A half-wave dipole resonant at 1420 MHz (the hydrogen line) has essentially the same impedance as at any other frequency:
- ≈ 73 Ω resistive in free space at resonance
- Very small reactive component (≈ 0 Ω) when correctly trimmed
In practice
The actual feedpoint impedance will vary depending on construction and environment:
- Typical real-world range: 65–75 Ω
- Over ground (≥ ½ λ high): often 60–70 Ω
- Thicker elements: slightly lower impedance
- Nearby structures / balun / feedline: can shift it further
Matching notes (useful at 1420 MHz)
- Directly feeding with 50 Ω coax gives a mismatch of about 1.5:1 VSWR (usually acceptable)
- For best sensitivity (important for radio astronomy):
- 1:1.5 impedance transformer, or
- Folded dipole (~300 Ω) with a 4:1 balun, or
- Quarter-wave matching section