With the repetition of some schemes of antenna amplifiers is that the viewer does not get positive results, enthusiastically described in the publication of the author. And it is not even the fact that the scheme collected or otherwise configured. Even the author of the amplifier being placed in the operating conditions existing at the viewer, will show poor results.
And the focus can be what it is. Applied by the viewer of the antenna and (or) the electromagnetic environment in the workplace is different. The input of the antenna amplifier, in addition to the useful signal which is intended to strengthen, can get interfering signals and noise. In some schemes, mainly broadband amplifiers, no input filter, except that the coupling capacitor. And if the noises have considerable value, then, amplified along with the desired signal, resulting in distortion or complete "clogged" even if the bandwidth of the interference is significantly lower than the signal frequency. Sources such powerful preventing interference can be: radio transmitters long medium and short waves, electric and power equipment companies, lightning. Significantly reduce the magnitude of such interference, a simple high pass filter, which can add without problems for almost any amplifier as, for example, here. To calculate such a filter is quite simple - at the cutoff frequency (this is the frequency at which the attenuation of the filter is 3 dB) resistance capacitance and inductance must be equal to each other and equal to the input impedance of the amplifier. Thus, when the input impedance of the amplifier Rin Ohms:
C=1/2*pi*F*Rin, L=Rin/2*pi*F
The cutoff frequency is enough to choose just below the frequency of the 1st channel meter waves, amplifier for TV. If necessary, the number of turns of a homemade coil, can be determined with the help of my program for radio computations calculated inductance. Such filters are useful not only for protection from a powerful out-of-band interference, but also to protect the input transistors against damage when lightning strikes near the antenna. To protect the amplifier during lightning strike to the antenna, as well as to ensure personal safety during lightning discharges, these filters are, of course, unable.
Author: E. shoustikov, UO5OHX ex RO5OWG; Publication: www.shustikov.by.ru, www.cxem.net