Antenna

An antenna

To transfer maximum power from a transmission line (often 50 Ω) to the antenna, the antenna impedance must match the transmission line's characteristic impedance (ZO).

Antenna impedance

Antenna impedance (ZA) is the total impedance an antenna presents to the AC current at its input terminals, expressed as a complex quantity (ZA = RA + jXA), it consists of reactive and resistance components:

Reflections and VSWR

Voltage standing wave ratio (VSWR) 

is a ratio that measures the efficiency of radio frequency (RF) power is transmitted from a power source (like a transmitter) through a transmission line and into a load (like an antenna).

VSWR is mathematically defined as the ratio of the peak standing wave voltage to the minimum standing wave voltage: VSWR = Vmax/Vmin.

It also has a second formula, using the reflection coefficient (Γ):
VSWR = (1 + |Γ|)/(1 - |Γ|)

• Γ = Vreflected/Vforward = |(ZA-ZO)/(ZA+ZO)|

  • ZA = antenna impedance

  • ZO = transmission line impedance 

Impedance mistmatches cause RF energy to bounce back to the transmitter, creating standing waves and increase the VSWR, causing power loss and damage equipments.

• Matched system: If the transmission line's characteristic impedance matches the antenna impedance, 100% of the RF energy is absorbed and radiated.

• Mismatched system: If impedances doesn't match, a portion of the forward RF energy reflects back toward the transmitter.

• Standing waves: The forward wave and reflected wave interfere to each other along the transmission line, producing a stationary wave pattern with points of max and min voltage.

References

• [1] Wikipedia