You never know what radio signals you might pick up if you just stop and listen. That's why Hans built a simple and tiny receiver capable of detecting QRSS (extreme slow speed continuous wave) transmissions on a fixed frequency, such as 10.14MHz. The device is powered by a computer's USB port and the audio output feeds into the PC's sound card and it can hook straight into a laptop for top secret missions.
Hans used a toroidal transformer as matching and input filter, connected directly to a 30m (10MHz) dipole antenna and a useful oscillator/mixer IC as a crystal oscillator and mixer. Introduction QRSS experimenting is a unique way to experiment with extraordinarily weak signal radio communications. Whilst "QRS" is a very old code for "send slowly" used by morse code operators of old, "QRSS" is an adaption by modern radio amateurs [Ref1] meaning "sent even MORE slowly"! So slowly, in fact, that in equivalent morse code terms, one "dit" can take 10 seconds to send or longer, meaning that an average word (such as "morse") takes over 8 minutes to send!
Why send so slowly, you may ask? The answer is that the slower you send, the less bandwidth you use, known by the professionals as the Nyquist theorem [Ref2]. By the time you send as slowly as QRSS, your communications bandwidth is sub 1Hz. The obvious disadvantage is that it takes an incredibly long time to transfer information. Yet there are also advantages, namely that the amount of noise (atmospheric and man-made interference) present in that same fraction of a Hz fragment of the radio spectrum is dramatically reduced compared to the usual bandwidth of say 9kHz occupied by commercial AM radio stations. This massive improvement in signal-to-noise ratio makes reception of flea-powered transmitters possible literally on the other side of the world.
For example, 0.5mW transmissions from Maryland, USA on the QRSS experimenter's favourite 10.14MHz frequency were received in Perth, Western Australia at a distance of 11,500 miles (18,600 km) [Ref3] Whilst the improvement in signal-to-noise ratio with decreasing bandwidth has been well understood for many decades, modern computing power has made it possible to enjoy experimentation in your home or even when mobile. Freely available Fourier analysis software running on your PC makes it possible to resolve bandwidths at a fraction of a Hz [Ref4] QRSS is a fascinating way to learn more about propagation effects and signal transmission over great distances.
Many interesting phenomena become visible, such as splitting of the signal due to doppler effects in the shifting ionosphere. The very simple receiver described here is capable of detecting QRSS transmissions on a fixed frequency, such as 10.14MHz. If you don't fancy monitoring QRSS transmissions, then building it for another frequency would enable reception of many other interesting communications on the short wave bands, aided by your PC and free software. The receiver is powered by your computer's USB port and the audio output feeds into your PC sound card. So all you need is this simple and tiny receiver, and your computer - or laptop if you want to go walkabout with it - and you're all set!
This circuit grew from a project by Paolo Saia IZ1KXQ, which itself was inspired by a 1997 article in QST magazine by Daniel Wissell N1YBT ("The 40m SLR - a Shielded Loop receiver"). [Ref5] Like Paolo, I used a toroidal transformer as matching and input filter, connected directly to a 30m (10MHz) dipole antenna. The very useful SA602 oscillator/mixer IC by Philips is used as crystal oscillator and mixer, directly to audio.
This receiver is therefore categorised as a direct conversion receiver, and receives both sidebands. In practice when monitoring slow QRSS transmissions, the presence of the unwanted sideband doesn't usually interfere with the wanted signal, it merely raises the noise floor by 3dB. The input transformer matches the antenna to the SA602 input and the additional tuned circuit creates a bandpass filter function, to reduce the strength of nearby broadcast band stations which would otherwise contribute to overloading of the mixer.
Grounding one side of the tuned circuit permits the variable shaft to be grounded, which helps reduce the effect of hand capacitance when adjusting the capacitor. The capacitor should be adjusted for best received signal strength. The crystal oscillator section of the SA602 includes a 10uH inductor in series with the crystal, which was found to be necessary in order to bring its oscillation to the correct frequency. Some experimentation may be required here to determine the correct value, if a specific exact frequency is desired. The balanced output from the mixer is fed into a low noise NE5534 op-amp. The gain of the op-amp is set at 45dB. The output is fed directly to the line input of the laptop computer. Despite the NE5534 datasheet specifying a minimum supply voltage of +/- 3V, it seems to work fine at just 5V single rail.
For convenience and portability, I powered the receiver with 5V from one of the PC's USB ports. The incoming +5V is filtered by an arrangement of two L-C filter sections, each consisting of a 10uH inductor and 220uF / 0.22uF capacitor combination. In operation I tried swapping the USB power source for a 9V (PP9) battery but could not detect any difference in noise level, proving the effectiveness of this filtering arrangement. With a laptop, I did notice a substantial increase in noise when the laptop was being charged. As for construction: I purchased a tin of Ozon mints. You'll notice from the photographs, that it's a very small tin. The mints are also tiny and tasty (all 70 of them). You can eat them at your leisure, or gobble them all in one sitting - but be warned, your mouth will sting for a while afterwards.
I built the receiver "ugly" style on a small offcut of PCB, which I soldered into the tin base. After application of the soldering iron for a few seconds, solder adheres very well to the tin. I drilled two small holes in the sides of the tin, and fed through 7-stranded wire as the two arms of the 30m dipole. Each is 7.1m long. The receiver therefore sits directly at the centre point of the dipole, i.e. there is no feeder between antenna and receiver. Power from the laptop, and audio back to the laptop, come and go via about 7m of twin screened cable (separately screened).
A standard USB cable contains four wires, with the 0 and +5V lines very sensibly colour-coded black and red. The receiver seems to work well considering its simplicity and ease of construction. The audio can be monitored simply by plugging earphones into the computer's sound output socket. The design is fully differential and whilst some broadcast breakthrough from powerful nearby shortwave stations is occasionally audible, it is infrequent and barely noticeable. The antenna can be strung between two available supports, such as guttering on your house or two appropriately spaced trees. If the receiver is exposed, then on windy days or when the sun is intermittently hidden by clouds, the temperature variation will cause the crystal to drift slightly and this can easily be observed on the spectral analysis screenshot.
One can experiment with boxes made from polystyrene to provide some thermal insulation and reduce this effect. Now for some portable operation! Andrea IW0HK visited London on holiday with his young family, and we spent a couple of hours walking to Hampstead Heath in North London to try some reception from Parliament Hill. Perfect weather made for an enjoyable mini-expedition and there were plenty of trees on the hill to use as antenna supports. The receiver and antenna wire coil up very small and fit easily into the laptop bag along with the computer itself. In a short while we were able to monitor four different stations: two Italian transmissions (Paolo I1DFS and Andrea IK4IDP) and one US station from Maryland (Larry WB3ANQ).
Also visible was my own experimental transmission from across London to the south [Ref8], about 15 miles away. In some of the images of my own transmission, ghost images rapidly converge to the main trace. These are due to doppler shift effects, as the signal is reflected from rapidly moving aircraft descending on their final approach to London's busy Heathrow Airport. Just some of the fun to be had with this interesting communications mode and simple receiver! References and further reading [Ref1] QRSS Knights website [Ref2] Wikipedia information on Nyquist [Ref3] WB3ANQ website and VK6DI website [Ref4] ARGO and other popular audio fourier analysis software [Ref5] 1997 article in QST magazine by Daniel Wissell N1YBT ("The 40m SLR - a Shielded Loop receiver"). [Ref6] More on QRSS receiver [Ref7] 10.140MHz crystals [Ref8] Simple 10.140MHz transmitter (requires amateur radio license to operate).
|Quantity||RS Part #||Part description|
|1||454-255||3.5mm stereo plug|
|1||127-307||Variable capacitor (or polyvaricon tuning capacitor)|
|1||435-484||Copper-clad unetched PCB|
Other parts 1 T37-6 toroidal former 1 10.14MHz quartz crystal 1 Ozon mint tin 15m wire for antenna Assorted resistors and capacitors.