HFU HF Underground
Technical Topics => Equipment => Topic started by: chipre99 on May 26, 2020, 0546 UTC
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Hello everyone. I am building an antenna loop for vlf reception (65 turns of wire in a 4 ft diagonal square loop). The final impedance is 1300 ohms aprox. How can I connect it to my sdr receiver whose antenna input is 50 ohms? Is a balun suitable? Tnx
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Just off the top of my head but a 5:1 or 6:1 transformer should get you in the ballpark. 5^2=25*50ohm=1250ohm, or 6^2=36*50=1800ohm. You'd need to pick a toroid core mix that is effective at those low frequencies. Generic ferrite types might be fine or even an old AM radio loopstick ferrite. Maybe something like 4 turns primary and 20 or 24 turns secondary. The ratio is squared and primary turns might require some experimenting.
The other way to go about it is to use the loop itself for the transformation using a few turns around it as a sense wire, then a variable capacitor to resonate the 65 turns. I believe this is far more common, or at least how I built a loop for MW ages ago. It will not be wideband though.
I'm curious how you came to the 1300ohm impedance value? If I remember right loops are often much lower impedance only needing an autotransformer voltage balun or a transformation of 50 to around 450ohms?
Wouldn't mind building myself one again soon.