TinyCoupler for EFHW dipole

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). 

2022-05-14 12.20.36 (Kopie)

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.

2022-05-14 12.34.37

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.

tiny_coupler

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.

2022-06-21 13.26.21_2
Antenna set up as inverted Vee (or rather inverted „L“) on a sloping meadow.

 

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3 Gedanken zu “TinyCoupler for EFHW dipole

  1. dl1hbt 26. September 2022 / 08:25

    Roman, it`s always good to read about practical experience as well as hints and tricks to see what works with others in the greater SOTA-environment. Keep up your good work… cu, Tom

    • DL3TU 27. September 2022 / 10:12

      Thanks, Tom!
      I like to write about the things that work for me – or sometimes what doesn’t work 🙂
      In this particular case I’d like to point out that the efficiency of this transformer is not high, but the focus was really on reducing size and weight as much as possible. As time is often limited on long hikes, it won’t hurt if there is no monster pile-up 😉

      73 and cu,
      Roman

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