HF antenna bench

Short dipole coil calculator

How many close-wound turns to put on each loading coil when the coils sit at the midpoint of a shortened dipole.

Frequency

MHz

Length of each leg

16 ft 0 in. A full-size leg at this frequency is 32.73 ft.

Coil wire

Close-wound pitch uses the insulated diameter, 0.068 in. The leg wire is taken as the same conductor (0.064 in bare).

PVC form

Schedule 40 and 80 share this outside diameter. The winding sits on the outside.

Example: 40 m, each leg 16 ft, about half size.

Each loading coil

closely spaced turns on each of the two coils

Continuous solution 22.2 turns. Nearest half-turn is shown.

Target inductance: 19.9 µH
This winding: 19.6 µH−1.2%
Reactance: 893 Ω
Winding length: 1.500 in
Wire each coil: 11.34 ft
Wire, both coils: 22.67 ft
From feedpoint: 8 ft 0 in
Of full size: 49%

Inductance at nearby half-turn counts
Turns	Inductance	Vs target
21.5	19.0 µH	−4.3%
22wind	19.6 µH	−1.2%
22.5	20.3 µH	+1.9%

PVC adds a few percent over this air-core estimate. If the antenna resonates low, take off half a turn and check again.

Coils sit at the midpoint of each leg — 8.00 ft from the feedpoint and 8.00 ft out to the tip. Wind two identical coils.

How the turns are figured

Inductance is the off-center-loaded dipole equation published by Jerry Hall, K1PLP, in QST, September 1974, and later in the ARRL Antenna Book. With the coil fixed at the midpoint of each leg, the distance from the feedpoint to the coil is half the leg.

Turns come from Wheeler’s 1928 single-layer formula, solved so the turns touch. Pitch is the insulated wire diameter. Mean turn diameter is the PVC outside diameter plus that same wire diameter. The 234/f quarter-wave constant is the one inside Hall’s equation.

Enamel diameters are typical heavy-build overalls; THHN diameters are typical finished outsides. A different spool of insulation will move the answer by a fraction of a turn. Add about a foot of tails beyond the winding length of wire.