QRP L- tuner for 5 MHz … 28 MHz

Recently I came across an article from Andrew, VK1AD, in which he describes a QRP L- tuner for 3.5 MHz to 52 MHz. It can be found here.
Andrew’s article is based on an essay by Peter, VK3YE, published in VK’s Amateur Radio magazine, Edition 5, May 2017.
I really wanted to try this easy to build L- tuner, because I thought it might perform even better than my EFHW- transformers (UnUns) and the EFHW- dipole I’m currently using for my SOTA activations.

Design:

My initial design covered the bands 40m to 10m. Portable operation wasn’t allowed on 6m in Germany and the 80m band is scarcely used for SOTA in Central Europe. The 60m band  I completely overlooked in the beginning.
Edit (2021-07-06): Operating from a temporarily fixed station on 6 m is now allowed in DL.

Hence I chose a wire length of 22 metres (about half a wavelength for 7 MHz) in inverted- L configuration on a six metre squid pole. In addition, about 10 metres wire were used as a single radial (this is just a random length which was at hand).
Edit (2021-05-03): With a radial length of 20 metres, the VSWR on 60m can be as low as 1 : 1.

L-tuner_schematic
Figure 1: Schematic of the L- tuner according to VK3YE.

I largely adhered to VK3YE’s design. The inductors were wound according to the instructions in the circuit diagram. After the test run they were fixed in position with hot glue. Only for the capacitor I used a different type which was at hand (~200 pF) .

A ground plane made of a piece of PCB was placed next to the capacitor in order to reduce hand sensitivity. A screw (visible between toroid # 2 and #3) provides the option to connect a counterpoise.

2019-09-29 12.33.49
Figure 2: The L- tuner mounted in a plastic box.

Field test:

For the traditional bands (7, 14, 28 MHz), the length of my wire is a multiple of a half wavelength. This means, the impedance at the end can reach values of 3 – 4 kOhm. Nevertheless, a VSWR of 1.1 : 1 could be achieved without any problems.

On the WARC bands (10, 18, 24 MHz) conditions are different: The antenna impedance is not as high but shows a significant imaginary component. But still my wire could be matched to 50 Ohm with the available L/C combinations.

Even on the 60m band, which I didn’t plan on originally, a VSWR < 2:1 could be achieved using the full inductance and about 100pF. Edit (2021-05-03): with a radial length of 20 metres, the VSWR can be as low as 1 : 1.

At an output power of ten watts, there was no arcing in the capacitor whatsoever. No tests were performed to determine where the power limit is.

Conclusion:

  • This handy L-Tuner and a single piece of wire cover a total of seven amateur radio bands.
  • In most cases, sufficient matching can be achieved by selecting capacitance and inductance according to the values determined initially. A SWR meter is not mandatory, though useful.
  • The transceiver requires no ATU.
  • After almost 40 activations and 1000+ QSOs the performance is convincing: End-fed wire and L- tuner are my favourite setup.

Many thanks to VK1AD and VK3YE for this very clever design!

2019-09-29 17.42.32
Figure 3: The L- tuner in the outside view
QRP L- tuner
Figure 4: L- tuner attached to a squid pole with two pipe clips. Radial (yellow) and coax can be seen below and antenna wire in front of the blue sky 🙂
2020-08-18 15.34.39
Figure 5: Alternative solution to attach the tuner to the squid pole with an adjustable rubber cord. Found on WB3GCK’s web site (Link).

40 Gedanken zu “QRP L- tuner for 5 MHz … 28 MHz

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