If we only ask about the device, you'll know for sure to define what is
an antenna, or at least have ever seen it. We also know that changing
conditions or characteristics, for example targeting them, they improve
the communication link.
But if someone asked to describe what is an antenna technically
speaking, how would you describe its work? That's what we'll talk about
today.
Basics
Before we begin to define the antenna work, we need to learn (or
remember) some basic concepts. By understanding these concepts, it will
be much easier to understand how antennas works.
Wavelength
Radio waves (electromagnetic) is physical, of which we highlight the
frequency. We know it is not easy viewing. So let's make our first
analogy: imagine a drop of water falling on the flat surface of a bucket
of water.
After the droplet hits the water at rest, we can see the waves
formed. In telecom we specifically describe the pattern of sine waves,
the wavelength is distance between two peaks.
Mathematically, the wavelength (λ) is defined by the speed with
which the wave propagates (c) divided by frequency (f) thereof.
λ = c / f
- wavelength (λ): is represented by the Greek letter λ;
- 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:
λ = (300 Mm / s) / (900 MHz) = 0.33333 ... or
33.33 cm.
Polarization
When we talk about electromagnetic waves, another important concept
is the polarization, ie what the plan of the electrical component in
which the wave propagates. Ok, started to complicate things? So let's
try to explain better. Electromagnetic waves are composed of two
planes - vertical and horizontal. These plans represent 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. 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 and horizontal polarizations, and also the phase
differences. The concept of polarization is very important in
antennas, mainly because when a signal is transmitted in one
polarization must be received in the same polarization, otherwise we
will have an attenuation (loss), known as cross-polarization. To better
understand the polarization of waves, let's see some examples, in
which we highlight only the E component - electric field. (Remember
though that there are always a magnetic field 90 degrees to the
electric field).
And see how looks the wave (the electric component E) for Cross
polarization - a combination of vertical and horizontal polarizations,
electrically in phase.
Let's stop here, our artistic ability (?!?) limits us! But a wave
with Circular polarization (electric component E) - a combination of
two polarized waves - one vertical and one horizontal, electrically out
of phase by 90 degrees, but with the same magnitude, it would "more or
less" as we draw down. Surely the real wave is at least less "shaky."
As an example of antenna with Circular polarization we have
Helical Antennas or Cross Yagi with Circular polarization (left or
right), better known as RHCP (Right Hand Circular Polarization) and
LHCP (Left Hand Circular Polarization). We'll see more of their
applications in due course.
Antennas
Okay, after
briefly introducing some basic concepts, let's talk about antennas. By
definition, 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 radiates this 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 demodulation (and other treatments),
recovering the original information. Note: Just as an example, we are
not considering existing losses. Sure, but how the antenna works? How
she radiates the information? To understand this, we need a little
atomic review! Calm down, let's just talk about atoms: Atoms are the
smallest possible share of any chemical element. All that exists is
made up of elements. Put simply, most of them are formed by the atoms:
protons, electrons and neutrons. At the core of the atom have the
neutrons and protons. The electrons stay moving around this nucleus,
like cars on a trajectory as in a crazy race.
An attraction (positive-negative) is what makes it possible that all elements exist.
But what does this have to do with the antenna? 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 a set 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.
Pause: Are You caughting up how energy is radiated by the antenna?
Well then you've got it all. Because now, just the opposite happens.
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 field reaching 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 conductors that generate these fields. For example in the
transmission, the electromagnetic field corresponds to a specific
voltage and alternating current. In the reception, the same reference
voltage and alternating current is induced.
A Simple Antenna
Further, consider the representation of the simplest type of antenna:
a dipole antenna. As the name suggests, is an 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 totally against 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!
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)!
It's beginning to be clear? Note: You may wonder: "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. Okay so far? After many pauses for further explanations, let's
continue talking about further concepts.
Resonance
Recalling what we have seen so far, the electric waves in antennas
usually have a fixed wavelength. We also saw that an antenna can be
considered as an RLC circuit, where definition of these features are
given by the environment where the antennas are, and their physical
properties - especially its size. Ready for another term? So here we
go: Resonance! In general, 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 the maximum 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.
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,
as explained in another tutorial. The impedance of the antenna ceases
to be purely resistive, with a complex impedance - in both meaning of
the word, which gets her an unwanted behavior. 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
Just
to finish by today, you 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 as an 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.
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!
Properties and Types of Antennas
After our brief summary, focused mainly on the functioning of the
antennas, we can proceed with several other concepts, types of
antennas, etc.. Some concepts - for example Impedance - were also
mentioned, but were not well described.
But for today, our tutorial is already extended too much,
and is also very difficult to absorb more knowledge than what was
exposed here, at once. So lets take this supplement ,as well as
continuing the subject of antennas, for the next tutorials. Much remains
to be said, many questions to be eliminated. Hopefully you have
managed to understand at least some of the basics of antennas.
Now, do you have a minute?
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Conclusion
Today we had a first approach on antenna,
an undeniably important subject, and a essential system for the good
performance of any network. As always in a more informal way, we try
to flow explanations in a simplified manner, as a matter of course is a
foundation for other studies and further refinements as necessary. New
tutorials on the subject will be published in due course, always with a
focus ever deeper.
Download
Chap1Radiation and Antennas.pptChapter4_Antenna Arrays.pptChapter3_loopAntenna.pptHelical Antenna4.pptHelical Antennas.ppt
