Raspberry Pi WSPR Transmitter

Sometimes several things come together that make a really easy project...

Having read several copies of the Official Raspberry Pi Magazine, I was intrigued that one of the issues this magazine actually gave away a Raspberry Pi Zero on the front cover in the same way that computer magazines in the 90's used to have CD-Roms and floppies on the front!

The current incarnation of the Raspberry Pi Zero W has onboard WiFi and Bluetooth networking and with the addition of a Raspberry Pi Stem becomes a complete OTG (On the Go) USB stick - ideal for working away in Lancashire without the need to cart a mouse/keyboard/monitor up to your hotel room!  You can even get a 3D printed case online for a few pounds...

Then I found WsprryPi - a WSPR transmitter that uses the Raspberry Pi's onboard frequency generator to transmit WSPR and uses NTP (Network Time Protocol) to discipline the oscillator frequency - all that is needed is an external Low-Pass Filter and you have a little transmitter.

Enter Language Spy with a neat little filter module.  I managed to get one for 17m (another one of my favourite bands) before the stock ran out.  

The Raspberry Pi output pin is a square-wave switched from 0V to 3.3V and therefore is approximately 1.65V RMS, there are obviously losses in the low-pass filter as the higher harmonics that make up the square wave are lost, however I measured the output at approximately 0.9V(RMS) into a 50Ω load

This works out to approximately 10 mW (or 10 dBm) so QQRP.

Unfortunately a recent upgrade to Windows 10 appears to have broken the raspberrypi.local shortcut, so whilst the board still works, the OTG capability doesn't and a mouse and keyboard are needed once more.  

Further Adventures in QRSS...

Having decided to buy the Yaesu SCU-17 USB Sound Card Interface for the FT-817, which  provides a dedicated USB sound card, serial port and PTT switching etc.,  I tuned my receiver to 10.13870 MHz and shortly afterwards received an off-air signal from IK2CMN over several minutes (as shown in the WSJT waterfall display below).

Setting my own callsign was a doddle and shortly afterwards I managed to get the OpenBeacon to transmit "G0VRM IO93QT" in QRSS into the dummy load, however it's not quite as clean as the off-air signal.  Following the instructions, I uploaded a WSPR string and it worked first time!

Notice that there are two entries for G0VRM, however one is 35 dB down and is probably some oddity of the radio being so close to the transmitter, although it appears to be very stable!  Looking promising, however connecting the beacon to an external antenna via a Z-match proved to be unsucessful.

Adventures in QRSS

Whilst wandering around the 2016 G-QRP Convention at Rishworth with Richard G0GLZ, I spotted a box on one of the stalls, and having talked to the stallholder established it was a QRSS (very slow Morse) beacon transmitter that had at one time been used in Senegal in West Africa.  At this point I decided that perhaps this project wasn't for me, it was promptly bought (partly on my behalf) by Richard G0GLZ - I did consider "The one that got away" as the title of this entry...

Although the manufacturer appears to have retired the OpenBeacon kit and have removed it from their website, the documentation and driver software are still available from http://etherkit.github.io/openbeacon_landing_page.html.  As the name suggests, the OpenBeacon is an open-source beacon transmitter that is capable of transmitting a variety of modes which include QRSS, DFCW, Sequential Multi-tone Hellschreiber and WSPR.

The first challenge for this project was the installation of the unsigned driver software under Windows 10 Advanced Boot Menu, which required a change to the startup settings in order to disable Driver Signature Enforcement, several reboots later with the driver successfully installed, I was then able get the settings showing the previous callsign of 6W7RV.

Looking up the locator IK14LL on http://qthlocator.free.fr/index.php, it appears to be almost as West as it is possible to be and still be in Africa; and from the QRZ.com database, the callsign once belonged to a holiday rental "shack" located in a Wildlife Reserve with its own lagoon and Baobab forest (sorry, no pictures I'm afraid).

Finally, back to my own shack, after connecting the beacon up to a 50Ω dummy load (a BNC thin-wire Ethernet terminator in its previous life) and was able to hear the beacon transmission on my FT-817.