L-network tuner for end-fed antennas from Ebay tuner kit

02 Oct 2018:

A simple kit for a QRP T-Network tuner for around 10EUR from Ebay is modified into a L-network tuner for end-fed / random wire antennas.

The Kit

There is a QRP Tuner Kit on Ebay which one can use to build all kinds of QRP-Tuner projects. It contains a knife switch for switching the inductors and two variable capacitors. As a nice addon there is a SWR-measuring circuit in there. It is using a wheatstone brigde, which is always ensuring a low SWR when tuning, and a simple LED to indicate the SWR (the darker the better).

I ordered one and modified it to build a tuner for an end-fed or so called random-wire antenna.

There are some reviews on Youtube. E.g. these two here:

Since there are plenty sellers on ebay just search for

“QRP Manual 1-30MHz Antenna Tuner” or similar and you will find it.

My modifications

Since I wanted to built a tuner for an endfed antenna I modified the build slightly. Here shown in ascii-art until I find the time to properly draw it:

    |            |           |        |
    |         +------+      ___       |
  ( o )       |VSWR  |      ---       +---o
      |       |Bridge|       |        +---o
      |       +------+       |        |
      |          |           |        |

Here ‘nnnn’ mark the variable inductance and the two — lines mark the capacitance in the asciiart.

The inductance has the same total of turns: 36. But instead of having a fixed inductance I wanted to be able to tune the inductor through the full range and also be able to turn it off.

So I tapped it the following way:

1 2 3 4 5 6 7 8 9 10 11 12 center
0 2 3 3 4 3 3 4 3 3 4 X 4

The taps are used to short portions of the inductor. One end is connected to the center of the switch and the other end is connected to the first pin. So when in position 1 the inductor is completely shorted out and when in position 12 (which is left free) it is using the full inductance.

Here are some photos of the final build:

Photo of L-network tuner

Photo fo L-network tuner

To be continued … I will post experiences here later.