ANTENNAS Basics

The basic ANTENNAS are the 1/4 wavelength monopole ANTENNAS and the 1/2 wavelength dipole ANTENNAS.

• ANTENNAS Dimensions

The length of ANTENNAS is determined by the wavelength of the radio waves used; the longer the wavelength, the longer ANTENNAS, and the shorter the wavelength, the shorter ANTENNAS.
An induced electromotive force is generated in ANTENNAS placed in the air by numerous radio waves.
It is necessary to resonate with the desired radio wave frequency from among these.

• 1/4 wavelength monopole ANTENNAS

As shown in Figure 1, a conductor with a length of 1/4 wavelength is installed vertically.
When high-frequency current is applied, the current distribution is as shown in Figure 1, with the current being maximum at the base and zero at the tip.
For a grounded ANTENNAS, the frequency at 1/4 wavelength is called the natural frequency, and the wavelength at that time is called the natural wavelength.

• 1/2 wavelength dipole ANTENNAS

A half-wavelength dipole ANTENNAS is an ungrounded ANTENNAS with a half-wavelength wire stretched in the air, as shown in Figure 2.
When high-frequency current is applied, the current distribution becomes as shown in Figure 2, with the current being maximum at the center and zero at the tip.
A 1/2 wavelength dipole ANTENNAS has a fundamental frequency that corresponds to λ/2, where λ (lambda) is the wavelength, but it also resonates at frequencies that are integer multiples of λ/2.

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  Figure 1. 1/4 wavelength monopole ANTENNAS

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  Figure 2. 1/2 wavelength dipole ANTENNAS

The directivity of ANTENNAS indicates how much radio waves can be concentrated and radiated in a specific direction.

The directivity of ANTENNAS ANTENNAS the degree to which it can concentrate and radiate radio waves.
An omnidirectional (non-directional) ANTENNAS as shown in Figure 3 is suitable for ANTENNAS used by general broadcasting stations and taxi radios.
On the other hand, when communicating with a specific person who is not moving, it is preferable to use a unidirectional ANTENNAS as shown in Figure 4, which is less likely to interfere with other radio stations.
In Figure 4, the largest radiation pattern in the 0° direction is called the main lobe, the radiation pattern behind in the 180° direction is called the back lobe, and the other lobes are called side lobes.

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  Figure 3 Omnidirectional (non-directional) ANTENNAS

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  Figure 4 Unidirectional ANTENNAS

When power P0 [W] is supplied to a reference ANTENNAS and power P [W] is supplied to ANTENNAS under test, and the same field strength is generated at the same point, P0/P is called ANTENNAS gain.

• Relative and Absolute Gains

ANTENNAS gain is one of the indicators that shows the performance of ANTENNAS. ANTENNAS with high gain can transmit radio waves over long distances even with low power.
The gain can be calculated as the ratio of the electric field strength at the same distance in the direction of maximum radiation when the same power is applied to the reference ANTENNAS and ANTENNAS under test, or it can be calculated from the ratio P0/P of the power P [W] radiated from ANTENNAS ANTENNAS the power P0 [W] radiated from the reference ANTENNAS when an arbitrary antenna radiates with the same electric field strength at the same distance in the direction of maximum radiation.
The gain when a half-wave dipole ANTENNAS is used as the reference ANTENNAS is called the relative gain Gr, and the gain when an isotropic ANTENNAS is used as the reference antenna is called the absolute gain Ga.
The relationship between Ga and Gr is Ga = 1.64Gr or Ga = Gr + 2.25 [dB]

• ANTENNAS gain as defined by the Radio Law Enforcement Regulations

Article 2, paragraph 74 of the Radio Law Enforcement Regulations stipulates the following:
"An antenna gain is the ratio of the power supplied to the input of a given antenna to the power required at the input of a reference antenna to produce the same field at the same distance in a given direction. In this case, unless otherwise specified, the figure expressing the gain of an antenna refers to the gain in the direction of the main radiation."

As wireless technology advances, a wide variety of ANTENNAS are used for a variety of purposes.
The basic structure of a linear ANTENNAS is shown below.

Source: NTT Docomo Technical Journal Vol.5 No.3

Figure 5. Diagrams of various ANTENNAS structures

The features and applications of the structure shown in Figure 5 are shown in Table 1.

Source: NTT Docomo Technical Journal Vol.5 No.3

Table 1 ANTENNAS characteristics and uses