Quiz!!

ZT75 System 50 Line

Determine ZT for a matched load.

Half Wave Dipole

lZT

For l = /2, ZT = 73 + 42.5j

For l = 0.95/2, ZT ~ 72

Strongest Radiation

Weaker Radiation

Weaker Radiation

Negligible Radiation

Negligible Radiation

Balanced Transmission LineGain ~ 1.64 ~ 2.15 dBi

Radiation Pattern exhibits line (cylindrical) symmetry with respect to Dipole Axis.

Quarter Wave Dipole(Vertical Marconi)

lZT

Balanced Transmission Line

Perfectly Conducting Planar Reflector(Ground Plane)

Unbalanced Transmission Line(Co-ax)

Reflected image of driven element.

For l = 0.95/4, ZT ~ 37

Radiation pattern is identical to half wave dipole above ground plane, zero radiation below ground plane.

Practical Quarter Wave Whip

Imperfectly Conducting Ground Plane

Unbalanced Transmission Line(Co-ax)

Strongest Radiation

Weaker Radiation

Weaker Radiation

Ground Plane Systems:1.Large Metal Sheet 2.Highly conductive terrain3.Radial conductors buried in earth4.Radial Conductors emanating from antenna mast (Counterpoise)

Folded Half Wave Dipole

l ~ /2ZTBalanced Transmission Line

• ZT ~ 288 • Provides consistent gain over broader bandwidth.• Pattern same as half wave dipole

Yagi-Uda Antenna

Strongest Radiation

The Yagi-Uda antenna is constructed with one driven element and several parasitic elements.

Driven element

Reflector

Directors

Directors Gain

3 9 dBi

6 11.5 dBi

11 13 dBi

Log Periodic Antenna

Log Periodic Antenna produces less gain than Yagi-Uda, but has broader bandwidth.

Loop Antennas

Direction of Propagation – Magnetic Field lines shown

dv N

dt

Yields maximum response when magnetic field lines are perpendicular to plane of loop – yields sharp null when magnetic field lines are parallel to plane of loop. Used for direction finding.

Dish Antennas

D

2

2 2

3

; 4

44 ;

16 4 (radians)

73 (degrees)

e

eD D

dBD

DA A A

AAG G G

G D

D

Problems in Text

X

XGD X

X

X

X

X

X X X X

X73

3dBX

X

X