Antennas and Propagation

1. Impedance Matching and Measurement Techniques Antennas and Propagation

2. 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

3. 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:

4. Impedance Matching • Voltage Standing Wave Ratio (VSWR)

5. Impedance Matching • Lumped Element Matching

6. 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.

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

8. Impedance Matching The input impedance is simply RS.

9. 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

10. Impedance Matching

11. Impedance Matching • lumped circuit Distributed Circuit • Equivalent Circuit

12. Impedance Matching • Transmission line matching • Shorted Stub Open Stub

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

14. 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 armsunbalance the • antenna and the coaxial feed • Balun’s are used to balance the • currents

15. 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 I3 • Does not affect antenna input impedance • Frequency dependent

16. 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 I4 is approximately equal to I3 which means a balanced feed

17. 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