Multiband Wire Antennas: Fan Dipoles, Trapped Dipoles, and OCF Designs

You want HF on multiple bands but you've got one support point, limited space, and a budget that doesn't include a SteppIR. Fair enough. Wire antennas have been solving this problem since before most of us were licensed, and three designs keep showing up in conversations: fan dipoles, trapped dipoles, and off-center-fed (OCF) dipoles.

This article breaks down how each one works, when to pick one over the others, and the gotchas that nobody mentions until you've already strung 150 feet of wire through the trees.

The Baseline: Why Multiband Matters

A single-band half-wave dipole is hard to beat for simplicity. Cut to 468/f(MHz), feed with coax, done. But HF operators rarely stay on one band. Propagation shifts, contests span multiple bands, and POTA activators want to log contacts wherever the action is.

Running separate feedlines to separate antennas works if you have the real estate. Most of us don't. So we compromise, and the question is which compromise costs you the least.

Fan Dipoles: Multiple Elements, One Feedpoint

How They Work

A fan dipole is literally multiple dipoles sharing a single feedpoint. You cut pairs of wires for each band you want, connect all of them to the center insulator, then spread them out in a fan shape. The antenna on each band that's resonant takes most of the current; the others present high impedance and stay mostly out of the picture.

Typical configuration for a 3-band fan (20/40/80):

• 80m elements: 2 × 33.5 ft (66 ft total, tuned for ~3.55 MHz CW segment)
• 40m elements: 2 × 16.6 ft (33.2 ft total, for 7.15 MHz)
• 20m elements: 2 × 11.2 ft (22.4 ft total, for 14.175 MHz)

Spacing between element tips should be at least 12-18 inches to reduce interaction. More is better.

Pros and Cons

Fan dipoles are cheap, no-tune, and broadband on each individual band. I built my first one in 2009 with hardware store wire and some dog bone insulators from a hamfest. Total cost was maybe $30.

The catch? Interaction between elements detunes them from the calculated lengths. You'll spend an afternoon with an antenna analyzer tweaking. The 40m legs especially tend to shift when you add 80m elements below them. Trim in small increments, a quarter inch at a time near resonance.

Also, they're wide. An 80/40/20 fan dipole needs the same linear span as an 80m dipole (roughly 130 feet tip to tip) plus the fanning angle. If you're space-limited, this might not shrink your footprint at all.

Build Tip

Use different colored wire for each band pair or mark them with tape every few feet. When you're up on a ladder adjusting elements in February, you don't want to guess which wire is which. Trust me on this.

Trapped Dipoles: Resonant Elements in Series

How They Work

Trapped dipoles use LC traps (parallel resonant circuits) inserted along the wire. At the trap's resonant frequency, it presents high impedance and electrically isolates the wire beyond it. Below that frequency, the trap looks like a loading coil, adding electrical length.

So a typical 20/40m trapped dipole has two traps resonant at 14.1 MHz placed about 11 feet from center on each side. On 20m, the traps block current and the antenna acts like a ~22 ft dipole. On 40m, current flows through the traps (which add some inductance) and continues to the element tips.

Real-World Dimensions

A commercial 20/40/80 trapped dipole (like the old Hy-Gain DX-88 or the Hustler BTV series verticals, though those are verticals, not dipoles, so let me correct myself) — for wire trapped dipoles, something like the Alpha Delta DX-CC gives you a reference:

• Total length: approximately 82 feet
• That's significantly shorter than a full-size 80m dipole (130 ft)

The tradeoff is bandwidth. Traps narrow your usable bandwidth on each band. Where a full-size 80m dipole might give you 2:1 SWR across 200 kHz, a trapped version might only cover 75 kHz before needing a tuner.

The Trap Question: Buy or Build?

Commercial traps work fine. Companies like Unadilla and MFJ sell them. Building your own from coax stubs or wound coils with capacitors is a rite of passage for some hams, but the Q of homebrew traps varies wildly depending on construction.

I'll get heat for this, but for most operators, just buy the traps. The money you save building your own gets eaten up by the analyzer time, failed first attempts, and weather-proofing challenges. A pair of commercial traps runs $40-60 and they're sealed against moisture.

Nope. The homebrew romantics will disagree. That's fine.

Losses

Traps dissipate some power as heat. In a well-built trap, losses are small, maybe 0.2-0.5 dB per trap at HF. Poorly constructed traps with low-Q coils or corroded connections can eat 1-2 dB per trap. That adds up when you have four traps in a three-band design.

OCF Dipoles: One Wire, Multiple Harmonics

How They Work

An off-center-fed dipole (also called a Windom or OCF) is a single wire fed not at the center but at approximately the 1/3 point. This offset feed point hits a moderate impedance (around 200-300 ohms) that happens to repeat on harmonic bands.

Feed a 135-foot wire at the 1/3 point (roughly 45 ft from one end, 90 ft from the other) through a 4:1 current balun, and you get usable SWR on 80m, 40m, 20m, and often 10m without traps or multiple elements.

The magic is that the feedpoint impedance on the fundamental and even harmonics stays in a range that a 4:1 balun can transform close to 50 ohms. Not perfectly, but close enough that a tuner handles the rest.

Performance Notes

OCF dipoles radiate. They're not a compromise antenna in terms of gain. On the fundamental frequency, they perform identically to a center-fed dipole. On harmonics, the pattern breaks into multiple lobes, which can actually help for DX work on higher bands.

The weak spot is usually 40m, where SWR can land around 3:1 even with the balun. An internal tuner handles this effortlessly. External tuners too, obviously.

(I ran an OCF for six years at my previous QTH and worked 250+ DXCC entities with it. Not bragging, just saying the antenna works. My buddy KD9XYZ ran a similar setup and had comparable results, minus the time he forgot to weatherproof the balun connection and spent three months wondering why his SWR was climbing.)

Critical Detail: The Balun

Don't cheap out here. A 4:1 current balun is mandatory, not optional. Voltage baluns at this impedance ratio tend to saturate on higher power and cause common-mode currents that mess up your radiation pattern and generate RFI.

The Balun Designs 4114ocf is specifically made for OCF dipoles and handles 3 kW. DX Engineering's version is solid too. Budget $50-80 for a proper balun.

Head-to-Head: Which One Should You Build?

Factor Fan Dipole Trapped Dipole OCF Dipole Length needed Full size of lowest band 60-70% of lowest band Full size of lowest band Bandwidth per band Wide Narrow Moderate (tuner helps) Band coverage You choose specific bands 2-4 bands Fundamental + harmonics Efficiency Highest Slightly lower (trap loss) High Complexity Low (just wire) Medium (traps) Low (one wire + balun) Cost $20-40 $60-120 $50-90 Tuner needed? Usually not Sometimes Usually yes on some bands

Practical Decision Framework

Choose a fan dipole if: You have the full span available and want maximum bandwidth on your chosen bands without a tuner. Three bands is the practical maximum before interaction becomes unmanageable.

Choose a trapped dipole if: Space is your primary constraint. You need to fit a multiband antenna in a shorter span. Accept the bandwidth tradeoff.

Choose an OCF if: You want the most bands from the simplest single wire and you already have a tuner. Especially good for contesters and anyone who band-hops frequently.

Installation Considerations

All three designs work as inverted-V configurations, which only needs one tall center support. Height matters more than the specific design choice. A fan dipole at 50 feet outperforms a trapped dipole at 25 feet every time.

Minimum recommended heights for the center point:

• 80m operation: 40+ feet (higher is always better)
• 40m as lowest band: 30+ feet
• 20m as lowest band: 20+ feet

For inverted-V setups, aim for a 90-120 degree included angle between the legs. Drooping below that steepens the angle and raises the radiation angle, which hurts DX.

Wrapping Up

All three designs have been putting signals on the air for decades. None of them are magic, and none of them are junk. Your specific situation dictates the best pick: available space, how many bands you need, whether you own a tuner, and how much fiddling you're willing to do.

Start with what fits your space and budget. You can always try another design next year. That's half the fun of wire antennas — a new experiment is always just a spool of wire away.