Antennas/Antenna Arrays

Objectives Explain how the elementary dipole antenna transmits electromagnetic waves. Calculate optimum antenna height for a given frequency both grounded and ungrounded. Identify the two types of antenna polarization. Interpret the beam power distribution pattern for a single dipole antenna and a linear antenna array (broadside and end fire) and identify sidelobes. Describe how quasi-optical systems increase antenna directivity.

Maxwell’s Theory A time varying electric field (E) (from an alternating source - oscillator) will generate a time-varying magnetic field (B). A magnetic field will generate an electric field. So, an E-field will produce a B-field that produces an E-field that produces …. The E-field / B-field interaction produces a propagating electromagnetic wave perpendicular to E & B fields. There exists a continuous transfer of energy between fields.

Generation of Electromagnetic Radiation E difference along conducting dipole produces electrical field Field lines represent field strengths of same magnitude Outer lines further apart due to repulsion of like poles Field lines collapse when voltage difference between ends decreases Delay in reaction and repulsion causes outer field to separate before collapsing Electric field reversed, opposite field lines produced. Since field lines are in same direction, looped field pushed away at speed of light

Formation of Electric and Magnetic Fields Conducting Dipole Antenna

Electromagnetic Field Orientation Vertically Polarized Antenna Electric Field determines Polarization

Elementary Dipole Antenna Simplest form of Antenna Optimum Length is l/2 If mounted into ground (vertical), length reduced to l/4 Reflection off of ground gives it virtual l/2 length

Polarization & Antenna Orientation Linear Polarization (horizontal or vertical) Circular Polarization (for 3D maneuvering) Antenna Polarized in direction of Electric Field Transmit and receive antennas should have same polarization in order to receive optimum energy Vertically polarized lens allows only vertical component of light through. When Horizontally polarized lens is placed between, no light passes through.

Circularly Polarized Antenna

Antenna Beam Forming High sensitivity/High Directivity increases radar detection range and exploits weak signals Radars separated by ½ wavelength to utilize constructive/destructive interference Directionality is a function of either waveguide physical design or constructive/destructive interference.

Directivity Ability to focus energy in a specific direction (azimuth and elevation) Power Density of beam not uniform Beamwidth measured at 3 dB down point in az/elev Search Radar - larger beamwidth for detection and tracking Fire Control Radar – smaller beamwidth for accurate targeting solution

Linear Array

Linear Array Broadside – Propagates on Z-Axis, perpendicular to antenna array. Signal from individual elements transmitted in phase. End Fire – Radiates only on X-Axis, in-line with antenna array. Element signals transmitted 180 degs out of phase Individual phase shifts between 0 – 180 degs will result in antenna pattern shifting

Linear Array Electronic Steering By inducing phase shift between elements, can electronically focus beam and steer it No mechanical breakdown, physical restrictions, and can use same system for multiple roles (Search, Tracking, Fire Control) simultaneously

Phased Array

Quasi-Optical Systems Parasitic Elements - used to concentrate the beam in one direction only . A current is induced in the element to cause destructive interference in specific direction. Reflectors Reflective material placed near radiating antennas. Parabolic shapes (dishes) used to concentrate energy into a narrow beam (i.e. radar reflectors).

Objectives Explain how the elementary dipole antenna transmits electromagnetic waves. Calculate optimum antenna height for a given frequency both grounded and ungrounded. Identify the two types of antenna polarization. Interpret the beam power distribution pattern for a single dipole antenna and a linear antenna array (broadside and end fire) and identify sidelobes. Describe how quasi-optical systems increase antenna directivity.