The End Effect in Verticals, Wire Antennas, OCFD, and End-Fed Matching

The physical end of an antenna wire is not always the same as its electrical end. That small difference is called the end effect, and it explains why practical antennas often need to be shorter than simple textbook wavelength formulas suggest.

The end effect helps us build shorter dipoles, useful top-loaded verticals, and high-impedance end-fed half-wave antennas. But it also makes multiband wires imperfect, shifts resonances, interacts with nearby objects, and often exposes common-mode current problems when the antenna system has no clean return path.

What the End Effect Really Is

At the open end of a wire antenna, RF current falls toward a minimum and RF voltage rises toward a maximum. That high-voltage point capacitively couples to the surrounding world: ground, trees, buildings, masts, support ropes, the other half of the antenna, the operator, and sometimes the outside of the coax shield.

This added capacitance makes the wire behave electrically longer than its tape-measure length. In practice, that means the physical wire needed for resonance is usually shorter than a simple free-space wavelength calculation.

For reference, full wavelength is often approximated as:

A free-space half wave is therefore often approximated as:

But a common practical starting point for a thin HF dipole is closer to:

The difference is roughly five percent. That difference is not caused by one single effect. It includes end effect, conductor diameter, insulation, height above ground, nearby objects, feed hardware, and installation details. The formula is a starting point, not a law.

At 7.1 MHz, the free-space half-wave calculation gives:

The common practical dipole starting formula gives:

So a practical 40-meter half-wave dipole often ends up several feet, or roughly one meter, shorter than the simple free-space number.

The same principle applies to vertical antennas. A free-space quarter wave is often approximated as:

But a practical quarter-wave vertical often starts closer to:

At 7.1 MHz, this gives approximately:

After that, it must still be trimmed in its real location, with its real ground system, real feedpoint hardware, and real surroundings.

Why the End Effect Helps

The end effect is not only a problem. It is one reason practical antennas can be a little shorter than their textbook electrical length. A 40-meter dipole is not normally 70 feet, or about 21.3 meters, long in a real installation. It is often closer to 66 feet, or about 20.1 meters. A 40-meter vertical is not always 35 feet, or about 10.7 meters, tall. It may tune nearer 33 feet, or about 10.1 meters, depending on conductor diameter, top hardware, radial system, and surroundings.

The effect can also be increased deliberately. A capacity hat, T-top, umbrella wire system, top wire, or large metal tip adds capacitance at the high-voltage end of a vertical. That makes the vertical electrically longer without making it physically taller.

This is why top-loading is so useful on short low-band verticals. It raises current higher on the radiator, improves radiation resistance, and can reduce loading-coil loss compared with placing all the loading at the base.

End feeding also depends on end behavior. A half-wave wire fed at the center has high current and relatively low voltage, so the feed impedance is modest. The same half-wave wire fed at the end has low current and high voltage, so the feed impedance is very high.

That is the useful side of the end effect. It gives us shorter practical antennas, top-loaded verticals, high-impedance end-fed half waves, and convenient matching opportunities.

Why the End Effect Makes Life Difficult

The problem is that the end effect does not apply equally to every part of a multiband antenna. It is strongest at physical, unterminated, high-voltage ends. Internal current and voltage nodes along a harmonic antenna are not physical open wire ends, so they do not receive the same shortening effect.

This is why a wire that is resonant on 80 meters is not automatically resonant at perfect arithmetic multiples on 40, 20, 15, and 10 meters. The harmonic relationship may be close, but it is not exact.

The external wire ends are shortened by end effect. The internal half-wave sections on harmonic operation do not behave exactly like separate physical antennas with their own open ends. The result is that higher-band resonances often drift from the simple paper prediction.

This matters a lot for multiband dipoles, off-center-fed dipoles, end-fed half waves, and so-called random wires. The wire may be calculated beautifully on paper and still show SWR dips in unexpected places. The wire was not wrong; the simplified model was incomplete.

The difficulty becomes worse because high-voltage ends are sensitive to their surroundings. A wire end close to a wet tree, metal gutter, mast, fence, wall, roof edge, balcony railing, or support pole can shift resonance. Rain, ice, insulation, drooping ends, bends, and nearby conductive objects all change the effective capacitance.

Verticals: The End Effect Plus the Other Half

A vertical antenna is only half of the story. The other half is the ground, radial system, counterpoise, vehicle body, balcony railing, metal roof, mast, or coax shield.

In a quarter-wave vertical, current is highest near the base and voltage is highest near the top. The end effect at the top makes the vertical electrically longer. But the feed impedance and real-world efficiency are dominated by the return system.

A perfect vertical over a poor ground system is not a perfect antenna. It is a vertical plus a loss resistor.

This is why a low SWR at the base of a vertical is not proof of efficiency. A poor ground system can make the antenna easier to match because ground loss adds resistance. The transmitter may be happy, while part of the RF power is warming the soil, mast, coax shield, or station wiring.

A good vertical installation normally needs radials, elevated tuned radials, a counterpoise, a ground screen, a metal vehicle body, or another deliberate conducting reference. A common-mode choke is then used to prevent the coax shield from becoming an uncontrolled extra radial.

The Rybakov Vertical as a Practical Example

The Rybakov-style antenna is usually built as a non-resonant vertical wire around 7.5 to 8 meters long, or about 24.6 to 26.2 feet, often supported by a fishing pole, fed through a 4:1 unun, and used with radials or a counterpoise plus a tuner.

The Rybakov works because it avoids being a simple quarter-wave or half-wave radiator on most HF bands. A common 7.6-meter radiator is about 24.9 feet long.

At 14.2 MHz, near the 20-meter band, the wavelength is approximately:

So a 7.6-meter, or 24.9-foot, radiator is about 0.36 wavelength. That is close to 3/8 wave and can give a useful low-angle vertical pattern.

At 7.1 MHz, near the 40-meter band, the wavelength is approximately:

So the same 7.6-meter, or 24.9-foot, radiator is only about 0.18 wavelength. It is short and capacitive, so efficiency depends heavily on the radial system and matching loss.

At 28.5 MHz, near the 10-meter band, the wavelength is approximately:

So the same 7.6-meter, or 24.9-foot, radiator is about 0.72 wavelength. At that point it is electrically long, and the pattern becomes more complex, with multiple lobes and nulls.

The end effect shifts all of these electrical lengths slightly. A 7.6-meter wire is not just 7.6 meters electrically. Its top end capacitively couples to the environment. Its bottom end couples through the transformer, radial field, ground, mast, and coax shield.

This is where common mode enters. A Rybakov has one obvious vertical radiator, but RF current must return somewhere. With a good radial field, most of the return current flows in the radials. With a poor or missing counterpoise, the outside of the coax shield becomes part of the antenna.

That can make the Rybakov seem to tune better, but it also means the feed line is radiating, receiving local noise, changing the pattern, and possibly bringing RF into the shack.

The 4:1 unun is not magic. It transforms impedance by a factor of four. If the antenna feed impedance is around 200 ohms resistive, a 4:1 transformation brings it near 50 ohms. If the feed impedance is 600 − j800 ohms, or 25 + j300 ohms, the 4:1 unun does not magically make it a 50-ohm antenna. It only gives the tuner a different problem to solve.

Off-Center-Fed Antennas and Why the Ends Matter

An off-center-fed dipole, often called an OCFD or Windom-style antenna, is a half-wave or near-half-wave wire fed away from the center. Moving the feed point changes the impedance. Center feed gives a lower impedance. Moving outward raises the impedance. End feed gives a very high impedance.

A common OCFD feed point is chosen to produce something near 200 ohms on several bands, so a 4:1 transformer can bring that toward 50 ohms. But the antenna is asymmetric: one side is long and the other side is short.

That asymmetry makes common-mode current more likely than in a well-balanced center-fed dipole. A 4:1 transformer may transform the differential impedance, but it may not provide enough common-mode isolation by itself. In many real installations, especially low OCFDs on the lower HF bands, an additional 1:1 choke is needed.

End effect also explains why OCFDs are not perfect harmonic antennas. The physical wire ends receive end-effect shortening. The internal harmonic sections do not receive the same treatment. So a carefully cut 80-meter OCFD may not place all higher-band resonances exactly where simple arithmetic says they should be.

What 4:1, 9:1, 49:1, 64:1, and 81:1 Really Mean

The ratio printed on a transformer is usually the impedance ratio, not the turns ratio. Impedance is expressed in ohms, so there is no imperial or metric version of these values.

A 4:1 transformer is common for OCFDs, some loops, and Rybakov-style verticals. For an OCFD, it should normally be part of a balanced or hybrid feed arrangement because the antenna is fundamentally a balanced wire antenna fed with coax. For a vertical or random wire, the system is unbalanced, so an unun is normally the correct transformer type.

A 9:1 unun is usually used with non-resonant end-fed wires or random wires. It assumes the tuner will finish the job. It is not an EFHW transformer. It works best when the wire length avoids exact half-wave multiples on the bands of interest, because a half-wave end-fed wire can present thousands of ohms, far above what a 9:1 unun was meant to handle.

A 49:1 transformer is normally associated with an end-fed half-wave antenna. A half-wave wire fed at the end has high voltage and low current, so its impedance is often in the thousands of ohms. A 49:1 transformer maps about 2450 ohms to 50 ohms, which explains why it is popular.

A 64:1 or 81:1 transformer may be useful when the actual end impedance is higher. This can happen with certain wire heights, thinner conductors, different layouts, or higher harmonic operation. But higher ratios are not automatically better. They increase voltage stress, can increase core heating, and make stray capacitance more troublesome on the upper HF bands.

The Common-Mode Problem: Where the Missing Half Goes

Common-mode current is often described as RF current flowing on the outside of the coax shield. That is a useful practical description, but the deeper idea is broader.

This is especially important with end-fed, off-center-fed, and vertical antennas because they are naturally unbalanced or asymmetric.

A choke does not match the antenna. A choke blocks an unwanted current path. Matching and choking are different jobs.

For a vertical with radials, the system should ideally look like this:

For an end-fed wire with a separate counterpoise, the system should look like this:

For an EFHW with no separate counterpoise, the coax shield becomes the counterpoise whether we admit it or not. Choking directly at the transformer can sometimes make the antenna unstable because the return path has been removed. A better approach is often to provide a defined counterpoise, or allow a short controlled length of coax shield to act as the return path and then place a choke beyond that point.

For an OCFD, the feed point should normally include real common-mode control. A voltage balun alone can make the SWR look acceptable while allowing feedline radiation.

How the End Effect and Common Mode Interact

The end effect is strongest where RF voltage is high. Common-mode trouble is strongest where the antenna system has not been given a clean return path. Those two problems often meet at the same place: the feed point of an end-fed antenna, the base of a vertical, or the feed point of an off-center-fed dipole.

A Rybakov with too few radials may use the coax shield as the missing radial. The operator sees an acceptable tune, but the coax is radiating.

An EFHW with a 49:1 transformer may tune beautifully, but the transformer, coax shield, and station wiring are still part of the return system. The RF voltage at the feed end is high, so nearby objects and feedline layout affect tuning.

An OCFD with only a 4:1 voltage balun may show reasonable SWR on several bands, while the feedline carries common-mode current because the two antenna legs are unequal.

A random wire with a 9:1 unun may work on many bands, but if no counterpoise is provided, the tuner, radio chassis, microphone cable, power supply, operator, and coax shield may all become part of the antenna.

Practical Rules That Save Trouble

• Cut wires a little long. Install the antenna in its final location and trim afterward. End effect depends on the real environment.
• Do not choose a transformer ratio by antenna name alone. A 4:1 transformer belongs near 200 ohms, a 9:1 near 450 ohms, a 49:1 near 2450 ohms, and a 64:1 near 3200 ohms.
• Do not use SWR as the only success measurement. A lossy ground, lossy transformer, or radiating feedline can produce a friendly SWR.
• Give every unbalanced antenna a deliberate return path. Verticals need radials or a counterpoise. End-fed wires need a counterpoise or controlled coax-shield return. OCFDs need common-mode isolation.
• Use a choke for common-mode current, not as a substitute for matching. The transformer handles differential-mode impedance. The choke controls the outside of the coax shield.
• Expect multiband wires to be imperfect. End effect skews harmonic resonances. This is normal, especially on OCFDs and EFHWs.

Conclusion

The end effect is both friend and enemy.

It lets practical wires be shorter than textbook lengths. It makes top-loaded verticals possible. It gives end-fed half-wave antennas their useful high impedance. It helps compact antennas work.

But it also ruins perfect harmonic assumptions, shifts resonances, makes wire ends sensitive to nearby objects, raises feedpoint voltage, and exposes common-mode current when the return path is not well defined.

The Rybakov, OCFD, random wire, and EFHW can all be good antennas when understood as complete systems: radiator, end effect, feed impedance, transformer, return path, feedline, choke, and environment.

Ignore any one of those pieces and the antenna may still work, but the coax, shack wiring, ground loss, or nearby environment may be doing more of the work than expected.

Interested in more technical content? Subscribe to our updates for deep-dive RF articles and lab notes.

Questions or experiences to share? Feel free to contact RF.Guru for practical antenna and RF system advice.