PMG 1968






 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
          (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.




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

           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

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

              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

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