FD4 or One-End Offset Feed: Fit the Whole Antenna

The FD4 earned its place: one long off-centre-fed wire offered several useful bands without a separate antenna for each. The practical difficulty is getting that whole wire, its offset feedpoint and the coax route into a real garden. My objection is not to its age. It is to treating the familiar layout as the answer before looking at the site.

Where the full span and a suitable feedpoint route are available, a properly installed FD4 remains a sensible choice. Where they are not, I prefer to redistribute the antenna into a shorter main branch fed from an accessible end and a deliberately routed return branch. That can put the useful wire where the supports allow it, instead of forcing two conventional arms into poor positions. The advantage is installation freedom with a declared current path—not a claim that part of the antenna has vanished.

The Useful Comparison Is Span and Routing

An 80 m FD4-type wire is roughly a 40-metre-class structure. A one-end offset-fed system can devote less of its total current path to the main elevated span and put the remainder into a separate counter-conductor or a defined section of coax exterior. That is redistribution, not automatic electrical miniaturisation.

Keep the Historical Names and the Actual Circuits Separate

Loren Windom’s 1929 article described a horizontal wire fed off centre by a single-wire feeder. That feeder was part of the radiating system. A modern FD4 is normally a two-arm, coax-fed off-centre-fed dipole. A third architecture deliberately uses a counterpoise or a declared section of coax exterior as a return branch. These are not interchangeable drawings.

The distinction is practical. If the feed-line exterior is meant to be quiet, movement of the coax should not become an informal tuning control. If a defined exterior section is the return branch, choking it at the transformer would suppress part of the intended antenna. A sloper, inverted-L or flattop main branch can each be useful, but bending or lowering a conductor changes its coupling and pattern; it is not permission to ignore the return route.

Off-Centre Does Not Automatically Mean Unbalanced

Moving a dipole feedpoint away from the centre changes the impedance sampled along its standing-current distribution. It does not repeal the differential-current relationship at a two-terminal feedpoint. In an ideal two-arm OCF dipole, the wanted current entering one arm has an equal return through the other arm even though the physical lengths differ.

Installed balance is a separate question. Unequal height, ground clearance, nearby trees or metalwork, transformer capacitance and a feed line routed close to one arm can couple the two terminals differently to their surroundings. That mode conversion can drive current on the coax exterior. The antenna may then include the feed line, mast and station wiring even though the sketch shows only two arms.

A 4:1 Ratio Is Not a 200-Ohm Guarantee

The familiar arithmetic is limited: an ideal 4:1 impedance transformer maps 200 + j0 Ω to 50 + j0 Ω. An installed FD4 does not present 200 + j0 Ω on every desired band. Its feedpoint impedance is complex and varies with wire length, feed position, frequency, conductor diameter, height, ground, nearby objects and any current on the feed line.

A transformer scales the load within its operating limits; it does not remove reactance or create resonance. On a multiband antenna, the same physical feedpoint samples a different current distribution on each band. A ratio useful on one band can present an awkward impedance on another.

Choose the ratio from measured or credibly modelled R + jX at the declared reference plane across all required frequencies. Then include transformer loss, tuner range, feed-line attenuation, voltage and current stress. A low SWR at the transmitter is not by itself an efficiency, temperature or pattern measurement.

A Moderate Feed Impedance Is Worth Keeping

The useful off-centre-feed idea is to choose a feed position and current distribution that avoid the extreme impedance of a near-end half-wave feed over the bands of interest. When the resulting load calls for 4:1 transformation, the ideal voltage/turns ratio is 2:1; a 49:1 impedance transformation is 7:1. The lower required transformation is a real design simplification compared with that kilohm-class end-feed problem.

It gives the designer a less extreme voltage ratio and winding task to solve, but does not prove lower loss merely by counting turns. Magnetising inductance, voltage per turn, material, leakage, capacitance and the actual load still set the useful frequency range. Fair-Rite’s transformer guidance describes that low-frequency inductance versus high-frequency parasitic compromise.

A moderate ratio is an advantage the two architectures can share. A conventional OCF/FD4 arrangement can also present a load suited to a moderate-ratio transformer. The reason to choose the one-end architecture here is how its main and return branches fit the installation while retaining an appropriate feed impedance. Their different conductor layouts—not the number 4:1—decide which gives the more useful installation.

Transformation and Common-Mode Control Are Separate Jobs

A transformer transfers the wanted signal between impedance levels. A common-mode choke adds impedance to an exterior-current loop. Some assemblies perform both functions, but the measurements still need to establish both behaviours under the intended load.

• For a genuinely balanced coax-fed OCF dipole, a suitable measured current balun can transform impedance while presenting common-mode impedance at the balanced port.
• For a deliberately unbalanced transformed port or return branch, my practical default is a 4:1 UNUN for transformation plus a separately specified 1:1 choke for common-mode control.
• For an installation that does not preserve textbook balance, separating the functions makes the transformer transfer and the choke boundary independently testable. It does not make a weak transformer or choke acceptable.

The 4:1 value in my default is conditional on the measured load calling for that ratio. It is not a promise that every FD4, OCF or offset-fed wire wants a 4:1 transformation.

The Choke Position Defines the Return Branch

A choke is finite impedance inserted into a complete common-mode circuit. Its useful position depends on which conductor is meant to radiate, the impedance of the exterior-current path, coupling to ground and nearby structures, and the choke’s complex impedance at each operating frequency.

For a coax-fed FD4 intended as a two-arm dipole, common-mode control normally begins at the feedpoint boundary. For a design that intentionally uses a counterpoise or coax-exterior section, the choke belongs where that declared branch ends. There is no universal quarter-wave distance that works on every band and installation.

A second choke near the station can protect a different boundary, but it cannot erase radiation, pickup or loss on the coax section ahead of it. Make cable-route changes only with the transmitter de-energized, then stand clear of the antenna and feed system and repeat the measurements at a safe low power. A substantial tuning change after rerouting is evidence that the installed current boundary needs investigation. Never touch an energized antenna or feed system.

Multiband Behaviour Includes More Than SWR

The FD4 earned its reputation because one long wire can offer usable impedances on several bands. “Usable” still depends on the transformer, feed line, tuner and exact operating frequencies. One fixed arm split does not guarantee every amateur band, and a successful match does not prove that the same current path is preserved everywhere.

As the wire becomes electrically longer, its far-field pattern develops more lobes and nulls. Suppressing feed-line current can make the installed pattern more repeatable, but it cannot turn a multi-wavelength wire into a frequency-independent radiator. An intentional return branch produces another three-dimensional pattern that must be evaluated as part of the antenna.

Receive noise is equally site-specific. Controlling one feed-line pickup route may help, yet local fields can couple directly to the radiator, mast, feed line and station wiring. No balun, UNUN or choke label guarantees a lower noise floor.

Make the Better-Fitting Layout a Complete Design

For the two-arm FD4, first establish that both arms and the feed route can occupy suitable positions. For the one-end alternative, lay out the main branch and the entire return before calling it compact. A return running through a noisy room, against metalwork or too close to lossy ground may surrender the very benefit sought by moving the main wire.

Then qualify that chosen layout. These checks support the installation decision; they are not a reason to postpone making it:

Compare the complete systems at the same reference planes:

• measure complex feedpoint impedance across every required band;
• state whether the coax exterior is excluded or intentionally used;
• measure exterior current at repeatable points while changing feed-line routing;
• verify transformer and choke loss, temperature, voltage and current under representative power, waveform and duty cycle;
• include tuner and feed-line loss rather than comparing transmitter SWR alone; and
• compare received signal, noise and field response with controlled A/B/A tests when those outcomes matter.

Primary and Authoritative Sources

• Loren G. Windom, “Notes on Ethereal Adornments,” QST, September 1929—the original single-wire-fed architecture and its intended feeder-current distribution.
• Roy Lewallen, W7EL, “Baluns: What They Do and How They Do It”—coaxial current modes, imbalance and current-versus-voltage balun behaviour.
• Tom Rauch, W8JI, Windom and Off-Centre-Fed Antennas—historical and modern architectures, feedpoint impedance and common-mode boundaries.
• Tom Rauch, W8JI, Common-Mode Current—installed current paths and the coax exterior as a separate conductor.
• ITU-R Report SM.2158-3—differential/common-mode definitions and mode conversion caused by imbalance.
• C. L. Ruthroff, “Some Broad-Band Transformers”—primary transmission-line-transformer analysis and practical bandwidth limitations.

Practical Conclusion

Keep the FD4 when its full two-arm structure fits well and does the job on the bands and paths you use. When that layout forces poor wire placement or awkward feedpoint access, I prefer the one-end offset-fed arrangement with a shorter main span and a deliberately allocated return route. That is a concrete improvement in how the antenna can be installed, not an assertion that it radiates more power in every direction.

The return must count in the drawing, the clearance and the current model. A shorter visible span is useful only when the complete antenna fits better. Do not turn the coax beyond the intended boundary, the mast or the shack into the missing half by accident.

For a deliberately unbalanced transformed port or return branch, I prefer to keep the jobs explicit: use a 4:1 UNUN when the measured load supports that ratio, then place a separately specified choke at the intended antenna boundary. For a genuinely balanced OCF dipole, a measured current balun may be exactly right. Preserve the moderate-impedance advantage where the load provides it, put the conductors where they can work, and choose the box for that actual system.

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