Manual antenna tuner

29-4-2017: The antenna tuner is something you can do yourself, variable capacitors, roller inductor and knobs are all used components and they come from the Rosmalen radiomarket. The case is from CONRAD. Unfortunately there is no step counter with the inductor, but it has a stop which you can feel, so just count the left hand turns.

The antenna tuner (also called matchbox or antenna coupler) is a T-shaped network, the inductor does something like 35 microhenry (and probably more), and the capacitors are 30 to 300 pf. I mounted the components on wood because they need to be isolated from the metal case. Straps keep them on the board, and rubber thules are used to isolate the components from the metal case.

Performance-wise this tuner is able to adapt a 200 dummy load to 50 Ohms between 1.8 MHz and 30 MHz, I checked this with my antenna analyzer, it works. There is still space to store a 1:1 or 1:4 balun if needed (see entry on 7-5-2017 how I solved this). I’m not sure how much power the tuner can dissipate, the heat could go in the inductor, or, the capacitors could spark, etc. These capacitors are rated for 1200 Volt, so maybe we can go up to a kilowatt. (hi) (see the entry on 4-5-2017 that discusses where the dissipation could occur in the tuner)

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Added on 4-5-2017, a note:

 

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7-5-2017: Added a 1:1 balun to be able to handle twin-lead lines. Glue, polycarbonate, green tape, some banana jacks and an extra SO-239 case part do the job. The balun is separate from the tuner, you need a patch cable to connect the output of the tuner that goes to the right most SO-239 connector to the left most connector which is the input of the balun. The input of the tuner goes via the middle antenna connector. The balun is nothing more than a common mode filter. The magnetic field lines will stay inside the ferrite core, and any forward magnetic field leaving the tuner will cancel against a returning current that has an opposite magnetic field. This will work fine up to 30 MHz which is the design area for ferrite (it is a 4c65 material), beyond this point you need another trick to design your balun. I added some green tape around the 4c65 core although it is not strictly necessary, on both sides you glue two pieces of polycarbonate and with two parker screws you attach the balun to the aluminum back plate. Check the end result with an antenna analyzer and a multimeter.

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Bill of materials:

  • three SO239 connectors, (relatively easy to get via stores in Rotterdam or Delft)
  • a 300 by 200 by 100 mm aluminum case (difficult to get, consider CONRAD)
  • two banana jacks and the matching case parts (a store in Delft had them)
  • a 4C65 ferrite core, you need it for HF and it shouldn’t saturate below 1 kW (got it from dx-wire in Germany, see also my article on the 1:4 guanella balun)
  • two 30-300pf variable capacitors (got it from a hamradio market)
  • roller inductor (got it from a radio market)
  • wooden ground plate, cut it as required
  • three tules, three knobs (hamradio markets have them)
  • 0.8mm or thicker copper wire (again, the ham radiomarket)
  • 30 cm of 50 Ohm patch cable with two SO239 connectors (I found a store in Rotterdam that I refuse to reveal, it is the only one in town, otherwise, CONRAD or radiomarkets)
  • various M4 nuts and bolts and straps (via any hardware store)

Options to consider: possibly an integrated SWR power meter, this is what I currently solve with a separate AV-CN600 cross-needle meter. The schematics is well known, however, I could not find a cross-needle microamp meter, the electronics is not very difficult, perhaps the calibration is.  Also, many transceivers already have a SWR/Power meter, so perhaps you can skip this step as well.

Last update: 7-May-2017

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