Toroid Testing

I started building a K9YA RX loop antenna but only had a few small #2 material iron powder toroids to make the required 9:1 impedance transformer. The design calls for a small 43 ferrite toroid, so I decided to try what I had on hand and do some comparison testing.


Test Jig

First I made up a trifilar winding and wound 5 turns per the design instructions, then hooked it up on a breadboard as a 9:1 impedance auto transformer.  The high Z winding was terminated with a 470 ohm resistor, and the 259B antenna analyzer was put across the low (50 ohm) winding.  I swept each toroid on 1.815 MHz, 3.515 MHz, 7.015 MHz and 10.110 MHz and recorded the match.

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Not bad but added to the antenna might put me over the 2:1 SWR target.  So I started thinking about what I could do to get a closer match with the transformer alone.    I re-read the MFJ-259B manual and noticed that it mentioned using a test coax less than 1 electrical degree long.  I thought the 2 foot jumper I was using to the bread board was OK, but when I did the math for 30M band ,  one electrical degree came out to less than 3.5 inches for the jumper!  So I made up a better adapter fitting which you can see in the next slide show.

Using the improved connector I got sweeps at 1:/1.1 for 1815 and 3015 center frequencies, so the transformer part of the K9AY project is completed and the number 2 material toroid tested well.  But while I had all this out on the bench I decided to finish matching work on a 30M vertical loop I have been planning. The following slides show a summary of that process.

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A few tips I learned:

  •  use simple 18-20 gauge hookup wire to do initial testing, adding or subtracting turns to get in the ballpark. It’s much easier to re-use than stiff magnet wire when trying different winding types.
  • keep your load resistor value very close to  what the turns ratio would call for.
  • for odd turns ratios like the 3:2 I used, an auto-transformer type winding is easy to do
  • the bifilar or trifilar winding type is known for reducing stray capacitance in the winding.  Trying to overlay a secondary with a separate primary  winding gave more erratic results for me.
  • keep the connection to the analyzer as short as possible to keep transmission line effects and stray capacitance/inductance from skewing your test results.
  • dressing the leads attaching the HiZ winding to the load resistor did not seem as critical.
  • I’ll be adding a choke balun made of #31 ferrite material cores with turns of the coax looped through it.