Antenna Length Calculator
Calculate dipole, monopole, loop, and other antenna lengths from frequency. Includes velocity factor correction.
Formula
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Common Antenna Length Reference
| Band | Frequency | Dipole (ft) | Dipole (m) |
|---|---|---|---|
| 160m | 1.8 MHz | 247.0 | 75.3 |
| 80m | 3.5 MHz | 127.0 | 38.7 |
| 40m | 7.0 MHz | 63.5 | 19.4 |
| 20m | 14.0 MHz | 31.8 | 9.7 |
| 10m | 28.0 MHz | 15.9 | 4.8 |
| 6m | 50.0 MHz | 8.9 | 2.7 |
| 2m | 144.0 MHz | 3.2 | 1.0 |
| 70cm | 430.0 MHz | 1.1 | 0.3 |
| Wi-Fi | 2.4 GHz | 0.2 | 0.06 |
| 5 GHz | 5.0 GHz | 0.09 | 0.03 |
Understanding Antenna Length Calculations
The Basic Relationship
The length of an antenna is directly determined by the wavelength of the radio wave it is designed to transmit or receive. The fundamental relationship is λ = c / f, where λ is the wavelength in meters, c is the speed of light (299,792,458 m/s), and f is the frequency in Hz. For practical antenna design, we use a simplified formula: wavelength in meters = 300 / f(MHz).
Why Velocity Factor Matters
Radio waves travel slightly slower in a conductor than in free space, typically 95-97% of the speed of light for bare copper wire. This means the physical antenna must be shortened by the velocity factor. The classic formula 468 / f(MHz) for a half-wave dipole already incorporates a standard velocity factor of 0.95. If you use insulated wire or a different conductor, the velocity factor changes and the length must be adjusted accordingly.
Dipole vs. Monopole vs. Loop
- Half-Wave Dipole (λ/2): Two symmetric elements, center-fed. Total length = λ/2. Natural impedance ~73 Ω. Most common wire antenna for HF bands.
- Quarter-Wave Monopole (λ/4): Single element with ground plane. Total length = λ/4. Natural impedance ~36 Ω. Used for mobile and VHF/UHF base stations.
- Full-Wave Loop (λ): Continuous loop antenna. Total length = λ. Natural impedance ~100-120 Ω. Quiet, directional pattern with lower noise.
- 5/8 Wave (5λ/8): Longer than quarter-wave with more gain. Common for VHF/UHF base and mobile antennas. Provides lower radiation angle.
Practical Antenna Construction Tips
When building an antenna, always cut the elements slightly longer than the calculated length and trim for best SWR. Factors like height above ground, nearby objects, and feed line routing all affect the resonant frequency. Use an antenna analyzer or SWR meter to tune the antenna by trimming equal amounts from both ends of a dipole. Remember that the velocity factor for insulated wire is lower than bare wire — typically 0.80-0.90 versus 0.95-0.97.
Feed Line and Baluns
A half-wave dipole is a balanced antenna and requires a balun (balanced-to-unbalanced transformer) for use with coaxial cable. A 1:1 current balun is standard for dipoles fed with coax. Quarter-wave monopoles are naturally unbalanced and connect directly to coax, but they require a ground plane of at least four radials, each λ/4 long. The radials should slope downward at about 45 degrees for optimal impedance matching.
Verwandte Tools
Frequently Asked Questions
Can I use the same antenna for transmit and receive?
Yes, antennas are reciprocal — they work identically for transmit and receive. A dipole that is resonant at 14.0 MHz for transmitting will also be resonant for receiving at that same frequency. The length, impedance, and radiation pattern are the same in both directions. This reciprocity principle applies to all passive antenna types.
How do I tune a dipole antenna to resonance?
Start by cutting each leg about 5% longer than the calculated length. Use an antenna analyzer or SWR meter to find the resonant frequency (lowest SWR). If the resonance is too low in frequency, the antenna is too long — shorten both legs equally and recheck. If resonance is too high, the antenna is too short — add length or use capacitive loading. Aim for an SWR of 1.5:1 or lower at your target frequency.
Does height above ground affect antenna length?
Height above ground does not change the resonant length of an antenna, but it significantly affects the feed point impedance and radiation pattern. A dipole at 0.1 wavelengths above ground has a feed impedance around 20-30 ohms, while the same dipole at 0.5 wavelengths is closer to 70 ohms. The calculated physical length for resonance stays the same regardless of height.
What is the best antenna for a beginner ham radio operator?
The half-wave dipole is the best starter antenna for HF bands. It is simple to build, requires only wire, connectors, and a center insulator, and performs well. For VHF/UHF, a quarter-wave ground plane is easy to build from scrap wire and coax. Start with a 20m (14 MHz) dipole for worldwide HF communication, or a 2m (144 MHz) ground plane for local repeaters.
How does loading coil make an antenna shorter?
A loading coil adds inductive reactance to the antenna, electrically lengthening it without increasing its physical size. This allows a physically short antenna to resonate at a lower frequency. Loading coils are common in mobile HF antennas where a full-size quarter-wave on 40m (10 meters tall) is impractical. The trade-off is reduced efficiency and narrower bandwidth compared to a full-size antenna.