Part of our Building Your First Ham Radio Station guide.
A standard center-fed dipole is fed at its exact midpoint, which is clean and simple but limits you to essentially one band, plus whatever narrow harmonics happen to fall in a usable range. Sliding that feedpoint off center changes the whole picture, unlocking multi-band operation from a single wire, but introducing an impedance that a plain 1:1 balun cannot handle and a radiation pattern that is not quite as tidy as a center-fed design.
Why moving the feedpoint changes everything
A center-fed half-wave dipole presents an impedance in the neighborhood of 50 to 75 ohms at resonance, a comfortable match for standard coax. Move the feedpoint to roughly a third of the way along the wire, the common OCFD split ratio, and the impedance at that point rises to somewhere around 200 to 300 ohms, depending on height and the specific split used, which is why an OCFD needs a 4:1 or sometimes 6:1 current balun rather than the simple 1:1 balun a center-fed dipole gets away with.
The multi-band payoff
The real reason hams build an OCFD at all is that the off-center feedpoint happens to present a reasonably tunable impedance on several harmonically related bands at once, commonly 80, 40, 20, 17, and 10 meters from a single wire cut for 80m, depending on the specific design and split ratio used. This gives you genuine multi-band coverage from one wire and one feedpoint, no separate antennas, traps, or switching required, which is the entire appeal over a single-band center-fed dipole, or over the harmonic bands an EFHW already covers well.
What you actually give up
- Common-mode current is a real, persistent issue. The asymmetric feedpoint tends to push current onto the outside of the coax shield more than a center-fed design does, which is exactly why a good current balun at the feedpoint, not just any 4:1 transformer, matters so much here.
- SWR is not perfectly low on every supported band. An OCFD’s SWR match on its various bands is a genuine compromise, workable with most modern radios’ internal tuners or an external tuner, but rarely as clean as a purpose-built single-band dipole would achieve.
- The radiation pattern skews slightly asymmetric compared to a textbook center-fed dipole’s clean figure-eight, since the current distribution along the wire is no longer perfectly symmetrical.
Where an OCFD genuinely makes sense
If you have room for one wire antenna and want coverage across several HF bands without the complexity of traps or a switched antenna system, an OCFD is a genuinely reasonable, well-proven compromise, and it is a popular choice for exactly that reason among hams with a single available antenna run. If you have room and interest in optimizing a single band, though, a purpose-built center-fed dipole or EFHW for that specific band will always outperform an OCFD’s compromise match on that same band.
A practical note on the balun
Do not skimp on the transformer here. A cheap, poorly wound 4:1 balun on an OCFD is one of the most common causes of frustrating, hard-to-diagnose RF-in-the-shack problems this specific antenna design produces, and spending a bit more on a well-built current balun from a reputable maker genuinely pays for itself in avoided headaches down the line.