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.
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| 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.
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| 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 dx-wire.de. The sealed ABS box (item G302) is from Velleman.
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| Figure 3. The balun is housed inside an ABS box. |
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
Visit my homepage: http://oz1bxm.dk
