Dipole, EFHW or EF-OCF: Same Wire, Different Feed Problems
Dipole, EFHW or EF-OCF: Same Wire, Different Feed Problems
Hang the same wire at the same height, then move the feed connection. What changes first: the radiation pattern, the voltage, or the job you have given the feedline?
RF.Guru working definition: Common-mode current is the non-cancelling phasor-sum current in a specified set of conductors, evaluated at a defined cross-section and using a declared current-direction convention. In the intended differential transmission-line mode, the outgoing and return currents are equal and opposite, so their phasor sum is zero. When they do not cancel, the remaining current must close through another reference or return path—such as the outside of a coax shield, a mast, equipment chassis, station wiring, nearby structures, earth, the operator, or distributed coupling through the environment.
This broader working definition is especially useful in practical antenna systems. On transmit, non-cancelling current on the outside of the coax can make the feedline and connected structures part of the radiating antenna system unless that path is intentional, clearly defined and properly controlled—for example by providing the required return path and placing a suitable common-mode choke at the correct boundary.
“An EFHW is just a dipole fed at the end.” That is a useful starting point, but a poor stopping point. And declaring an off-centre-fed antenna “definitely not a dipole” does not solve the problem either. I want to separate what the wire does from what we ask its matching and return system to do—first without the maths, then with the numbers that make the practical differences obvious.
The useful distinction: the same current pattern in the same conductor geometry gives the same radiation-pattern shape. Moving the feedpoint changes the impedance we must match. Changing the physical return branch can also change the radiating structure. Those are separate effects, and they lead to different reasons for choosing a dipole, EFHW or EF-OCF.
The No-Maths Story: Keep the Wire Still
Imagine a straight wire roughly half a wavelength long: about 40 m for an 80 m-band example, not an exact cutting instruction. Keep its height, direction, surroundings and shape unchanged. A half-wave standing-wave mode is a repeating RF-current shape with its largest current near the middle and very little at the open ends.
Feed It in the Middle
A centre-fed dipole makes the connection near that current maximum. The two physical arms form the intended current path. The feed impedance is relatively low, which usually makes the matching job less extreme than feeding near an end. A suitable feed arrangement still has to stop the coax exterior from becoming an unintended extra conductor.
Feed It Near an End
An end-fed half-wave, or EFHW, can excite substantially the same current hump from a high-impedance region near one end. If the main wire carries the same relative current and phase, and the other conductors contribute little to the field, its pattern can resemble the centre-fed dipole's. Think of them as twins in the sky with different problems at the feedpoint.
The EFHW needs a matching network and a real complementary return path. Its end voltage can be much higher than the centre-feed voltage. A transformer and choke do not make those facts disappear. Nor does a choke at an arbitrary fixed distance guarantee that only the main wire radiates.
Feed It Off-Centre—or Route Part of It Differently
An off-centre-fed dipole, abbreviated OCF, divides the wire into unequal physical arms. It can offer a feed impedance between the centre and end values. But the dominant half-wave current hump can still remain centred on the whole straight wire: unequal arm lengths do not automatically pull the pattern toward the long side. W8JI's OCF discussion makes this distinction explicitly.
An end-fed off-centre-fed arrangement, or EF-OCF, can look different mechanically. The connection is at the end of the obvious elevated wire, while another conductor provides the other branch. In a coax-return arrangement, an intentional section of coax exterior serves that role up to a defined choke boundary. It is part of the antenna, not invisible plumbing.
Route that branch down a mast or away through the garden and you have changed more than the location of a connection on one straight wire. The complete shape, current distribution and interaction with the surroundings can now differ. That is a physical reason for a different pattern—not a special rule that “the longer arm wins”.
Same Pattern Does Not Mean the Same Feeding Job
This is where the practical advantage of a moderate-impedance feed becomes worth discussing. If the installed load falls in a suitable range, we can use less-extreme impedance transformation and lower terminal voltage than a kilohm-class end feed requires. That can make the matching and insulation task easier without claiming a new radiation law or a guaranteed efficiency ranking.
For an intentionally unbalanced installed port whose measured load calls for 4:1 transformation, my practical default is an appropriate UNUN for that job and a separately specified common-mode choke for the current boundary. A suitable current balun remains a valid choice where the actual load and feed arrangement call for it. The point is to choose the functions deliberately, not to expect one label to explain the installation.
The Numbers Behind the Voltage Difference
Take the same 100 W of accepted real power at the antenna-side terminals in three illustrative, purely resistive loads. These are examples, not measured product impedances or the transmitter's forward-meter reading. Resistance is in ohms, voltage in volts and current in amperes. The voltage and current below are RMS values—the effective values used for average-power calculations with a sinusoidal signal.
I = √(P/R)V = √(P × R)
| Illustrative feed condition | Resistance | RMS current at 100 W | RMS voltage at 100 W |
|---|---|---|---|
| A low-impedance centre-feed example | 70 Ω | 1.20 A | 83.7 V |
| A moderate-impedance off-centre example | 200 Ω | 0.707 A | 141 V |
| A high-impedance end-feed example | 3000 Ω | 0.183 A | 548 V |
The 3000 Ω example needs about 3.87 times the terminal voltage of the 200 Ω example for the same accepted power. Conversely, the 200 Ω case carries more current. Lower voltage does not mean that all component stress has vanished, but it is a concrete reason to prefer a less-extreme load when the site and antenna objective allow it.
In ideal real-resistance arithmetic, 200 Ω needs 4:1 impedance transformation to reach 50 Ω; 3000 Ω needs 60:1. A nominal 49:1 transformer maps 2450 Ω to 50 Ω, not every possible EFHW end impedance. A real matching assembly also has losses and parasitic reactances, and the antenna load usually includes reactance.
For a complex load Z = R + jX, where R is its resistive part and X its reactance, use P = |I|²R and |V| = |I| × |Z|. Do not substitute the magnitude of impedance for resistance in the accepted-power calculation. These values are not touch-safe operating instructions; keep RF conductors and high-voltage ends inaccessible during transmission.
The Technical Layer: Current Shape Before Antenna Names
For an ideal straight thin wire of length L = λ/2, put the coordinate z = 0 at its centre. Here λ is wavelength. A useful approximation to the fundamental current mode is:
I(z) ≈ Imax cos(2πz/λ), for −λ/4 ≤ z ≤ λ/4
Imax is the centre-current amplitude. This idealised hump has a maximum at the centre and zeros at the ends. Real end geometry, loading and the finite feed arrangement perturb it; an actual end feed is not a magical connection to an isolated mathematical zero-current point.
The far field is found by adding the contributions of all those current elements with their direction and phase. MIT's treatment of wire-antenna radiation develops that connection between distributed current and pattern. If the complete relative current distribution and geometry are unchanged, the normalised pattern is unchanged. The absolute field strength can still differ because matching and conductor losses change the power available to radiate.
The Two Arms Do Not Have Independent Radiation Budgets
Split that same wire at an off-centre point. The longer portion may contain more of the high-current region, but the fields from both portions overlap. For one observation direction, write their complex vector field contributions as Eshort and Elong. Then:
Etotal = Eshort + Elong|Etotal|² = |Eshort|² + |Elong|² + 2 Re(Eshort · Elong*)
The star denotes complex conjugation and Re means the real part. The last term is their interference contribution. This is why adding separate current-squared integrals over the two arms does not assign independent shares of radiated power or establish a tilted beam. Current-squared weighting is useful for conductor-heating calculations when the resistance per unit length is specified; radiation requires the coherent field sum.
Moving a bookkeeping boundary along an otherwise unchanged wire does not move its current maximum or shrink the radiating structure. Changing the actual current or physical branch geometry can. That distinction lets us keep the useful off-centre design idea without inventing a long-arm steering mechanism.
High Feed Voltage and the Environment
Voltage also drives electric-field coupling to nearby conductors. For a fixed small coupling capacitance C at frequency f, the current magnitude is |Ic| = 2πfC |Vcoupling|. Here Vcoupling is the voltage between those particular conductors—not automatically the antenna's terminal voltage.
So a high-voltage feed deserves attention to insulation, spacing, mounting and return routing. But a moderate terminal voltage does not prove that both OCF arms are at low voltage relative to a mast or earth, and it does not guarantee a quiet coax exterior. W8JI's end-fed examples illustrate why terminal impedance and the complete return system belong in the same analysis.
On the Higher Bands, Count the Whole Structure
A 40 m wire is approximately 3.8 wavelengths long at 28.5 MHz. It can support several current maxima and phase reversals, producing multiple lobes and nulls. Those are structured interference patterns, not random behaviour. The same warning applies to a long centre-fed wire, an EFHW and an OCF: a match on another band does not recreate the simple fundamental-mode pattern.
Do not discard the short arm from that picture merely because it was labelled a counterpoise. It can be electrically substantial on an upper band. An OCF is not guaranteed fewer or broader lobes than an equal-length EFHW, and neither is guaranteed a preferred beam direction from its name. A genuinely different EF-OCF layout may produce a different useful pattern because its complete current-bearing geometry is different.
What I Would Choose—and Why
- A centre-fed dipole when centre access and feedline routing are convenient: it offers a straightforward low-impedance feed near the fundamental current maximum.
- An EFHW when one-end access solves the support problem and the intended half-wave or bounded dual-band job is clear: accept the high-impedance matching, voltage and return-path requirements as part of the design.
- An OCF or EF-OCF when an intermediate feed impedance and the available main/return routing fit the site better: use that smaller transformation burden and mechanical flexibility deliberately, while retaining the whole return branch in the antenna model.
If you want the one-minute explanation for another operator, it is this: the current makes the pattern; the feedpoint makes a matching problem; the installation decides what else joins the antenna. I would happily choose a moderate-impedance EF-OCF for a site where that solves the real problem. I would not justify it by pretending the short branch does not radiate, or that the long branch automatically points the beam.
Mini-FAQ
- Can an EFHW have the same pattern as a centre-fed dipole? Yes, approximately, when both excite the same wire-current mode in the same geometry and other conductors contribute negligibly. Equal pattern shape does not establish equal matching loss or equal radiated power from the transmitter.
- Does feeding a wire off-centre automatically tilt its pattern? No. Off-centre feeding can change impedance without materially changing the dominant half-wave mode. Pattern differences require a change in the complete current distribution, its phase or the conductor geometry.
- What is different about a coax-return EF-OCF? Part of the coax exterior is an intended radiating or return branch up to a defined choke boundary. Its route belongs to the antenna geometry, so it must be compared with the complete dipole or EFHW system rather than just the visible wire.
- Why does a moderate-impedance feedpoint help? At a specified accepted power and purely resistive load, a lower resistance requires less terminal voltage and more current. A moderate load can reduce the required transformation ratio, but does not by itself prove lower total antenna loss.
- Are the 70, 200 and 3000 ohm examples product specifications? No. They are illustrative purely resistive loads accepting 100 W. Real antenna impedance is complex and changes with frequency and installation.
- Can I calculate each arm's radiated power by adding its current-squared integral? Not independently. Far fields add with phase before their squared magnitude determines power flow, so interference between the arms must be included.
- Does a choke make either antenna independent of its surroundings? No. A suitable choke restricts a defined common-mode path. It does not remove the need for a return path or eliminate coupling to ground, supports and nearby conductors.