Broadband Antennas宽带天线.ppt

Broadband Antennas Neal Mansfield Joseph Pete Matthew Robinson Steven Summers Overview Typical antennas are designed for a specific narrow band of operation Broadband antennas are designed to operate effectively over a wide range of frequencies Public two-way-radio VHF covers 130-174Mhz Public two-way-radio UHF 406-512Mhz The challenge is to create an antenna which can operate in both the VHF and UHF bands Bandwidth Broad Spectrum of Types Helical Log Periodic Biconical Sleeve Spiral Biconical Antenna Types of Biconical Infinite Biconical Finite Biconical Discone http://www.ik8uif.it/biconical-10ghz.htm Log Periodic Antenna The antenna is ideally suited for reception of VHF/UHF point-to-point communication where its directional characteristics can significantly improve rejection of interfering signals. In professional applications, this antenna is ideally suited for EMC pre-testing, surveillance and monitoring. The antenna covers a frequency range of 230 to 1600 MHz (a much wider frequency range can be received with reduced gain). /home/ax31b.htm Sleeve Antenna The sleeve antenna is used primarily as a receiving antenna. It is a broadband, vertically polarized, omnidirectional antenna. Its primary uses are in broadcast, ship-to-shore, and ground-to-air communications. Although originally developed for shore stations, there is a modified version for shipboard use. /inch/32.htm http://www.emartin.it/it9vky/Risorse/opensleeve.htm Spiral Antenna The spiral antenna is used primarily as a receiving antenna Vertically polarized Frequency Independent Designed to minimize finite lengths and maximize angular dependence Helical Antenna Directional Circularly Polarized Polarization changes with time Both high gain and wide band Geometry Normal Mode Radiation pattern similar to linear dipole The dimensions of the helix are small compared to the wavelength Narrow in bandwidth Radiation efficiency is small Rarely used Axial Mode Circular Polarization ?C/λ4/3 C/λ=1:nea