Ismyer, I'll be glad to help you with ferrite antennas. 1) No, there is no
optimal ferrite rod length. For your Sangean 505 try a ferrite plus coil
antenna such as the Terk or Select-a-Tenna. These antennas couple to your
receiver inductively (magneticially-no wire connections).
2) Ferrite rods are expensive so the bigger rods are on the better receivers.
But, you can easily add one. But, you really don't need to if you use one of
the above MW antennas. See Univesal Radios web site & look up the Palomar LC-1
& RF Systems AA-2 add on ferrite rods.
3) Yes, Ferrite rods are expensive & hard to find. For MW you can easily make
one. See the web site of "National Radio Club", a MW DX Club with rod designs.
4) If you use a Select-A-Tenna the 505 will out perfrom the smaller one. It is
all in knowing how to couple an exterior loop to the receiver. You need an
exterior loop that couples (transfers signals to) your 505.
5) Flux is the type of ferrite used. It is sold by an assigned number. #61 is
good for MW.
6) "Q" stands for the "Quality" of the circuit, not always agood thing. For
example a "Hi-Q" provides a narrow band width. You want a wider band width.

Need more help let me know. You really need a Select-A-Tenna with the mini jack
on it's base, or the locally available Terk.

73 John T. Wagner, Ohio