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EFHW Antenna and 49:1 UnUn

Discussion in 'Amateur Radio Antennas' started by archjeb, Jan 31, 2019.

  1. archjeb

    archjeb Active Member

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    Folks,

    I built a couple of 49:1 unun transformers attached to a 130' wire. Amazing of how well this works.
    unun49-to-1.jpg

    Primary has 2 bifilar turns, and secondary is total of 14 turns.

    Some great references out there such as https://elginradio.wordpress.com/2017/10/17/end-fed-half-wave-antennas/

    The above picture does not show the 100pF cap across the feedpoint and ground. Having this cap installed does help with the higher frequency bands.

    I know its a bad comparison, but testing between my CHA-250B Vertical and this EFHW antenna with an A/B switch - I go from signals that are in the S3 range to S15+. It is an order of magnitude of difference. This is especially the case with 80/40M bands. For 20 and 10, I don't notice as much of a dramatic difference.

    Most of the things (box, stainless screws) from Home depot and fairly cheap. The two torroids are mix 43 and are superglued together and then turns of #14 enameled wire. Use at least 2 FT240-43 torroids to improve efficiency. Overall, pretty cheap in cost and great performance.

    10 turns of the coax through a mix 31 torroid before feedline goes into the shack to deal with any common mode. Don't put the choke by the transformer as this will impact performance as the coax from Unun to choke is used as a Counter-Poise.

    Overall, I think I now have a favorite portable antenna...



    If you haven't had an opportunity to build and play with one of these, its not a bad weekend project.

    -J
     
    Shadetree Mechanic likes this.

  2. The DB

    The DB Sr. Member

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    Just a note, if you make one of these yourself, you need to twist together the first two wraps of the 14 winding wire with both wraps of the two winding wire. I made one without doing this and it didn't work so well. Upon further research, I remade it tightly twisting those two separate wires together, just as it shows in archjeb's picture, and it worked surprisingly well. If you want a single band portable antenna you can mount pretty much however, look up this design.

    Also, you need something on the other side. It doesn't need to be very long, a few inches is enough unless you are talking 60 meters or lower frequencies, but you do need something there. More doesn't hurt, but after a few inches it didn't help any either.

    The one that I played with had no CMC issues that you normally see in an end fed design. This is likely due to the full isolation that this "unun" design utilizes.

    Actually, looking at how this works, I'm about 95% sure this is a balun, not an unun. The full isolation I mentioned above is only part of why I think this.

    I'm surprised more people haven't played with this very simple antenna design. Its great for a diy project and very portable. That being said, the biggest thing working against this antenna is likely the fact it is a single band antenna.


    The DB
     
    Slowmover and Mudduckmobile like this.
  3. archjeb

    archjeb Active Member

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    DB,

    Yes, the primary is bifilar, so the two conductors are tightly twisted together.

    As far as band use....this works great from 80-10m. You're using a wire that is multiple-harmonic of the half wave frequency. So if you have something in the 130-134' range, you'll get from 80-10m.

    Since I'm a general and only use phone, I only care about the phone band which is higher, so I've cut the wire in the 130' range. I use the touch up tuner in the radio to get me what I need for SWR to the adjacent freqs of resonance. This really is a great antenna for my geography with lots of trees and the ability to pull a line up a tree with an end fed. Simple, easy and cheap...

    I use a bow and arrow with a fishing line attached to shoot my lines up. Easy and quick...

    -J
     
    Shadetree Mechanic likes this.
  4. HomerBB

    HomerBB Sr. Member

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    I use this 8010 EFHW at home, too. Work all bands from 80 through 10 on it.
    I have been using a 4010 version while away from home making contacts on multiple bands.
     
  5. HomerBB

    HomerBB Sr. Member

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    A few more comments about this 49:1 autotransformer EFHW design.

    It seems to work best when the transformer is from ~1m to 2m from the ground.

    A ground rod from the braid side of the feedline at the transformer is a good best practice.

    My experience is a coax length of 1/4 wavelength of the lowest band is helpful in some setups.

    The antenna wire can be used in numerous configurations, although the inverted L and sloper seems to work best. I have had to bend the wire radiator in zig zag and multiple 90° bends to fit it in the available area and it still worked well.

    If setting it up as an inverted L go vertical as high as you have means, then horizontal, or sloped up or down or however your "best you can do" is. It will work.

    Use a half wave length of wire on your lowest band, however, make it too long and start folding it back on itself to tune it.

    While some have gotten a "no need for a tuner" result on multiple harmonic bands, environmental influences may work against that possibility so use of a tuner is sometimes necessary to trim up the SWR.

    To DB's point of a single band antenna, this 49:1 transformer is a good choice for a single band matcher. It is not a good choice for working lower bands from a too short wire, that is, if you have cut your wire to a 20m halfwave, it ain't much punkin' for 40m, 80m, or 160m. It will be good for 15m, and 10m. I have used a tuner to work the same wire on 17m, 12m, and 11m as well. All on one end fed wire.

    A radial/counterpoise field is not necessary with this EFHW wire antenna. A very long counterpoise wire can work against this antenna as it begins to appear as a dipole resulting in the transformer that is designed to match impedances in ~2450 Ohms range is now the wrong type.

    If you get lonely for a radial field because your grand daddy elmer said you had to do it that way then change the 49:1 out for a 9:1, change the wire to a non-resonant length on the bands you wish to work, and go for it. . . I have one of these, also, and it works, too.

    Homer
    KG5SQX
     
    Shadetree Mechanic and The DB like this.

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