End Fed Half Wave (EFHW) Antenna Length Calculator

Estimated EFHW Lengths for Common Ham Bands (using 96% Velocity Factor)

Band	Frequency Range (MHz)	Typical Half Wave Length (ft)	Typical Half Wave Length (m)
80m	3.5 – 4.0	~66.3 – 58.0	~20.2 – 17.7
40m	7.0 – 7.3	~33.1 – 31.8	~10.1 – 9.7
30m	10.1 – 10.15	~23.0 – 22.9	~7.0 – 6.98
20m	14.0 – 14.35	~16.6 – 16.2	~5.05 – 4.95
17m	18.068 – 18.168	~12.8 – 12.8	~3.9 – 3.8
15m	21.0 – 21.45	~11.1 – 10.8	~3.38 – 3.30
12m	24.89 – 24.99	~9.3 – 9.3	~2.84 – 2.83
10m	28.0 – 29.7	~8.3 – 7.8	~2.53 – 2.38

What is an End Fed Half Wave Calculator?

An end fed half wave calculator is a specialized tool designed for amateur radio operators and electronics hobbyists to determine the precise length of an End Fed Half Wave (EFHW) antenna for a specific operating frequency. Unlike a center-fed dipole, an EFHW antenna is fed from one end, which offers significant convenience for portable operations, SOTA (Summits on the Air), and installations where a center support is not feasible. Who should use it? Any ham radio operator, from newcomer to seasoned expert, who wants to build a simple, effective, and multi-band capable antenna with minimal fuss. A common misconception is that these antennas don’t need a ground or counterpoise; while they are more forgiving, a small counterpoise wire or using the coax shield is necessary for proper performance and to prevent common mode currents. This end fed half wave calculator simplifies the process, ensuring your antenna is resonant exactly where you want it. For more on antenna theory, check out our guide on HF Antenna Basics.

End Fed Half Wave Calculator Formula and Mathematical Explanation

The core of any end fed half wave calculator is a simple yet fundamental physics formula. The calculation determines the physical length of a wire that will be electrically resonant at a half wavelength of the desired frequency. The standard formula is:

Length (in feet) = 468 / Frequency (in MHz)

This formula is a slight modification of the free-space calculation. The number 468 (instead of 492) already accounts for the “end effect” and an approximate velocity factor of a typical wire antenna, which is usually around 95-97%. The velocity factor accounts for the fact that RF energy travels slightly slower along a conductor than it does in a vacuum. Our end fed half wave calculator allows you to adjust this for even greater precision.

Variables Table

Variable	Meaning	Unit	Typical Range
L	Antenna Length	Feet or Meters	Depends on Frequency
f	Frequency	Megahertz (MHz)	1.8 – 30 (HF Bands)
Vf	Velocity Factor	Percentage (%)	94 – 98%
Constant	Imperial Constant	–	468

Practical Examples (Real-World Use Cases)

Let’s see how the end fed half wave calculator works in practice.

Example 1: Building a 40-Meter EFHW for Portable Use

• Goal: Create a portable antenna for the 40m band, targeting the popular digital portion around 7.074 MHz.
• Inputs for the end fed half wave calculator:
  • Frequency: 7.074 MHz
  • Velocity Factor: 96% (for standard insulated wire)
• Calculation: Length = (491.8 * 0.96) / 7.074 = 472.128 / 7.074 ≈ 66.74 feet.
• Interpretation: You would cut a wire approximately 66 feet and 9 inches long. This single wire, paired with a matching unit, provides access to the 40m, 20m, 15m, and 10m bands. You can learn more about matching with our article, Understanding SWR.

Example 2: Creating a 20-Meter Antenna for a Home Station

• Goal: Install a stealthy antenna for the 20m band, focusing on the voice (SSB) segment at 14.225 MHz.
• Inputs for the end fed half wave calculator:
  • Frequency: 14.225 MHz
  • Velocity Factor: 96%
• Calculation: Length = 472.128 / 14.225 ≈ 33.19 feet.
• Interpretation: A wire just over 33 feet long can be easily configured as a sloper or inverted ‘L’ from a second-story window, making it an excellent choice for homes with limited space. This is a great starting point for anyone new to the hobby, as discussed in our Portable Radio Guide.

How to Use This End Fed Half Wave Calculator

Using this end fed half wave calculator is straightforward:

1. Enter Frequency: Input your desired operating frequency in MHz into the first field. Be specific; for example, use 14.250 instead of just 14.
2. Adjust Velocity Factor: If you know the specific velocity factor of your wire (found in the manufacturer’s datasheet), enter it as a percentage (e.g., 95 for 95%). If unsure, 96 is a safe starting point for insulated wire.
3. Read the Results: The calculator instantly provides the primary result in feet and several key intermediate values in the results box.
4. Cut and Tune: Always cut the wire slightly longer than the calculated length. Use an antenna analyzer to trim it down to the exact length for the lowest SWR. This tuning process is vital for optimal performance. The functionality of an antenna tuner can be a helpful reference.

Key Factors That Affect End Fed Half Wave Calculator Results

While the end fed half wave calculator provides a very accurate starting point, several environmental factors can influence the final resonant length of your antenna.

• Height Above Ground: The antenna’s height relative to the ground affects its feedpoint impedance and radiation pattern. A lower height can make the antenna appear electrically longer.
• Wire Insulation & Diameter: The thickness and material of the wire’s insulation change the velocity factor. Thicker insulation results in a lower velocity factor and a shorter antenna. The calculator accounts for this.
• Nearby Objects: Proximity to buildings, trees, and other conductive objects can de-tune the antenna. Try to keep the wire as clear of obstructions as possible.
• Configuration (Sloper, Inverted-V, Horizontal): The shape of the antenna impacts its resonance. An inverted-V configuration often requires slightly longer wire lengths than a horizontal one.
• The Matching Unit (Unun): An EFHW requires a high-impedance transformer (an “unun,” typically 49:1 or 64:1) to match the antenna’s high end-impedance (2000-5000 ohms) to your radio’s 50-ohm output. The quality of this unun is critical. Learn more about Balun vs Unun differences.
• Counterpoise Length: A counterpoise wire of about 0.05 wavelengths of the lowest frequency band is recommended to provide a stable ground reference and reduce common mode currents, which can cause RFI (Radio Frequency Interference). You can learn about selecting proper feedlines in our guide to Choosing Coax Cable.

Frequently Asked Questions (FAQ)

1. Why is an EFHW a good multi-band antenna?

An EFHW antenna presents a high impedance at its fundamental frequency and at its harmonic multiples. This means an antenna cut for 7 MHz (40m) will also be resonant on 14 MHz (20m), 21 MHz (15m), and 28 MHz (10m), making it naturally multi-band without complex traps or coils.

2. Do I absolutely need a counterpoise with this antenna?

While some operators get away with just using the coax shield, it is highly recommended to use a dedicated counterpoise wire of about 0.05 wavelengths. This improves efficiency, creates a more stable SWR, and prevents the coax from radiating, which can cause interference (RFI) in your shack. Using the end fed half wave calculator helps get the main element right, but don’t forget the counterpoise.

3. What is a 49:1 Unun and why do I need it?

An unun (unbalanced-to-unbalanced) is a transformer. The feedpoint impedance of an EFHW is very high (around 2450 ohms). A 49:1 unun transforms this high impedance down to approximately 50 ohms (2450 / 49 ≈ 50), which is the standard impedance for most amateur radio transceivers and coaxial cables.

4. How accurate is the end fed half wave calculator?

The formula used is a well-established standard and provides a very accurate starting point. However, you should always cut the wire about 5% longer and trim it to perfection using an SWR meter or antenna analyzer, as local factors always play a role.

5. Can I use this calculator for a center-fed dipole?

No. While the overall length of a full half-wave dipole is similar, it is comprised of two quarter-wave elements fed in the center. This end fed half wave calculator is specifically for a single-wire, end-fed antenna.

6. What’s the best way to install an EFHW antenna?

Popular configurations include a sloper (running from a high point down to a low point), an inverted-V (center raised on a mast, ends sloping down), or a horizontal run. A sloper is often easiest and very effective for DX (long-distance communication).

7. Why does my SWR look good on some bands but not others?

This can be due to the harmonics not falling perfectly in-band, or the effect of the compensation capacitor in the unun. Often, a small adjustment to the main wire length will shift the SWR dips for all bands. A tuner can address minor mismatches. This is a common query beyond a simple end fed half wave calculator.

8. Is a higher antenna always better?

Generally, yes. Higher antennas tend to have lower radiation angles, which is better for long-distance communication (DX). They are also less likely to be affected by ground losses and nearby objects.

Related Tools and Internal Resources

If you found this end fed half wave calculator useful, you might also be interested in our other resources:

• HF Antenna Basics: A comprehensive guide to the fundamentals of high-frequency antennas.
• SWR Tuning Guide: Learn what Standing Wave Ratio means and how to minimize it for best performance.
• Amateur Radio Antenna Calculator: A collection of calculators for various antenna types.
• Choosing the Right Coax Cable: Understand the different types of coaxial cable and how they impact your station.
• Balun vs. Unun Explained: A detailed comparison of these two critical antenna components.
• HF Antenna Design: An article on the principles of designing your own HF antennas.