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The antenna mechanism

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We introduced the formula for the wave length of radio waves in the previous section. An antenna which will be most efficient is an antenna with a length half the wave length of the frequency used. For example, if you are using a frequency of 433 MHz, the wave length is about 70 cm, therefore an antenna with a length of about 35 cm will be most efficient. The transmitter must emit radio waves using limited power, and the receiver must capture efficiently the radio waves that are emitted. With the antenna at this length, the antenna and transmitted radio waves achieve a resonant state and maximum power is emitted. At the receiver too, the received radio waves and antenna achieve a resonant state, and can capture the maximum power. The antenna should be kept as straight as possible and should not be bent into a circle. Today equipment tends to be compact, and antennas with a length ¼ (λ/4) that of the wave length are frequently used. The thinking behind λ/4 ground antennas is the same as for λ/2 dipole antennas. However as the function of one side is changed to earth, the antenna length is halved making a 1/4 λ antenna. For this reason, this earth is very important. The whip antennas of radio modules, mobile phones and so on use this mechanism, with the case serving the function of the ground.

Wave length λ λ/4 ground antenna

Types of antenna

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The types of antenna include whip antennas, dipole antennas, Yagi-Uda arrays, parabola antennas, loop antennas and so on.

Whip (rod) antennas Antennas used for mobile phones and the like. Non-directional antenna with equal sensitivity in any direction.
Dipole antenna Used for amateur radio and so on.
Yagi-Uda array Used as television antennas and so on. They have strong directivity, and must be aligned in the direction of the transmitting station. This is a dipole antenna with director and reflector elements attached to direct and reflect the radio wave.
Parabola antenna Used for receiving satellite broadcasts. These antennas have very strong directivity and require fine directional adjustment, but they can use the power of the radio waves efficiently.
Loop antenna Loop antennas capture the changes in the magnetic field of the radio waves. The radio waves propagate in the direction at right angles to the circle of the loop.

Similarly the receiving antenna is placed in such a way as to be perpendicular to the magnetic field of the radio waves.
Dielectric antenna Antennas using high frequency dielectric ceramics can be compact and achieve high performance.

Directional pattern of antennas

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There are both directional antennas and non-directional antennas. Antennas with directivity are used in cases where the direction of the other party in communication is fixed. This avoids unwanted radio wave emission in the environment and does not pick up noise from other directions. It is convenient as it allows efficient transmission with low power. Radio waves radiating in a specific direction are called a beam. Non-directional antennas radiate unwanted radio waves in the environment, and conversely pick up noise from every direction. However, communication is possible wherever the other party in communication goes, so they are suited to mobile applications. Directional antennas include Yagi-Uda arrays, parabola antennas and the like. Non-directional antennas include whip antennas and so on. The following diagrams show directivity. Although it is not shown here, naturally radio waves radiate in three dimensions, so we should also consider the directivity pattern when seen from the side too. The directional pattern diagrams show the relative intensity of the maximum field strength in any direction, thus indicating electric field directivity.

Directivity of typical antennas

Directional antennas and non-directional antennas
In the diagram above, with the whip antenna the radio waves are radiating in every direction equally, so it is a non-directional antenna. With the Yagi-Uda array and parabola antenna, the radio waves are radiating in a specific direction, so they are said to be directional antennas (beam antennas)

Main lobe, side lobe, and back lobe
If we take the Yagi-Uda array as an example, the largest radiation beam in the intended direction is the main lobe, and in the opposite direction the unwanted radiation that occurs is called the side lobe. A side lobe occurring in space in the direction opposite to the main lobe is called back lobe.

FB ratio
If we look at the directional pattern of the Yagi-Uda array, there are a main lobe and a back lobe generated. The ratio between the main lobe and back lobe, called the FB (Front/Back) ratio, is calculated to express the level of directivity of the antenna, and this is shown in decibels (dB). Therefore the larger this value, the better the performance of the antenna.


As the directional pattern diagram shows the field strength, we use 20 1og for the calculation.

Gain of antennas

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When choosing an antenna, directivity and gain are concerns. Furthermore, depending on the specification, the unit of gain is expressed variously as dBd, dB, dBi and it is difficult to make a decision on which to choose.
Also, because the antenna is made of metal and there is no circuit for electrical amplification, the fact that there is gain may seem a little strange.
Antennas can concentrate input energy in a certain direction, but there are differences in the method of concentration according to the type and between different antennas. In other words, antennas that spread the input power in directions other than that of the other party in communication, and antennas with directivity that concentrate the power efficiently, show differences in range. This difference is the difference of gain, and the higher the gain, the more acute directivity becomes, and this means that directional alignment becomes more difficult.
Antenna gain is expressed as he ratio of received power at the maximum electric field direction when the same power is input to an antenna under test and a reference antenna
To express antenna gain, there are two methods, one using an isotropic antenna as reference, the other using another type of antenna (usually a λ/2 half wave length dipole antenna) as reference.

When using an isotropic antenna as reference, the gain is called absolute gain, and the unit used is dBi.
When using an ideal half wave length (λ/2) dipole antenna as reference, the gain is called relative gain, and the unit used is dBd.

With relative gain, the ratio of the absolute gain of the antenna used as reference, and the absolute gain of the antenna in question is equivalent. As the absolute gain of the half wave length (λ/2) dipole antenna used as reference is 2.14 dBi, the relative gain Gr dBd of an antenna with absolute gain of Ga dBi is found by relative gain Gr dBd = absolute gain Ga dBi – 2.14 dB.
In other words, between dBd and dBi, the relationship 0 dBd = 2.14 dBi obtains.
If an antenna specification is 2.14 dBi, it means that it is equivalent to an ideal half wave length dipole antenna.

For antenna gain, the expressions dBd and dB mean the same thing, with dBd being the formal designation.
Isotropic antennas are theoretic, formulaic, virtual antennas, that radiate radio waves in all directions with equal intensity, and that have spherical directivity.
 

Impedance matching

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When connecting an antenna from a high frequency circuit it is necessary to transfer power efficiently and ensure that no problems arise with reflection of the radio waves. Reflection occurs when the signal source impedance and the impedance of the antenna do not match, and making them match is called impedance matching. Reflection means the situation in which part of the signal sent in the direction of the antenna returns towards the signal source, and if it combines with the incident signal, adverse effects may arise.
 The specification of an antenna will always include ″Input impedance: 50 Ω or the like, so impedance matching should be implemented at the connection circuit so as to match this value. It is also necessary for the impedance of the cable used to match. The impedance of the cable is decided by the per unit impedance and capacitance, and the impedance of cables on the market will always be indicated.
 There are various methods of impedance matching. However, as it is a very involved subject, we would refer you to a specialist textbook.

Horizontally polarized waves and vertically polarized waves

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The radio waves radiated from antennas standing vertically propagate vertically in relation to the ground, and are called vertical waves. In the same way, with horizontally placed antennas the electric field is horizontal in relation to the ground, so the waves are called horizontal waves. Circularly polarized waves are also used, for satellite broadcasts and so on.
 Naturally, if the plane of polarization of both antennas does not match, there will be a lot of loss in capturing the radio waves.

Horizontally polarized waves / Vertically polarized waves

Antenna material

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Because high frequency currents flow through antennas, naturally they must be made of metal. Therefore, metals with a low specific resistance are used as the material for antennas. However, silver and gold are not appropriate from the perspective of cost accounting, and steel rusts and is heavy so it is not suitable for antennas. Ordinarily, aluminum is used for antennas for its low specific resistance and low cost, but this is often used for relatively large antennas. For compact equipment such as mobile phones and radio modules, antennas made of shape memory alloys (titanium-nickel alloy) or of stainless steel are used, or dielectric antennas. Even simple antennas made of piano wire are used.

How to use antennas

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Antennas should be attached to the outside of the product, on the top if possible. The antenna should be attached in a position as far as possible from the human body. Radio waves above 750 MHz are particularly easily absorbed by the human body, so caution is required. For equipment that fits against the human body, allowance should be made for a distance of at least 2 to 3 cm. The case in which the radio module is incorporated should be ABS plastic. When using a metal case that attenuates radio waves, only the main unit of the radio module should be built in, and the antenna should be outside. Furthermore, you should ensure that the case of the module and the metal case have the same electric potential. The antenna should be kept as straight as possible and should not be bent into a circle. Make the plane of polarization of the radio waves match for both antennas. If the antenna is located externally, always use coaxial cable, and implement impedance matching.

Mobile phone antennas

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When the antenna used in a mobile phone is extended, it is a λ/4 whip (rod) antenna, but when it is retracted, the coil in the tip becomes a helical antenna. Compared with the whip antenna, the sensitivity of this helical antenna is inferior, so mobile phones are used with the antenna extended. A receive only F type antenna is built into the inside of the phone, and the internal and external antennas receive using spatial diversity, and internal functions for power control and so on are applied to the signal. As this is a mobile application, directivity is non-directional. The frequency of the radio waves used in mobile phones (in Japan), is 800 MHz outgoing and 900 MHz incoming. If we calculate the length of the antenna (excluding the helical antenna) using the medial frequency of 850 MHz, the wave length λ with 850 MHz is

resulting in a λ/4 antenna of about 9 cm.

There are mobile phones that use the 800 MHz band, and those that use the 1,500 MHz band. The antenna of the latter is shorter. There are also mobile phones in the 800 MHz band with short antennas. There are also antennas in which, when extended, the upper helical antenna is electrically connected, becoming a λ/2 wave length antenna.

How to use mobile phones efficiently

For the reasons above, the following are suggestions for how to use a mobile phone without experiencing problems, and for high quality voice and data transmission.

· Extend the antenna fully when using the phone, and take care not to cover it in any way. · As there is an internal antenna near the top of the phone, hold the phone near the bottom. · Try to keep the antenna as far from your body as possible. · If the state of the signal seems bad, trying moving or turning around. · Use an antenna of an appropriate length. Do not modify or replace the antenna. · Do not use a metal strap or metal fittings. · Make sure the antenna is vertical. · We do not recommend antennas with flashing illumination.

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