Inspired by a paper from HB9BCB I built my own version of a small and lightweight coupler for an EFHW dipole. It helps reducing the weight of my radio equipment for long hikes to alpine summits (see here).
The coupler is made of a tiny toroid (T50-43) and 24 turns of 0.4mm enamelled Cu-wire. There is a tap after three turns, where the inner conductor of the coax (RG-174 or the like) is connected (see photo for details). The result is a transformer with a turns ratio of 3 : 24 (or 1 : 8) which translates any impedance 1 : 64. Since this coupler is intended to operate from 7MHz to 14MHz I did not add a capacitor for compensation as suggested. This capacitor is only required for higher frequency bands like 21MHz or 28MHz.
It would have been advantageous to use a slightly thicker wire in order to reduce power loss, but this was the only one available in my junk box.
I did not test how much power this tiny toroid can handle. It works with the max. output power of my MTR-3B which lies in the range of 5 – 6 watts at 12V supply voltage.
The coupler is protected by shrinking tube. The antenna wire is connected to the brass screw using a cable lug. I prefer them over banana plugs as it is much easier to connect a wire in case it’s broken and you have to fix it on a summit.
The antenna consists of an approx. 20m long wire with a link at 14.5m for 10MHz. On 7MHz (when the link is closed) the wire is half a wavelength, on 14MHz the wire is a full wavelength.
So far I used this setup on 7 activations and made about 250 QSOs, thereof one with JA. My impression is that its performance is somewhat reduced compared to my usual setup with EFHW and QRP-L-tuner. But there is a significant improvement in terms of weight and size in return.