Transcript EEE412 Antennas and Propagation

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 dB
1:1
1.1:1
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
0.60
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 I3
• 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 I4 is approximately equal to I3 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