Antenna Length Calculator
Calculate optimal antenna length for FM, LoRa, WiFi, and more
Quick Frequency Presets
Antenna Parameters
Antenna Structure Diagram
Calculation Results
Ground Radials
Cut Tolerance Band — see the safe range at a glance
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Four Antenna Types Compared (same frequency)
| Type | Length | Gain | Note |
|---|
How do I confirm my cut antenna is correct?
Use an SWR meter: ideal is 1:1, but SWR < 1.5 is fine in practice. A NanoVNA or SWR meter lets you measure live — trim 1–2 mm at a time and re-measure.
Why is it shorter than the theoretical value?
Radio waves travel slower in wire than in vacuum — this ratio is the velocity factor (VF). Bare copper VF≈0.95, PVC≈0.92, Teflon≈0.95. Lower VF means a shorter antenna. The main result here already includes the VF correction.
Common Frequency Bands Reference
| Band | Frequency | Wavelength | 1/4 Wave |
|---|
Table 1/4-wave values are theoretical (no VF) — use the result above for actual cutting
Click a table row to apply that frequency
About Antenna Length Calculator
This antenna length calculator helps you quickly calculate the optimal antenna length for FM radio, amateur radio (2m/70cm bands), LoRa IoT (915MHz/868MHz), WiFi (2.4GHz/5GHz), and other frequency bands. It supports quarter-wave monopole, half-wave dipole, 5/8 wave, and full-wave loop antenna types, with velocity factor correction for different wire materials.
How to Use
- Select a preset frequency band (FM, LoRa, WiFi, etc.) or enter a custom frequency
- Choose the antenna type (1/4 wave is best for simple DIY projects)
- Select the wire type to apply the correct velocity factor correction
- View the results and cut your antenna wire to the recommended length
Calculation Formula
Antenna length is calculated based on the wavelength formula: λ = c/f, where c is the speed of light (299,792,458 m/s) and f is frequency. Different antenna types use different multipliers: 1/4 wave = λ/4, 1/2 wave = λ/2, 5/8 wave = 5λ/8, full wave = λ. The result is then multiplied by the velocity factor (VF) to get the actual length.
FAQ
Q: Why is the actual antenna length shorter than theoretical?
A: Because radio waves travel slower in wire than in vacuum. This ratio is called the Velocity Factor (VF). Bare copper wire is about 0.95 (95%), and insulated wire is about 0.92 (92%).
Q: Does a 1/4 wave antenna need a ground plane?
A: Yes, a quarter-wave monopole antenna requires a good ground plane to work properly. The ground plane can be a metal plate, multiple ground radials (at least 4, each 1/4 wavelength long), or a conductive surface like a car roof.
Q: How do I confirm the antenna is properly tuned?
A: Use an SWR meter (Standing Wave Ratio meter). The ideal value is 1:1, but SWR < 1.5 is acceptable in practice. If SWR is too high, adjust the antenna length: shorten it if the lowest SWR frequency is too low, lengthen it if too high.
Q: What are the advantages of a 5/8 wave antenna?
A: A 5/8 wave antenna provides about 3dB more gain than a 1/4 wave antenna, with a lower radiation angle suitable for long-distance ground communications. However, it requires a matching coil and is more complex to build.
References
- ARRL Antenna Book - The definitive guide to antenna design (American Radio Relay League)
- ITU Radio Regulations - International Telecommunication Union
- Velocity Factor Data - Wikipedia
