The impedances, at the transmitter end of the two-wire 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 two-wire line,
which is summarized as voltage U 2 , above the complex impedance. At the
Two-wire 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%