fredag den 30. november 2012

Homemade 1:2 balun

My loop antenna will show about 100 ohms impedance at the 14 MHz resonance frequency. My coax-cable is a 50 ohm type. Transforming 100 ohms to 50 ohms (and vice versa) can be accomplished by an antenna tuner, or by a HF balun. The inexpensive approach is a balun.

Having searched the Internet for 1:2 baluns, I found two sites describing materials and construction. One is by DH1PLY and the other one is by OE7OPJ. The inner parts of my homemade balun can be seen in this picture. The toroid core (TX36/23/15-4C65 from Ferroxcube) and the teflon wire (AWG18) were purchased from The sealed ABS box (G302) is from Velleman.

Homemade 1:2 balun.

Testing an 1:2 balun is easy. You connect a resistor at the balanced terminals. Then you measure the impedance at the unbalanced end. This impedance should be ½ the resistance at the balanced terminals, because the impedance ratio is 2:1.

I connected a 50 ohms 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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