Long transmission line
In the case of high-frequency electromagnetic waves spread along the transmission line, because of very short wavelengths of the electromagnetic wave, transmission lines and transmission occurs when different audio signals. Must use the other for a set of high-frequency analysis method.
When the transmission lines along the transmission wavelength of electromagnetic wave transmission line with the geometry comparable to the length of time, at this time of the transmission line known as long-term.
1.'s Long-term with the uneven distribution of current and voltage: a transmission line to check the length of 10 meters, when the line-to high-frequency current (long), such as F = 150MHZ. 000 (wavelength) = 2 m due to a shorter wavelength In this transmission line in the current cycle, there are a few changes, as shown in Figure A. Online at the same time and therefore the points of the current large and small direction is different; in the case of low-frequency (that is, short-term cases) If F = 50HZ AC, its wavelength is 6000 km, a difference of 3,000,000 prepared, with the 10-meter-long transmission line current major changes in the small change that can be very small.
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2. Is a long-term distribution system parameters: the long-term, with the length of the transmission line or a different wavelength of electromagnetic waves spread along the different transmission line itself has distributed capacitance and inductance parameter distribution. And the distribution of these parameters influence , In the long-term, with the result of a long line of different or the work of a different wavelength, the transmission line itself would show the different nature of the resistance (capacitive reactance, or inductive reactance pure resistance). But there is only short-term only with the electric field in the capacitor, coil Only in the magnetic field produced, with a long line does not matter.
Open-end transmission line
When receiving the signal transmission line, will signal the speed of light by the cosine distribution terminals spread. Due to open end, the signals can not continue to spread, it will start to the end of the formation of the direction of total reflection, this time the equivalent of transmission lines in the terminal access A source so that the electrical signal to start again from the terminal end of the speed of light to spread. The general said that by the beginning of the end-to-end transmission of radio waves that incident; by the end of the reflected waves that echo. Incident and reflected waves are traveling wave . Here's the so-called traveling wave that is the same voltage and current phase of the waves.
Transmission line also discuss the introduction of the concept of a standing wave, the so-called standing wave voltage and current phase is different and the difference between a quarter of the wavelength, amplitude of voltage and current are unevenly distributed. In the open end of the transmission line, at the same time there is a reflection of the incident wave and Wave, reflected wave and the current incident current phase of the RP each other, so that the transmission line there is a standing wave, transmission lines are usually at the same time there is a traveling wave and standing wave, the wave, standing by the incident and the reflected wave The current form.'s Open-end transmission lines due to the formation of total reflection, so a large component of its standing and thus does not have the energy transmission (only a good state of wave energy transmission line only) the assumption that this lossless transmission line. That time is just In a certain period of time the energy stored in a period of time to release energy. Therefore, it is the source of the signal is a pure electric load resistance, in the open end of the transmission line has the following characteristics of the transmission line less than a quarter of the wavelength of its time For the capacitive reactance; wavelength equal to a quarter of the reactor at zero; more than a quarter of the wavelength when the reactor power for emotional; wavelength equal to one-half hours of pure resistance.
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Terminal short-circuit transmission line
End of the short-circuit transmission line and the open end of the transmission line on the contrary, its current standing wave is characterized by the incident and reflected waves with the current phase, the incident reflected wave voltage and phase voltage on the contrary. Impedance of its properties is less than a quarter of the wavelength when the transmission line was emotional ; Equal to a quarter of the wavelength when it was a purely resistive; more than a quarter of the wavelength when it was capacitive; equal to one-half wavelength when the reactor is zero.
Under normal circumstances, the transmission line terminal then there are the load, the load resistance element usually have both reactor components, and thus signal output from the energy absorption of 1 db load and the other part of the times will be reflected, so traveling wave transmission line with the standing wave at the same time Co-exist, in order to further describe the online traveling wave and standing wave distribution, we introduced a number of specific indicators:
1. Reflection coefficient: P = reflected wave amplitude / = incident wave amplitude transmission line characteristic impedance - the load impedance / transmission line characteristic impedance load impedance
2. Traveling wave coefficient: K = voltage minimum / maximum voltage = reflected wave amplitude - the amplitude of incident / reflected wave amplitude incident wave amplitude
At the same time, because transmission lines in the incident and reflected waves exist, so that any transmission lines are the voltage and the amplitude of the two.
3. VSWR: S = maximum voltage / minimum voltage
To sum up, in the end have a transmission line load, the transmission line input impedance nature of the following:
1. Transmission line from the end of a quarter wavelength Qi Department several times equal to the equivalent impedance characteristic impedance divided by the square of the load terminals.
2. Transmission line from the terminal one-half wavelength resistance Integer Office, the equivalent equal to the load impedance.
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Antenna of the general principles
When the conductor on high-frequency current to pass, in the space around them will have electric and magnetic fields. According to the electromagnetic field in the distribution of space can be divided into areas close to the middle zone, far zone. Set up a space for the R distance away from the conductor, In R "λ/2π region, said at the time of the last areas in the region of the electromagnetic field and conductor of current, voltage close.
In R "λ/2π far as the regional district, the electromagnetic fields in the region to leave the conductor to the dissemination of space, it changes relative to the conductor on the current and voltage will be some time lag, when the electromagnetic waves spread out and no longer Wire on the current, voltage direct links, and this region of the electromagnetic fields known as the radiation field.
Antennas is the use of the radiation field of this nature, so that the transmission of signals through antennas after launch to space radiation to the fullest, how to become an effective conductor-guided radiation that time? We are here to look at transmission lines, in The two parallel transmission lines in order to enable the transmission of energy only and no radiation, must ensure that the structure of the symmetry of the two lines, online counterparts and the size of the current point in the opposite direction. And the distance between the two lines "π. Electromagnetic fields to be able to effectively go out radiation , It is necessary to destroy this symmetry of the transmission lines, such as the use of two conductors separated into a certain point of view, or to remove one side and so on, so that the conductor can have a symmetry and radiation damage.
Figure TX, plans will pave the way transmission from the terminal or π / 4 Department into a straight-conductor separated from the terminal at this time on the current conductor is not the same but with the RP, so that the paragraph in the conductor's point of space radiation field Compared with the superposition. Constituted an effective system of radiation. This is the simplest, most basic unit of the antenna, known as the symmetrical half-wave dipole antenna, the impedance characteristics for 75Ω. Electromagnetic radiation from the antennas out, spread out to all sides, The direction of electromagnetic wave propagation put on a symmetrical oscillator, in the role of the electromagnetic wave, the antenna would have a vibrator on the force sensors. So when connected to an antenna and receiving equipment, receiving input devices will have a high frequency current, so that the antenna To receive and play the role of high-frequency electromagnetic waves into a current, that is to say at this time plays a receiving antenna of the antenna, the reception quality of radio waves in addition to the strong or weak, but also depends on the direction of the antenna and symmetrical half oscillator and receiving The devices match.
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