I have had good results with an ordinary (unbalanced) air-core loop of 10'
diameter. Nulls are approaching 50 db on groundwave signals. The loop uses a
standard single-turn pickup loop out to the rx via 50 ohm coax. I agree with
Jan that a preamp is of no value once your limiting noise floor is
background sky/atmospheric noise. For those interested, construction plans
for my 10-footer are here:  http://www.imagenisp.ca/jsm/Loop.html

Steve / VE7SL