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FIRST YEAR EXAMINATION in BASIC RADIO PRINCIPLES


A.A.E.   (I)   (48)
CRANWELL (1)

            4

ROYAL AIR FORCE — AIRCRAFT APPRENTICES

No. 1 RADIO SCHOOL, CRANWELL


INTERMEDIATE EXAMINATION IN EDUCATIONAL SUBJECTS
JANUARY 1947 ENTRY

JANUARY, 1948


BASIC RADIO PRINCIPLES

Time allowed — Three hours

Attempt SIX questions only

1.

(a)

What do you understand by the following:— (i) power, (ii) the specific resistance of a material, (iii) a 240-volt 60-watt lamp?

 

(b)

Make a diagram of the following circuit :—
Four 12-volt 36-watt lamps, joined in parallel, are connected in series with a variable resistor and a fixed resistor of 0.34 ohm to a battery of E.M.F. 24 volts and total internal resistance of 1/6 ohm.
Two pilot lamps each of resistance 8.5 ohms are connected in series and supplied by the potential difference across the 0.34 ohm resistance.

 

(c)

For the circuit at (b), if the variable resistor is adjusted so that the 36-watt lamps operate at their rated current, find:—

(i)

the value of the rated current and the resistance of each 36-watt lamp,

(ii)

the potential difference across the 0.34 ohm resistance,

(iii)

the power of one of the pilot lamps,

(iv)

the value to which the variable resistor is adjusted,

(v)

the terminal potential difference of the battery,

(vi)

the energy dissipated by one of the 36-watt lamps in 5 minutes.


2.

(a)

With the aid of a clear, well-labelled diagram describe the construction and explain the action of an attraction-type moving-iron ammeter.

 

(b)

The coil of a moving coil ammeter has a resistance of 0.498 ohm and gives full scale deflection with a current of .005 ampere. How could this movement be used to measure currents up to 25 amperes ? Calculate the value of any resistor used.

 

(c)

The above instrument is to be used as a voltmeter reading from 0—50 volts. State how this could be arranged and calculate the value of any resistor needed.


3.

(a)

Explain briefly the following terms :—

 

(i)

Self induction.

 

(ii)

Mutual inductance.

 

 

Fig.1

 

(b)

In the circuit of Fig. 1 the switch S is first moved to position 1 for two seconds and then to position 2 for two seconds. With the aid of carefully drawn diagrams of current and voltage, describe and explain what happens in the above case. Assume that the time of change over from position 1 to 2 is negligible.

 

(c)

What was Faraday's discovery in connection with electro-magnetic induction ? Show how it applies in the case of a moving-coil microphone.


4.

(a)

Sketch the theoretical diagram of a shunt-wound direct current motor, and explain the basic principles of its action.

 

(b)

Show in detail how the direction of rotation of the armature may be determined, and also explain what would happen if the supply voltage were reversed.

 

(c)

Explain what would be the effect of increasing the value of the resistance in series with the field winding.

 

(d)

What do you understand by the term armature reaction in the case of an electric motor ?


5.

(a)

State clearly what you understand by the terms :—

 

(i)

Phase difference.

 

(ii)

Reactance.

 

(iii)

Impedance.

 

(b)

An alternating voltage of constant value but variable frequency is applied in turn to each of the elements in Fig. 2.

 

 

Fig.2

 

 

Explain briefly with the aid of graphs how the resistance or reactance of each of these varies with change of frequency.

 

(c)

 

 

 

Fif.3

 

 

 

(i)

Calculate the R.M.S. value of the voltage across the inductance in the circuit shown in Fig. 3.

 

(ii)

Supposing that a condenser of capacity 1/3600 F. were added in series with the circuit shown, calculate the new value of the voltage across the inductance.

     6.    Assuming that the circuits shown in Fig. 4 are both tuned to the same frequency :—
 

(a)

State whether the circuits are behaving as series or parallel tuned circuits. Show by vectors the relation between current and voltage in each circuit when the frequency of the applied voltage is below the resonant frequency.

 

(b)

If L1 = 125H, C1 = 500F and L2 = 500H, find the value to which C2 must be adjusted.

 

(c)

If the effective resistances of the circuits R1 and R2 are 10 ohms and 62.5 ohms respectively, calculate the "Q" of each circuit at resonance and compare their selectivity.

 

(d)

Define "coupling factor, K," and show by a set of curves the effect on the overall response of the circuit of increasing progressively the value of K.

 

 

Fig.4

7.

(a)

Describe carefully the construction and action of a hard diode valve, indicating what is meant by (i) thermionic emission, (ii) space charge, (iii) saturation current, (iv) A.C. resistance.

 

(b)

Compare the characteristic curves of .(i) a hard diode, (ii) a mercury vapour diode, (iii) a metal rectifier. Mention one advantage of each of the above.

  8.      The following table of results refers to a triode valve.
           Anode current in mA with Vg at:—

  Vg -0 -1 -2 -3 -4 -5 -6
Va                
175   14.1 12.0 10.0   8.0   5.9   3.8   2.0
150   12.0 10.0   8.0   6.0   4.0   2.0   0.6
125     9.9   7.9   5.9   3.9   2.0   0.5  
100     7.8   5.8   3.8   1.8   0.4    
75     5.6   4.0   1.9   0.4      
50     3.2   2.0   0.7        
 

(i)

Plot the Ia — Va curves of the valve.

 

(ii)

Calculate the values of Ra, and g m of the valve (showing how they are obtained from the curves) for values of Va = 100 volts and Va = 0 volts.

 

(iii)

Calculate the voltage amplification factor of the circuit when the anode load is 25 kilohms and the supply voltage is 175 volts.

 

(iv)

Plot the relevant load line, and then with Vg = -3 volts and a signal voltage of 2 volts peak, find from the graph the resulting swings of anode current and voltage.


Note that in question 6(a) the term VECTORS is used. This is the mathematical description used at the time, but nowadays the term PHASORS would be used.
When explaining the action of a device like a microwave Phase Shifter or Phase Follower it becomes necessary to distinguish between the relationship of electric field components in SPACE (VECTORS) and in TIME (PHASORS).




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