2:1 Balun Schematic Question

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Eugene J. Crist, Jr.:
I'm interested in building the 2:1 balun in Jerry Sevick's book "Understanding Baluns and Ununs." The schematic is on page 67 figure 10-2A, and consists of a 1:2 unun connected to a 1:1 current balun. the last wire in the trifilar unun winding is #6, and according to the schematic, you can take a tap 6 turns from the unun's terminal 5 to connect to the #3 terminal of the 1:1 balun to get a 50-100 ohm TLT. My question - what does terminal 6 connect to? If I'm reading it correctly, it appears you can connect to terminal 6 rather than the tap to get a 50:112.5 TLT. Is that correct? If so, should you only wind the third wire of the trifilar winding 6 turns (while winding the first two 7 turns) around the core if you're only interested in making a 50:100 ohm TLT? That's the way I understand it but wanted to get a second opinion prior to building.

Thanks, Gene (KB3ONA)

Dale Hunt:
The impedance ratio is the square of the turns ratio.  With the 50 ohm load connected
across 2 of the three windings (7 turns each), the full 3rd winding gives a turns ratio
of 21 : 14 = 1.5, an impedance ratio of 1.5 * 1.5 = 2.25, for an impedance of
2.25 * 50 = 112.5 ohms.

To lower the ratio from 2.25 : 1 to 2 : 1 you need to use fewer turns on the last winding.
By using only 6 turns the ratio becomes 20 : 14, or about 1.43 : 1, giving an impedance
ratio of 2.04 : 1, which is very close to the target of 2 : 1.


So if you are only interested in the 2 : 1 ratio, you leave terminal #6 unconnected and
hook your load 1 turn back up the winding.  You could just remove the last turn, but
it often is easier to wind all three wires together onto the core and ignore the last turn.

Eugene J. Crist, Jr.:
Thanks for thorough explanation - very much appreciated. That makes perfect sense.

Eugene J. Crist, Jr.:
Two additional questions:
1.  In the book "Understanding Baluns and Ununs" the schematic on page 67 figure 10-2A referred to in the original post, is the ground from the balun component connected to the ground of the unun?
2. Jerry recommends using a 250 permeability core, and in the "Transmission Line Transformers Handbook" from Amidon page 18 which has the info for the voltage transformer section in which Jerry recommends using the FT-150-K core. If you want to use a different core, such as a FT-150-61 would the number of turns through the voltage transformer core change? I'm thinking not since the voltage transformer is only providing the impedance transformation and it only matters for the current transformer component since it needs a high enough impedance to choke common mode currents. If I'm wrong then how does one go about selecting a core for a voltage transformer? I understand choke/current transformer selection starts with the amount of impedance you want at the lowest operating frequency, and from that you can calculate the number of turns through each core, but I'm lost on what you base voltage transformer core selection on. Any info would be greatly appreciated.

Thanks, Gene (KB3ONA)

Dale Hunt:
Quote from: KB3ONA


1.  In the book "Understanding Baluns and Ununs" the schematic on page 67 figure 10-2A referred to in the original post, is the ground from the balun component connected to the ground of the unun?



If the balun is doing it's job, neither side is "ground" where it connects to the unun.

The two output wires from the balun have to connect across the two full windings of the
unun.  It shouldn't matter which way they are connected.



Quote


2. ... If you want to use a different core, such as a FT-150-61 would the number of turns through the voltage transformer core change?..



It depends...

You need to keep the turns ratio constant to get the 2 : 1 impedance ratio.  Other
combinations of numbers of turns can get you close to 2 : 1, but might not be as convenient.

Whether or not you need to change the number of turns depends on the relative
permeability of the cores and the operating frequency.  Core materials not only have
a permeability, but they also have an optimum frequency range over which they are the
most effective.

The Amidon site rates the K material as being usable from 1 to 50 MHz, while the type 61
material is rated up to 25 MHz.  If you aren't planning to use your balun above 20m or so,
the type 61 material should be OK.

If you were using a core with lower permeability then you might have to increase the
number of turns to keep the shunt inductance of the winding to a reasonable level.  (I learned
the hard way on a filament transformer that too few turns on a transformer doesn't work well
even when the turns ratio is correct!)  But going from K to 61 doesn't look like a problem.

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