The impedances, at the transmitter end of the twowire line, are in the range of 90 ohms
up to 530 ohms. With the impedances above 150 ohms, the internal coupler is overwhelmed.
The 4: 1 transformation brings the impedances to values the coupler can handle.
For example, the impedance in the 40m band is 798 ohms (measured on wet ground). Divided by 4 results
133 ohms, so that the internal coupler is clear. The picture shows the conditions for 40m again.
The Balun 4: 1 brings losses. The transmitter emits, in the example, 10W. By the
Difference method, in the voltage measurement, one measures two voltages on the twowire line,
which is summarized as voltage U _{ 2 }, above the complex impedance. At the
Twowire line one calculates the active power P _{ W } on the real part of the complex
Impedance R _{ A } to 7W. The losses are 30%, that is 3W. Dividing the 10W
through 4, you get to 2.5W. So you would only at a quarter of the power at
QSO partners will be weaker to hear an S level. The loss is not that significant.
An LC coupler, without a 4: 1 balun, only lost 9%
