Yagi advantages and 3-element 3 GHz design

Advantages of Yagi antenna

1. Good gain (≈ 7 dBi for 3 elements, up to ~15 dBi) and directivity with a single feed point.
2. Unidirectional beam with good front-to-back ratio, reducing interference and ghost images.
3. Light, compact, cheap and easy to build from rods and a metal boom (no insulation needed at element centres).
4. Folded-dipole driven element gives easy matching to 300 Ω/75 Ω lines.
5. Can be stacked for more gain; easy to rotate and point.

Design of 3-element Yagi at 3 GHz

3 elements = 1 reflector + 1 driven dipole + 1 director. Driven element length is not given, so the usual value 0.48λ is assumed.

Design rules used (standard empirical Yagi rules):

λc   = c / fc
L_DE = 0.48 λc               (driven, assumed)
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

λ = 3×10⁸ / 3×10⁹ = 0.1 m = 10 cm

Step 2: Element lengths

L_DE = 0.48 × 10   = 4.80 cm
L_R  = 1.05 × 4.80 = 5.04 cm
L_D  = 0.95 × 4.80 = 4.56 cm

Step 3: Spacing and boom

S_R  = 0.25 × 10 = 2.5 cm
S_D  = 0.15 × 10 = 1.5 cm
Boom = 2.5 + 1.5 = 4.0 cm

   R      DE     D
   |      |      |
  5.04   4.80   4.56 cm
   |     (o)     |   ===> beam
   |<2.5->|<1.5->|
   <-- boom 4.0 cm -->

At 3 GHz the elements are small rods (diameter ≈ 0.5–1 mm, ≈ 0.005–0.01λ), often printed on a PCB.

Answer: λ = 10 cm; reflector 5.04 cm, driven 4.80 cm, director 4.56 cm; spacings 2.5 cm and 1.5 cm; boom 4.0 cm.