The Efhw (End-Fed Half-Wave) Antenna Calculator helps amateur radio users determine the correct wire length needed to build an end-fed half-wave antenna for specific frequencies. These antennas are widely used because they are simple, efficient, and require fewer installation components. The calculator uses a mathematical formula to provide the length of wire you need, either in meters or feet, based on the frequency you plan to transmit or receive on.
This tool is essential for ham radio operators, field communicators, and emergency service groups that need quick and accurate measurements for antenna setup. It saves time, reduces trial and error, and ensures the antenna operates properly on the desired frequency.
Formula of Efhw Antenna Calculator
Total Length of Wire (in meters):
L = 143 / f
Total Length of Wire (in feet):
L = 468 / f
Where:
L is the total wire length
f is the frequency in MHz
For insulated wire, the length changes slightly because of the wire’s velocity factor:
L = (143 × VF) / f (in meters)
L = (468 × VF) / f (in feet)
Where:
VF is the velocity factor (commonly between 0.95 and 0.98 for insulated wire)
The velocity factor helps adjust the length to match the speed at which the radio waves move through the wire. A slightly lower VF means slightly shorter wire is needed.
Quick Reference Table for Popular HF Bands
This table shows commonly used HF amateur radio bands and the wire length needed for a standard Efhw antenna using bare wire (VF = 1.00). This makes it easy to find the value without calculating every time.
| Frequency (MHz) | Band Name | Length (meters) | Length (feet) |
|---|---|---|---|
| 3.5 | 80 meters | 40.86 | 133.71 |
| 7.0 | 40 meters | 20.43 | 66.86 |
| 10.1 | 30 meters | 14.16 | 46.34 |
| 14.2 | 20 meters | 10.07 | 32.96 |
| 18.1 | 17 meters | 7.90 | 25.86 |
| 21.2 | 15 meters | 6.75 | 22.07 |
| 24.9 | 12 meters | 5.74 | 18.79 |
| 28.5 | 10 meters | 5.02 | 16.42 |
Use this table as a handy reference for your projects or during field deployment.
Example of Efhw Antenna Calculator
Let’s say you want to build an Efhw antenna for the 20-meter band, which operates around 14.2 MHz.
Using the formula:
L = 143 / f
L = 10.07 meters
If you’re using insulated wire with a velocity factor of 0.96, then:
L = (143 × 0.96) / 14.2
L = 9.67 meters
So, for a bare wire, use 10.07 meters. For insulated wire, use 9.67 meters.
Most Common FAQs
The Efhw antenna is simple to build, easy to deploy, and does not require a center feed point. This makes it ideal for portable and temporary use. It also performs well across a wide range of frequencies when properly tuned.
The velocity factor adjusts the wire length to match the slower speed of radio waves in insulated wire compared to bare wire. If you ignore the VF, your antenna may not perform as expected.
Yes, with a suitable transformer and proper tuning, a single Efhw antenna can operate on several bands. This makes it versatile for users who want to communicate on multiple frequencies.