Log-Periodic Antenna
Understanding Log-Periodic Antennas
The log-periodic antenna achieves wideband operation by having an active region that shifts along the array as frequency changes. At any frequency, the elements near half-wavelength are active while shorter and longer elements are nearly transparent to the wave.
LPDA Design
- Scale factor (tau): Ratio of consecutive element lengths. Typically 0.8-0.95. Higher tau = better performance but more elements.
- Spacing factor (sigma): Relative spacing between elements. Typically 0.05-0.2.
- Gain: 6-10 dBi. Nearly constant across the band.
- Bandwidth: Set by the longest element (lowest frequency) and shortest element (highest frequency).
LPDA Advantages
- Very wideband (10:1 or more achievable).
- Consistent gain and pattern across the band.
- Linear polarization with good cross-pol.
- Moderate gain without requiring a ground plane.
tau = L(n+1)/L(n) = 0.8 to 0.95 (scale factor)
sigma = d(n)/(2 x L(n)) = 0.05 to 0.15 (spacing)
Number of elements: N = 1 + log(f_max/f_min)/log(1/tau)
Example: 200 MHz to 2 GHz, tau = 0.9:
N = 1 + log(10)/log(1/0.9) = 1 + 21.9 = 23 elements
Gain: ~7 dBi across band
Frequently Asked Questions
What is a log-periodic antenna?
A log-periodic antenna uses multiple dipole elements of progressively increasing size to achieve wideband operation. At any frequency, the elements near resonance are active. This provides nearly constant gain over bandwidths of 2:1 to 10:1.
Where are log-periodic antennas used?
EMC compliance testing (replacing multiple narrowband antennas with one wideband antenna), broadband monitoring and surveillance, HF communications, TV reception (covering all UHF channels), and spectrum analysis.
What is the gain of a log-periodic antenna?
Typical gain is 6-10 dBi, nearly constant across the band. This is lower than a Yagi of similar size at a single frequency, but the LPDA operates over a much wider bandwidth. The trade-off is gain vs bandwidth.