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End-fed Halfwave Matching Transformer

Discussion in 'Home Brew' started by DTB Radio, Sep 24, 2020.

  1. DTB Radio

    DTB Radio Well-Known Member

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    I've been puttering around with end-fed half-wave antennas for 40 through 10 meters (concentrating on 40 for now), and have had trouble coming up with a good match-box design. I've tried different ideas from an array of online sources ranging from auto-transformers in various turns ratios to basic L-match circuits. I finally found a design which, at least in the test phase (using fixed-value non-reactive loading resistors) shows an extremely good SWR spread. I do not use any caps at all in the design, just a mix-43 torroid and a 3 to 17 turns ratio transformer. Using a vector network analyzer and a 10-inch jumper, with a test load of 3.3k ohms, the SWR is 2.1 or below from 160 meters all the way through 10 meters. With a load of 2.2K, the SWR is about 1.7 on 160 meters, and 1.5 or below on 80 through 10 meters. With an 18-foot cable connected, the readings with a 2.2k load were 2.1 on 160 meters, and under 2 on 80 through 10. With 18' and a 3.3k load, the readings are 1.6 or below across the HF band. With 18' and a 1.5K load, the readings were again 1.6 or below across the HF bands. With a 4.7K load the SWR spread was quite a bit higher, with 160 meters being around 2.9, 80 meters at about 2.5, 40 meters at around 2.5, 20 meters around 2, and the rest of the band at under 2 with 10 meters being in the 1.6 or below range. I have not put the transformer into use with a practical antenna installation yet, but of all the designs I've tried, this one shows the most promise.



    I plan on posting updates.
     

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  2. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    3/17 turns is a 32:1 ratio. I'm using three stacked T240-52 cores with 2/14 turns for a 49:1 ratio. I do use a pair of 220pf caps in series at the low impedance point. Works great on 80-20m with a bit higher SWR on 15 and 10m. I never bothered to fine tune however as I only put it up a couple days before knee surgery.
     
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  3. nomadradio

    nomadradio Analog Retentive

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    Just bought a commercial end fed HW, 120 feet long. Haven't put it up yet. Uses the 49 to 1 transformer ratio.

    Pics I see of this transformer in the DIY articles show half the secondary turns on one side of the toroid, the wire travels across the open center of the toroid, and the rest of the turns on the opposite side of the toroid. Tried to wrap my head around this and decided the detail level of the pics wasn't sufficient. Sure would appreciate finding an explanation of why the turns are wound this way. Just looks odd.

    Gonna mount the feedpoint on four feet of a 8-foot ground rod that still protrudes above ground level. Hoping this will stave off issues with hot coax.

    73
     
  4. Justme

    Justme Sr. Member

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    Using that way of winding has the benefit the ends are opposite to each other and easier mounting is made that way.
    you can just as well wind them all even over thee core, it would not matter as long you keep the 49;1 transformation intact.
    But the other method has the advantage to use the whole core and still ending up with the input/output on the other side of the core.
     
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  5. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    Exactly. You have better isolation from input to output that way and you have to reverse the windings at the cross over point to maintain proper phasing.
     
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  6. DTB Radio

    DTB Radio Well-Known Member

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    I got a chance last night to do a quick and dirty test using a wire length for 80 meters. I didn't get time to fine-tune, but with a modest counterpoise was seeing less than a 2 SWR in the general vicinity of the 80, 40, and 20 meter bands. SWR climbs after 20, so I may need to "tweak" the box design a bit. Captain Kilowatt mentioned 2 220PF caps in series, which is something I haven't seen in any of the schematics I've found online. May give that a try with a 2-turn primary.
     
  7. Justme

    Justme Sr. Member

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    Normally a 110pF c is used, it will help to keep the swr down, using 2 220 pF in series is also 110 pF but higher voltage can be put over these 2 C's and more power used.
    In all schematics and home brew the 110 pF c was needed, and helped to bring theswr down.
    Use high voltage C 500 volts or higher or 2 x 220 pF in series.
    https://www.hyendcompany.nl/
    Tip:
    use a small piece of wire as counterpoise, a 10 feet length will do in most cases.
     
  8. binrat

    binrat WDX Club Coordinator
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    I have made about 10 this year for 40-10 or specific freq.
    I have found that it maybe good on 40, it will be slightly out of band on the others, especially if its tuned on 40 around 7.5 to 7.3 mHz.
    I used minimum 1kv 100 pF cap close to feed point.
    5 watt and 12 watt pictured. rich efhw.jpg stephen efhw.jpg
     
  9. blasphemy000

    blasphemy000 Active Member

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    I've had good luck with link coupled, parallel resonant matching circuits, for single band EFHW antennas. I followed the design laid out on this person's website for designing the match box.

    http://www.aa5tb.com

    On the left side of the page there are a few links that cover EFHW antennas. I used a decent sized #2 (red) iron powder toroid with somewhere between 1:7 & 1:8 turns ratio (1:49 to 1:64 impedance). I can't find my notes page at the moment, but I think it was 3:22 or 3:23 turns, my apologies. I then have an air variable cap across the secondary of the toroid transformer to resonate out the transformer's inductance. The air variable has range for 40, 20, 17, and 15m, but I only have 40m and 20m wires that I use with it. Each band has its own counterpoise cut to 0.05 of a wavelength as explained here: http://www.aa5tb.com/efha.html

    That page also describes tuning the capacitor on the bench with a resistive load, then adjusting the length of your antenna to get the best match without moving the capacitor. After the wire is trimmed to resonance any remaining small mismatch can be tuned out by slight tweaking of the variable.

    Be mindful of the voltage rating of the variable capacitor at higher power levels. The RF voltage across the secondary can get very high due to the higher impedance.

    I have my 40m and 20m wires mounted horizontally about 25 feet of the ground at my house and they work well for the low height. I'd prefer to have them vertical, but I don't have anything talk enough to do that at my house.

    This type of box is great for portable use also. If you have your different wires already trimmed to length, you can quickly throw one up into a tall tree and be in the air in minutes.

    73 - Brad
     
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  10. binrat

    binrat WDX Club Coordinator
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    I just noticed fat finger typing. It should read 7.25 to 7.3 mHz effects 20,15 metres if using it as a multi band. A simple transformer about 6' from the matching transformer works well to adjust the frequencies. I use #14 wire wrapped around 1" outside dia pvc pipe. Anywhere from 5-7 wraps does it.
     
  11. HomerBB

    HomerBB Sr. Member

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    I wanted to make a QRP 49:1 autotransformer for my travel pack.
    The previous builds made with FT240-43 were larger than I wanted or needed. Besides that, I was bored seeing the weather had shut me in. This time I used FT140-43. Instead of a 2/14 turns method, I went with 3/21 turns, both being 1:7 ratio, or 49:1.
    With 3000 Ohms resistor (3×1k in series) it tested out really good in all the bands I want to use in the field. I will be tuning a wire soon.
    I needed a box, and was out of those I prefer, so I fell back only my repurposing tendencies. I had 3 boxes of this style, and use only one.
    2WR183_Homer
    K5HBB

    20210218_164228-01.jpeg 20210218_164232-01.jpeg 20210218_164247-01.jpeg
     
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  12. HomerBB

    HomerBB Sr. Member

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    When I put my antenna wire together with my 49:1 transformer I will place a small coil at the 72" distance from the transformer. It will be 5 wraps of 14 awg wire on a 3/4" pvc nipple. This is also to assist in bringing SWR on the higher bands down. Then at the mid point of the entire wire length from the transformer out I will place a capacitor to bring the resonance into the upper 80m band (75m?) Because I am a General and use phone. See the attached graphic.
    Screenshot_20210218-181106_Facebook-01.jpeg
     
    #12 HomerBB, Feb 19, 2021
    Last edited: Feb 19, 2021
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  13. binrat

    binrat WDX Club Coordinator
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    My 50 watt efhw has a FT114-43 with a 49:1 (21:3 wrap) 16gauge wire. Works well for 40-10 with the transformer at 72"-74". I got about 80 SOTA activations using it. Why 114-43, it was what I had at the time.
     
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  14. HomerBB

    HomerBB Sr. Member

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    Jealous.
    No tuner?
     
  15. binrat

    binrat WDX Club Coordinator
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    No tuner. It took what felt like forever to get it right, but dang does it work.
     
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