# Basics of Antennas.docx

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La Trobe University

ELE

ELE 5PMC

Eddie Custovic

Spring

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BASICS OF ANTENNAS – Lecture Note 1
INTRODUCTION
Antennas are devices that are capable of launching RF (radio frequency) energy into space and detect it as
well. How well an antenna is able to launch RF energy and how well it can detect, depends on some
important parameters that will be introduced here. Many people find it hard sometimes to understand
some or all of these parameters, but I have found out that explanations with examples can help. I have
tried my best to choose the examples close to the concepts. This Lecture note does not contain any
mathematical formulae, instead I have concentrated on only the concepts of antennas. Mathematical
considerations will be dealt in the following lecture notes.
So first, how do antennas radiate RF energy? To understand this, check out the picture of the leaf below.
The leaf can be thought equivalent to an antenna, the ripples are the radiation & the circular nature of
ripples is the radiation pattern. If the ripples were to be seen, somehow hovering from the top of the
water surface, you would see concentric circles forming from the point where the stick was inserted in
water and moving away from it. As ripples (radiation) travel away from this point, they die out. In antenna
theory, this is equivalent to RF energy eventually fading out as it moves away from antenna. According to
physics, ripples do not travel, it is the disturbance that caused the ripples that travels. This travel of
disturbance is what makes the ripples. In antenna theory, this is equivalent to the radiation travelling.
The Omnidirectional antenna: The word ‘Omnidirectional’ means ‘in all directions’. So an omnidirectional
antenna is the one that radiates equal RF energy in all directions. Of course, such an antenna does not
exist! But it is very useful to understand the many complicated parameters of a real world antenna.
Omnidirectional antennas used in real world are not perfect i.e. they do not radiate the same energy in all
directions. An omnidirectional antenna is not a very good antenna to use in real world scenarios because it
spreads out the RF energy in all directions. Mostly directional antennas which direct RF energy in the
direction of interest (say in the direction where a TV receiver is location) are used. An example of such an
antenna is the one shown in Figure 3. Some applications of omnidirectional antennas are in data transmission base stations for secure WIFI communications. More about this is not in the scope of this
lecture note and can be made available on request.
Figure 3 : Omnidirectional Antenna
IMPORTANT PARAMETERS OF AN ANTENNA :
Frequency : The very word frequency means the number of times any event occurs. That is precisely what
it means even in electromagnetic and antenna theory. The best part about our world is that any type of
energy (there are lots of different types out there) can simply be divided into sine or cosine waves. Any
sine or cosine wave has an amplitude (defines power), frequency (number of complete cycles/sec or Hertz
(named after the scientist who discovered it)), wavelength (distance between two heights). The following
figure 1 will help get these terms right. Figure 2 helps in understand low frequency and high frequency sine
waves. Figure 1 : Frequency, Amplitude & Wavelength of a Sine Wave
Figure 2 : Low and High Frequency Sine waves
Frequency Bands : Just like every country has a government, there are also governing bodies (e.g. ITU –
International Telecommunications Union) which are responsible for dividing all the frequencies available
into what we call as ‘frequency bands’. By dividing all frequency or ‘spectrum’ into bands, the governing
bodies then auction these bands to communications companies to use. If this regulation was not in place,
then we would receive TV signals on our phones and calls would come to our TVs! If an antenna operates
in the frequency range of 400Mhz to 500Mhz then its bandwidth is 100Mhz! So if someone asks you to
design an antenna, ask them which frequencies they want it to cover. You will then get the bandwidth of
the antenna!
Antenna Gain : Antenna gain in dB is a number which defines how much the antenna amplifies the RF
signal before launching it in space. For a receiving antenna, the gain defines how much the receiving
antenna amplifies the detected RF energy before future processing. Good gain in the direction of interest
helps in better transmission and reception of RF signals. Good gain during transmission is more important
than during reception simply because the transmitted signal needs to be strong enough to travel the
distance and reach the receiv

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