Antennas and Wave Propagation

Antenna

The interface between the transmitter and free space and between free space and the receiver.

Radio Waves

Radio signals are called an electromagnetic wave because it made up of both electric and magnetic fields.

Magnetic Field

An invisible force filed created by a magnet. An antenna is a type of electromagnet.

Electric Field

An invisible force field produced by the present of a potential difference between two conductors.

Volts per meter

SI unit for electric field strength.

Permittivity

The dielectric constant of the material between the two conductors.

Transmission Line

Made up of a conductor or conductors. Do not radiate signals efficiently.

Wave Impedance or Impedance of Space

The ratio of the electric filed strength of a radiated wave to the magnetic field strength.

Near Field

Describes the region directly around the antenna where the electric and magnetic fields are distinct.

Far Field

Approximately 10 Wavelengths from the Antenna.

Polarization

The orientation of magnetic and electric fields with respect to the earth.

Antenna Reciprocity

Means that the characteristics and performance of an antenna are the dame whether the antenna is radiating or intercepting an electromagnetic signal.

Dipole Antenna

Also called a doublet, formally known as the Hertz Antenna. A two piece of wire, rod, or tubing that are one-quarter wavelength long at the operating resonant frequency.

End Effect

A phenomenon caused by any support insulators used at the ends of the wire antenna.

Conical Antenna

A common way to increase the bandwidth is to use a version of the dipole antenna known as _____. The advantage being the tremendous bandwidth. Can maintain a constant impedance and gain over a 4:1 range.

Radiation Pattern

The shape of the electromagnetic energy r adiated from or received by that antenna.

Beam Width

The measure of the antennas directivity.

Folded Dipole

It consists of two parallel conductors connected at the ends with one side open at the center for connection to the transmission line. 300 Ohm impedance. Usually offer higher bandwidth compared to standard dipoles.

Directivity

The antenna’s ability to send or receive signals over a narrow horizontal directional range.

Parasitic Array

Consists of a basic antenna connected to a transmission line plus one or more additional conductors that are not connected to the transmission line.

Parasitic Elements

Extra conductors in the parasitic array.

Yagi Antenna

Made up of a driven element (antenna) and one more parasitic elements.

Driven Arrays

An antenna that has two or more driven elements. Each element receives RF energy from the transmission line.

Colinear Antennas

Usually consist of two or more half wave dipoles mounted end to end. Generally used only on VHF and UHF bands because their length becomes prohibited at lower frequencies.

Broadside Array

A stacked collinear antenna. Highly directional radiation pattern that is broadside or perpendicular to the plane of the array.

End Fire Array

Used two HW Dipoles spaced one half wavelength apart. Has a bidirectional radiation pattern but narrower beam widths and lower gain.

Wide Bandwidth log periodic antenna

Special type of driven array. Very wide bandwidth compared to other antennas. Most TV antennas use this kind of antennas.

Impedance Matching

Most critical aspects of any antenna system. SWR should be 1:1, maximum power transfer will take place.

Q Section or Matching Stub

A one quarter wavelength of a coaxial or balanced transmission line of specific impedance that is connected between a load and source is used to match impedances.

Balun

Transformer used to match impedances.

Antenna Tuner

A variable inductor, one or more varactors, or combinations connected in various configurations to tune the antenna.

Reflection

The angle of reflection is equal to the angle of incidence.

Refraction

The bending of a wave due to the physical make up of the medium through which the wave passes.

Diffraction

Bending of waves around an object.

Huygen’s Principle

Assuming that all EM waves radiate as spherical waveforms from a source, each point on a wave front can be considered as a point source for additional spherical waves.

Ground Waves

Leave an antenna and remain close to the Earth. Follow the curvature of the earth and can travel at distances beyond the horizon. Must have vertical polarization.

Sky Waves

Signals radiated by the antenna into the upper atmosphere where they are bent back to Earth.

Direct Wave or Space Wave

Travels in a straight line directly from the transmitting antenna to the receiving antenna. Line of sight communication. Frequencies above 30MHz.

Repeater Stations

Extend the communication distance at VHF, UHF, and microwave frequencies.

Trunked Repeater System

Multiple repeaters are under the control of a computer system that can transfer a user from an assigned but busy repeater to an available repeater.

Communication Satellites

Act as fixed repeater stations

Transponder

The receiver-transmitter combination within the satellite.

Fading

Variation in signal amplitude at the receiver caused by characteristics of the signal path and changes in it.

Diversity System

Uses multiple transmitters, receivers, or antennas to mitigate the problems caused by the multipath signals.

Frequency Diversity

Two separate sets of transmitters and receivers operating on different frequencies are used to transmit the same information simultaneously.

Spacial Diversity

Uses two receive antennas spaced as far apart as possible to receive signals.