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Yagi parasitic elements and LPDA comparison

Short answer

The parasitic elements are one reflector and several directors. The longer inductive reflector cancels backward radiation, while shorter capacitive directors provide end-fire phasing, increasing gain and narrowing the beam. Unlike the narrowband Yagi with one driven element, the LPDA feeds all scaled elements and gives very wide bandwidth.

Name the parasitic elements used in Yagi-Uda array. Explain their significance in the array. Compare Yagi-Uda antenna with Log periodic dipole array.

Parasitic elements of a Yagi-Uda array

The Yagi-Uda array has one driven element (fed λ/2 or folded dipole) and two kinds of parasitic elements (not fed; excited by mutual coupling):

  1. Reflector – one element, slightly longer than λ/2, placed behind the driven element.
  2. Directors – one or more elements, slightly shorter than λ/2, placed in front.
  R      DE     D1     D2     D3
  |      |      |      |      |
  |      |      |      |      |  ====> beam
  |     -o-     |      |      |
  |      |      |      |      |
 longest  fed   shorter ->

Significance

  • Reflector: being longer than resonant length, it is inductive; its induced current lags so that its field cancels radiation in the backward direction and reinforces the forward direction. It sets the front-to-back ratio. Usually one reflector is enough; more add little.
  • Directors: shorter, so capacitive; their currents lead and give a progressive phase towards the front, guiding the wave like an end-fire array. Each extra director increases gain (about 1 dB per director for the first few) and narrows the beam.
  • Parasitic coupling lowers the input impedance of the driven element, so a folded dipole (≈ 300 Ω alone) is often used to restore a convenient impedance.

Yagi-Uda vs log periodic dipole array (LPDA)

PointYagi-UdaLPDA
Elements fedOnly one (driven); rest parasiticAll elements fed by a transposed line
Element sizesNearly equal (~0.45–0.5λ)Grow by ratio τ along the antenna
BandwidthNarrow (≈ 2–5 %)Very wide (10:1 possible), frequency independent
GainHigher for same boom, 7–15 dBiModerate, 6–10 dBi, almost constant
Beam directionTowards the directorsTowards the shortest elements
Input impedanceVaries with frequencyNearly constant over band
DesignSimple, empiricalUses τ, σ, apex angle α
UseTV for one channel, fixed linksMultiband TV, EMC testing, HF wideband

✓ Answer checked by Claude Opus 5.5 (high effort)

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