JPH022324B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an antenna device for receiving FM broadcasts or television broadcasts.

In Japan, FM broadcasting ranges from 76 to 90MHz.
90-108MHz and 170MHz for VHF television broadcasting
A frequency of ~222MHz has been allocated, and a folded dipole antenna is used as the receiving antenna. This is because if the total length of the folded dipole antenna is approximately λ/2 of the reception frequency (λ: wavelength), it can be easily matched to a feed line with an impedance of 300Ω.

However, this folded dipole antenna requires more than twice as much material as a linear dipole antenna, and the product price is high because it requires machining for folding.
In addition, when used as the radiator 1 of the Yagi antenna shown in Fig. 1, an insulating support base 2 that commonly supports the primary side terminal to which the feed line is connected and the opposing secondary side terminal is provided. Due to the large size, this also leads to an increase in the product price, and from the viewpoint of electrical characteristics, the primary side should be located on the reflector 3 side or the waveguide 4
There were drawbacks such as the need for installation instructions because the characteristics differ depending on which side it is located on. On the other hand, although a linear dipole antenna does not have such drawbacks, it is difficult to match it well to a feed line with an impedance of 300Ω.

This invention was made in view of the above circumstances.
Impedance of linear dipole antenna is 300Ω
An object of the present invention is to provide an antenna device that can be well matched to the feed line of the antenna.

Hereinafter, the present invention will be described in detail with reference to the drawings, taking as an example the case where the present invention is implemented in a receiving antenna for VHF television broadcasting. FIG. 2 is a circuit diagram showing this embodiment. In the figure, 6 and 6 are linear dipole antennas, 7 is a coil, 8 is a capacitor, 9 and 9 are terminals for connecting a feed line (hereinafter referred to as feed terminals), and 10 is a distance from the linear dipole antennas 6 and 6. d shows linear elements arranged in parallel in line symmetry.

By the way, the total length of the linear dipole antennas 6 and 6 is set to a center frequency of 99MHz with a low frequency of 90 to 108MHz.
If approximately 0.55λ is selected, the impedance characteristic will be as shown in the Smith Chart in Figure 3. The impedance Z・ at 0.5λ of a linear dipole antenna is Z・=73.13+j42.55 [Ω], but Smithchart showed that in order to match the impedance to 300Ω, it is better to increase the overall length. This can be known from the impedance characteristics. That is, the Smith Chart in Figure 3 is the normalized impedance Z ₀
The frequency characteristics of impedance at =30Ω are shown, but the frequency near the center point of Smith Chart is
It is around 120MHz, and 0.55λ at 99MHz corresponds to 0.67λ at 120MHz, so the longer the antenna length, the closer the impedance is to 300Ω. 90~108MHz
In the band , the impedance has inductive reactance, so if a capacitive reactance is connected in parallel, the inductive reactance is canceled out and the impedance moves on a Smith Chart along a constant circle with the conductance. in this case,
It is necessary to minimize the impact on the 170-222MHz band. This is because as the frequency increases, the value of the capacitive reactance decreases, and the power supply terminals 9 become almost short-circuited. In other words, if you connect capacitors in parallel, the capacitors are capacitive reactances, and 1/2πfC
Since it can be expressed as (f: frequency), as the frequency increases, the value of capacitive reactance decreases in proportion and approaches 0. In other words, a short circuit occurs with the connecting terminal.

Connecting a capacitor in parallel to compensate for the inductive reactance from 90 to 108 MHz, the 170 to
In the 222MHz band, there is a possibility of shorting the power supply terminals 9, 9, but this must be avoided.
To do this, connect a coil in series with the capacitor.

That is, since the coil has an inductive reactance and can be expressed as 2πfL (f: frequency), the value of the inductive reactance increases in proportion to the frequency. 170 by appropriately selecting the value of coil L.
It exhibits a large inductive reactance value in the band of ~222 MHz, resulting in a characteristic in which the circuit is essentially in an open state. If only the 90 to 108 MHz band is to be received, there is no need to take such consideration, and it is sufficient to connect only the desired capacitive reactances in parallel.

The series resonant circuit consisting of coil 7 and capacitor 8 satisfies the above conditions and exhibits the desired capacitive reactance in the band of 90 to 108 MHz, and
The values were chosen to show a very large inductive reactance in the 222MHz band. results of the experiment,
The optimum values were approximately 1.2 μH for the coil 7 and approximately 1.5 pF for the capacitor 8, but as a result of connecting a series resonant circuit of the coil 7 and the capacitor 8 having these values, the impedance at the feed terminals 9, 9 was as follows. As shown in the Smith Chart in Figure 4, 90~
The 108MHz band was matched to 300Ω. That is,
The Smith chart in Figure 4 is the normalized impedance
Since Z ₀ =300Ω, the impedance at the center point is 300Ω, and the band from 90 to 108 MHz is concentrated near the center point. Generally, a method is adopted in which 99MHz, which is the center frequency of 90 to 108MHz, is adjusted to be near the center point, and in the same figure, the vicinity of 100MHz is closest to the center point. Note that the total length of the antenna is the same as in FIG. 2.

Next, when the linear elements 10 are spaced apart by a distance d and disposed line-symmetrically on the linear dipole antennas 6, 6, the impedance Z _io at the feed terminals 9, 9
is expressed by the following formula.

However, Z・₁₁ ; Self-impedance of the linear dipole antennas 6, 6 Z・₂₂ ; Self-impedance of the linear element 10 Z・₁₂ ; Mutual impedance of the linear dipole antennas 6, 6 and the linear element 10 90 to 108MHz The impedance in the band has already been matched to 300Ω at the stage mentioned above, so to avoid adversely affecting this, Z・₂₂ and Z・
Decide on ₁₂ . For this purpose, the total length of the linear element 10 is selected to be short, for example, with respect to the wavelength of the center frequency of 99 MHz in the band of 90 to 108 MHz. Fortunately, 170-222MHz
Since the center frequency of the band 196MHz is at a frequency ratio of approximately 1:2, if the total length of the linear element 10 is
If it is λ/4 of 99MHz, it will be about 1/2λ at 196MHz, so it will not affect the band from 90 to 108MHz.
It is possible to have a large impact on the ~222MHz band. But actually 90~
There will also be some negative impact on the 108MHz band, so
The total length of the linear element 10 will be determined while looking at the state of impedance with the band of 170 to 222 MHz.

Z・₁₂ is the total length of the linear dipole antennas 6, 6,
It is almost determined by the total length of the linear element 10 and the distance d, but if values are selected using these as parameters, Z· _io in equation (1) will match 300Ω as shown in the Smith chart in Figure 5. can be done. That is, since the normalized impedance Z ₀ of the Smith Chart shown in FIG. 5 is 300Ω, the closer it is to the center point, the closer it is to 300Ω, and the band from 170 to 222 MHz is concentrated near the center point. That is,
Although Z· ₁₁ has capacitive reactance in the band of 170 to 222 MHz, it can be considered that this capacitive reactance is compensated by the second term of equation (1).
Since the total length of the linear element 10 is less than λ/2 with respect to the wavelength, Z· ₂₂ is an impedance having capacitive reactance. Therefore, the impedance with which reactance Z・₁₂ has can be calculated backwards from the value of Z・_io , but since it is easier to find Z・_io experimentally, the value of Z・₁₂ is I didn't dare ask.

The optimum value for matching the band from 170 to 222MHz to 300Ω is approximately 0.38, where the total length of the linear element 10 is 196MHz.
~0.48λ, similarly d=0.033~0.085λ.

On the other hand, the total length of the linear dipole antennas 6, 6 and the linear element 10 is limited from the viewpoint of antenna directivity, but if only the linear dipole antennas 6, 6 are considered, the length should be approximately 1.3λ or less at 222MHz. Ba,
As shown in FIG. 6, the directional side lobe is smaller than the main lobe and is within a practically acceptable range. Furthermore, since the total length of the linear element 10 is such that it operates as a waveguide element in the band of 1700 to 222 MHz, the directivity including the linear element 10 is as follows.
As shown in FIG. 7, the antenna is unidirectional, and the gain of the antenna is improved compared to the folded dipole antenna, as shown in FIG. In FIG. 8, the broken line shows the directional characteristic in the case of a folded dipole antenna, and the solid line shows the directional characteristic in the case of a dipole antenna having linear elements. The total length of the linear element 10 can be considered to be freely selected as long as it does not serve as a reflector.

As described above, according to the present invention, a linear dipole antenna can be well matched to a feed line with an impedance of 300Ω, which eliminates all of the drawbacks of the folded dipole antenna mentioned at the beginning, resulting in a low product price. Antenna equipment can be provided.

Also, since the gain of the antenna is improved, it is possible to receive a good television image. Furthermore, the linear shape of the antenna allows for completely free design applications; for example, depending on the purpose of use, it can be folded into a telescoping rod antenna that is convenient to carry, or as shown in Figure 9. It is also possible to create a small antenna with a zigzag shape. Further, as shown in FIG. 10, the coil in FIG. 2 may be formed into a parallel resonant circuit of a capacitor and a coil.

[Brief explanation of drawings]

Fig. 1 is an external perspective view of a five-element Yagi antenna using a conventional folded dipole antenna, Fig. 2 is a circuit diagram showing an embodiment of the present invention, and Fig. 3 shows the impedance characteristics of a linear dipole antenna. Fig. 4 is a Smith chart showing the state when a coil and a capacitor are connected to the antenna, and Fig. 5 is a Smith chart showing the state when a linear element is arranged in the antenna.
Fig. 6 is a directivity diagram of a linear dipole antenna, Fig. 7 is a directivity diagram when a linear element is arranged in the above antenna, Fig. 8 is a gain characteristic diagram, and Figs. 9 and 10 are FIG. 6 is a circuit diagram showing other embodiments of the present invention. 6... Linear dipole antenna, 7... Coil, 8... Capacitor, 9... Power supply terminal, 10...
... Linear element.

Claims (1)

[Claims] 1. A linear dipole antenna having an inductive reactance in a low frequency band whose total length is 1.3 times or less the wavelength of the frequency used, and a capacitive reactance connected in parallel to the output terminal of this antenna to compensate for the inductive reactance. and an impedance circuit consisting of a combination of two or more reactance elements that exhibit large inductive reactance in a high frequency band, and an impedance circuit that is symmetrical to the linear dipole antenna and whose spacing is 0.033 to 0.085 of the wavelength of the center frequency in the high frequency band. 1. An antenna device comprising: a linear element having a length equal to or less than 0.5 times the wavelength of the center frequency and arranged so as to be twice as long as the wavelength of the center frequency.