← MES Jr. Grade
Past Questions — 2012
Information content of a message has the characteristic that
A)
it is additive
B)
Increases linearly with time
C)
It increases monotonically
D)
All of the above
In information theory, the information content of a message is additive for independent events, and in many practical contexts it is treated as increasing monotonically with the amount of uncertainty or data. It also increases linearly with time when information is accumulated at a constant rate. Therefore, all the given statements are considered correct.
Data rate R and information rate Rt are different because of
A)
channel capacity C, being high
B)
equivocation
C)
entropy h(x)
D)
none of these
In information theory, the data rate R and information rate Rt differ due to the presence of uncertainty in the received message, called equivocation. Equivocation measures the ambiguity remaining after reception, so the effective information rate is reduced. Hence R and Rt are different because of equivocation. Option (a) is not the reason, and entropy h(x) is only a measure of source uncertainty, not the direct cause of the difference between data rate and information rate.
A parallel plate capacitor is charged so that the electric field in the dielectric is E. If the permittivity of dielectric is ε, the energy density i.e. energy per unit volume is
A)
0.5 ε E
B)
0.5 ε E2
C)
0.5 ε2 E
D)
0.5 E2/ε
For a dielectric, the energy density stored in the electric field is given by u = 1/2 εE^2. This comes from the general expression u = 1/2 E·D, and since D = εE, we get u = 1/2 εE^2. Therefore, option (b) is correct. The other options have incorrect dependence on E and ε.
Sometimes microwave signals follow the earth’s curvature. This is due to
A)
ionospheric reflection
B)
faraday rotation
C)
tropospheric scatter
D)
ducting
Microwave signals normally travel in straight lines, but under certain atmospheric conditions they can be trapped in a layer of the atmosphere and guided along the earth’s curvature. This phenomenon is called ducting. Ionospheric reflection is mainly for HF radio waves, Faraday rotation is the rotation of polarization in the ionosphere, and tropospheric scatter is a different long-distance propagation mechanism but not the one that makes microwaves follow the earth’s curvature.
Two spherical drops of water each having a charge Q and surface potential V are combined to form a bigger drop. The surface potential of the bigger drop will be
A)
V
B)
2V
C)
< V but > 2V
D)
none of these
For a spherical drop, surface potential is V = kQ/R. When two identical drops combine, the total charge becomes 2Q and the volume doubles, so the new radius becomes R' = 2^(1/3)R. The new potential is V' = k(2Q)/R' = k(2Q)/(2^(1/3)R) = 2^(2/3)V. However, this is not among the given options. In many exam settings, the intended result is that the potential remains the same only if charge and radius scale proportionally, but for two identical charged drops the exact value is 2^(2/3)V, which is greater than V. Since that exact value is not listed, the correct choice from the options is 'none of these'.
The main advantage of using microwaves for communication is
A)
Large bandwidth
B)
Small bandwidth
C)
Low power
D)
High power
Microwaves are widely used in communication because they offer a very large bandwidth, which allows transmission of a high amount of information. They also support highly directional antennas, reducing interference and enabling point-to-point links. The other options are incorrect because microwaves do not have small bandwidth as an advantage, and low power or high power are not the main communication benefit.
In microwave detector used in laboratory measurement of relative power levels, standing wave ratio is about
A)
0.1
B)
1.25
C)
5
D)
l0
In laboratory microwave measurements, a detector is used for relative power-level measurement and should have a reasonably low standing wave ratio (SWR) so that it does not disturb the transmission line significantly. The typical SWR of a microwave detector is about 1.25. Higher values like 5 or 10 indicate poor matching, while 0.1 is not a meaningful SWR value.
In microwave resistor the wall thickness is made as small as possible to
A)
have high R
B)
have low R
C)
reduce capacitor
D)
reduce inductance
In a microwave resistor, the wall thickness is kept as small as possible so that the current path length is minimized and the lead/wall inductance becomes very low. At microwave frequencies, even a small inductance can cause significant impedance, so reducing inductance is important. Hence, the correct choice is (d) reduce inductance. Options (a) and (b) are about resistance, but wall thickness is not reduced for that purpose here, and (c) capacitor is not the relevant effect.
The force between two charges in 120N. If the distance between the charges is doubled, the force will be
A)
120N
B)
60N
C)
30N
D)
15N
By Coulomb’s law, electrostatic force is inversely proportional to the square of the distance between two charges: F ∝ 1/r². If the distance is doubled, the force becomes 1/4 of the original. So, new force = 120/4 = 30 N. Therefore the correct option is (c) 30N.
Taking the earth’s radius as 6370Km and electrosphere height as 25Km, the capacitance of earth electrosphere combination is about
A)
1 F
B)
0.2F
C)
0.01F
D)
1µF
Treat the earth and the electrosphere as a spherical capacitor. For a spherical capacitor, C = 4πϵ0ab/(b−a), where a is Earth’s radius and b is the electrosphere radius. Here, a = 6370 km and b = 6395 km, so the separation (b−a) = 25 km = 2.5×10^4 m. Since the gap is very small compared to Earth’s radius, we can use the approximation C ≈ 4πϵ0a^2/d, which gives a value of the order of 10^-6 F. Numerically this comes out close to 1 µF. Hence the correct option is (d).
Which mode has the minimum cutoff frequency in rectangular wave guides?
A)
TE11
B)
TE10
C)
TE01
D)
TE20
In a rectangular waveguide, the cutoff frequency is lowest for the dominant mode. For the usual rectangular guide, the dominant mode is TE10, because it has the smallest cutoff frequency among the listed modes. TE11, TE01, and TE20 all have higher cutoff frequencies than TE10.
What is the numerical aperture in an optical fiber when critical angle is 30°?
A)
0.5
B)
0.70
C)
0.886
D)
0.2
For an optical fiber, the numerical aperture is related to the critical angle by the common exam formula NA = \u221a(1 - cos^2\u03b8c) = sin\u03b8c when taken in the simplified form used in such questions. With critical angle \u03b8c = 30\u00b0, NA = sin 30\u00b0 = 0.5. Hence option (a) is correct. The other options do not match this standard result.
The rate at which information can be passed through a tele-communication channel depends on the
A)
carrier frequency
B)
bandwidth
C)
transmission time
D)
transmitter power
The information-carrying capacity of a telecommunication channel is primarily determined by its bandwidth. By Shannon’s channel capacity principle, a wider bandwidth allows more information to be transmitted per unit time. Carrier frequency only sets the operating range, transmission time affects total data sent but not the channel’s rate, and transmitter power influences signal strength/SNR but the key factor asked here is bandwidth.
If the carrier of a 100% modulated AM transmitter is suppressed, the percentage power saving will be
A)
50%
B)
66.67%
C)
78%
D)
100%
For a conventional AM transmitter at 100% modulation, the total power is P_t = P_c(1 + m^2/2). With m = 1, P_t = 1.5P_c. The sideband power is P_s = P_t - P_c = 0.5P_c. If the carrier is suppressed, only sideband power is transmitted, so the new power is 0.5P_c. Power saving = (1.5P_c - 0.5P_c) / 1.5P_c × 100 = 66.67%. Hence the correct option is (b).
The semiconductor diode which can be used in switching circuit in microwave range is
A)
PIN code
B)
varactor diode
C)
Tunnel diode
D)
Gunn diode
A PIN diode is widely used as a microwave switch because its intrinsic layer gives it low capacitance and good high-frequency performance. It can act as an effective RF/microwave switch by changing its resistance with bias. Varactor diodes are mainly used for voltage-controlled capacitance/tuning, tunnel diodes for negative resistance oscillators, and Gunn diodes for microwave oscillators, not switching circuits.
If a 240 watt carrier is simultaneously modulated by two sine waves with modulation indices of 0.3 and 0.4, its total modulation index is
A)
0.5
B)
0.7
C)
0.12
D)
0.75
For simultaneous modulation by two independent sine waves, the total modulation index is the root-sum-square of the individual indices: m = √(m1² + m2²). Here, m1 = 0.3 and m2 = 0.4, so m = √(0.3² + 0.4²) = √(0.09 + 0.16) = √0.25 = 0.5. Hence, the correct option is (a). The carrier power value 240 W is not needed to find the modulation index.
Which mode has the lowest cutoff frequency in circular wave guides?
A)
TE01
B)
TE10
C)
TE11
D)
TE20
In a circular waveguide, the cutoff frequency depends on the mode indices. Among the given modes, TE11 has the lowest cutoff frequency and is the dominant mode of a circular waveguide. TE01, TE10, and TE20 all have higher cutoff frequencies than TE11.
Microwave resonators are used in
A)
microwave oscillators
B)
microwave narrow band amplifiers
C)
microwave frequency meters
D)
All of the above
Microwave resonators are used in microwave oscillators to provide frequency-selective feedback, in microwave narrow band amplifiers to achieve selective amplification over a narrow frequency range, and in microwave frequency meters to measure frequency by resonance. Therefore, all the listed applications are correct.
By properly coding and grouping longer sequence, it is possible to
A)
increase efficiency
B)
reduce delay in transmission
C)
increase delay
D)
make H = C
In information theory, proper coding and grouping of longer sequences helps reduce redundancy and improve the utilization of the channel. This increases coding efficiency. The other options are not the general result of coding longer sequences: delay is not necessarily reduced or increased, and H = C is not a standard consequence of grouping.
In VHF oscillator using butterfly capacitor
A)
only L
B)
only C
C)
both L and C
D)
none of these
A butterfly capacitor is a variable capacitor used for tuning in VHF oscillators. In VHF range, the oscillator frequency is generally controlled by the capacitance, while the inductance is usually fixed or less commonly varied. Therefore, the correct choice is only C. Options involving only L or both L and C are not the standard answer here.
The noise which assumes great importance at high frequencies is
A)
flicker noise
B)
transit time noise
C)
Johnson noise
D)
shot noise
Transit time noise becomes significant at high frequencies because the finite time taken by carriers to travel through a device is no longer negligible compared with the signal period. This causes phase lag and fluctuations that dominate at high frequency. Flicker noise is mainly important at low frequencies, while Johnson noise and shot noise are broadband noises not specifically associated with high-frequency dominance.
For a 50Ω resister for 3 GHz application, the stray capacitance should be less than
A)
1µF
B)
1nF
C)
1pF
D)
0.1pF
At 3 GHz, a resistor used in microwave applications must have extremely small stray capacitance so that its reactance remains very high compared with 50Ω. A capacitance of 1 pF would already have a very low reactance at 3 GHz, so it is too large. The required stray capacitance should therefore be much less than 1 pF, and among the given options, 0.1 pF is the correct choice.
An FM signal with a modulation index mf is passed through a frequency tripler. The wave in the output of the tripler will have a modulation index of
A)
mf /3
B)
mf
C)
3mf
D)
9mf
In frequency modulation, the modulation index is proportional to the frequency deviation. A frequency tripler multiplies both the carrier frequency and the frequency deviation by 3. Therefore, the new modulation index becomes 3 times the original: mf' = 3mf. Hence, the correct option is (c).
The length of the directors of a yagi antenna should be .............. that of the driven element .
A)
greater than
B)
less than
C)
equal to
D)
independent to
In a Yagi antenna, the directors are made slightly shorter than the driven element so they act as parasitic elements that direct radiation forward. If a director were equal to or longer than the driven element, it would not function properly as a director. Hence, the correct answer is 'less than'.
As wavelength decreases, the size of the high directivity antenna
A)
decreases
B)
increases
C)
not affected
D)
either (b) or (c)
For a high-directivity antenna, the physical size is generally related to the wavelength. As wavelength decreases, the same electrical size can be achieved with a smaller physical antenna. Therefore, the size of a high-directivity antenna decreases. Options (b) and (c) are incorrect because the antenna does not need to increase in size or remain unaffected when wavelength decreases.