Different types of Signals:
Electronics circuits deals with many types of signals. Signals are broadly divided into two categories and they are electrical signals and electronic signals. Electrical signals are mainly power carrying signals like AC or DC power lines. They are mainly used to deliver power to the device. They are either high voltage or high current signals. Alternating voltage can range up to a frequency of 50-60Hz. Mainly these are very low frequency signals or DC signals having no frequency or phases.
Electronic signals however are different and they mainly have low amplitude (below 15V) and they mainly used for audio, video, or on-off signals and digital signals used in computers. These signals can have a frequency range of audio signals 20hz-20kHz or Video signals of bandwidth of few Mhz. They can be transmitted via copper wire for few kilometers. Telephone lines, cable tv network etc. are such examples.
Signals transmissions & radio waves:
Copper wire for audio signals and co-axial cable for video signals are good for transmission of these signals upto few kilo meters. Signals attenuate as the distance increases. Repeaters are required to transmit these for longer distances. Repeaters will amplify the signals and boot the signal strength.
These electronic signals are low frequency signals. They are called baseband signals and can not propagate in space. However radio frequency signals have the property to propagate longer distances in space. This property of radio signal is utilised in signals transmissions. Radio signal used as carrier for the baseband signals. Some signal properties like amplitude, frequency or phase are changed as per baseband signals. This is called modulation process. Radio signals are then feed to transmitter.
Receiver is the other end of the signal transmission. Here carrier signals are received and they go for a reverse modulation process. This is known as demodulation and baseband signals are extracted and reconstructed. This is the entire process of modulation and demodulation what happens in our radio, TV, modems etc.
Antenna
In
radio communication system the receiving antenna is linked to the transmitting
antenna through the electromagnetic wave. This arrangement is somewhat similar
to that we find in transformer circuits. In the case of transformer, the
coupling is strong and the field involved is entirely magnetic. In the case of
antenna, however, the coupling is weak and the field involved is
electromagnetic.
The antenna coupling system can be represented by a four terminal network. This
representation is very useful because we can then apply the well-known network
theorems to solve antenna problems. The important general results obtained there
by are applicable to all kinds of antennas. A network theorem which is
particularly useful in antenna theory is the reciprocity theorem.
Fig shows a high frequency generator, connected to a two parallel wire
transmission line. If the line is kept sufficiently far away from any metallic
or conducting objects, equal and opposite current will flow in the two wires at
any given position on the line. Therefore, at any appreciable distance, from the
line, the field effects of two wires will almost cancel each other. The end of
the line, remote from the generator, is sorted with a straight segment of wire.
The field effect of this wire may be observed at any distance remote from the
line, because there is no source of equal and opposite fields to cancel them.
This
shorted segment is known as elemental antenna.
Radiation resistance:
When a high frequency current flows through an antenna, there is also an
energy-loss due to the radiation of electromagnetic energy. This in turn,
concludes that the transmission line, feeding the elemental antenna, must see a
resistance component of load. If the load were all-resistive, the average power
delivered to the antenna would have to be zero. It is un-important that our
energy is not dissipated as heat. There is still an energy-loss to be accounted
for and the amount of energy lost is given by
Pavg=789(dl/λ)2I02 or Pavg=789(dl/λ)2Irms2
or Pavg=½RradI02
The radiation resistance of elemental antenna is
R rad =789(dl/λ)2
Antenna Directivity And Gain:
Directivity and gain is two very useful terms in Antenna System. For a
particular direction the ratio of power per unit solid angle to the power per
unit solid angle for a Reference isotropic antenna is called the Directivity.
Gain is the ratio of maximum radiation intensity of an antenna to the maximum
radiation intensity of a reference antenna provided both antennas have the same
power input.
Polarization:
Polarization is the direction of electric field of the incoming electromagnetic
waves.
The Half-Wave Dipole
There is only one part of a receiving aerial that is active, i.e. does the
receiving and is connected to the TV/radio set. This active element is called
the dipole. The simplest design of antenna would consist of a dipole only:
A half-wave dipole
In the diagram above, there are two wires marked 'to receiver.' For UHF and VHF,
one wire will be the copper-core and the other the copper braiding of a co-axial
cable.
Before we precede, a quick word about gain. Although having a technical
definition, for us 'gain' can mean "the effectiveness with which a receiving
aerial receives a signal."
The diagram below shows the reception pattern of a half-wave dipole. The blue
area is where the gain is higher than a certain value; the dipole is in the
center:
We can change the directivity of the aerial by adding other elements. Any other
elements that we add to the basic half-wave dipole are called passive elements
and are not connected electrically to the dipole.
There are two types of passive elements:
Directors
Directors alter the directivity of the aerial so that the aerial's gain is
improved in front of the dipole. Most aerials have more than one director, and
the more directors the aerial has the better the aerial is at picking out the
signal from the required source and rejecting signals from other angles.
These diagrams do not show the cross-bar that holds all the elements in place as
it does not much affect the characteristics of the aerial.
The spacing between the directors, diameter of the tubing used and the spacing
between the first director and the dipole are important in practice but will be
disregarded here. The length of the directors governs the bandwidth of the
aerial (over which channels it is effective), but suffice it to say that it is
about 75% the length of the dipole.
The gain of the dipole with directors in place looks like this:
Notice how the gain is now more focused in the direction of the directors.
As stated earlier, the more directors an aerial has the more focused the gain is
in the direction of the directors. Every new director added becomes less
effective though, and in practice it is only worth adding 18-20 directors to the
aerial, as any more than this wouldn't increase the gain very much.
On the diagram above, the aerial still has some gain at the rear - in other
words, it can still receive signals from behind. This is known as a low
front-to-back ratio.
The Reflector
To improve the front-to-back ratio we can add the second type of passive
element, a reflector. The reflector reflects signal coming in from the back of
the aerial whilst improving the forward gain.
This design is called a Yagi-Uda array, after its creators.
Again, the length, size and position of the reflector affect the aerial's
properties, but we won't go into that here.
The reflector can take the shape of a metal plate (with holes in it, making the
aerial more impervious to wind) or several rods spaced equidistant from the
center of the dipole.
The result is that there is less gain behind the aerial and more, where we want
it to be, in front:
Folded Dipoles
In order to minimize signal loss it is important that the impedance (a sort of
resistance for AC) of the dipole matches that of the feeder cable and the
receiving set.
The impedance for the type of dipole discussed above is about 75 ohms. More
often than not though the impedance needs to be altered to match the cable and
receiving set characteristics.
This change of impedance is achieved by folding a rod over so that its folded
length is still half-a-wavelength:
Now we know what each constituent part of an aerial is called and what its
function is, let's look at some examples in the field.
Monopole Antenna:
Consider a transmitter perpendicular to the ground. The electrons in the
antenna, when a signal is applied, are changing their velocities continuously
(i.e. moving up and down very quickly) in response to the applied signal.
For a station that broadcasts at a wavelength of 1500m, the antenna needs to be
750m long. This is because there is a 'virtual antenna' caused by the aerial
being earthed in the ground:
The transmitting aerial (and the receiving aerial) need only be
half-the-wavelength tall.
Now if this transmitter has no directional properties (i.e. it radiates in all
directions equally), it has a coverage area, assuming completely flat ground
that is a perfect circle:
(View from above - antenna in center; blue is coverage area)
Electromagnetic wave propagation
Different mechanisms are involved in the propagation of radio waves from
transmitting to receiving antennas, the important ones being:
- Ground wave or Surface wave propagation
- Space wave or Tropospheric wave propagation
- Sky wave or Ionospheric wave propagation
Ground wave or Surface wave propagation:
Due to the presence of the ground, near the transmitting and receiving antennas, the propagation of the ground waves takes place along the surface of the earth. In the case of long and medium wave signals, the ground wave propagation is common. Daytime reception of all radio signals is possible due to the ground wave propagation.
Space wave or Tropospheric wave propagation:
The portion of the earth's atmosphere situated in the first 15 km adjacent to the earth's surface is known as the earth's Troposphere. The propagation of the space wave takes place through the earth's troposphere. In case of radio waves from television, radar and frequency modulated transmitter, where the frequencies are above 50Mhz, the tropospheric space waves are the important means of radio communications.
Sky wave or Ionospheric wave propagation:
An ionized region situated at height of 90km or more is known as the Ionosphere, which contains electrons, positive ions and neutral atoms. The sky wave propagation takes place due to reflection of the radio wave from the lower surface of the ionosphere and earth's surface. All long distance radio communications are possible due to the sky wave reflection from the ionospheric and as well as reflection from the satellites.
