End Fed Half Wave Calculator

Calculate precise antenna lengths for resonant multiband performance

Resonant Harmonic Table

Harmonic	Frequency (MHz)	Wavelength	Wire Efficiency

Note: Real-world resonance may shift slightly higher on upper harmonics due to end effects.

What is an End Fed Half Wave Calculator?

An end fed half wave calculator is an essential tool for amateur radio operators and antenna enthusiasts. It allows you to precisely determine the physical length of a wire antenna that is resonant at a specific frequency and its integer harmonics. Unlike a center-fed dipole, an EFHW is fed at one end through a high-ratio impedance transformer (typically a 49:1 or 64:1 unun).

Using an end fed half wave calculator ensures that your antenna will perform efficiently across multiple bands. Because the wire is a half-wavelength at the fundamental frequency, it naturally presents a half-wavelength resonance at multiples of that frequency (e.g., a 40m EFHW is also resonant on 20m, 15m, and 10m). This makes it one of the most popular “stealth” and portable antennas in the ham radio community.

Common misconceptions include the idea that any length of wire will work with a tuner. While a tuner can match many lengths, using an end fed half wave calculator to find the “natural” resonant length minimizes feedline radiation and maximizes the efficiency of your 49:1 unun, which can overheat if forced to handle high SWR.

End Fed Half Wave Calculator Formula and Mathematical Explanation

The calculation of an antenna’s length is based on the speed of light and the behavior of electromagnetic waves in a vacuum, adjusted for the medium of the wire. The fundamental formula used by this end fed half wave calculator is:

The Formula:

Length (Feet) = (468 * Velocity Factor) / Frequency (MHz)

Length (Meters) = (142.5 * Velocity Factor) / Frequency (MHz)

Variable	Meaning	Unit	Typical Range
f	Target Frequency	MHz	1.8 to 54.0 MHz
VF	Velocity Factor	Decimal	0.90 to 0.99
λ (Lambda)	Wavelength	Meters/Feet	N/A
L	Physical Wire Length	Meters/Feet	Determined by f

The “468” constant is a derivation of the speed of light adjusted for typical wire “end effects” and the transition from theoretical physics to practical antenna building. When using the end fed half wave calculator, the Velocity Factor (VF) is the most critical variable. Wire insulation slows down the speed of the signal, meaning the wire must be physically shorter than a bare wire to be electrically resonant.

Practical Examples (Real-World Use Cases)

Example 1: The Classic 40-10m Multiband Antenna

A user wants to build an antenna for the 40-meter band, specifically centered at 7.150 MHz. They are using standard 14 AWG stranded wire with PVC insulation. By inputting these values into the end fed half wave calculator with a VF of 0.95:

• Input Frequency: 7.150 MHz
• Velocity Factor: 0.95
• Calculated Length: 62.18 feet

This result allows the operator to cut the wire slightly long (e.g., 64 feet) and prune it back until the SWR is perfect at 7.150 MHz, knowing it will also resonate near 14.300 MHz (20m) and 21.450 MHz (15m).

Example 2: 20-meter Monoband Portable Antenna

For a SOTA (Summits on the Air) activation, an operator needs a lightweight 20-meter EFHW for 14.060 MHz using thin teflon wire (VF 0.97).

• Input Frequency: 14.060 MHz
• Velocity Factor: 0.97
• Calculated Length: 32.32 feet

The end fed half wave calculator provides the exact starting point for a successful field deployment.

How to Use This End Fed Half Wave Calculator

1. Enter the Frequency: Type in the lowest frequency you intend to use. For a multiband antenna, this is the fundamental frequency (e.g., 3.5 MHz for 80m).
2. Select Velocity Factor: If using bare copper, use 0.97. If using insulated wire, use 0.95. For very thick insulation, 0.92-0.94 may be necessary.
3. Choose Units: Toggle between feet and meters based on your preference.
4. Read the Main Result: The large highlighted number is your total wire length from the unun to the end insulator.
5. Review Harmonics: Check the generated table to see where the antenna will resonate on higher bands.
6. Apply Counterpoise: Use the suggested 0.05λ counterpoise length if you are not using the coax shield as a return path.

Key Factors That Affect End Fed Half Wave Calculator Results

When using an end fed half wave calculator, remember that the environment heavily influences the final resonance:

• Height Above Ground: Antennas closer to the ground (less than 1/4 wavelength) will have a lower resonant frequency than predicted.
• Wire Insulation: Thick PVC jackets can lower the resonance by 3-5% compared to bare wire.
• Nearby Objects: Trees, buildings, and metal gutters act as parasitic elements, shifting the results of the end fed half wave calculator.
• Unun Efficiency: A poor 49:1 transformer can introduce reactance that makes the wire appear shorter or longer than it is.
• Antenna Topology: An “Inverted L” or “Sloper” configuration will resonate slightly differently than a horizontal wire.
• End Effects: The insulators used at the ends of the wire add a small amount of capacitance, effectively lengthening the antenna.

Frequently Asked Questions (FAQ)

1. Why does the end fed half wave calculator use 468 instead of 492?

The number 492 is for a theoretical wave in free space. 468 accounts for the “end effect” and the fact that waves travel slower on a physical conductor, providing a more accurate real-world starting point.

2. Do I need a tuner with an EFHW designed by this calculator?

If the end fed half wave calculator results are followed and the antenna is pruned correctly, you often won’t need a tuner on the fundamental and even harmonics. However, a tuner helps on the band edges.

3. Can I use the calculator for a 9:1 unun “random wire”?

No, this is specifically for a half-wave resonant antenna. Random wires use non-resonant lengths to avoid high impedance points.

4. How long should my counterpoise be?

A common rule of thumb is 0.05 wavelength, which our end fed half wave calculator provides. Many users also rely on the coax shield as the counterpoise.

5. Why is my SWR high on the higher harmonics?

Harmonics on an EFHW are not perfectly integer-multiples due to wire diameter and end effects. You may need a small compensation coil or “lumped constant” to align all bands perfectly.

6. Does the gauge of the wire matter?

Thicker wire has a slightly wider bandwidth but may require a very minor adjustment to the total length compared to thin wire.

7. What is the best height for an EFHW?

Ideally, at least 1/4 to 1/2 wavelength above ground for the lowest band of operation to ensure a good radiation pattern.

8. Can I fold the ends of the wire to shorten it?

Yes, folding the wire back on itself is a great way to “prune” without cutting. The folded portion no longer contributes significantly to the resonant length.