[CQ-Contest] Balun design

[Sinisa Hristov]

Guy Molinari wrote:
 
> I need a balun design to match 50 ohm unbalanced to 100 ohm balanced (2:1).
>   I have a T-200-2 core and am winding it as follows:
 
> A bifiliar winding with one long and 1 short wire.   This goes for 12 turns
> and then the shorter wire stops at the halfway point.   The longer wire
> continues for 12 more turns.


Exact single-step 50 Ohma unbalanced to 100 Ohms balanced is difficult.
Following approximations may be useful.

50 Ohms unbalanced to 89 Ohms balanced.
Make a quintifilar winding and connect all 5 "coils" in series.
Connect 50 Ohms port "hot" to one end, and ground 3 "coils" later.
For 89 Ohms balanced use the first connection after the 50 Ohms hot, and the other end.
That makes a voltage ratio of 3 : 4, or impedance ratio of 9 : 16 = 50 : 88.9.

50 Ohms unbalanced to 112.5 Ohms balanced.
Make a 7-filar winding and connect all 7 "coils" in series.
Connect 50 Ohms port "hot" to one end, and ground 4 "coils" later.
For 112.5 Ohms balanced use the first connection after the 50 Ohms hot, and the other end.
That makes a voltage ratio of 4 : 6, or impedance ratio of 16 : 36 = 50 : 112.5.

Unbalanced-to-unbalanced 50 : 112.5 Ohms is simpler.
Make a trifilar winding and connect all 3 "coils" in series.
Use only 2 of them for 50 Ohms port, and use all 3 for 112.5 Ohm port.

Adjust the number of turns for the desired frequency coverage.


73,

Sinisa YT1NT, VE3EA