This project used to be born for using transceivers without IF CW filter in CW mode more effectively. Even it is also very effective when the trx has a narrow band IF filter.

The audio CW filter is not a replacement of a good IF CW filter but it helps to receive weak CW signals much better having only a wide (SSB) IF bandwidth.

The principle is the use of negative feedback. On resonant frequency (suggested to be 800 Hz or near) the amplification is on the maximum. When the audio signal differs from the resonant frequency, the amplification cuts back according to the fall of the resonance curve. This means a high gain on the top of curve. The bandwidth is about 10 Hz , -3 dB but that does not mean you do not hear far signals. Personally it is an advantage for me because I can follow the events on the neighboring frequencies while I am operating with my partner station or searching the band.

The bandwidth can be regulated with a variable resistor (VR2). VR1 compensates some noise and can as well be used for a limited volume control. The input comes from my trx volume control so I use the volume knob there. To change for wideband mode I use the trx audio output so there is no switch over from CW to wideband facility built in the AF filter.

The circuit diagram can be found here. There is a sample audio clip here showing reception while I am changing the bandwidth. If you listen to it carefully you will hear how an almost unreadable station gets to be heard much better and his/her partner station is not using the same frequency (about 20-40 Hz away from it). By the way, I no longer use heavy headphones to make my ears pain in long operations. Instead I use my mp3 player earphones, the best of those I have ever tried.

Field or backyard antenna

This is a half Delta Loop. Needs good grounding. Resonant on 1/2, 1, 2, 3, and so on lambda.

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