Antenna amplifier input protection. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Antenna amplifier input protection. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Antenna amplifiers

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When repeating some antenna amplifier circuits, it happens that the viewer does not receive positive results, described with enthusiasm in the author's publication. And the point is not even that the circuit is otherwise assembled or configured. Even the author's copy of the amplifier, when placed in the operating conditions that this viewer has, will show unimportant results.

And the focus may be here. The antennas used by this viewer and (or) the electromagnetic environment in his place of residence is different. At the input of the antenna amplifier, in addition to the useful signal that it is designed to amplify, interfering signals and interference can enter. In some circuits, mainly broadband amplifiers, there are no input filters, except perhaps a coupling capacitor. And if the interference is significant, then, amplifying along with the useful signal, they lead to its distortion or complete "clogging" even if the interference frequency band is much lower than the signal frequency. Sources of such powerful interfering interference can be: broadcasting transmitters of long medium and short waves, electric transport and power equipment of enterprises, lightning discharges. A simple high-pass filter will help to significantly reduce the magnitude of such interference, which can be added without problems to almost any amplifier, such as, for example, here. Calculating such a filter is quite simple - at the cutoff frequency (this is the frequency at which the filter attenuation is 3 dB), the capacitance and inductance resistances must be equal to each other and equal to the input impedance of the amplifier. Thus, with the input resistance of the amplifier Rin Ohm:

C=1/2*pi*F*Rin, L=Rin/2*pi*F

It is enough to select the cutoff frequency just below the frequency of the 1st channel of meter waves, for a television amplifier. If necessary, the number of turns of a homemade coil can be determined using my radio calculation program by its calculated inductance. Such filters are useful not only to protect against powerful out-of-band interference, but also to protect the input transistors from damage when lightning strikes near the antenna. Of course, these filters cannot protect the amplifier during a lightning strike into the antenna, as well as ensure personal safety during a lightning discharge.

Author: E.Shustikov, UO5OHX ex RO5OWG; Publication: shustikov.by.ru

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