WSPR using Raspberry Pi

Recently, I put into operation a 100 mW WSPR transmitter, based on the QRPi shield (https://www.tapr.org/pdf/DCC2015-QRPi-HA7DCD-1.pdf), installed on a Raspberry Pi ver. 1B and using a 40m longwire antenna. Constantly, the WSPR signals are heard by many European stations, but, from time to time, some signals succeed to cross the ocean.