Short note: Parasitic array antenna

A parasitic array is an antenna array in which only one element (the driven element) is fed from the transmitter; the other elements, called parasitic elements, are not connected to the feed line. They get current by mutual coupling (induction) from the driven element and re-radiate, so that the total field is strengthened in one direction. The best-known example is the Yagi–Uda antenna.

 Reflector  Driven   D1    D2    D3
    |         |      |     |     |   -> max radiation
    |        =|=     |     |     |
    |         |      |     |     |
 <-0.25λ-> <-0.1 to 0.3λ spacing->

Elements

• Driven element: a half-wave dipole or folded dipole (about 0.46–0.48λ), connected to the feeder.
• Reflector: about 5% longer than the driven element (≈0.5λ). Its reactance is inductive, so its re-radiated field cancels radiation behind it.
• Directors: about 5% shorter (≈0.43–0.45λ). Their reactance is capacitive, so their fields add in the forward direction.
• Spacing: 0.1λ to 0.3λ between elements.

Working: the current induced in a parasitic element depends on its length (its self-reactance) and its spacing (the phase delay). With the right length and spacing, the fields from all elements add in phase toward the directors and nearly cancel toward the reflector, giving an end-fire, unidirectional beam.

Features

• Gain about 7 dB for 3 elements, rising with more directors (but slowly).
• Narrow bandwidth (a few %), since element lengths are resonant.
• Feed impedance of the dipole drops (to about 20–25 Ω), so a folded dipole is used to raise it.
• Simple, light and cheap; used for TV reception, VHF/UHF links and amateur radio.