At a given frequency only part of the structure is active. This active region moves along the antenna as frequency changes.
Antennas · F1SYH
Log-periodic antenna
A log-periodic antenna is a wideband directional array whose element lengths and spacing follow a geometric progression.
HF
VHF
Large bande directive
Operating principle
Main characteristics
Typical gain
≈ 6 à 10 dBi
Impedance
Conçue généralement pour 50 Ω
Polarization
Selon orientation
Difficulty
Avancée
Dimensions and design
Dimensions are controlled by scale and spacing factors. The whole structure and feed line must be calculated together.
Construction and tuning
Step 1
Define the frequency range precisely.
Step 2
Choose the progression parameters.
Step 3
Calculate every element and spacing.
Step 4
Build a balanced boom transmission line.
Step 5
Check impedance across the complete range.
Advantages
- Wide frequency range.
- Relatively constant directivity.
- Stable impedance when correctly designed.
- One antenna for several bands.
Limitations
- Less gain than a single-band optimized Yagi.
- Large number of elements.
- Complex construction and feeding.
- Long boom.
History and origin
Log-periodic antennas were developed during the 1950s through university research into frequency-independent structures.
Safety
Wind loading and numerous fasteners require a strong mechanical structure.