Much of today’s excellent free decoding software only needs a basic computer with a sound card and a connection between radio and computer to supply the audio.
One of the main problems with using any computer around shortwave radios is the amount of interference they produce in the form of unwanted radio signals. This only gets worse when directly connecting the two together with a high level of noise coming back into the shortwave radio along the audio connection.
To do away with the interference completely you must isolate the radio from the computer electrically while still passing the audio signal so your computer can decode all that lovely data. Building an interface is easy and can be done very cheaply by recycling a part or two from broken household items.
Using a common transformer found in many electrical products that take power from the main supply is the simplest way to construct this interference
Parts and Tools Needed
2 3.5mm stereo jack plugs or buy 2 pairs of cheap in ear headphones (this does save time and much soldering)
A mains drop down transformer (110v to 12v or 240 to 12v are ideal for this interface)
A soldering iron and solder
As some shortwave radios have a mono audio output and some have stereo you are going to have to change the wiring slightly to get the right amount of volume into the computer for the decoding program to do its work. Giving a program like Multipsk poor quality or low volume audio will just increase the amount of errors during decoding and most likely make it unusable.
Because there are only 2 connections on the step down transformer any stereo audio output from your shortwave radio has to be turned into mono before going into the transformer.
This is easily done by joining the left and right outputs together leaving just two wires (common + joined left/right). Repeat this joining on the other side of the transformer on the wires that plug into the computer.
In the simplest terms the transformer will work much better one way than the other by providing a degree of amplification of the audio for your radio. If your transformer converts 110 volt into 12 volt then you must attach the wires coming from the shortwave receiver on the output (12 volt) of the transformer for this interface to work properly.
If you have taken this transformer out of a old appliance make note which side the mains voltage is going on the way in. If the transformer is new the audio input should be connected to the primary winding which (usually) should be marked
It doesn’t matter which way the wires are soldered onto the transformer as there is no polarity to worry about either on the input of output.
Things to Remember
Apart from making sure that you get the transformer (cant state this enough) the right ways round its good to only use as much wire either side of the transformer as necessary. Every bit of extra wire is just another route for interference from your computer to find its way into the shortwave radio.
Please make sure any appliance you take the transformer out of is unplugged first, yeah I know its common sense but something would be amiss if I didn’t bring it to your attention 🙂
If you have any problems or questions about this interface feel free to send me an Email.