WSPR Without Tears
FCC happy. Second, the output power is around 10 mW. We talked about the thrill of using low power
to make contacts, but it's nice to start with a little more power than that.
Our WSPR Pi is a circuit board (pcb) that plugs into the Pi 40-pin header. It boosts the signal level to
about 200 mW and contains a seven pole low pass filter to keep out-of-band transmissions down by 43
dB. It also contains an LED that lights when transmitting and a crude output power indicator. (We
found it annoying to not know when and how well our system was working without connecting a 'scope
or power meter and wanted something simple as a sanity check.)
The Pi is a computer, and most computers need to be shutdown properly rather than just have the power
turned off. We added a push button to the circuit board to initiate a shutdown sequence in the Pi. We
wrote a program to run in the background that monitors the push button and shuts the Pi down when it's
pressed.
Raspberry Pi
The Pi is a computer and is subject to all of the headaches associated with computers. We wanted to
make life easier for anybody trying to duplicate our WSPR experience by limiting the amount of
computer exposure you're subjected to. Loading the software into the Pi is not difficult, but there are
quite a few areas that can go wrong and it can be frustrating if you've never done it before. It only takes
one problem to kill the whole process. There are a lot of steps involved in getting to a working WSPR
transmitter.
We created a version of the software that runs automatically when the Pi is turned on. There are several
parameters that you (the user) have to set: callsign, grid square and WiFi (if you're using wifi). We
programmed a web server in the Pi (we told you it's a computer :-) that lets you fill this stuff in from a
web page onyour PC. In other words, you bring up Internet Explorer or Chrome or Firefox and type in
the Pi's address (we'll get to that) and the web page comes up and you enter the data. This means no
programming of the Pi at all. Bring up the web page, type in your callsign, grid square, and WiFi
parameters and you're up and running.
The operating system (Linux) and all of the software are stored on a micro SD card. The SD card is
plugged into the Pi. Set it and forget it. You need a card reader (<$10) and software (from the TAPR
website) and an SD card to do this. We'll repeat the important part: you have to burn the image onto the
SD card (similar to burning a CD/DVD) and then plug it into the Pi. There's *no* programming.
What you'll need to complete the assembly
1.
Raspberry Pi 2.0 Model B or Raspberry Pi 3.0
2. 2 amp (minimum) power supply
w/micro USB connector.
TAPR (c) 2018-2019 “WSPR Without Tears” 3 Of 20 Version 2.1 05/20/2019
