I built this End Fed Half Wave Antenna coupler in 2009 for medium power use on 40m only. It is fixed tuned to the QRP frequency of 7030 kHz.
I constructed it from 8 turns of a B & W 2 inch diameter coil stock of 16 AWG wire, 1 turn of 16 AWG enameled wire for a primary, and several capacitors that comprise a total capacitance of about 251 pF. This makes a turns ratio of 8:1 which in turn makes an impedance ratio of 64:1. See the schematic below.
The capacitors values where determined empirically to resonate at 7030 kHz using my MFJ-249A antenna analyzer. Carbon resistors totaling a resistance of 3200 Ohms were used as a load and the circuit was adjusted using this method. When the coupler was placed at the based of the antenna the antenna was adjusted until a resonance was once again measured to be 7030 kHz. At this time the feed point impedance was about 35 Ohms. If my simplified calculations are correct then this means that the antenna's actual impedance before the transformation is 2240 Ohms. The 2:1 SWR bandwidth is about 100 kHz.
This antenna has performed very well whether using 2 Watts or 100 Watts. Whenever I venture into the upper phone portion of the band I use my home brew antenna tuner in the shack to make the transmitter happy. This is fine for 40 m but would be terribly inefficient to use this setup on other bands due to the high Q nature of the parallel tuned circuit.
Here are some more photos of this coupler:
