Antenna Basics (4)

If the dipole antenna diameter thick, the antenna input impedance versus frequency to smaller, easier to maintain the match and feeder, then the antenna on a wide frequency range. On the contrary, is narrow.
In practice, the antenna input impedance will be affected by the surrounding objects. To make a good match with the antenna feeder, will also be required by the erection of the antenna measurement, appropriate adjustments to the antenna of the local structure, or installation of matching devices.

3.5 Return Loss

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       As noted, when the feeder and antenna match, there is no reflected wave on feeder, only the incident wave, which is transmitted to the feeder on the direction of travel of the wave antenna. At this time, the voltage throughout the feeder on the amplitude and current amplitude are equal, any point on the feeder impedance is equal to its characteristic impedance.
       The day does not match the feeder lines and when the antenna impedance is not equal to the characteristic impedance of the feeder, the feeder on the load transfer can only absorb part of high-frequency energy, and can not absorb all the energy is not absorbed will reflect back that part of the formation of reflected wave.
       For example, in right, because the impedance of the antenna with different feeders, one for 75 ohms, a 50 ohm impedance matching, the result is

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3.6 VSWR
       In the case of mismatch, feeders exist incident and reflected waves. Phase of incident and reflected waves in the same place, the voltage amplitude of the sum of the maximum voltage amplitude of Vmax, the formation of wave loop; incident and reflected waves in opposite phase relative to the local voltage amplitude was reduced to the minimum voltage amplitude Vmin, the formation of wave section. Other points of the wave amplitude value is between abdominal and wave between sessions. This standing wave is called the line.
       Incident voltage and reflected voltage wave amplitude ratio is called the reflection coefficient, denoted R
    Reflected wave amplitude (ZL-Z0)
R = ─ ─ ─ ─ ─ = ─ ─ ─ ─ ─ ─ ─
    Incident wave amplitude (ZL + Z0)
Antinode voltage and nodal voltage amplitude ratio as VSWR, also called the voltage standing wave ratio, denoted as VSWR
            Antinode voltage amplitude Vmax (1 + R)
VSWR = ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ = ─ ─ ─ ─
            Nodal degree of convergence voltage Vmin (1 - R)
Terminal load impedance ZL and the closer the characteristic impedance Z0, the smaller the reflection coefficient R, standing wave ratio VSWR closer to 1, the better the match.

3.7 balance device
Signal source or load, or transmission line, according to their relationship to the land, can be divided into two types of balance and imbalance.
If the source voltage between the ends and in the same size, opposite polarity, called the balanced signal source, otherwise known as the unbalanced signal source; if the load voltage to ground at both ends of the same size, opposite polarity, is called load balancing, otherwise known as the unbalanced load; if the transmission line impedance between two conductors and ground the same, is called balanced transmission line, otherwise unbalanced transmission line.
Imbalance in the load imbalance between the signal source should be used with coaxial cable, the balance between source and load balancing with parallel wire transmission line should be connected, so as to effectively transmit signal power, or they do not balance or balance will be destroyed and can not work properly. If you use the unbalanced transmission line and balance the load connected, the usual approach is to install in the grain between the "balanced - unbalanced," the conversion device, commonly referred to as balun.

3.7.1 half-wavelength balun
Also known as "U" shaped tube balun, which is used to balance the load feeder coaxial half-wave symmetry and balance of the connection between the oscillator. "U" shaped tube balun impedance transformation of 1:4 there role. Mobile communication system using coaxial cable characteristic impedance is usually 50 ohms, so the YAGI antenna, using the equivalent half-wave oscillator to adjust the impedance to 200 ohms or so, to achieve the ultimate with the main feeder impedance 50 ohms coaxial cable .

3.7.2 quarter-wavelength balanced - unbalanced device
Quarter-wavelength short-circuit transmission lines using the terminal for the open nature of the high-frequency antenna to achieve the balanced input port and output port coaxial feeder imbalance between the balance - imbalance in transformation.

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Antenna Basics (4)
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3 transmission line a few basic concepts
Connect the antenna and transmitter output (or receiver input) of the cable as a transmission line or feeder. Transmission line's main task is to effectively transmit the signal energy, therefore, it should be able to power the transmitter signals with minimal loss of transmission to the input of the transmitting antenna, or antenna to transmit signals with minimum loss to the receiver input, while it itself should not pick up or stray interference signals, so that it requires transmission lines must be shielded.
Incidentally, when the physical length of t

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ransmission line is equal to or greater than the wavelength of the transmitted signal, the transmission line is also called long-term.

3.1 the type of transmission line
FM section of the transmission line there are two: the parallel wire transmission lines and coaxial transmission line; microwave band of the transmission lines are coaxial transmission line, waveguide and microstrip. Parallel wire transmission line formed by the two parallel wires is symmetrical or balanced transmission line, this feeder loss, could not be used for the UHF band. Coaxial transmission line of the two conductors were wire and copper mesh shielding, due to copper mesh ground, two conductor-to-ground asymmetry, so called asymmetric or unbalanced transmission lines. Coaxial cable operating frequency range, a small loss, coupled with some of the static shielding effect, but do little to interfere with the magnetic field. Avoid use with a strong current parallel to the line, not near the low-frequency signal lines.

3.2 The characteristic impedance of transmission line
Around an infinitely long transmission line voltage and current ratio is defined as the transmission line characteristic impedance, with Z0 said. Coaxial cable characteristic impedance is calculated as
Z. = [60 / √ εr] × Log (D / d) [Europe].
Where, D is the diameter of the coaxial cable outer conductor copper mesh; d for the coaxial wire diameter;
εr is the relative dielectric between conductor dielectric constant.
Usually Z0 = 50 Europe, there are Z0 = 75 ohm.
Easy to see from the above equation, only the feeder characteristic impedance and the conductor and the conductor diameter D and d between the dielectric constant εr, but not with the length of the feeder, the feeder terminal frequency and connected load impedance.

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3.3 feeder attenuation coefficient
       Signal transmission inside the feeder, in addition to resistive losses in the conductor, there are insulating materials, dielectric loss. The two losses increase with the length of the feeder and the operating frequency increases. Therefore, we should try to shorten the rational distribution feeder length.
       Loss per unit length generated by the size of the attenuation coefficient β is used, the unit of dB / m (dB / m), cable technology, most of the instructions on the unit with the dB / 100 m (dB / meters).
Let the power input to the feeder for the P1, from the length L (m) of feeder output power P2, the transmission loss TL can be expressed as:
TL = 10 × Lg (P1 / P2) (dB)
Attenuation coefficient
β = TL / L (dB / m)
      For example, NOKIA 7 / 8 英寸 low-power cable, 900MHz attenuation coefficient β = 4.1 dB / 100 m, can also be written as β = 3 dB / 73 m, that is, the signal power at 900MHz, each after 73 m length of this cable, the power to less than half.
      The average non-low-power cable, for example, SYV-9-50-1, 900MHz attenuation coefficient of β = 20.1 dB / 100 m, can also be written as β = 3 dB / 15 m, that is, a frequency of 900MHz signal power, each 15 m long through this cable, the power would be less than half!

3.4 Matching concept
      What match? Simply put, the feeder terminal connected load impedance ZL is equal to the characteristic impedance Z0 feeder, the feeder terminal is called the matching connections. Match, there is only transmitted to the feeder on the load terminal incident, but not the load generated by the end of the reflected wave, therefore, the same day as the termination load line, matching the antenna can be made to ensure that all the signal power. As shown below, the day when the line impedance of 50 ohms, and 50 ohm cable is matched, while in Europe the day when the line impedance of 80, and 50 ohm cables do not match.

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