What Is an Off-Center Fed Dipole?
Definition and Basic Operating Principle
An off-center fed dipole is exactly what its name suggests: a half-wave wire dipole that is fed not at its center, but at a point displaced from the center toward one end of the wire. By deliberately moving the feedpoint away from the electrical midpoint of the antenna, the designer exploits the fact that the feedpoint impedance of a dipole changes significantly along its length, and that certain offset positions yield similar impedance values across several harmonically related amateur radio bands. The result is a single wire antenna capable of multiband HF operation with a single coaxial feedline.
Like most HF antennas, the OCF is based on the dipole. The benefit of an off-center fed antenna is that if you move the feed point away from the center, you can find a spot that will allow the antenna to resonate on multiple bands.
How It Differs from a Standard Center-Fed Dipole
A standard center-fed half-wave dipole presents a theoretical feedpoint impedance of approximately 73 ohms in free space, making it an excellent match for 50-ohm coaxial cable with modest SWR on a single band. When the feedpoint is moved off-center, the impedance rises substantially. Feeding the OCF dipole at a point that is one-third of its length from one end typically yields a higher feedpoint impedance, approximately 200–300 ohms, as compared to the center-fed half-wave dipole at 73 ohms. This elevated impedance is the key that unlocks multiband capability, but it also means that a balun or impedance transformer is mandatory to interface with 50-ohm coax.
Brief History and Origins of the OCF Dipole in Amateur Radio
The off-center fed dipole has deep roots in amateur radio history. It was Loren Windom and several others at Ohio State University who discovered how changing the feedpoint would affect the coverage and performance of an antenna. The original Windom antenna from the late 1920s was an off-center fed resonant dipole with a single wire feedline of any length. It was intended to be used on one frequency only. Its main advantage was that it could easily be matched to a tube transmitter. Today's versions, based on the original Windom, include the common OCF and Carolina Windom. Over the decades, advances in ferrite core technology and computer antenna modeling allowed builders to refine the OCF concept into the reliable multiband antenna it is today.
How the Off-Center Fed Dipole Works
Feedpoint Impedance Explained
Understanding why the OCF works requires a basic understanding of standing wave behavior on a dipole wire. When offsetting the feed position of a dipole antenna away from its center, at some point similar feed impedances can be obtained for a number of frequency bands. This occurs at the fundamental (λ/2 dipole) resonant frequency as well as a number of harmonic resonant frequencies. This is possible because a standing wave is present along the dipole which causes the feed impedance to change. In the span of a quarter wavelength, it varies from a very high value at the antenna ends (several kΩ) to the value of the radiation resistance at the corresponding frequency.
For the OCF, the goal is to determine a point where the impedance is low enough to be usable on multiple bands. You can accomplish this with a feedpoint placed somewhere between 45 and 20 percent of the total length from one end of the antenna.
Why the 1/3 – 2/3 Split Is the Most Popular Ratio
If we move our feedpoint so that one leg of the antenna is 1/3 long and the other is 2/3 long, the antenna will now resonate on 80, 40, 20, 10 and 6 meters usually without a tuner. This 33%–67% split (also described as approximately 36%–64% in many practical designs) has become the de facto standard for OCF dipoles because it places the feedpoint at a location where the impedance on multiple harmonic bands converges to a workable range around 200 ohms — a ratio that can be conveniently transformed to 50 ohms using readily available 4:1 baluns. Some hams have reported choosing a 20%–80% split in conjunction with a 4:1 current balun, providing a usable antenna on 80, 40, 30, 20, 15, 12, and 10 meters, with SWR readings less than 2:1 on these bands.
Radiation Patterns and Efficiency Across Bands
At its fundamental frequency (typically 80 meters for the most common OCF design), the radiation pattern resembles a classic dipole with broadside lobes perpendicular to the wire. On higher bands, the pattern becomes more complex. The off-center feed taps into the standing wave so that currents and phases on the two legs are not equal, especially on harmonics. That unbalance is what bends the pattern and moves the hot directions on different bands. On 80 m, the pattern is still somewhat "dipole-ish" but skewed. On 40, 20, 17, 15, 12, 10 m the wire is multiple wavelengths long. The off-center feed taps into different parts of the standing wave, so you get odd lobes and nulls that do not match a simple center-fed or well-behaved EFHW. In practice, this means the OCF will show some gain over a dipole in certain directions on the higher bands while exhibiting some directional asymmetry.
Understanding Current and Voltage Distribution Along the Wire
On a half-wave dipole, current is maximum at the center and approaches zero at the ends, while voltage is minimum at the center and maximum at the ends. When the feedpoint is displaced off-center, the current maximum is no longer at the feedpoint — it lies somewhere between the feedpoint and the center of the wire. This offset between the feedpoint location and the current maximum is what raises feedpoint impedance. Placing the feed point away from the center increases the resistive part of the feed impedance and source load more than the reactive (imaginary) part of the resonant antenna, which is nearly resonant. This effectively lowers the loaded Q-factor of the antenna at the feed point. The lower Q-factor contributes to the OCF's broader 80-meter bandwidth compared to a conventional center-fed dipole — a significant practical advantage.
Multiband Performance of the OCF Dipole
Which Amateur Radio Bands the OCF Covers
The OCF dipole presents a reasonably good match to the transmitter across multiple bands, which are even harmonics of the fundamental frequency, including 80, 40, 20, 12, 10 and 6 meter bands. More advanced designs or slightly different feedpoint ratios can also add 30, 17, and 15 meters, sometimes with the help of the rig's internal antenna tuner. DX Engineering Multi-Band Off Center Fed Dipole Antenna Kits cover the 80, 40, 30, 20, 17, 15, 12 and 10 meter bands by taking advantage of the practice of feeding two different length wire dipole legs with a 4:1 balun.
Comparing OCF Performance on 80m, 40m, 20m, 15m, and 10m
Performance varies by band on the OCF. On 80 meters, the OCF is the fundamental resonant frequency, and performance is strong across both the CW (3.5 MHz) and phone (3.8 MHz) portions. The broader impedance bandwidth offered by the offset feedpoint is an advantage on 80m compared to a center-fed dipole. On 40 meters, performance is excellent — it is the first harmonic, and SWR is typically very low. On 20 meters, the second harmonic, performance is similarly strong. On 15 meters, the OCF is operating on an odd harmonic (third harmonic of 40m rather than a direct even harmonic of 80m), which can result in higher SWR, and a tuner is often recommended. On 10 meters, as the fourth harmonic of 80m, the OCF generally works well, though the more complex radiation pattern means the antenna is more directional.
Why the OCF Is Attractive for HF Multiband Operation
Compared to a single-band resonant dipole with equal length legs, the OCF dipole offers the advantage of HF multi-band operation at the "cost" of slightly to somewhat elevated SWR. The key attraction of the OCF is its simplicity: one wire, one coaxial feedline, and a single balun cover the bulk of the HF spectrum. There are no traps to corrode, no matching sections of ladder line to maintain clearance from nearby objects, and no multiple feedlines to manage. The off-center fed dipole is an excellent multiband antenna that is relatively simple to construct, yet gets quite decent performance.
SWR Expectations Across Supported Bands
With a well-built 4:1 current balun and a properly sized wire, a well-installed 80m OCF dipole typically achieves SWR below 2:1 on 80, 40, and 20 meters, and often below 2:1 on 10 and 12 meters as well. Depending upon band and antenna surroundings, in many cases the automatic antenna tuner built into the transceiver will provide the desired band coverage at good SWR. In some cases a low-cost external tuner will provide better band coverage, while a more capable external tuner is required for high power operations. Bands like 30, 17, and 15 meters typically require tuner assistance unless the feedpoint ratio has been optimized specifically to include them.
Baluns and Feed Systems for the OCF Dipole
Why a Balun Is Essential for the OCF Antenna
Using a quality 4:1 balun at the feedpoint is crucial to the overall performance of the antenna when they are fed with coaxial cables. The feedpoint impedance at the offset is at or about 200 ohms and the balun will provide good transformation to the coax feedline impedance of 50 ohms. Beyond impedance transformation, the balun serves a second critical role: it suppresses common-mode current on the outer surface of the coaxial feedline shield. Without adequate common-mode suppression, the feedline itself becomes part of the radiating system, degrading the radiation pattern, creating RF in the shack, and causing interference to household electronics.
4:1 Balun vs 6:1 Balun — Which Is Right for Your OCF?
This is one of the most hotly debated topics in OCF construction. The choice of balun ratio depends on the feedpoint ratio used and the actual impedance encountered at that point. The 6:1 is the type utilized in the commercial Buckmaster off-center fed dipole fed at the 33% mark; some people prefer the 4:1 (easier to build) balun, with feeding anywhere from the 33% point to the 38% point. The 4:1 is much easier to build. The 6:1 may provide a better match. As a general guideline, the 4:1 balun works well with feedpoint ratios closer to the 36%–64% range, while the 6:1 is best suited for the classic 33%–67% split. Feedpoint height greater than 60 feet will require a 6:1 balun in some designs, as height changes the actual impedance seen at the feedpoint.
Voltage Balun vs Current Balun Considerations
Ham radio operators debate current baluns versus voltage baluns for OCF use. A current balun (Guanella design) forces equal and opposite currents in both antenna legs, which is the correct behavior for a balanced antenna fed with unbalanced coax. A voltage balun (Ruthroff design) forces equal and opposite voltages across the output terminals. For OCF dipoles, the current balun is generally preferred because the antenna is inherently somewhat unbalanced due to the asymmetric leg lengths — and the current balun provides superior common-mode rejection under those conditions. You need a 4:1 current balun at the feed point to bring it down from 200 ohms to 50 ohms. Google 4:1 Guanella balun. Simple to make and better than the voltage balun.
Common Coaxial Feedline Lengths and Their Effects
Unlike ladder-line-fed antennas such as the G5RV, the OCF dipole uses standard 50-ohm coaxial cable as its feedline, which simplifies installation enormously. Any length of coax can theoretically be used, but it is good practice to add a 1:1 choke balun or several ferrite snap-on chokes near the feedpoint and again where the feedline enters the shack. This prevents the coax from radiating and keeps the radiation pattern predictable. Avoid using very long runs of small-gauge coax (such as RG-58) at high power levels, as the elevated SWR on some bands can cause measurable feedline loss; RG-8X or RG-213 is preferred for runs over 50 feet.
Building Your Own Off-Center Fed Dipole
Materials List and Wire Selection
One of the great virtues of the OCF dipole is its low cost and minimal parts count. To build a standard 80–10m OCF dipole, you will need:
- Approximately 135 feet of copper or copper-clad antenna wire (14 AWG stranded insulated wire is ideal)
- One quality 4:1 current balun rated for your power level
- Three end insulators
- Dacron poly
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