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A single inductive loading coil in a vertical or a pair of loading coils in a dipole are commonly used to match a given total antenna length parameter to a given frequency wavelength (where the ratio may not be a nice N/4) for either resonance or low SWR. This question only concerns dipoles with a pair of loading coils or inductors, one in each leg.

If a loading coil is of non-zero dimension and non-zero wire length, then the difference in standing wave current (forward + reverse) would be different across the inductor, which should cause the inductor to radiate a magnetic field (act as a mini loop antenna).

Given this, what should be the relative direction of windings of the two inductive loading coils be for a dipole? Same or opposite winding directions?

Will one winding relationship tend to cancel the (E)M fields and the other constructively add? Or will the directional pattern change?

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  • $\begingroup$ Unless it has a diameter on the order of $0.01\lambda$ or more, the small (antenna) aperture of an air-core solenoid will not act as an effective radiator. By reciprocity, it will also not act as an effective transducer of electromagnetic waves. $\endgroup$
    – Brian K1LI
    Jul 21, 2019 at 11:45
  • $\begingroup$ Helically wound antenna’s (ducky’s,slinky’s) do radiate. $\endgroup$
    – hotpaw2
    Jul 21, 2019 at 13:40
  • $\begingroup$ Yes, but the circumference and spacing of their windings are a substantial fraction of a wavelength at their operating frequencies. For example, the diameter, length and turns spacing of a loading coil for an 80m dipole would be in the range of 5cm, 10cm and .2cm, respectively - i.e., less than $.001\lambda$ and down. This would an extremely poor helical radiator. $\endgroup$
    – Brian K1LI
    Jul 21, 2019 at 19:24
  • $\begingroup$ A helically-wound vertical is entirely different from a loading coil. I've never seen a loading coil that used a half-wavelength of wire! $\endgroup$
    – Brian K1LI
    Jul 21, 2019 at 23:40
  • $\begingroup$ A helical is just a loading inductor where the wire stubs go to zero. Perhaps wider wind spacing $\endgroup$
    – hotpaw2
    Jul 22, 2019 at 3:33

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Practically speaking, the size and winding direction of the loading coil doesn't really matter.

Unless the coils are monstrously huge compared to the wavelength involved, there will be a negligible effect on radiation.

As for winding direction, if the coils are near the feed point of the dipole, you might see a difference if they are close enough to interact with each other's magnetic fields (essentially becoming a loosely-coupled transformer).

But, since it is more efficient to place loading coils in the middle or ends of the element, that's not typically the case.

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