In article ,
Charly wrote:

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...

The impedance ratio of your transformer is 50 ohms to 800 ohms.

p= primary
s=secondary

Turns p/Turns s = the square root of (impedance p / impedance s)
so
2/8= square root(50/s)
800 = s

The turns ratio is 1:4 and the impedance ratio is 1:16. In other words
the impedance ratio is the square of the turns ratio. This is a little
high and I would go for more like turns ratio 1:3 for impedance ratio
of 1:9, which would be 50 ohms : 450 ohms. For example you could use 3
turns primary and 9 turns secondary.

The sloping wire characteristic impedance will change with the height
above ground so I expect that you would want an impedance number at the
coax/wire junction.

The core will have a frequency range it will work over so test it at
three places. Test it at the lowest, highest and some place in the
middle of the intended frequency range. You may find the response falls
off on the low or the high end. When you do this wind the primary and
secondary apart from each other so you get the core response.

When you are done testing the core range wind them together so the two
windings couple better enhancing whatever coupling the core is
providing for you.

--
Telamon
Ventura, California