Introduction to Antenna Design

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15 Νοε 2013 (πριν από 3 χρόνια και 10 μήνες)

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Introduction to Antenna



Design

Definition

-
A device that radiates or intercept
electromagnetic waves is called an
antenna.


-
An antenna not necessarily perform
equally well in all directions.


Radiation Pattern

-
A polar diagram indicates how well an
antenna transmits or receives in different
directions .

Beam Width

-
The angle within which the power radiated
is above one
-
half of what it is in the most
preferential direction.



Antenna Resistance

-
Radiation Resistance.

A factious resistance which would be
dissipate as much power as the antenna in
question is radiating if it were connected to
the same transmission line.

-
Ohmic Resistance.

Actual loss in term of heat

Due to conversion of electrical

e
nergy into heat.

Antenna as resonant circuit

-
The impedance presented by an antenna
has a reactive component due to currents
and voltages being out of phase.

-
As the frequency of the signal being
presented to the antenna is changed, the
impedance presented to the transmission line
become reactive, just as with a tuned circuit.



Bandwidth = f
0
/ Q



Velocity Factor

-
The velocity of electromagnetic waves is
slightly different in a conductor such as the
material the antenna is made of:aluminium,
copper etc.

-
The velocity factor is that number which
when multiplied by the speed of light in the
free space gives us the speed of light in the
medium in question


Antenna Types

Some of the important antennas are
discussed as follow.


-
Half
-
Wave Dipole and Marconi Antennas

-
Beam Antennas

-
Folded dipole Antennas

-
Turnstile Antennas

-
Loop Antennas


Antenna Gain

-
A comparison of the output, in a particular
direction, of the antenna in question and a
reference antenna. (Reference is generally an
omnidirectional antenna)


-
A antenna with let say 10 db gain means that
the antenna in question improves upon the
reference antenna in that direction by 10 db.


-
Frequently, one antenna is compared to the
other, avoiding the need of a reference.

Front
-
To
-
Back Ratio

-
The ratio expressed in db of the output in
the most optimum direction to the output
180 degree away from the optimum
direction.

Reflectors and Directors

-
Reflectors and directors are additional
conducting elements used to obtain
improved directivity of an antenna.

-
The directors are placed in front and
reflectors are placed in back of the
antenna in question.

Antenna Traps

-
It is often desirable that a
station transmits different
frequencies effectively
using same transmission line
and antenna .

-
It can be achieved by
installing different units
called trap (LC networks)
at different places along
the antenna which
changes length of antenna
accordingly.



Wave Propagation

-
The radiated signal travels along one of the
following routes

1)Along the ground (Ground Wave)

2)Straight out in a straight line (Line
-
of
-
sight
wave)

3)Up to the ionosphere and back to earth (Sky
wave)


-
Maximum distance between transmitter and
receiver is given as



-
Where hr and ht are heights of transmitting
and receiving antenna respectively


2 2
d h h
t
r
 
Antenna Modeling Software

Antenna Modeling Software

-
CST MICROWAVE
STUDIO

-
Antenna Magus

-
VE3SQB Programs

-
HFSS

-
EZNEC antenna design tool

-
Remcom Tools


Numerical Technique of HFSS

-
works on
finite element method

-
Finite Element Analysis (FEA) is a numerical method which provides solutions to
problems that would otherwise be difficult to obtain
.

-
FEA uses a complex system of points called

nodes

which make a grid called
a

mesh


.
This mesh is programmed to contain the material and structural
properties which define how the structure will react to certain loading conditions.
Nodes are assigned at a certain density throughout the material depending on
the anticipated stress levels of a particular area. Regions which will receive large
amounts of stress usually have a higher node density than those which
experience little or no stress. Points of interest may consist of: fracture point of
previously tested material, fillets, corners, complex detail, and high stress areas.
The mesh acts like a spider web in that from each node, there extends a mesh
element to each of the adjacent nodes. This web of vectors is what carries the
material properties to the object, creating many elements