Ferrite core : yes it works... Now for the antenna...
 

In article . com,
"N9NEO" wrote:

Telamon -

Can you please explain mutual inductance? Is that like coupling
coeficcient sp?

Yes. Here is an interesting physics experiment with explanation.
http://www.allaboutcircuits.com/vol_1/chpt_14/6.html

If we short one winding out and measure the other winding's inductance
then we will have a measure of the coupling between the two coils.

The problem with this is if you short either coil the other will also be
shorted. The transformer through the mutual inductance coupling between
them effects an impedance transfer dependent on the turns ratio. You
short one coil and guess what? The 0 ohm short is coupled to the other
coil. If you have the equipment you can measure the self inductance of
one coil and watch that value drop when you short the ends of the other
coil together. Better coupling will result in a bigger change in
inductance.

The lower the inductance measured then the lower the stray inductance
- or supposedly the better the coupling. With fewer turns then I
think maybe the lower the stray leakage we will measure.

Am I on the right track?

The idea here is high mutual inductance or coupling between the coils
and have the self inductance of each coil approximate the impedance of
the wire on one coil and the value of coax on the other.

Also you want the path through the windings to be an electrically short
path so the UNUN will work on the high frequency end.

I think the balun design can use the very small cores as long as we
dont want to transmit. Correct? I am thinking that since the voltage
we will see across the coil In receive mode is microvolts then we don't
have to worry too much about wasting power magnitizing the core. So
then use only a very small number of turns.

I have to build a balun too for a T2FD I am putting in the attic. I
have Doug DeMaw's ferrite design book that I will open tonight when I
get home.

Not so many good places to find info on receive baluns on www.

There is stuff around on the net but a good book is better bet.

BALUN and UNUN design can get a little complicated when you want broad
bandwidth. If you have a book with a proven design worked out for an
antenna that is a great way to hack a BALUN or UNUN.

The thread started out with you grabbing what was on hand and just
following a few rules will get an UNUN working pretty close to optimal
with just the stuff on hand on at least part of the SW spectrum.

With the unknown core on hand the hack would be the minimum or nearly
minimum number of turns in both primary and secondary windings to get
the impedance ratio you need to get.

The winding technique changes the performance character of the
transformer.

If you wind primary and secondary apart from each other you will cause
all or nearly all the flux linkage to go through the core. The
consequence of this is the cores have an efficient frequency range and
chances are it will not work so well through the entire SW band range
of 3 to 30 MHz.

If you wind them together then the two coils couple to each other
directly in addition to the common flux linkage generated in the core.
If the core response is good on the low end and drops off on the high
end the close coil to coil coupling will help on the high end as an
example.

--
Telamon
Ventura, California

Ferrite core : yes it works... Now for the antenna...
 

I wonder if magnet wire is a good idea? Generally magnet wire is
designed to get the turns as close as possible. I'm not sure that is
best for high frequency use since you want to minimize stray
capacitance.

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...

Charly