tags: balun, long wire antenna
Here I show my realization of a 1: 9 “BalUn” (actually a UnUn) for the 3-50 MHz band based on two FT114-61 toroids. The use of two toroids instead of one makes it possible to increase the power handling as well as to reduce the number of revolutions required due to the increase in the AL factor. AL measurement, made with a 5-turn winding, determined that the AL of the two tori is about 170 and the resulting inductance is 4 uH.
By putting its values in LTSpice and by putting a load of 450 ohm (transformation 1: 9) at the end of the balun, we see that the latter allows a bandwidth between 3 MHz and 60 MHz for a SWR of 2: 1 (S11 < -10 dB) in the simulation.
The realization of this balun is made according to the wiring illustrated in the figure below. The winding is made with a copper wire of 1 mm in diameter. To ensure a certain seal, the balun is placed in a PVC tube.
To validate the operation of the balun in terms of impedance transformation, a measurement of S11 is made when the balun is loaded by a resistor of 470 ohm. The measurement confirms the result of the simulation and shows that the bandwidth of the balun is between 3 and 50 MHz.
Regarding power handling,
this balun has been tested several times with a transmission power of up to 50 W without any observed problems.