EFHW Antenna Calculator
Your expert tool for designing End-Fed Half-Wave antennas.
Formula Used: Length (ft) = (468 / Frequency_MHz) * VelocityFactor. Results are slightly trimmed for real-world use.
| Band | Frequency Range (MHz) | Approx. Half-Wave Length (ft) | Approx. Half-Wave Length (m) |
|---|---|---|---|
| 80m | 3.5 – 4.0 | 113 – 130 | 34.4 – 39.6 |
| 40m | 7.0 – 7.3 | 62 – 65 | 18.9 – 19.8 |
| 30m | 10.1 – 10.15 | 45 | 13.7 |
| 20m | 14.0 – 14.35 | 32 – 33 | 9.7 – 10.1 |
| 17m | 18.068 – 18.168 | 25 | 7.6 |
| 15m | 21.0 – 21.45 | 21 – 22 | 6.4 – 6.7 |
| 12m | 24.89 – 24.99 | 18 | 5.5 |
| 10m | 28.0 – 29.7 | 15 – 16 | 4.6 – 4.9 |
What is an EFHW Antenna Calculator?
An EFHW antenna calculator is a specialized tool designed for amateur radio operators and electronics enthusiasts to determine the precise physical length of an End-Fed Half-Wave (EFHW) wire antenna. Unlike a standard dipole which is fed in the center, an EFHW is fed at one of its ends. This makes it incredibly versatile for various deployment scenarios, especially portable operations. The calculator takes a desired frequency and the wire’s velocity factor to compute the resonant length, saving time and guesswork during antenna construction.
This tool is invaluable for anyone building their own antennas, from seasoned hams aiming for optimal performance on a specific frequency to beginners taking their first step into antenna construction. A common misconception is that any random wire length will work if you have a powerful enough tuner. While a tuner can force a match, a resonant antenna like one built using this efhw antenna calculator will always be more efficient, radiating more of your power instead of losing it as heat in the tuner.
EFHW Antenna Formula and Mathematical Explanation
The core principle behind any efhw antenna calculator is based on the relationship between the speed of light, frequency, and wavelength. The fundamental formula for a half-wavelength in free space is `Length (feet) = 468 / Frequency (MHz)`. This formula is an approximation derived from `Length (meters) = (c / f) / 2`, where `c` is the speed of light and `f` is the frequency. The number 468 (instead of 492) already includes a slight shortening factor (around 5%) to account for “end effect” where the antenna wire interacts with the air and its surroundings.
However, radio waves travel slightly slower in a physical wire than in a vacuum. This is accounted for by the Velocity Factor (VF). Insulated wire has a lower VF (e.g., 0.95-0.98) than bare wire. The final, practical formula used by the calculator is:
Adjusted Length (feet) = (468 / Frequency in MHz) * Velocity Factor
Using an accurate velocity factor is crucial for precision. If your antenna is too long, its resonant frequency will be too low; if too short, it will be too high.
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| Frequency (f) | The target operating frequency | Megahertz (MHz) | 1.8 – 30 (for HF bands) |
| Velocity Factor (VF) | The speed of RF energy in the wire relative to a vacuum | Dimensionless ratio | 0.95 – 0.99 (for insulated wire) |
| Length (L) | The calculated physical length of the antenna wire | Feet or Meters | Varies with frequency |
Practical Examples (Real-World Use Cases)
Example 1: 40-Meter Band for Digital Modes
An operator wants to build an EFHW for the popular FT8 portion of the 40-meter band, centered at 7.074 MHz. They are using standard insulated copper wire with a known velocity factor of 0.97.
- Input – Frequency: 7.074 MHz
- Input – Velocity Factor: 0.97
Using the efhw antenna calculator, the output is a primary half-wave length of approximately 64.12 feet (19.54 meters). The operator can confidently cut the wire to this length, attach it to their DIY 49:1 unun, and expect a very low SWR near their target frequency, ensuring maximum power transfer for their digital transmissions.
Example 2: 20-Meter Band for Portable Voice (SOTA)
A portable operator preparing for a Summits on the Air (SOTA) activation wants a lightweight antenna for the 20-meter voice segment, around 14.285 MHz. They are using thin, lightweight insulated wire with a VF of 0.96.
- Input – Frequency: 14.285 MHz
- Input – Velocity Factor: 0.96
The calculator recommends a half-wave wire length of 31.54 feet (9.61 meters). This compact and efficient antenna is perfect for carrying up a mountain. The ability to calculate the length precisely beforehand means less time spent tuning and more time making contacts from the summit. This is a key benefit of using a reliable efhw antenna calculator.
How to Use This EFHW Antenna Calculator
Using this calculator is a straightforward process designed for accuracy and ease of use.
- Enter Frequency: Input your desired operating frequency in the “Frequency (MHz)” field. For best results, choose the center of the frequency range you intend to use most often. For example, for the 40m band (7.0-7.3 MHz), you might enter 7.150.
- Enter Velocity Factor (VF): In the “Velocity Factor” field, enter the VF for your specific antenna wire. If you are unsure, 0.97 is a safe starting point for common insulated hookup wire.
- Read the Results: The calculator automatically updates. The primary result shows the optimal half-wave (λ/2) length, which is what you’ll cut your main wire to. Intermediate values for full-wave and quarter-wave are also provided for reference.
- Decision-Making: Always cut your wire slightly longer than the calculated length. It’s much easier to trim a wire that is too long than to add length to one that is too short. Use an antenna analyzer to find the lowest SWR point and trim the wire in small increments until the SWR minimum is at your desired frequency. An online SWR calculator can also help you understand the values.
Key Factors That Affect EFHW Antenna Results
While an efhw antenna calculator provides a fantastic starting point, several real-world factors can influence the final resonant frequency of your antenna.
- Height Above Ground: The proximity of the antenna to the ground (and other objects) can capacitively load the antenna, making it seem electrically longer. This usually means a shorter physical wire is needed than the calculator predicts.
- Wire Insulation: The thickness and material of the wire’s insulation (the dielectric) changes the velocity factor. Thicker insulation generally leads to a lower VF and a shorter wire.
- Nearby Objects: Buildings, trees, and other conductive objects can couple with the antenna and shift its resonant frequency. Try to keep your antenna as clear of obstructions as possible.
- Configuration (Sloper, Inverted-V, Horizontal): The shape of the antenna affects its impedance and resonance. An inverted-V configuration will often require a slightly different length than a flat-top horizontal dipole. An EFHW installed as a sloper is a very popular and effective configuration.
- Impedance Matching Transformer (Unun): An EFHW has a very high impedance (2000-5000 ohms) and requires a transformer (typically a 49:1 or 64:1 unun) to match it to a 50-ohm coaxial cable. The quality and characteristics of this transformer can slightly alter the required wire length. Poorly made transformers can be lossy.
- Counterpoise/Coax Length: Although an EFHW theoretically doesn’t need a counterpoise, the coax shield often acts as one. The length and routing of the coax can impact the antenna’s tuning and performance. Adding a short (0.05 wavelength) counterpoise wire can help stabilize the system.
Frequently Asked Questions (FAQ)
A: The calculator provides a highly accurate mathematical starting point based on the provided inputs. However, you should always cut the wire slightly longer and trim it to perfection using an antenna analyzer due to real-world environmental factors.
A: An EFHW antenna has a naturally high impedance at its feedpoint (around 2500-3000 ohms). A 49:1 unun transforms this high impedance down to about 50-60 ohms, providing a good match for your radio’s 50-ohm output. Without it, the SWR would be extremely high, and very little power would be transferred.
A: Yes! An EFHW cut for a specific frequency will also be resonant on its harmonic multiples. For example, an antenna cut for 7.1 MHz (40m) will also work well on 14.2 MHz (20m), 21.3 MHz (15m), and 28.4 MHz (10m), making it a great multi-band option. This is a major advantage of the design.
A: A counterpoise is a wire or system of wires that provides the “other half” of the antenna system for the RF current to flow against. For an EFHW, the shield of your coax feedline often acts as the counterpoise. Adding a short, dedicated counterpoise wire (e.g., 0.05 wavelengths long) can help to decouple the coax and stabilize the SWR.
A: An EFHW is specifically cut to be a resonant half-wavelength at the fundamental frequency of operation. A “random wire” is a non-resonant length of wire that relies entirely on a wide-range antenna tuner to create a match. A resonant EFHW is generally more efficient. The efhw antenna calculator is designed for resonant antennas.
A: The manufacturer often provides the VF. If not, you can find it experimentally. Cut a wire for a known frequency using a VF of 1.0, measure the resonant frequency with an analyzer, and then calculate the true VF with the formula: VF = (Actual Resonant Freq / Target Freq).
A: Yes, you can. The half-wave length result is exactly what you need for a standard dipole. However, remember that a dipole consists of two quarter-wave elements, so you would cut the total calculated half-wave length in half for each leg of the dipole. Check our dipole vs efhw guide for more.
A: The power handling is determined by the transformer (unun). The end of an EFHW is a high-voltage point. Your unun must be built with components (ferrite core, wire, capacitor) rated for your desired power level to avoid overheating and failure.