Antennas and Propagation Impedance Matching and Measurement Techniques

Antennas and Propagation Impedance Matching and Measurement Techniques

Impedance Matching Problem: • Measurement systems have fixed impedance usually 50Ω • However most of the antennas have characteristic impedances that can not be adjusted during design • Antenna feeds can be unbalanced, balun circuitry is needed to balance them For maximum power transfer • Reactive components should be eliminated • Active Components Should be equal to 50Ω

Impedance Matching Input Impedance and Reflection (Г) The reflection coefficient from the Antenna Input is: Reflected Power from the Antennas is given by And Transmitted power to the antenna is given by Return loss can ve evaluated by:

Impedance Matching Voltage Standing Wave Ratio (VSWR) VSWR Reflection Return loss d. B 1: 1 1. 2: 1 1. 5: 1 1. 9: 1 2. 0: 1 3. 0: 1 4. 0: 1 5. 0: 1 6. 0: 1 10: 1 infinity: 1 0. 00 0. 05 0. 09 0. 20 0. 31 0. 33 0. 50 0. 67 0. 71 0. 82 1. 000 infinity 26. 44 20. 83 13. 98 10. 16 9. 54 6. 02 4. 44 3. 52 2. 92 1. 71 0. 00

Impedance Matching Lumped Element Matching

Impedance Matching Lumped Element Matching • • • Lumped element networks are used to cancel the reactive component of the load and transform the real part so that the full available power is delivered into the real part of the antenna Can be used to match antennas whose resistance is less than that of the transmission line, and whose reactance can be set by shortening the length of the radiating element from the resonant length. If the reactance is capacitive, adding a shunt inductor will cancel the reactive part of the antenna admittance and result in a match to the transmission line.

Impedance Matching • We want to match RP to RS and cancel reactances with a conjugate match.

Impedance Matching The input impedance is simply RS.

Impedance Matching Distributed Elements: • Matching with transmission line elements • This method is preferred when; – at higher frequencies – when parasitics of lumped elements cannot be controlled – when very small capacitors or inductors are required • Transmission line characteristic impedance • Transmission line equivalent circuit

Impedance Matching

Impedance Matching lumped circuit Equivalent Circuit Distributed Circuit

Impedance Matching Transmission line matching Shorted Stub Open Stub

Impedance Matching Transmission line matching For a stub of length λ/8 Short Stub: Inductor Open Stub: Capacitor

BALUN Balun: (Balanced-to-unbalanced feed) • Even if the impedance of the antenna and the transmission line are matched, unbalanced field current distributions of the coaxial line may cause reflections • The unequal currents on the dipole’s arms unbalance the antenna and the coaxial feed • Balun’s are used to balance the currents

BALUN Bazooka Balun • A sleeve is added to the coaxial antenna • The sleeve and the outer coaxial line form another coaxial line with impedance Z’c • Since it is shorted, its impedance at the antenna terminals is infinite (λ/4) • Large input impedance suppresses I 3 • Does not affect antenna input impedance • Frequency dependent

BALUN Folded Balun • A λ/4 coaxial line is added to the feed line forming a twin-lead transmission line with infinite impedance at antenna side. • The current I 4 is approximately equal to I 3 which means a balanced feed

BALUN Broadband baluns • Gradually changing the transmission line from unbalanced transmission line to balanced one. • Ferromagnetic transformers can be used at lower freuencies where changing the transmission line type is not practical