Thanks to all for the replies...

With the hints received here and elsewhere, I finaly made my firt unun with the
piece of ferrite I found...

I used magnet wire retrieved from an old broken AC/DC adaptor :
- 2 turns for the radio side,
- 8 turns of wire antenna side, making a 4:1 unun.
Currently the unun is just beside the radio (no coax or whatever : just a few
centimeter of wire connected in the radio), grounded via the outer radio/antenna
connector.

And yes it works ! At least connected to the ATS909 : it increases the signal
level by two bars and the sound is clearer. The result is not that obvious with
the SW100, perhaps due to a different antenna connection impedance (anyone knows
the Sony SW100 or AN100 antenna connection impedance ?).

Next step : mounting an inverted L antenna between my house the one of the tree
near-by : connection in the attic, going through the attic length (to have a
longer wire), passing through a tiny hole between the wall and the roof,
reaching the tree at something like 5 meters above the ground, then going down
the tree.

At what heigth should I stop the vertical leg above ground ?

The horizontal leg will around 12 meters (say 8 meters in the attic, 4 meters
outside : I can't go longer in this city environment).

Feeding will be a 75 ohms coax grounded via some house's water pipe : it seems
that the 909 has a 100 ohm antenna connection impedance... so it will be a
better match than 50 ohms (unless I put another 2:1 unun just before the radio
of course).

In radio with my (poor) electronic knowledge, the more fun is building the
antenna stuff... :-)

Oh yes : I need to try the snake-in-the-attic antenna as well...

Charly