EFHW Calculator (End-Fed Half-Wave)
The ultimate tool for precise antenna length calculation.
Antenna Parameters
— ft
— ft
— MHz
Harmonic Length Comparison
Visual representation of antenna lengths for the fundamental frequency and its harmonics.
Harmonic Length Details
| Harmonic | Frequency (MHz) | Half-Wave Length (ft) | Half-Wave Length (m) |
|---|
Calculated lengths for multi-band operation on harmonic frequencies based on your input.
What is an EFHW Calculator?
An efhw calculator is a specialized tool designed for amateur radio operators (hams) to determine the precise physical length of an End-Fed Half-Wave (EFHW) antenna. This type of antenna is extremely popular for both home and portable use (like for SOTA and POTA activations) due to its simplicity, effectiveness, and ease of deployment. Unlike a center-fed dipole, an EFHW is fed at one end, requiring only a single support point and simplifying installation. The efhw calculator is crucial because an antenna’s length must be resonant with the target frequency for optimal performance, ensuring maximum power transfer and minimal signal reflection (low SWR).
This calculator is used by anyone from beginners building their first antenna to seasoned experts planning a multi-band setup. A common misconception is that any long wire will work as an EFHW antenna. While any wire will radiate some signal, only a properly sized half-wave wire, calculated with an efhw calculator, will be resonant and highly efficient without requiring an extensive antenna tuner for every band.
EFHW Calculator Formula and Mathematical Explanation
The core of any efhw calculator is a simple yet fundamental physics formula derived from the speed of light and frequency. The basic formula for a half-wave antenna in free space is:
Length (feet) = 492 / Frequency (MHz)
However, in the real world, electrical waves travel slightly slower along a physical wire than in a vacuum. This is known as the “end effect” and “velocity factor.” To account for this, the formula is adjusted to:
Length (feet) = 468 / Frequency (MHz)
Our efhw calculator takes this one step further by allowing you to input a specific Velocity Factor (VF), which depends on the wire’s insulation. This provides a much more accurate result. The final formula is:
Length (ft) = (468 / Frequency [MHz]) * Velocity Factor
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| Frequency (f) | The target operating radio frequency. | Megahertz (MHz) | 1.8 – 54 (HF/6m bands) |
| Velocity Factor (VF) | The ratio of signal speed in the wire to the speed of light. | Dimensionless | 0.94 – 0.98 |
| Length (L) | The calculated physical length of the half-wave antenna wire. | Feet (ft) or Meters (m) | Varies with frequency |
Practical Examples (Real-World Use Cases)
Example 1: 40-Meter Band for CW Operation
An operator wants to create an EFHW for the CW (Morse code) portion of the 40-meter band, centered at 7.050 MHz. They are using standard insulated copper wire with a velocity factor of 0.95.
- Inputs: Frequency = 7.050 MHz, Velocity Factor = 0.95
- Calculation: Length = (468 / 7.050) * 0.95 = 66.38 * 0.95 = 63.06 feet.
- Interpretation: The operator should cut the wire to approximately 63.06 feet (or 19.22 meters). This antenna will also be resonant on the 20-meter band (14.100 MHz), making the efhw calculator essential for multi-band operation.
Example 2: 20-Meter Band for Portable POTA Activation
A portable operator wants a light and simple antenna for the 20-meter data/SSB portion, around 14.225 MHz. They are using a very thin, insulated wire with a velocity factor of 0.96.
- Inputs: Frequency = 14.225 MHz, Velocity Factor = 0.96
- Calculation: Length = (468 / 14.225) * 0.96 = 32.90 * 0.96 = 31.58 feet.
- Interpretation: A wire of 31.58 feet (9.63 meters) is needed. This short length is ideal for quick deployment in a park. Using an efhw calculator ensures they don’t need to waste time trimming the antenna in the field. This antenna will also work on the 10-meter band (~28.450 MHz).
How to Use This EFHW Calculator
- Enter Frequency: Input your desired operating frequency in MHz. For best results, choose the center of the band portion you use most often.
- Enter Velocity Factor: Input the velocity factor of your antenna wire. If unsure, 0.95 is a safe starting point for most PVC-insulated wires.
- Read the Primary Result: The large display shows the most important value: the total length your antenna wire should be, shown in both feet and meters. This is your cutting length for a half-wave antenna.
- Analyze Intermediate Values: The calculator also provides the full and quarter-wavelengths. The quarter-wavelength is particularly useful if you plan to add a quarter-wave counterpoise wire.
- Check Harmonic Data: The chart and table show the lengths and frequencies for the 2nd and 3rd harmonics. This is a key feature of the efhw calculator, helping you plan for using a single antenna across multiple bands (e.g., a 40m antenna on 20m and 10m).
- Cut and Tune: Always cut the wire slightly longer than the calculated value. You can then trim it down in small increments to achieve a perfect SWR at your desired frequency.
Key Factors That Affect EFHW Calculator Results
- Frequency: This is the most critical factor. Antenna length is inversely proportional to frequency; a lower frequency requires a longer wire.
- Wire Insulation (Velocity Factor): The type and thickness of the wire’s insulation change how fast radio waves travel along it. Thicker insulation leads to a lower velocity factor and a shorter physical antenna. Using an accurate VF in the efhw calculator is vital.
- Wire Gauge: While a minor factor, thicker wires can have a slightly different velocity factor and may require minor length adjustments compared to thinner wires.
- Surrounding Environment: Proximity to buildings, trees, and the ground will detune the antenna. Always try to install the antenna as high and in the clear as possible. The efhw calculator provides a starting point, but final tuning must be done in-situ.
- Impedance Matching (Unun): An EFHW antenna has a very high impedance (2000-4000 ohms) at its feedpoint. A broadband transformer, typically a 49:1 or 64:1 Unun (unbalanced-to-unbalanced), is required to match this to the 50-ohm coaxial cable from your radio. A poor quality Unun can affect performance. Consider using an SWR calculator to verify your match.
- Counterpoise: While many EFHWs are used without one, adding a short (0.05 wavelength) or quarter-wavelength counterpoise can help stabilize the SWR and provide a more efficient RF ground, especially with portable setups.
Frequently Asked Questions (FAQ)
Its popularity comes from its simplicity. It requires only one support, can be installed in various configurations (sloper, inverted-L, horizontal), and is inherently multi-band on its harmonic frequencies. Our efhw calculator helps unlock this multi-band potential.
It’s an impedance transformer. Your radio expects to see a 50-ohm load, but the end of a half-wave wire presents a very high impedance (around 2500 ohms). The 49:1 Unun transforms this 2500 ohms down to about 51 ohms (2500 / 49), providing an excellent match for your radio.
Technically, the coaxial cable shield acts as the counterpoise. This can sometimes cause RF to flow back into the shack. Adding a short counterpoise wire (about 0.05 * wavelength) to the ground side of the Unun can help mitigate this and stabilize the match.
Yes, the formula is based on physics and works for any frequency. You could use it to calculate an EFHW for the 2-meter or 70cm bands, although other antenna types are more common at VHF/UHF frequencies.
The efhw calculator provides a theoretical starting point. Real-world factors like height above ground, proximity to objects, and the exact velocity factor of your specific wire will influence the final resonant frequency. Always cut long and trim for the best SWR. Check our guide on antenna tuners for more info.
It’s a very good general estimate for common PVC-insulated stranded copper wire. For ultimate precision, you would need to measure the VF of your specific wire spool, but starting with 0.95 will get you very close. See our article on understanding velocity factor for details.
All antennas are compromises. While a full-size, center-fed dipole mounted high in the air might perform better, the EFHW’s ease of installation and multi-band capability make it a highly effective and practical choice for most operators, especially in restricted spaces. Using a precise efhw calculator maximizes its performance.
Not efficiently. An EFHW works on odd and even harmonics. An antenna cut for 40m (7 MHz) works well on 20m (14 MHz, 2nd harmonic), 15m (21 MHz, 3rd harmonic), and 10m (28 MHz, 4th harmonic). The 15m band is an odd harmonic, but it often requires a tuner.