On Dec 25, 10:02*am, Wimpie wrote:
On 25 dic, 03:33, JIMMIE wrote:





On Dec 24, 9:34*am, Wimpie wrote:

On 24 dic, 03:15, JIMMIE wrote:

I want to contruct a 4:1 BalUn but instead of stepping the impedance
up from 50 to 200 I want to step it down from 50 to 12.5. Is there any
difference in design between the two.

Jimmie

Hello Jimmie,

It depends on the design.

In case of a classical transformer, it will be the same, except that
the inductance can be divided by 4. Practically this means you can
half the number of turns.

For example if the 200 Ohms side have 8 turns, you can reduce this to
4 and this will be the 50 Ohms side. *As current increases (with same
power), you may use thicker wire, or use two windings in parallel.

In case of a real transmission line transformer, you may need
transmission lines with unpractical low characteristic impedance.
Maybe you can provide some details about the type of balun you want to
make.

Best regards,

Wim
PA3DJSwww.tetech.nl
without abc, PM will reach me very likely.

I had thought about the increased current and decided the best way to
compensate for this may be to use a transformer designed for more
power than I had palnned on using As I am only going to be running
100W PEP I figure using a design for 1000W would be more than
sufficent. I had not considered the need for fewer turns. Is this
important?

Jimmie

Hello Jimmie,

When you take a transformer type 200 to 50 Ohms balun, the lower
frequency limit is determined by the inductance of the 200 Ohms
winding (no load connected). This inductance is in parallel with the
200 Ohms source (or load).

When you short-circuit the 50 Ohms side, you will not measure 0 Ohms,
but an inductance. This is the leakage inductance due to limited
coupling between sec. and prim. winding.

When you use this same balun for a 50 to 12.5 Ohms circuit, the lower
frequency limit will decrease. *However the upper frequency limit will
decrease also, as the leakage inductance is in series with the load.

You can imagine that 150nH (j28 Ohms at 30 MHz) in series with 50 Ohms
load is acceptable in many situations, but this same 150nH in series
with 12.5 Ohms load may give problems.

Regarding power handling
*If you are using your balun with a load with bad VSWR (referenced to
12.5 Ohms), you have to derate power. *If the transformer is in a
voltage maximum, the core dissipation will increase due to higher flux
in it. When the transformer is in a current maximum, wire dissipation
will increase.

Best regards,

Wim
PA3DJSwww.tetech.nl
without abc, PM will reach me- Hide quoted text -

- Show quoted text -

Thanks, would it be safe to assume that a transformer designed for 1
to 30 Mhz at 1KW could be used as I have discribed without
modification on 40M. I think the load side should be very close to
12.5. VSWR was 4.1:1 when measured with an Anritsu analyzer a few
weeks ago with no balun attatched. Unfortunately I no longer have
access to that piece of test equipment. The best I can presently
manage is a cheap VSWR meter. Its measurement also agrees with the 4:1
mismatch.

Jimmie

Jimmie