Short vertical, made resonant with a coil.

When a full quarter wave will not fit — 80m, 160m, or anything mobile — a loading coil makes up the electrical length the wire is missing.

How to use it

• Set the frequency and the element length you can actually put up.
• Coil position is how far up the element the coil sits. 0 is base-loaded.
• Give the element conductor and, separately, the winding wire. These are two different conductors and the tool needs both.
• Set the coil form radius, then choose whether you are winding to a set length (you have a form of a given size) or close-wound (turns touching, and the length falls out).

What you get

• The inductance needed, the turns to wind, the resulting winding length and pitch, and a length/diameter check.

The thing that looks wrong but isn’t

Change the winding length and the turns change while the inductance stays put. That is correct. The inductance is a property of the antenna — it is however much electrical length your element is missing at that frequency — and it does not care what the coil looks like. A longer coil simply needs more turns to reach the same inductance, so a short fat coil and a long thin one can both be right. On a 16.4 ft element at 3.5 MHz, 61.9 µH is 26 turns over 1 inch or 61 turns over 12.

Tips

• Start big and trim. Wind a few extra turns and take them off one at a time against an analyzer. The calculation gets you close, not resonant.
• Watch the length/diameter row. The turns figure comes from Wheeler’s formula, which is only accurate down to about 0.8× — below that the coil is a pancake and the real inductance drifts from the number shown.
• A center coil is more efficient than a base coil, at the cost of having to support its weight up the element.

Good to know

Loading shortens the antenna at a real cost: narrower bandwidth and lower efficiency, and both get worse the shorter you go. Radials or a good ground matter as much here as the coil does — a very short loaded vertical over poor ground puts most of your power into warming the dirt.