Microwave Components

Introduction: Just like other systems, the Microwave systems consists of many Microwave components, mainly with source at one end and load at the other, which are all connected with waveguides or coaxial cable or transmission line systems.

Following are the properties of waveguides.

• High SNR • Low attenuation • Lower insertion loss

Waveguide Microwave Functions Consider a waveguide having 4 ports. If the power is applied to one port, it goes through all the 3 ports in some proportions where some of it might reflect back from the same port. This concept is clearly depicted in the following figure.

Scattering Parameters For a two-port network, as shown in the following figure, if the power is applied at one port, most of the power escapes from the other port, while some of it reflects back to the same port. In the following figure, if V1 or V2 is applied, then I1 or I2 current flows respectively.

If the source is applied to the opposite port, another two combinations are to be considered. So, for a two-port network, 2 × 2 = 4 combinations are likely to occur. The travelling waves with associated powers when scatter out through the ports, the Microwave junction can be defined by S-Parameters or Scattering Parameters, which are represented in a matrix form, called as "Scattering Matrix".

E-Plane Tee:

An E-Plane Tee junction is formed by attaching a simple waveguide to the broader dimension of a rectangular waveguide, which already has two ports. The arms of rectangular waveguides make two ports called collinear ports i.e., Port1 and Port2, while the new one, Port3 is called as Side arm or E-arm. T his E-plane Tee is also called as Series Tee.

As the axis of the side arm is parallel to the electric field, this junction is called E-Plane Tee junction. This is also called as Voltage or Series junction. The ports 1 and 2 are 180° out of phase with each other. The cross-sectional details of E-plane tee can be understood by the following figure.

The following figure shows the connection made by the sidearm to the bi-directional waveguide to form the parallel port.

H-Plane Tee:

An H-Plane Tee junction is formed by attaching a simple waveguide to a rectangular waveguide which already has two ports. The arms of rectangular waveguides make two ports called collinear ports i.e., Port1 and Port2, while the new one, Port3 is called as Side arm or H-arm. This H-plane Tee is also called as Shunt Tee.

As the axis of the side arm is parallel to the magnetic field, this junction is called H-Plane Tee junction. This is also called as Current junction, as the magnetic field divides itself into arms. The cross-sectional details of H-plane tee can be understood by the following figure.

The following figure shows the connection made by the sidearm to the bi-directional waveguide to form the serial port.

Magic Tee:

An E-H Plane Tee junction is formed by attaching two simple waveguides one parallel and the other series, to a rectangular waveguide which already has two ports. This is also called as Magic Tee, or Hybrid or 3dB coupler.

The arms of rectangular waveguides make two ports called collinear ports i.e., Port 1 and Port 2, while the Port 3 is called as H-Arm or Sum port or Parallel port. Port 4 is called as E-Arm or Difference port or Series port.

The cross-sectional details of Magic Tee can be understood by the following figure.

The following figure shows the connection made by the side arms to the bi-directional waveguide to form both parallel and serial ports.

Characteristics of Magic Tee:

• If a signal of equal phase and magnitude is sent to port 1 and port 2, then the output at port 4 is zero and the output at port 3 will be the additive of both the ports 1 and 2.

• If a signal is sent to port 4, E−arm then the power is divided between port 1 and 2 equally but in opposite phase, while there would be no output at port 3. Hence, S34 = 0.

• If a signal is fed at port 3, then the power is divided between port 1 and 2 equally, while there would be no output at port 4. Hence, S43 = 0.

• If a signal is fed at one of the collinear ports, then there appears no output at the other collinear port, as the E-arm produces a phase delay and the H-arm produces a phase advance. So, S12= S21 = 0.

Directional Couplers:

A Directional coupler is a device that samples a small amount of Microwave power for measurement purposes. The power measurements include incident power, reflected power, VSWR values, etc.

Directional Coupler is a 4-port waveguide junction consisting of a primary main waveguide and a secondary auxiliary waveguide. The following figure shows the image of a directional coupler.

Directional coupler is used to couple the Microwave power which may be unidirectional or bi-directional.

Properties of Directional Couplers The properties of an ideal directional coupler are as follows.

• All the terminations are matched to the ports. • When the power travels from Port 1 to Port 2, some portion of it gets coupled to Port 4 but not to

Port 3. • As it is also a bi-directional coupler, when the power travels from Port 2 to Port 1, some portion of

it gets coupled to Port 3 but not to Port 4. • If the power is incident through Port 3, a portion of it is coupled to Port 2, but not to Port 1. • If the power is incident through Port 4, a portion of it is coupled to Port 1, but not to Port 2. • Port 1 and 3 are decoupled as are Port 2 and Port 4.

Ideally, the output of Port 3 should be zero. However, practically, a small amount of power called back power is observed at Port 3. The following figure indicates the power flow in a directional coupler.

Where

• Pi = Incident power at Port 1 • Pr = Received power at Port 2 • Pf = Forward coupled power at Port 4 • Pb= Back power at Port 3

Waveguide Bend:

Waveguide bends are used to direct high frequency signals propagating through a waveguide in a specific direction. These bends allow the change in direction of a signal within a waveguide, with minimal loss, reflection and distortion of the electric and magnetic fields.

Unlike coaxial cables which can be bent on demand, waveguide structures are more rigid and require the use of bends to get the signal from one point to another.

Types of Waveguide bends:

Waveguide E bends - An E-bend changes or distorts the E-Field (Electric Field) of the propagating signal. In order to minimize reflections, the radius of the bend should be greater than two wavelengths of the signal.

Waveguide E bend

Waveguide H bends - An H-bend changes or distorts the H-Field (Magnetic Field) of the propagating signal. In order to minimize reflections, the radius of the bend should be greater than two wavelengths of the signal.

Waveguide H bend

The bend angle is usually 30 degrees, 45 degrees or 90 degrees, however this can be customized based on the requirement. When selecting the waveguide bend the waveguide size and the flange type need to be selected - these usually vary based on the frequency at which you are planning to use the specific waveguide bend.

If you are looking for Waveguide Bend products, everything RF has compiled complete catalogs from the leading manufacturers and made then searchable by specification i.e Waveguide Type, Waveguide Size, Bend Angle, Frequency etc.

Waveguide twists:

There are also instances where the waveguide may require twisting. This too, can be accomplished. A gradual twist in the waveguide is used to turn the polarisation of the waveguide and hence the waveform.

In order to prevent undue distortion on the waveform a 90° twist should be undertaken over a distance greater than two wavelengths of the frequency in use. If a complete inversion is required, e.g. for phasing requirements, the overall inversion or 180° twist should be undertaken over a four wavelength distance.

Waveguide bends and waveguide twists are very useful items to have when building a waveguide system. Using waveguide E bends and waveguide H bends and their srap bend counterparts allows the waveguide to be turned through the required angle to meet the mechanical constraints of the overall waveguide system. Waveguide twists are also useful in many applications to ensure the polarisation is correct.

Tunable Detector/ detector mount:

The tunable detector is a detector mount which is used to detect the low frequency square wave modulated microwave signals. The following figure gives an idea of a tunable detector mount.

The following image represents the practical application of this device. It is terminated at the end and has an opening at the other end just as the above one.

To provide a match between the Microwave transmission system and the detector mount, a tunable stub is often used. There are three different types of tunable stubs.

• Tunable waveguide detector • Tunable co-axial detector • Tunable probe detector

Also, there are fixed stubs like −

• Fixed broad band tuned probe • Fixed waveguide matched detector mount

The detector mount is the final stage on a Microwave bench which is terminated at the end.

Waveguide attenuator:

The attenuators are basically passive devices which control power levels in microwave system by absorpsion of the signal. Attenuator which attenuates the RF signal in a waveguide system is referred as waveguide attenuator. There are two main types fixed and variable. They are achieved by insertion of resistive films.

Fig.1 coaxial line attenuator

This figure depicts coaxial line based fixed type of attenuator. Here resistive film is fixed at the centre conductor which absorb the power and as a result of power loss and hence microwave signal gets attenuated. This is referred as coaxial line attenuator.

Fig.2 waveguide attenuator

This figure depicts waveguide based fixed type of attenuator. Here thin dielectric strip with coated resistive film is placed at the centre of waveguide. Film is placed in the waveguide parallel to the maximum E field.

In a variable type of waveguide attenuator, resistive vane is moved from one side of the wall to the centre by using screw where E field is considered to be maximum. This resistive film is shaped to give linear attenuation variation.

Microwave isolator:

An RF isolator is a two-port device which prevents RF energy from coming back to its source. It is analogous to a diode which only allows electrical current to flow in one direction, or a check valve in a fluid-flow system. Note that the RF isolator has absolutely no relationship to galvanic (ohmic) isolation used in non-RF signal and power-supply designs, and it is not galvanically isolated.

It is primarily used to protect other RF components from excessive signal reflection. For example, isolators are used in test applications to provide electrical separation between a device under test (DUT) and a sensitive signal source. This is done in the event that some of the energy at the DUT reflects back to the source (Figure 1).

Fig 1: One major use of the isolator is to prevent “backflow” of RF energy from a load back to the

sensitive source, which may be due to impedance mismatch.

Microwave circulator:

As the larger “sibling” of the isolator, the circulator is a three-port device used to control the direction of signal flow (RF energy) in a circuit. Note that an isolator is a circulator with its third port terminated (Figure 2).

Fig 2: The isolator and circulator are very similar in structure, except that the isolator has its third port terminated in the characteristic impedance.

The most common application is where a transmitter and receiver share an antenna, and the higher-power transit signal must not reach the sensitive receiver input as it would overload and possibly damage it (Figure 3). (The general term for this function is a duplexer.) Ideally, the transmit signal energy output would go only to the antenna port and not be “seen” by the receiver. (Note that there are other ways to implement the duplexer function, including using a more-expensive cavity duplexer.)

Fig 3: A circulator is used to separate transmitter and receiver signal-energy paths and flow

Slotted section:

In a microwave transmission line or waveguide, the electromagnetic field is considered as the sum of incident wave from the generator and the reflected wave to the generator. The reflections indicate a mismatch or a discontinuity. The magnitude and phase of the reflected wave depends upon the amplitude and phase of the reflecting impedance.

The standing waves obtained are measured to know the transmission line imperfections which is necessary to have a knowledge on impedance mismatch for effective transmission. This slotted line helps in measuring the standing wave ratio of a microwave device.

Construction

The slotted line consists of a slotted section of a transmission line, where the measurement has to be done. It has a travelling probe carriage, to let the probe get connected wherever necessary, and the facility for attaching and detecting the instrument.

In a waveguide, a slot is made at the center of the broad side, axially. A movable probe connected to a crystal detector is inserted into the slot of the waveguide.

Operation

The output of the crystal detector is proportional to the square of the input voltage applied. The movable probe permits convenient and accurate measurement at its position. But, as the probe is moved along, its output is proportional to the standing wave pattern, which is formed inside the waveguide. A variable attenuator is employed here to obtain accurate results.

The output VSWR can be obtained by

VSWR=√VmaxVmin

Where, V is the output voltage.

The following figure shows the different parts of a slotted line labelled.

Horn antenna:

To improve the radiation efficiency and directivity of the beam, the wave guide should be provided with an extended aperture to make the abrupt discontinuity of the wave into a gradual transformation. So that all the energy in the forward direction gets radiated. This can be termed as Flaring. Now, this can be done using a horn antenna.

Frequency Range

The operational frequency range of a horn antenna is around 300MHz to 30GHz. This antenna works in UHF and SHF frequency ranges.

Construction & Working of Horn Antenna The energy of the beam when slowly transform into radiation, the losses are reduced and the focussing of the beam improves. A Horn antenna may be considered as a flared out wave guide, by which the directivity is improved and the diffraction is reduced.

The above image shows the model of a horn antenna. The flaring of the horn is clearly shown. There are several horn configurations out of which, three configurations are most commonly used.

Sectoral horn

This type of horn antenna, flares out in only one direction. Flaring in the direction of Electric vector produces the sectorial E-plane horn. Similarly, flaring in the direction of Magnetic vector, produces the sectorial H-plane horn.

Pyramidal horn

This type of horn antenna has flaring on both sides. If flaring is done on both the E & H walls of a rectangular waveguide, then pyramidal horn antenna is produced. This antenna has the shape of a truncated pyramid.

Conical horn

When the walls of a circular wave guide are flared, it is known as a conical horn. This is a logical termination of a circular wave guide.

The above figures show the types of horn configurations, which were discussed earlier.

Flaring helps to match the antenna impedance with the free space impedance for better radiation. It avoids standing wave ratio and provides greater directivity and narrower beam width. The flared wave guide can be technically termed as Electromagnetic Horn Radiator.

Flare angle, Φ of the horn antenna is an important factor to be considered. If this is too small, then the resulting wave will be spherical instead of plane and the radiated beam will not be directive. Hence, the flare angle should have an optimum value and is closely related to its length.

Combinations

Horn antennas, may also be combined with parabolic reflector antennas to form special type of horn antennas. These are −

• Cass-horn antenna

• Hog-horn or triply folded horn reflector

In Cass-horn antenna, radio waves are collected by the large bottom surface, which is parabolically curved and reflected upward at 45° angle. After hitting top surface, they are reflected to the focal point. The gain and beam width of these are just like parabolic reflectors.

In hog-horn antenna, a parabolic cylinder is joined to pyramidal horn, where the beam reaches apex of the horn. It forms a low-noise microwave antenna. The main advantage of hog-horn antenna is that its receiving point does not move, though the antenna is rotated about its axis.

Radiation Pattern

The radiation pattern of a horn antenna is a Spherical Wave front. The following figure shows the radiation pattern of horn antenna. The wave radiates from the aperture, minimizing the diffraction of waves. The flaring keeps the beam focussed. The radiated beam has high directivity.

Advantages

The following are the advantages of Horn antenna −

• Small minor lobes are formed • Impedance matching is good • Greater directivity • Narrower beam width • Standing waves are avoided

Disadvantages

The following are the disadvantages of Horn antenna −

• Designing of flare angle, decides the directivity • Flare angle and length of the flare should not be very small

Applications

The following are the applications of Horn antenna −

• Used for astronomical studies • Used in microwave applications

Microwave Communication systems

Microwave Communication systems Link:

Microwave signal are used for communication over long distance continental or intercontinental. Microwave is the communication link which make the communication possible. The basic block diagram of microwave communication system is shown in figure.

Construction: Antenna:- Mostly a parabolic refractor types of antenna are used which is used to transmit and receive the signal. Circulator: A circulator is used to isolate transmitter with the receiver input and to couple transmitter to antenna and antenna to receiver input. Protection Circuitry: It provides safety to the mixer from overloads. Mixer (Receiver): It has two outputs. One is the incoming signal and other is the signal from lower band pass filter (BPF).The mixer gives an IF signal of 70Mhz. Band pass filter (BPF): It provides the necessary selectivity to the receiver and it prevents the interference. IF amplifier and AGC:- It amplifies the signal up to a intermediate frequency of 70Mhz. and its gain is controlled through AGC (automatic gain control) Amplitude limiter: As the signal is frequency modulated one so as amplitude limiter is used to avoid unwanted amplitude variations . Mixer (Transmitter): It is used to convert IF frequency to transmitting microwave frequency band to pass through it and hence prevent interference.

Power amplifier:-This amplifier amplifies the transmitted power from a repeater section in the range of 0.2W to 10W. Microwave source:- Klystron & Gunn Oscillators were used as microwave source. Now, V H F transistor crystal oscillators are used for microwave source. Power splitter:- It divides the output power from a microwave source and feeds a large portion to the transmitter mixer, which converts it into transmitting microwave frequency. Shift oscilator:- It provides one of the inputs to the balanced mixer so that it produces 70MHz IF at the output of receiver mixer. This microwave link communicates with 600 to 2700 channels per carrier. Thus the number of carriers in each direction can be four to twelve. Troposcatter communications:

Troposcatter Communications are used for beyond line of sight (over the horizon) point to point communications between remote geographic areas where cable links are not feasible.

Developed in the 1950s, this technology uses particles that make up the lower portion of the earth’s atmosphere (Troposphere) as a reflector for microwave radio signals. In troposcatter propagation, when a signal is aimed towards the troposphere, some part of the radio signal is scattered back to the Earth due to forward scattering in the troposphere. This returning signal is received by a receiver at the other end. But in troposcatter propagation, only a small portion of the signal is scattered forward and reaches the receiver, a major part of the signal is either lost in space or reflects in other directions.

A troposcatter system has an antenna at both ends, capable of both transmitting and receiving signals, aimed at a fixed point in the troposphere slightly above the horizon. The common region where the antenna beams intersect is the area where the forward scattering phenomenon takes place.

As the name implies, troposcatter uses the troposphere as the region that affects the radio signals being transmitted, returning them to Earth so that they can be received by the distant receiver. Troposcatter relies on the fact that there are areas of slightly different dielectric constant in the atmosphere at an altitude of between 2 and 5 kilometres. Even dust in the atmosphere at these heights adds to the reflection of the signal. A transmitter launches a high power signal, most of which passes through the atmosphere into outer space. However a small amount is scattered when is passes through this area of the troposphere, and passes back to earth at a distant point. As might be expected, little of the signal is "scattered" back to Earth and as a result, path losses are very high. Additionally the angles through which signals can be reflected are normally small.

The area within which the scattering takes place is called the scatter volume, and its size is dependent upon the gain of the antennas used at either end. In view of the fact that scattering takes place over a large volume, the received signal will have travelled over a vast number of individual paths, each with a slightly different path length. As they all take a slightly different time to reach the receiver, this has the effect of "blurring" the overall received signal and this makes high speed data transmissions difficult.

It is also found that there are large short term variations in the signal as a result of turbulence and changes in the scatter volume. As a result commercial troposcatter propagation systems use multiple diversity systems. This is achieved by using vertical and horizontally polarised antennas as well as different scatter volumes (angle diversity) and different frequencies (frequency diversity). Control of these systems is normally undertaken by computers. In this way troposcatter radio communications systems can run automatically giving high degrees of reliability.

Radar Systems Introduction:

RADAR is an electromagnetic based detection system that works by radiating electromagnetic waves and then studying the echo or the reflected back waves.

The full form of RADAR is RAdio Detection And Ranging. Detection refers to whether the target is present or not. The target can be stationary or movable, i.e., non-stationary. Ranging refers to the distance between the Radar and the target.

Radars can be used for various applications on ground, on sea and in space. The applications of Radars are listed below.

• Controlling the Air Traffic • Ship safety • Sensing the remote places • Military applications

In any application of Radar, the basic principle remains the same. Let us now discuss the principle of radar.

Basic Principle of Radar:

Radar is used for detecting the objects and finding their location. We can understand the basic principle of Radar from the following figure.

As shown in the figure, Radar mainly consists of a transmitter and a receiver. It uses the same Antenna for both transmitting and receiving the signals. The function of the transmitter is to transmit the Radar signal in the direction of the target present.

Target reflects this received signal in various directions. The signal, which is reflected back towards the Antenna gets received by the receiver.

Terminology of Radar Systems

Following are the basic terms, which are useful in this tutorial.

• Range

• Pulse Repetition Frequency • Maximum Unambiguous Range • Minimum Range

Now, let us discuss about these basic terms one by one.

Range

The distance between Radar and target is called Range of the target or simply range, R. We know that Radar transmits a signal to the target and accordingly the target sends an echo signal to the Radar with the speed of light, C.

Let the time taken for the signal to travel from Radar to target and back to Radar be ‘T’. The two way distance between the Radar and target will be 2R, since the distance between the Radar and the target is R.

Now, the following is the formula for Speed.

Speed=Distance / Time

⇒Distance=Speed×Time

⇒2R=C×T

R=CT / 2

We can find the range of the target by substituting the values of C & T in Equation above.

Pulse Repetition Frequency

Radar signals should be transmitted at every clock pulse. The duration between the two clock pulses should be properly chosen in such a way that the echo signal corresponding to present clock pulse should be received before the next clock pulse. A typical Radar wave form is shown in the following figure.

As shown in the figure, Radar transmits a periodic signal. It is having a series of narrow rectangular shaped pulses. The time interval between the successive clock pulses is called pulse repetition time, TP

The reciprocal of pulse repetition time is called pulse repetition frequency, fP, Mathematically, it can be represented as

fP=1 / TP

Therefore, pulse repetition frequency is nothing but the frequency at which Radar transmits the signal.

Maximum Unambiguous Range

We know that Radar signals should be transmitted at every clock pulse. If we select a shorter duration between the two clock pulses, then the echo signal corresponding to present clock pulse will be received after the next clock pulse. Due to this, the range of the target seems to be smaller than the actual range.

So, we have to select the duration between the two clock pulses in such a way that the echo signal corresponding to present clock pulse will be received before the next clock pulse starts. Then, we will get the true range of the target and it is also called maximum unambiguous range of the target or simply, maximum unambiguous range.

Minimum Range

We will get the minimum range of the target, when we consider the time required for the echo signal to receive at Radar after the signal being transmitted from the Radar as pulse width. It is also called the shortest range of the target.

Types of RADAR:

Radars can be classified into the following two types based on the type of signal with which Radar can be operated.

• Pulse Radar • Continuous Wave Radar

Now, let us discuss about these two types of Radars one by one.

Pulse Radar

The Radar, which operates with pulse signal is called the Pulse Radar. Pulse Radars can be classified into the following two types based on the type of the target it detects.

• Basic Pulse Radar • Moving Target Indication Radar

Let us now discuss the two Radars briefly.

Basic Pulse Radar

The Radar, which operates with pulse signal for detecting stationary targets, is called the Basic Pulse Radar or simply, Pulse Radar. It uses single Antenna for both transmitting and receiving signals with the help of Duplexer.

Antenna will transmit a pulse signal at every clock pulse. The duration between the two clock pulses should be chosen in such a way that the echo signal corresponding to the present clock pulse should be received before the next clock pulse.

Moving Target Indication Radar

The Radar, which operates with pulse signal for detecting non-stationary targets, is called Moving Target Indication Radar or simply, MTI Radar. It uses single Antenna for both transmission and reception of signals with the help of Duplexer.

MTI Radar uses the principle of Doppler effect for distinguishing the non-stationary targets from stationary objects.

Continuous Wave Radar

The Radar, which operates with continuous signal or wave is called Continuous Wave Radar. They use Doppler Effect for detecting non-stationary targets. Continuous Wave Radars can be classified into the following two types.

• Unmodulated Continuous Wave Radar • Frequency Modulated Continuous Wave Radar

Now, let us discuss the two Radars briefly.

Unmodulated Continuous Wave Radar

The Radar, which operates with continuous signal (wave) for detecting non-stationary targets is called Unmodulated Continuous Wave Radar or simply, CW Radar. It is also called CW Doppler Radar.

This Radar requires two Antennas. Of these two antennas, one Antenna is used for transmitting the signal and the other Antenna is used for receiving the signal. It measures only the speed of the target but not the distance of the target from the Radar.

Frequency Modulated Continuous Wave Radar

If CW Doppler Radar uses the Frequency Modulation, then that Radar is called the Frequency Modulated Continuous Wave (FMCW) Radar or FMCW Doppler Radar. It is also called Continuous Wave Frequency Modulated Radar or CWFM Radar.

This Radar requires two Antennas. Among which, one Antenna is used for transmitting the signal and the other Antenna is used for receiving the signal. It measures not only the speed of the target but also the distance of the target from the Radar.

Pulse Radar:

The Radar, which operates with pulse signal for detecting stationary targets is called Basic Pulse Radar or simply, Pulse Radar. In this chapter, let us discuss the working of Pulse Radar.

Block Diagram of Pulse Radar

Pulse Radar uses single Antenna for both transmitting and receiving of signals with the help of Duplexer. Following is the block diagram of Pulse Radar −

Let us now see the function of each block of Pulse Radar −

• Pulse Modulator − It produces a pulse-modulated signal and it is applied to the Transmitter. • Transmitter − It transmits the pulse-modulated signal, which is a train of repetitive pulses. • Duplexer − It is a microwave switch, which connects the Antenna to both transmitter section and

receiver section alternately. Antenna transmits the pulse-modulated signal, when the duplexer connects the Antenna to the transmitter. Similarly, the signal, which is received by Antenna will be given to Low Noise RF Amplifier, when the duplexer connects the Antenna to Low Noise RF Amplifier.

• Low Noise RF Amplifier − It amplifies the weak RF signal, which is received by Antenna. The output of this amplifier is connected to Mixer.

• Local Oscillator − It produces a signal having stable frequency. The output of Local Oscillator is connected to Mixer.

• Mixer − We know that Mixer can produce both sum and difference of the frequencies that are applied to it. Among which, the difference of the frequencies will be of Intermediate Frequency (IF) type.

• IF Amplifier − IF amplifier amplifies the Intermediate Frequency (IF) signal. The IF amplifier shown in the figure allows only the Intermediate Frequency, which is obtained from Mixer and amplifies it. It improves the Signal to Noise Ratio at output.

• Detector − It demodulates the signal, which is obtained at the output of the IF Amplifier. • Video Amplifier − As the name suggests, it amplifies the video signal, which is obtained at the

output of detector. • Display − In general, it displays the amplified video signal on CRT screen.

Doppler Effect in Radar Systems:

If the target is not stationary, then there will be a change in the frequency of the signal that is transmitted from the Radar and that is received by the Radar. This effect is known as the Doppler effect.

According to the Doppler effect, we will get the following two possible cases −

• The frequency of the received signal will increase, when the target moves towards the direction of the Radar.

• The frequency of the received signal will decrease, when the target moves away from the Radar.

CW Radar:

Basic Radar uses the same Antenna for both transmission and reception of signals. We can use this type of Radar, when the target is stationary, i.e., not moving and / or when that Radar can be operated with pulse signal.

The Radar, which operates with continuous signal (wave) for detecting non-stationary targets, is called Continuous Wave Radar or simply CW Radar. This Radar requires two Antennas. Among which, one Antenna is used for transmitting the signal and the other Antenna is used for receiving the signal.

Block Diagram of CW Radar

We know that CW Doppler Radar contains two Antennas − transmitting Antenna and receiving Antenna. Following figure shows the block diagram of CW Radar −

The block diagram of CW Doppler Radar contains a set of blocks and the function of each block is mentioned below.

• CW Transmitter − It produces an analog signal having a frequency of fo • The output of CW Transmitter is connected to both transmitting Antenna and Mixer-I. • Local Oscillator − It produces a signal having a frequency of fl • The output of Local Oscillator is connected to Mixer-I. • Mixer-I − Mixer can produce both sum and difference of the frequencies that are applied to it. The

signals having frequencies of fo and fl are applied to Mixer-I. So, the Mixer-I will produce the output having frequencies fo+fl or fo−fl

• Side Band Filter − As the name suggests, side band filter allows a particular side band frequencies − either upper side band frequencies or lower side band frequencies. The side band filter shown in the above figure produces only upper side band frequency, i.e., fo+fl

• Mixer-II − Mixer can produce both sum and difference of the frequencies that are applied to it. The signals having frequencies of fo+fl and fo±fd are applied to Mixer-II. So, the Mixer-II will produce the output having frequencies of 2fo+fl±fd or fl±fd

• IF Amplifier − IF amplifier amplifies the Intermediate Frequency (IF) signal. The IF amplifier shown in the figure allows only the Intermediate Frequency, fl±fd and amplifies it.

• Detector − It detects the signal, which is having Doppler frequency, fd • Doppler Amplifier − As the name suggests, Doppler amplifier amplifies the signal, which is

having Doppler frequency, fd • Indicator − It indicates the information related relative velocity and whether the target is inbound

or outbound.

CW Doppler Radars give accurate measurement of relative velocities. Hence, these are used mostly, where the information of velocity is more important than the actual range.

FMCW Radar:

If CW Doppler Radar uses the Frequency Modulation, then that Radar is called FMCW Doppler Radar or simply, FMCW Radar. It is also called Continuous Wave Frequency Modulated Radar or CWFM Radar. It measures not only the speed of the target but also the distance of the target from the Radar.

Block Diagram of FMCW Radar

FMCW Radar is mostly used as Radar Altimeter in order to measure the exact height while landing the aircraft. The following figure shows the block diagram of FMCW Radar −

FMCW Radar contains two Antennas − transmitting Antenna and receiving Antenna as shown in the figure. The transmitting Antenna transmits the signal and the receiving Antenna receives the echo signal.

The block diagram of the FMCW Radar looks similar to the block diagram of CW Radar. It contains few modified blocks and some other blocks in addition to the blocks that are present in the block diagram of CW Radar. The function of each block of FMCW Radar is mentioned below.

FM Modulator − It produces a Frequency Modulated (FM) signal having variable frequency, fo(t)nand it is applied to the FM transmitter.

FM Transmitter − It transmits the FM signal with the help of transmitting Antenna. The output of FM Transmitter is also connected to Mixer-I.

Local Oscillator − In general, Local Oscillator is used to produce an RF signal. But, here it is used to produce a signal having an Intermediate Frequency, fIF. The output of Local Oscillator is connected to both Mixer-I and Balanced Detector.

Mixer-I − Mixer can produce both sum and difference of the frequencies that are applied to it. The signals having frequencies of fo(t) and fIF are applied to Mixer-I. So, the Mixer-I will produce the output having frequency either fo(t)+fIF or fo(t)−fIF

Side Band Filter − It allows only one side band frequencies, i.e., either upper side band frequencies or lower side band frequencies. The side band filter shown in the figure produces only lower side band frequency. i.e., fo(t)−fIF

Mixer-II − Mixer can produce both sum and difference of the frequencies that are applied to it. The signals having frequencies of fo(t)−fIF

and fo(t−T) are applied to Mixer-II. So, the Mixer-II will produce the output having frequency either fo(t−T)+fo(t)−fIF or fo(t−T)−fo(t)+fIF

IF Amplifier − IF amplifier amplifies the Intermediate Frequency (IF) signal. The IF amplifier shown in the figure amplifies the signal having frequency of fo(t−T)−fo(t)+fIF. This amplified signal is applied as an input to the Balanced detector.

Balanced Detector − It is used to produce the output signal having frequency of fo(t−T)−fo(t) from the applied two input signals, which are having frequencies of fo(t−T)−fo(t)+fIF and fIF. The output of Balanced detector is applied as an input to Low Frequency Amplifier.

Low Frequency Amplifier − It amplifies the output of Balanced detector to the required level. The output of Low Frequency Amplifier is applied to both switched frequency counter and average frequency counter.

Switched Frequency Counter − It is useful for getting the value of Doppler velocity.

Average Frequency Counter − It is useful for getting the value of Range.

MTI Radars:

If the Radar is used for detecting the movable target, then the Radar should receive only the echo signal due to that movable target. This echo signal is the desired one. However, in practical applications, Radar receives the echo signals due to stationary objects in addition to the echo signal due to that movable target.

The echo signals due to stationary objects (places) such as land and sea are called clutters because these are unwanted signals. Therefore, we have to choose the Radar in such a way that it considers only the echo signal due to movable target but not the clutters.

For this purpose, Radar uses the principle of Doppler Effect for distinguishing the non-stationary targets from stationary objects. This type of Radar is called Moving Target Indicator Radar or simply, MTI Radar.

According to Doppler effect, the frequency of the received signal will increase if the target is moving towards the direction of Radar. Similarly, the frequency of the received signal will decrease if the target is moving away from the Radar.

Types of MTI Radars

We can classify the MTI Radars into the following two types based on the type of transmitter that has been used.

• MTI Radar with Power Amplifier Transmitter • MTI Radar with Power Oscillator Transmitter

MTI Radar with Power Amplifier Transmitter

MTI Radar uses single Antenna for both transmission and reception of signals with the help of Duplexer. The block diagram of MTI Radar with power amplifier transmitter is shown in the following figure.

The function of each block of MTI Radar with power amplifier transmitter is mentioned below.

• Pulse Modulator − It produces a pulse modulated signal and it is applied to Power Amplifier. • Power Amplifier − It amplifies the power levels of the pulse modulated signal. • Local Oscillator − It produces a signal having stable frequency fl. Hence, it is also called stable

Local Oscillator. The output of Local Oscillator is applied to both Mixer-I and Mixer-II.

Coherent Oscillator − It produces a signal having an Intermediate Frequency, fc. This signal is used as the reference signal. The output of Coherent Oscillator is applied to both Mixer-I and Phase Detector.

Mixer-I − Mixer can produce either sum or difference of the frequencies that are applied to it. The signals having frequencies of fl and fc are applied to Mixer-I. Here, the Mixer-I is used for producing the output, which is having the frequency fl+fc

Duplexer − It is a microwave switch, which connects the Antenna to either the transmitter section or the receiver section based on the requirement. Antenna transmits the signal having frequency fl+fc, when the duplexer connects the Antenna to power amplifier. Similarly, Antenna receives the signal having frequency of fl+fc±fd. when the duplexer connects the Antenna to Mixer-II.

Mixer-II − Mixer can produce either sum or difference of the frequencies that are applied to it. The signals having frequencies fl+fc±fd and fl are applied to Mixer-II. Here, the Mixer-II is used for producing the output, which is having the frequency fc±fd

IF Amplifier − IF amplifier amplifies the Intermediate Frequency (IF) signal. The IF amplifier shown in the figure amplifies the signal having frequency fc+fd. This amplified signal is applied as an input to Phase detector.

Phase Detector − It is used to produce the output signal having frequency fd from the applied two input signals, which are having the frequencies of fc+fd and fc. The output of phase detector can be connected to Delay line canceller.

MTI Radar with Power Oscillator Transmitter

The block diagram of MTI Radar with power oscillator transmitter looks similar to the block diagram of MTI Radar with power amplifier transmitter. The blocks corresponding to the receiver section will be same in both the block diagrams. Whereas, the blocks corresponding to the transmitter section may differ in both the block diagrams.

The block diagram of MTI Radar with power oscillator transmitter is shown in the following figure.

As shown in the figure, MTI Radar uses the single Antenna for both transmission and reception of signals with the help of Duplexer. The operation of MTI Radar with power oscillator transmitter is mentioned below.

• The output of Magnetron Oscillator and the output of Local Oscillator are applied to Mixer-I. This will further produce an IF signal, the phase of which is directly related to the phase of the transmitted signal.

• The output of Mixer-I is applied to the Coherent Oscillator. Therefore, the phase of Coherent Oscillator output will be locked to the phase of IF signal. This means, the phase of Coherent Oscillator output will also directly relate to the phase of the transmitted signal.

• So, the output of Coherent Oscillator can be used as reference signal for comparing the received echo signal with the corresponding transmitted signal using phase detector.

The above tasks will be repeated for every newly transmitted signal.

RADAR Display:

An electronic instrument, which is used for displaying the data visually is known as display. So, the electronic instrument which displays the information about Radar’s target visually is known as Radar display. It shows the echo signal information visually on the screen.

Types of Radar Displays

In this section, we will learn about the different types of Radar Displays. The Radar Displays can be classified into the following types.

A-Scope

It is a two dimensional Radar display. The horizontal and vertical coordinates represent the range and echo amplitude of the target respectively. In A-Scope, the deflection modulation takes place. It is more suitable for manually tracking Radar.

B-Scope

It is a two dimensional Radar display. The horizontal and vertical coordinates represent the azimuth angle and the range of the target respectively. In B-Scope, intensity modulation takes place. It is more suitable for military Radars.

C-Scope

It is a two-dimensional Radar display. The horizontal and vertical coordinates represent the azimuth angle and elevation angle respectively. In C-Scope, intensity modulation takes place.

D-Scope

If the electron beam is deflected or the intensity-modulated spot appears on the Radar display due to the presence of target, then it is known as blip. C-Scope becomes D-Scope, when the blips extend vertically in order to provide the distance.

E-Scope

It is a two-dimensional Radar display. The horizontal and vertical coordinates represent the distance and elevation angle respectively. In E-Scope, intensity modulation takes place.

F-Scope

If the Radar Antenna is aimed at the target, then F-Scope displays the target as a centralized blip. So, the horizontal and vertical displacements of the blip represent the horizontal and vertical aiming errors respectively.

G-Scope

If the Radar Antenna is aimed at the target, then G-Scope displays the target as laterally centralized blip. The horizontal and vertical displacements of the blip represent the horizontal and vertical aiming errors respectively.

H-Scope

It is the modified version of B-Scope in order to provide the information about elevation angle of the target. It displays the target as two blips, which are closely spaced. This can be approximated to a short bright line and the slope of this line will be proportional to the sine of the elevation angle.

I-Scope

If the Radar Antenna is aimed at the target, then I-Scope displays the target as a circle. The radius of this circle will be proportional to the distance of the target. If the Radar Antenna is aimed at the target incorrectly, then I-Scope displays the target as a segment instead of circle. The arc length of that segment will be inversely proportional to the magnitude of pointing error.

J-Scope

It is the modified version of A-Scope. It displays the target as radial deflection from time base.

K-Scope

It is the modified version of A-Scope. If the Radar Antenna is aimed at the target, then K-Scope displays the target as a pair of vertical deflections, which are having equal height. If the Radar Antenna is aimed at the target incorrectly, then there will be pointing error. So, the magnitude and the direction of the pointing error depends on the difference between the two vertical deflections.

L-Scope

If the Radar Antenna is aimed at the target, then L-Scope displays the target as two horizontal blips having equal amplitude. One horizontal blip lies to the right of central vertical time base and the other one lies to the left of central vertical time base.

M-Scope

It is the modified version of A-Scope. An adjustable pedestal signal has to be moved along the baseline till it coincides the signal deflections, which are coming from the horizontal position of the target. In this way, the target’s distance can be determined.

N-Scope

It is the modified version of K-Scope. An adjustable pedestal signal is used for measuring distance.

O-Scope

It is the modified version of A-Scope. We will get O-Scope, by including an adjustable notch to A-Scope for measuring distance.

R-Scope

It is a Radar display, which uses intensity modulation. The horizontal and vertical coordinates represent the range and height of the target respectively. Hence, it is called Range-Height Indicator or RHI display.

Plan Position Indicator or the PPI display:

It is a Radar display, which uses intensity modulation. It displays the information of echo signal as plan view. Range and azimuth angle are displayed in polar coordinates. Hence, it is called the Plan Position Indicator or the PPI display.

• This is an intensity- modulation type displays system which indicates both range and azimuth angle of the target simultaneously in polar co-ordinate as shown in figure.

• The Demodulated echo signals from the receivers is applied to the grid of the CRT which is biased slightly beyond cutoff.

• Only when Blips corresponding to the targets occur, a saw tooth current applied to a pair of coils (on opposite side of the neck of the tube) flows.

• Thus, a beam is made to deflect radially outward from the center and also continuously around the tube(mechanically) at the same angular velocity as that of the antenna.

• The brightness spot at any point on the screen indicates the presence of an object there. • Normally PPI screens are circular with a diameter of 30cm or 40cm. Long persistence phosphors

are used to ensure that the PPI screen dose not flicker.