wideband simple dieletric resonator antenna

8/12/2019 wideband simple dieletric resonator antenna

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Basics :

1. Wavelength : In telecom we specifically describe the pattern of sine waves, thewavelength is distance between two peaks.

Mathematically, the wavelength () is defined by the speed with which the wave propagates

(c) divided by frequency (f) thereof.

= c / f

wavelength ():is represented by the Greek letter ; speed (c): Considering that our waves propagate in air, we can consider as the speed of

light in vacuum - c - 300,000,000 m / s (which may be represented by 300M m / s);

frequency (f): frequency of the signal will be using.

For example, on a 900 MHz system, we have: = (300 Mm / s) / (900 MHz) = 0.33333or 33.33 cm.

2. Polarisation : Polarization (alsopolarisation) is a property of wavesthatcan oscillatewith more than one orientation. Electromagnetic waves, such as light,andgravitational waves exhibit polarization whereas this is not a concern with sound

wavesin a gas or liquid which have only one possible polarization, namely the

direction in which the wave is travelling.

let's try to explain better.

Electromagnetic waves are composed of two planes - vertical and horizontal. These plansrepresent the electric and magnetic fields. These components are always orthogonal,

vectors off by 90 degrees. They vary in phase - or zero - degrees of electrical phase shift.

The propagating speed (also vector) for these two fields in turn spreads in 90 degrees of

the two.


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The following figure helps us visualize these vectors.

So depending on how the signal coupling is done - the antenna is oriented - we have a

definition of polarization.

If the transmitter is such that the wave is completely in the vertical plane (Electrical plane E),

then we have Vertical polarization. If the wave is in the horizontal plane (Magnetic plane B),

we have Horizontal polarization. There are other types of polarization, as Cross polarization

and Circular polarization (right and left), that actually are combinations of vertical andhorizontal polarizations, and also the phase differences.

The concept of polarization is very important in antennas, mainly because when a signal istransmitted in one polarization must be received in the same polarization, otherwise we will

have an attenuation (loss), known as cross-polarization.

In this we highlight only the E component - electric field.


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Now, the wave (the electric component E) for Cross polarization - a combination of vertical

and horizontal polarizations, electrically in phase.

Antennas :

An antenna (or aerial) is an electrical device which converts electric power into radio waves,and vice versa. It is usually used with a radio transmitter or radio receiver. In transmission, a

radio transmitter supplies an oscillating radio frequency electric current to the antenna's

terminals, and the antenna radiates the energy from the current as electromagnetic waves(radio waves). In reception, an antenna intercepts some of the power of an electromagnetic

wave in order to produce a tiny voltage at its terminals that is applied to a receiver to be

amplified.

Or in simple words, an antenna is a device designed to transmit or receive electromagnetic

energy, matching these sources of energy and the space. Also often called radiant systems.

Note that the same device can be used to transmit or receive.

Let's start by looking at a simplified representation of a system for transmission and reception.

The original information is changed, for example through some kind of modulation and

treatment, and still conveyed or guided by a cable to the antenna. The antenna then radiatesthis information by the medium (air) until it reaches the other antenna, which in this case will

make receiving the signal, making it still the way the cable to the device that will make such


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demodulation (and other treatments), recovering the original information. Note: Just as an

example, we are not considering existing losses.

How does antenna works ?

The antennas are usually made with metallic materials (aluminum / brass). These metals are

formed by atoms. When all the atoms are brought together - to form the metal, then we have aset of free electrons.

And when this series of free electrons is subjected to an electric voltage (electric field), they

begin to move and vibrate.

When electrons vibrate from one side to another antenna, they create an electromagnetic

radiation in the form of radio waves.

How energy is radiated by the antenna?

The electromagnetic radio waves that leave the transmitting antenna travel through the

medium, eg air, and reach the other antenna - reception. The effect of electromagnetic fieldreaching the other antenna is to make the free electrons vibrate in the same - which now

generates an electric current corresponding to what was sent from the transmitting antenna.

So now we can conclude: the transmission antennas convert the electrical current (electrons)into electromagnetic waves (photons), and the reception do the reverse - convert

electromagnetic waves (photons) into electrical current (electrons).

The information is preserved because the antenna acts as a transducer matching conductorsthat generate these fields. For example in the transmission, the electromagnetic field


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corresponds to a specific voltage and alternating current. In the reception, the same reference

voltage and alternating current is induced.

A Simple Antenna :

The representation of the simplest type of antenna: a dipole antenna. As the name suggests, isan antenna with two poles.

It is a model of the antenna easy to make, and consists of two pieces of wire of equal length,separated from each other by a center insulator and may have an insulator on each end to

attach it to a support.

In the figure below is an example of a dipole antenna (insulators shown in red in figure).

Let's use this example to talk about antennas, but now we're basically with simple question,but that many people can NOT explain:

"How can there be a current flowing in antenna, if both parts are open? This runs totallyagainst what we learn, where have current, we need a closed circuit, no?"

To answer this, we again return to the familiar concepts of electrical circuits.

You must remember the concept of capacitance (C), defined through the use of capacitors.

And there is a kind of unavoidable capacitance that arises between compontent always close to

each other on the circuit - and often unwanted: parasitic capacitance.

Only in our case, this capacitance is what allows the antenna to work!


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At high frequency, the parasitic capacitance between the two arms of the antenna has a low

impedance, and represents the current return path.

In short: a tuned antenna can be considered as an RLC circuit - with resistance R, inductance(L) and capacitance (C)!

And in the case of antennas with only one arm?" Do not worry, the antenna will always seek a

reference plane to act as "ground", such as a metal rod next.

From what was shown, we can say that every antenna requires two parts to radiate energy.

And that energy is proportional to the dipole current.

Resonance :

Resonance is the phenomenon that occurs in a particular frequency where we have a maximum

possible transfer of energy.

In the case of antennas, so there to be resonance, its size (physical length) must be a multiple

of its wavelength. In this case, we will have a main frequency where the antenna delivers themaximum amount of energy possible - resonant. And the larger the size (length) of elements of

the antenna, the lower the resonant frequency.

In more technical terms, we have the resonance frequency where the inductive and capacitive

reactances cancel each other out - we have a purely resistive impedance.


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Most antennas are used in its resonance frequency. That's because when we turn from this

resonance frequency, the reactances levels give rise to parameters that may jeopardize the

operation, for example the SWR, the impedance of the antenna ceases to be purely resistive,

with a complex impedance - in both meaning of the word, an unwantd behaviour of antenna isobserved.

It is clear that a non-resonant antenna also works - transmit and receive. But it needs a more

powerful transmitter (because a smaller part of the input energy will be present at the output).And for the same reason, you need a receiver with a sensitivity much higher. So: the system

efficiency will be much lower.

Wavelength X Length of Antenna :

We should remember what we taught to be the resonance of the antenna physical size must be

multiple of its wavelength.

Let's try to understand why exactly this value? As always, let's remember more concepts...

Remember that an electrical circuit - which we has also mentioned that a tuned antenna acts asan RLC circuit - the Voltage (Potential Difference):

in a Short Circuit is equal to Zero; in an Open Circuit is Maximum.

Well, the antenna end, we have an Open Circuit - so the point with the Highest Voltage.

And considering the two ends - one with the maximum positive voltage and one with the

maximum negative voltage - we have the center point with Zero voltage.


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This distance between the end and the central point is the distance between the point of

maximum voltage (yellow circle in figure) and point of zero voltage (green circle in figure) -

and is a quarter wavelength!

Types of Antennas :

1. Wire AntennasShort Dipole Antenna Dipole Antenna Half-Wave Dipole

Broadband Dipoles Monopole Antenna Folded Dipole Antenna

Loop Antenna

2. Travelling Wave AntennasHelical Antennas Yagi-Uda Antennas Spiral Antennas


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3. Reflector AntennasCorner Reflector Parabolic Reflector (Dish Antenna)

4. Microstrip AntennasRectangular Microstrip (Patch) Antennas

Planar Inverted-F Antennas (PIFA)

5. Log-Periodic AntennasBow Tie Antennas Log-Periodic AntennasLog-Periodic Dipole Array

6. Aperture AntennasSlot Antenna Cavity-Backed Slot Antenna Inverted-F Antenna

Slotted Waveguide Antenna Horn Antenna Vivaldi Antenna

7. Other AntennasNFC Antennas Fractal Antennas Wearable AntennasMHD Antennas

Waveguides :

A waveguide is a structure that guides waves, such as electromagnetic waves or sound waves.There are different types of waveguides for each type of wave. The original and most common

meaning is a hollow conductive metal pipe used to carry high frequency radio waves,

particularly microwaves.

The geometry of a waveguide reflects its function. Slab waveguides confine energy to travel

only in one dimension, fiber or channel waveguides for two dimensions. The frequency of the

transmitted wave also dictates the shape of a waveguide: an optical fiber guiding high-frequency light will not guide microwaves of a much lower frequency. As a rule of thumb, the

width of a waveguide needs to be of the same order of magnitude as the wavelength of the

guided wave.

Some naturally occurring structures can also act as waveguides. The SOFAR channel layer in

the ocean can guide the sound of whale song across enormous distances.

Or in simple words, Waveguide is the type of transmission line which carries microwavefrequencies from source to load. The energy is in the form of electric field and magnetic fieldwhich are perpendicular to each other the electric field and magnetic field inside. The


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waveguide is also perpendicular to the direction of propagation as the energy is in the form of

electric/magnetic field. Therefore, it is capable to handle the high power of micro wave length

of frequencies.

Types of Wave guide:There are the basic two types of the wave guide.

Circular wave guide

Rectangular wave guide

Construction:

1. Circular wave Guide:As shown in the given diagram the circular waveguide is designedfrom a conducting pipe which is hollow from the center and polished

from interior portion. The outer surface of the wave guide is coded

with the insulated paint in order to avoid dust and rust. These types ofwave guide are available in different lengths and sizes in order to

fulfill the requirement of the circuit.

2.Rectangular Wave Guide

:

As shown in the given diagram, the rectangular wave guide is

designed from conducting material in rectangular shape which is

hollow from the center and fully polished from interior. The outer

surface of the wave guide is coded with insulating material orpaint in order to avoid dust and rust. These types of wave guides

are available in different lengths and sizes in order to fulfill therequirements of the circuit.


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Direction of Propagation:

As shown in the given diagram, there are two dimensions of the rectangular waveguide where

one is the broad dimension and the second is the narrow dimension. The broad dimension is

denoted by a and the narrow is by b. In the wave guide the electric -field and the magnetic-

field carry the signal from source to load. In this case electric-filed and the magnetic-field isperpendicular to each other and at the same time. These fields are perpendicular to the

direction of propagation. Inside side the wave guide horizontal field is perpendi cular to a

and Vertical field is perpendicular to b according to the frequency both Horizontal andVertical fields changes its amplitudes and direction as shown in the diagram.

Flexible Waveguide :

It is the type of waveguide which can be easily turn and twisted in circuits to connect the

source with the load. This type of wave guide is designed in such a way that internal portion is

made of conducting material in spring shape. The external portion is covered with the rubberto avoid dust, rust and humidity.


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Higher Mode

It is the mode of operation which deals with the high range of frequency. The higher mode is

from 30 MHz to 300 MHz in the higher mode the transmission or propagation is carried outthrough the waveguide in form of electromagnetic and horizontal field. TE mode and TM

mode are higher modes.

Lower Mode

The mode of operation which deals with the frequency less than 3 MHz is called the lower

mode. In this case the transmission is carried out through the micro strip coaxial cable. The

movement of electrons for the flow of current is taken in the lower mode of operation to carrythe energy from the source to load.

Circular Waveguide:

1. TE Modes of Circular Waveguide :


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The Maxwells equations can be expanded in the cylindrical coordinate as

The and components can be expressed in terms of and as

For TE waves, , and satisfies the equation

(1)

or

where .To solve this equation, we make use of the separation-of-variablestechnique, namely,

, and substitute into (1) to have


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The general solution has the form

Thus,

The boundary condition at = arequires that the tangential component of be zero, namely,

or

Where,

.

Note that . The first few roots of are listed in the followingTable.


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The allowed values of are expressed as , and the associated modes are

designated as TEn,mmodes. Explicitly


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2. TM Modes of Circular Waveguide :


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Field patterns in circular waveguide in different modes of operation:


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wideband simple dieletric resonator antenna

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