Recently I was on air on 15m almost everyday at 1100z to 1200z.
It was very good propagation on the band. The Europe to JA path was good. Many stations QRV on JT65 mode.
Some freq had happen the tragic jam.
In this situation. Sometimes I found evil signal. That was very strong and occupied some freq width.
Almost -1dB signal level. If this signal received then the AGC function of transceiver operations. Then blocking weak signal, I cannot decode weak signal.
Really it was evil signal.