# The input impedance of a Car Satellite TV Antenna

The input impedance of a Car Satellite TV Antenna

Definition: the ratio of the input signal voltage to the signal current in the car satellite TV antenna, which is called the input impedance of the satellite TV antenna. The input impedance has the resistance component Rin and the reactance component Xin, namely Zin = Rin + j Xin. The existence of the reactive component will reduce automobile satellite dishes from feeder to extract the signal power, therefore, must make as far as possible to zero reactance components, namely should as far as possible make the car satellite TV antenna input impedance is pure resistance. In fact, even a well-designed and well-tuned Car Satellite TV Antenna has a small electrical component in its input impedance.

Satellite TV antenna input impedance and vehicle structure, size and the working wavelength is related to the half-wave symmetric oscillator is one of the most important basic car satellite TV antenna, the input impedance of Zin = 73.1 + j42.5 (o). As to shorten the length of its (3 ~ 5) %, can eliminate the reactive component, that car satellite TV antenna input impedance is pure resistance, at this time the input impedance of Zin = 73.1 (o), (nominal 75 euro). Note that strictly speaking, the pure resistive satellite TV antenna input impedance is only for the point frequency.

By the way, the input impedance of a half wave oscillator is four times that of a half-wave symmetrical oscillator, that is, Zin = 280 (euro300).

It is interesting to note that for any car satellite TV antenna, people can always through automobile satellite TV antenna impedance debugging, the requirements of the operating frequency range, the input impedance of the imaginary part is small and real component is quite close to 50 euro, making the car satellite TV antenna input impedance for Zin = Rin = 50 o -- -- -- -- -- - is this car can satellite TV antenna and feeder in good impedance matching is a must.

Critical parameters affecting automobile satellite TV antenna performance has a lot of, usually in the process of automobile satellite TV antenna design can be adjusted, such as resonant frequency, impedance, gain, aperture or radiation pattern, polarization, and bandwidth efficiency, etc. In addition, the transmitting satellite TV antenna has the maximum rated power, while the receiver of the satellite TV antenna has the noise suppression parameters.

The resonant frequency

The "resonant frequency" and "electric resonance" are related to the electrical length of the satellite TV antenna. The electrical length is usually the physical length of the wire divided by the ratio of the wave velocity in the free space to the velocity in the wire. The electric length of a Car Satellite TV Antenna is usually expressed by wavelength. The car satellite TV antenna is generally tuned at a certain frequency and is effective in the frequency band of this resonant frequency. But other car satellite TV antenna parameters, particularly the radiation pattern and impedance change with frequency, so the resonant frequency of automobile satellite TV antenna may simply close to these more important parameter of the center frequency.

An automobile Car Satellite TV Antenna can be resonant at the frequency corresponding to the length of the target wavelength component. Some car satellite TV antennas are designed to have multiple resonant frequencies, while others are relatively effective at wide frequency bands. The most common broadband satellite TV antenna is a log-periodic satellite TV antenna, but its gain is much smaller than that of a narrow-band satellite TV antenna.

gain

Car satellite TV antenna design, "gain" refers to the strongest satellite TV antenna radiation direction of car satellite TV antenna radiation pattern intensity and automobile satellite TV antenna intensity of reference than a logarithm. If you refer to the satellite TV antenna of the automotive satellite, the gain units are dBi. For example, the gain of the dipole satellite TV antenna is 2.14 dBi [1]. Dipole automotive satellite TV antenna is commonly used for reference the satellite TV antenna (this is due to the perfect omni-directional reference automobile satellite TV antenna can't manufacture), in this case car satellite TV antenna gain is an unit with dBd.

Automobile satellite TV antenna gain is a passive phenomenon, automobile satellite TV antenna does not increase incentives, but only to redistribute and has made in one direction than the more energy to the car satellite TV antenna radiation. If the car satellite TV antenna gains positive gain in some directions, the gain in the other direction is negative because of the energy conservation of the satellite TV antenna. Thus, the gain achieved by the satellite TV antenna is balanced between the coverage of the satellite TV antenna and its gain. For example, the spacecraft's satellite TV antenna has a large gain, but its coverage is narrow, so it must point to earth precisely. And the broadcast of a Car Satellite TV Antenna, because it needs to radiate in all directions, has a very small gain.

The gain and aperture of the satellite TV antenna of the disc car satellite (reflecting area), the surface accuracy of the reflector surface of the automobile satellite TV antenna, and the frequency of transmitting/receiving are proportional. Generally speaking, the larger the aperture, the higher the frequency, the higher the gain, but the error of surface accuracy at higher frequencies leads to a significant decrease in gain.

"Aperture" and "radiation direction map" are closely related to gain. Aperture refers to the shape of the "beam" section in the highest gain direction, which is two-dimensional (sometimes the aperture is expressed as the radius of the circle of the circle or the Angle of the cone of the beam). The radiation direction map is a three-dimensional diagram of the gain, but usually only the horizontal and vertical dimensions of the radial direction graph are considered. High gain satellite TV antenna radiation pattern is often accompanied by "secondary lobe". The secondary lobe is the beam of the gain that is removed from the main lobe (gain the highest "beam"). The secondary lobe will affect the quality of the satellite TV antenna when the system needs to determine the direction of the signal in the direction of the radar and so on, because the power distribution of the secondary flap will also reduce the main lobe gain.