Need help trimming a wire antenna

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KI5OMM, Jul 4, 2021.

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  1. KI5OMM

    KI5OMM Platinum Subscriber Platinum Subscriber QRZ Page

    I am ready to trim a EFHW wire for the low end of 80M and I have a few basic questions. Typically how much should I trim at a time? 2" - 6" or more each try? Also do I need to fully erect the antenna up high each time to get an accurate SWR measurement? Lastly my wire goes through a clamp then the dogbone insulator and back through the other side of the clamp. Does it matter if I leave the excess wire hanging off the end of the antenna as I proceed to shorten it to tune it? I would permanently trim it and secure it after testing of course, but does it matter if I leave a foot or so excess dangling while measuring for SWR?
  2. KI4ZUQ

    KI4ZUQ Ham Member QRZ Page

    Shorten without cutting. Just try doubling back the end of the wire to get resonance or SWR you are seeking.
    AK5B, KU3X, KB2SMS and 1 other person like this.
  3. KE0EYJ

    KE0EYJ Ham Member QRZ Page

    Fold it back over itself... start with about 4 inches and see where it goes on the various bands you use. Less wire will make 80m go higher. More wire makes it lower. Just use electrical tape, fold the wire over, then tape it down. Cutting and folding will affect the signal differently. For example, if you find folding about 4 inches works, don't cut 4. it won't be the same and you'll cut too much. Having too much wire folded over (as in several more inches) may create a kind of capacitance hat, I have learned.

    I fold all of the time.

    And yes, to be accurate, you should erect it to the same position each time.
    SM6CJB, KI4ZUQ, AK5B and 2 others like this.
  4. KI5OMM

    KI5OMM Platinum Subscriber Platinum Subscriber QRZ Page

    I'm not sure I am understanding what you mean by folding over and taping. My antenna needs a clamp to support the weight of the wire. My main question is how much excess can I leave dangling under the clamp and will it affect my SWR reading?
    Thanks for your help!
  5. KE0EYJ

    KE0EYJ Ham Member QRZ Page

    Sorry, I misunderstood.

    I have only done that with a dipole, but it works. Yes, it does mess with SWR a bit, but you can fold that extra tail of wire over if you need to adjust the frequency. As for how much excess.... I wouldn't be surprised if you could have almost 1/3 of the wire hanging and still make it work (although it would take some experimenting to adjust the swr).

    FYI -- when I am portable, I am often folding or even tying a little extra on with a pigtail if I need more wire. Works just fine. No need to even solder, if temporary. Solder and seal, if you want to make it permanent.
  6. WA7ARK

    WA7ARK Ham Member QRZ Page

    The wire has to be elevated while trimming... If antennas could be pruned to length while laying on the ground, then hams would never have any fun...

    If you expect to use an EFHW for its multi-band capability, e.g. 80m/40m/20m/15m/10m, then you have to determine the min SWR frequency on all bands at any given wire length. If you just prune the antenna to your favorite spot in the 80m band, then you will likely shift all of the harmonics such that they are no longer useful on those bands...
    WA9UAA, AK5B and WB5YUZ like this.
  7. KI5OMM

    KI5OMM Platinum Subscriber Platinum Subscriber QRZ Page

    OK. Sounds reasonable. How about a bit of advice... I have a 49:1 UnUn and I am starting with 132 ft. of wire at 60ft on the high end sloping to about 6ft on the low end. Sound like a good starting point?
  8. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    Beware of "paralysis from over-analysis!" One of the biggest mistakes a newcomer to wire antenna construction can make is to spend too much time trying to do calculations that will make sure the antenna won't need pruning. There is no formula or calculator that will give such exact results!

    Use a simple formula, like 468/f for a half-wave, where f is the desired frequency of operation, cut your antenna, making a careful note of the exact length, and put it up.

    Then, using your whatever, find out if the point of lowest SWR (not to be confused with the point of resonance; you can cut your antenna for resonance, but you have to measure reactance, not SWR, to get there) is higher or lower in frequency than you want it to be. Then you use the classic ratio-proportion equation of cross-multiplying and dividing, using your planned frequency, the frequency that shows lowest SWR, and the original length of the antenna, with the final length as the variable for solving, to determine how much length needs to be removed/added. (As a shortcut, I've always added a little less than ten percent to the length from the original calculation, so the antenna is always a little too long. Then I just fold the wire back on itself until it is the right length.)

    Finally, remember that 1.5:1 or lower is good enough. Don't let perfect be the enemy of completion!

    Use the above, and you will almost certainly get close enough that the antenna will only need to be lowered once to adjust. Once the antenna and its support are actually built, the process usually takes less than an hour.

    Good luck. It may seem daunting, but once you've done it two or three times, you will be amazed at how simple it all really is, as long as we don't let paralysis from over-analysis set in!
    Last edited: Jul 4, 2021
    AK5B likes this.
  9. WA7ARK

    WA7ARK Ham Member QRZ Page


    To properly align the harmonic responses so that they all fall near the center of their respective bands requires a "compensating network" to be added to the wire. There are several different methods; using a coil close to the transformer, a coil close to the far end, or a capacitor near the center of the wire. Most of the store-bought versions of the EFHW antenna include the compensation network. Read my recent posting on this topic, here:
    The ARRL End Fed Half Wave Antenna Kit and doing it cheaper and better

    Which method are you planning to use? If none, then the antenna is likely to be usable on one or maybe two bands...

    What are you trying to do?
    WB5YUZ likes this.
  10. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    Forgot about this part. Don't just leave it hanging; it will act as a capacitance hat and make the antenna appear longer than it really is. Instead of just leaving it hanging, wrap it loosely around itself at the rate of about an inch or two per turn (not turns per inch, which is what we usually deal with when winding coils!).

    Folding the antenna ends back on themselves does add a very small amount of reactance to the antenna, but not nearly as much as letting the ends hang. People frequently fold the ends of the antenna back on itself without being able to accurately measure the small amount of reactance added, whereas letting the ends hang is often used to intentionally and significantly change the frequency of lowest SWR.
    AG6QR and AK5B like this.

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