Elecraft K2 Internal 4:1 balun

March 8th, 2009 by m0rgb

Whilst looking around at the various mods done to K2's I discovered the following balun.

It is a small efficient QRP balun designed some time ago by Charles Greene W1CG. see www.njqrp.org for further details, including full building instructions.

My K2 is setup on internal batteries with it's main goal to be a portable radio. As I use mainly doublets through a 4:1 balun I thought building this inside the K2 would suit my needs perfectly.

I purchased the toriods from www.qrpproject.de you can buy these ready made but I wanted to build it! The toriods purchase were Amidon FT82-43.

Main components used

Main components used

As I had already performed a mod to add a separate charging socket to my K2 I knew I would have to move this to allow me to add the balun.

The balun consists of two toriods wound with 6 turns on way, a cross over and 6 turns the other way. Then wired together to give 4:1

Winding was pretty easy compared to the many toroids in the K2 :-)


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Testing with 200 ohm resister on the K2 gave me 1:1 swr so I guessed it was working OK.

I had used one of the transverter cutouts for my charger connector but now wanted to use both of these for the feeder posts, so another hole was required! Ouch!!

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No big deal, mark carefully and drill ha ha I added some heatshrink over the connections inside just to be safe.

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I then cut the posts down to fit better and added heatshrink where they go through the chassis to avoid them shorting out. These posts were not ideal but they were what I had!

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I connected the output of the balun directly to these posts ensuring the enamel was fully removed from the wire and then used a short piece of RG58 (again it's what I had) to go the the output of antenna 2 on the tuner.

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The balun is very light and is held in place with the coax and connections to the posts. I tried cable ties but they didn' seem to be needed and just looked messy!

Finally a few shots of the posts and the back of the K2.

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I'm sure I'll get the chance to try it in the field sometime soon. Overall I am pleased with this small modification as I continues with the theme of my K2. QRP and fully portable.

Have a go it's not difficult.

73 Gary M0RGB

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