This is a useful device for coupling an external antenna to a radio that has no antenna jack. Particularly useful for use with a medium wave loop and a standard table or portable radio as most of them don't have external antenna jacks and rely solely on there internal loop stick. The ferrite rod  I used is 5.5x.375 inches taken out of an old radio. Size of the rod is not very critical here nor is type of ferrite material. I use about 20 turns of standard insulated hook up wire. Experiment with the number of turns and position in relation to the radio. I used a DX 398  and a Radio Shack loop for the initial test and double sided taping the coil to the top of the radio produced best results. Using the coil actually seemed to reduce the noise somewhat compared to the loop alone but that could have been just my imagination. Give it a try and see what you think.