EXAMINATION FOR THE POSTMASTER GENERAL’S CERTIFICATE
OF COMPETENCE IN RADIOTELEGRAPHY

FIRST CLASS

 RADIOCOMMUNICATION

 March, 1968

    Three hours are allowed for this paper.
   Two of the questions marked with an asterisk must be attempted.
   All three of these questions may be answered if desired.
   Six questions should be answered in all.

*1       With the aid of a simple circuit diagram, explain the action of
    oscillator and frequency multiplier stages of a marine h.f. transmitter
    in producing a radiated frequency of 16712 kc/s (16712 kHz). 

     The permitted frequency tolerance at this frequency is ±200 parts in
 10⁶ (0.0002%). Calculated the tolerance in cycles per second (hz).
 

*2       With the aid of vector and/or waveform diagrams, explain how the
    following arise in a half-wave dipole transmitting aerial when it is
    energised:
    (a)   travelling wave,
    (b)   standing wave,
    (c)   node, and
    (d)   antinode,
 

*3       Explain the meaning of the following terms as applied to a
    communications receiver:
    (a)   selectivity,
    (b)   sensitivity, and
    (c)   signal-to-noise ratio, 

   With the aid of a sketch, describe how the passband of a communications
 receiver may be varied between “wide” and “very narrow”. Explain the action
 on “very narrow”.  Would this position be suitable for the reception of
 radiotelephony?   Give reasons for your answer.

 4       Draw a simple circuit diagram to show how the static characteristics
    of a transistor in the common-emitter mode could be determined.  Show
    the necessary meters and voltage supplies in your diagram. 

              Explain the meaning of TWO of the following terms as applied to a
         transistor connected in the common-emitter mode:
         (a)   input characteristic,
         (b)   output characteristic,
         (c)   transfer (mutual) characteristic. 

5        What are the basic, essential, functions of a selector unit in a
    marine radiotelegraph auto-alarm apparatus?

        With the aid of a simplified circuit diagram, explain the action of
    one type of selector unit in accepting a correct and complete signal.

                                                                             (PTO)

6          With the aid of sketches, explain why a loop aerial has directional
    characteristics.

           On what does the magnitude of the resultant e.m.f. induced into
         a loop aerial depend?

           Explain how the outputs of an open aerial and a loop can be
         combined to give a heart-shaped polar diagram of reception.  What
         advantage does this particular shape have over the directional
         characteristics of a loop alone? 

7          With the aid of a simplified diagram, explain the action of a
    power unit, including that of a minimum speed indicator and voltage
    regulator, suitable for use as part of a Man-Powered Portable Equipment
    for Survival Craft. A detailed explanation of the generator action is
    required. 

8          Explain the factors which render pentode valves particularly
    suitable for use for r.f. voltage amplification at medium and high
      frequencies.

      Why are triodes often used in preference to pentodes in the first
  stages of v.h.f. receivers?  Describe circuits which are used to
  avoid the need for neutralization of the triode’s inter-electrode
 capacitance. 

           What precautions should be taken in the wiring and layout of
            Components in v.h.f. equipment?