Folded Dipole Calculator & Resource Hub
Folded Dipole Antenna Dimensions Calculator
Enter your target frequency to calculate the optimal dimensions for a half-wave folded dipole antenna. This tool provides the total length, leg lengths, and estimated wavelength for your project.
Enter the center frequency you want the antenna to be resonant on. E.g., 14.2 for the 20-meter band.
Diameter of the wire or tubing. This has a minor effect on length but is important for bandwidth.
Center-to-center distance between the parallel conductors. Affects impedance.
Total Antenna Length (L)
9.99 m / 32.77 ft
Formula Used: Length (feet) = 468 / Frequency (MHz). This is a standard approximation for half-wave dipoles that accounts for end effects. The actual resonant length can be influenced by wire diameter, insulation, and height above ground.
Antenna Length vs. Frequency
Dynamic chart showing how total antenna length (in meters and feet) changes with frequency. The center point reflects the values in the folded dipole calculator.
Lengths for Common Amateur Radio Bands
| Band | Frequency Range (MHz) | Typical Center Freq (MHz) | Calculated Length (m / ft) |
|---|---|---|---|
| 80 meters | 3.5 – 4.0 | 3.75 | 37.84 m / 124.13 ft |
| 40 meters | 7.0 – 7.3 | 7.15 | 19.85 m / 65.12 ft |
| 20 meters | 14.0 – 14.35 | 14.175 | 9.97 m / 32.72 ft |
| 15 meters | 21.0 – 21.45 | 21.225 | 6.66 m / 21.86 ft |
| 10 meters | 28.0 – 29.7 | 28.5 | 4.98 m / 16.34 ft |
| 6 meters | 50.0 – 54.0 | 52.0 | 2.73 m / 8.96 ft |
Table of pre-calculated lengths for popular HF bands, useful for quick reference. Use the folded dipole calculator for precise measurements for your specific frequency.
What is a Folded Dipole Calculator?
A folded dipole calculator is an essential tool for radio amateurs and electronics hobbyists that computes the physical dimensions needed to construct a folded dipole antenna for a specific resonant frequency. Unlike a standard dipole, a folded dipole consists of two parallel conductors, shorted at the ends and fed at the center of one of the conductors. This construction gives it unique properties, most notably a higher feed impedance and a wider bandwidth. Our folded dipole calculator simplifies the complex process of determining the correct length for your project.
This type of antenna is widely used by amateur radio operators (hams), shortwave listeners (SWLs), and as a driven element in more complex Yagi antennas. The main advantage is its impedance multiplication. A standard half-wave dipole has a theoretical feedpoint impedance of about 73 ohms. A folded dipole made with two identical conductors multiplies this impedance by a factor of four, resulting in approximately 288-300 ohms. This makes it a perfect match for 300-ohm balanced twin-lead feeder lines and allows for easy matching to 50-ohm coaxial cable using a 4:1 balun. A common misconception is that a folded dipole has more gain than a standard dipole; in reality, their gain and radiation pattern are virtually identical. The key differences are impedance and bandwidth. Using a folded dipole calculator is the first step in harnessing these benefits.
Folded Dipole Formula and Mathematical Explanation
The core calculation for any half-wave antenna starts with the wavelength of the radio signal. The formula for wavelength (λ) is:
λ (meters) = 300 / f (MHz)
A theoretical half-wave antenna would be exactly half of this wavelength. However, in the real world, radio waves travel slightly slower in a conductor than in free space, and there are capacitive effects at the ends of the antenna (known as “end effect”). To compensate, the physical length must be shortened. A widely accepted empirical formula for the total length (L) of a half-wave dipole in feet is:
L (feet) = 468 / f (MHz)
This formula, used by our folded dipole calculator, incorporates a velocity factor of approximately 95% (0.95), which is a reliable estimate for typical wire antennas. While a folded dipole’s structure is different, this length formula remains the standard and most effective starting point for construction. The resonant length of a folded dipole can be slightly shorter than a simple dipole, especially with wide spacing, but this formula provides a dimension that is very close and usually requires minimal trimming. For an accurate build, using a precise folded dipole calculator is crucial.
Variables Table
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| L | Total Antenna Length | Meters / Feet | Depends on frequency |
| f | Frequency | MHz | 1 – 1000+ |
| λ | Wavelength | Meters | Depends on frequency |
| Zin | Feedpoint Impedance | Ohms (Ω) | ~288-300 |
Practical Examples (Real-World Use Cases)
Let’s see how the folded dipole calculator applies to real-world scenarios.
Example 1: Building an antenna for the 20-meter Ham Radio Band
An operator wants to build a folded dipole for the center of the 20m phone (voice) portion, which is around 14.250 MHz.
- Input: Frequency = 14.250 MHz
- Calculation: L (feet) = 468 / 14.250 = 32.84 feet
- Calculator Output (Primary): Total Length = 9.95 meters / 32.84 feet
- Interpretation: The builder would cut a total conductor length of approximately 32.84 feet. The antenna would be fed at the center, and the impedance would be around 300 ohms, ideal for use with a 4:1 balun and 50-ohm coax.
Example 2: Creating a high-performance FM broadcast receiving antenna
A user wants to improve reception for their favorite FM station at 98.1 MHz.
- Input: Frequency = 98.1 MHz
- Calculation: L (feet) = 468 / 98.1 = 4.77 feet
- Calculator Output (Primary): Total Length = 1.45 meters / 4.77 feet
- Interpretation: A compact folded dipole just under 5 feet long can be constructed. Its wider bandwidth compared to a simple dipole makes it excellent for receiving adjacent stations as well. This is a common design for commercial FM and TV antennas, showcasing the versatility of the design beyond amateur radio. Using a folded dipole calculator ensures optimal reception.
How to Use This Folded Dipole Calculator
Our folded dipole calculator is designed for simplicity and accuracy. Follow these steps:
- Enter Target Frequency: In the “Frequency (MHz)” field, input the desired resonant frequency for your antenna. This is the most critical parameter.
- Enter Physical Properties (Optional): Input the diameter of your conductor (wire/tube) and the planned spacing between the two conductors. While these have a smaller impact on the overall length, they are crucial for determining the antenna’s impedance and bandwidth. Our calculator assumes a standard impedance transformation but having these fields helps in planning the mechanical build.
- Review the Results: The calculator instantly provides the most important dimension: “Total Antenna Length”. This is the end-to-end length your antenna should have. It also shows the “Length per Leg” (from the end to the center feedpoint) and the full wavelength for reference.
- Make a Decision: Use these dimensions to build your antenna. It’s always a good practice to cut the wire slightly longer (1-2%) than the calculated value. You can then trim it down to achieve a perfect SWR (Standing Wave Ratio) match using an antenna analyzer. The value from the folded dipole calculator is an extremely close starting point.
Key Factors That Affect Folded Dipole Results
While our folded dipole calculator provides a very accurate starting point, several environmental and physical factors can influence the final resonant frequency and performance.
- Frequency: This is the single most important factor. As frequency increases, wavelength decreases, and therefore the required antenna length becomes shorter.
- Height Above Ground: An antenna’s height significantly impacts its feedpoint impedance and radiation pattern. A dipole placed very close to the ground (less than a half-wavelength) will have a lower impedance and a radiation pattern that directs more energy upwards.
- Velocity Factor of Wire: The standard `468/f` formula assumes bare wire. If you use insulated wire (like common THHN), the electrical velocity factor is lower, meaning radio waves travel slower. This requires the antenna to be physically shorter by 2-5% compared to what the folded dipole calculator shows. Always start longer and trim down.
- Conductor Diameter: A thicker conductor (or “fatter” antenna) results in a wider SWR bandwidth. This means the antenna will perform well over a broader range of frequencies, a key advantage of the folded dipole design.
- Spacing-to-Diameter Ratio: For a folded dipole, the ratio of the spacing between the conductors to the diameter of the conductors is the primary determinant of the impedance step-up ratio. While a 4:1 ratio (to ~300 ohms) is standard for equal-sized conductors, this can be altered by using conductors of different diameters.
- Nearby Objects: Proximity to trees, buildings, metal gutters, and other conductive objects can detune the antenna and alter its radiation pattern. For best results and to match the folded dipole calculator‘s output, install the antenna in a clear, open space as high as possible.
Frequently Asked Questions (FAQ)
1. Why is a folded dipole’s impedance 300 ohms?
A folded dipole acts as a 1:1 current divider and a transformer. When made with two identical conductors, it creates a 4:1 impedance transformation. Since a standard dipole’s impedance is ~73 ohms, 4 x 73 ohms is ~292 ohms, which is a near-perfect match for 300-ohm twin-lead transmission line.
2. What is the main advantage of a folded dipole over a simple dipole?
The two main advantages are a higher feed impedance (making it easy to match with certain feedlines) and a wider operating bandwidth. This means it can maintain a low SWR over a larger slice of the frequency band.
3. How do I feed a folded dipole with 50-ohm coax cable?
You must use a 4:1 balun (BALanced-to-UNbalanced transformer). The balun efficiently matches the 300-ohm balanced antenna feedpoint to the 50-ohm unbalanced coaxial cable, ensuring maximum power transfer.
4. Does running the calculator again for a different frequency give me a multi-band antenna?
No. A folded dipole is a monoband antenna. The folded dipole calculator provides dimensions for resonance on a single frequency band. While it can be used on odd harmonics with a tuner, it is not efficient as a multi-band antenna without traps or other complex modifications.
5. Why is my SWR high even after using the folded dipole calculator?
The calculator provides a theoretical length. Real-world factors like height above ground, wire insulation (velocity factor), and nearby conductive objects heavily influence the resonant frequency. Always cut the wire a little longer and trim for the lowest SWR at your desired frequency.
6. Is a folded dipole directional?
Like a standard dipole, it is bidirectional. It radiates most strongly broadside (perpendicular) to the length of the wire, with deep nulls off the ends of the wire.
7. Can I make a folded dipole from TV twin-lead?
Yes, this is a very common and effective construction method. You simply cut the twin-lead to the length provided by the folded dipole calculator, short the ends, and split the center of one wire to create the feedpoint.
8. Does the spacing between the wires matter?
Yes. While it doesn’t change the 4:1 impedance ratio if the conductors are identical, wider spacing can further increase the antenna’s bandwidth. However, the spacing must remain very small relative to the wavelength.