OCF Dipole Antenna Calculator — OCFD Dimensions, Design & Plans

An OCF, OCFD or off-center-fed dipole is approximately a half-wavelength antenna with its feedpoint moved away from the electrical center. Because the two legs are unequal, the calculator must determine both the overall wire length and the position of the feedpoint.

Do not treat these numbers as final cut lengths. Wire insulation, conductor diameter, surroundings and mounting geometry can shift resonance. Leave enough extra wire at both ends to fold back during tuning instead of making irreversible cuts.

Use the table below for quick access to common OCF antenna designs. Dimensions should be generated from the chosen design frequency rather than assigning one fixed length to an entire amateur band.

Choose a band and calculate exact dimensions

An off-center-fed dipole is a wire antenna in which the feedpoint is located away from the physical center. It still starts from the same basic half-wavelength dipole concept, but one leg is shorter than the other.

Moving the feedpoint changes the impedance presented at the feedpoint and changes how the antenna behaves on harmonic frequencies. This is why an OCF dipole can be useful as a multiband HF antenna when the wire length, feedpoint location, transformer and feedline system are chosen together.

OCF and OCFD are commonly used as abbreviations for off-center-fed dipole. You will also see the spelling off-centre-fed dipole.

An OCFD should not be treated as a conventional center-fed dipole that was simply cut into unequal pieces. Feedpoint position is part of the electrical design and can affect impedance, SWR, harmonic-band coverage and common-mode current.

The calculator begins with the wavelength corresponding to the selected design frequency and derives an approximate half-wave conductor length.

The basic free-space relationship is:

Wavelength = speed of propagation ÷ frequency

A practical wire dipole is normally somewhat shorter than one-half of the corresponding free-space wavelength because the conductor is not an ideal infinitely thin wire in free space. Insulation, wire diameter, nearby objects and the installation environment can all affect the final electrical length.

The calculator therefore uses:

Calculated total length = half-wave reference length × correction factor

It then divides the resulting wire according to the selected feedpoint split.

For a 33/67 design:

Short leg = total length × 0.33

Long leg = total length × 0.67

For a custom design:

Short leg = total length × selected feedpoint percentage

Long leg = total length − short leg

Advanced calculations may additionally account for conductor type, insulation, diameter and installation geometry. Modeling software for OCFD designs can likewise use wire and dielectric properties when estimating dimensions.

The feedpoint split is one of the most important OCF antenna design choices. There is no single position that is automatically best for every combination of bands, height, conductor and matching system.

33/67 — the classic OCFD split

A feedpoint around one-third of the overall length is the familiar OCFD arrangement. It provides a useful starting point for multiband designs, but it should not be assumed to give the lowest impedance or SWR on every harmonic band.

20/80 — an alternative multiband split

Moving the feedpoint closer to one end changes where the feedpoint intersects the standing-current distributions on higher frequencies. A 20/80 position has been used as an alternative when designers want a different multiband impedance compromise, including designs intended to bring additional classic HF bands into a practical range.

About 29.5/70.5

Feedpoints near 29–30% are another multiband design option. DJ0IP documents an 80m OCFD using a feedpoint around 29.7%, illustrating that useful OCFD designs do not have to use exactly one-third.

Custom feedpoint position

Use a custom split when you are working from NEC/EZNEC modeling, a proven published design, measured current distribution or a specific set of target bands.

Changing the feedpoint should be treated as a design change, not merely a mechanical adjustment. Recheck impedance, SWR, common-mode behavior and band coverage after changing the split.

An off-center feedpoint usually does not present the same impedance as the center of a conventional resonant dipole. The actual value varies with frequency, feedpoint position, height, surroundings and the behavior of the feedline.

This is why an OCF dipole is commonly used with an impedance-transforming network between the balanced antenna element and 50-ohm coax.

What Is the Feedpoint Impedance of an OCF Dipole?

There is no single feedpoint impedance that applies to every OCFD. Moving the feedpoint changes the relationship between voltage and current at that location, while operation on different bands changes the current distribution along the entire wire.

A value measured in the shack can also differ from the actual impedance at the antenna terminals because a mismatched coaxial feedline transforms impedance along its length. DJ0IP specifically distinguishes true feedpoint impedance from measurements made elsewhere along the coax.

For meaningful comparison, state where the measurement was made and whether feedline transformation has been removed or modeled.

4:1 vs 6:1 Balun for an OCF Dipole

Do not select a transformer ratio only because it is commonly associated with the words “OCF dipole.”

The correct ratio should be based on the impedance range the antenna presents across the bands you actually intend to use.

A transformer converts impedance by the square of its turns ratio. Its job is to bring the antenna-side impedance into a range that is more practical for the 50-ohm coax, transmitter and tuner.

Use the measured or modeled antenna system to choose between available transformer ratios.

Current Balun vs Voltage Balun

An OCF dipole is electrically unbalanced around its feedpoint even though the radiating element itself is a two-wire dipole. That makes control of common-mode current especially important.

A transformer should therefore be evaluated not only by impedance ratio but also by how the complete device handles unwanted current on the outside of the coax shield.

Do You Need a Common-Mode Choke?

A common-mode choke can help prevent the outside of the coax from becoming an unintended radiating conductor. Excess common-mode current can alter the radiation pattern, affect resonance and carry RF toward the station. DJ0IP documents these effects in OCFD systems.

Choking requirements depend on the antenna, feedline length, transformer construction, frequency and installation.

Can an OCF Dipole Be Fed Directly With Coax?

Physically, coax can be connected to many wire antennas. Electrically, direct connection does not solve either the impedance transformation or common-mode-current problem.

For a practical OCFD, treat the wire, transformer/balun, choke and coax as one antenna system rather than independent components.

An OCF dipole is popular because one full-size wire can present useful operating conditions on the fundamental frequency and selected higher bands. However, “multiband” does not mean every HF band will automatically have a low SWR.

Coverage depends on:

• total electrical length;
• feedpoint split;
• transformer ratio;
• antenna height and geometry;
• wire characteristics;
• common-mode current;
• coax type and length;
• the SWR range accepted by the transmitter or tuner.

A classic OCF feedpoint may not provide the same impedance compromise on 15m as it does on several other harmonic bands. Alternative feedpoint positions can change that behavior, which is one reason split selection belongs inside the calculator instead of being fixed at 33/67.

Always verify the finished antenna with an analyzer across each intended operating band.

A 160 meter OCF dipole requires substantial wire length and installation space. For that reason, the physical layout is just as important as the mathematical dimensions.

Design frequency: [160M_SELECTED_FREQUENCY] Total length: [160M_TOTAL_LENGTH] Short leg: [160M_SHORT_LEG] Long leg: [160M_LONG_LEG] Feedpoint split: [160M_SPLIT] Suggested transformer: [160M_ENGINEERED_MATCHING_DATA]

Before choosing a 160m OCF antenna, check:

• available end-to-end space;
• feedpoint support height;
• safe clearance from power lines and structures;
• whether the legs must slope;
• the planned coax route;
• transformer and choke power handling;
• whether operation is required on 160m only or on higher bands as well.

A 160m OCFD used as a multiband antenna should be evaluated separately on every intended HF band. Higher-frequency radiation patterns can develop multiple lobes and nulls as the wire becomes several wavelengths long.

Do not scale a commercial 160m antenna mechanically without also considering its feed system and intended band coverage.

The 80m OCF dipole is a common starting point for a full-size multiband HF wire antenna because its total wire length can also support useful harmonic-band operation.

Design frequency: [80M_SELECTED_FREQUENCY] Total length: [80M_TOTAL_LENGTH] Short leg: [80M_SHORT_LEG] Long leg: [80M_LONG_LEG] Feedpoint: [80M_SPLIT] Expected bands: [80M_PREDICTED_BANDS]

For an 80m OCF dipole, give particular attention to the feedpoint position and common-mode choking. Designs around 20%, one-third and approximately 29–30% do not behave identically on the higher bands.

Install the wire at its intended height before making final length adjustments. Resonance measured close to the ground can change after the antenna is raised.

A 40 meter OCF dipole uses roughly half the wire of an 80m version and is easier to fit on many properties. Depending on the design, it can also be used on selected higher-frequency bands.

Design frequency: [40M_SELECTED_FREQUENCY] Total length: [40M_TOTAL_LENGTH] Short leg: [40M_SHORT_LEG] Long leg: [40M_LONG_LEG] Feedpoint: [40M_SPLIT]

A “40–6 meter OCF” should not be interpreted as a promise of identical SWR or radiation performance on every band between 40m and 6m. Check the actual modeled or measured response band by band.

For example, Buckmaster's current 4-band OCF is specified for 40, 20, 10 and 6 meters rather than every amateur allocation within that frequency range.

A 20 meter OCF dipole is considerably shorter than 40m, 80m and 160m designs, making it useful where available span is limited.

Design frequency: [20M_SELECTED_FREQUENCY] Total length: [20M_TOTAL_LENGTH] Short leg: [20M_SHORT_LEG] Long leg: [20M_LONG_LEG] Feedpoint: [20M_SPLIT]

Because there are fewer lower-frequency harmonics available below the design band, a 20m OCF is not a direct substitute for an 80m or 160m multiband wire antenna. Choose it for the bands and physical dimensions you actually need rather than simply for the smallest possible OCFD.

An OCF dipole can be installed horizontally as a flat-top or with the center/feedpoint elevated and the ends sloping downward as an inverted V.

Flat-top OCF dipole

END ─────────────── ◆ ───────────────────────── END
│
│ COAX

A flat-top provides a simple geometry for modeling and usually makes the physical relationship between the two unequal legs easy to reproduce.

Inverted-V OCF dipole

\ ◆ /
\ │ /
\ │ COAX /

An inverted V can reduce the required horizontal span and may be easier to support from one high central point. Changing the included angle and end heights also changes the antenna geometry, so do not expect an inverted-V version to tune at exactly the same dimensions as a flat-top.

Feedpoint height: [INPUT] End height: [INPUT] Included angle: [INPUT] Ground model: [INPUT]

Model this configuration

An accurate calculator cannot compensate for a poor installation. Build and tune the antenna as a complete RF system.

1. Choose the antenna orientation

Decide where the wire can run with safe clearances and as little interaction as practical with large conductive objects.

The strongest low-band radiation from a simple horizontal dipole is generally broadside to the wire, while the pattern becomes more complex on higher harmonics.

2. Set the feedpoint support

The feedpoint carries the transformer and coax, so use a support system that handles both vertical load and wire tension without transferring excessive strain to electrical connections.

3. Support the wire ends

Use suitable nonconductive line and insulators. Provide enough movement for wind and tree motion rather than tensioning the conductor like a rigid cable.

4. Route the coax deliberately

Route coax away from the radiating wire where practical. The feedline should not be treated as an invisible component: common-mode current can make the outside of the shield radiate.

5. Keep clear of conductive structures

Gutters, metal roofs, towers, fences, guy wires and long parallel conductors can detune the antenna. Buckmaster likewise warns that nearby long metallic objects can negatively affect its multiband OCF antennas.

6. Weatherproof the feedpoint

Prevent water from entering coax connectors, transformer enclosures and wire insulation. Water ingress and degraded insulation can change losses and SWR.

7. Test before permanent trimming

Raise the antenna to its intended operating geometry, sweep the required bands, then tune gradually.

Tune the antenna only after it is installed close to its final height and configuration.

Step 1 — Start slightly long

Leave extra conductor at both ends. Fold excess wire back rather than cutting immediately.

Step 2 — Measure the fundamental band

Use an antenna analyzer or VNA to locate the frequency of minimum SWR and, separately, the point of resonance if your instrument provides impedance data.

Minimum SWR and zero reactance are related but are not necessarily the same frequency.

Step 3 — Adjust total electrical length

If the relevant resonance is too low in frequency, shorten the antenna gradually. If it is too high, additional electrical length is required.

Step 4 — Recheck every target band

An OCFD is a multiband compromise. A change made for the fundamental frequency can also affect the higher bands.

Step 5 — Evaluate the feedpoint split

If one important band remains problematic, the issue may not be solved by changing total length alone. Feedpoint location affects the impedance seen on harmonic bands.

Step 6 — Check common-mode current

Unexpected resonance shifts, RF in the shack or strong sensitivity to coax routing can indicate that the feedline is participating in the antenna.

Step 7 — Confirm the system at operating power

After low-power measurements are satisfactory, verify that the transformer, ferrite components, coax and connectors remain within their intended operating limits.

Recalculate dimensions before trimming

Low SWR at the transmitter does not automatically prove that the antenna is resonant or efficient.

Three separate questions matter:

1. What impedance exists at the antenna feedpoint?

This describes the load before the feedline transforms it.

2. What SWR exists on the coax?

A mismatch causes standing waves on the transmission line.

3. What does the transmitter see?

The impedance at the shack end depends on the antenna, coax characteristic impedance, electrical length and feedline loss.

A tuner can transform the impedance seen by the transmitter, but it does not eliminate mismatch loss that has already occurred in the coax.

Long feedlines and higher frequencies make coax attenuation more important. When SWR is high, effective transmission-line loss can be greater than the matched-loss figure alone suggests.

This is why an advanced OCF antenna calculator should distinguish:

• feedpoint impedance;
• feedpoint VSWR;
• coax type;
• coax length;
• matched attenuation;
• mismatch loss;
• transmitter-end impedance;
• transmitter-end SWR.

Coax type: [SELECT] Coax length: [INPUT] Frequency: [AUTO] Feedpoint impedance: [MODEL_RESULT]

Estimated matched loss: [RESULT] Estimated total line loss: [RESULT] Transmitter-end SWR: [RESULT]

An OCF dipole does not have one fixed radiation pattern for all bands.

On its fundamental frequency, a horizontal half-wave-like antenna has a relatively simple broadside pattern. As operating frequency increases and the same physical wire becomes electrically longer, additional lobes and nulls can appear.

Height above ground also changes the elevation pattern and take-off angles.

Band:

160 | 80 | 40 | 20 | 15 | 10 | Custom

Height:

0.25 λ | 0.5 λ | 1 λ | Custom

View:

Azimuth | Elevation

Maximum modeled gain: [NEC_RESULT] Broadside direction: [NEC_RESULT] Primary elevation angle: [NEC_RESULT] Beamwidth: [NEC_RESULT] Polarization: [NEC_RESULT]

Do not reuse one 80m radiation diagram as a representation of the antenna on 20m or 10m. Each frequency requires its own model.

Numerical modeling helps answer questions that a basic length calculator cannot.

An NEC-based model can estimate:

• feedpoint resistance and reactance;
• current distribution along the wire;
• resonance;
• SWR with a defined reference impedance;
• azimuth radiation pattern;
• elevation pattern;
• gain;
• feedpoint-height effects;
• changes caused by wire geometry.

Advanced OCFD software can also incorporate conductor and insulation properties when calculating antenna dimensions.

Engine: [NEC_ENGINE] Wire conductivity: [INPUT] Wire diameter: [INPUT] Insulation/dielectric: [INPUT] Ground conductivity: [INPUT] Ground dielectric constant: [INPUT] Height: [INPUT] Feedpoint position: [INPUT]

Modeling is a prediction, not a substitute for final measurements. Real installations contain supports, feedlines, buildings, trees and conductive objects that may not be represented in a simplified NEC model.

A homebrew OCF dipole is straightforward mechanically, but the matching and common-mode-control system deserves as much attention as the wire lengths.

Materials

• antenna wire suitable for the intended span and environment;
• center/feedpoint insulator or transformer enclosure;
• impedance transformer/balun selected for the design;
• common-mode choke if required by the feed system;
• 50-ohm coaxial feedline;
• end insulators;
• support rope or line;
• weatherproof connectors and sealing materials;
• strain relief hardware.

Step 1 — Calculate the dimensions

Choose the design frequency and feedpoint split. Record total length, short-leg length and long-leg length.

Step 2 — Cut the wire with tuning allowance

Make both legs slightly longer than the calculated final dimensions.

Step 3 — Build the feedpoint

Connect the two unequal antenna legs to the antenna-side terminals of the selected transformer. Keep mechanical strain off solder joints and transformer windings.

Step 4 — Add feedline choking

Install the chosen common-mode-control solution according to the electrical design. Do not assume the coax shield will remain RF-neutral by itself.

Step 5 — Add end insulators and supports

Provide strain relief and sufficient mechanical flexibility.

Step 6 — Raise the antenna

Install it at its intended height and geometry before final tuning.

Step 7 — Sweep all target bands

Record SWR and impedance rather than judging the antenna from one spot frequency.

Step 8 — Tune incrementally

Fold back or shorten wire in small steps. Re-measure after every change.

Calculate your homebrew OCFD dimensions

When a full-size OCF dipole will not fit, electrical loading can reduce its physical length. The trade-off is that a shortened antenna is no longer equivalent to the full-size design.

Depending on the loading method, expect changes in:

• current distribution;
• feedpoint impedance;
• resonance;
• bandwidth;
• loss;
• efficiency;
• multiband behavior.

A shortened OCF dipole therefore needs its own modeled dimensions rather than a simple percentage reduction of a full-size antenna.

An off-center-fed wire can also be used with balanced or open-wire transmission line, but this becomes a different feed system from the typical coax-fed OCFD.

Low-loss ladder line can tolerate high SWR more efficiently than many coaxial feedlines, while a transmatch or balanced matching system is then used to transform the impedance presented at the station.

Important design questions include:

• feedline characteristic impedance;
• electrical length;
• routing and spacing from conductive objects;
• balanced-to-unbalanced transition;
• tuner matching range;
• common-mode current.

If the goal is wide-range operation through a balanced line and tuner, also compare the design with a conventional doublet before choosing an off-center feedpoint.

An OCF dipole can be oriented vertically if sufficient height and a suitable support arrangement are available.

Vertical orientation changes the polarization and the way the antenna interacts with the ground. The feedpoint remains off-center electrically, so the same matching and common-mode considerations still apply.

A vertical OCFD is not automatically equivalent to a quarter-wave vertical with radials. It remains a two-leg dipole system rather than a monopole using the ground or radial field as its return conductor.

Model the vertical version separately for:

• ground interaction;
• feedpoint height;
• end height;
• polarization;
• elevation pattern;
• feedline routing.

Both OCF and end-fed antennas can provide multiband operation from a single wire, but they solve the feeding problem differently.

Choose an OCF dipole when an off-center feedpoint is practical and you want to optimize the feedpoint around a particular multiband impedance compromise.

Choose an end-fed design when locating the feedpoint near one end materially simplifies the installation and the required high-ratio matching network is acceptable.

The terms “Windom” and “OCF dipole” are sometimes used loosely, but not every antenna sold or described as a Windom uses the same feed arrangement.

A conventional coax-fed OCF dipole consists primarily of two unequal wire legs connected at an off-center feedpoint through an appropriate matching system.

A Carolina Windom-type design may intentionally include a vertical radiating feedline section as part of the antenna concept. That makes its feedline arrangement and radiation behavior different from a simple OCFD.

When comparing the two, use the actual schematic rather than relying on the product name alone.

A center-fed dipole places the feedpoint at the electrical center. An OCF dipole deliberately moves it away from that position.

On the fundamental frequency, moving away from the current maximum changes the feedpoint impedance. On harmonic bands, the current maxima and minima occur in different places along the wire, so the off-center position can be selected to obtain a useful multiband compromise.

Neither design is universally “better.” The practical choice depends on the required bands, available supports, feedpoint location and matching strategy.

Commercial off-center-fed dipoles package the wire dimensions, feedpoint hardware and impedance-matching system into a ready-made antenna. When comparing models, look beyond the number of advertised bands.

Compare:

• overall wire length;
• leg dimensions;
• supported bands;
• maximum specified power;
• tuner requirements;
• transformer design;
• feedline requirements;
• minimum installation geometry;
• weatherproofing and mechanical construction.

Buckmaster OCF Dipoles

Buckmaster currently lists 4-band, 7-band and 8-band OCF designs. The manufacturer specifies 68 ft for its 4-band antenna, 135 ft for the 7-band and 270 ft for the 8-band version.

MFJ OCF Dipoles

RadioWavz DX80 OCF Wire Antenna

Maxcon, NI4L and N9SAB OCF Antennas

Do not compare antennas by brand name alone. Two OCFDs of similar length can use different feedpoint positions, transformers and band-coverage strategies.

The following dimensions are manufacturer-specific and should not be treated as universal OCF dipole formulas.

These dimensions and band listings are from Buckmaster's current product information.

The 4-band, 7-band and 8-band models scale to progressively longer fundamental wavelengths, but copying the wire dimensions alone does not reproduce the full commercial antenna. The matching system and installation requirements are part of the design.

For a homebrew antenna, use the calculator and validate the resulting OCFD with measurements rather than assuming that commercial dimensions are interchangeable with a different balun or feedpoint system.

High SWR on every band

Possible causes: incorrect dimensions, transformer wiring error, damaged coax, connector fault, unsuitable transformer ratio or severe interaction with nearby metal.

Check: disconnect and test components separately where practical, verify dimensions, inspect connections and sweep the antenna at the feedpoint or with feedline effects accounted for.

One band tunes, but the other bands do not

Possible causes: unsuitable feedpoint split, incorrect overall electrical length or an expectation that the chosen design should cover bands it was not optimized for.

Check: compare measured current/impedance behavior with the intended OCF design before repeatedly trimming the ends.

Resonance moves when coax is repositioned

Likely issue: common-mode current is making the feedline part of the radiating system.

Check: choking, transformer construction and coax routing.

RF in the shack

Possible causes: common-mode current, insufficient feedline isolation or an antenna/feedline system that is using station wiring as part of the RF return path.

Check: common-mode choke performance and station grounding/bonding strategy.

Balun or transformer gets hot

Heating indicates dissipation in the matching system. Causes may include excessive current, high reactive loading, core loss, insufficient power rating, saturation or operation outside the transformer's intended impedance/frequency range.

Stop transmitting and diagnose the cause rather than treating heat as normal.

SWR changes after rain

Moisture can affect damaged coax, connectors, transformer enclosures, wet insulation and nearby materials.

Inspect weatherproofing before retuning the antenna.

Antenna works differently after being raised

Height changes the antenna's interaction with the ground and surroundings, so resonance and impedance can shift.

Make final tuning adjustments at the intended operating height.

Good SWR but disappointing performance

Low SWR only tells you about impedance matching at the measurement point. It does not directly measure radiation efficiency, feedline loss, radiation pattern or take-off angle.

Evaluate the entire system rather than optimizing one SWR number.

This calculator provides starting dimensions for an OCF/OCFD antenna. Final results should be verified by measurement or a validated antenna model.

Calculation assumptions:

• the antenna is modeled from a half-wave fundamental design unless otherwise specified;
• the selected correction or velocity factor is an approximation;
• short- and long-leg dimensions are determined from the selected feedpoint percentage;
• calculated impedance, SWR, gain and radiation patterns require a defined electromagnetic model and cannot be inferred from wire length alone;
• feedline calculations require the actual coax type, length, velocity factor and attenuation data;
• transformer recommendations require an engineering rule based on modeled or measured feedpoint impedance;
• nearby conductive objects may significantly change real-world results;
• a tuner changes the impedance presented to the transmitter but does not remove losses already occurring in a mismatched feedline;
• all modeled radiation data should identify frequency, height, ground assumptions and antenna geometry.

Published OCFD research and practical modeling show that feedpoint position is a design variable rather than a universal fixed value, and that feedline/common-mode behavior must be considered when evaluating impedance and resonance.

Commercial dimensions shown above are product-specific and are based on the manufacturer's currently published Buckmaster OCF information.

The page structure, target query clusters and required semantic coverage follow the supplied SEO brief and recommended OCFD calculator architecture.

Safety note:

Never install an antenna, support line, mast or feedline where it can contact overhead electrical conductors. Maintain appropriate electrical and structural clearances for the installation site.