The image below shows my End-Fed antenna with coupler for 20 m and 40 m

Usage of this coupler
Goal is to maximise the antenna current (i.e. to maximise the power to the antenna). This is done by varying the tuning of the resonance circuit L1 and C1. This has to be done every time the frequency of the Tx is changed. After tuning L1 and C1, minimise the SWR by changing the coupling between L1 and L2 (change distance between L1 and L2). Retune C1,L1 etc. till maximum antenna current and minimum SWR has been achieved. The remaining SWR has to be nulled with the aid of an antenna tuner. It is however possible to get a SWR of 1 by solely changing C1, L1 and the coupling. This will in general not maximise the antenna current and leads to increased power loss in L1!
The antenna current is monitored by MES1 and a suitable oscilloscope or other current indicating instrument.
Although the antenna is for 20 and 40 m, it can be tuned from 160 to 10 m!
When properly adjusted, the power loss in L1 is relative low.
Attention: when using this tuner with high power (e.g. 100 W or higher), very high RF voltage (several hundred V to several kV) will be across L1 and C1.
So only adjust with minimal power applied!
E.g: when the antenna impedance is, say 2500 ohm and P = 100 W, the voltage across L1-C1 is about 500 Veff => more than 700 Vpeak! With 5 W, V = > 100 Veff
With no antenna attached: the voltage across L1-C1 can easily be several tenths of kV, depending on the Q-factor of mainly L1 . Possible leading to flashovers to whatever is in the neighbourhood of this circuit (e.g. coax to your transceiver).
So use this tuner only when you understand the possible dangers.
I explicitly do not take any responsibility, claims of damage or personnel injuries or even loss of life or whatsoever. You use it on your own responsibility.
Calculation of the behaviour of this tuner (condition to maximise antenna current, power loss in, mainly, L1 etc.):
Follows in due time