Efhw Antenna Calculator

What is an EFHW Antenna Calculator?

An EFHW (End-Fed Half-Wave) antenna calculator is a specialized tool designed for radio amateurs and electronics enthusiasts to determine the precise physical length of wire required to build an effective half-wavelength antenna that is fed from one end. Unlike a center-fed dipole, an EFHW is incredibly convenient for portable operations and installations where a central feed point is impractical. This calculator not only provides the starting length for your antenna wire but also gives key values for constructing the essential impedance matching unit.

The primary challenge with an EFHW antenna is its extremely high feed point impedance (typically 2500-5000 Ohms), which must be transformed down to the 50 Ohms that modern transceivers expect. Our efhw antenna calculator simplifies this by providing the necessary values to get you started on both the radiating element and the matching transformer, which is a critical component for any successful build.

The EFHW Antenna Calculator Formula and Explanation

The calculations are based on fundamental radio wave principles, adjusted for real-world materials. The core of any antenna length calculation is the frequency it’s designed for.

1. Radiating Element Length

The main formula used by the efhw antenna calculator for the wire length is:

Adjusted Length = (142.5 / Frequency in MHz) * Velocity Factor

This formula gives the length in meters. The constant 142.5 is derived from the speed of light and is a standard constant for calculating a half-wavelength in meters from a frequency in MHz. For more on antenna math, see our guide on multiband antenna theory.

Antenna Length Calculation Variables

Variable	Meaning	Unit	Typical Range
Frequency	The target resonant frequency of the antenna.	MHz	1.8 – 54.0 (HF Bands)
Velocity Factor (VF)	A multiplier that accounts for the slower speed of RF energy in a wire compared to a vacuum. It depends on the wire’s insulation.	Unitless ratio	0.92 – 0.99
Adjusted Length	The final physical length to cut the wire. This is the primary result of the efhw antenna calculator.	Meters / Feet	Depends on frequency.

2. Matching Unit Capacitor

The impedance transformer (often a 49:1 or 64:1 unun) requires a capacitor across the primary side to compensate for winding inductance and improve the SWR on lower frequencies. The formula to find the required capacitance is:

C (pF) = 1 / ( (2πf)² * L )

This calculator assumes a primary inductance (L) of 2.4 microhenries (µH), which is typical for a 2-turn primary on a popular FT240-43 ferrite core. You can learn more about cores in our FT240-43 core guide.

Practical Examples

Example 1: 40-Meter Band General Class Voice Portion

• Input Frequency: 7.225 MHz
• Input Velocity Factor: 0.96
• Results:
  • Antenna Wire Length: 18.94 meters (or 62.15 feet)
  • Matching Capacitor: approx. 202 pF

Example 2: 20-Meter Band Technician Class Data Portion

• Input Frequency: 14.100 MHz
• Input Velocity Factor: 0.95
• Results:
  • Antenna Wire Length: 9.59 meters (or 31.46 feet)
  • Matching Capacitor: approx. 53 pF

How to Use This EFHW Antenna Calculator

1. Enter Frequency: Input the center frequency you want your antenna to be resonant on, in MHz.
2. Enter Velocity Factor: Input the VF of your specific antenna wire. If you don’t know it, 0.95 is a safe starting point. Check our velocity factor explained page for more info.
3. Select Units: Choose whether you want the final length in meters or feet. The efhw antenna calculator will convert automatically.
4. Review Results: The calculator instantly provides the “Required Antenna Wire Length.” This is your starting point. Always cut the wire slightly longer (e.g., 2-3%) to allow for fine-tuning.
5. Interpret Other Values: The “Free Space” length shows the theoretical ideal, while the “Matching Capacitor” value is your target for the impedance transformer. The “1/4 Wave Length” is also provided for reference.
6. Build and Tune: Cut your wire to the calculated length. Assemble your antenna and matching unit. Use an antenna analyzer or SWR meter to find the lowest SWR. If the SWR is lowest on a frequency *below* your target, the antenna is too long. Trim it in small increments until the lowest SWR point is at your desired frequency. For help, see our guide on how to measure SWR.

Key Factors That Affect EFHW Antenna Performance

• Antenna Height: The height of the wire above ground significantly impacts the feedpoint impedance and the radiation pattern. Higher is generally better.
• Installation Shape: Whether the antenna is installed as a straight horizontal wire, an inverted V, or a sloper will change its resonant frequency and performance characteristics. The length calculated here is for a straight wire.
• Nearby Objects: Buildings, trees, and metal objects can detune the antenna, requiring length adjustments. Try to keep the wire as far from other objects as possible.
• Wire Insulation: The type and thickness of the wire’s plastic insulation determine its velocity factor. Thicker insulation leads to a lower VF and a shorter antenna. Our guide to the best antenna wire can help you choose.
• Transformer Construction: The quality of the matching unit (unun) is critical. The choice of ferrite core (e.g., FT240-43 vs FT140-43), winding technique, and capacitor quality all affect efficiency and bandwidth.
• Ground and Counterpoise: While an EFHW doesn’t need tuned radials like a vertical antenna, having a short (e.g., 0.05 wavelength) counterpoise wire connected to the shield side of the coax at the feedpoint can help stabilize SWR and reduce RF on the feedline.

Frequently Asked Questions (FAQ)

1. Why is my SWR so high after cutting the wire to the exact length?

The efhw antenna calculator provides a mathematically ideal starting point. Real-world factors like height, nearby objects, and ground conductivity always affect the final tuning. Always cut the wire a little long and trim it down for a perfect match.

2. What value capacitor should I use if I can’t find the exact one?

Use the closest standard value capacitor you can find. It’s common to use a high-voltage ceramic or mica capacitor with a value of 100pF or 150pF for general-purpose HF EFHW transformers. The calculated value is a theoretical target for perfect resonance.

3. What is Velocity Factor (VF) and why does it matter?

Velocity Factor is the ratio of the speed of a radio wave in a wire compared to its speed in a vacuum. Because insulation slows the wave down, the physical length of the wire needs to be shorter than the theoretical free-space length. An incorrect VF is a common reason for initial length inaccuracies.

4. Can I use this antenna on multiple bands?

Yes! An EFHW antenna is harmonically related. This means an antenna cut for 7 MHz (40m band) will also be resonant on or near 14 MHz (20m), 21 MHz (15m), and 28 MHz (10m), making it a great multiband option without a tuner.

5. What kind of wire should I use?

Any copper wire will work, but 14 AWG or 16 AWG insulated stranded copper wire is a popular choice for its strength, flexibility, and durability.

6. What impedance ratio should I choose for my transformer?

A 49:1 ratio transformer is the most common choice, as it transforms the high impedance of the antenna (around 2450 ohms) down to 50 ohms. A 64:1 ratio is also used and can sometimes provide a better match, depending on the antenna’s height and environment.

7. Does this efhw antenna calculator work for a vertical orientation?

Yes, the length calculation is the same. However, a vertical EFHW will have a different radiation pattern (lower angle, good for long-distance) and its feedpoint impedance may be slightly different, potentially requiring more tuning.

8. Do I need a counterpoise wire?

It is highly recommended. A short counterpoise (around 0.05 wavelengths long, or 4-5 meters for the 80m band) connected to the transformer’s ground lug can help prevent the coax shield from becoming part of the antenna and reduce RF in the shack.

Related Tools and Internal Resources

Explore our other resources to expand your antenna building knowledge:

• 49:1 Unun Transformer Guide: A deep dive into building the most critical part of the EFHW system.
• Multiband Antenna Theory: Understand why the EFHW works on multiple harmonic bands.
• SWR Calculator: Calculate SWR from forward and reflected power readings.
• The Best Antenna Wire for 2026: A review of different wire types for your projects.
• Velocity Factor Explained: Learn how to find the VF for any wire.
• FT240-43 Core Guide: Datasheets and winding tips for this popular ferrite core.