14-20dB active antenna circuit
A very simple active antenna circuit can be designed using this circuit diagram.
This active antenna circuit is based on transistors and other few common electronic components .
This RF circuit provide a 14-20dB gain at the popular short-wave and radio-amateur frequencies of 1-30MHz.
Because in practice of frequency short-wave reception, the general rule is that if the used antenna is longer, the stronger the received signal will be we need in some cases ( when we can not built a big antenna , because of some factors ).
Antennas that are much shorter than 1/4 wavelength present a very small and highly relative impedance that is dependent on the received frequency. It is difficult to match impedance over a decade of frequency coverage. For this RF circuit a gain of 20dB is typical from 1-18 MHz, decreasing to 14dB at 30MHz.
For this circuit many different types of JFET's may be used , such as the MPF102, NTE451, ECG451 or 2N4416 and 2N3904 or 2N2222 transistor works well for Q2, Q3, and Q4 .
The most important thing of Q3 transistor circuit parameter is the voltage drop across R8: the greater the drop, the greater the gain.
Transistor Q4 transform Q3's relatively moderate output impedance into a low impedance, thereby providing sufficient drive for a receiver's 50-ohm antenna-input impedance.