I have been using an Ettus B200 SDR to receive FT8 transmissions in the 2-meter band (144.174 MHz) using the WSJT-X software. Signals are received using a telescopic antenna inside on a window ledge with approximately 1 m of cable. I have made no effort to ensure the antenna is tuned, e.g., by adjusting the length.
I am located in the UK in grid square IO91. As expected, I mostly receive signals from the same grid square, and occasionally the grid squares adjacent. I see replies to messages which originated further away (based on the call sign).
Some received messages are obviously "incorrect", for example:
210113_084530 144.174 Rx FT8 -18 1.0 1545 Z50H+TSKY-JHO 210115_114630 144.174 Rx FT8 -17 -1.6 685 SIDE LOBE
Others seem like they might be real, but they are located in grid squares more than 10,000 km away.
210111_195445 144.174 Rx FT8 -16 0.3 1728 VI7JOW/P TO7JRB R BO96 210113_163545 144.174 Rx FT8 -17 1.3 1247 J06NQY A68ITS/R R QM30 210114_100830 144.174 Rx FT8 -18 1.0 2653 YI7ORO VD7XLO R CK63 210115_051015 144.174 Rx FT8 -23 1.8 2388 8L9VCM 3K9WKZ/P MF32
The call signs don't appear to be real. Is it safe to assume the message is corrupt? Or is there another explanation?