Building radio receivers is the classic introduction to electronics, usually by constructing simple detector type receivers for medium wave reception. However during the day there is not much to listen to. Almost as simple to build but much more interesting is the classic direct-conversion type shortwave receiver. Not only can it receive stations from around the world; this set pulls in SSB and CW transmissions as well.Back in the mists of time when radio communication was still in its infancy, the earliest radio sets used just one valve, either a triode or pentode, and were based on the direct conversion design, often employing ‘Audion’ demodulators. Apart from the tuning knob there would be a second knob that controlled regeneration in the tuning circuit. Those skilled at using the radio set were able to draw down signals from the ether: The BBC Home Service, Radio Luxembourg…