Basic Antenna Theory
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Active In SP

Posts: 1,124
Joined: Jun 2010
24-11-2010, 10:21 AM

This article is spresented by:Professor R. Struzak

Basic Antenna Theory

to refresh basic concepts related to the antenna physics
needed to understand better the operation and design of microwave links and systems

Review of basic antenna types
Radiation pattern, gain, polarization
Equivalent circuit & radiation efficiency
Smart antennas
Some theory

Intended & unintended radiators

Antennas intended to produce specified EM field
Radiocommunication antennas; Measuring antennas; EM sensors, probes; EM applicators (Industrial, Medical, Scientific)
Radiators not intended to generate any EM field, but producing it as an unintended side-effect
Any conductor/ installation with varying electrical current (e.g. electrical installation of vehicles)
Any slot/ opening in the screen of a device/ cable carrying RF current
Any discontinuity in transmission medium (e.g. conducting structures/ installations) irradiated by EM waves
Stationary (e.g. antenna masts or power line wires); Time-varying (e.g. windmill or helicopter propellers); Transient (e.g. aeroplanes, missiles)

Antenna purpose
Transformation of a guided EM wave in transmission line (waveguide) into a freely propagating EM wave in space (or vice versa) with specified directional characteristics
Transformation from time-function in one-dimensional space into time-function in three dimensional space
The specific form of the radiated wave is defined by the antenna structure and the environment

for more information abut this topic please follow the link:
Active In SP

Posts: 1,124
Joined: Jun 2010
26-11-2010, 11:51 AM

.ppt   Antennas.ppt (Size: 1.01 MB / Downloads: 67)
A good antenna works
A bad antenna is a waste of time & money
Antenna systems can be very inexpensive and simple
They can also be very, very expensive

Antenna Considerations
The space available for an antenna
The proximity to neighbours
The operating frequencies you will use
The output power

Antenna Types
High Frequency
1.6 - 30 Mhz + 50 Mhz
160 - 6 metres
An antenna’s size/length depends on the frequency
It’s functionality largely depends on the height above ground, as well as the polarity and it’s configuration

Types of Antennas
Simple wire
Folded dipole
Trap dipole
Offset or Windom antenna
Phased dipoles
Vertical or horizontal (both)
Beverage wave antenna

Types of Antennas
Trap Yagi
Phased arrays
Vertical or Horizontal
Horns for super ultra high frequencies
Mobile antennas

Antenna Polarization
Vertical or horizontal

Electrical vs Magnetic radiation

Vertical waves travel @ 90◦ to the earths surface

Horizontal waves travel parallel to the earth’s surface

Usually wire antennas are horizontal but an inverted ‘V’ dipole has a vertical component

Yagi type antennas can be either vertical or horizontal
Circular antennas can be both

Usually, horizontally polarized antennas hear less noise

Isotropic Antenna
The isotropic antenna is a hypothetical point source.

It does not exist in reality but is considered as an important starting point considering different

antennas from the theoretical to the practical

The pattern is a Cardioid - a donut shape or a sphere

Polarization - Practical
Antennas radiating a vertical polarization are best received by an antenna of like polarization

Cross polarization reduces reception by as much as 30 db

Bouncing DX signals probably have both polarizations

Designing antenna polarization usually depends on the frequency being used - at 70 cm in th eUHF band the elements are very short so either polarization is possible. Usually vertical is used as repeaters are vertically polarized.

Antenna length is dependant on frequency

The lower the frequency the longer the antenna elements

80 metres 3.750 Mhz 124 ft
40 7.055 66
10 28.5 16.4
6 52 9
2 145 3.2

Isotropic Source
An isotropic antenna is a: hypothetical point source

What is the antenna radiation pattern for an isotropic radiator? A sphere

Polarization of an antenna is determined by: the electric field

What does horizontal wave polarization mean? The electric lines of force of a radio wave are parallel to the earth's surface

What does vertical wave polarization mean? The electric lines of force of a radio wave are perpendicular to the earth's surface

What electromagnetic wave polarization does a Yagi antenna have when its elements are parallel to the earth's surface? Horizontal

What electromagnetic wave polarization does a half-wavelength antenna have when it is perpendicular to the earth's surface? Vertical

VHF signals from a mobile station using a vertical whip antenna will normally be best received using a: vertical ground-plane antenna

A dipole antenna will emit a vertically polarized wave if it is: Parallel with the ground mounted vertically

If an electromagnetic wave leaves an antenna vertically polarized, it will arrive at the receiving antenna, by ground wave: vertically polarized

Compared with a horizontal antenna, a vertical antenna will receive a vertically polarized radio wave: at greater strength

Wavelength vs Physical Length
The speed of a radio wave: is the same as the speed of light

The velocity of propagation of radio frequency energy in free space is: 300 000 kilometres per second

If an antenna is made longer, what happens to its resonant frequency? It decreases

If an antenna is made shorter, what happens to its resonant frequency? It increases

The resonant frequency of an antenna may be increased by: shortening the radiating element

To lower the resonant frequency of an antenna, the operator should: lengthen it

Adding a series inductance to an antenna would:
decrease the resonant frequency

The wavelength for a frequency of 25 MHz is:
12 metres (39.4 ft)

The wavelength corresponding to a frequency of 2 MHz is: 150 m (492 ft)

At the end of suspended antenna wire, insulators are used. These act to: limit the electrical length of the antenna

One solution to multi-band operation with a shortened radiator is the "trap dipole" or trap vertical. These "traps" are actually: a coil and capacitor in parallel

Gain, Directivity, etc.
What is meant by antenna gain? The numerical ratio relating the radiated signal strength of an antenna to that of another antenna

The gain of an antenna, especially on VHF and above, is quoted in dBi. The "i" in this expression stands for: isotropic

Approximately how much gain does a half-wave dipole have over an isotropic radiator? 2.1 dB

What is a parasitic beam antenna? An antenna where some elements obtain their radio energy by induction or radiation from a driven element

If a slightly shorter parasitic element is placed 0.1 wavelength away from an HF dipole antenna, what effect will this have on the antenna's radiation pattern? A major lobe will develop in the horizontal plane, toward the parasitic element

If a slightly longer parasitic element is placed 0.1 wavelength away from an HF dipole antenna, what effect will this have on the antenna's radiation pattern? A major lobe will develop in the horizontal plane, away from the parasitic element, toward the dipole


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