Calculator Inputs
Example Data Table
| Band | Frequency | Mode | Velocity Factor | Approximate Wire Length |
|---|---|---|---|---|
| 40 m | 7.150 MHz | Half wave | 0.95 | 62.2 ft |
| 20 m | 14.200 MHz | Half wave | 0.95 | 31.3 ft |
| 10 m | 28.400 MHz | Quarter wave | 0.95 | 7.8 ft |
Formula Used
The calculator first converts the entered frequency to hertz. It then finds the full wavelength with this formula:
Wavelength = Speed of light / Frequency
The selected antenna mode sets the wavelength fraction. A quarter wave uses 0.25. A half wave uses 0.5. A full wave uses 1.0.
Wire Length = Wavelength × Fraction × Velocity Factor × End Correction × Trim Factor
Feedline power is estimated with:
Power at Antenna = Transmitter Power × 10(-Feedline Loss dB / 10)
SWR mismatch is estimated from the reflection coefficient:
Reflection Coefficient = (SWR - 1) / (SWR + 1)
These formulas provide planning estimates. Real antennas shift with height, nearby objects, ground quality, insulation, bends, and feed method.
How to Use This Calculator
Enter the target frequency first. Select the frequency unit carefully. Choose the antenna mode that matches your plan. Use half wave for many simple wire designs. Use quarter wave when a ground or counterpoise system is included.
Set the velocity factor for your wire. Bare wire is often near 0.95. Insulated wire can be lower. Adjust the end correction if your experience suggests a different trimming allowance.
Add feedline data if you want a power estimate. Enter SWR, gain, and field distance for advanced output. Press the calculate button. The result appears above the form.
Single Wire Antenna Planning Guide
Why Wire Length Matters
A single wire antenna can work well when its length suits the chosen frequency. The wire does not need to be perfect at first cut. It should be close enough to allow safe trimming. This calculator gives that starting point. It also shows useful values that support tuning decisions.
Frequency and Wavelength
Frequency controls wavelength. Lower frequencies need longer wire. Higher frequencies need shorter wire. A half wave wire is common because it offers a simple resonant target. A quarter wave wire can work with a good counterpoise. A long wire may cover several bands with a tuner.
Velocity and End Effects
Real wire does not always behave like an ideal conductor in free space. Insulation, thickness, height, and nearby objects change the electrical length. That is why velocity factor and end correction are included. They help you create a more practical estimate. Final trimming should still be done slowly.
Feedline and Power
The feedline can waste power before it reaches the antenna. Loss rises with length and frequency. A high SWR can add more mismatch loss. The calculator estimates accepted power after these effects. This helps compare different cable choices and installation layouts.
Counterpoise Planning
A single wire often needs something to work against. That may be a radial system, ground wire, vehicle body, or station ground. The counterpoise output gives a simple quarter wave reference. More radials can improve stability and reduce unwanted feedline current.
Practical Tuning
Cut the wire slightly long when possible. Install it in the intended location. Measure resonance with an analyzer. Trim small pieces each time. Recheck after every change. Keep notes. A careful record makes future antenna builds faster and more accurate.
FAQs
1. What is a single wire antenna?
It is an antenna made from one main radiating wire. It may be used as a quarter wave, half wave, random wire, or long wire. Many designs need a tuner, counterpoise, or matching network.
2. Should I cut the wire exactly to the result?
No. Cut it slightly long first. Install it at the planned height and shape. Then trim slowly while measuring resonance, SWR, and bandwidth.
3. What velocity factor should I use?
Bare wire is often near 0.95. Insulated wire may be lower. Manufacturer data, analyzer testing, and past builds give better values than general guesses.
4. Why does the calculator include end correction?
Wire ends create capacitance and make the antenna act slightly longer. End correction shortens the estimated physical wire length to reflect practical behavior.
5. Do I need a counterpoise?
Many single wire antennas need a return path. A counterpoise or radial system can improve tuning, reduce RF in the shack, and stabilize performance.
6. Can this calculator replace an antenna analyzer?
No. It provides a strong planning estimate. An analyzer or SWR meter is still needed for safe final tuning and real installation checks.
7. Why is my real antenna different from the estimate?
Height, bends, nearby metal, trees, ground quality, wire insulation, and feed method can shift resonance. Real installations rarely match free-space math exactly.
8. What does the PDF download include?
The PDF includes the calculated values shown in the result section. It helps save antenna notes for trimming, field work, or later comparison.