End-Fed Half-Wave Design Tool

EFHW antenna calculator: end-fed half-wave design and length

An EFHW (end-fed half-wave) antenna is a resonant wire antenna approximately one half wavelength long on its design band and fed at one end. Because the feed point is near a voltage maximum, its impedance is much higher than 50-ohm coax and normally requires an impedance transformer. A properly designed EFHW can also operate on several harmonically related amateur bands, making it popular for portable, field, and compact multiband installations.

Calculate your EFHW How an EFHW works
EFHW calculator

Calculate your EFHW antenna

Set the principal band, exact design frequency and the wire correction factor you want to use. The calculator returns the free-space wavelength, half-wave reference length, corrected radiator length and harmonic-frequency references.

The band provides operating context; the exact frequency controls the calculated length.
Enter the exact design frequency in MHz.
Enter the practical correction factor for the selected wire and construction. K is normally less than 1.
Used as installation context in the calculated summary.
Operating principle

How an end-fed half-wave antenna works

An EFHW uses the same basic half-wave resonance as a center-fed dipole, but the feed point is moved to the end of the radiator. Current is low and RF voltage is high at that point, so the feed-point impedance is typically in the thousands of ohms rather than near the 50–75 ohms seen around the center of a half-wave dipole. The complete system therefore includes the wire radiator, a matching transformer or unun, the feed line, and a return path that may involve a short counterpoise and/or the outside of the coax shield.

Resonance and radiator length

The starting EFHW antenna length comes from the wavelength at the selected design frequency. A free-space half wave is then shortened by a practical correction factor because conductor diameter, insulation, end effects, installation height, bends, and nearby objects change the electrical length. The calculator should therefore provide a starting dimension, not a promise of the final cut length.

Impedance and matching

At the end of a resonant half-wave wire, resistance is very high and some reactance may remain in a real installation. A high-ratio impedance transformer converts that load into a range that 50-ohm coax and the transceiver can handle. A 49:1 transformation is common, but the best ratio depends on the measured antenna impedance, transformer construction, and installation.

Operation on harmonics

A wire that is one half wavelength on the principal band becomes multiple half wavelengths long at harmonically related frequencies. That can produce additional high-impedance resonances and useful multiband operation. Real resonance positions do not follow perfect integer multiples, however, so SWR, impedance, and radiation pattern should be checked separately on every band you plan to use.

Design approaches

Resonant EFHW vs non-resonant end-fed wire

A resonant EFHW and a non-resonant end-fed wire may look similar in the field, but they are designed around different electrical goals. The EFHW length is intentionally related to a half wavelength on a principal band, while a non-resonant end-fed wire is deliberately chosen to avoid troublesome resonant lengths on the bands of interest. That difference changes the impedance range, transformer ratio, tuner requirements, and the way multiband coverage is obtained.

Resonant EFHW

A resonant EFHW is cut for a selected frequency and is expected to show a resonance near that frequency plus possible resonances on harmonically related bands. Its end impedance is usually very high, so a high-ratio transformer such as 49:1 is common. Some installations can operate on intended bands without a tuner, while others still benefit from final matching.

Non-resonant end-fed wire

A non-resonant end-fed wire uses a length selected to keep its impedance within a more manageable, but still elevated, range across several bands. It normally relies more heavily on an antenna tuner and often uses a lower transformation ratio. The exact behavior depends strongly on wire length, counterpoise, feed line, installation geometry, and frequency.

Parameter Resonant EFHW Non-resonant end-fed wire
Length selection Calculated around 1/2 wavelength on a principal band Chosen to avoid problematic resonances on intended bands
Feed-point impedance Very high near end-fed resonance; often several kΩ Usually lower than EFHW resonance but varies widely by band
Typical transformation High-ratio transformer/unun; 49:1 is common Often a lower-ratio unun, depending on design
Tuner role May be optional on well-aligned resonant bands Usually an important part of multiband matching
Multiband behavior Uses harmonically related resonances Uses a compromise impedance profile across bands
SWR curve Expected minima near designed resonances SWR minima are not the design target
Adjustment priority Trim resonance positions and verify matching Choose a workable wire length, then tune each band
Calculator parameters

EFHW antenna length calculator

Choose the principal amateur band and the exact design frequency, then set the wire correction factor appropriate for your conductor. The calculator should return the estimated half-wave radiator length and identify harmonically related frequencies or bands for further checking. Use the result as a cut-length starting point and leave a small trimming margin for field tuning.

Operating band and design frequency

The band selection gives the calculator a useful operating context, while the design frequency sets the exact point around which the EFHW is sized. Choose a frequency near the part of the band you expect to use most often rather than relying only on the nominal band label.

Calculated wire length

The initial radiator length is based on one half of the corrected wavelength. In practice, the final EFHW antenna length may change after installation because height, bends, insulation, nearby conductors, transformer leads, and the surrounding environment alter resonance. Cut slightly long when possible and trim only after measurement.

Wire correction factor

The correction factor accounts for the difference between an ideal free-space half wave and a real wire antenna. Conductor diameter, conductivity, insulation dielectric, and end effects all influence electrical length. If the exact factor is unknown, use a conservative starting value and expect final adjustment with an antenna analyzer.

Harmonic bands

The calculator can flag frequencies related to the fundamental by integer harmonics and map them to amateur bands where appropriate. Treat these as candidates for multiband operation, not guaranteed low-SWR bands: transformer behavior, geometry, and installation can shift each resonance.

Calculation method

EFHW calculation formulas

The calculator should keep the basic physics separate from practical wire correction. First determine the free-space wavelength from frequency. Then convert one half wavelength into a starting physical wire length using a correction factor. Harmonic frequencies can be generated as integer multiples of the fundamental, but the finished antenna must still be measured because real resonances shift with construction and installation.

Wavelength

For frequency f in MHz, the free-space wavelength is:

λ (meters) = 299.792458 / f (MHz)

Here, λ is wavelength and f is the design frequency. For quick field work, 300/f is often close enough, while the calculator can retain the more precise constant.

Radiator length

Start with one half of the free-space wavelength, then apply a practical correction factor K for the selected wire and construction:

Lwire = (λ / 2) × K

K is normally less than 1 for a practical wire. The final installed length should remain adjustable because environmental effects are not captured perfectly by a single factor.

Harmonic frequencies

Ideal harmonically related frequencies are calculated from the principal frequency f0:

fn = n × f0, where n = 2, 3, 4 …

These values are reference points. An actual EFHW multiband antenna can show resonance offsets, especially on higher bands, so harmonic coverage must be verified with measured impedance and SWR.

Geometry

EFHW antenna configurations

The same EFHW radiator can be installed in several geometries. The total electrical length remains the design starting point, but the shape changes height requirements, polarization components, coupling to ground and nearby objects, and the azimuth/elevation pattern. When comparing configurations, model or measure the antenna in the geometry you will actually use.

Horizontal EFHW

Run the wire approximately horizontal between two supports, with the transformer at one end. This layout is straightforward when span is available and generally emphasizes horizontal polarization. Height above ground strongly affects the elevation pattern, especially on the lowest design band.

Inverted-V EFHW

Support the wire near its center region and slope both portions downward. An inverted-V reduces the required horizontal span and is convenient for portable deployment from a single mast or tree. The included angle and end heights change feed impedance and the mix of radiation angles.

Sloper EFHW

Raise one end and slope the radiator toward the other end. A sloper needs only one high support and can be quick to install for field operation. The slope introduces both horizontal and vertical field components, and the pattern can become directional depending on height and frequency.

Inverted-L or vertical EFHW

Use a vertical section with the remaining wire horizontal or sloping, or deploy most of a shorter-band EFHW vertically when sufficient support height exists. This saves ground footprint and adds vertical polarization, but feed-point placement, ground proximity, common-mode current, and RF voltage require careful installation.

Physical installation

Wire, height, and EFHW antenna installation

A useful EFHW design calculator should treat geometry and materials as more than cosmetic inputs. Real resonance depends on the conductor, insulation, height, bends, support positions, soil and nearby structures. These variables can shift impedance and radiation even when two antennas use the same nominal wire length, so the final installation should be measured in place.

Conductor

Copper and copper-clad wire are common because of their conductivity and availability. Diameter changes mechanical strength, RF resistance, bandwidth, and end effects. Insulation adds dielectric loading and can make the physical length required for resonance different from bare wire.

Installation height

Height controls ground coupling and has a major effect on the elevation pattern. A low EFHW may favor higher-angle radiation on the lower bands, while greater height can lower some radiation angles. The feed point must also be positioned safely because the end of a resonant EFHW can carry high RF voltage.

Angles and section layout

In an inverted-V, inverted-L, or sloper, the section lengths and angles determine the wire’s coordinates in space. Changing those angles can alter feed impedance, polarization, lobe direction, and resonance. Use the intended geometry during both modeling and final analyzer measurements.

Nearby objects

Metal roofs, gutters, fences, masts, vehicles, wet foliage, building wiring, and even the ground can couple to the antenna. These effects are difficult to predict with a simple length formula and are a common reason the measured resonant frequency differs from the calculated value.

Matching system

EFHW transformer, unun, and impedance matching

The end of a half-wave radiator is a high-voltage, low-current feed point with an impedance commonly in the thousands of ohms. Direct connection to 50-ohm coax would create severe mismatch, so most practical EFHW antenna designs use a broadband impedance transformer or unun at the feed point. The transformer ratio should be treated as a design choice confirmed by measurement, not as a fixed property of every EFHW.

Feed-point impedance

At resonance, the resistive component at the very end of a half-wave wire is high, while a real antenna may also show residual reactance. The actual R + jX value depends on geometry, height, ground, the return path, and where the feed connection sits relative to the electrical end of the radiator.

Transformation ratio

A 49:1 impedance ratio transforms about 2,450 ohms toward 50 ohms and is therefore widely used in EFHW transformers. Other ratios, including 64:1 or higher, can be appropriate when the measured feed-point resistance differs. Select the ratio to suit the antenna and transformer bandwidth rather than copying a number without testing.

Ferrite and transformer construction

Ferrite material, core size, winding geometry, turns ratio, interwinding capacitance, and lead length all affect loss and broadband behavior. A transformer that works at QRP power may overheat or saturate at higher power. High-power or 1.5 kW EFHW designs require appropriately rated cores, wire, spacing, enclosure, and thermal margin.

Antenna tuner

A well-aligned resonant EFHW may present an acceptable match on its intended bands without a tuner, but a tuner can still be useful for small resonance offsets or bands that are only approximately harmonic. A tuner cannot recover power already lost in a poor transformer or lossy feed line, so first correct the antenna system itself.

Feed system

Counterpoise, coax feed line, and common-mode current

An end-fed antenna still needs a return path for RF current. Depending on the design, that path can involve a dedicated counterpoise, the outside surface of the coax shield, grounding capacitance, and coupling to nearby objects. Because the feed system can become electrically active, changes in coax length or routing may change the measured impedance and SWR.

Counterpoise

A short counterpoise can provide a more defined return path and may reduce dependence on the coax. There is no single counterpoise length that is correct for every EFHW; practical designs often start with a small fraction of a wavelength and then verify the result. Keep the counterpoise clear of people and conductive objects during testing.

Coaxial cable

The coax is not always electrically invisible in an EFHW system. Its length, loss, shield current, and routing can influence analyzer readings and RF in the station. Measure the antenna with the feed line arrangement you intend to use, and avoid assuming that a good reading at the radio proves the feed point itself is well matched.

Common-mode current and choke

Common-mode current flows on the outside of the coax shield when the feed line becomes part of the RF return path. It can cause pattern distortion, RF on equipment, noise pickup, and measurement changes. A properly designed common-mode choke can reduce that current, but its placement and choking impedance should match the actual installation.

Performance

Radiation pattern and performance

Once the wire geometry and installation are defined, radiation plots help explain where RF energy is likely to go. EFHW patterns change substantially with frequency because a multiband wire becomes electrically longer on the higher bands. A configuration that looks almost dipole-like on its principal band can develop multiple lobes and nulls on harmonic bands.

Azimuth pattern

The azimuth plot shows gain around the horizon at a selected elevation angle. Use it to identify broadside directions, lobes, and nulls. The result is meaningful only for the modeled frequency, geometry, ground, and reference elevation angle.

Elevation pattern

The elevation plot shows how radiation is distributed from the horizon upward at a selected azimuth. Lower-angle lobes are generally relevant to longer-distance HF paths, while high-angle energy can support shorter-range ionospheric coverage on suitable bands.

Polarization

Polarization analysis separates horizontal and vertical field components. A horizontal EFHW is mainly horizontally polarized, a vertical EFHW mainly vertically polarized, and slopers or inverted-L layouts can produce a mixed result that varies by direction and frequency.

Gain and directivity

Calculated gain, lobe direction, and beamwidth describe the modeled pattern rather than transmitter power. Compare these values only under the same ground and geometry assumptions, and remember that loss in the transformer, feed line, or ground reduces realized system performance.

Frequency response

EFHW impedance, SWR, resonance, and bandwidth

Frequency-domain measurements connect the physical antenna to the matching system. The most useful view is not a single SWR number but the combination of resistance, reactance, transformed impedance, and SWR across a frequency range. This makes it easier to see whether a low-SWR point is a true usable resonance or simply the result of loss and transformation.

Impedance

Feed-point impedance is written as R + jX. R is the resistive component and X is reactance: positive X is inductive and negative X is capacitive. At a simple resonance, reactance crosses or approaches zero, but the resistance at the end of an EFHW remains much higher than 50 ohms before transformation.

SWR

SWR describes mismatch relative to the reference impedance, normally 50 ohms after the matching network. A lower SWR means less reflected power at that measurement point, but it does not by itself measure antenna efficiency. Loss in coax or ferrite can reduce reflected power and make the transmitter-end SWR look better.

Resonance

On an impedance or Smith chart, resonance is associated with reactance passing through or near zero. The measured resonant frequency can shift from the calculated frequency because the installed wire has a different electrical length than the ideal model. Trim or adjust only after the antenna is in its normal geometry.

Operating bandwidth

Bandwidth is the frequency span over which the system meets a chosen matching or performance criterion. Conductor diameter, transformer response, height, loss, and the impedance slope around resonance all influence the result. Define the threshold used by the calculator instead of presenting bandwidth as an absolute property.

Multiband operation

EFHW multiband operation and harmonic bands

Multiband EFHW operation comes from using one radiator as an integer number of half wavelengths on higher, harmonically related frequencies. A 40-meter EFHW is a familiar example because useful resonances can appear on 20, 15, and 10 meters. This relationship is only a starting model: end effects, transformer capacitance, geometry, and feed-system behavior can move the higher-band resonances enough to require adjustment.

For every candidate band, check the actual transformed impedance, SWR, and radiation pattern. Higher harmonics also produce more lobes and nulls, so a band can match well while radiating very differently from the principal-band pattern.

Band / reference Calculated / analyzed frequency Harmonic number Resonance status Calculated SWR / match status
Run the calculator with harmonic analysis enabled to generate this table.

Harmonic-frequency arithmetic does not predict the installed SWR by itself. The table therefore identifies calculated frequency references only; resonance and matching must be verified from the actual antenna or a suitable model.

System efficiency

Feed-line loss, transformer loss, and real system efficiency

SWR measured at the transmitter can differ from SWR at the antenna feed point. A lossy coaxial cable attenuates both forward and reflected waves; when the reflected wave is attenuated on its trip back to the radio, the displayed SWR can look lower even though part of the transmitted power has been lost as heat. The same principle applies to other lossy parts of the system.

Coax attenuation

Feed-line loss rises with frequency and can increase further under high mismatch. A long or small-diameter coax run may therefore reduce the apparent SWR at the radio while also reducing the power that reaches the antenna.

Ferrite and winding loss

Transformer cores and windings dissipate power through magnetic and conductor losses. Heating is a practical warning sign. Core mix, size, turns, flux density, frequency, duty cycle, and power level all determine whether the transformer remains efficient.

Common-mode suppression loss

A choke intentionally presents high impedance to current on the outside of the coax. A properly designed choke suppresses unwanted current with limited loss, but an undersized or poorly chosen ferrite can heat and add avoidable dissipation.

Tuner and matching-network loss

Inductors, capacitors, relays, traces, and connectors in a tuner have finite Q and resistance. Loss tends to become more important when the tuner must transform an extreme impedance or carry high circulating current or voltage.

Ground and object coupling

RF coupled into lossy soil, building materials, vegetation, wiring, or metal structures may be absorbed rather than radiated usefully. Installation height and geometry can therefore affect efficiency even when the analyzer shows a convenient match.

Connector and conductor loss

Poor solder joints, corroded connectors, undersized wire, and long transformer leads add series resistance and can create heat or unstable readings. Good mechanical and electrical construction matters more as current, frequency, or power increases.

A lower transmitter-end SWR is not proof of higher radiation efficiency. Matching and efficiency are separate properties of the antenna system.

Resonance correction

Inductive loading for higher-band resonance correction

Some EFHW designs add a small series inductance to shift one or more higher-band resonance points without changing the entire wire length. This is a tuning technique rather than a universal requirement. The coil value and position should be selected from the behavior of the specific antenna, then confirmed by measurement across all bands of interest.

Purpose of the loading coil

A series inductor adds positive reactance at its location and changes the electrical behavior of the wire above and below it. In a multiband EFHW, this can move a higher-band SWR minimum toward the desired part of the band while preserving a useful principal-band resonance.

Coil calculation

A coil calculator can use target inductance, mean coil diameter, winding length or spacing, wire diameter, and number of turns to estimate a single-layer air-core coil. Treat the geometry as a starting point: lead length, winding pitch, nearby materials, and self-capacitance affect the real RF inductance.

L(µH) = N²R² / (9R + 10W)

Wheeler form for a single-layer air-core coil, with the dimensions in inches.

Loss and Q

Coil Q compares stored reactive energy with resistive loss. Larger conductor diameter and a physically larger coil can reduce RF resistance, while tight winding, small wire, unsuitable form materials, or excessive current can increase heating. A tuning improvement is useful only if it does not introduce excessive loss.

Field adjustment

How to tune a DIY EFHW antenna after installation

The calculated dimensions are a starting point. Final tuning should be done with the complete EFHW antenna installed in its normal position, using the transformer, counterpoise, coax length, and geometry you intend to operate with. Make small changes, record the result, and recheck all relevant bands after each adjustment.

  1. Build slightly long

    Cut the radiator with a modest trimming margin and assemble the intended transformer, counterpoise, and feed line. Avoid final trimming on the ground or in a temporary geometry.

  2. Install in the real configuration

    Raise the EFHW as the planned horizontal, inverted-V, sloper, inverted-L, or vertical arrangement. Keep high-voltage wire ends away from people and accessible conductive objects.

  3. Measure with an analyzer

    Sweep the principal band and each intended harmonic band. Record resonance position, R, X, and SWR rather than looking only at the lowest SWR number.

  4. Adjust the radiator length

    If the principal resonance is too low in frequency, shorten the wire in small increments. If it is too high, restore length or adjust the construction. Re-measure after every change.

  5. Refine matching and higher bands

    If the principal band is correctly placed but matching is poor, investigate transformer ratio, counterpoise, coax common-mode current, and transformer construction. Use an inductive correction only when the higher-band behavior justifies it.

  6. Verify the complete system

    Recheck all operating bands at low power, then confirm transformer/choke heating and RF behavior under normal transmit conditions. Save the final wire, geometry, feed-line, and analyzer values as the build reference.

Calculated result

Calculated EFHW antenna parameters

This summary should combine the selected inputs and the values actually calculated or modeled for the current EFHW design. Show only fields for which the application has reliable input or output data, and keep modeled values clearly distinguished from measured values.

Parameter Calculated / selected value Unit or note
Run the EFHW calculator to populate the current design summary.
FAQ

EFHW antenna FAQ

How long should an EFHW antenna be?

Start with one half of the wavelength at the chosen design frequency, then apply a correction factor for the real wire. The installed antenna normally needs final trimming because insulation, conductor diameter, height, bends, transformer leads, and nearby objects shift the electrical length. An EFHW antenna calculator should therefore give a starting length, not an absolute final dimension.

What transformer ratio does an EFHW antenna use?

A 49:1 impedance ratio is one of the most common choices because it transforms roughly 2,450 ohms toward 50 ohms. However, an actual end-fed half-wave can present a different resistance and reactance, so 64:1 or another ratio may work better in some designs. Choose the transformer from measured impedance, bandwidth, ferrite performance, and power requirements.

Does an EFHW antenna need a tuner?

Not always. A resonant EFHW with a suitable transformer can often reach an acceptable SWR on its principal band and some harmonic bands without an antenna tuner. A tuner can correct modest residual mismatch, but it cannot fix a lossy transformer, excessive common-mode current, or a badly placed resonance. Measure and correct the antenna system before relying on the tuner.

Does an EFHW need a counterpoise?

Every end-fed antenna needs an RF return path, but that path can be distributed among a dedicated counterpoise, the outside of the coax shield, and capacitive coupling to the surroundings. A short counterpoise often makes behavior more repeatable and can reduce feed-line involvement. The best length is installation-dependent, so verify it with analyzer and common-mode measurements.

Can one EFHW work on several amateur bands?

Yes, that is one reason the EFHW multiband antenna is popular. A radiator resonant as a half wave on its principal band can become multiple half wavelengths long at higher harmonic frequencies. Useful examples often occur on 40/20/15/10 meters, but the exact resonance, SWR, and radiation pattern must be checked on each band rather than assumed from harmonic math alone.

How high should an EFHW antenna be installed?

There is no single correct EFHW antenna height. Greater height generally reduces ground interaction and can change the elevation pattern, while low installations can favor higher-angle radiation on lower HF bands. Practical height is also limited by available supports and safety. Model or measure the antenna at the height you can actually use, and keep high-voltage ends out of reach.

Can I install an EFHW vertically?

Yes. An EFHW vertical antenna is still a half-wave radiator fed near a high-impedance end, so the matching principle remains similar. A vertical or near-vertical installation changes polarization, ground interaction, feed-line routing, and radiation angle compared with a horizontal wire. On lower HF bands the required support height can be substantial, making slopers and inverted-L layouts more practical.

Why did the resonant frequency change after I installed the antenna?

The simple length formula does not include every real-world coupling effect. Wire insulation, diameter, wet conditions, height, bends, trees, metal structures, the transformer, counterpoise, and coax can all shift resonance. Tune the antenna in its final position with the complete feed system attached, and make small length changes while watching both reactance and SWR.

Can this calculator be used for EFHW kits and high-power designs?

It can be used to estimate radiator length for a DIY EFHW antenna or to sanity-check the geometry of an EFHW antenna kit, including products such as an ARRL EFHW kit, PackTenna end-fed half-wave, or MyAntennas EFHW-8010. Do not override manufacturer dimensions or power ratings from the calculator alone. A 1.5 kW EFHW antenna also requires a transformer, ferrite, wire, insulation, connectors, and spacing specifically engineered for that power level.