1. Antennas and Propagation
Impedance Matching and Measurement Techniques

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) VSWR Reflection
Return loss dB
1:1
0.00
infinity
1.1:1
0.05
26.44
1.2:1
0.09
20.83
1.5:1
0.20
13.98
1.9:1
0.31
10.16
2.0:1
0.33
9.54
3.0:1
0.50
6.02
4.0:1
0.60
4.44
5.0:1
0.67
3.52
6.0:1
0.71
2.92
10:1
0.82
1.71
infinity:1
1.000

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 arms unbalance 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