V Broadband Antennas Broadband Antenna Bandwidth Broadband Nonresonant

V. Broadband Antennas

Broadband Antenna • Bandwidth • Broadband • Non-resonant type -traveling wave antenna, vee, rhombic antenna -helical, log-periodic antenna -conical antenna

Traveling wave antenna For short-wave band: 2~30 MHz

Traveling wave antenna in free space Unnormalized pattern factor Pattern with n forward lobe Elemen t factor

Traveling wave antenna

Traveling wave antenna Traveling wave VEE antenna in free space

Traveling wave antenna Rhombic antenna in free space

Traveling wave antenna Ground effect (perfect ground)

Traveling wave antenna Beverage antenna: Traveling wave ant. above the earth with imperfect conducting properties Tangential Electric field on earth is not zero =>1. inducing a current on the antenna conductor. 2. due to unbalanced operation, can radiate the field. Loss and standing wave

Beverage antenna Loss: radiation and dissipation in earth Reflection from load end

Helical antenna Mode of Helix

Normal mode Helix Direction of maximum directivity: Normal to the axis of Helix Normal mode

Normal mode Helix For circular polarization, Normal mode helix is usually operated as linearly polarized

Normal mode Helix Resonant stub helix antenna on ground plane

Axial mode Helix Hansen. Woodyard

Axial mode Helix Useful axial mode helix design

With tapered helix, bandwidth and axial ratio are improved uniform end taper continous taper nonuniform Polarization LHCP RHCP

Biconical Antenna Analysis Spherical TEM wave Infinite bicone

Biconical Antenna

Finite biconical Antenna Non-ideal open: non TEM higher mode

conical monopole antenna monopole bandwidth

Discone antenna disc+cone

Sleeve antenna(monopole) Virtual feed

Sleeve antenna(dipole)

Open-sleeve dipole

Frequency independent antennas • Bandwidth over 10: 1 -pattern, impedance, polarization, phase center. . • Frequency-independent antennas are designed to minimize finite length and maximize angular dependence. Helix - geometry of angular dependent • Self-complimentary structure • Thick metal biconical, bow-tie • Self-scaling property: most radiation takes place from the active region

Self complimentary: A antenna and its complimentary are identical complimentary Frequency independent

Equiangular antenna of geometry enlarged antenna For enlarged surface coincides with original one

Spiral antennas Equiangular spiral Expansion ratio truncated

Equiangular spiral Frequency range(bandwidth) Antenna impedance Finite thickness Coaxial feed Pattern HPBW Polarization: circular as helix

Archimedean spiral 1. feed to active region ->act as transmission line 2. active region ->radiation takes place 3. Beyond the active region ->currents can be neglected active region: portion of the antenna that is one wavelength in circumference Circular polarization

Cavity-backed Archimedean spiral For unidirectional beam

Conical equiangular spiral Main beam

Log-periodic Antennas Logarithmically periodic structure The electrical properties vary periodically with the logarithm of the operating frequency Scaling of antenna dimension Scaling of operating frequency Spiral antenna: continuously scaled structure scaling->rotation of the structure

Log-periodic Self complementary Active region: tooth with length of quarter wavelength

Main beam direction: normal to the plane of antenna(bidirection) Polarization: linear polarization normal to that of bow-tie(without teeth) Operating band: determined by the length of the largest and the smallest tooth

Log-periodic toothed wedge(unidirectional) Log-periodic toothed trapezoid Log-periodic wire Trapezoid zig-zag Log-periodic wire Trapezoid(circular pol. )

Log-periodic dipole array(LPDA) Series-fed array of log periodically scaled parallel wire dipoles Main beam

Log-periodic dipole array(LPDA) Log-periodic