Dipole Antenna — Half-Wave Resonance
A half-wave dipole is a fundamental antenna whose length L = λ/2 = c/(2f) resonates at the design frequency. Feed point impedance is approximately 73 Ω in free space. Radiation pattern is a figure-8 in the E-plane with maximum gain 2.15 dBi.
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Half-wave length: L = 468/f (MHz) in feet, or λ/2 in meters Feed point impedance rises with height above ground Velocity factor 0.95–0.98 accounts for wire insulation Bandwidth inversely proportional to Q factor
Ready to run the numbers?
Why: Dipole antennas are the basis of most RF systems—broadcast, amateur radio, and wireless. Understanding length-frequency relationship and impedance enables proper matching and efficient radiation.
How: Enter frequency; length follows from λ = c/f with velocity factor. Impedance varies with height above ground. Radiation resistance and loss resistance determine efficiency.
Run the calculator when you are ready.
📻 HF 20m Band Dipole
Amateur radio 20-meter band (14.0-14.35 MHz) half-wave dipole
📡 VHF 2m Band Dipole
Amateur radio 2-meter band (144-148 MHz) half-wave dipole
🎵 FM Broadcast Dipole
FM broadcast band (88-108 MHz) center frequency dipole
🔊 Amateur Radio 40m Dipole
40-meter band (7.0-7.3 MHz) half-wave dipole for DX
📊 Measurement Reference Dipole
Precision measurement dipole at 100 MHz for antenna testing
📶 UHF 70cm Band Dipole
Amateur radio 70cm band (420-450 MHz) half-wave dipole
Dipole Antenna Parameters
📋 Key Takeaways
- • A half-wave dipole has approximately 73Ω feed point impedance in free space
- • Dipole length formula: L = 143/f (MHz) × velocity factor for optimal resonance
- • Doubling distance reduces signal by 6 dB for point sources (inverse square law)
- • Velocity factor (0.95-0.98 typical) accounts for slower wave propagation in wire vs free space
💡 Did You Know?
📖 How Dipole Antennas Work
A dipole antenna consists of two conductive elements fed at the center. When RF current flows, it creates a standing wave pattern along the antenna length. At resonance (half-wavelength), the current distribution maximizes radiation efficiency.
Resonance and Length
For a half-wave dipole, each element is λ/4 long (total length = λ/2). At this length, the antenna resonates, creating maximum current at the feed point and optimal radiation.
Impedance Matching
The 73Ω impedance of a free-space dipole closely matches 75Ω coaxial cable, minimizing reflections and maximizing power transfer. Height above ground affects impedance - higher mounting typically increases impedance toward 73Ω.
Radiation Pattern
The half-wave dipole has a figure-8 pattern in the vertical plane with maximum radiation perpendicular to the antenna axis. In the horizontal plane, it's omnidirectional, making it ideal for general-purpose communications.
🎯 Expert Tips
💡 Mount Higher for Better Performance
Mount at least λ/4 above ground (higher is better). This reduces ground losses and brings impedance closer to the ideal 73Ω.
💡 Use a Balun for Balanced Feed
A balun (balanced-unbalanced transformer) prevents common-mode currents on the feedline, reducing RFI and improving pattern.
💡 Thicker Wire = Wider Bandwidth
Use thicker wire (2-3mm) for wider bandwidth. Thinner wire has narrower bandwidth but may be easier to install.
💡 Fine-Tune Length for SWR
After construction, measure SWR and trim/adjust length by 2-5% to achieve minimum SWR at your operating frequency.
⚖️ Dipole vs Other Antenna Types
| Feature | Half-Wave Dipole | Quarter-Wave Vertical | Yagi-Uda |
|---|---|---|---|
| Gain | 2.15 dBi | 0 dBi | 8-15 dBi |
| Impedance | 73Ω | 36Ω | 50Ω |
| Pattern | Omnidirectional (H) | Omnidirectional (H) | Directional |
| Bandwidth | Narrow | Narrow | Moderate |
| Complexity | Simple | Simple | Complex |
| Size | λ/2 | λ/4 | Multiple λ |
❓ Frequently Asked Questions
Q: Why is the dipole length shorter than λ/2 in practice?
A: The velocity factor accounts for slower wave propagation in wire. Typical values (0.95-0.98) make the physical length 2-5% shorter than the theoretical λ/2.
Q: What happens if my dipole is too long or too short?
A: Too long: impedance becomes capacitive, SWR increases. Too short: impedance becomes inductive, SWR increases. Both reduce efficiency.
Q: Can I use a dipole for multiple bands?
A: Yes! A dipole works on odd harmonics (3×, 5× frequency). A 40m dipole also works on 15m (3rd harmonic) and 10m (5th harmonic).
Q: What is the difference between a dipole and a folded dipole?
A: A folded dipole has ~300Ω impedance (4× higher) and wider bandwidth. It's often used in Yagi arrays and requires a matching transformer.
Q: How does height above ground affect dipole performance?
A: Higher mounting reduces ground losses, improves impedance match, and increases gain. Minimum recommended height is λ/4, but λ/2 or higher is better.
Q: What wire gauge should I use for a dipole?
A: 14-12 AWG (1.6-2.0mm) is ideal for HF. Thicker wire provides wider bandwidth but is heavier. Thinner wire is lighter but has narrower bandwidth.
Q: Do I need a balun with a dipole?
A: Highly recommended! A balun prevents common-mode currents on the feedline, reducing RFI and maintaining the antenna's balanced pattern.
Q: Can a dipole work indoors?
A: Yes, but performance is reduced due to reflections and losses. Outdoor mounting is always preferred for best performance.
📊 Key Statistics
📚 Official Data Sources
IEEE Antennas and Propagation Society
Professional antenna engineering standards
Last Updated: 2026-02-01
NIST Electromagnetic Standards
US National Institute of Standards electromagnetic measurements
Last Updated: 2026-01-15
⚠️ Disclaimer: This calculator provides theoretical estimates based on standard antenna formulas. Actual performance may vary due to ground conditions, nearby objects, feedline losses, and environmental factors. Always measure SWR and adjust antenna length for optimal performance. Not a substitute for professional antenna design services.
What is a Dipole Antenna?
A dipole antenna is one of the simplest and most widely used antenna types in radio communications. It consists of two conductive elements (typically wires or rods) arranged in a straight line, fed at the center. The most common form is the half-wave dipole, where each element is approximately one-quarter wavelength long, making the total length about half a wavelength.
📡 Basic Structure
A dipole consists of two equal-length conductive elements fed at the center. The current distribution creates a standing wave pattern along the antenna.
Key Characteristics:
- Balanced antenna
- Omnidirectional in horizontal plane
- ~73Ω feed point impedance
⚡ Feed Point Impedance
At resonance, a half-wave dipole in free space has an impedance of approximately 73 ohms (real), making it well-matched to 75-ohm coaxial cable.
Impedance Factors:
- Height above ground
- Wire diameter
- Ground conductivity
📊 Radiation Pattern
The half-wave dipole has a figure-8 radiation pattern in the vertical plane, with maximum radiation perpendicular to the antenna axis.
Pattern Features:
- Maximum gain: ~2.15 dBi
- Beamwidth: ~78°
- Nulls along axis
How to Calculate Dipole Length
Calculating the correct dipole length is crucial for optimal antenna performance. The length depends on the operating frequency and the velocity factor of the wire material.
📏 Length Calculation Steps
Step 1: Determine Operating Frequency
Identify the center frequency of your operating band. For amateur radio, this might be the middle of a band (e.g., 14.175 MHz for 20m band).
Step 2: Apply Velocity Factor
The velocity factor accounts for the fact that electromagnetic waves travel slower in wire than in free space. Typical values:
- Bare wire: 0.95-0.97
- Insulated wire: 0.80-0.95
- Thick wire: Higher velocity factor
Step 3: Calculate Length
Use the formula: L = (143 / f) × vf, where f is in MHz and vf is the velocity factor.
Step 4: Fine-Tuning
After initial construction, measure SWR and adjust length slightly (typically ±2-5%) to achieve minimum SWR at your operating frequency.
When to Use a Dipole Antenna
Dipole antennas are versatile and suitable for many applications, from amateur radio to commercial communications. Understanding when to use them helps optimize your antenna system.
📻 Amateur Radio
Ideal for HF, VHF, and UHF amateur radio operations. Simple to construct, easy to tune, and provides good performance for general-purpose communications.
Best For:
- HF bands (3-30 MHz)
- VHF/UHF (30-1000 MHz)
- Portable operations
🏢 Commercial Applications
Used in FM broadcast, two-way radio, and point-to-point communications. Provides reliable performance with simple installation.
Applications:
- FM broadcast (88-108 MHz)
- Land mobile radio
- Base stations
🔬 Measurement & Testing
Dipoles serve as reference antennas for antenna measurements and testing. Their predictable characteristics make them ideal calibration standards.
Uses:
- Antenna range calibration
- Gain measurements
- Pattern measurements
Dipole Antenna Calculation Formulas
Understanding dipole antenna formulas is essential for antenna design and optimization. These formulas relate physical dimensions to electrical characteristics.
📊 Core Dipole Formulas
Half-Wave Dipole Length
Where L is length in meters, f is frequency in MHz, and vf is velocity factor (typically 0.95-0.98).
Wavelength
Where λ is wavelength in meters, c is speed of light (299,792,458 m/s), and f is frequency in Hz.
Feed Point Impedance
Approximate impedance for half-wave dipole in free space. Actual impedance varies with height, wire diameter, and ground conditions.
Radiation Pattern
Normalized electric field pattern for half-wave dipole, where θ is elevation angle from horizontal.
Bandwidth
Where BW is bandwidth in Hz, f is center frequency, and Q is quality factor (typically 10-50 for wire dipoles).
Quality Factor (Q)
Q factor determines bandwidth. Higher Q means narrower bandwidth. Depends on wire diameter and height above ground.
For educational and informational purposes only. Verify with a qualified professional.
🔬 Physics Facts
Half-wave dipole has 2.15 dBi gain and 73 Ω resonant impedance in free space.
— NIST
Impedance varies from ~36 Ω at λ/4 height to ~90 Ω at λ/2 above ground.
— ARRL
Velocity factor of 0.95–0.98 shortens physical length vs. free-space λ/2.
— HyperPhysics
468/f (MHz) gives length in feet for half-wave dipole.
— ARRL
📋 Key Takeaways
- • A half-wave dipole has approximately 73Ω feed point impedance in free space
- • Dipole length formula: L = 143/f (MHz) × velocity factor for optimal resonance
- • Doubling distance reduces signal by 6 dB for point sources (inverse square law)
- • Velocity factor (0.95-0.98 typical) accounts for slower wave propagation in wire vs free space
💡 Did You Know?
📖 How Dipole Antennas Work
A dipole antenna consists of two conductive elements fed at the center. When RF current flows, it creates a standing wave pattern along the antenna length. At resonance (half-wavelength), the current distribution maximizes radiation efficiency.
Resonance and Length
For a half-wave dipole, each element is λ/4 long (total length = λ/2). At this length, the antenna resonates, creating maximum current at the feed point and optimal radiation.
Impedance Matching
The 73Ω impedance of a free-space dipole closely matches 75Ω coaxial cable, minimizing reflections and maximizing power transfer. Height above ground affects impedance - higher mounting typically increases impedance toward 73Ω.
Radiation Pattern
The half-wave dipole has a figure-8 pattern in the vertical plane with maximum radiation perpendicular to the antenna axis. In the horizontal plane, it's omnidirectional, making it ideal for general-purpose communications.
🎯 Expert Tips
💡 Mount Higher for Better Performance
Mount at least λ/4 above ground (higher is better). This reduces ground losses and brings impedance closer to the ideal 73Ω.
💡 Use a Balun for Balanced Feed
A balun (balanced-unbalanced transformer) prevents common-mode currents on the feedline, reducing RFI and improving pattern.
💡 Thicker Wire = Wider Bandwidth
Use thicker wire (2-3mm) for wider bandwidth. Thinner wire has narrower bandwidth but may be easier to install.
💡 Fine-Tune Length for SWR
After construction, measure SWR and trim/adjust length by 2-5% to achieve minimum SWR at your operating frequency.
⚖️ Dipole vs Other Antenna Types
| Feature | Half-Wave Dipole | Quarter-Wave Vertical | Yagi-Uda |
|---|---|---|---|
| Gain | 2.15 dBi | 0 dBi | 8-15 dBi |
| Impedance | 73Ω | 36Ω | 50Ω |
| Pattern | Omnidirectional (H) | Omnidirectional (H) | Directional |
| Bandwidth | Narrow | Narrow | Moderate |
| Complexity | Simple | Simple | Complex |
| Size | λ/2 | λ/4 | Multiple λ |
❓ Frequently Asked Questions
Q: Why is the dipole length shorter than λ/2 in practice?
A: The velocity factor accounts for slower wave propagation in wire. Typical values (0.95-0.98) make the physical length 2-5% shorter than the theoretical λ/2.
Q: What happens if my dipole is too long or too short?
A: Too long: impedance becomes capacitive, SWR increases. Too short: impedance becomes inductive, SWR increases. Both reduce efficiency.
Q: Can I use a dipole for multiple bands?
A: Yes! A dipole works on odd harmonics (3×, 5× frequency). A 40m dipole also works on 15m (3rd harmonic) and 10m (5th harmonic).
Q: What is the difference between a dipole and a folded dipole?
A: A folded dipole has ~300Ω impedance (4× higher) and wider bandwidth. It's often used in Yagi arrays and requires a matching transformer.
Q: How does height above ground affect dipole performance?
A: Higher mounting reduces ground losses, improves impedance match, and increases gain. Minimum recommended height is λ/4, but λ/2 or higher is better.
Q: What wire gauge should I use for a dipole?
A: 14-12 AWG (1.6-2.0mm) is ideal for HF. Thicker wire provides wider bandwidth but is heavier. Thinner wire is lighter but has narrower bandwidth.
Q: Do I need a balun with a dipole?
A: Highly recommended! A balun prevents common-mode currents on the feedline, reducing RFI and maintaining the antenna's balanced pattern.
Q: Can a dipole work indoors?
A: Yes, but performance is reduced due to reflections and losses. Outdoor mounting is always preferred for best performance.
📊 Key Statistics
📚 Official Data Sources
IEEE Antennas and Propagation Society
Professional antenna engineering standards
Last Updated: 2026-02-01
NIST Electromagnetic Standards
US National Institute of Standards electromagnetic measurements
Last Updated: 2026-01-15
⚠️ Disclaimer: This calculator provides theoretical estimates based on standard antenna formulas. Actual performance may vary due to ground conditions, nearby objects, feedline losses, and environmental factors. Always measure SWR and adjust antenna length for optimal performance. Not a substitute for professional antenna design services.
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