Here we have a signal injector for the medium wave range, which uses only one transistor. This unmodulated injector was found in a 1982 documentation with its diagram shown in the figure. The circuit is quite simple. It is a Hartley oscillator with a BF494 transistor, which is powered with a voltage of just 1.5 V, coming from a common battery. The frequency of this oscillator is basically determined by the capacitor in parallel with the coil and by the coil. The coil consists of 95 turns of enameled wire 28 on a 1ocm ferrite rod. The tap is made on the 5th turn, as shown in the diagram. A common commercial coil for the medium wave range can also be used. The power of this injector is good, which means that in most cases it does not need a connection to the test receiver, just by staying connected in the vicinity. Readers who want more versatility in the use of this injector can replace the fixed 47 pF capacitor with a miniature variable capacitor.
