*One of the most important parts of the tv is the part that receives and sends the signal between the transmitter and the receiver .this part is called the antenna .The design of these devices requires a lot of care .In the coming subsections we will discuss the basic considerations for such designs and the types of antennas.
A)Ghosts:-
They are multiple images caused by almost simultaneous reception of the same signal from two or more directions. The two received signals can be the direct and the reflected signals. The solution to this problem is to use highly directional antennas to receive the signal from one direction only and reject the coming signals from the other directions.
B)Sky waves :-
These waves are transmitted from antennas with certain angle ,hit the ionosphere and hence are subjected to reflection from this layer. This reflection is limited to certain conditions most important of which are the angle of incidence and the frequency of the wave .At frequencies above 35 to 40 Mhz ,these waves cannot be reflected and only direct wave travelling in straight line can be received. At tv frequencies, reception is only possible when the transmitter antenna directly intercepts the signal as it travels away from the transmitter .The maximum distance for the antenna to be away and still receives a signal is called line of sight and is calculated as follows:-
From the fig. Shown below R^2+d^2=(R+h)^2
Rearranging the previous equation we get d^2=2Rh ……….(1)
where R is the radius of the earth and is equal to 4,000miles.substituting this value in equation (1)we get
d=1.23(h)^0.5
Where d= line of sight distance (miles)
h=is the height of the transmitting antenna(feet)
C)Unwanted signal paths:-
Signals may take other paths than direct waves for example (waves reflected from the earth).This problem can be compensated by the fact that reflected wave suffers phase shift due to two phenomenas :-
1)Phase shift due to reflection from the surface of the earth .
2)Phase shift due to longer path traveled by the reflected wave .
These two phase shifts are additive and have an approximate value of 360 degree which is equivalent to no phase shift at all.
The design of antennas varies considerably from signal to another but most of the antennas have common characteristics that have to be considered. In the coming section we will introduce some of the most important of these properties.
A)Polarization:-
The polarization of a certain wave is determined by the direction of the electric field with respect to some fixed coordinates .A horizontal antenna will produce a horizontally polarized E.M wave and a vertical antenna will produce a vertically polarized E.M wave .For antennas located close to the earth ,vertically polarized rays produce better signals. When the receiving antenna is raised about one wave length above the ground ,however, either vertical or horizontal antennas may be used .When the antenna is several wavelengths above the ground , horizontally polarized waves give a more favorable SNR. Horizontally polarized waves are standard for tv industry ,and so all tv receiving antennas are mounted horizontally.
B)Directivity:-
Antennas are either unidirectional (radiates or receives the stronger signal from only one direction)or omnidirectional(radiates or receives equally well in all directions).It is hence expected that sending antennas are omnidirectional and receiving antennas are unidirectional.
C)Gain:-
The antenna gain is defined as the effectiveness of a unidirectional antenna relative to the effectivness of a half wave dipole having the same polarization when both antennas are oriented for max. pickup.
D)Bandwidth:-
It is known that the gain -bandwidth product =constant so for higher gain it is expected that the antenna will have more limited bandwidth. A practical consideration is that for greater conductor diameter the bandwidth increases.
E)Impedance:-
Antenna impedance is generally refer to the point at which the transmission line is attached.For tv receiving antennas ,this point is at the center of the connected element.
Generally tv antennas are made of wires and rods of specific length that determines the resonant frequency at which we have the highest voltage signal. At the matter of fact antennas have inductance and capacitance ,therefore it has a resonant frequency.
A)Half wave antenna :-
Any rod undergrounded whose length is half of the wavelength of the received wave is called a half wave antenna. An example of such antennas is the dipole antenna shown in the figure below: Two metallic rods are used for the antenna itself. T hey are mounted horizontally on a supporting structure . Each of the rods is one quarter of the wavelength and thus the total length of the rods is half the wave length. An important property of the dipole is that they receive signals with greatest intensity when the rods are at right angles to the approaching signal.
TUNNING THE ANTENNA :-
An antenna must be tuned in order to have the strongest signal developed along its length tuning is usually accomplished by cutting the wires or rods to a specific length. The length is determined by the frequency of the signal . Antennas are similar to other tuned circuits in that they have specific band width. Thus bandwidth is related to the effective “Q” of the antenna. Hence tv antennas have a bandwidth with enough to accommodate all VHF &UHF channels. To compute the length of the dipole needed to receive a frequency f the following formula is applied:
L=468/f
Where
L is the length of the dipole(feet)
f is the frequency at which the dipole resonates.
ANTENNA WITH RTEFLECTORS AND DIRECTORS:
The reflector is also called the parasitic element. The signal received generates a voltage in the driven element .As a result , signal current flows in the reflector and the signal is reradiated .The reradiated signal induces a signal voltage in the driven element. If the reflector and dipole are properly spaced , the reradiated signal will be in phase with the original signal and therefore it will reinforce the original .If the signal is coming from the reflector side the reradiated signal will be out of phase with the original signal and the gain will be reduced considerably. In the shown fig the dipole antenna with reflector and response pattern are shown:-
The director is a wire or rod whose length is slightly less than half wavelength. Like the reflector it is a parasitic element .When the dipole is placed perpendicular to the incoming signal, the director is the first to intercept the signal. The director picks up part of the signal and then reradiates it with phase relationship that strengthens the signal arriving at the driven element. An example of an antenna with multiple reflectors and directors is the Yagi antenna shown in figure .
B)UHF Antenna :-
UHF signals are often weaker than VHF signals .Furthermore ,the losses in transmission are greater in UHF . We will now present some of the types of the UHF antennas.
FAN DIPOLE ANTENNA :-
The half wave dipole , also known as the bow tie or di-fan antenna ,is the simplest type of VHF antenna .It is also the simplest UHF antenna .By using triangular sheets of metal(fans )instead of rods the unit becomes a broad band affair capable of receiving all those signals within the UHF band. For maximum gain , a fan dipole should be slightly loner than a rod dipole .The response pattern of a fan dipole is a figure eight unless a screen reflector is placed behind the array. A figure of the fan dipole antenna is shown below.
Note that !!A lot of types of reflector designs exist and here are some examples
1-Mesh reflector 2-parabolic reflector 3-corner reflector
An antenna widely used in the past is the folded dipole an.This antenna consists of two dipole antennas connected in parallel .The has the same bidirectional pattern as the simple dipole discussed before. Dipole antenna and the folded dipole antenna have approximately the same gain but the response of the folded dipole is more uniform over agiv3en bandwidth of frequencies than that of the simple dipole .The folded dipole response is increased by adding reflectors and directors to it .The figures coming show the basic dipole antenna and some popular tv receiving antennas
In addition to the folded dipole antenna is the log periodic antenna .This type of antenna is extensively used for color t v signal reception .It has a very wide bandwidth .The term log periodic does not refer to the relative lengths of the elements or to the distance between them. Instead, it refers to their electrical characteristics. A graph of an antenna impedance versus the logaritm of the frequency is shown in fig .Note that the maximum value of the impedance repeats periodically .
The log periodic antenna is shown in figure .It is called dipole array log periodic antenna because it consists of a number of dipoles which vary in spacing and length. The antenna is designed such that the ratio of the distances between dipoles and the lengths of dipoles is constant. .For any particular frequency being received only one or two log periodic elements are active .
*In addition to the above mentioned antennas we can add the conical array antennas which reduce the number of lobes in the response pattern.
Another type is the eight -bay bow tie which provides large gain and sharp directivity with the covering of UHF band but its practical installation is difficult and there are more practical antennas .
D)Combination antennas:-
In a lot of places we have both VHF and UHF stations. Combination antennas are hence very useful. They gather all the advantages needed for antennas .These advantages are high gain ,good directivity ,broad bandwidth and excellent mechanical characteristics .Nevertheless, a question may arise :”how can the antenna discriminate between UHF and VHF and FM?” .The answer is that a three way signal splitter is used with each of these antennas .Its function is to separate the thrre bands so that they are fed to their appropriate terminal. An example of these antennas and their response pattern is shown below.
*In some places very low signal level is provided, so we need a high gain antenna and the gain of an antenna increases with the number of elements .These kinds of antennas have sharp directivity which can be shown by their response patterns. It should be noted that in some places where strong signals are available we use indoor antennas such that the rabbit ears antenna.
11.4.4 TYPES OF TRANSMISSION LINES:-
Now that we have seen various types of antennas we need to discuss how signals will flo flow from the driven element to the receiver .Sure their must be a transmission line s and this is the subject discussed in this section.
*If for example a dipole presents an impedance of 72 ohms ,for max. power transfer we need a transmission line to have the same impedance so we need an antenna with a uniform response in a frequency range .
*It is also desirable to use as high an impedance as possible because line loss is inversely proportional to the characteristic impedance .Also length and area must be taken into consideration..
*There are a lot of transmission lies types but the most important is to choose the best for the location. I will now demonstrate the kinds of transmission lines and if there is a special use it will be mentioned.
1)Parallel wire line :the line is balanced which means both wires have the same average potential with respect to the ground.
2)Tubular twin lead: is less affected by weather conditions and signal losses in bad weather are not th
considerable.
3)Airlead: it has very low losses.
4)Open wire line: it has the least losses and is relatively unaffected by changes in weather
5)Twin cable :the two wires are enclosed in a dielectric and the entire setup is shielded by a copper grounded wire which acts as an electrostatic shield that prevents stray interference. This type of wire is used in highly electrical noisy areas.
6)Coaxial cables: they are unbalanced cables and they need special modifications in the receiver circuit.
In the figure below we illustrate the above mentioned cables.
The signals received may suffer a lot of attenuations due to the transmitting medium and to other factors that are related to the antenna impedance .to amplify the received signals we have two methods :1-Signal booster
2-antenna preamplifier
1)Antenna boosters:
They are basically RF amplifiers .Their function is to strengthen the incoming signal to produce a picture having the full contrast range .Also boosters improve the SNR so that the picture will be free of noise spots. The two objectives must exist in the same time time .The noise must be eliminated before the booster to prohibit the amplification of the noise with the signal and this is done by increasing antenna height and using shielded lead-in line
This is simply a broad band transistor mounted on the antenna or the antenna must.
END