VK3EDW

Original QRSS MEPT Design

FSKCW MEPT & MY FIRST TRANSMITTER (2016)

QRSS is an extreme slow speed CW transmission mode. By slowing down the CW signal send rate the detected signal is decoded using Audio Spectrum Analyzer software instead of the traditional 'by ear' method. Slowing the CW dot and dash times to typically 3 second for a dot and 18 second for a dash increases the minimum required SNR ratio for detection considerably, typically 10dB (20dB with a dot length of ~ 10 seconds).

The transmitter used for QRSS is called an MEPT (Manned Experimental Propagation Transmitter). There are numerous kits and designs which can be used, the friendly Mr Google will come up with plenty of links, also see my links page.

My MEPT operates in FSKCW mode (Morse code with frequency-shift keying), where "key up" is one frequency, "key down" is another frequency and the transmit duty cycle is 100%. Frequency shift is typically 5Hz and at 3 second per dot rate gives an optimum bandwidth of 0.33Hz. Reference

My design is based around an Arduino Uno R3 which I taught myself to code in September 2016 following drawn out knee surgery recovery. My code is not particularly elaborate but is a work in progress and seems to work well.

The signal generator is an Adafruit SI5351 board which is fairly easy to use especially as Jason Milldrum, NT7S, has produced a very useful library complete with a calibration sketch. I have provided a link for Jason's code on the "links page" of this website.

A VKEL VK16E GPS module was bought on Ebay for AU$8.40 and again I sourced a library on github to get the data input to my sketch for precise timing. The GPS module is mounted outside the house as the metal roof prevents satellite acquisition inside.

I used WA4DSY's online filter calculator ( on links page ) to design a good low pass filter.

I bought a LCD display from Jaycar Electronics which is a 84 x 48 pixel display type typically used in the old Nokia phones. This was the most expensive item at AU$19.20 as I did not want to wait for a really cheap one from China on Ebay (AU$3.20).

Coil 32 is a great windows freeware 'coil inductance calculator' program for both air and toroid core designs (links page for source). I used this to design the low pass filter tailored to suit capacitors I had in the junk box.

I have built the MEPT into a box that used to contain a rotator controller for satellite work.

Costs:-

Arduino UNO R3 from Ebay AU$ 3.40

Adafruit SI5351 from Ebay AU$14.95

GPS module AU$8.40

Basic components not in the junk box

including the LCD from Wiltronics and Jaycar AU$25.60

Total AU$52.35

If one was to buy everything from scratch I would think you could do it easily for under AU$45.00 if you take advantage of Ebay purchasing for boards, display, case and components including cores, not forgetting a good look through your junk box.

The following pictures show my MEPT in it's basic form. The old case has numerous connectors and switches that remain from it's previous use but are not used in this project but may be useful at a later date.

The power supply is an old laptop 15V 'block'. within the MEPT are both 12V and 5V regulators. The 12V rail is used by the Arduino and RF stages whilst the 5V runs the Si5351, GPS and LCD.