Amateur Radio WB3GCK The 4:1 Un-Un
Craig LaBarge, WB3GCK

I first did some experimenting with the "un-un" when I built a new vertical antenna to use with my pop-up camper.  I was so impressed with the performance of that antenna that I built a second un-un for general-purpose portable operations.  Before I get too far into this, let me be clear --  I didn't invent this thing.  I merely built one based on information from others and adapted it for my own purposes.  Check the references below for a couple of sites that inspired my version.

So, what is it?  As you probably know, a balun is a transformer that is designed to go between a balance load (like a dipole fed with balance line) and an unbalanced source (like the coax from your rig).  Hence the name balun.  Well, the un-un is very similar except that it is designed to work with unabalance load, something like a random wire up in a tree with a counterpose wire laying on the ground or a vertical with ground radials.  Hence, the name "un-un."  This particular un-un is a 4:1 transformer, which seems to work very well in a variety of antenna configurations.  When used with an auto-tuner, the un-un turns a simple piece of wire and a counterpoise wire or radials into a multi-band antenna.  The popular 43-foot vertical antenna configuration typically uses a 4:1 un-un at the feedpoint.  Several vendors offer these commercially.

Here are the parts I used:
  • T-200-2 powdered iron toroid core from Amidon
  • About 7.5 feet of #18 solid hook up wire (I've also used #20 wire with good results)
  • Red and black binding posts (Marlin P. Jones & Assoc. Stock# 14492)
  • SO-239 UHF connector (chassis mount)
  • 4" x 3" x 1.6" plastic box  (Marlin P. Jones & Assoc. Stock# 15523 BX)
  • About 6 inches of 50-ohm coax
  • Some ring terminals to facilitate coil and coax connections (crimped and soldered)
  • 2 strips of double-sided foam tape to hold the coil in the box

It's very easy to build.  This one took about 2 hours to put together.  IW7EHC's web page shows how to wind the toroid and make all of the internal connections, so I won't try to duplicate that here.  You can build it with a smaller toroid and lighter guage wire, but I went with the larger toroid and heavier wire to minimize losses.

Here are a couple of examples of how I have used it so far:
  • 25-foot vertical.  Using a 28-foot Jackite pole for a support, I had a 25-ft vertical wire connected to the hot side (red binding post) on the un-un and 6 16-ft radials connected to the ground side.  The un-un was about 3 feet above ground, so the radials gently sloped downwards and laid on the ground.  This works well from 40-10 meters.
  • 50-foot inverted L.  I supported the antenna with my trusty 28-foot Jackite pole.  25-ft vertical and the remaining 25-ft running horizontal to the nearest tree.  I used 6 16-foot radials an a couple of 25-foot radials.  The un-un, of course, was located at the feed point and fed with 28-ft of RG-8x coax.  This was my antenna for Field Day 2011 and it worked great on 80-10 meters.
  • A "No Counterpoise" antenna.  This is basically a 50-foot length of speaker wire with 25-feet removed from one of the conductors.  Connect the 50-foot side to the red binding post and the 25-foot side to the black binding post.  It loads up fine from 40-10 meters but I haven't really used it on the air yet.
  • I've been told by another ham that my "Dollar Store Special" wire antenna works well with an un-un.
Just remember to keep the coax run between your tuner and the un-un as short as possible to minimize losses and have fun!

73, Craig WB3GCK


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References:
  1. "The UNTENNA," Rick McKee KC8AON, http://www.angelfire.com/electronic2/qrp/unun.html
  2. "UN-UN FOR LONGWIRE ANTENNA", IW7EHC, http://iw7ehc.altervista.org/ununEN.htm

 

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Un-Un Winding on T200-2 core

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Inside the Un-Un

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Completed Un-Un
© 2011 Craig A. LaBarge

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