## Fundamentals of Electricity & Radio Communications sample paper-1

*By*

**The Radio Society of Sri Lanka**

The copy right of the information contained in this sample exam are with the Radio Society of Sri Lanka (RSSL).

These questions are mainly targeting the Radio Amateur exam for Novice and General class. The recommended time duration is 1 hour. For more information please contact Mr. Jayasiri Wijeratne (4S7VJ) 4s7vj@rssl.lk

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Question 1 |

The circuit is shown in the figure for

A | voltage multiplication |

B | reverse bias protection |

C | full wave rectification |

D | half wave rectification |

Question 1 Explanation:

As shown in the diagram, when a diode connected in series with an a.c. supply it’s a half wave rectification. If you connect a bridge rectifier with the a.c. supply it is Fullwave rectification. If a diode connected with a d.c. supply as forward bias, it is a reverse bias protection. That means, if you connect a d.c. supply with wrong polarity by mistake, protect your equipment. There is a special system to connect few diodes and capcitors with an a.c. supply you can multiply the voltage accordingly.

Question 2 |

Ground wave communication is most effective in the frequency range of

A | 30 MHz to 300 MHz |

B | 300 kHz to 3 MHz |

C | 3 MHz to 30 MHz |

D | above 300 MHz |

Question 2 Explanation:

Ground wave propagation occurs on MF (300kHz to 3MHz) and HF (3 to 30MHz) only. Above 30 MHz, propagate mostly with line of sight.

Question 3 |

The most widely used conducting materials are

A | copper and aluminum |

B | copper and silver |

C | copper and gold |

D | gold and silver |

Question 3 Explanation:

The best conductor is Silver, next copper, next gold , next aluminium. But silver and gold are too expensive for common use. That is why copper and aluminium are commonly used.

Question 4 |

As the frequency rises the reactance of a capacitor

A | none of these |

B | stays constant |

C | decreases |

D | increases |

Question 4 Explanation:

Apply the following formula for the reactance of capacitor, X = 1 / 2 π f C, When "f" increasing "X" is decreasing.

Question 5 |

The unit of resistance is

A | Ampere |

B | Farad |

C | Ohm |

D | Watt |

Question 5 Explanation:

The unit of resistance is Ohm. The unit of current is Ampere. The unit of capacitance is Farad. The unit of power is Watt.

Question 6 |

In the smoothing circuit of a power supply capacitors of 8 μF, 4 μF and 2 μF are connected in parallel, the effective capacitance is

A | 8 μF |

B | 14 μF |

C | 8/7 μF |

D | 4 μF |

Question 6 Explanation:

Formula for the equivalent value of parallel capacitors is C = C₁ + C₂ + C₃ = 8+4+2 =14 μF

Question 7 |

When constant D. C. voltage applied to capacitor, it is acts as

A | A infinite resistor |

B | Zero resistor |

C | Current source |

D | A finite resistor |

Question 7 Explanation:

Electric charges do not pass through a capacitor. Therefore it acts as a infinite resistor.

Question 8 |

The maximum current that may be safely passed through a 10000Ω resistor rated at 25 W is

A | 0.005 A |

B | 1 A |

C | 0.05 A |

D | 0.5 A |

Question 8 Explanation:

Use the formula, P = I² R , P = 25W, R = 10000Ohm, Therefore I² = P / R = 25 / 10000.
Therefore I = √( 25 / 10000) = 5 / 100 = 0.05 A or 50 mA

Question 9 |

To minimise unwanted radiation of sub-harmonics and harmonics, a VHF transmitter should be followed by

A | a low pass filter |

B | a notch filter |

C | a high pass filter |

D | a band pass filter |

Question 9 Explanation:

Low pass filter is filtering only higher frequencies and it allow to pass low frequencies. Band pass filter allow to pass all frequencies within the specified frequency range. All lower frequencies and all higher frequencies get cut off. High pass filter stops all lower frequencies and allows to passing of higher values only. Notch filter cuts only one perticular frequency.

Question 10 |

A moving coil meter depends on which of the following in order to operate

A | an electric only |

B | interaction of an electric and magnetic field |

C | the interaction of two permanent magnetic fields |

D | interaction of a permanent and electromagnetic field |

Question 10 Explanation:

The moving coil generates an electro-magnet ic field, while a current flow through the coil. This coil installed on a permanent magnetic field. This coil is turning (moving) due to the interaction between those fields.

Question 11 |

An SWR meter is inserted into a perfectly matched transmitter/antenna system the value shown should Indicate

A | 1:1 SWR |

B | 0:1 SWR |

C | 10 W reflected power |

D | 1:0 SWR |

Question 11 Explanation:

If SWR equal to 1 (or 1:1) the antenna system is perfectly matched. If it is totally mismatched, SWR = infinity, If it is less than 2, it is practically acceptable. If it is 3, only 50% of power will radiate. Always SWR exist between 1 and infinity.

Question 12 |

Which one of the following frequencies is in the VHF range ?

A | 5MHz. |

B | 250 MHz. |

C | 950 MHz. |

D | 25 MHz. |

Question 12 Explanation:

VLF (very low frequency) = 10 to 30 kHz. ; LF (low frequency or long wave) = 30 to 300 kHz. ; MF (medium frequency or medium wave) = 300 to 3000 kHz. ; HF (high frequency or short wave) = 3 to 30 MHz. ; VHF (very high frequency) = 30 to 300 MHz. ; UHF (ultra high frequency) = 300 to 3000 MHz. ; SHF (super high frequency) = 3 to 30 GHz. ; EHF ( extra high frequency or micro wave) = 30 to 300 GHz.

Question 13 |

Which of the following could be attached to a moving coil meter in an attempt to measure power ?

A | a thermistor |

B | a resistor |

C | a thermocouple |

D | a thermostat |

Question 13 Explanation:

1. If you convert a moving coil meter to a voltmeter and measure the voltage across the resistor and calculate the power using P = V² / R. 2. If you convert a moving coil meter to an ammeter, measure the current through the resistor and calculate the power with using P = I² R 3. If you don't know the resistance, measure the voltage across the resistor and current through the resistor and calculate the power using, P = V I. Thermister is a special kind of resistor, it's resistance varies with the temperature. Thermocouple generates an electric current , when it is heating up. Thermostat is a switch operating according to the temperature.

Question 14 |

The Reactance of a 15 H smoothing choke at a frequency of 50 Hz is approximately

A | 1500 Ω |

B | 3000 Ω |

C | 4700 Ω |

D | 750 Ω |

Question 14 Explanation:

Apply X = 2 π f L = 2 x 3.14 x 50 x15 = 4700 Ω (approximately:- 2x3x50x15 = 4500)

Question 15 |

The input power of a transmitter stage running at 24V, 2.5A is

A | 60 W |

B | 600 W |

C | 300 W |

D | 150 W |

Question 15 Explanation:

Apply P = V I formula. Therefore, P = 24 x 2.5 = 60W

Question 16 |

3GHz is equivalent to

A | 300 MHz |

B | 3000 MHz |

C | 30 MHz |

D | 30000 MHz |

Question 16 Explanation:

1 GHz = 1000 MHz

Question 17 |

For a silicon transistor the base emitter voltage for biasing must be above

A | 1.0 V |

B | 0.3 V |

C | 0.7 V |

D | 0.8V |

Question 17 Explanation:

For silicon transistors, base-emitter bias voltage must be above 0.7 V. For germanium transistors, it is above 0.3 V. Same principle apply for diodes also.

Question 18 |

The current at the center of a given λ/2 antenna is found to be 0.5 A. If this antenna has a radiation resistance of 70 Ω find the radiated power.

A | none of these |

B | 50 W |

C | 70 W |

D | 17.5 W |

Question 18 Explanation:

Apply P = I² R formula, I = 0.5 A, R = 70Ω Therefore P = 0.5 x 0.5 x 70 = 17.5 W

Question 19 |

The typical accuracy of a dip oscillator might be

A | 0.001% |

B | 10% |

C | 0.05% |

D | 1% |

Question 19 Explanation:

From a dip meter, frequency is measured with an analog dial. Error is not more than ±10% . It operates by extracting a very small amount of energy from the oscillating circuit to be measured.

Question 20 |

Volt is a measuring unit of

A | Resistance |

B | Current |

C | Power |

D | Electrical potential |

Question 20 Explanation:

The unit of Power is Watt. The unit of potential or voltage is Volt. The unit of current is Ampere. The unit of resistance is Ohm.

Question 21 |

What is the power consumed by a transmitter taking 0.5A at 24V ?

A | 12W |

B | 2.4W |

C | 4.8W |

D | 18W |

Question 21 Explanation:

Apply P = V I, V = 24 Volt, I = 0.5 A, therefore P = 24 x 0.5 = 12 W

Question 22 |

The output signal of a balanced modulator is

A | FM |

B | AM |

C | DSB |

D | SSB |

Question 22 Explanation:

SSB transmitter produces a DSB (Double Side Band) signal from the balanced modulator and passes through the side band filter for USB or LSB signal and the non selected side band gets filtered out.

Question 23 |

The typical accuracy of a moving coil meter is

A | 10% |

B | 3% |

C | 0.3% |

D | 0.03% |

Question 23 Explanation:

The question says accuracy, but it would have been better to say the error of the moving coil meter is about 3%. That means accuracy is 97%.

Question 24 |

The conductivity of a current carrying conductor can be increased by

A | increasing its length |

B | none of the above |

C | adding a resistor |

D | increasing its diameter |

Question 24 Explanation:

Consider R = ρ l / A formula, R = resistance, ρ = resistivity of the material, l = length of
conductor, A = cross section of the conductor. The conductivity is increased by the decrease of the resistance, R. For decrease R; ρ and l should decrease or A should increase.

Question 25 |

The tolerance of a resistor is given as 10%. If the nominal value is 4700Ω, then its value must b be lie Between

A | 4653 and 4747 Ω |

B | 4230 and 5170 Ω |

C | 4656 and 5170 Ω |

D | 4230 and 4747 Ω |

Question 25 Explanation:

If the tolerance is 10% , The value is 4700±10%. 10% of 4700 is 470. Therefore 4700±10% is equals to 4700±470. Therefore 4700+ 470 = 5170 and 4700 - 470 = 4230.

Question 26 |

For ideal amplitude modulation the modulating index must be

A | greater than one |

B | unity |

C | zero |

D | smaller than one |

Question 26 Explanation:

If modulating index = 1, it is 100% modulated. If it slightly increased, it is over modulated. Then signal becomes distorted. Therefore always modulating index < 1

Question 27 |

There are two basic forms of transistors, these are

A | NPP and PNN |

B | PNP and NNP |

C | PPN and NNP |

D | PNP and NPN |

Question 28 |

Resistors of 100Ω and 150 Ω are connected in parallel the effective value is

A | 150 Ω |

B | 100 Ω |

C | 250 Ω |

D | 60 Ω |

Question 28 Explanation:

For two parallel resistors, Equivalent resistance is R = R₁ R₂ / (R₁ + R₂), therefore R = 100 x 150 / ( 100+150) = 100 x 150 / 250 = 60 Ω

Question 29 |

The symbol in the figure is that of a

A | Switching diode |

B | varactor diode |

C | Zener Diode |

D | Light Emitting Diode (LED) |

Question 30 |

A varactor diode acts like

A | a variable inductor |

B | a variable capacitor |

C | a variable regulator |

D | a variable resistor |

Question 30 Explanation:

Variable capacitor is a tuning condenser or a varactor diode; Variable resistor is a potentiometer or variac or reostat; There is no meaning on variable regulator because regulator is a constant supply.

Question 31 |

To check that a crystal is working on its correct overtone the simplest piece of equipment is

A | a dip oscillator |

B | an ammeter |

C | a Voltmeter |

D | an absorption wave meter |

Question 31 Explanation:

Voltmeter and Ammeter can't do anything on this. An absorption wave meter can use only if there is enough power output. But dip meter (dip oscillator or GDO) can measure the frequency in the oscillator circuit. It operates by extracting a very small amount of energy from the oscillating circuit to be measured.

Question 32 |

A good dummy load is constructed from

A | wire-wound resistors |

B | light bulb |

C | a column |

D | non-reactive resistors |

Question 32 Explanation:

A good dummy load must have a pure resistance, without having a reactance. Only then will it have a 1:1 SWR. Wire wound resistors or electric bulbs have some reactance. But in practically, testing an amateur TX having about 50 Watts we can use a filament type bulb (100 W 230 V) as a dummy load.

Question 33 |

A light bulb is rated at 12V , 3W. The current drawn when used on a 12V source is

A | 250 mA |

B | 750 mA |

C | 36 A |

D | 4 A |

Question 33 Explanation:

Use the formula of P = V I, P = 3W, V = 12V, therefore I = P/V = 3/12 = 0.25 A = 250 mA

Question 34 |

The effective resistance between A and B in the circuit shown is

A | 60Ω |

B | 115Ω |

C | 225 Ω |

D | 55Ω |

Question 34 Explanation:

Two resistors of 110 Ohm are in parallel. It is equivalent to 55 Ohms. This 55 Ohm is connected in series with 5 Ohms. This is equivalent to 60 Ohms.

Question 35 |

Transformers operates on

A | a dc. supply only |

B | both on ac. and dc |

C | all the above are correct |

D | an ac. supply only |

Question 35 Explanation:

If the current through primary winding varies, a voltage is induced on the secondary. This action continues, only while an A.C. supply connected to the primary.

Question 36 |

The effective capacitance between A and B is

A | 55 pF |

B | 24 pF |

C | 100 pF |

D | 45 pF |

Question 36 Explanation:

parallel connection of 33pF and 27pF is equivalent to 60 pF ( 33+27 =60), same way other two equivalent to 40 pF. Then these 60pF and 40 pF connected as series. Their equivalent value is 60x40 / (60+40) =24pF.

Question 37 |

Which of the following represents 100% amplitude modulation ?

A | (b) |

B | (c) |

C | (d) |

D | (a) |

Question 38 |

What is the power consumed by a transmitter taking 1.5 A at 12V ?

A | 18W |

B | 12W |

C | 1.5W |

D | 8W |

Question 38 Explanation:

apply the formula of, P = V I, V = 12V, I = 1.5 A, therefore, P = 12 x 1.5 = 18 Watt

Question 39 |

A transformer has a coil 8Ω resistance and a reactance of 6Ω The impedance is

A | 12Ω |

B | 10Ω |

C | 6Ω |

D | 8Ω |

Question 39 Explanation:

Use the formula, Z² = X² + R², Therefore Z² = 6² + 8² = 36 + 64 = 100, therefore Z = √100 = 10 Ohm.

Question 40 |

Which of the instrument below has highest accuracy ?

A | An oscilloscope |

B | A digital frequency counter |

C | An absorption wave meter |

D | A heterodyne wave meter |

Question 40 Explanation:

A good frequency counter is accurate as ± 1 Hz. When using an Oscilloscope we have to measure the wave length according to scale on the screen an calculate the frequency. The error will be about 10%. An absorption wave meter has an analog dial, it operates by extracting a small amount of energy from the oscillating circuit to be measured. Because of that the error will be more ( about 10%) .

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