Antenna Theory Basics

Understanding antenna theory is crucial for successful amateur radio operations. This document covers the essential concepts every operator should know.

Fundamental Concepts

Wavelength and Frequency

The relationship between wavelength (λ) and frequency (f) is fundamental:

$$ \lambda = \frac{c}{f} $$

Where c is the speed of light (≈ 300,000,000 m/s).

Resonance

An antenna is resonant when its electrical length matches the operating frequency. For a half-wave dipole:

• Length ≈ 468 / frequency (MHz) in feet
• Length ≈ 142.65 / frequency (MHz) in meters

Common Antenna Types

Dipole

The half-wave dipole is the fundamental antenna type:

• Simple construction
• Omnidirectional in horizontal plane
• Feed impedance: ~72Ω

Vertical

Vertical antennas offer low-angle radiation:

• Requires ground plane
• Better for DX contacts
• Smaller footprint

Yagi

Directional beam antenna:

• High gain
• Narrow beamwidth
• Requires rotation

Practical Considerations

Height Above Ground

Antenna height significantly affects performance:

• HF antennas: 1/2 wavelength or higher
• VHF/UHF: Height is critical for line-of-sight

Feed Line Selection

Choose appropriate coaxial cable:

Type	Impedance	Loss @ 100MHz	Use Case
RG-58	50Ω	4.4 dB/100ft	Low power, short runs
RG-8X	50Ω	2.8 dB/100ft	General purpose
LMR-400	50Ω	1.5 dB/100ft	High power, long runs

Testing and Tuning

SWR Measurement

Standing Wave Ratio indicates antenna match:

• SWR 1:1 - Perfect match
• SWR < 1.5:1 - Acceptable
• SWR > 2:1 - Needs adjustment

Use an antenna analyzer for best results.

References

• ARRL Antenna Book
• ON4UN’s Low Band DXing
• Antenna Modeling Software (EZNEC, 4NEC2)