>Do other 505 's exhibit several beats with no antenna on 15 and 10?
Yes, they do. The beat at 21.3333 is a tough one, at least S1, and
the one on the low edge of 10M CW will tear your ear out (27.980 on a
Triton or 515. But under real conditions with real antennas I have
never missed a QSO as a result any birdie in a T-T radio.
Dick