Impedance of simple dipole 📡
The impedance of a simple dipole antenna is the measure of how much resistance and reactance it offers to the flow of alternating current (AC) at a given frequency. It depends on several factors, including the length of the dipole and the frequency of operation.
For a half-wave dipole antenna, which is the most common configuration, the impedance can be approximated as:
Zdipole≈73 Ω + j⋅42 ΩZ_{dipole} \approx 73 \, \Omega \, + \, j \cdot 42 \, \Omega
Here:
- ZdipoleZ_{dipole}Â is the impedance of the dipole antenna
- 73 Ω73 \, \Omega is the real part, representing the resistance (which corresponds to the radiative resistance of the antenna)
- 42 Ω42 \, \Omega is the imaginary part, representing the reactance (which is due to the antenna’s capacitance and inductance)
This is for a center-fed half-wave dipole antenna in free space, operating at the resonant frequency.
Key Points:
- Resonant Frequency: At the resonant frequency of the antenna, the impedance is purely real (or close to it), and the reactance component (imaginary part) becomes very small.
- Impedance Matching: To maximize power transfer from the transmitter to the antenna, you need to match the antenna impedance to the characteristic impedance of the transmission line, typically 50 Ω.
If the dipole is not resonant (i.e., not operating at the correct frequency), the impedance will shift, and the reactance will increase.