Folded Dipole Antenna Calculator
Antenna Design Calculator
Please enter a valid, positive frequency.
— m —
— ft —
~300 Ω
The total length of a half-wave folded dipole antenna is estimated using the standard empirical formula: Length (feet) = 468 / Frequency (MHz). This formula accounts for the end effect, which makes the antenna electrically longer than its physical length.
Antenna Length vs. Frequency Chart
This chart illustrates how the required antenna length (in feet and meters) changes with frequency.
Common HF Band Dimensions
| Amateur Band | Typical Frequency (MHz) | Total Length (ft) | Total Length (m) |
|---|---|---|---|
| 80 Meters | 3.800 | 123.16 | 37.54 |
| 40 Meters | 7.200 | 65.00 | 19.81 |
| 20 Meters | 14.200 | 32.96 | 10.05 |
| 15 Meters | 21.300 | 21.97 | 6.70 |
| 10 Meters | 28.500 | 16.42 | 5.00 |
Reference table showing pre-calculated lengths for popular High Frequency (HF) amateur radio bands.
What is a Folded Dipole Antenna?
A folded dipole antenna is a variation of the standard half-wave dipole antenna. It consists of two parallel conductors, connected at both ends, and fed at the center of one of the conductors. This “folded” design creates a loop, which gives the antenna unique properties compared to a single-wire dipole. While its physical length is determined by the same principles, its electrical characteristics, particularly its feed impedance, are significantly different. This makes the folded dipole antenna calculator an essential tool for radio amateurs and engineers.
This type of antenna is primarily used by amateur radio operators (hams), shortwave listeners, and in commercial broadcasting (like FM radio). The main reason to use a folded dipole is its higher feed impedance (around 300 ohms) and broader bandwidth. A common misconception is that “folded” means the antenna is physically smaller; in reality, its overall length is nearly identical to a standard dipole for the same frequency. The folded design refers to the electrical path, not a compact physical form.
Folded Dipole Antenna Formula and Mathematical Explanation
The core calculation for a folded dipole’s length is straightforward and relies on an empirical formula that approximates a half-wavelength in free space, adjusted for the “end effect” of electrical currents on a real-world conductor. A folded dipole antenna calculator automates this process.
The step-by-step process is:
- Determine Wavelength (λ): The wavelength is the speed of light divided by the frequency.
- Calculate Half-Wavelength: The fundamental length of a dipole is a half-wavelength (λ/2).
- Apply Velocity Factor: Radio waves travel slightly slower on a conductor than in free space. The constant 468 (for feet) or 142.6 (for meters) in the formula L = 468 / f (MHz) conveniently combines the speed of light with a typical velocity factor (~0.95-0.98).
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| L | Total Antenna Length | Feet or Meters | Depends on frequency |
| f | Frequency of Operation | Megahertz (MHz) | 1.8 (160m band) to 30 (10m band) for HF |
| Z_in | Input Impedance | Ohms (Ω) | ~288-300 Ω |
Practical Examples (Real-World Use Cases)
Example 1: 40-Meter Band Voice Operation
An amateur radio operator wants to build a folded dipole for the voice portion of the 40-meter band, centering on 7.225 MHz.
- Input: Frequency = 7.225 MHz
- Calculation: Length = 468 / 7.225 = 64.78 feet
- Interpretation: The operator needs to construct a folded dipole with a total end-to-end length of approximately 64.78 feet (or 19.74 meters). This antenna will be resonant around 7.225 MHz and provide a good match to 300-ohm twin-lead feedline. For an even better match to standard 50-ohm coaxial cable, a 4:1 balun would be used at the feedpoint. Many enthusiasts use a folded dipole antenna calculator to quickly get these numbers.
Example 2: 10-Meter Band Technician Licensee
A newly licensed Technician wants to explore the 10-meter band, targeting 28.4 MHz for SSB (Single Side-Band) contacts.
- Input: Frequency = 28.4 MHz
- Calculation: Length = 468 / 28.4 = 16.48 feet
- Interpretation: A much more manageable antenna, the total length required is only 16.48 feet (or 5.02 meters). This compact size makes it ideal for attic or backyard installations. The wider bandwidth of the folded dipole is also an advantage on the 10m band, which is quite broad. This is a perfect project for someone new to antenna building, and a DIY antenna design guide can provide step-by-step instructions.
How to Use This Folded Dipole Antenna Calculator
Using this calculator is simple and provides instant, actionable results for your antenna project.
- Enter Frequency: Input your desired center frequency in the “Frequency (MHz)” field.
- Review Results: The calculator instantly displays the primary result—the total required length in feet. It also shows key intermediate values like the length in meters and the half-wave dimension, which is the distance from the center feedpoint to the end.
- Analyze Chart: The dynamic chart shows how antenna length corresponds to a range of frequencies around your input, helping you visualize how small frequency changes affect the required dimensions.
- Decision Making: Use the calculated length as your starting point. It’s standard practice to cut the wire slightly longer and then trim it down to achieve the lowest SWR (Standing Wave Ratio) at your desired frequency. An accurate SWR calculator can be used alongside an antenna analyzer for precise tuning.
Key Factors That Affect Folded Dipole Antenna Results
Several factors beyond the basic formula can influence the final resonant frequency and performance of your antenna. A good folded dipole antenna calculator provides a great starting point, but these factors are key for fine-tuning.
- Height Above Ground: The height of the antenna, measured in wavelengths, significantly impacts its feedpoint impedance and radiation pattern. Lower heights generally lower the impedance.
- Velocity Factor of Wire: Insulated wire has a different velocity factor than bare wire, causing it to be electrically longer. Antennas made with insulated wire will need to be physically shorter (by 2-5%) than the formula suggests.
- Conductor Diameter and Spacing: The diameter of the conductors and the spacing between them affect the antenna’s impedance and bandwidth. Wider spacing and fatter conductors lead to broader bandwidth.
- Nearby Objects: Proximity to buildings, trees, and other conductive objects can detune the antenna. Always try to install antennas in as clear a space as possible. For optimal performance, it is vital to understand antenna tuning basics.
- Feedline Type: Using a 4:1 balun is crucial for matching the folded dipole’s ~300 ohm impedance to standard 50 ohm coaxial cable, minimizing signal loss and SWR. Neglecting this can lead to significant inefficiencies.
- End Insulators and Sag: The type of insulators used and the amount of sag in the wire can slightly alter the physical length, which in turn affects the electrical length.
Frequently Asked Questions (FAQ)
The folded design acts as a 4:1 impedance transformer. It effectively steps up the ~72 ohm impedance of a standard dipole to approximately 288 ohms, which is a near-perfect match for 300-ohm twin-lead transmission line.
Its primary advantages are higher impedance and wider bandwidth compared to a standard dipole. The wider bandwidth means it maintains a low SWR over a broader range of frequencies. This topic is covered in depth in our guide to understanding impedance matching.
Yes, 300-ohm TV twin-lead is an excellent and convenient material for constructing a folded dipole antenna, especially for receive-only or low-power applications.
Yes, the spacing affects the impedance and bandwidth. However, for a folded dipole made with two identical conductors, the 4:1 impedance transformation ratio remains constant regardless of spacing. Using a consistent spacing is more important.
High SWR can be caused by many factors: incorrect balun (or no balun), proximity to other objects, incorrect height, or issues with your feedline. Our coax loss calculator can help diagnose feedline issues.
Like a standard dipole, it is bidirectional, radiating most of its signal broadside (perpendicular) to the length of the antenna wire.
A balun (BALanced to UNbalanced) is a device that interfaces a balanced antenna (like a dipole) to an unbalanced feedline (like coaxial cable). For a folded dipole fed with 50-ohm coax, a 4:1 balun is essential for a proper impedance match.
It’s a very reliable starting point, usually within 1-2% of the final resonant length. Real-world environmental factors (height, nearby objects) require final tuning by trimming the antenna length for the lowest SWR, often done with help from the best antenna analyzers on the market.