- 2081 Chaitra · 3+6 marks
Folded dipole and Yagi design 785–895 MHz
Short answer
Folded dipole: in-phase currents on both conductors, half-sine distribution, figure-8 pattern like a dipole, directivity 1.64, impedance 292 Ω. Yagi design: fc=840 MHz, λc=35.71 cm; reflector=18.00 cm, driven=17.14 cm, directors=16.29,15.47,14.70 cm; reflector–driven spacing 8.93 cm, director spacing 5.36 cm, boom 25.0 cm.
Sketch the current distribution and radiation pattern of folded dipole antenna. Design a 5 element Yagi-Uda antenna for the operating frequency band of 785 MHz to 895 MHz and dipole as a driven element. Take an effective length of the dipole 0.48λc.
Folded dipole: current distribution and pattern
A folded dipole is two parallel λ/2 conductors joined at both ends, with one of them split at the centre for feeding. The currents in both conductors are in the same direction and phase, each following a half-sine distribution, so it radiates like a dipole carrying double current.
Current distribution (both arms in phase)
_________________________
| --> --> I --> --> |
| |
|__--> --> ~ --> -->__|
max at centre, zero at ends
Pattern (plane containing the dipole)
90°
.-""-.
\ /
0° ----[==]---- 180° nulls along
/ \ dipole axis
'-..-'
270° figure-8
- Input impedance: Z = N² × 73 Ω = 4 × 73 ≈ 292 Ω for 2 equal conductors.
- Pattern: same figure-8 as a λ/2 dipole in the plane containing it; omnidirectional in the plane perpendicular; directivity 1.64.
- Wider bandwidth than a simple dipole; ideal driven element of a Yagi.
Design of 5-element Yagi for 785–895 MHz
5 elements = 1 reflector + 1 driven folded dipole + 3 directors.
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: Centre frequency and wavelength
fc = (f₁ + f₂)/2 = (785 + 895)/2 = 840 MHz
λc = 3×10⁸ / 840×10⁶ = 0.3571 m = 35.71 cm
Step 2: Element lengths
L_DE = 0.48 × 35.71 = 17.14 cm
L_R = 1.05 × 17.14 = 18.00 cm
L_D1 = 0.95 × 17.14 = 16.29 cm
L_D2 = 0.95 × 16.29 = 15.47 cm
L_D3 = 0.95 × 15.47 = 14.70 cm
Step 3: Spacings and boom
S_R = 0.25 × 35.71 = 8.93 cm
S_D = 0.15 × 35.71 = 5.36 cm
Boom = 8.93 + 3 × 5.36 = 25.00 cm
| Element | Length (cm) | Distance from R (cm) |
|---|---|---|
| Reflector | 18.00 | 0 |
| Driven (folded dipole) | 17.14 | 8.93 |
| Director 1 | 16.29 | 14.29 |
| Director 2 | 15.47 | 19.64 |
| Director 3 | 14.70 | 25.00 |
R DE D1 D2 D3
| | | | |
18.0 17.14 16.29 15.47 14.70 cm
| (o) | | | ===> beam
|<8.93>|<5.36>|<5.36>|<5.36>|
<------- boom 25.0 cm ------>
The band is 110 MHz wide (≈ 13 % of fc), which is large for a Yagi; the folded dipole driven element helps by its wider bandwidth. Element diameter ≈ 0.005λ ≈ 2 mm.
Answer: fc = 840 MHz, λc = 35.71 cm; R = 18.00 cm, DE = 17.14 cm, D1 = 16.29 cm, D2 = 15.47 cm, D3 = 14.70 cm; spacings 8.93 cm and 5.36 cm; boom 25.0 cm.
✓ Answer checked by Claude Opus 5.5 (high effort)
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