Antenna Basics

Antenna Basics

Theory
Basic terms
Basic antenna types

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©Kathrein/Scholz 07/04

Antenna Basics / Theory

What is an antenna ?

An antenna is the converter between two kinds of electromagnetic waves : cable bounded waves ⇔ free space waves

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©Kathrein/Scholz 07/04

Antenna Basics / Theory

antenna principle shown by bending a coax cable open

the pulsing electrical field, created by the transmitter‘s high frequency power, cannot leave the cable

the field lines become longer and are orthogonal to the wires

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©Kathrein/Scholz 07/04

Antenna Basics / Theory

the field lines have reached the maximum length and allow a wave to free itself from the cable
⇒ basic radiating element : λ/2 dipole

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4m band dipole (68-87 MHz) side-mounted on a pipe mast
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©Kathrein/Scholz 07/04

Antenna Basics / Theory
The resonance frequency of the dipole is determined by its mechanical length, which is half of the corresponding wave length

Max.
Frequency :
Number of cycles per second
Wavelength :

+

Length of one cycle

Zero

_

t [sec]

wavelength

Relation between frequency and wave length :

300 λ [m] = f [MHz]

Example : f =935 MHz ⇒ λ = 0.32 m ⇒ dipole length ~ 160 mm

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Antenna Basics / Theory
Electrical and magnetical field on a dipole

maximum voltage is between the ends of the dipole; the electrical field lines occur between these two charge centers voltage (U)

current (I)

the current on the dipole causes a magnetical field with an opposite amplitude distribution (max. at the feeding point, min. at the dipole ends electric field (E)

magnetic Field (H)

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©Kathrein/Scholz 07/04

Antenna Basics / Reality
Electrical and magnetical field on a dipole (Simulation of the original dipole)

Voltage (e-plane)

Current (h-plane)

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©Kathrein/Scholz 07/04

Antenna Basics / Theory

Wave propagation :
Permanent