I've built and blogged one of these before, but now I've developed the idea a little. The original suffered with a poor antenna and although it worked, it was more than a little deaf. The new USBrx has eighteen components, including the connectors, antenna and panel.
- Loop antenna wire (32feet, 9.75m) - any insulated wire will do.
- Loop antenna connectors male (2 required)
- Loop antenna connectors female (2 required)
- Tuning capacitor, 20-200pF, polyvaricon
- Knob to suit tuning capacitor
- Ferrite toroid, 9mm 4c65, FT37 type, any small HF ferrite
- SA602 mixer / oscillator
- Quartz crystal, frequency at band edge
- Emitter capacitor, 30pF
- Feedback capacitor, 30pF
- Bypass capacitor, 3n3 (3300pF)
- Coupling capacitor, 100nF
- Supply decoupling capacitor, 10uF
- USB extension cable
- Old earbud cable
- Copperclad panel (FR4), 55x55mm
The antenna is a loop, 32 feet long, and works as a quarter-wave magnetic loop for 40 meters. It was first described by Ben Edginton G0CWT, to whom I'm indebted for the idea. It's basically an opened-out version of the 'original' USBrx antenna, itself a copy of the Poundshop antenna, first used by me in 2005 as part of the Poundshop Radio. These antennas are directional, and very compact. An eight-foot square for forty metres is something which can be occasionally strung across a room (I use lengths of nylon string and small bulldog clips), and taken down when business is done. The loop is resonated with the polyvaricon variable capacitor, which can have one side connected to the panel. I made a 64-foot version in 2007, and with extra windings on the transformer I used it on Topband.
The antenna has a very low impedance, in the region of 2.5 ohms, so a transformer is used to match the loop antenna to the 1500 ohms of the SA602. One turn (a single pass through the toroid's hole) for the antenna and 25 turns of 30SWG enamelled copper wire for the SA602 input.
The output, using the tip and body contacts of the 'earbud' cable, will drive the microphone input of a PC soundcard. You can listen to it with headphones directly from the soundcard output, but you won't like it. Everything from the crystal's frequency to 25kHz in both directions will be coming at you, and it's quite a cacophony. To make sense of it, you need a little help from one of my favourite pieces of software, the SAQrx. You can download this from here. This is intended for use with LF antennas for monitoring the famous Alexanderson Alternator transmissions, which use the callsign SAQ, but it has a host of other uses, including rudimentary digital signal processing. Because the SAQrx software can filter out any narrow band from near DC to over 20kHz, you can use it to choose a spot frequency to listen to. You can choose narrow, medium and wide bandwidths, and the gain can be adjusted to suit. The program works well under Wine in a Linux machine, as shown in the photograph. Comment if you need any help with this one!