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|30m QRSS receiver|
|Written by Hans Summers|
|Thursday, 18 June 2009 18:07|
30m QRSS Receiver
The inspiration for this project was Paolo IZ1KXQ, who used a simple SA602-based homebrew direct conversion receiver to produce some very nice QRSS reception reports, of my Junkbox QRSS beacon and others. Paolo's receiver was based on a 1997 QST article by Daniel Wissell, N1YBT: "The 40m SLR - a Shielded-Loop Receiver". Paolo didn't use a shielded loop, he used a toroid as matching and input filter connected directly to a 30m dipole.
My circuit (see above left) uses Paolo's input matching filter, followed by a simple SA602 as mixer and crystal oscillator. John VK6JY sent a nice computer-drawn diagram (above middle) and also a version with an LM386 audio amplifier for driving a small loudspeaker (above right).
I grounded one side of the bandpass filter tuning capacitor, the side connected to the tuning capacitor shaft: this is so that when adjusted with a screwdriver the tuning is not affected by the connection to the screwdriver. 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. 10.140MHz crystals are available from Peter DL6NL, see his contact details at the bottom of my my Junkbox QRSS beacon page.
Above are four photographs (CLICK for larger versions!) showing (from left to right):
1. The tiny Ozon mint box, with the lid closed.
2. Open the lid, to see the tiny receiver!
3. The antenna wires (blue and green) and power/audio cable wind up into a compact and portable package.
4. Operating mobile in my garden. The receiver can be seen hanging in the middle of the dipole, against the blue sky background.
The last of these pictures, shows the receiver enclosed in an additional box made from white polystyrene packaging material intercepted on its way to the rubbish bin. This was hastily cut up and formed into a polystyrene box to provide some degree of thermal insulation. Without this, the crystal drifted up and down by a few 10's of Hz quite rapidly, as the sun came and went behind clouds. With the polystyrene box this problem was greatly reduced, resulting in a slower drift over several minutes. With indoor operation this would be less of a problem.
The receiver seems to work well. 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 is occasionally audible, it is very infrequent and barely noticeable.
My first test of the receiver was to listen for my own 30m QRSS beacon signal (image above left). Reception of the 360mW signal from just a few metres away was of course no great feat of DX - but useful as it allowed me to easily tune the front-end bandpass filter for maximum signal strength, and test out the various settings of the ARGO spectrum analysis software. ARGO is the most popular of the available free software packages for decoding QRSS signals and can be downloaded here. Image second left above shows my first decoding of my own beacon signal. Notice the rapid frequency drift in the receiver causing the trace to look very wobbly. That was before I put the receiver board in its tin. The following day I installed the receiver in its tin and the rapid drift disappeared, see the trace in image (above second right). I switched off the transmitter and connected the receiver to the attic dipole - I was able to receive my first remote beacon station (above far right, that's the weak trace of Andrea IK4IDP in Bologna, Italy - locator JN54QL.
G0UPL Garden reception
On 3'rd May 2006 I switched off my beacon transmitter again for a few hours (since it is impossible to receive any remote signals whilst a local signal is booming in at point blank range). I hung the dipole antenna between a tree in the front garden and an upstairs window, at about 4 - 5m height from the ground. I was able to copy three stations as shown below. Note that the drift is all in the receiver, rather than the transmit stations.
G0UPL/IW0HK QRSS mini-DXpedition in London!
On 5'th May Andrea IW0HK visited London on holiday with his XYL and two young sons. We met at 1pm (12:00UT) at Tufnell Park tube station and walked to Hampstead Heath. I have never been there before. Not far into the park is a nice hill named "Parliament Hill". WX condx were perfect. We climbed the hill and found some trees near the top on the South side, with an excellent view southwards over the whole of London. I climbed the low branches of two perfectly spaced trees and tied the two ends of the dipole at about 4m above ground. The receiver at the dipole's centre hung at about 2m above ground.
09. From the top of parliament hill, a beautiful view to the South across the whole of London towards G0UPL's QTH 20km South.
G0UPL (left) and Andrea IW0HK. Photographer anonymous, borrowed from the bench in the background.
|Last Updated on Tuesday, 12 April 2011 02:32|