OCF Dipole Calculator

Why the feedpoint is deliberately offset

An off-center-fed dipole moves the feedpoint away from the electrical centre of a half-wave radiator. The photograph shows the physical form of the antenna as a wire dipole with an off-centre feed arrangement rather than a centre-fed connection. That difference is small mechanically but significant electrically: the feedpoint impedance and current distribution change, which is why an OCF design can provide useful multiband behaviour when paired with an appropriate transformer or balun.

The calculator estimates the two wire sections from the chosen frequency and feedpoint position. Treat those dimensions as a starting point. The ideal ratio is not universal because conductor diameter, height, surrounding objects and the feed system influence the final impedance. The transformer ratio also needs to match the actual antenna impedance rather than an assumed textbook number. If the feedpoint is moved substantially, the multiband impedance pattern can change enough that the original transformer is no longer the best choice.

Keep the two radiator sections mechanically secure and maintain the intended feedpoint position. The feedline should leave the transformer without immediately running alongside one radiator, because common-mode current can change the measured behaviour. A current choke can be useful where feedline radiation is suspected. Tune the antenna in its final installation and check several bands rather than optimizing only one frequency. Record both the resonant frequencies and the measured feedpoint impedance if the analyser provides it. The image is useful because it connects the calculation to a real Windom-style installation: the antenna is still a dipole, but the feedpoint is deliberately offset and the supporting environment becomes part of the practical design.