An End-Fed Half-Wave (EFHW) antenna is a popular wire antenna configuration used by amateur radio operators because it can provide a relatively simple way to build an HF antenna without requiring a center feed point. However, determining the appropriate wire length is an important first step, and the theoretical length is not always the same as the final length required after installation.

The End-Fed Half-Wave Antenna Calculator helps estimate the wire length required for a target resonant frequency. Enter the desired frequency in MHz and a wire shortening factor, and the calculator provides the free-space wavelength, ideal half-wave length, practical starting cut length, trimming allowance, quarter-wavelength height reference, and common EFHW feed-system references.

For example, at 7.1 MHz, the calculator determines an ideal half-wave length of approximately 21.13 meters. With the default shortening factor of 0.95, the recommended starting cut length is approximately 20.07 meters (65.8 feet).

The calculated value should be treated as a starting point for antenna construction and tuning, rather than an exact final dimension. Actual resonance can change because of wire characteristics, antenna geometry, height, surrounding objects, and installation conditions.

What Is an End-Fed Half-Wave Antenna?

An End-Fed Half-Wave antenna, commonly abbreviated EFHW, is a wire antenna with an electrical length of approximately one-half wavelength at its intended operating frequency and a feed point located near one end of the wire.

Unlike a conventional center-fed half-wave dipole, where the feed point is approximately in the middle, an EFHW is fed at or near one end. This changes the impedance at the feed point significantly. A half-wave wire has a relatively high impedance near its ends, so an impedance transformation system is commonly used between the antenna and transmission line.

The basic concept is straightforward:

Radio frequency → impedance transformer → EFHW wire

The wire provides the radiating element, while the transformer helps match the antenna's high feed-point impedance to the lower impedance normally expected by the feed line and transceiver.

EFHW antennas are popular for several practical reasons. They can be constructed from relatively simple wire, installed in different configurations, and used in portable or fixed amateur-radio setups. Their physical layout can be adapted to available space, although changing the geometry can also change the antenna's electrical behavior.

The key design parameter is the resonant frequency. Once the target frequency is known, the approximate wavelength can be calculated and the corresponding half-wave length estimated.

That is where an EFHW antenna calculator becomes useful.

How the End-Fed Half-Wave Antenna Calculator Works

The calculator uses the target resonant frequency and a wire shortening factor to estimate a practical starting length.

There are two primary inputs:

• Resonant Frequency: entered in MHz
• Wire Shortening Factor: a multiplier between 0.85 and 1.00

The calculator uses 0.95 as its default shortening factor.

1. Enter the Resonant Frequency

Start by entering the frequency at which you want the EFHW to operate.

For example:

7.100 MHz

Frequency has a direct relationship with wavelength. As frequency increases, wavelength decreases. As frequency decreases, wavelength increases.

This means a lower-frequency EFHW generally requires a longer wire than an EFHW designed for a higher frequency.

The calculator accepts frequencies from 0.1 MHz to 300 MHz, although the practical application of an EFHW design depends on the intended antenna system and operating frequency.

2. Calculate the Free-Space Wavelength

The calculator first determines the approximate free-space wavelength using:

Wavelength (m) = 300 / Frequency (MHz)

For a target frequency of 7.1 MHz:

Wavelength = 300 / 7.1

Wavelength ≈ 42.25 meters

This is the approximate free-space wavelength corresponding to 7.1 MHz.

3. Calculate the Ideal Half-Wave Length

An EFHW uses approximately half of that wavelength as its theoretical electrical length.

The calculator therefore uses:

Ideal Half-Wave Length = Wavelength / 2

This can also be expressed as:

Ideal Half-Wave Length (m) = 150 / Frequency (MHz)

For 7.1 MHz:

Ideal Half-Wave Length = 150 / 7.1

Ideal Half-Wave Length ≈ 21.13 meters

This is the theoretical half-wave dimension before applying the calculator's shortening factor.

4. Apply the Wire Shortening Factor

The calculator then converts the theoretical half-wave dimension into a practical starting cut length:

Starting Cut Length = Ideal Half-Wave Length × Shortening Factor

Using the default shortening factor of 0.95:

Starting Cut Length = 21.13 × 0.95

Starting Cut Length ≈ 20.07 meters

The calculator also converts this result into feet:

20.07 meters ≈ 65.8 feet

This is why the calculator describes the value as a starting cut length. It should not be interpreted as a guaranteed final resonant wire length.

EFHW Antenna Length Formula

The basic theoretical calculation is simple.

First calculate the wavelength:

λ = 300 / f

Then calculate the half-wave length:

L = 150 / f

Where:

• λ = wavelength in meters
• L = theoretical half-wave length in meters
• f = frequency in MHz

For a practical starting estimate using the calculator's shortening factor:

Starting Cut Length = (150 / f) × Shortening Factor

For example, at 7.1 MHz with a 0.95 shortening factor:

Starting Cut Length = (150 / 7.1) × 0.95

Starting Cut Length ≈ 20.07 meters

The formula provides a useful starting dimension, but antenna construction is not purely a mathematical exercise. The actual resonant frequency can be influenced by the physical environment and construction details.

Therefore, the most effective workflow is:

Calculate → Cut → Install → Measure → Trim → Re-measure

Understanding the Wire Shortening Factor

The wire shortening factor is a multiplier applied to the theoretical half-wave length to produce a practical starting estimate.

The calculator allows a value between 0.85 and 1.00, with 0.95 as the default.

For example:

A factor of 1.00 leaves the theoretical half-wave length unchanged. A factor of 0.95 reduces the theoretical length by approximately 5%.

However, there is no single shortening factor that will produce the exact resonant length for every EFHW installation.

The effective electrical length can be affected by factors such as:

• Wire diameter
• Wire insulation
• Antenna height
• Antenna configuration
• Nearby buildings
• Trees and other objects
• Conductive structures
• Ground and surrounding environment
• Feed-system characteristics

For that reason, the 0.95 default should be viewed as a starting assumption, not a universal EFHW constant.

If your measured resonance differs from the desired frequency, the antenna can be adjusted incrementally.

7.1 MHz EFHW Calculation Example

Let's use the calculator's default values to demonstrate a complete calculation.

Input

Resonant Frequency: 7.100 MHz

Shortening Factor: 0.95

Step 1: Calculate wavelength

Wavelength = 300 / 7.1

≈ 42.25 meters

Step 2: Calculate ideal half-wave length

Half-Wave Length = 42.25 / 2

≈ 21.13 meters

Step 3: Apply the shortening factor

Starting Cut Length = 21.13 × 0.95

≈ 20.07 meters

Step 4: Convert to feet

20.07 × 3.28084 ≈ 65.8 feet

So, for a 7.1 MHz target frequency and a 0.95 shortening factor, the calculator produces:

• Free-Space Wavelength: 42.25 m
• Ideal Half-Wave Length: 21.13 m
• Starting Cut Length: 20.07 m
• Starting Cut Length: 65.8 ft

The actual wire should be installed and measured before making final tuning decisions.

Understanding the Calculator Results

The calculator provides several outputs designed to help with both antenna planning and initial construction.

Resonant Frequency

This is the frequency entered by the user and represents the intended operating or tuning frequency.

For example:

7.100 MHz

The target frequency is the starting point for calculating wavelength and antenna length.

Free-Space Wavelength

The calculator determines:

Wavelength = 300 / frequency

At 7.1 MHz, the result is approximately 42.25 meters.

This value is used to derive the theoretical half-wave length.

Ideal Half-Wave Length

The ideal half-wave length is half the calculated wavelength.

At 7.1 MHz:

≈ 21.13 meters

This represents the theoretical starting point before applying the shortening factor.

Starting Cut Length

This is the practical estimate produced after applying the shortening factor.

At 7.1 MHz with a 0.95 factor:

≈ 20.07 meters

The calculator also provides the value in feet:

≈ 65.8 feet

This is one of the most useful outputs when physically constructing the antenna.

Suggested Trimming Allowance

The calculator calculates a trimming allowance equal to approximately 2% of the starting cut length:

Trimming Allowance = Starting Cut Length × 0.02

For a 20.07-meter starting length:

20.07 × 0.02 ≈ 0.40 meters

This provides a reference for allowing some additional wire during the initial tuning process.

In practical antenna construction, it is generally preferable to begin with sufficient wire and make controlled adjustments rather than cutting the antenna shorter than required from the beginning.

Wire Shortening Factor

The calculator displays the selected shortening factor so that users can clearly see which multiplier was applied to the theoretical length.

The default is:

0.95

Users can change this value within the calculator's supported range.

Suggested Minimum Apex Height

The calculator calculates a quarter-wavelength reference:

Height Reference = Wavelength / 4

At 7.1 MHz:

42.25 / 4 ≈ 10.56 meters

This value should be understood as a quarter-wavelength reference for planning, not a universal minimum height that guarantees optimal EFHW performance.

Actual antenna installation height and geometry depend on the available site, intended configuration, safety considerations, and operating requirements.

Typical Feed Transformer

The calculator lists:

49:1 impedance transformer

A 49:1 transformer is a common reference for EFHW systems because the feed point can present a much higher impedance than the transmission line and transceiver are designed to handle directly.

The exact transformer implementation should be appropriate for the antenna and operating system.

Typical End-Fed Impedance

The calculator provides a reference range of:

2,500–5,000 ohms

This represents the high-impedance nature associated with an end-fed half-wave feed point.

The actual feed-point impedance can vary, so this should be treated as a typical reference rather than a guaranteed value for every antenna.

Why EFHW Antennas Need an Impedance Transformer

One of the biggest differences between an EFHW and a conventional center-fed dipole is the impedance at the feed point.

Near the end of a half-wave wire, the RF voltage can be relatively high while current is relatively low. This results in a high feed-point impedance.

A typical transceiver and coaxial feed system are designed around much lower impedances. Connecting a high-impedance EFHW feed point directly to a conventional low-impedance system would therefore not provide the desired impedance relationship.

An impedance transformer can bridge that difference.

A commonly used reference is a 49:1 impedance transformer.

The transformer provides impedance transformation between the high-impedance antenna feed point and the lower-impedance feed system.

However, it is important not to interpret "49:1" as meaning that every EFHW will have exactly the same feed-point impedance. Real antennas vary with frequency, geometry, construction, surroundings, and installation.

The calculator therefore provides the 49:1 transformer as a typical feed-system reference, rather than a guarantee of a particular antenna design.

How to Use the End-Fed Half-Wave Antenna Calculator

Using the calculator is straightforward.

Step 1: Choose your target frequency

Determine the frequency you want the antenna to be designed around.

Enter the value in MHz.

For example:

7.100 MHz

Step 2: Select a shortening factor

The calculator defaults to:

0.95

You can adjust the value within the supported range of 0.85 to 1.00 if you have a specific reason to use a different starting assumption.

Step 3: Calculate the antenna length

The calculator will provide:

• Wavelength
• Ideal half-wave length
• Starting cut length
• Starting length in feet
• Trimming allowance
• Quarter-wavelength height reference
• Feed transformer reference
• Typical high-impedance feed-point reference

Step 4: Prepare the wire

Use the calculated starting length as a construction reference.

Avoid treating the number as the guaranteed final length.

Step 5: Install the antenna

Install the antenna in the configuration you intend to use for operation.

The physical installation matters because changing the geometry or surroundings can affect the antenna's measured behavior.

Step 6: Measure the antenna

Use appropriate RF measurement equipment, such as an antenna analyzer, to determine the actual resonant behavior.

Step 7: Trim gradually

If adjustment is required, make small changes rather than removing a large amount of wire at once.

Step 8: Measure again

Repeat the measurement after each meaningful adjustment.

This iterative approach is more reliable than expecting a mathematical formula to predict the exact final wire length in every real-world installation.

Why Calculated EFHW Length May Differ From Actual Resonance

A calculator can provide an excellent starting point, but real antennas operate in physical environments rather than ideal mathematical conditions.

Several factors can cause the actual resonant frequency to differ from the calculated value.

Wire Diameter

The physical characteristics of the conductor influence its electrical behavior. Changing conductor dimensions can affect the relationship between physical length and electrical length.

Wire Insulation

Insulated wire can behave differently from bare wire because the insulation changes the electromagnetic environment around the conductor.

Antenna Height

The height of the antenna above the ground and surrounding structures can affect its electrical characteristics.

Installation Geometry

An EFHW can be installed in different configurations, including sloping and other practical wire arrangements.

Changing the shape of the wire can change the antenna's electrical behavior.

Nearby Objects

Metal structures, buildings, other wires, trees, and other surrounding objects can interact with the antenna.

This means two antennas made using exactly the same wire length can behave differently when installed in different environments.

Feed System

The transformer, feed line, grounding arrangement, and other components can also influence measurements and overall system behavior.

For these reasons, the calculator should be viewed as a design and planning tool, not as a replacement for actual antenna measurements.

EFHW Antenna Tuning: From Calculated Length to Resonance

Calculating the initial wire length is only the first stage of building an EFHW.

A practical tuning workflow is:

Calculate → Cut → Install → Measure → Trim → Re-measure

Start with the calculated wire length and install the antenna in its intended configuration. Then measure the antenna's resonant behavior.

If the measured resonance is not where you want it, make a small physical adjustment and measure again.

As a general tuning principle, if the resonant frequency is lower than the desired frequency, shortening the electrical length will generally move resonance upward. If resonance is higher than desired, additional electrical length may be required.

The exact response can depend on the antenna system and installation, so measurements should guide the tuning process.

This is also why cutting the wire significantly shorter than the calculated starting length can create unnecessary problems. It is much easier to remove small amounts of excess wire than to add wire back after cutting.

EFHW Antenna Height and Installation Considerations

Antenna height is an important consideration when planning an EFHW installation.

The calculator provides a quarter-wavelength reference height:

Reference Height = Wavelength / 4

For a 7.1 MHz target:

42.25 / 4 ≈ 10.56 meters

This value is useful for planning, but it should not be interpreted as a universal minimum installation height.

There is no single height that guarantees optimal performance for every EFHW.

The practical installation depends on:

• Available space
• Antenna configuration
• Target frequency
• Surrounding structures
• Ground conditions
• Safety requirements
• Intended operating environment

For example, a portable operator may have to work with a much different installation than someone constructing a permanent antenna system.

The most important point is consistency during tuning. If you measure the antenna in one configuration and then substantially change its height or geometry, the antenna's resonant characteristics can also change.

Common EFHW Calculator Mistakes

Even a straightforward calculator can produce misleading results if the inputs or outputs are misunderstood.

Mistake 1: Treating the calculated length as exact

The calculated value is an estimate.

The starting cut length should be used as a practical construction reference, followed by measurement and tuning.

Mistake 2: Entering the wrong frequency

The calculator expects frequency in MHz.

For example:

7.1 MHz

should not be entered as a value representing 7.1 GHz or another unit.

Mistake 3: Ignoring the shortening factor

The shortening factor directly changes the starting wire length.

A factor of 0.95 produces a shorter starting length than a factor of 1.00.

Mistake 4: Cutting the wire too short

Removing too much wire at the beginning can make tuning difficult.

Starting with sufficient wire and trimming gradually provides more flexibility.

Mistake 5: Changing the installation while tuning

Changing antenna height, orientation, or geometry can change its measured characteristics.

Try to keep the physical configuration consistent while making tuning adjustments.

Mistake 6: Assuming every EFHW requires exactly the same transformer

A 49:1 transformer is a common reference, but antenna feed-point impedance and transformer requirements depend on the particular system.

Frequently Asked Questions About EFHW Antennas

What is an End-Fed Half-Wave antenna?

An End-Fed Half-Wave antenna is a wire antenna approximately one-half wavelength long at its target frequency and fed near one end. Because the feed point can have relatively high impedance, an impedance transformer is commonly used.

How do you calculate EFHW antenna length?

A basic theoretical half-wave length can be calculated using:

Length (m) = 150 / Frequency (MHz)

For a practical starting estimate, the result can be multiplied by a shortening factor:

Starting Length = (150 / Frequency) × Shortening Factor

The final antenna length may require adjustment after installation and measurement.

What is the formula for a half-wave antenna?

First calculate wavelength:

Wavelength = 300 / Frequency (MHz)

Then calculate half-wave length:

Half-Wave Length = 150 / Frequency (MHz)

These formulas provide an approximate free-space starting point.

How long is a 40-meter EFHW antenna?

The exact physical length depends on the chosen operating frequency and practical shortening factor. For example, at 7.1 MHz, the calculator produces an ideal half-wave length of approximately 21.13 meters and a starting cut length of approximately 20.07 meters using a 0.95 shortening factor.

What shortening factor should I use for EFHW?

The calculator uses 0.95 as its default shortening factor and supports values from 0.85 to 1.00. However, a shortening factor is an estimate, not a universal value. Actual antenna resonance should be checked after installation.

Does an EFHW need a 49:1 transformer?

A 49:1 impedance transformer is a common EFHW feed-system configuration because the antenna's end feed point can have relatively high impedance. However, the appropriate transformer depends on the specific antenna and feed system.

What impedance does an EFHW have?

An EFHW feed point can present a relatively high impedance. This calculator uses 2,500–5,000 ohms as a typical reference range. Actual impedance can vary significantly depending on antenna design, frequency, geometry, and installation.

How do I tune an EFHW antenna?

Start with a calculated wire length, install the antenna in its intended configuration, measure its resonant behavior, make small adjustments, and measure again. A practical workflow is:

Calculate → Install → Measure → Trim → Re-measure

Does antenna height affect EFHW resonance?

Yes. Antenna height and installation geometry can affect the antenna's electrical behavior. This calculator provides a quarter-wavelength value as a planning reference, but it is not a universal minimum height for every EFHW installation.

Is the calculator result the final antenna length?

No. The calculator provides a theoretical half-wave length and a practical starting cut length based on the selected shortening factor. The final length should be determined through measurement and incremental tuning after installation.

Final Takeaway: Calculate First, Tune Second

Designing an End-Fed Half-Wave antenna starts with understanding the relationship between frequency, wavelength, and physical wire length.

The End-Fed Half-Wave Antenna Calculator simplifies that initial calculation by determining the free-space wavelength, ideal half-wave length, and a practical starting cut length based on a user-selected shortening factor.

For example, at 7.1 MHz, the calculator produces an ideal half-wave length of approximately 21.13 meters. With the default 0.95 shortening factor, the starting cut length is approximately 20.07 meters (65.8 feet).

The calculator also provides a trimming allowance, quarter-wavelength height reference, 49:1 transformer reference, and typical high-impedance feed-point range.

The key principle is simple: calculate first, then tune based on real measurements. Real-world antenna behavior depends on wire characteristics, installation geometry, height, nearby objects, and the feed system. Therefore, calculated dimensions should be treated as a starting point rather than a guaranteed final specification.

Use the End-Fed Half-Wave Antenna Calculator to establish your initial wire length, install the antenna, measure its performance, and make controlled adjustments until it reaches the desired operating range.