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Tuesday, 20 April 2021

Logic Devices for Interfacing

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A microcomputer system consists of four components namely microprocessor, memory, input devices and output devices. In order to design a microprocessor based system for a particular application, the designer has to select suitable memories and I/O devices, and interface them to the microprocessor. But these memories and I/O devices must be compatible with the microprocessor, both in speed and timing characteristics. If a particular device is not compatible, additional electronic circuit has to be designed, through which the device may be connected to the CPU. Before discussing interfacing circuits for connecting memory and I/O devices to the microprocessor, let us have a review of logic devices used for interfacing.

1. Tristate devices

Normally a logic circuit has only two states: Logic 1 and Logic 0. Tristate logic device has three states: Logic 1, Logic 0 and high impedance (high Z state). The term "Tristate" is the trademark of National Semiconductor Company and is used to represent the three logic states. A tristate logic device has a third input called enable input.

Here the enable input is active low input i.e., inverter is active only if a low is given to the enable input. If the enable input is disabled (logic high), the inverter will enter a high impedance state. That is, the inverter will not respond for the input.

2. Buffer

The buffer is a logic circuit that amplifies the current or power. It has one input and one output line. The logic level of output is the same as that of the input i.e., logic 1 input provides logic 1 output. The symbol for buffer is shown in Figure.

The buffer is commonly used to increase the driving capability of a logic circuit. It is also known as driver.

3. Decoder

A decoder is a logic circuit used for detecting the presence of a specified combination of bits on its inputs and to indicate that code by a specified output level. Generally a decoder has n inputs and 2n outputs. Figure shows a 2:4 decoder with active low output lines.

For example, if the input is 01, the output line 1 will be low and all other output lines are high. Here, the decoder will function only if a low is given to the enable input (enable input is active low). Decoders are commonly used in interfacing I/O devices and memory to the microprocessor.

4. Semiconductor Memory

A memory unit is an integral part of a microcomputer system. It is used to store programs, data and result. A microprocessor based system uses a number of memory devices of different technologies such as magnetic memory, semiconductor memory and optical memory. The speed of the memory must match with the operating speed of the CPU. If the memory is slow, the CPU has to wait for data and instructions. Memory devices are broadly classified into two groups.

(i) Primary memory

(ii) Storage memory

Primary memories are fast semiconductor memories. While storage memory refers to the storage medium comprising slow devices such as magnetic tapes, hard disks, floppy disks, compact disks (CD) etc. These devices are used to hold large data files and huge programs such as compilers, application programs etc. The primary features of these devices are high storage capacity, low cost and slow access. The access time for these devices is of the order of millisecond.

Primary memory

It refers to the storage area which can be directly accessed by the processor. All programs and data must be stored in primary memory prior to execution. In primary memories the access time must be compatible with the read/write time of the processor. Access time is the time to access any particular memory location. Therefore semiconductor memories are used as primary memories. (The access time for semiconductor memory is only 50 ns. Also, CPU is a semiconductor device.

Semiconductor memories are broadly classified into two.

(1) RAM (Random Access Memory)

(2) ROM (Read Only Memory)


In a random access memory, any memory location can be accessed in a random way. That is, the access time is same for each and every memory location. RAM is also called read/write memory (R/WM), since the processor can write into or read from this memory. RAM is also a volatile memory. That is, it stores information as long as the power is supplied to it. Its contents are lost when power supply is switched off.

RAM is again classified into two.

(a) Static RAM (SRAM)

(b) Dynamic RAM (DRAM)

Static RAM retains the stored information as long as power supply is ON. But DRAM loses its stored information in a few milliseconds even though its power supply is ON. A DRAM stores information in the form of charge on a capacitor; which leaks away in very short time. Therefore its content must be periodically refreshed for restoring the capacitor charge (usually every 2ms). Thus DRAM requires a refreshing and control circuitry which will increase the cost of the system.

A DRAM requires only one transistor and a capacitor. That is, DRAM requires only one transistor per memory cell. (Memory cell is an electronic circuitry which stores a binary bit 0 or 1). But SRAM uses six transistors in a memory cell. Therefore, packing density is more for DRAM compared to SRAM. Also, DRAM consumes less power.


ROM is a non volatile memory. That is, it retains the stored information even if the power is OFF. This memory is used for storing programs and data permanently (i.e., need not be altered later). It is cheaper than RAM. Different types of ROM available in market are

Masked ROM

In masked ROM, the information is stored permanently by the manufacturer at the time of manufacturing.

Example: Audio CD of a movie.

PROM (Programmable ROM):

This memory can be programmed by the user with a special 'PROM Programmer'; which selectively burns the fuses (within the PROM) according to the bit pattern to be stored.

EPROM (Electrically Programmable ROM):

The content of an EPROM can be erased and can be reprogrammed more than once. To erase its content, it is exposed to ultraviolet radiation for about 20 minutes. To facilitate the exposure of ultraviolet radiations, the EPROM chips are packed in a case which has transparent window.

Limitations of EPROM chips are

(a) It must be taken out of the circuit to erase it.

(b) The entire chip must be erased.

(c) The erasing process takes 15 to 20 minutes.

EEPROM or EEPROM (Electrically Erasable Programmable ROM):

They are also called Electrically Alterable ROM (EAROM). They need not be removed from the circuit board for erasure. Also, EEPROM is byte erasable. That is, selective erasure of its content is possible. Its content can be erased and programmed on the system board itself very easily on a byte by byte basis (need not be taken out of the circuit). Its disadvantage is that different voltage levels are required for erasing, writing and reading the stored information.

Flash Memory

It is also electrically erasable and reprogrammable. The major difference between Flash memory and EEPROM is in the erasure procedure. In Flash memory, the entire content is erased in one operation. That is, it is not byte by byte erasable like EEPROM. Unlike EEPROM, flash memory uses one transistor memory cell resulting in high packing density, lower cost and higher reliability.

In a microprocessor based product, programs are generally written in ROM and data that are likely to vary are stored in RAM.

For example, in a microprocessor controlled microwave oven, the program that 'runs' the oven is permanently stored in ROM; and the data such as starting time, baking period and temperature are entered in RAM through key pad.

Saturday, 17 April 2021

Basic Operations of Microprocessor

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A microprocessor is a programmable electronic device that has computing and decision making capability similar to that of a central processing unit (CPU) of a computer. Today, the applications of the microprocessor are increasing at a faster rate. Microprocessor can be embedded in a large system or it can be a stand alone unit capable of controlling different processes. It can also function as CPU of a computer. At a very elementary level, we can draw an analogy between microprocessor operations and functions of a human brain. The brain gets inputs from eyes, ears etc (input devices) and sends processed information to "output devices" such as face in the form of expressions and emotions. Generally a microprocessor is a programmable device, which accepts binary data from an input device, process the data according to the instructions stored in the memory, and provides the result to an output device. Prior to going into the details of a microprocessor, we must have a basic knowledge of computers.

Basic elements of a computer

All computer systems consist of three basic functional blocks as shown in figure.

(1) Central Processing Unit (CPU)

(2) Memory

(3) Input and Output devices

A system designed with a microprocessor as its central processing unit is called a microcomputer. A microprocessor based system includes a microprocessor as CPU, semiconductor memories, input and output devices, interfacing devices and so on. The organization of a microcomputer (microprocessor based system) is shown in Figure. These functional blocks are linked together with three internal buses. Bus is a communication path between the CPU and peripherals (devices) with a group of wires to carry bits.

The three buses are the data bus, the address bus and the control bus. The input and output devices are connected to the input and output ports respectively. A port is a physical interface on a computer through which data are passed to and from the peripherals. An instruction is a command given to the computer to perform a given task. A group of instructions designed to solve a specific problem is called a program. These instructions and data are stored in specific locations in the memory. Each location has a unique address associated with it.


Instructions are obtained by the CPU by placing the address (of the memory location where instruction is stored) in the address bus. Now the instructions are transferred to the CPU through the data bus. The CPU executes these instructions sequentially to get the final result. The processed data (result) is stored back in the memory or sent to peripheral devices like monitor, connected to the computer. All these processes are controlled and coordinated by the signals on the control bus generated by the CPU.


As stated earlier, microprocessor can function as CPU of a computer. It includes all logic circuitry necessary for performing various operations specific to that processor. For the sake of clarity, the microprocessor can be divided into three segments.

(a) Arithmetic and logic unit (ALU)

(b) Registers

(c) Control Unit.


This is the area of the microprocessor where various computing functions are performed. The ALU performs arithmetic operations like addition and subtraction, and logical operations like AND, OR and NOT. We can consider ALU as a section of CPU which contains all the logic circuits needed for performing different operations specific to that processor. A particular instruction (i.e., machine code of a particular instruction) will activate the appropriate logic circuit in the ALU so that a particular task is performed. This is called microprogramming which is done in the design stage of the microprocessor. We can compare the operation of microprocessor with the operation of our brain. In early childhood, we learn a word "sit" and the physical motion needed for the action "sit" are embedded in our brain. Later, when we hear the word "sit", our brain activates a series of actions for our muscles and bones, so that we perform the action "sit". In this analogy, the word "sit" is like an instruction to the microprocessor and action initiated by the brain are like microprogram.

The bit pattern (machine code of instruction) required to initiate these microprograms are given to the programmer in the form of instruction set of the microprocessor. The programmer selects the appropriate bit pattern (instruction) from the instruction set, for a given task and enters them sequentially to the memory through an input device. When CPU reads these bit patterns (instruction) one at a time, it initiates the appropriate microprogram through the control unit and performs the task specified in the instruction.


We can consider registers as temporary memory locations seen inside the processor. These registers are identified by the letters A, B, C, D, E, H, and L.

Control Unit

The control unit provides the necessary control signals for all the operations in the processor. It controls the flow of data between the microprocessor, the memory and the peripheral devices like keyboard and monitor.

Advantages of Microprocessor based systems

(i) Processing speed is high.

(ii) Automation of industrial processes and office administration.

(iii) Since the device is programmable, there is flexibility to alter the system by changing the software alone.

(iv) Compact and low cost.

(v) It is more reliable.

(vi) Operations and maintenance are easier.


Applications of Microprocessor based systems

Microprocessors are widely used in control applications.

(i) Microprocessor based systems are widely used in frequency meters, frequency synthesizers, spectrum analyzers etc.

(ii) In industry, they are widely used for controlling various parameters like speed, temperature and pressure.

(iii) In telephone industry they are widely used in digital telephone sets, telephone exchange and modems.

(iv) They are used in automobiles for monitoring various quantities like air-fuel mixture, temperature, speed etc.

(v) 32-bit microprocessors are widely used in CAD machines.

(vi) The car, maruti wagonR also uses a 32 bit processor for controlling the MPFI unit.

Tuesday, 13 April 2021

Microprocessor Programming Languages

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Taming the microprocessor, for a particular application is done by giving instructions to the microprocessor. Instruction is a command given to the microprocessor to carry out a particular task. An instruction activates the particular logic circuit, so that a specific task will be performed. The instruction set is the set of all instructions that a specific microprocessor is intended to execute. That is a microprocessor understands only an instruction present in the instruction set of that particular microprocessor.


To communicate with the microprocessor, the programmer must give the instructions in the binary language because the microprocessor understands only the binary language (1s and 0s). These binary code instructions are called machine language.


It is tedious and error prone for people to recognize and write instructions in binary language; so these instructions are written in hexadecimal code and entered in a computer by using Hex keys.


For example, the binary instruction 0011 1100 is equivalent to 3C in hexadecimal. This instruction can be entered in a computer with a Hex keyboard by pressing two keys; 3 and C. The monitor program of the system translates these keys into their equivalent binary pattern.


Assembly language


It is not easy to recognize a program written in hexadecimal numbers. Therefore, every maker of a microprocessor has set up a symbolic code for every instruction called mnemonics. The mnemonic of a particular instruction consists of letters that suggest the operation to be performed by that instruction.


Example: The binary code 0011 1100 (3CH in hexadecimal) is represented by the mnemonic INR A. (Hexadecimal numbers are usually followed by the letter H)


Here INR stands for increment and A represent accumulator. This instruction suggests the operation of incrementing the accumulator content by one.


The complete set of 8085 mnemonics is called assembly language and a program written in these mnemonics are called assembly language program. Writing a program in assembly language is much easier and faster than writing a program in machine language. Also assembly language is specific to each microprocessor. That is, the assembly language for 8085 microprocessor is entirely different from the assembly language for Motorola 68000. A microprocessor specific language is called a low level language. Thus machine language and assembly language are microprocessor specific and are both considered as low level languages. The mnemonics are convened to machine Ianguage (binary pattern) by using a program called assembler.

High level languages


Programming languages that are intended to be machine independent are called high level languages. BASIC, PASCAL, C, C++, JAVA etc are examples for high level languages. Instructions written in these languages are called statements rather than mnemonics. A program written in BASIC for a computer with Intel 8085 microprocessor can generally be run on another computer with a different microprocessor (Motorola). A program written in its high level language is termed as source code.


The instructions written in high level language is converted to machine language by using a program called a complier or an interpreter. The compiler or Interpreter translates the source code into the machine language compatible with the microprocessor being used in the system. The machine language equivalent of the source code is also called object code.

The primary difference between compiler and interpreter lies in the process of generating machine.


Machine Language


• The program developed using 1’s and 0’s is called machine language program. The machine can understand only machine language programs.

• The program developed using mnemonics is called assembly language program. Assembler is a conversion software, which can convert assembly language programs to machine language programs.

• The machine language and assembly language programs are machine (processor) dependent.

• The language which can be used to develop software independent of the hardware (processor) are called High Level Languages.

• The compiler or interpreter is a software which can convert high level language programs to machine language programs.

Applications of Microcontrollers

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Microcontrollers are also called embedded controllers or single chip microcomputers. Microcontroller is simply a computer on a chip. Microcontrollers find applications in environment where personal computers (PC) cannot be used. Following are some of its applications.

Important applications of in microcontrollers are

1. Used to measure and control the temperature of a furnace and ovens, speed of an electric motor, pressure of boilers etc.

2. Used in automobiles for automatic control of fuel and air mixture, ignition system, to test various conditions of engine, brakes etc.

3. Used in military equipments, RADAR, missiles etc.

4. Used in medical instrumentations like patient monitoring in ICU, pathological analysis, measurement of physiological parameters etc.

5. Used in home appliances like washing machines, microwave ovens etc where some physical parameters are monitored and controlled.

6. Used in electronic toys making them more entertaining and easy to use.         

7. They are widely used in controlling pumps, alarms, and other power applications. It is also used in communication equipment.

8. The Microcontroller output is binary values, but application equipments display, motor, speakers etc. work in analog signal.

9. There are many applications where you have to display numbers. The most popular display device used for displaying number is seven segment LED displays.

10. Microcontroller keeps the speed of the motor as constant. It adjusts the speed of the motor by changing the duty cycle of the signal applied to it. Hence constant speed can be maintained.

11. The electro mechanical relays have been used for many years in industry to control high dc or ac voltages and currents. Relays also provide isolation between the controller and the circuit under control.

12. It helps in Keyboard interfacing. When a key is pressed, the microcontroller identifies the pressed key by using either a software based or hardware based technique and then performs the assigned operation.

13. It helps in Stepper Motor interfacing. In order for getting accurate position control of rotating shafts, the Microcontroller 8051 is connected to it for constant speed.

Monday, 12 April 2021

Communication System Block Diagram Explanation

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Communications Systems


The process of transmission of information or message from one end to another is known as communication. When electromagnetic or radio waves are utilized for communication purpose, then it is called as radio communication. Electronic communication is started with wire telegraphy in the first half of nineteenth century. After a decade of years, telephony was developed, In the beginning of the twentieth century, radios come into existence. Radio communication made possible by the invention of the triode tubes. 


The modern communication system involves the process of sorting, processing and storing of information before conveying the message. During transmission, the filtering of noise takes place. Finally there includes the processing steps like decoding, storage and interpretation. Some types of communication include radar, telecommunications, mobile, computer, radio telemetry etc. The essential requirements of any communication system are fundamentally same.


Communication System Block Diagram with Explanation


The general block diagram of a general communication system is shown in figure. Any communication system is consist of an information source, transmitter, receiver, channel and noise.




The purpose of communication system is to communicate a message. This message comes from an information source. The message from an information source may be a speech from an individual or a numerical data from a computer. The total number of messages consists of individual messages which can be distinguished from one another. The amount of information contained in any given message is measured in bits depending upon the method of communication.




The message comes from an information source may not be an electrical signal. Unless the message that comes from the information source is electrical in nature, it will be unsuitable for sending. So the physical quantity must be converted into an electrical signal before it is applied to the transmitter. This is done with a transducer. The transducer will convert the physical quantity (in which the information is presented) into a corresponding electrical signal. For example, a carbon microphone will convert sound into electrical signal. The output from the transducer is known as signal. The signals are of two types — analogue type or digital type. According to the type of signals used, communication systems are also classified into either analogue or digital systems. In modem communication systems, the analogue signals are converted into digital signals, and thereafter transmitted through a digital system.


Even though the message that comes from the information source is electrical in nature, it is unsuitable for immediate sending. A lot of process must be done on the message to make it suitable for transmission. The transmitter is required to process the incoming information. The main process to be done is known as modulation. Modulation is a process in which some characteristics of a high frequency sine wave (carrier) is varied in accordance with the instantaneous values of the message signals. In a transmitter, the information modulates a carrier. That is, the information is impressed on a high frequency sine wave. The method of modulation may be analog or digital, high level or low level. The system may be amplitude modulator, frequency modulator, phase modulator, or pulse code modulator or combination of these.




Channel is the medium through which the information is transmitted from the transmitter to the receiver. The channel may be free space, air, wire, or fiber optic channel. In radio communication, the medium is free space where as in line communication it is a cable or a wire. In radio communication, information is transmitted as electromagnetic waves into the free space. In line communication it is transmitted as electric signals through cable. The acoustic channel is not used for long distance communication.


Noise source


The reason for noise is


1. Some distortion in the system

2. Because of introduction of noise, which is present in a transmission system.


Noise is defined as any unwanted form of energy tending to interfere with the proper and easy reception and reproduction of wanted signals. Noise can interfere with the signal at any point in a communication system. There are many ways of classifying noise, It is most convenient here to divide noise into two broad groups — internal noise and external noise.


External noise is the noise whose sources are external to the receiver. For example, atmospheric noise and industrial noise. Internal noise is the noise created within the receiver itself. For example, thermal agitation noise.




The transmitted signal finally reaches at the receiver. The main function of a receiver is to demodulate the modulated incoming signal so as to retrieve the original message. The signal from the channel is amplified and the information is extracted in the desired form with the help of a transducer.

Friday, 9 April 2021

Modulation and Need for Modulation

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Modulation and Need of Modulation

Modulation may be defined as a process by which any characteristics of a wave is varied as a function of the instantaneous value of another wave. The first wave, which is normally a high frequency sine wave, is known as carrier wave. The second wave is known as the modulating wave and the resultant wave is known as the modulated wave. The rate at which the variation takes place is equal to the modulating wave frequency. Modulating signal is an electrical signal having sound, picture or any other type of information.

Audio frequencies ranges from 20 Hz to 20 KHz. Sound waves propagated in air directly beyond a few hundred meters. Radio frequency means anything above 20 KHz. Radio waves are electromagnetic in nature, are capable of being propagated upto infinite distance in space. Sound waves can be able to propagated upto infinite distance in space. Sound waves can be able to propagated upto infinite distance in space by superimposing them on radio waves. This is what is done by the modulation process.

A carrier sine wave can be represented by the equation, e = Esin(ωt+φ), where ‘e’ is the instantaneous value of carrier sine wave, E is the maximum amplitude, ω is the angular frequency and φ is the phase. An unmodulated carrier has constant amplitude, a constant frequency and a constant phase relationship with respect to some reference. In modulation process one of these three parameters of the carrier may be varied by the modulating wave or signal. Hence at any moment its variation from the unmodulated value is proportional to the instantaneous value of modulating voltage. Depending on which characteristics of the carrier is varied by the modulating signal, there are three types of modulation – amplitude, frequency or phase modulation. The need for modulation is explained below.

1. The height of the antenna required for transmission and reception of a signal must be equal to ¼ th of the wavelength of the signal used for communication. We know, f = C/λ, where f is the frequency used, C is the velocity of the electromagnetic wave and λ is the wavelength. If frequency is small, λ is high and hence height of the antenna required must be comparatively large. If frequency is large, λ is small and hence height of the antenna required must be comparatively small. For example, a 15 KHz electromagnetic wave has a wavelength of λ = 3 x 108/15 x 103 = 20,000 m. Therefore the height of the antenna is λ/4 = 20,000/4 = 5000 m or 5 Km. Therefore the 15 KHz signal requires an antenna of 5 Km height. A vertical antenna of this size is unthinkable. Since the frequency is inversely proportional to the antenna height, a higher frequency must be used to reduce the antenna height.

2. At low frequencies, the transmitting power must be very large. The construction of a transmitter having such a huge power handling capacity is a difficult task. A radio frequency signal will travel a greater distance than the same amount of energy transmitted as sound signal.

3. All sound signals are within the range of 20 Hz to 20 KHz. If we transmit these sound waves directly, all waves from different stations and sources would be inseparably mixed up. In order to separate the various signals from different stations, it is necessary to transmit them at different portions of electromagnetic spectrum. This will also overcome the poor radiations at low frequencies. A tuned circuit is used in the receiver to select a desired transmission within a predetermined range and to reject all the other unwanted signals.

Thursday, 8 April 2021

Fundamentals of Electromagnetic Waves

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Electromagnetic waves are oscillations that transmit through free space with the velocity as that of light. Free space is the space that does not interfere with the normal radiation and propagation of the radio waves. Free space has no magnetic or gravitational field, no solid bodies and no ionized particles. Free space is unlikely exist anywhere. However the concept of free space is used because it simplifies the approach to wave propagation. Radio waves are electromagnetic in nature, has an electric field and magnetic field. Electromagnetic are transverse wave, that the oscillations are perpendicular to the direction of propagation. Also the direction of electric field, magnetic field and the direction of propagation are mutually perpendicular to each other.

Polarization: Polarization is refers to the physical orientation of the radiated wave in space. A wave is said to be vertically polarized, when all its electric intensity vectors are vertical. A wave is said to be horizontally polarized when all its electric intensity vectors are horizontal. A vertical antenna will radiate vertically polarized wave and a horizontal antenna radiates horizontally polarized waves.

Reflection: Electromagnetic waves will be reflected by a conducting medium. They will be reflected by ground, mountains and buildings. This is much similar to the reflection of light by a mirror.

Refraction: Refraction takes place when electromagnetic waves pass from one propagating medium to another medium having a different density. They will get refracted as they pass through the layers of the atmosphere having different degrees of ionization.

Diffraction: Diffraction of electromagnetic waves occurs due to the presence of small slits in a conducting plane or sharp edges of obstacles. The electromagnetic waves may be diffracted around the tall massive objects.

Attenuation and absorption: The power density of the wave diminishes rapidly with distance from the source of electromagnetic waves. The attenuation is proportional to the square of displacement. In free space, absorption of radio waves does not occur because there is nothing to absorb them. However atmosphere is tend to absorb radio waves because some of the energy from the wave is transferred to the atoms and molecules of the atmosphere. Thus the energy of the waves may be absorbed quite significantly.

Ground Wave Propagation:

Frequency below high frequency range { very low frequency (3 KHz to 30 KHz), low frequency (30 KHz to 300 KHz) and medium frequency (300 KHz to 3 Mhz)} will travel along the curvature of the earth. So these waves are called the ground waves or surface waves. Ground waves are propagated by means of a type of waveguide effect, which uses the earth surface and the lowest ionized layer of the atmosphere as the two waveguide walls. Ground wave propagation is one of the two means of the beyond the horizon propagation. Ground waves must be vertically polarised to prevent the short circuit of the electric field components.

A ground wave is attenuated in two ways.

1. A wave induces currents in the ground over which it passes and they loses some of energy by absorption.

2. Because of the diffraction the wave front gradually tilts over as the wave propagate over the earth, its tilts increases more and more and increasing the tilt causes greater short circuiting of the electric field component of the wave and hence the field strength reduction. Eventually at some distance from the antenna the waves lies down and dies.

Use: Medium wave radio communication

Sky wave propagation

The ionosphere is the upper portion of the atmosphere which absorbs a large quantities of the energy from the sun, it becoming get heated and ionized due to the heat. There were several degrees of ionization at different height. The various layers of ionosphere have specific effects on the propagation of radio waves particularly at high frequency. Under certain conditions, waves in the high frequency range (3 MHz to 30 MHz) are return to the earth by the ionized layers of atmosphere so they are called sky waves. The mechanism involved in this process is refraction. The ionization density increases for a wave approaching the given layer (ionosphere) at an angle, so the refractive index of the layer is reduced. Hence the incident wave gradually bends further and further away from the normal. At a particular layer it will bend downward and finally emerging from the ionized layer at an angle equal to the angle of incidence as shown in figure. This is the second method for the beyond the horizon propagation.

Skip distance

It is the distance upto which sky waves of given frequency cannot be received or minimum distance from transmitting antenna to the point at which sky wave of a given frequency is returned to earth by ionosphere. It depends on frequency of transmission, critical frequency, and height of layer and increases as ionization in the layer reduces. It is the shortest distance measured along the surface of the earth at which a sky wave of fixed frequency will be returned to the earth.

Use: Short wave communication.

Frequency above high frequency range generally travels through the troposphere, the portion of the atmosphere closest to ground. So space waves are sometimes called troposphere waves. Space wave propagation depends on line of sight (LOS) conditions. So the space waves are limited in this propagation by the curvature of earth. When earth curvature can be neglected, space wave propagation take place in the manner illustrated is shown in the figure.

Here energy reaches the receiver in two ways

1. By a ray travelling directly between transmitting and receiving antennas (Direct wave).

2. By a ray that reaches the receiver after reflection from the surface of the ground (Ground reflected wave) or Satellite reflected wave.

The field strength at the receiving antenna is the vector sum of the fields represented by the two rays.