JPH0324804B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a structure of a radiating element of a directional antenna. More specifically, the present invention relates to a wideband directional antenna capable of widening the directivity of the antenna and widening the VSWR characteristic.

As the use of radio waves increases, a wideband antenna that can be used for multiple channels has become required. In particular, the UHF broadcast wave band has a wide bandwidth.
Channel changes or multi-channel sharing are often performed. In this case, unless the directivity characteristics of the antenna are made constant with respect to frequency changes, the service area etc. will change, causing problems.

Conventionally, turn style antennas, super gain antennas, etc. have been used as antennas for TV broadcasting, but since these have relatively narrow bands, recently a dipole with a reflector, which is a combination of a dipole of about 1 wavelength and a reflector, has been used. Antennas are also being used. Although this antenna can have relatively good VSWR characteristics over a wide band by making the radiating element thicker, it is difficult to widen the directional characteristic, that is, to reduce the frequency dependence of the directional characteristic.

The reason for this is that if the radiating element is shortened, the gain will decrease at low frequencies, and if the radiating element is made long, the directivity will become sharper at higher frequencies and unnecessary side lobes will occur.

Therefore, when forming an omni-directional antenna by attaching reflector-equipped antennas to the four sides of a steel tower, for example, it has been difficult to achieve desired directional characteristics over a wide band.

The reason for this is that when conventional dipole antennas with reflectors are combined, the directivity of each dipole antenna with a reflector has a frequency characteristic, and the half-power width tends to be wide at low frequencies and narrow at high frequencies. This tendency becomes even more inconvenient depending on the distance phase.

An object of the present invention is to provide a dipole antenna with a reflector that has uniform directional characteristics over a wider band than conventional antennas.

The present invention will be described below with reference to the drawings.

FIG. 1 is a conceptual cross-sectional view of a super gain antenna and its current distribution.

Since the super gain antenna is a dipole antenna with a reflector, which is a combination of a dipole antenna 1 of about 1/2 wavelength and a reflector 2, the current distribution basically becomes the current distribution curve _I1 shown in FIG. However, in order to use the same antenna over a wide band, if a current with a frequency doubled is supplied, the current distribution becomes the current distribution curve I ₂ shown in FIG. 1.

FIG. 2 is a conceptual cross-sectional view of a dipole antenna with a reflector, which is a combination of the dipole antenna 3 of the first wavelength and the reflector 2, and its current distribution.

The current distribution when a current corresponding to a frequency of about one wavelength is supplied is a current distribution curve I ₂ in FIG. 2. If a double frequency current is supplied to this antenna, the current distribution will be the current distribution curve _I3 in FIG. 2.

FIGS. 3 and 4 are diagrams showing examples of the directivity of radiated electromagnetic waves corresponding to the antennas shown in FIGS. 1 and 2, respectively. Each broken line is the calculated directional characteristic for a low frequency (470MHz), and the solid line is the calculated directional characteristic for a high frequency (860MHz).

As can be seen from FIGS. 3 and 4, the directional characteristics change depending on the frequency in either case. An object of the present invention is to solve the drawback of the conventional dipole antenna with a reflector that the directivity characteristics change depending on the frequency. The above problem can be solved by using a bent dipole antenna to make the directivity of each dipole antenna with a reflector wider at higher frequencies than at lower frequencies, thereby minimizing the cut in the directivity due to distance phase. It is something to be solved.

5 to 7 are conceptual cross-sectional views of a dipole antenna with a reflector according to the present invention and diagrams showing the flow of current at a certain moment. FIG. 8 is an equivalent current distribution curve of the dipole antenna of the present invention.

FIGS. 5 to 7 show a certain moment when the antenna of the present invention is supplied with a current having a frequency corresponding to the current distribution curves I ₁ , I ₂ , I ₃ of FIGS. 1 and 2, respectively. The distribution of current and direction of flow at is shown.

The current distributions in FIGS. 5 to 7 and the current distributions in FIGS. 1 and 2 differ in the vicinity of the tip of the dipole as follows. That is, the current near the tip of the dipole shown in FIGS. 1 and 2 disappears, and the current flowing through the tip bent portion 4 shown in FIGS. 5 to 7 takes its place. Note that the longitudinal direction components of the dipole of the current flowing through the tip bent portion 4 and the current flowing through the dipole tip adjacent to the tip bent portion 4 are in opposite directions. Therefore, the electromagnetic waves radiated from the bent tip portion 4 and the dipole tip portion 5 each have components with opposite phases and weaken each other. As a result, the substantial current distribution in the dipole antenna of the present invention corresponding to the current distribution curves I ₁ , I ₂ , I ₃ is 8th
The current distribution curves are I ₁ ′, I ₂ ′, and I ₃ ′ in the figure.

Since there is no significant change in waveform symmetry between the current distribution curves I ₁ and I ₁ ′ and between I ₂ and I ₂ ′, the radiation directivity characteristics are almost the same. Note that since the average distance of the current distribution from the center of the dipole becomes small, the directivity characteristics become somewhat isotropic.

On the other hand, there is a large change in waveform symmetry between I ₃ and I ₃ ′, and the waveform of I ₃ ′ is close to the symmetry of I ₂ . As a result, the directional characteristics of the radiated waves generated by the current distribution of I ₃ ' are close to those of I ₂ . Therefore I ₃ ′ and
The change in the directivity characteristic of I ₂ ' is small, that is, the frequency dependence of the directivity characteristic of the radiation wave radiated from the antenna of FIG. 8 is small.

FIG. 9 is an example of calculating the directivity characteristics of an embodiment of the dipole antenna of the present invention. Similar to Figures 3 and 4, the dashed line in Figure 9 indicates a low frequency (40M
Hz), and the solid line shows the antenna's directional characteristics for high frequencies (860MHz). As shown in FIG. 9, the directivity characteristics of the antenna of the present invention have almost no frequency dependence (at least in the range of 470 to 860 MHz). Naturally, the decrease in the half-power angle at high frequencies is also small.

FIG. 10 shows the directivity characteristics for high frequencies of the conventional dipole antenna with a reflector and the dipole antenna with a reflector according to the present invention when combined as an omnidirectional antenna. The broken line is the directional characteristic of the conventional antenna, and the solid line is the directional characteristic of the antenna of the present invention.

FIG. 10 shows that the antenna of the present invention has a more isotropic directivity characteristic.

Directional characteristics were calculated as follows.

Figure 11 is for explaining the calculation of directional characteristics.
1 is a conceptual cross-sectional view of a dipole antenna with a reflector according to the present invention.

x is the coordinate axis in the longitudinal direction of the dipole, θ is the radial direction, H ₁ is the length of the tip bent portion 4 of the radiating element,
H-H ₁ is the distance between the center of the dipole and the tip 6 of the dipole, h is the distance between the reflector and the dipole, and 1' is the mirror image of the dipole.

Using the angles and dimensions shown in FIG. 11, the electric field directivity characteristic D(θ) of the present invention is approximately calculated by the following equation.

D(θ)≒jηI ₀ /λcos(mh cos θ) ×∫ ^H ₀ f cos(mx sin θ)dx where f=sin(m ₁ (H-x)) ...0x<H ₁ f=-sin( m ₂ (x-H ₁ )) ...H ₁ xH λ: Wavelength of electromagnetic wave in free space m, m ₁ , m ₂ : Propagation constant in free space and dipole, respectively η: Spatial impedance 376Ω I ₀ : Abdominal current of antenna The radiation resistance R _r can be obtained from D(θ) using the following formula.

R _r =1/I ₀ ² ∫ ² 〓 ₀ ∫〓 ^/2 ₀ D(θ) ² /120πr ² sinθ
dθ d Since the frequency dependence of D(θ) is small, the radiation resistance
The frequency dependence of R _r is small. Therefore VSWR
The characteristics are also wideband. Figure 12 shows an example of VSWR frequency characteristics.

In this manner, the present invention achieves equivalently reducing the current distribution at the tip of the dipole and reducing the frequency dependence of the directional characteristics of dipole radiation.

[Brief explanation of drawings]

Figure 1 is a conceptual cross-sectional view of a super gain antenna, Figure 2 is a conceptual cross-sectional view of a dipole antenna with a reflector that combines a first wavelength dipole antenna and a reflector, and Figures 3 and 4 are A diagram showing an example of the directivity of radiated electromagnetic waves corresponding to the antennas of FIGS. 1 and 2, respectively, FIGS. 5 to 7 are conceptual cross sections of the antenna according to the present invention, and FIG. 8 is an equivalent diagram thereof. Fig. 9 shows the directivity characteristics of the antenna according to the present invention, Fig. 10 shows the directivity characteristics of the antenna composed of a dipole antenna as an omnidirectional antenna, and Fig. 11 shows the directivity of the antenna of the present invention. Conceptual cross-sectional view of the antenna showing the parameters necessary to calculate the characteristics, 12th
The figure shows an example of VSWR frequency characteristics. DESCRIPTION OF SYMBOLS 1...Dipole antenna, 2...Reflector, 3...Dipole antenna, 4...Bend part, 5...Dipole tip, 6...Bend point, 7...Dipole end, _I1 , _I2 , _I3 ...Current distribution Curves, I ₁ ′, I ₂ ′, I ₃ ′...Equivalent current distribution curves.

Claims (1)

[Claims] 1. Direct both sides of the length of the 1/8 to 1/6 length of the distance between both ends of the antenna from the tip of the rod-shaped or flat dipole radiating element toward the center of the dipole on the opposite side of the electromagnetic wave radiation direction. 180° or more
A dipole antenna with a reflector comprising a dipole bent at an angle of less than 270° and a reflector 2 provided parallel to the dipole radiating element on the opposite side of the electromagnetic wave radiation direction.