Yagi driven element and 760 MHz design

Role of the driven element

The driven element is the only element of a Yagi-Uda antenna connected to the feed line; usually a resonant λ/2 dipole or folded dipole.

• It receives RF power from the transmitter (or delivers received power to the receiver) — the single feed point of the array.
• Its radiated field induces currents in the reflector and directors by mutual coupling; without it the parasitic elements would carry no current.
• It fixes the operating (resonant) frequency and the input impedance; a folded dipole (≈ 292 Ω alone) compensates for the impedance drop caused by the close parasitic elements and matches the feeder.
• Its position between reflector and directors sets the phase relationships that produce the forward beam.

Design of 5-element Yagi at 760 MHz

Design rules used (standard empirical Yagi rules):

λc   = c / fc
L_DE = 0.48 λc               (driven, given)
L_R  = 1.05 L_DE  (≈ 0.5λc)  (reflector 5 % longer)
L_D1 = 0.95 L_DE, L_Dk+1 = 0.95 L_Dk  (5 % shorter each)
S_R  = 0.25 λc   (reflector to driven)
S_D  = 0.15 λc   (driven to D1 and between directors)

Step 1: Wavelength

λc = 3×10⁸ / 760×10⁶ = 0.3947 m = 39.47 cm

Step 2: Element lengths

L_DE = 0.48 × 39.47 = 18.95 cm
L_R  = 1.05 × 18.95 = 19.89 cm
L_D1 = 0.95 × 18.95 = 18.00 cm
L_D2 = 0.95 × 18.00 = 17.10 cm
L_D3 = 0.95 × 17.10 = 16.25 cm

Step 3: Spacings and boom

S_R  = 0.25 × 39.47 = 9.87 cm
S_D  = 0.15 × 39.47 = 5.92 cm
Boom = 9.87 + 3 × 5.92 = 27.63 cm

  R     DE    D1    D2    D3
  |     |     |     |     |
 19.89 18.95 18.00 17.10 16.25 cm
  |    (o)    |     |     | ===> beam
  |<9.87>|<5.92>|<5.92>|<5.92>|
  <------ boom 27.63 cm ------>

Answer: λc = 39.47 cm; R = 19.89 cm, DE = 18.95 cm, D1 = 18.00 cm, D2 = 17.10 cm, D3 = 16.25 cm; S_R = 9.87 cm, S_D = 5.92 cm; boom ≈ 27.6 cm.