WSPR on 50 MHz

[VK4ADC]

Leigh et all

My Raspberry Pi Zero W is alive and well and about ready for 6M WSPR use.   The PA PCB was built yesterday, tested with a sig gen at +10dBm to check the LPF action ( > 25dB down at 72MHz, each LPF section tested separately) and fed with the Pi this morning. My RF power meter shows +25dBm out of the amp stage after the output LPF, a little over 200mW.    My main concern was frequency error as it was jumping all over the place ( +/- 300 - 400 Hz) as it did the WSPR software's internal frequency correction but I have put it into free-run mode and adjusted the output frequency via the command line. Now there is just a few HZ drift noticeable at the moment over the course of the 110 seconds.  It will undoubtedly need re-calibration after it is boxed and has been running a while (so is temperature stable).  The whole assembly (Pi plus PA) runs from +12V, drawing around 300-500mA depending on what it is doing at the time (only 200mA is drawn by the PA PCB itself).

The PA PCB was designed to be the same size (65 x 30mm) and to sit over/under the Pi Zero's mounting holes and use a 10 pin header (dual 5) connector to interconnect the PCBs.  An SMA edge connector can fit to the end of the board in lieu of the current RG316 teflon coax out.  The input LPF can be set up as a 5 or 7 pole, as can the output LPF, and could be configured for any of the lower bands by changing the values of L and C.   I used the LPF calculator at http://www.wa4dsy.net/filter/filterdesign.html to work out the values for 6m, using a 60MHz Fc and a 5-pole Butterworth as that calculated as close to standard values. The SHF-0289 GaAs HFET was bought in a pack of 5 via eBay a few years ago, datasheet here : http://web.rfoe.net:8000/SIRENZA/PDF/gaa...f-0289.pdf, and while rated 50MHz to 6GHz, SHOULD work on HF too. It runs self-biassed (source resistor style) so only needs a single power supply.  The +6V three-terminal regulator is used for the PA power but was making the PCB a bit warm at +12V applied so I now have an external +8V regulator in series with the +12V supply to reduce the on-board power dissipation / temperature.

I contemplated making it multiband but the physical logistics (ie changing LPFs) was too difficult, and they are cheap enough to make to build a separate one for each band desired - OR - since the 6M LPF will pass anything up to 55MHz, why not just make up some in-line LPFs or BPFs (in small metal boxes) for the lower bands and put them in series from the RF Out connection ???  BNC or SMA in and out ??

I will "box it" in the next few days so that only power and antenna connections are required but will have a DC power switch plus a reset switch for the Pi on the outside.  It looks like it will have to be plastic as the Zero W has an internal WLAN antenna - unfortunately.

Costing : Raspberry Pi Zero W : $22 (posted),  16GB microSD card (type 10) @ $8, rest of the bits from my project box.. maybe $10-$20. All up say $50..

My command line (for now) is "sudo wspr -r -f vk4adc qg62lg51 25 50294110" to achieve an output at 50.294500 +/-. { repeat and free-run }


The images below show the details so far:

   

   

   

I will add a photo of the completed "sandwich" in the next day or two..