Build an Off-Center-Fed Dipole (OCFD) Antenna
The off-center-fed dipole — also known as the Windom or OCF dipole — achieves five-band coverage on 80m, 40m, 20m, 15m, and 10m from a single 135-foot wire and one coax feedline. The key is the feedpoint position: placed 1/3 from one end rather than at the center, the asymmetric wire lengths create multiple resonances across the HF bands. A 4:1 current balun handles the roughly 200Ω feedpoint impedance and manages common-mode current. The result is a mechanically simple multi-band antenna that requires no traps, no tuner on the primary bands, and only two end supports plus a center mast.
The Asymmetric Feedpoint
A standard center-fed dipole has equal-length legs that both resonate at the same fundamental frequency and its odd harmonics. Moving the feedpoint off-center creates two legs of different lengths — each with its own fundamental resonance. The combination of the two legs' resonances, plus the harmonics of both, produces multiple resonant points across the HF bands from one wire.
For the standard 135-foot OCFD fed at the 1/3 point (45 ft from one end, 90 ft from the other):
Why the 4:1 Balun Is Non-Negotiable
The OCFD feedpoint has two characteristics that make a 4:1 current balun essential — not optional:
Impedance transformation: the feedpoint impedance is approximately 200–300Ω, depending on the operating band. A 4:1 balun steps this down to 50–75Ω for 50Ω coax connection, keeping SWR at a manageable level on most covered bands.
Common-mode rejection: the asymmetric wire lengths create unequal currents in the two legs — the OCFD is inherently unbalanced. Without a current choke, significant RF flows on the outside of the coax shield, causing RF in the shack, pattern distortion, and unreliable SWR readings. The "current" part of the current balun (rather than a voltage balun) is what actually suppresses this common-mode current.
- A 1:1 balun is wrong — it does not provide impedance transformation
- A 4:1 voltage balun (W2AU style) is also wrong — it does not suppress common-mode current
- A 4:1 current balun (choke balun) is correct — both impedance transformation and common-mode suppression
- The difference between a voltage balun and a current balun is fundamental — using the wrong type produces an OCFD that seems to work but has common-mode problems
OCFD Wire Length — The Standard and the Variants
The "standard" OCFD dimensions that most builders use are based on published designs that have been refined over decades. The total wire length of 135 feet and the feedpoint at 45 feet from one end (1/3 of 135) is not the only possible OCFD design but it is the most widely tested and published. Variations exist:
Expected SWR by Band
The OCFD is not a perfect-SWR antenna on every covered band — the asymmetric geometry and single feedpoint compromise means some bands are better matched than others. Realistic SWR expectations after installation:
Most modern transceivers operate into 2.5:1 SWR without protection activation. A small tuner makes 15m and 10m fully comfortable when needed. The 40m performance is typically the best of the five bands — this is the band the asymmetric feedpoint most naturally favors.
4:1 Current Balun Design — How It Works
A 4:1 current balun for an OCFD is built as a choke balun: a high-impedance common-mode choke that simultaneously provides 4:1 impedance transformation. The most common implementation uses two stacked FT-240-43 toroid cores with a bifilar winding — two parallel wires wound together through the cores.
The bifilar winding creates two equal inductors in series on the unbalanced (high-impedance) side, which together with the connection topology produces the 4:1 ratio. The high choking impedance of the wound core prevents common-mode current from flowing regardless of the impedance transformation ratio.
Winding the 4:1 Current Balun Step by Step
Gather: 2 FT-240-43 toroid cores, approximately 3 feet of #14 AWG two-conductor zip cord or two parallel strands of #14 AWG enameled wire, and a weatherproof enclosure (Hammond 1590C die-cast aluminum or similar).
- Stack the two FT-240-43 cores on top of each other to form a double core
- Cut two parallel conductors approximately 36 inches long — strip and tin both ends of both conductors
- Hold the two conductors side by side (this is the bifilar pair) and wind them together through the stacked core, keeping them parallel and snug. Count 9 full turns through the core hole
- Mark one end of each conductor with tape: conductor A start and conductor B start
- Connect: conductor A start → coax center conductor (50Ω input). Conductor A end + conductor B start → connected together (this is the center tap that creates the 4:1 ratio). Conductor B end → antenna wire terminal on the high-impedance side
- The coax braid connects to the common point of the center tap
- Verify with a 200Ω resistor across the output: SWR at the coax input should be below 1.5:1 at 7 and 14 MHz
| Dimension | Standard value | Cut to (3% long) | Notes |
|---|---|---|---|
| Total wire length | 135.0 ft | 138.0 ft total | Trim during tuning; keep off-cuts for splicing if needed |
| Short leg (feedpoint to short end) | 45.0 ft | 46.4 ft | This leg primarily sets 40m and upper-band resonances |
| Long leg (feedpoint to long end) | 90.0 ft | 92.7 ft | This leg primarily sets 80m resonance |
| Feedpoint position | 45 ft from short end = 1/3 of 135 ft | — | Exact position is critical — measure carefully |
| Minimum apex height | 35 ft recommended | 50 ft preferred | Higher apex → lower SWR and better DX angle on all bands |
| Minimum short leg end height | 8 ft above ground | — | Keep both ends clear of people and animals |
| Minimum long leg end height | 6 ft above ground | — | Long leg end can be lower since it runs further from center |
Ocfd Calculator
This design has published dimensions for more than one band, or this page has no dedicated dimensions table this script could confidently locate. The default shown below is the most frequently cited frequency on the whole page -- verify it before trusting the result.
Complete materials for a 135-foot OCFD with homebrew 4:1 current balun
Building the OCFD — From Wire to First Contact
Build the balun first, then cut and attach the wire. Allow 3–4 hours for the complete build including balun construction and all-band SWR verification.
Build and Test the 4:1 Current Balun
Before cutting any antenna wire, build and verify the 4:1 current balun. Stack two FT-240-43 toroids and wind 9 bifilar turns through both cores simultaneously. Make the center-tap connection (conductor A end connects to conductor B start), with the coax center conductor going to conductor A start and the coax braid going to the center tap node. The two ends of conductor B are the two antenna wire terminals — one connects to the short leg, the other to the long leg.
Test the completed balun before installation: connect a 200Ω resistor between the two antenna terminals and measure SWR at the coax input with the NanoVNA across 3–30 MHz. Well-wound baluns show SWR below 1.8:1 at 3.5 MHz and below 1.5:1 from 7–30 MHz when loaded with the correct 200Ω.
Mount the Balun in a Weatherproof Enclosure
Mount the SO-239 in one side of the Hammond 1590C enclosure. Mount two stainless binding post terminals in the opposite side — one for the short leg wire, one for the long leg wire. Mount the wound balun toroid inside the box with RTV silicone adhesive. Solder the coax connections to the SO-239 and the antenna wire terminals per the winding diagram. Leave a weep hole (1/8" drill) in the bottom of the enclosure for moisture drainage — even sealed enclosures accumulate condensation.
Cut the Wire Legs
Cut two wire legs from the spool: one at 46.4 feet (short leg, 3% over the 45-foot standard) and one at 92.7 feet (long leg, 3% over the 90-foot standard). Use a steel measuring tape on flat ground. Label both ends of the short leg with red tape and both ends of the long leg with blue tape — the color coding identifies which wire connects to which terminal at height without needing to remeasure.
Strip and form loops at one end of each wire for the balun terminal connections. The other ends will receive egg insulators after the connection lengths are verified.
Attach Wire Legs to the Balun
Connect the short leg wire to the "SHORT LEG" terminal on the balun enclosure. Connect the long leg wire to the "LONG LEG" terminal. Each connection: form a loop in the stripped wire end, place over the terminal stud, add washer and nut, tighten firmly, and apply No-Ox-Id to the thread. Both connections should be mechanically secure under firm hand tension — the balun enclosure will carry the full weight of the antenna during installation.
Mount a support bail or hook on the top of the enclosure — this is the center support attachment point. A stainless U-bolt through the enclosure top edge, with a closed ring to attach the center support rope, is a clean solution. Alternatively, commercial balun housings have integrated attachment loops.
Install End Insulators
At the far end of each leg, thread the wire through an egg insulator and secure with the wrap-and-solder method: double back 4 inches of wire, wrap 5 times around the main wire, solder. Attach 24 inches of Dacron rope to each insulator. The short leg end has a shorter rope than the long leg end — the short leg is likely to be more steeply angled and needs more length to reach its anchor point comfortably.
Plan the Physical Layout
The OCFD is not symmetric — the short leg runs 45 feet from the center on one side, and the long leg runs 90 feet on the other. When planning the installation, consider which direction each leg runs:
- The long leg (90 ft) needs more room — run it toward open space with at least 90+ feet available
- The short leg (45 ft) can run toward a fence, building, or other constraint
- Both legs should maintain at least 6 feet of clearance above ground at their ends
- In an inverted-V configuration, the center (balun) is at maximum height and both legs slope down to end anchors
- The two legs will slope at different angles in an inverted-V — the long leg slopes less steeply, the short leg more steeply, at the same apex height
Route the Coax and Raise the Antenna
Pre-route the coax from the feedpoint to the shack before raising the antenna. On a 135-foot wire, the feedpoint is at height and the coax run may be 50–150 feet long depending on the installation. Raise the center support (the balun enclosure) to the apex height. Form a drip loop in the coax just below the balun enclosure — the coax should drop 12 inches, then loop upward, then run down from the loop to the shack. This prevents water from running along the coax directly into the SO-239.
Once the center is at height, pull the short leg to its end anchor and secure, then pull the long leg to its end anchor. The long leg will need a longer horizontal run to reach its anchor — ensure the anchor is far enough from center before raising.
Initial Five-Band SWR Sweep
Connect the NanoVNA at the radio end of the coax. Sweep all five target bands in order from lowest to highest frequency. Record the SWR minimum and its frequency on each band:
- 80m sweep (3.3–4.1 MHz): look for SWR dip in the 3.5–4.0 MHz range
- 40m sweep (6.8–7.5 MHz): look for SWR dip near 7.150 MHz
- 20m sweep (13.5–15.0 MHz): look for SWR dip near 14.200 MHz
- 15m sweep (20.5–22.0 MHz): look for SWR dip in 21 MHz range
- 10m sweep (27–30 MHz): look for SWR dip in 28–29 MHz range
The initial sweep rarely shows all five bands at exactly the target frequencies — the legs are cut long for trimming. Document all five dip frequencies before any trimming.
Trim the Long Leg for 80m
The long leg (90 ft) primarily controls 80m resonance. Trim the long leg to move 80m resonance to the target frequency. On 80m with a 90-foot leg, each 3 inches trimmed from the long leg raises 80m resonance approximately 3–4 kHz.
After trimming the long leg for 80m, check that 20m resonance (which also uses the long leg) has shifted slightly. Re-note the 20m resonant frequency before proceeding.
Trim the Short Leg for 40m
The short leg (45 ft) primarily controls 40m resonance. Trim the short leg to move 40m resonance to 7.150 MHz. On 40m with a 45-foot leg, each 2 inches trimmed raises resonance approximately 8–10 kHz.
Trimming the short leg also shifts 15m resonance (a harmonic of 40m). After 40m trimming, re-check 15m and 10m resonances. These upper bands may need final adjustment after both legs are at their primary target frequencies.
Verify 20m, 15m, and 10m
After both legs are trimmed for their primary bands (80m and 40m), check 20m, 15m, and 10m. These upper bands are harmonics of the lower bands and will have shifted with the trimming. Typical results after trimming 80m and 40m to target:
- 20m: should be close to 14.200 MHz — may need small additional long-leg trim of 2–4 inches
- 15m: typically falls near 21.1–21.3 MHz after 40m tuning — usable without further trimming in most cases
- 10m: resonance in the 28.5–29.5 MHz region — useful for 10m DX even if not perfectly centered
If 15m or 10m SWR is above 3:1 and the radio's ATU cannot handle it, small adjustments to the short leg (a few inches) may improve 15m at the cost of slightly shifting 40m. The compromise tuning of an OCFD is real — optimize for your most-used bands. A small tuner handles the remaining variation.
Weatherproof and Document
The balun enclosure is already weatherproof from its Hammond die-cast housing. Additionally:
- Seal the SO-239 entry and coax connection with self-amalgamating tape on the exterior of the enclosure
- Apply self-amalgamating tape to each wire terminal where it exits the enclosure
- Seal the coax with self-amalgamating tape at the drip loop junction
- Apply a thin bead of RTV silicone around the lid of the enclosure where it meets the body — not enough to prevent future access, but enough to prevent water from running in along the seam
Record final trimmed lengths of both legs, resonant frequency and SWR on all five bands, balun core material and turns, apex height, end heights and end-to-center distances, and installation date. Photograph the balun assembly and the complete antenna. Inspect the balun enclosure lid seal annually.
Carolina Windom — The OCFD with a Vertical Section
The Carolina Windom (manufactured by Radio Works) adds a specific-length vertical radiating coax section below the feedpoint. Instead of routing the coax horizontally away from the balun, a 22-foot section of coax hangs vertically below the balun — this section is intentionally allowed to radiate and adds a vertical polarization component to the antenna's radiation pattern.
The claimed advantage is improved low-angle DX performance compared to a standard OCFD. The vertical coax section radiates RF on all five covered bands, adding a vertical component that fills in the pattern asymmetry of the horizontal OCFD and produces lower average takeoff angles.
- Construction is identical to a standard OCFD except the coax drops vertically 22 feet before the drip loop and route to the shack
- The 22-foot vertical coax section must be RG-8X or similar — a current choke is installed at the bottom of the 22-foot section to prevent the radiating section from extending further down the feedline
- The vertical section slightly shifts resonances from the standard OCFD — expect to re-tune both legs after adding the vertical coax section
- Many operators report improved DX signal reports with the Carolina Windom configuration versus a standard OCFD at the same installation height
OCFD Performance Compared to Other Multi-Band Options
Honest assessment of where the OCFD fits among multi-band HF antenna options:
- vs Fan Dipole: fan dipole has better per-band SWR and slightly higher efficiency on each covered band; OCFD is simpler to install (one straight wire). For clean SWR, fan dipole wins. For simplicity, OCFD wins.
- vs Trap Dipole: OCFD has no traps and no trap losses; trap dipole is shorter (105 ft vs 135 ft). For efficiency, OCFD wins. For physical length, trap dipole wins.
- vs All-Band Doublet: doublet with ladder line covers all HF bands at maximum efficiency; OCFD covers 5 bands without a tuner. For flexibility, doublet wins. For convenience, OCFD wins.
- vs ZS6BKW: both cover similar bands from similar wire lengths. ZS6BKW requires a ladder line section; OCFD feeds directly with coax. For installation simplicity, OCFD wins. For SWR consistency, similar.
- Sweet spot: the OCFD is the right choice for operators who want 5-band coverage without a tuner, with a single straight wire, feeding with coax, and who don't mind the OCFD's modest SWR variations across bands.
Why does an OCFD need a 4:1 balun instead of a 1:1?
Two reasons. First, the OCFD feedpoint impedance is approximately 200–300Ω on most bands — a 4:1 balun steps this down to 50–75Ω for comfortable coax connection. A 1:1 balun provides no impedance transformation — the SWR would be unacceptably high on most bands with a 1:1 balun. Second, the OCFD's asymmetric current distribution creates very strong common-mode current tendency. A 4:1 current balun (not voltage balun) provides high choking impedance to suppress this. Using a 1:1 balun or a 4:1 voltage balun (rather than current balun) produces an antenna that seems to work but has RF in the shack and unreliable SWR readings.
Does it matter which leg points which direction?
To a small extent. The asymmetric current distribution in the OCFD produces a slightly asymmetric radiation pattern — the antenna is not equally directional in all azimuths. The long leg side tends to have slightly more gain in that direction on some bands. For most operators this effect is small enough to ignore — orient the antenna based on available space and support points. If you have a choice, pointing the long leg toward Europe (from North America) or toward your primary DX target region extracts the modest directional advantage the asymmetry provides.
Can I operate on 30m, 17m, and 12m with an OCFD?
These WARC bands are not primary resonances of the standard 135-foot OCFD. The antenna presents variable impedances on 30m, 17m, and 12m that usually require a tuner. With a good antenna tuner, the OCFD can be operated on WARC bands — the 135-foot wire is long enough to be a useful antenna on all HF bands with tuner assistance. The feedline loss penalty on WARC bands depends on how far off-resonance the antenna is — if the SWR is below 5:1 at the feedpoint, coax loss is manageable. For regular WARC band operation, a dedicated antenna or a doublet with ladder line is preferable.
My OCFD SWR is good on 40m but high on all other bands — what is wrong?
High SWR on all bands except one is the classic symptom of reversed antenna terminals — the short and long legs are connected to the wrong balun terminals. When the short leg (45 ft) connects to the long-leg terminal, the antenna is still resonant on 40m (because 45 ft resonates near 7 MHz) but the harmonic resonances that create 80m, 20m, 15m, and 10m coverage are all wrong. Lower the antenna, swap the two wire connections at the balun terminals, and re-sweep. This fix resolves the problem immediately.
Can I use a commercial 4:1 balun from an antenna kit?
Only if it is specifically a 4:1 current balun. Many commercial "4:1 baluns" sold for antenna use are actually 4:1 voltage baluns (W2AU style) — these do not suppress common-mode current and will not work correctly for an OCFD. Check the product description carefully. Look for the words "current balun" or "choke balun" — not just "4:1 balun." DX Engineering, Palomar Engineers, Balun Designs, and Antennas by W6LVP all make quality 4:1 current baluns that are correct for OCFD use.
What is the difference between an OCFD, a Windom, and a Carolina Windom?
The terms are related but distinct. The original Windom (Loren Windom W8GZ, 1929) was fed with a single wire connected at the 1/3 point — no coax at all. The modern "off-center-fed dipole" or "OCF dipole" uses coax with a 4:1 current balun at the same 1/3 feedpoint — the direct descendant of the Windom concept updated for coax operation. The Carolina Windom is a commercial variant of the OCFD that adds a 22-foot vertical radiating coax section below the feedpoint for improved low-angle radiation. All three cover the same five bands from the same 135-foot wire, differing mainly in the feedline arrangement and the presence or absence of the vertical coax section.