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Vertical Antenna w/ Loading Coil

[last updated: 2024-05-26]
ham radio home page
coil-loaded vertical antenna experiment:
vertical antenna tests sans coil
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https://en.wikipedia.org/wiki/Loading_coil

  • A friend, Kim W7PCT, does a lot of POTA and SOTA. He mostly uses a MFJ-1979 with base loading coil.
    I am impressed and intrigued by his setup and wanted to duplicate it.
    link to MFJ-1979 ... https://mfjenterprises.com/products/mfj-1979 ($69.66 Sep-2023)
    -----------------------------------------------


    But first I wanted to understand what was going on with how the loading coil does its magic.

  • When you send a signal into an antenna, and your signal is a lower frequency (longer wavelength) than the antenna's resonant frequency,
    the antenna presents capacitive reactance to the feedline. In order to counteract that and balance/resonate the antenna at the frequency you're sending it,
    you add an inductance, from a "loading coil."
      While at the current time I have no understanding of how to apply this to the actual process of designing a loading coil,
      supposedly the relevant equation is:
      f = 1 / ( 2 pi (LC)1/2 )
      which reduces to: L C = 1 / ( f2 4 pi2

    • Loading Coils:
      • The coil designed to work with the MFJ-1979 is (I think...): MFJ-67
            https://www.hamradio.com/detail.cfm?pid=H0-016541&oi=0
        It is an air-wound coil, 2" diam x 6" long,
        with a clip that allows "shortening" the active coil.
        Theoretically this allows tuning the MFJ-1979 from 2 to 40m.
        ----------------

      • Adding a coil (inductance) in series with your vertical antenna
        effectively "lengthens" the antenna, making it resonant on a lower base frequency.
          The process of building a vertical antenna with a loading coil:
            the quick version:
            • get an antenna
            • decide your desired frequency
            • decide where you'll mount the coil (base, mid, or top)
            • calculate inductance needed
            • procure or design and build your coil
            • assemble and test

            -----------------------------

        • First you get an antenna, and find its exact length.
          This may get more complicated if you have a telescoping antenna that you can "shorten" by sliding some sections,
          but ultimately you need a length (in feet for the calculation link listed below), to use in the inductance calculator.
          -----

        • Then you must decide what frequency you want to use.
          Be aware you can use a single antenna-coil pair for multiple frequencies/bands if you add a shorting jumper wire, that allows you to electrically shorten the coil (changing its inductance by doing so) by shorting out some number of turns.
          But bottom line is you must decide what frequency or frequencies you're going to use, because you have to enter those values into the inductance calculator.
          -----

        • Then you must make a decision as to where on the antenna you want to put the coil.
          • The calculated inductance needed to tune your antenna to your desired frequency will be different
            depending on where you mount the coil.
          • The coil will always be electrically connected in series with the antenna.
            But it can be placed at the bottom of the antenna, at the top ("top-loaded"), or anywhere in between.
          • OTOH it may be that, because of the way your antenna is built, your only option is to put the coil at the bottom ("base-loaded").
          • There are consequences/advantages/disadvantages to where you put the coil.
            It's reported that the farther away from the feedpoint you put your coil, the larger the inductance that will be needed there to tune to a given frequency.
            In addition, there are consequences to the values of the harmonic frequencies that result from different positions.
          • Bottom line however is that the inductance calculator requires you to enter a value representing the coil position along the length of the antenna.
            For a base-loaded antenna, the position value is "0".

          -----

        • Then you calculate the inductance needed for each frequency you want to use.
          There are several online calculators to do this for you. I have not yet found a workable equation to allow doing this calculation off-line.
          • This link
                (link to:) Coil-shortened Vertical
            You enter:
            • the length (ft) of your antenna,
            • the "conductor diam" (inches), presumably coil wire diam,
            • the position (ft) of the coil where it is placed along the length of the antenna,
            • and your desired freq (MHz),

            It then calculates the inductance needed in series to make your antenna resonate on the desired freq.

              Note that the title, "coil-shortened" antenna, can be confusing:
                The title is worded from the standpoint of a desired freq. Standard calculations will tell you what length of antenna you need for that freq,
                From that perspective, it would be more precise to call it: "coil-shortened required length for a desired frequency."

                -------------
                But if you add an in-series inductance, you shorten that required length.
                Thus, "coil-shortened."
                But worded from a different perspective:
                The premise is that the given length antenna is too short to properly resonate on the desired freq.
                Adding the calculated inductance increases the effective length of the antenna,
                so now it will resonate on a lower frequency (higher band).
                From this perspective, your added coil lengthens (makes it act as if it's longer) your given length of antenna.

              -----

        • For a dipole:
          This link
              (link to:) Coil-shortened Dipole antenna
          was presented in a 1974 QST article:
          https://dxc.wc2l.com/QST_Sep_1974_p28-34_58.pdf

        • Design your coil:
          • Once you've determined the inductance value for the coil that you need for your antenna and freq of choice,
            use this site to calculate coil dimensions to produce desired inductance:
            (link to:) calculate coil dimensions
          • For a "single-layer" coil:
            • You'll enter: coil diam and length, and number of turns, and it will calculate resulting inductance.
              This is an iterative process, as you'll need to enter repeatedly slightly modified values until you home in on something that gives you the inductance you want.
            • This on-line calculation used the formula:
                L = (R2 * N2) / ((9*R)+(10*L))
          • For a "multi-layer, multi-row" coil:
            • This uses the formula:
                L = ( 0.8 * R2 * N2 ) / ( (6*R) + (9*L) + (10*D) )

      • Addendum:
        • In that article, BTW, there is a brief mention of:
          "... [in an off-center loaded dipole] ... the inductors do little actual radiating themselves ... in contrast to helically wound or continuously loaded elements, where a long thin inductor is the radiator as well as the loading element."
        • There is another interesting note:
          [as distance of loading coil from feedpoint increases (ie. moving it up higher on the vertical antenna), the size of required inductor increases. As position of loading coil gets near to the ends, this becomes impractical, however addition of capacitive hats can reduce this somewhat]

        • Regarding effect of moving the coil:
          The size of the required coil increases with height along the antenna.

        • link to calculate inductance for dipole:
          https://www.homepages.ed.ac.uk/jwp/radio/software/loading.html

      eof

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