The Folded Dipole Antenna Calculator helps estimate the key dimensions and reference characteristics of a folded dipole antenna from its operating frequency. Enter the frequency and a practical shortening factor to calculate the free-space wavelength, approximate total antenna length, length of each side, nominal feed-point impedance, reference directivity, and a wavelength-based mounting-height reference.

A folded dipole is a balanced antenna that uses two parallel conductors connected together at their ends. It is widely used in RF applications because of its relatively high nominal feed impedance and practical construction.

The calculator provides a starting point for antenna design. Actual resonant frequency and impedance can vary with conductor diameter, conductor spacing, installation height, nearby objects, feed-line configuration, and other construction details. For best results, the finished antenna should be measured and tuned after installation.

What Is a Folded Dipole Antenna?

A folded dipole antenna is a variation of the conventional half-wave dipole. Instead of using a single straight conductor on each side of the feed point, a folded dipole uses two parallel conductors that are connected together at their ends.

The feed point is normally located at the center of one of the conductors. Because the conductors are electrically coupled, current flows along both sections of the antenna.

One important characteristic of a standard folded dipole is its nominal feed-point impedance of approximately 300 ohms under commonly used reference conditions. The actual impedance, however, depends on the antenna's physical geometry and surrounding environment.

How Does a Folded Dipole Work?

When an RF signal is applied to the feed point, current flows through the parallel conductors. The antenna's dimensions are related to the wavelength of the operating frequency, allowing the structure to resonate around its design frequency.

Like other dipole antennas, a folded dipole has linear polarization. Its polarization depends on the physical orientation of the antenna. A horizontally installed folded dipole produces horizontal polarization, while a vertically installed antenna produces vertical polarization.

Folded Dipole vs. Standard Half-Wave Dipole

Although both antennas are based on the half-wave dipole concept, their physical structures and feed characteristics differ.

These impedance values should be treated as reference values rather than guaranteed measurements. Real antennas can have substantially different impedance depending on their dimensions and surroundings.

How the Folded Dipole Antenna Calculator Works

The calculator uses the operating frequency to determine the free-space wavelength and then estimates a practical half-wave antenna length.

The main calculation sequence is:

1. Enter the operating frequency.
2. Calculate the free-space wavelength.
3. Apply the practical shortening factor.
4. Calculate the approximate total antenna length.
5. Divide the total length by two to obtain the approximate length of each side.
6. Provide reference values for impedance, directivity, balun transformation, and mounting height.

1. Enter the Operating Frequency

The first input is the antenna's operating frequency in megahertz (MHz).

Frequency has a direct relationship with wavelength. As frequency increases, wavelength decreases. Consequently, an antenna designed for a higher frequency is generally physically shorter than an antenna designed for a lower frequency.

For example, a folded dipole designed around 145 MHz will be considerably shorter than one designed around 14 MHz.

2. Calculate the Free-Space Wavelength

The calculator uses the following approximation:

λ = 300f

Where:

• λ = wavelength in meters
• f = frequency in MHz

For example, at 145 MHz:

λ = 300145

This produces a wavelength of approximately 2.069 meters.

The wavelength is the fundamental reference used to determine the physical dimensions of the antenna.

3. Calculate the Practical Antenna Length

A theoretical half-wave dipole has a length related to half of the wavelength. In practical antenna construction, the physical length is usually somewhat shorter than the simple theoretical value because of electrical effects associated with the conductor and its environment.

The calculator uses:

L ≈ 143f × K

Where:

• L = approximate total antenna length in meters
• f = operating frequency in MHz
• K = practical shortening factor

The calculator uses 0.95 as its default shortening factor.

This should be understood as a design approximation, not a universal correction factor. The optimal value can vary with conductor size, conductor spacing, construction, mounting arrangement, and the surrounding environment.

4. Calculate the Length of Each Side

Once the total antenna length is calculated, the approximate length of each side is:

L_side = L2

This gives a practical starting dimension for constructing the two sides of the folded dipole.

The two sides should not automatically be treated as permanently fixed to the calculated dimension. Final tuning may require small adjustments after the antenna is assembled and installed.

Folded Dipole Antenna Calculator Inputs

The calculator uses two primary inputs.

Frequency

Enter the desired operating frequency in MHz.

For example:

• 7.1 MHz
• 14.2 MHz
• 50 MHz
• 145 MHz
• 433 MHz

The frequency determines the wavelength and therefore strongly influences the physical size of the antenna.

Practical Shortening Factor

The practical shortening factor accounts for the fact that a real antenna's resonant physical length does not necessarily equal the ideal mathematical half-wavelength.

The default value is:

K = 0.95

A value below 1 reduces the calculated physical length relative to the basic 143/f approximation.

However, this factor should not be confused with the velocity factor of a coaxial cable or transmission line. Transmission-line velocity factor and antenna physical-length correction are different concepts.

Folded Dipole Antenna Formulas

Here are the main formulas used by the calculator.

Free-Space Wavelength

λ = 300f

Practical Total Length

L ≈ 143f × K

Length of Each Side

L_side = L2

Half-Wavelength Mounting Reference

H = λ2

The final equation is only a wavelength-based reference. It should not be interpreted as a universal optimum antenna height.

Worked Example: 145 MHz Folded Dipole

Suppose you want to design a folded dipole for 145 MHz and use the default practical shortening factor of 0.95.

Step 1: Calculate Wavelength

λ = 300145λ ≈ 2.069 m

So the free-space wavelength is approximately 2.069 m.

Step 2: Calculate Practical Total Length

Using:

L ≈ 143145 × 0.95

The resulting practical total antenna length is approximately:

L ≈ 0.936 m

Step 3: Calculate Each Side

L_side = 0.9362L_side ≈ 0.468 m

So the calculator provides approximately:

• Wavelength: 2.069 m
• Total antenna length: 0.936 m
• Each side: 0.468 m
• Nominal feed impedance: 300 Ω
• Reference directivity: 2.15 dBi
• Half-wavelength height reference: approximately 1.034 m

These dimensions are starting values. The physical antenna may need adjustment after construction.

Understanding the Calculator Results

Free-Space Wavelength

The free-space wavelength represents the distance an electromagnetic wave travels during one complete cycle in free space.

It provides the fundamental scale for antenna dimensions.

The relationship is straightforward:

Higher frequency → shorter wavelength → shorter antenna

Lower frequency → longer wavelength → longer antenna

Total Antenna Length

The Total Antenna Length is the calculator's practical starting estimate for the folded dipole's overall resonant conductor length.

The result is based on the operating frequency and selected shortening factor.

Because real-world antennas are affected by their construction and environment, you should not assume that this dimension will produce perfect resonance without adjustment.

Length of Each Side

The Length of Each Side is simply half of the calculated total length.

This provides an easy starting point when constructing the two sides of the antenna.

Nominal Feed-Point Impedance

The calculator uses approximately:

Z ≈ 300Ω

as the nominal reference impedance for a standard folded dipole.

Actual feed-point impedance can differ because of factors such as:

• Conductor diameter
• Spacing between conductors
• Antenna geometry
• Mounting height
• Nearby conductive structures
• Feed-line arrangement
• Ground and environmental effects

Therefore, 300 Ω should not be interpreted as an exact measured impedance for every folded dipole.

Reference Directivity

The calculator provides 2.15 dBi as a reference value associated with the ideal half-wave dipole radiation characteristic.

This value should not be interpreted as guaranteed real-world gain.

Actual antenna gain can be affected by:

• Conductor losses
• Installation height
• Ground interaction
• Nearby structures
• Feed-line losses
• Construction accuracy

This is why the calculator labels the value as Reference Directivity rather than measured antenna gain.

Balun and Matching

A folded dipole is a balanced antenna, while coaxial cable is an unbalanced transmission line. A suitable balun or matching arrangement may therefore be required depending on the feed system.

For a nominal 300 Ω antenna connected to a 75 Ω system, a 4:1 impedance transformation is commonly relevant:

3004 = 75Ω

However, a 4:1 transformer should not automatically be used for every folded-dipole installation.

For example, a 50 Ω feed system requires a different matching consideration. The appropriate solution depends on the actual antenna impedance and the target feed-line impedance.

Mounting Height Reference

The calculator provides:

H = λ2

as a simple half-wavelength reference.

For example, if the wavelength is 2.069 m:

H = 2.0692H ≈ 1.0345 m

This is not a universal recommendation for antenna installation. Antenna height can significantly affect radiation pattern and performance, and the best height depends on the application, frequency, surrounding terrain, and nearby structures.

Polarization

A folded dipole produces linear polarization.

Its polarization is determined by its physical orientation.

A horizontal folded dipole is normally horizontally polarized, while a vertical installation is vertically polarized.

For reliable communication, the transmitting and receiving antennas generally need compatible polarization.

How to Build a Folded Dipole Antenna

A basic folded dipole can be constructed using conductive wire or tubing, appropriate insulators, a feed-point connection, and suitable support hardware.

A practical construction workflow is:

1. Select the operating frequency.
2. Calculate the wavelength.
3. Calculate the practical antenna length.
4. Cut the conductor slightly longer than the calculated dimension.
5. Form the folded-dipole structure.
6. Maintain consistent spacing between parallel conductors.
7. Install the feed point.
8. Connect the appropriate balun or matching network.
9. Mount the antenna in its intended orientation.
10. Measure its performance.
11. Make small dimensional adjustments if required.

Leaving some additional conductor length during initial construction can make final tuning easier.

How to Choose the Correct Balun for a Folded Dipole

What Is a Balun?

A balun is a device used to interface balanced and unbalanced circuits and, depending on its design, can also provide impedance transformation.

A folded dipole is a balanced antenna. Coaxial cable, commonly used as a feed line, is an unbalanced transmission line.

A suitable balun or current-choke arrangement can help provide an appropriate interface between the antenna and feed line.

When Is a 4:1 Balun Used?

A 4:1 impedance transformation is relevant when approximately 300 Ω needs to be transformed to approximately 75 Ω:

300Ω → 75Ω

This configuration can be useful with systems designed around 75 Ω.

The correct balun should be selected based on the actual antenna impedance, feed-line impedance, frequency range, power level, and desired matching arrangement.

What About 50 Ω Systems?

A common RF feed-line impedance is 50 Ω. A nominal 300 Ω antenna does not transform to 50 Ω through a simple 4:1 impedance ratio:

300/4 = 75Ω

Therefore, using a 4:1 transformer simply because the antenna is a folded dipole is not necessarily correct for a 50 Ω system.

A matching network, different transformer ratio, or another feed arrangement may be appropriate depending on the measured antenna impedance and operating frequency.

Factors That Affect Folded Dipole Performance

The calculator provides theoretical and practical starting values, but several physical factors can change the final antenna characteristics.

Conductor Diameter

The diameter of the conductor affects the electrical characteristics of the antenna. Larger conductors can also influence bandwidth and resonant behavior.

Conductor Spacing

The distance between the parallel conductors is an important part of a folded dipole's geometry.

Changing this spacing can affect impedance and other antenna characteristics.

Antenna Height

The antenna's distance from the ground and surrounding objects can influence its radiation pattern and electrical behavior.

The calculator's half-wavelength height is therefore only a reference point.

Nearby Objects

Metal structures, buildings, masts, other antennas, wiring, and other conductive objects can interact with the antenna.

This interaction can change the resonant frequency and feed-point impedance.

Feed Line

The feed line is also part of the overall RF system.

Cable impedance, cable losses, routing, common-mode current, and the type of balun or choke used can all influence system performance.

Construction Accuracy

A calculated antenna dimension is only useful if the physical antenna is constructed consistently.

Small dimensional differences can shift resonance, especially at higher frequencies where a small physical change represents a larger fraction of the wavelength.

Folded Dipole vs. Other Antenna Types

Different antenna designs are suitable for different applications.

The folded dipole is particularly useful when its impedance characteristics and physical configuration fit the intended RF system.

Common Folded Dipole Antenna Calculation Mistakes

Mistake 1: Using the Wrong Frequency

The antenna should be calculated around the intended operating frequency.

If you design an antenna for one frequency and operate it significantly away from that frequency, its electrical characteristics can change.

Mistake 2: Treating the Calculated Length as Exact

Calculator results are starting dimensions.

Real-world resonance depends on construction, conductor geometry, height, and surroundings. Final tuning may therefore be necessary.

Mistake 3: Confusing Velocity Factor With Antenna Shortening

A transmission-line velocity factor describes how the propagation speed in a transmission medium compares with propagation in free space.

The shortening factor used in a practical antenna-length calculation is a different concept.

For this reason, the calculator uses Practical Shortening Factor rather than Velocity Factor.

Mistake 4: Assuming the Impedance Is Always Exactly 300 Ω

Approximately 300 Ω is a useful nominal reference for a standard folded dipole, but the actual impedance can vary.

Don't design the entire feed system around an assumed exact value if measurement is available.

Mistake 5: Automatically Using a 4:1 Balun

A 4:1 transformation converts an impedance such as 300 Ω to approximately 75 Ω.

It does not convert 300 Ω directly to 50 Ω.

The matching solution should be selected based on the antenna and feed system.

Mistake 6: Ignoring the Installation Environment

An antenna in free space behaves differently from one installed close to a roof, mast, building, ground, or other conductive objects.

Always consider the actual installation environment when evaluating final performance.

Frequently Asked Questions

What is a folded dipole antenna?

A folded dipole is a balanced dipole antenna made from two parallel conductors connected together at their ends. It is commonly associated with a nominal feed-point impedance of approximately 300 Ω.

What is the impedance of a folded dipole?

A standard folded dipole is commonly treated as having a nominal impedance of approximately 300 Ω, but the actual feed-point impedance depends on its geometry and installation.

How do you calculate the length of a folded dipole?

A practical starting approximation is:

L ≈ 143f × K

where f is frequency in MHz and K is a practical shortening factor.

What is the formula for a folded dipole antenna?

The calculator uses:

λ = 300f

for wavelength and:

L ≈ 143f × K

for the practical antenna length.

How long is a folded dipole for 145 MHz?

Using a practical shortening factor of 0.95:

L ≈ 143145 × 0.95

The calculated total length is approximately 0.936 m, or about 93.6 cm.

Each side is approximately 46.8 cm.

What is the wavelength at 145 MHz?

Using:

λ = 300145

the free-space wavelength is approximately 2.069 m.

Does a folded dipole need a balun?

A folded dipole is a balanced antenna, while coaxial cable is unbalanced. Depending on the feed arrangement, a suitable balun or current-choke arrangement may be useful or necessary.

What balun is used for a 300-ohm folded dipole?

For a nominal 300 Ω antenna feeding a 75 Ω system, a 4:1 impedance transformation is appropriate in principle:

300Ω/4 = 75Ω

For a 50 Ω system, another matching approach may be required.

Can a folded dipole be connected to 50-ohm coax?

Yes, but a nominal 300 Ω folded dipole is not directly impedance-matched to 50 Ω simply by using a 4:1 transformer. The appropriate matching arrangement depends on the actual antenna impedance and feed system.

What is the difference between a folded dipole and a regular dipole?

A standard dipole uses two primary conductor arms extending from the feed point. A folded dipole uses parallel conductors connected at their ends. This different geometry produces different impedance characteristics.

Does antenna height affect folded dipole performance?

Yes. Installation height can affect the antenna's radiation pattern and interaction with the ground and nearby objects. The calculator's half-wavelength value is a reference rather than a universal optimum height.

Can I use the calculator for VHF and UHF frequencies?

The underlying wavelength and practical-length calculations can be used as starting estimates across many RF frequencies. At higher frequencies, however, construction details become increasingly important because small physical dimensions can represent a significant portion of the wavelength.

Folded Dipole Antenna Calculator Quick Reference

Conclusion

The Folded Dipole Antenna Calculator provides a convenient way to estimate the starting dimensions of a folded dipole from its operating frequency. By calculating the wavelength and applying a practical antenna-length approximation, you can quickly determine an initial total length and the approximate length of each side.

The calculator also provides useful reference information about nominal impedance, directivity, balun transformation, polarization, and mounting height.

However, calculated dimensions should not be treated as guaranteed final specifications. Conductor diameter, spacing, antenna height, nearby structures, feed-line configuration, and installation environment can all affect the final electrical characteristics. For a practical installation, use the calculated dimensions as a starting point and verify the completed antenna with appropriate RF measurements before finalizing the design.