Antennas and Propagation

Impedance Matching and Measurement Techniques

Impedance MatchingProblem:•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:

Voltage Standing Wave Ratio (VSWR)

Impedance Matching

VSWR Reflection Return loss dB1:1 0.00 infinity1.1:1 0.05 26.441.2:1 0.09 20.831.5:1 0.20 13.981.9:1 0.31 10.162.0:1 0.33 9.543.0:1 0.50 6.024.0:1 0.60 4.445.0:1 0.67 3.526.0:1 0.71 2.9210:1 0.82 1.71infinity:1 1.000 0.00

Impedance MatchingLumped Element Matching

Impedance MatchingLumped 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 MatchingDistributed 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 Matchinglumped circuit Distributed Circuit

Equivalent Circuit

Impedance MatchingTransmission line matching

Shorted Stub Open Stub

Impedance MatchingTransmission line matchingFor a stub of length λ/8Short Stub:

Inductor

Open Stub:

Capacitor

BALUNBalun: (Balanced-to-unbalanced feed)• Even if the impedance of the antenna

and the transmission line are matched, unbalanced field current distributions of thecoaxial 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

BALUNBazooka 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

BALUNBroadband 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