A couple of weeks back I got a Raspberry Pi setup with TAPR’s QRPi low pass/band pass filter board, running WsprryPi, providing a low-cost, low power, WSPR transmitter. For my first attempt everything was hooked together temporarily. I wanted to see if I could get the Pi and a USB battery pack packaged in a weatherproof box with connectors for an antenna so I can take it out and leave it outside in the yard transmitting for a day (or at least as long as my USB battery pack will last for – I currently have 10000mAh USB pack that seems to last at least 8 hrs or so with a 1/4 charge remaining).
This is what my second attempt looks like:
- 10″ Tactical Weatherproof Equipment Case, from MCM
- on the sides of the case I drilled through and added banana jacks to connect each side of the wire dipole on the outside of the case. On the inside I soldered a short wire from each jack which connects into the QRPi board connector
- the battery is a 10000mAh Anker USB battery pack (hard to see in the photo but it’s at the back of the case)
- battery and the Pi are velcro’d into the case
There’s plenty of free space on the inside of this case, I probably could have gone with a smaller one. I have some other Pi related projects in mind though, so will probably use this size case again for another Pi + Packet radio related project.
So, the results, how does it perform. Well the first couple of weekends I ran this I didn’t get any spots, so I suspected something was up with my antenna connections. I had already run with the same wire dipole connected directly to the board, so I knew it would radiate a signal. The first test I roughly cut the wire dipole using the 468/MHz formula but I didn’t bother to check it on an antenna analyzer. So next step, where is the resonant point of this antenna?
Hooking up my YouKits analyzer, the center line is showing 14.1 MHz, but the low point is clearly to the left, so the wire is too long:
Trimming off about 6 inches from each side got us more in the ballpark – now we’re looking good: 
Getting the Pi back out in the yard, I hung the antenna from a couple of bushes at approx 5ft off the ground. This is very low for a dipole on 20m, but I know from previously running with this antenna at the same height that I still got spots all the way out to the East coast, so I’m not too worried about getting it up higher.
To keep things in perspective, remember we’ve only running 100mW, so we’re running low power, definitely QRP by any definition. At this low power, any spot is a good spot for me.
So, what about the spots? I didn’t get anything all afternoon, but then I got a number of spots from Texas, so if anything, there was a good propagation path between California and Texas early evening, but the spots dropped out shortly after sunset:
Since every day on HF is different, this is definitely an ongoing experiment 🙂 So far, this works pretty good and I’m happy with the setup!
NOTE: to transmit on Amateur Radio frequencies requires an Amateur Radio license, with the appropriate privileges for the frequency/band you are using. Also, per recommendations for the WsprryPi software, ensure you are using appropriate low pass/band pass filters so the Pi is not generating unexpected harmonics on other frequencies.
For future reference, current solar conditions (from http://www.hamqsl.com/solar3.html ):
SFI=86, SN=25, A11, K=2. 20m day/night was fair