antenna

Antenna

An antenna is a device for converting electromagnetic radiation in space into electrical currents in conductors or vice-versa, depending on whether it is being used for receiving or for transmitting, respectively.

Isotropic RadiatorA hypothetical antenna that would radiate all the energy supplied to it with equal intensity in all directions

Omnidirectional Antennaeare isotropic in the sense that same power (radiation) is transmitted in all directions.

Directional antennae have preferred patterns (like an ellipse)

Antenna Characteristics

1. Radiation Patterndefines the variation of the power radiated by

an antenna as a function of the direction away from the antenna.

Antenna Characteristics2. Major and Minor Lobes

Main lobe is the lobe in the direction of maximum radiation

Minor lobe a lobe with less intensity than the main lobe.

Antenna Characteristics3. Directivity and Gain

Directivity refers to the ability of an antenna to send or receive signal over a narrow horizontal directional range.

Gain of the antenna is expressed as the ration of the power transmitted (Pt) to the input power of the antenna (Pi)

Antenna Characteristics4. Beam Width

the measure of an antenna’s directivity refers to the angle of the radiation pattern over which a transmitter’s energy is directed or received

is at least half of what it is in the most powerful direction.

• The 3dB down points are 70.7 percent of the maximum.

Antenna Characteristics5. Front-to-back Ratio

the ratio between the radiation intensity in an antenna’s direction of maximum radiation and intensity at an angle of 180° to this direction. (in dB)

Antenna Characteristics

6. Effective Radiated Power (ERP)simply the power applied to the antenna

multiplied by the antenna gain

Antenna Characteristics7. Impedance8. Polarization

the direction of the electric field vector of an electromagnetic wave

• An antenna is said to be vertically polarized (linear) when its electric field is perpendicular to the Earth's surface.

Antenna Characteristics9. Ground Effects

When an antenna is installed within a few wavelengths of the ground, the earth acts as a reflector and has a considerable influence in the radiation pattern

Hertz Antenna

A half-wave antenna (referred to as a dipole, Hertz, or doublet) consists of two lengths of wire rod, or tubing, each 1/4 wavelength long at a certain frequency.

The impedance for a half wave dipole antenna in free space is dipole 73 Ω which presents a good match to 70Ω coaxial feeder and this is one of the reasons why coax with this impedance was chosen for many applications.

468L= f

L= length of half wave dipole in feetF= operating frequency in MHz

142.5L= f

L= length of half wave dipole in metersF= operating frequency in MHz

Dipole Polarization

Polarization is same as axis of wire:•Vertical dipole is vertically polarized•Horizontal dipole is horizontally polarized

Folded Dipole Antenna

• Same length as half wave dipole•Uses 2 conductors• Impedance 4 times that of normal dipole• Approximately 300 ohms at resonance

• Less than 1 in spacing for high frequency antenna• Low frequency antenna, the spacing may be 2 or 3 in

Reference• Frenzel, L. E. (1995). Communication Electronics (Second ed.).

New York, NY: Glencoe.• Blake, R. (2002). Electronic Communication Systems (2nd ed.).

Albany, NY: Delmar.