88mH CW Filter

Many years ago there was some inductors known as "line chokes" which were used for the old copper landlines back in the day. They were installed at certain points in the chain in order to balance the audio frequency response. The AF response varied due to the distributed line capacitance and inductance along it's length.

The inductors were very high Q (low loss) and had four wires coming out of them. You had to wire them up so that each inductance would give you a value of 88mH.  These line chokes made their way into the amateur radio scene and popularised by W3NQN at the time, who published some simple designs to build them into a simple passive audio filter.

An old chum David G4EBT had some spare and sent me them back in 2007, and I turned them into a CW filter of my own topology. 

According to David -

W3NQN arranged for the donation of a large quantity of those surplus inductors from the Chesapeake and Potomac Telephone Company of Maryland.  The inductors were provided with the understanding that they would be distributed to radio amateurs at no cost except for packing and shipping expenses.  Over a period more than of thirty years he took on the task of distributing the inductors in accordance with this understanding.  As I recall, the G-QRP Club distributed quite a lot of them, and the late G3RJV ,George, featured them in various mags such as Radcom, SPRAT and PW.

The design that I used is shown above. It's a simple bandpass arrangment that shows the 88mH choke in series with a 570nF capacitor to resonate at about 700Hz (can vary slightly with termination impedance).

The 44uF capacitor provided some matching in my design because I chained four of these sections up to make a much narrower bandwidth filter that was about 40Hz wide. Such an arrangment is also lossy, so you really have to crank up the volume on the receiver to make it usable. If you prefer then you could simply add a small basic audio amplifer to boost things up.

ABOVE - You can see my first effort, a bit messy and if I had the time I would rebuild it a bit neater, but it worked so well that I used it immediately and had no time to make the neater version ! 

These line chokes even today are often sitting around in folks shacks still waiting to be utitilsed, so hopefully this little page might provoke a few of you into finally using them.

Potential experimenters might like to take note that attempting to build passive audio filters like these that use small moulded PCB chokes (often available for pennies and look like small resistors) are NOT suitable for these designs. The DC resistance is far too high, and you will end up not with a CW filter but one more suited for SSB.  It's doubtful if such line chokes are still available new these days so future builders might end up having to build their own chokes. So long as the DC resistance is about 3 Ohms then you should be OK.

Have fun.