For example, if 50 ohm coax cable is used, the impedance match will be 50 ohms to 200 ohms. If 75 ohm coax is used, the impedance match will be 75 ohms to 300 ohms.
It also transforms a balanced load to an unbalanced transmission line. This can also work in the opposite direction as well.
To test a balun, connect the end of the coax cable to the SWR analyzer. Connect a
noninductive resistor to the points labeled A and B. The SWR analyzer can then be
tuned to the frequency where the lowest SWR is found. You should expect a SWR very
close to 1:1.
Typical operation would be at a frequency where the length of the balun coax cable is one half of an electrical wavelength in the coax. This takes into account the velocity factor of the coax cable. If the frequency is too low the balun coax is too long and if the frequency is too high the balun coax is too short.
To understand how this type of balun works, consider the following:
Assume a transmitter is supplying 10 volts of RF to the coax cable. This 10 volts of RF travels down the transmission line to point A. At point A, one side of the load resistor is attached as well as another transmission line leading to point B.
The balun coax is connected to point A which has 10 volts of RF applied. Since the balun coax is one half wavelength it provides a 180 degree phase delay between the two points. Therefore the RF voltage at point A is 180 degrees out of phase with the voltage at point B.
The load resistor is connected between two 10 volt RF sources that are 180 degrees out of phase. This results in 20 volts of RF across the load resistor.
At the transmitter end of the coax cable the power applied would be the voltage squared divided by the resistance. This would be: 10 volts X 10 volts / 50 ohms which equals 2 watts.
At the load end the power applied would also be the voltage squared divided by the resistance. This would be: 20 volts X 20 volts / 200 ohms which also equals 2 watts.
Since the source power delivered to the coax is 2 watts and the power delivered to the load resistor is 2 watts there is no reflected power and the resulting SWR is 1:1.
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