## 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 |

As the frequency rises the reactance of a capacitor

A | stays constant |

B | decreases |

C | increases |

D | none of these |

Question 1 Explanation:

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

Question 2 |

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

A | a thermistor |

B | a thermocouple |

C | a thermostat |

D | a resistor |

Question 2 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 3 |

The typical accuracy of a dip oscillator might be

A | 10% |

B | 0.001% |

C | 1% |

D | 0.05% |

Question 3 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 4 |

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

A | an ammeter |

B | an absorption wave meter |

C | a Voltmeter |

D | a dip oscillator |

Question 4 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 5 |

Ground wave communication is most effective in the frequency range of

A | 30 MHz to 300 MHz |

B | above 300 MHz |

C | 3 MHz to 30 MHz |

D | 300 kHz to 3 MHz |

Question 5 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 6 |

The unit of resistance is

A | Watt |

B | Ampere |

C | Ohm |

D | Farad |

Question 6 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 7 |

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

A | 750 Ω |

B | 4700 Ω |

C | 1500 Ω |

D | 3000 Ω |

Question 7 Explanation:

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

Question 8 |

The typical accuracy of a moving coil meter is

A | 10% |

B | 3% |

C | 0.3% |

D | 0.03% |

Question 8 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 9 |

The circuit is shown in the figure for

A | voltage multiplication |

B | reverse bias protection |

C | full wave rectification |

D | half wave rectification |

Question 9 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 10 |

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

A | 4656 and 5170 Ω |

B | 4230 and 5170 Ω |

C | 4230 and 4747 Ω |

D | 4653 and 4747 Ω |

Question 10 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 11 |

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 | 14 μF |

B | 8 μF |

C | 8/7 μF |

D | 4 μF |

Question 11 Explanation:

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

Question 12 |

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

A | 1.0 V |

B | 0.8V |

C | 0.7 V |

D | 0.3 V |

Question 12 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 13 |

A varactor diode acts like

A | a variable capacitor |

B | a variable inductor |

C | a variable regulator |

D | a variable resistor |

Question 13 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 14 |

The most widely used conducting materials are

A | copper and gold |

B | copper and aluminum |

C | gold and silver |

D | copper and silver |

Question 14 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 15 |

The effective capacitance between A and B is

A | 24 pF |

B | 100 pF |

C | 55 pF |

D | 45 pF |

Question 15 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 16 |

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

A | Zero resistor |

B | A infinite resistor |

C | Current source |

D | A finite resistor |

Question 16 Explanation:

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

Question 17 |

There are two basic forms of transistors, these are

A | PPN and NNP |

B | PNP and NPN |

C | NPP and PNN |

D | PNP and NNP |

Question 18 |

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

A | the interaction of two permanent magnetic fields |

B | an electric only |

C | interaction of a permanent and electromagnetic field |

D | interaction of an electric and magnetic field |

Question 18 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 19 |

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

A | 0.5 A |

B | 1 A |

C | 0.05 A |

D | 0.005 A |

Question 19 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 20 |

3GHz is equivalent to

A | 300 MHz |

B | 30000 MHz |

C | 30 MHz |

D | 3000 MHz |

Question 20 Explanation:

1 GHz = 1000 MHz

Question 21 |

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

A | 250 Ω |

B | 60 Ω |

C | 100 Ω |

D | 150 Ω |

Question 21 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 22 |

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

A | 6Ω |

B | 8Ω |

C | 10Ω |

D | 12Ω |

Question 22 Explanation:

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

Question 23 |

A good dummy load is constructed from

A | wire-wound resistors |

B | non-reactive resistors |

C | a column |

D | light bulb |

Question 23 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 24 |

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 | 50 W |

B | 70 W |

C | 17.5 W |

D | none of these |

Question 24 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 25 |

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

A | 115Ω |

B | 225 Ω |

C | 55Ω |

D | 60Ω |

Question 25 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 26 |

Which of the instrument below has highest accuracy ?

A | An absorption wave meter |

B | An oscilloscope |

C | A digital frequency counter |

D | A heterodyne wave meter |

Question 26 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%) .

Question 27 |

The symbol in the figure is that of a

A | Switching diode |

B | varactor diode |

C | Light Emitting Diode (LED) |

D | Zener Diode |

Question 28 |

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

A | 12W |

B | 1.5W |

C | 8W |

D | 18W |

Question 28 Explanation:

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

Question 29 |

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

A | a low pass filter |

B | a band pass filter |

C | a notch filter |

D | a high pass filter |

Question 29 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 30 |

The output signal of a balanced modulator is

A | SSB |

B | DSB |

C | AM |

D | FM |

Question 30 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 31 |

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

A | 150 W |

B | 600 W |

C | 300 W |

D | 60 W |

Question 31 Explanation:

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

Question 32 |

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

A | 950 MHz. |

B | 250 MHz. |

C | 25 MHz. |

D | 5MHz. |

Question 32 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 33 |

The conductivity of a current carrying conductor can be increased by

A | none of the above |

B | adding a resistor |

C | increasing its diameter |

D | increasing its length |

Question 33 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 34 |

Transformers operates on

A | an ac. supply only |

B | a dc. supply only |

C | all the above are correct |

D | both on ac. and dc |

Question 34 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 35 |

For ideal amplitude modulation the modulating index must be

A | unity |

B | zero |

C | greater than one |

D | smaller than one |

Question 35 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 36 |

Which of the following represents 100% amplitude modulation ?

A | (d) |

B | (c) |

C | (a) |

D | (b) |

Question 37 |

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

A | 1:0 SWR |

B | 0:1 SWR |

C | 1:1 SWR |

D | 10 W reflected power |

Question 37 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 38 |

Volt is a measuring unit of

A | Current |

B | Resistance |

C | Power |

D | Electrical potential |

Question 38 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 39 |

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

A | 18W |

B | 12W |

C | 2.4W |

D | 4.8W |

Question 39 Explanation:

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

Question 40 |

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

A | 4 A |

B | 750 mA |

C | 36 A |

D | 250 mA |

Question 40 Explanation:

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

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