Don't do this at home... it may brick your Pi
First experimental setup. Just to see how it works.
No band filter. So a lot of harmonics... and QRM.
No buffering on GPIO4 pin, so it may brick the Pi. These GPIO pins work on 3V3 and are directly extracted from the Broadcom BCM chip. This is a so called SOC (System On a Chip) .
For this experiment I used a Raspberry Pi 2 model B. WSPR signal set to 40m.
Calculating output power
Signal measured on the Raspberry Pi pins (7, 9). Loaded with End-Fed antenna 50Ω.
Scale 1V/div. The signal is about 2V top-top = 0.707V RMS.
So the output power is about (0.7V * 0.7V) / 50Ω = 20 mW.
As the Tx and Rx antennas are in parallel and far to close ... the signal is booming ;-)
And ... producing a lot of QRM... ;-(
WSPR signal report
After five minutes and two Tx cycles the signal is reported !!
And on WSPRnet.org over a longer (one hour) period.
Not bad for 20mW on 40m at daytime...
Next step ... adding buffering for GPIO4 and a low pass or band filter.