Here’s another thread from the wonderful Elecraft mailling list:
Vic, K2VCO writes:
I made a simple 1:1 receive-only balun for 160 meters by winding 9 bifilar turns of no. 24 enameled wire on an FT37-77 toroid. I connected a 50-ohm resistor to the output and my MFJ antenna analyzer to the input. On 1.8 MHz it shows a resistance of 50 ohms, an inductive reactance of 1 ohm, and an SWR of 1.0:1.
My question is, did I make a ‘good’ balun? Are there other tests I should do?
Bob N6WG responded:
You can use your MFJ to measure the impedance of the balun. Connect one of of the balun (short both wires together) to the center of the MFJ coax connector and the other end, shorted together, to the shell of the connector. Use the MFJ to measure the impedance of the inductor formed at the frequency to be used, say 1850 kHz. If the impedance is 5 or more times the 50 ohm line impedance, you have a good balun for 160m. If that is the only band you expect to use it on, you are done.
If you expect to use it over more bands, say 80 and 40m as well, tune the MFJ to those bands also and make the same neasurement. With a good core, the impedance should hold through these bands as well, or even be higher.
Don W3FPR also replied:
If my calculations are correct, YES you should have a good balun. The reactance of the 9 turns is 809 ohms (AL value is 884 mH/1000 turns, inductance for 9 turns should be 71.6 uH) which is more than 16 times the impedance of an antenna feedline of 50 ohms. Greater than 10 times the line impedance is the value I usually use for an effective choking action.
I think this would also be a good test for any commercial balun that you might buy.
Leave a Reply