I have an old Sangean SG-786 analog that I bought about 15 years ago
in Venezuela and I have always notice the same when I tune into medium
wave. If I touch the antenna, the radio drops in sensitivity. I also
notice the opposite on shortwave (when I touch the antenna, reception
improves slightly). This doesn't occurr with my SG-606A, which is a
digital.

Marty
San Juan, Puerto Rico