fredag den 30. november 2012

Homemade 1:2 balun

I need an 1:2 balun
My 20m loop antenna has about 100 ohm impedance at the 14 MHz resonance frequency. My coax-cable is a 50 ohm type. Transforming 100 ohms to 50 ohms can be accomplished by an antenna tuner, or by an HF balun. The inexpensive approach is using a balun.

Figure 1. Balun 1:2. Picture credit: OE7OPJ.

Winding the toroid
Please refer to the drawing by OE7OPJ in figure 1. The red wire begins at the UHF-connector's center pin. The black wire begins at the UHF-connector's ground pin. The two  wires "follow each other" for 6 windings, and then the black wire is connected to one of the output terminals. The red wire continues with 6 more windings and then connected to the UHF-connector's ground pin. Finally, a short wire connects the UHF-connector's center pin to the other output terminal.

Figur 2. The finished 1:2 balun.

My homemade balun can be seen in figure 2. The toroid core is TX36/23/15-4C65 from Ferroxcube. An alternative toroid can be Amidon FT-140-43. The core's outer diameter is 34 mm and the inner diameter is 22 mm (not critical). The teflon wire (AWG18) was purchased from The sealed ABS box (item G302) is from Velleman.

Figure 3. The balun is housed inside an ABS box.
Testing the balun
Testing an 1:2 balun is easy. You connect a resistor at the balanced terminals. Then you measure the impedance at the unbalanced port. This impedance should be ½ the resistance at the balanced terminals due to impedance ratio 2:1.

I connected a 50 ohm resistor at the balanced terminals. Then I measured 25 ohms at 14 MHz at the unbalanced end. Conclusion: The balun's impedance transformation is just right!

73 from OZ1BXM Lars Petersen

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