Antenna Loading Coil Calculation

The calculation method is taken from a BASIC program which I got many years ago from OA4/KA2OIG who is hereby gratefully acknowledged. (And all my responsibility is hereby disclaimed!)

I haven't personally tested these designs, but others have and found them to be satisfactory. I would still appreciate some feedback. Please mail me.

The method appears to be based on an equation in J. Hall, "Off-center loaded dipole antennas", QST Sept 1974, 28-34.

I am grateful to Craig Pennington, NC4P,, for identifying this source and spotting an error in the original program which I have corrected in the current version.

Hall does not give a derivation. However, an apparently different method is described fully by L. Duarte Lopez, "Designing a shortened antenna", QST October 2003, 28-32. I have also programmed this but find that it gives inductor values about 40% higher than this calculation. (Thanks to Skip Behnke, W2OZ for this information.)

The calculation refers to either a loaded 1/4 wave (typically a vertical, in which case flip the picture below through 90o) or a loaded dipole, in which case the dimensions refer refer to one arm and two coils will be required.

Frequency: MHz

Length of antenna or arm: Coil position, distance from feedpoint: feet.

Diameter, US wire gauge: inches diameter

Loading coil inductance micro Henries

For designing the actual loading coil go here. If you want to use a toroid go here.

This corrected version installed 19 Sept 2003.