Off Center Fed Dipole: How an OCFD Covers Several HF Bands

The off center fed dipole, usually shortened to OCFD and often sold under the old name “Windom”, is one of the most popular multiband wire antennas on the HF bands. It looks like an ordinary half-wave dipole, but the coax does not connect in the middle. Instead the feed point sits roughly a third of the way along the wire, where the antenna presents a similar, moderately high impedance on several harmonically related bands. With the right balun, one wire and one run of coax can give a usable match on four or more bands without a tuner.

That is the promise. The reality is a little more nuanced: an OCFD is an asymmetric antenna, and asymmetric antennas tend to push current onto the outside of the coax. Builders who understand why the antenna works, which bands it really covers and how to keep the feed line quiet usually end up very happy with it. Builders who simply hang a kit and hope often end up chasing RF in the shack. This guide covers the theory in plain terms, the practical build, the balun choice, tuning and the common mistakes.

How an Off Center Fed Dipole Works

Every half-wave dipole has a current maximum in the middle and voltage maxima at the ends. At the centre the feed point impedance is low, somewhere in the region of fifty to seventy ohms depending on height and surroundings, which is why a centre-fed dipole is such a natural match for coax on its fundamental band. Move the feed point away from the centre and the impedance rises, because you are now connecting at a point where the current is lower and the voltage is higher.

The trick of the OCFD is to choose an offset where the impedance is similar on several bands at once. If you take a wire that is a half wave long on the lowest band, it is a full wave long at twice the frequency and two wavelengths long at four times the frequency. The current distribution along the wire changes on each band, but there are points along the wire where the impedance comes out in the same general range on the fundamental and on its even harmonics. A feed point at roughly one third of the length, give or take a few percent depending on the design, is the classic compromise.

At that point the impedance is typically a few hundred ohms rather than fifty. That is why an OCFD always needs an impedance transformation at the feed point, normally a 4:1 balun, sometimes a higher ratio in designs that use a different offset. The balun brings the antenna’s impedance down close enough to fifty ohms that the radio’s internal tuner, or no tuner at all, can handle it on the design bands.

Why some bands work and others do not

The same geometry that helps on the even harmonics works against you on the odd ones. On the third harmonic of the fundamental, the wire is one and a half wavelengths long, and a feed point at one third of the length falls very close to a voltage maximum. The impedance there is very high and very reactive, which is why a typical OCFD cut for the 80 or 40 meter band is poor on 15 meters, the band near the third harmonic of 40 meters. This is not a defect in a particular kit; it is the physics of the offset.

In practice, an OCFD designed for 80 meters is usually expected to load on 80, 40, 20 and 10 meters, often on 17 or 12 meters as well with some help from a tuner, and poorly on 15 meters. A version cut for 40 meters shifts everything up: 40, 20 and 10 meters as the main bands, with others depending on the exact design. Manufacturers’ band claims vary, so always read the actual SWR curves a seller publishes, or better, measure your own.

The Windom Name and Where It Comes From

Many hams call any off-centre-fed wire a Windom, and many commercial OCFDs are sold under that name. Strictly speaking the original Windom antenna was a related idea from the early days of amateur radio that used a single-wire feeder connected off centre, with the feeder itself radiating as part of the system. Modern versions replaced the single wire with a balun and coax, which is a different and more practical arrangement.

The name matters only so far as it helps you search for information. When you read an article or forum thread about a “Windom”, check whether it means the classic single-wire version or a modern coax-fed OCFD, because the advice on feed line and grounding differs. For the rest of this guide, OCFD means the coax-fed version with a balun at the feed point.

Building an OCFD: Wire, Lengths and Feed Point

An OCFD is mechanically simple: two lengths of wire, an insulator at each end, a balun box at the feed point and support ropes. The detail that matters is getting the two legs and the offset right for your target bands.

Choosing the overall length

Start from the lowest band you want. The total length is close to that of a normal half-wave dipole for that band, and the usual dipole formulas or an online calculator give a sensible starting point. Treat any formula as a starting point only. Wire insulation, height, nearby objects and the balun all shift the resonant frequency, and with an OCFD you are trying to place several resonances at once, so you will almost always trim on the air.

Choosing the offset

Published designs place the feed point anywhere from about a third of the length to a little less. The short leg and long leg ratio affects the feed impedance and the balance between bands. If you are building from a published design, follow its ratio exactly rather than mixing numbers from different sources. If you want to experiment, antenna modelling software lets you move the feed point and watch the impedance on each band before you cut any wire. Our antenna modelling walkthrough shows how to set up a simple wire model.

Wire and hardware

• Wire: stranded copper or copper-clad steel in a gauge strong enough for the span. Copper-clad steel stretches less over time, which helps an antenna that has to keep several resonances in place.
• Insulators: standard dog-bone or egg insulators at the ends. The ends of any dipole carry high RF voltage, so keep them clear of trees, gutters and people.
• Balun box: weatherproof, with strain relief so the wire tension is carried by the box and not by the solder joints.
• Support: the balun box adds weight off centre, so the wire sags more on one side. Plan your supports so the feed point is not dragged down.

The Balun: Ratio, Type and Power Handling

The balun is the heart of an OCFD and the part most often skimped on. It has two jobs at once. It transforms the feed impedance down towards fifty ohms, and it should present a high impedance to common mode current so the coax shield does not become part of the antenna.

For most OCFD designs the ratio is 4:1. Some designs use 6:1 to suit a different offset or a lower height. The type matters as much as the ratio. A current balun, also called a Guanella balun, forces equal and opposite currents into the two legs and generally behaves better on an asymmetric antenna than a voltage balun. Many builders now use a current balun for the transformation and add a separate choke for common mode suppression, because a single core rarely does both jobs perfectly over a wide frequency range. Our guide to baluns and ununs explains the difference between the main types in more detail.

Power handling is the other trap. Because the OCFD presents an imperfect match on some bands, the balun core can run warm, especially at high duty cycle modes such as FT8 or RTTY, and especially if you also use it on a band where the antenna is far from resonance. If you plan to run an amplifier, choose a balun rated with plenty of margin for continuous modes, and check the core temperature after a long transmission during testing.

Common Mode Current: The OCFD’s Weak Spot

Because the two legs are different lengths, they do not present a symmetrical load to the feed point. Even with a good current balun, some current tends to flow on the outside of the coax shield. When that happens, the coax radiates, the pattern changes, the SWR shifts as you touch the radio or move the cable, and RF can appear in microphones, USB audio interfaces and computer speakers.

The cure is a common mode choke. Many OCFD owners put one right at the feed point, built into or alongside the balun, and a second one further down the coax, roughly where the cable drops away from the antenna or enters the house. The second choke breaks up the remaining length of shield so it cannot act as a long vertical radiator on any band. Our article on common mode chokes covers how to build and test one.

A simple test shows whether you have a problem. Watch the SWR on each band while you hold and move the coax near the radio. If the reading changes noticeably, current is flowing on the shield. Another sign is RF feedback that appears only on some bands. Add choking until the effect goes away rather than living with it.

Height and Layout

An OCFD obeys the same height rules as any horizontal wire. Low to the ground, it radiates mostly upwards, which is good for regional contacts and poor for long-distance DX. Higher up, the main lobe drops towards the horizon. Height in wavelengths is what counts, so an antenna that is high on 20 meters can still be low on 80 meters. Our article on antenna height and takeoff angle explains the trade-off with examples.

Layout choices that work well:

• Flat top: the classic layout, with the best pattern on the design bands, if you have two supports at a good distance.
• Inverted V: one central support with both legs sloping down. Because the feed point is off centre, the support goes at the balun, not at the middle of the wire, so the two legs slope at different angles.
• Sloper or bent layouts: possible in a tight garden, but every bend shifts the resonances. Expect more trimming.

Keep the long leg and the short leg in a reasonably straight line where you can. Folding one leg back towards the other degrades the pattern and changes the impedance on the higher bands more than people expect.

Tuning an OCFD on Several Bands

Tuning an OCFD is a balancing act, because the resonances on the different bands are linked. Shortening the whole antenna raises every resonance; changing the ratio between the legs shifts the bands relative to each other.

1. Measure first. Put the antenna at its final height and sweep it with an antenna analyser. A sweep that covers all HF bands at once tells you far more than spot readings. A NanoVNA or similar analyser makes this easy once it is calibrated; our blog covers how to calibrate and read a sweep.
2. Tune the lowest band first. Trim both legs in proportion to keep the offset ratio, until the lowest band is where you want it.
3. Check the harmonics. Look at where the dips land on the higher bands. If they are all high or all low together, the overall length needs adjustment. If one band is off while others are fine, the cause may be the offset or local surroundings.
4. Accept compromise. An OCFD rarely gives a perfect match on every band. A low SWR across the parts of each band you use most, with the internal tuner smoothing the edges, is a good result.

OCFD Compared With Other Multiband Wires

The OCFD is one of several ways to get multiple bands from a single wire. Each approach makes different trade-offs.

If you want every band with the lowest losses and are happy to use an external tuner, the doublet with ladder line is hard to beat. If you want coax feed, no tuner on the main bands and a single run of wire, the OCFD is a strong candidate.

Common OCFD Mistakes

• Relying on the balun alone for choking. Add a dedicated common mode choke, and test for current on the coax.
• Expecting every band. Plan for the odd harmonic band to be poor, and use a different antenna or a wide-range tuner there.
• Undersized balun for digital modes. High duty cycle modes heat cores that cope fine with SSB.
• Trimming only one leg. That changes the offset ratio as well as the length, and can move higher bands in unexpected directions.
• Tuning at ground level. Resonances move once the antenna is at its final height, so make final adjustments with the antenna up.

The Bottom Line

The off center fed dipole earns its popularity: one wire, one coax run and a balun can put you on four or more HF bands with little or no tuning. Its secret is the offset feed point, which gives a similar impedance on the fundamental and its even harmonics. Its weakness is the same asymmetry, which pushes current onto the coax unless you choke it properly. Choose a good current balun with power margin, add a separate common mode choke, mount it as high and straight as you can, and tune it at its final height. Do that, and the OCFD is a dependable everyday antenna for DX, contesting and casual operating.

For further reading, the ARRL Antenna Book reference covers multiband wire antennas in depth, and the general background on dipole antennas and baluns is a useful refresher on the underlying theory.

Frequently Asked Questions

Which bands does an off center fed dipole cover?

An OCFD typically matches on its fundamental band and the even harmonics, so a version cut for 80 meters usually loads on 80, 40, 20 and 10 meters, with some other bands possible using a tuner. Bands near the odd harmonics, such as 15 meters for an 80 or 40 meter design, are usually poor.

What balun does an OCFD need?

Most designs use a 4:1 balun at the feed point, and some use 6:1. A current balun generally works better than a voltage balun on this asymmetric antenna, and many builders add a separate common mode choke as well.

Why does my OCFD put RF in the shack?

The two legs are different lengths, so current tends to flow on the outside of the coax shield. Add a common mode choke at the feed point and another further down the coax, then check whether the SWR still changes when you move the cable.

Is an OCFD the same as a Windom antenna?

Not exactly. The original Windom used a single-wire feeder connected off centre. Modern OCFDs use a balun and coax. Many products and articles use the two names interchangeably, so check which design is being described.

Can I put up an off center fed dipole as an inverted V?

Yes. Place the central support at the balun rather than at the middle of the wire, so the short and long legs slope down at different angles. Expect the resonances to shift compared with a flat top, and retune at the final height.

Do I still need an antenna tuner with an OCFD?

Often not on the design bands, where the SWR can be low enough for the radio alone. A tuner, internal or external, helps at band edges and on bands where the antenna is not designed to match.