1a is a lower part of the loop antenna, 1b is an upper part of the loop antenna, 3 is a capacitor, and 4 is a variable capacitor. The capacitor 3 and the variable capacitor 4 cancel the inductances of the lower part 1a of the loop antenna and the upper part 1b of the loop antenna for tuning. Reference numeral 5 is a capacitor for impedance matching, 6 is a transistor for amplifying a received signal, 7 is a choke coil for supplying a bias current to the transistor 6, and 2 is a printed circuit board on which the above-mentioned components and electronic circuits such as a receiver are mounted. is there.

Since the loop antennas 1a and 1b capture only the magnetic field when receiving radio waves, they have so-called 8-shaped directivity as shown in FIG. Therefore, when considering the normally used vertically polarized waves, the loop antennas 1a and 1b arranged as shown in FIG. 2 show the maximum sensitivity to the magnetic field in the left-right direction of the paper surface, and therefore penetrate the paper surface to move from the front surface to the back surface. It had the maximum sensitivity to radio waves traveling from the back to the front.

(Problems to be Solved by the Invention) However, the loop antenna described above has a drawback in that it is difficult to capture radio waves traveling in the left-right direction of the paper surface in FIG. 2, and a pager or the like has extremely poor sensitivity in this direction. It was

Actually, since the lower part 1a of the loop antenna and the upper part 1b of the loop antenna shown in FIG. 3 act as minute vertical antennas for the radio waves, the sensitivity does not disappear at all as shown in FIG. , As indicated by the broken line in FIG.
It becomes a phenomenon that the sensitivity is reduced by 20 to 30 dB, but it was still a practical problem for pagers and the like. The unit of the numbers shown in FIG. 5 is dBμV / m and represents the sensitivity in each direction.

The present invention has been made in order to eliminate the above-mentioned drawbacks, and an object of the present invention is to provide a loop antenna circuit which has a simple structure and prevents the sensitivity from decreasing in a specific direction.

(Means for Solving the Problems) In order to achieve the above-mentioned object, the present invention provides a built-in loop antenna in which a capacitor is connected between one end of a U-shaped conductor and ground and between the other end and ground. An inductance for canceling a capacitive component when the loop antenna is operated as a vertical antenna is connected between the substantially central portion and the ground.

(Function) A loop antenna with a built-in device is inductive because its loop perimeter is significantly shorter than the wavelength used, but a capacitor provided between each end of the loop antenna and ground cancels out the inductive component. . Further, the loop antenna becomes capacitive when it operates as a small vertical antenna, but the inductance provided between the substantially central portion of the loop antenna and the ground cancels the capacitive component.
In this case, since the central portion of the loop antenna is set to be the equilibrium point with respect to the ground, the inductance does not affect the operation as the loop antenna.
Therefore, the loop antenna can have directivity having sensitivity in each direction.

(Embodiment) FIG. 1 is a circuit diagram showing an embodiment of the present invention, in which the same reference numerals as those in FIG.
Is a coil connected between the substantially central portions of the loop antennas 1a and 1b and the ground.

Next, the operation of this embodiment will be described. A loop antenna with a built-in device used for pagers, etc. is inductive because the loop circumference is significantly shorter than the wavelength used due to the accommodation space (for example, "NATIONAL
TECHNICAL REPORT ” 19 [2] Matsushita Techno Research (197
3-4) P.147). Therefore, it is necessary to cancel this inductive component. In the present embodiment, the capacitor 3 is connected between the tip of the lower portion 1a of the loop antenna and the ground, and the variable capacitor 4 is connected between the tip of the upper portion 1b of the loop antenna and the ground to form the inductive component. Is canceled.

On the other hand, when the loop antenna shown in FIG. 3 is considered as a minute vertical antenna, its equivalent circuit is as shown in FIG. That is, the loop antennas 1a and 1b are common,
The capacitor 3 and the variable capacitor 4 are in parallel. Since the length (which can also be considered as height) of the micro vertical antenna is significantly shorter than the wavelength used, it is well known (for example, Keiji Endo, Gensada Sato, Atsushi Nagai "Antenna Engineering" ( Showa 44-9-30) Nikkan Kogyo Shimbun P.81),
It will be capacitive. Therefore, this capacitive component must be canceled. Inductance may be used to cancel this capacitive component, but it cannot be connected to the capacitor 3 and the variable capacitor 4. Capacitor 3 and variable capacitor 4
This is because it is provided in order to cancel the inductance component of the loop antennas 1a and 1b, and if the inductance is connected here, the operation as the loop antenna will be hindered.

By the way, the circuit composed of the lower part 1a of the loop antenna and the capacitor 3 and the circuit composed of the upper part 1b of the loop antenna and the variable capacitor 4 shown in FIG. 1 can be set to operate symmetrically. With this setting, the central portions of the loop antennas 1a and 1b become the equilibrium points with respect to the ground, and even if something is connected between the central portions and the ground, the operation as the loop antenna is hardly affected. Therefore, if an inductance (coil 8) for canceling the capacitive component of the small vertical antenna is connected between the central portion of the loop antenna and the ground, the small vertical antenna can be operated without affecting the operation as the loop antenna. be able to.

FIG. 7 shows an equivalent circuit as a minute vertical antenna for the first embodiment.

The solid line in FIG. 5 is an example of the measured directivity of the loop antenna according to this embodiment, and the loop size is
It is 10 m / m x 58 m / m and the measurement frequency is in the 280 MHz band.
The measurement is based on the "Survey Report on Measurement Method for Mobile Radios" (SHO 61-10) Japan Electronic Machinery Manufacturers Association P3-126 ~ P3-13
It was performed according to the measuring method described in 2. As is apparent from FIG. 5, the sensitivity of the loop antenna according to the present embodiment in the directions of 90 ° and 270 ° is improved by about 15 to 20 dB as compared with the conventional loop antenna indicated by the broken line, and 0 °, 180 °. The sensitivity in the direction of ° hardly changes.

As described above, according to the present embodiment, the sensitivity of the micro vertical antenna can be improved without deteriorating the sensitivity of the loop antenna, and the directivity suitable for a pager or the like can be obtained.

Needless to say, even if the positions of the capacitor 3 and the variable capacitor 4 shown in FIG. 1 are exchanged, the same effect as this embodiment can be obtained.

Further, the loop antennas 1a and 1b may be composed of a single member, or may be composed of two or more members, and in any case there is no change in the electrical characteristics. Nor.

(Effect of the Invention) As described above in detail, according to the present invention, the directivity of the loop antenna is improved by an extremely simple structure in which an inductance is connected between the substantially central portion of the loop antenna and the ground. Therefore, the practical effect is extremely large.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an embodiment of the present invention, FIG. 2 is an external view of a conventional loop antenna circuit, FIG. 3 is a circuit diagram of a conventional loop antenna circuit, and FIG. 4 shows the directivity of the loop antenna. 5 and 5 are diagrams showing the directivity of this embodiment, FIG. 6 is a circuit diagram of an equivalent circuit as the vertical antenna of FIG. 3, and FIG. 7 is a circuit of an equivalent circuit of the vertical antenna of FIG. It is a figure. 1a: lower part of loop antenna, 1b: upper part of loop antenna, 2 ... printed circuit board, 3 ... capacitor, 4 ... variable capacitor, 5 ... capacitor, 6 ...
Transistor, 7 ... Choke coil, 8 ... Coil

Claims (1)

[Scope of utility model registration request] 1. A substantially central portion of a loop antenna with a built-in device and a ground, each of which has a capacitor connected between one end of a U-shaped conductor composed of one or more conductors and the ground and between the other end and the ground. A loop antenna circuit, characterized in that an inductance for canceling a capacitive component when the loop antenna is operated as a vertical antenna is connected between the two.