In the case of high frequency, when the electromagnetic wave transmits along the transmission line, since the wavelength of the electromagnetic wave is very short, then the different phenomena with the transmission of audio signals will occur on the transmission line of lte antenna. When the wavelength of the electromagnetic wave propagating along the transmission line can be compared with the geometric length of the transmission line, the transmission line typically refers to the long line.
The characteristics of long line antenna
First, the current and voltage are distributed unevenly on the long line: use a transmission line with a length of 10 m, when the high-frequency current passing through line, such as F = 150MHz. (Wavelength) = 2 m, due to the shorter wavelength, there are several changes in the cycle in this transmission line current.
Second, long line is a distributed parameter system: for the long line, with the different length of the transmission line or the different wavelengths, distributed capacitance and inductance parameters are distributes on the transmission line. While in the case of long line, with the difference in line length or wavelengths, the transmission line will exhibit a different nature of the impedance (capacitance, inductance or pure resistance), while the short line only has an electric field in the capacitor and the coil.The Open-ended transmission line
When the transmission line receives the signal, the electric signal will be transmitted at the speed of light and by the cosine distribution law to the terminal. Due to open termination, signals cannot be transmitted forward continuously, and forms the total reflection to the direction of the starting end, which is equivalent to access a signal source to the transmission line terminal so that electrical signals travel from the terminal to the end at the speed of light. In general, the waves transmit from the beginning to the end of the terminal are called incident wave, and the waves reflected back from the terminal are called reflected wave. Both incident and reflected waves in the wifi panel antenna are traveling waves, which means the voltage and current are the same.
Terminal short-circuited transmission line
The terminal short-circuited transmission line is totally different from the open-end of the transmission line, it's VSWR characteristics is that the incident current and the reflected wave current are the same, while the opposite phase of the incident voltage and the reflected voltage wave. Its impedance is less than a quarter-wavelength transmission line, equal to a quarter of wavelength, and reactance is zero when it is equal to the half wavelength.
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