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HF medium power balun

I needed to build some small baluns for vertical dipoles and arrays, to be used on the T2GM dxpedition. We required the following:

  • Handle 500w
  • good choking impedance 14 to 30Mhz
  • Lightweight and waterproof

First I tried a W1JR type on a FT140-43 core. This was a nice balun, but was not easy to form even with thin teflon coax and the performance dropped off on 10m. After reading GM3SEK’s article I headed to the junkbox to see what other ferrite I had to hand. I have a good number of Ferroxcube CST19/10/29-4S2 cores which I picked up on ebay for a song. 4S2 material is very similar 43 material.

Two cores were glued together to form a binocular, 3 turns of RG316 teflon coax fits through neatly. RG316 is only 3mm diameter but can handle 1.1Kw on 10m! The choking impedance is > 2000 ohms 14 to 30Mhz, reaches 2550 ohms on 10m. This was fitted into a small 60 x 65mm polycarbonate waterproof box with a 6m flying lead of Aircell 5 coax.

 

Two identical baluns were connected back to back and tested for extended periods 500w key down. Both the cores and the RG316 coax became warm as you would expect, but nothing to get worried about. I am confident they will survive anything when used at this power level.