Version: 13. Sepe
,m
er 2004
24
as the smoke isn‘t exactly healthy! When smoke rises, move the solder blob
slowly towards the end of the wire, till you have soldered about 1 cm of the
wire. If the lacquer won‘t melt, it helps to add fresh solder. There melted
lacquer is shoved away with this method.
When you are done, check that the wire is soldered all the way around. This
is very important, as most errors in home brew transceivers are from badly
soldered coils made from lacquered copper wire. With thicker wires, you will
have to scrape off the lacquer with a knife. Please be careful not to nick
the wire, as it will break at the nicks!
Use the same technique on the other end of the wire. This completes the
toroid coil.
Often you will need a secondary winding, as in Tr7. This can be symmetrical
or unsymmetrical. Symmetrical
always means without ground
connection. No coil end will reach
ground or a bypass capacitor. Such
symmetrical coils always have the
secondary centred over the primary.
Now, let us focus on an example of
a symmetrical winding: The primary
should have 14 turns, the secondary
4. To have the secondary symmetri-
cally placed, we have to count on
the INSIDE of the ring.
14 divided by 2 is 7. The middle is at 7 turns. The 4 secondary turns must
start at the 5th turn of the primary, and end at the 9th, as shown in the
drawing. In another way: The secondary is symmetrical to the center of the
primary, before and after it, you will find 5 turns.
Should calculations ever result in half turns, you ignore the half turn, and
accept the slight asymmetry.
With unsymmetrical secondaries, you put the secondary turns between the
turns of the primary. You begin in principle at the cold end of the coil.
Which end is cold? Cold in an RF sense, is close to ground. This doesn‘t
necessarily mean galvanically connected to ground. From an RF point of
view the connection can be via a capacitor of low reactance. Don‘t be
scared by this!
Another type of transformer is made by twisting two wires and winding
bifilar turns. Such transformers are found at Tr7 and Tr8 in Section 7. Bifilar
technique gives a transformer of low stray capacitance.
Now for the real thing: Tr7 consists of 2 x 10 turns of 0.3 mm lacquered
copper wire on an Amidon FT37-43 core. FT37-43 means ferrite, material
43, 0.37 inch diameter. No colouring, ferrites are carbon coloured.
Take two pieces of 25 cm wire of different colours (I call
them red and gold), and twist the pieces at about two
twists per cm. This isn’t critical, but rather use more
than less turns per cm. With this „double wire“ put ten
turns on the toroid (The drawing only shows 8, not to be
too cluttered). Both end now have to be untwisted, and
now the coil should look a lot like the drawing on the
left (remember the drawing has 8 turns, you should have
10!).
[ ] Tr7
Amidon FT37-43
C4/5
And now we have a case of changing the geometry
of the pc board for the coil geometry. The coil 1/2
doesn’t mount from lower right to upper left, but
instead from lower left to upper right. You have to
wind opposite to Tr7! (Only 8 turns shown!).
[ ] Tr8
Amidon FT37-43
A4/5
Not that difficult, eh?
