One of the earliest mode of radio communications is the use of Morse Code on a continuous wave carrier(CW) which is interrupted to generate a dit and dah in forming an alphabet. Though it looks simple, this method is often make less reliable as the surrounding man-made noise and atmospheric noise caused interference to the receiver station. This project will help to filter out the interference signal and ensure that the signal received from the Morse code station stand out.
L choke should be in the region of 5H but this is not critical. Try using the winding of an audio transformer to make this choke. The output of this filter is connected to the earphones. On Switch SW and on the radio. Vary the VR1 until it goes into oscillation and then turn it back a bit. Tune across the code station and you will hear a sharp peak response effectively cutting off other noise and interference.
Figure below shows the filter's selectivity response curve. It is sharp at the peak at 20dB at the audio frequency of 800kHz - 900kHz.
Ham Radio Circuit Filter Parts List