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Homebrew 9:1 Unun for random length wire antennas

Discussion in 'Home Brew' started by Moleculo, Jan 26, 2016.

  1. Moleculo

    Moleculo Administrator
    Staff Member

    Apr 14, 2002
    Likes Received:
    This last weekend I decided to roll my own 9:1 Unun for use with my Elecraft KX3 and whatever piece of wire I happen to have handy. This project is really simple and most of it was made using stuff I found in my junk drawer. There are plenty of end fed wire antennas commercially available for around $75, but you can build the core of this antenna for less than $10. Here's my version.

    Parts: Here's the complete parts photo for the 9:1 Unun. I used 3 different colors of wire (to make it easy to keep track of each wire), a random iron core toroid (it looks like it might be T-200-2 core), a small project box, chassis mount BNC and binding posts


    The first step is to figure out a way to manage carefully wiring 3 lengths of wire around the coil. My solution was simply to use some scotch tape every couple of inches to organize the cable. The scotch tape will disintegrate over time, but that's OK - the only purpose is to help you wind the wire around the core.


    Wind 9 turns around the core with the three wires. I used a couple of small zip ties on either end of the windings to hold everything in place.


    Now, solder up the wires as follows:
    • Red wire on left side of unun winding and green wire on right side of winding goes to BNC "hot".
    • Blue wire on left side of unun winding is soldered to red wire on right side of winding. I used a little liquid electrical tape to cover the connection after soldering
    • Blue wire on right side of unun winding goes to "hot" antenna wire terminal
    • Green wire on left side of unun winding goes to bnc ground.
    • Optional: if you add a ground terminal for a counterpoise wire, jumper the bnc ground to the ground lug. The antenna will perform better if you can lay out a ground wire; otherwise it will use the coax shield and radio chassis for ground.
    That's it! I used a little hot glue to help keep the thing from rattling around.


    Testing the 9:1 Unun

    If you have an analyzer or any type of SWR meter, it's fairly simple to test your work. I twisted together 3 150 ohm resistors to create a pure resistive 450 ohm load (50 ohms x 9) and hooked it up across the wire terminals. A basic 470 ohm resistor will be close enough, as well. Here's a pic of my analyzer showing an SWR sweep:


    On 80 meters, the 9:1 unun shows about a 3:1 SWR, on 40 meters, it's right at 2:1 SWR, and it flattens out at 20 meters and stays under 2:1 SWR through most of 10 meters. Of course, results will vary depending on the actual load, but with a decent tuner, you'll be able to have fun with this simple impedance transformer with all kinds of random wire antennas.

    I'm only going to run about 10-15 watts through this unun, but if you used a little thicker wire, you could certainly run 100 watts with a core of this size. This whole project took less than an hour and most of that time was spent organizing the wire itself.

    Have fun!

  2. Justme

    Justme Active Member

    Jan 28, 2008
    Likes Received:
    Nice project, made a few myself using the FT 240 - 43 core which is a bit wider and will work uite nice from 80 - 10 meters.
    Well done ;)
  3. KF6IRK

    KF6IRK Kapt. Irk

    Mar 2, 2013
    Likes Received:
    Looks like you used 22AWG for the windings?

    18AWG should be fine for 100w, eh?

    Quit goofing off and get to work on my HR-50 - ;)
  4. 2RT307

    2RT307 Sr. Member

    Nov 22, 2011
    Likes Received:
    Very cool, Moleculo.

  5. RatsoW8

    RatsoW8 Supporting Member, W9WDX ARC Member - WD8T

    Nov 3, 2009
    Likes Received:
    Yes it is and thanks for reminding us to build and not just buy.
    #5 RatsoW8, Jan 29, 2016
    Last edited: Jan 29, 2016
    2RT307 likes this.
  6. vkrules

    vkrules Sr. Member

    Feb 18, 2012
    Likes Received:
    thxs for posting. saves me the trouble.
    Love mine.I use 63 ft of wire for the antenna. configured horizontal. coming straight from my tower to a pole in the back yard. About 10 ft down on the coax I wound an ugly choke for 20 meters to stop cmc on that band.
    I have to say this is another wire antenna that exceeded expectations. not only will it tune all hf bands including 160 meters ,but on 10 m. 15 m and 20m the swr is less than 1.5 without the tuner.
    On 40m performance is what you would expect from a 1/2 wave dipole.
    20 m is a real surprise better than my 20m loop but with the lobes 45 deg off the end not broadside. 15 m is also very good, again with similar lobes.10m is good not sure of the pattern. 75 is ok but it is down on a full size dipole and 160 works but not great as expected.
    Great project try different length wires and see what you get.Any thing over 27ft seems to work.longer the better for the low bands.
    HomerBB, 2RT307 and wavrider like this.

    KC2JLI NOODLES New Member

    May 1, 2016
    Likes Received:
    CAN YOU USE THE T130-2 Iron Powder core OR IS T-200-2 CORE THE BETTER CHOICE..
    DID you used 22AWG OR 18AWG FOR WRAP AROUND??
  8. W4CJB

    W4CJB W9WDX Amateur Radio Club Member

    Apr 17, 2012
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    Very nice. I gotta go build one for my Yaesu FT-817
  9. Jay Mojave

    Jay Mojave Active Member

    Nov 17, 2015
    Likes Received:
    Yo Moleculo: Wow good going, simple, parts list, construction photos, and test measurement.
    Had to throw a quick 20 meter dipole together as the locals had gathered on 20 due to the recent Sand Fire. Used a similar core as a 1 to 1 current Balun, RG58C Coax, and old wire I had in the Barn. Not pretty as yours but it worked, sprayed it with Klyrlon Clear spray to keep rain out of it.
    Taped it to the tower Nylon guy wires, so its about 20 to 30 feet high and works quit well, talked allover with it. Now the locals know I am listening when they bad mouth me.

    Jay in the Great Mojave Desert
    HomerBB and wavrider like this.
: unun

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