wrote:
How about adapting (somehow or the other) computer monitor VGA cables
that already have a big ferrite core on each end of the cables to use
with radio antenna hookups? Only wondering.
cuhulin

Yes, I'll try to get some of these...

Thanks,

Charly

Well,my computer to computer monitor fifteen feet long VGA has a big
ferrite core on each end of the cable.I paid sixty dollars plus ten
dollars shipping & handling for my cable,I bought it from
www.si87.com in Bozeman,Montana about three years ago.The two ferrite
cores on the cable help to reduce/prevent ghosting images on my 22 inch
screen CRT Multisync computer monitor.Sure,I could have connected two
cheap cables together,but I wont go any cheapo route as regards my
computer equipment.My computer monitor sits on my coffee table and my
computer is about fourteen feet away on an end table by doggys
couch.Three big eyes (two tv sets and my computer monitor) staring back
at me.I dont know if a VGA computer cable will work for hooking up to a
radio though,I guess it's worth a try if it doesn't blow up the radio.I
wont be the one to try it first.
cuhulin

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

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

Good morning Telamon,

Telamon wrote:
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.

Thanks for the formula...

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.

You mean that the impedance of the core will depend on its altitude ?

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.

Thanks, I will try that...

Charly

Telamon wrote:
[...]
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.
[...]

Another question please... Will a 2 / 6 turns balun behave the same as a 3 / 9
turns balun ?

I suspect not, but what will be the difference ?

Thanks again for teaching a newbie...

Charly

In article ,
Charly wrote:

Good morning Telamon,

Telamon wrote:
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.

Thanks for the formula...

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.

You mean that the impedance of the core will depend on its altitude ?

Snip

The impedance of the wire will change with its height above ground. You
are trying to transform that impedance to that of the coax.

Basically the higher the wire the higher the impedance. Smaller diameter
wire for the same height will be higher in impedance.

--
Telamon
Ventura, California

In article ,
Charly wrote:

Telamon wrote:
[...] 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.
[...]

Another question please... Will a 2 / 6 turns balun behave the same
as a 3 / 9 turns balun ?

I suspect not, but what will be the difference ?

Snip

In a voltage type transformer the impedance and resulting
voltage/current changes are based on the ratio of the windings. The core
and number of turns in the windings change the inductance of the primary
and secondary windings and the mutual inductance between them.

You are trying to make an RF voltage type so generally shorter windings
are better so go with the minimum turns to get the ratio you want.

2/6 and 3/9 are both 1:3 winding ratio so the impedance transformation
is the same. The impedance conversion is 1:9 so 50 ohms : 450 ohms.

The mutual inductance between the windings will be greater with the 3/9.

--
Telamon
Ventura, California

Thanks again Telamon for your time...

I will take your advices in account.

Charly

Telamon -

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

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
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?

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.

regards,
Bob
N9NEO