Dipole Length & Tuning Guide

A comprehensive guide to designing, building, and tuning dipole antennas for amateur radio, from the classic 468 formula to practical field adjustments.

What is a Dipole Antenna?

The half-wave dipole is the most fundamental antenna in amateur radio. Simple, effective, and proven over decades of use, it consists of:

• Two quarter-wave elements - Fed at the center point
• Balanced design - Electrically symmetrical
• Resonant structure - Self-tuning at the design frequency

Why Choose a Dipole?

• Simple construction - Just wire, insulators, and feedline
• Low cost - Can be built for under $20
• Excellent performance - Efficient radiator when properly installed
• Forgiving design - Works well even with minor errors
• Multi-band potential - Can operate on harmonics

The Classic 468 Formula

The most widely known formula for calculating dipole length is:

Where Does 468 Come From?

The theoretical formula for a half-wavelength is:

Where c = speed of light (299,792,458 m/s)

The reduction from 492 to 468 accounts for:

1. End effect - Wire acts slightly longer than physical length (~5%)
2. Velocity factor - EM waves travel slightly slower in wire than free space (~0.95)

Worked Examples

Example 1: 20-Meter Dipole

Design a dipole for 14.2 MHz (center of the 20m band):

Using the 468 formula:

• Length = 468 / 14.2 = 32.96 feet or 10.05 meters
• Each leg = 32.96 / 2 = 16.48 feet (from center to end)

In metric:

• Length = 143 / 14.2 = 10.07 meters
• Each leg = 5.035 meters

Example 2: 40-Meter Dipole

Design a dipole for 7.15 MHz:

• Length = 468 / 7.15 = 65.45 feet or 19.95 meters
• Each leg = 32.73 feet or 9.98 meters

Example 3: 10-Meter Dipole

Design a dipole for 28.5 MHz:

• Length = 468 / 28.5 = 16.42 feet or 5.00 meters
• Each leg = 8.21 feet or 2.50 meters

Why Adjustments Are Needed

The 468 formula gives a good starting point, but several factors affect the final length:

1. Wire Diameter (End Effect)

Thicker wire has more capacitance, making the antenna electrically longer:

| Wire Size | End Effect Factor | Adjustment | | -------------------- | ----------------- | ----------- | | Thin (#18-#22 AWG) | 0.98 | -2% shorter | | Medium (#12-#14 AWG) | 0.97 | -3% shorter | | Thick (#8-#10 AWG) | 0.96 | -4% shorter |

2. Height Above Ground

Height affects the resonant frequency due to ground coupling:

• Low dipole (< λ/4 height): Appears longer, resonates lower
• High dipole (> λ/2 height): Closer to free-space characteristics
• Very high dipole (> 1λ height): Minimal ground effect

3. Nearby Objects

Metallic objects near the antenna affect resonance:

• Buildings with metal siding - Lower resonant frequency
• Power lines - Unpredictable effects (also noisy!)
• Trees - Slight lowering of frequency (wet trees more effect)
• Other antennas - Mutual coupling

4. Insulator Effects

End insulators add slight capacitance:

• Ceramic insulators: Minimal effect (<1%)
• Plastic insulators: Moderate effect (1-2%)
• Rope/cord insulators: Can be significant (2-5%)

Inverted Vee Configuration

An inverted vee is a dipole with the center elevated and the ends sloping down, forming a "V" shape.

Inverted Vee Formula

Inverted vees are electrically shorter than horizontal dipoles:

| Apex Angle | Vee Factor | Typical Height | | ----------- | ---------- | ----------------- | | 180° (flat) | 1.00 | Horizontal dipole | | 120° | 0.97-0.98 | Most common vee | | 90° | 0.94-0.96 | Steep vee | | 60° | 0.92-0.94 | Extreme vee |

Example: 20m inverted vee at 120° apex angle

• Length = (468 / 14.2) × 0.97 = 31.97 feet (vs. 32.96 for horizontal)
• Reduction: ~3% shorter

Advantages of Inverted Vee

1. Single support - Only need one tall mast/tree
2. Compact footprint - Smaller ground space required
3. Lower wind load - Less likely to break in storms
4. Easier installation - Simpler to raise and lower

Disadvantages

1. Vertical polarization component - Slightly reduces horizontal gain
2. Closer to ground - Ends near ground can detune and collect noise
3. Narrower bandwidth - Typically 10-15% less bandwidth than horizontal

Step-by-Step Tuning Procedure

Tools Required

1. SWR meter or antenna analyzer
2. Wire cutters or pruning shears
3. Soldering iron (if using solder connections)
4. Measuring tape
5. Ladder or safe climbing method

Initial Installation

1. Cut 5% long - Start with wire 5% longer than calculated

   • For 20m: 33.6 feet instead of 32 feet
   • Gives room for trimming

2. Install at final height - Tuning at different heights gives different results

3. Check initial SWR across the band:

   • Measure at low, middle, and high end
   • Record all readings

Reading SWR Patterns

The SWR curve tells you what adjustment to make:

Pattern 1: Too Long

• SWR lowest below your target frequency
• SWR rises as frequency increases
• Fix: Trim wire shorter

Pattern 2: Too Short

• SWR lowest above your target frequency
• SWR rises as frequency decreases
• Fix: Can't add wire, may need to rebuild

Pattern 3: Resonant

• SWR minimum at target frequency
• SWR rises on both sides equally
• Perfect! - No adjustment needed

Trimming Procedure

1. Calculate trim amount:

   • Frequency shift needed = (Current minimum - Target) / Target
   • Length change = Frequency shift × Current length
   • Example: If resonant at 14.0 MHz but want 14.2 MHz:
     • Shift = (14.2 - 14.0) / 14.0 = 1.4%
     • Trim = 0.014 × 33 feet = 0.46 feet (5.5 inches) total
     • Remove 2.75 inches from EACH leg

2. Trim conservatively - Remove half the calculated amount first

3. Re-measure - Check SWR again

4. Repeat until SWR < 1.5:1 at target frequency

Fine Tuning Tips

• Trim from ends only - Keep center connection intact
• Cut equally - Remove same amount from both sides
• Use crimp connectors - Allows adjustments without resoldering
• Mark minimum SWR frequency - Track your progress
• Be patient - Multiple iterations are normal

Bandwidth Considerations

Dipole bandwidth depends on wire thickness:

| Wire Diameter | 2:1 SWR Bandwidth @ 20m | | --------------------- | ----------------------- | | Thin (#18 AWG, 1mm) | ~200 kHz (1.4%) | | Medium (#14 AWG, 2mm) | ~300 kHz (2.1%) | | Thick (#10 AWG, 3mm) | ~400 kHz (2.8%) | | Very thick (6mm) | ~500 kHz (3.5%) |

Common Mistakes and Fixes

Mistake 1: Tuning at Wrong Height

Problem: Tuned near ground, doesn't work when raised Solution: Always tune at final operating height

Mistake 2: Trimming Too Much

Problem: Antenna now too short, can't be lengthened Solution: Start long, trim conservatively (half the calculated amount)

Mistake 3: Asymmetric Trimming

Problem: Pattern becomes directional, high SWR Solution: Always trim equal amounts from both legs

Mistake 4: Poor Connections

Problem: High SWR, intermittent operation, power loss Solution: Use quality connectors, weatherproof all joints, solder properly

Mistake 5: Ignoring Feedline Loss

Problem: SWR looks good but signals are weak Solution: Use low-loss coax (LMR-400 or better) for runs over 50 feet

Advanced Topics

Multi-Band Dipoles

Operate a single dipole on multiple bands:

1. Harmonics - Odd harmonics work (e.g., 40m dipole on 15m)
2. Trap dipoles - LC traps isolate sections
3. Fan dipoles - Multiple dipoles from one feedpoint
4. Off-center fed (OCFD) - Operates on multiple bands with tuner

Broadbanding Techniques

Increase bandwidth:

1. Thicker wire - Use #8-#10 AWG or even thicker
2. Cage dipole - Multiple parallel wires (3-6 wires in circle)
3. Folded dipole - Higher impedance (300Ω), wider bandwidth
4. Tapered elements - Thicker at center, thinner at ends

Matching and Feed Systems

Standard dipole impedance:

• Free space: ~73Ω resistive
• λ/2 height: ~50-60Ω (good match to 50Ω coax)
• λ/4 height: ~35-40Ω (mismatch on 50Ω coax)
• Very low (<λ/8): ~20-30Ω (poor match, high ground loss)

Matching solutions:

• 50Ω coax direct - Works well at λ/2 height
• 1:1 balun - Prevents feedline radiation
• 4:1 balun - For low dipoles or folded dipoles

Verification and Testing

After tuning, verify performance:

1. SWR sweep - Check across entire band
2. Pattern check - Make contacts in all directions
3. Signal reports - Compare to other antennas
4. Dummy load test - Verify power output
5. Feedline inspection - Check for damage or water ingress

Further Reading

Classic References

• "The ARRL Antenna Book" - Chapter on Wire Antennas
• "Antenna Physics: An Introduction" by Robert J. Zavrel, Jr. W7SX
• L.B. Cebik, W4RNL writings - Extensive dipole analysis

Online Calculators

• 66pacific Dipole Calculator - Trusted reference calculator
• M0UKD Dipole Calculator - Includes inverted vee
• AA5TB Reference - Scientific approach to antenna math

Related HamCalc Articles

• Magnetic Loop Efficiency - Compare dipole vs. loop characteristics
• Materials Library - Wire specifications and selection
• RF Safety Guidelines - Safe installation and operation

Summary

The dipole antenna remains a cornerstone of amateur radio for good reasons:

• Simple formula: Start with 468/f (MHz) in feet or 143/f in meters
• Corrections needed: End effect (2-5%), inverted vee (~3-6%), height, and environment
• Tuning process: Start long, trim conservatively, measure SWR repeatedly
• Bandwidth: 2-4% of operating frequency with standard wire
• Performance: 95%+ efficiency when properly installed

Understanding these principles and following proper tuning procedures will result in an excellent-performing antenna that serves you well for years.

Ready to build your dipole? The Dipole Calculator provides all the dimensions and corrections you need!