Microwave and RADAR : Important Interview question Part-1

1. If a target is to be detected which is present within 200 meters which type of Rader is more suitable Pulse Rader or FMCW Rader to detect it?

Ans. In short range transmission and reception should be continuous so FMCW RADAR is suitable for short ranges.

1. Why FMCW RADAR is not used for long range? Explain.

Ans. As transmission and reception are continuous in FMCW RADAR so more power is required which can damage the sensitive receiver due to leakage from a transmitter.

1. What is the difference between dB and dBm?

Ans. Decibel (dB), unit for expressing the ratio between two physical quantities, usually amounts of acoustic or electric power, or for measuring the relative loudness of sounds. One decibel (0.1 bel) equals 10 times the common logarithm of the power ratio.

dB is ratio of power and is used less whereas dBm is absolute power and is used especially for low powers.

                                           1 dBm = 10log [P(in mW)]

1. Can you improve range resolution without decreasing transmitted power?

Ans. Yes, the technique is known as pulse compression. In this technique when pulses are sent, their frequency is linearly increased at a constant rate throughout its length so that enough can be transmitted and while receiving, echo is passed through a filter that introduces time lag so that pulse is compressed, improving range resolution.

1. Why frequency diversity is used in RADARs?

Ans. To get around the problem of target fading we should switch periodically from one to another of several different radio frequencies thereby providing what is called frequency diversity.

1. How range ambiguity can be resolved in RADAR?

Ans. Range ambiguity can be resolved by changing pulse repetition frequency (PRF) detecting the signal on the display of different PRF in the RADAR.

1. What is the difference between Coherent and non-Coherent transmission in term of phase?

Ans. In non-coherent transmission, the phase of successive pulse is not the same but in coherent transmission, the phase remains the same.

1. What is the Doppler effect?

Ans. When there is relative movement between target and RADAR then there will be a difference between the transmitted pulse frequency and received pulse frequency is Doppler frequency and effects known as the Doppler effect.

The Doppler effect (or the Doppler shift) is the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source

1. What do you mean by blind ranges with respect to RADAR?

Ans. When a target lies at such a distance from RADAR that the RADAR receives the echo while it is transmitting, it fails to detect the target. In such a case, the target is said to be at blind range.

RADAR blind range: The range that corresponds to the situation in which a RADAR transmitter is on and hence the receiver must be off so that the RADAR transmitted signal does not saturate, i.e., does not blind, its own receiver. Note: RADAR blind ranges occur because there is a time interval between transmitted pulses that corresponds to the time required for a pulse to propagate to the object, i.e. to the target, and its reflection to travel back. This causes an attempt to measure the range just as the RADAR transmitter is transmitting the next pulse.

10. Explain the difference between Monostatic and Bistatic RADAR System.

Ans. In monostatic RADAR system single antenna is used for both transmission as well as reception it as there is single antenna used for both the directions, duplexer is needed to separate the transmit chain from receive chain and vice versa while in bistatic RADAR system separate antennas are used for transmit and receive functions and are located at different locations.

1. What is the difference between Geosynchronous and Geostationary satellite?

Ans. A geosynchronous satellite is one whose orbital track on the earth repeats over a point on the earth over time. If such a satellite orbit lies over the equator and the orbit is circular then it is called geostationary.

1. What are the sources of atmospheric attenuation to the microwave signals during transmission?

Ans. Two sources of atmospheric attenuation are Scattering and Absorption. However, Rayleigh scattering (low-frequency scattering) effect is negligible until optical frequency. The other, the important one for microwave frequency range is the absorption, especially due to H₂O and O₂ molecules.

1. Is cell phone at 1 GHz safe to use?

Ans. No, because at this frequency range of cell phone usage, the heating effect is largest. Since the power is very high so the cell phone is not safe to use.

1. What is a power used in GSM and CDMA system?

Ans. In GSM maximum power used is 2 W and in CDMA it is only 200 mW but the power can vary from 2W to 12 W.

1. Does a small loop antenna behave as inductive or capacitive below the second resonance condition?

Ans. At DC, a small loop antenna is behaving as inductive. From DC to first resonance frequency the reach of a small loop goes from positive to zero. For second resources frequency, it will increase from negative to zero. So at frequency right below the second resonance, the reactance of loop is negative thus loop looks capacitive.

1. How to place a ring antenna over a ground plane if want to use it as the reflector?

Ans. Vertically, half wavelength above ground. Since the equivalent magnetic dipole and its image will be in-phase and radiate maximum along the direction normal to the ground.

1. What is circular polarization?

Ans. Polarization is the orientation of E- field when the wave is propagating When the E_x and E_y components of electric field E has the same magnitude and 90° difference then resultant polarization is circular.

 E = (xE₀ + y E₀ e^{± jπ /2}) e^{j(ω t – k.r)}

1. What is the IP3 of an amplifier? Explain.

Ans. IP3 is a measure of third order intermodulation distortion in an amplifier. It is the interception point on the output power versus input power diagram where the third order intermodulation product gain curve meets the signal gain curve.

1. What do you mean by transverse nature of electromagnetic waves? Explain.

Ans. A transverse wave is a moving wave whose oscillations are perpendicular to the direction of the wave. For an electromagnetic wave, E and H are perpendicular to each and both are perpendicular to the direction of propagation. That’s why electromagnetic waves are known as Transverse Electromagnetic (TEM) waves.

1. Which polarization is used for television broadcasting?

Ans. For TV broadcasting vertical polarization has been adopted.

1. What is the physical interpretation of curl? Explain with example.

Ans. The curl is a vector operator that describes the infinitesimal rotation of a vector field in three-dimensional Euclidean space. At every point in the field, the curl of that point is represented by a vector. The attributes of this vector (length and direction) characterize the rotation at that point. The curl of a vector function gives the measure of angular velocity at every point of the vector field. If curl of a vector is zero i.e. ∇ × F=0  then is called irrotational.

1. If the divergence of a vector is zero then what information about vector you can have?

Ans. When divergence of a vector is zero then the vector is said to be solenoidal, zero divergence means that the amount going into a region equals the amount coming out. 

1. Why circular polarization is popular in recent days?

Ans. Circular polarization the clutter echo received from raindrops, transmission and reception between artificial satellite and earth as there is no effect of Faraday rotation on the wave, so circular polarization is becoming popular these days.

1. Man-Made noise usually has ______ type of polarization.

Ans. vertical polarization.

1. Resonant cavity is equivalent to _________ type of filter.

 Ans. It is equivalent to a bandpass filter.

1. What do you mean by pointing vector?

Ans. It is a quantity describing the magnitude and direction of the flow of energy in EM waves.

The Poynting vector represents the directional energy flux (the energy transfer per unit area per unit time) of an electromagnetic field.

   P = E X H (W/m²) where E is the electric field intensity and H is the magnetic field intensity.

1.  What is waveguide describing and its cut-off frequency?

Ans. A waveguide is a structure that guides waves, such as electromagnetic waves, with minimal loss of energy by restricting expansion to one dimension or two. A waveguide is a special form transmission line consisting of a hollow metal tube.

The cut-off frequency is the lowest frequency for which a mode will propagate in it.

1. What are optical fibre and its advantage and principle?

Ans. An optical fibre is a flexible, transparent fibre made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibres are used most often as a means to transmit light between the two ends of the fibre and find wide usage in fibre-optic communications, where they permit transmission over longer distances and at higher bandwidths (data rates) than electrical cables

Advantages: (i) Immunity to EMI (ii) Data security, data can’t be tapped (iii) Non-conductive cables, to shockproof to maintenance  (iv) High bandwidth than conventional copper cables  (v) Less signal attenuation less than 1dB per kilometre.

Principle: Optical fibre works on the principle of total internal reflection. When light travels from one medium to another it changes speed and is refracted. If the light rays are travelling for a less dense material to a dense medium they are refracted towards the normal and if they are travelling from a dense to a less dense medium they have refracted away from the normal.

1. What are the different types of wave propagation?

Ans. Types of wave propagation :

• Ground wave propagation: Used for a low-frequency range transmission, mostly less than 1 MHz. This type of propagation employs the use of large antennas order of which is equivalent to the wavelength of the waves and uses the ground or Troposphere for its propagation. Signals over large distances are not sent using this method.  It causes severe attenuation which increases with increased frequency of the waves.
• Line of sight propagation: Used for the propagation of EM waves with a frequency range of 3 – 30 MHz. Make use of the ionosphere so called due to the presence of charged ions in the region of about 60 to 300 km from the earth surface. These ions provide a reflecting medium to the radio or communication waves within a particular frequency range. We use this property of the ionosphere for long-distance transmission of the waves without much attenuation and loss of signal strength.
• Skywave propagation: Used for a line of Sight communication also known as LOS. Space satellite communication and very high-frequency waves use this propagation method. It basically involves sending a signal in a straight line from the transmitter to the receiver. We must ensure that for very large distances, the height of the tower used for transmission is high enough to prevent waves from touching the earth curvature thus preventing attenuation and loss of signal strength.

1. What is the effect of using high PRF in RADAR?

Ans. Operating RADAR with high Pulse repetition frequency causes range ambiguities, which are not suitable for surveillance ambiguities or long-range tracking application of RADAR. 

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