- 2079 Chaitra · 3 marks
Antenna input impedance
Short answer
Input impedance Z_in = R_in + jX_in, where R_in includes radiation resistance Rr and loss resistance R_L. Antenna is resonant when X_in=0. For a λ/2 dipole Z_in ≈73+j42.5 Ω (shortened to ~73 Ω resonant). Matching ensures maximum power transfer.
Explain the input impedance of an antenna.
Input impedance is the impedance seen at the antenna's feed terminals, i.e. the ratio of voltage to current there (or of E to H at a reference point).
Z_in = R_in + j X_in
R_in = Rr + RL
- Rr (radiation resistance) represents power radiated; RL represents ohmic and dielectric loss.
- X_in represents energy stored in the near field.
- The antenna is resonant when X_in = 0.
Z_in depends on frequency, antenna length, feed point, and nearby objects/ground. Examples: λ/2 dipole ≈ 73 + j42.5 Ω (slightly shortened → 73 Ω), λ/4 monopole ≈ 36.5 Ω, folded dipole ≈ 292 Ω.
For maximum power transfer, Z_in must match the line Z0 (Γ = (Z_in − Z0)/(Z_in + Z0) → 0). Mismatch gives VSWR > 1 and reflected power; matching networks or baluns are used.
✓ Answer checked by Claude Opus 5.5 (high effort)
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