12th Class Physics Sample Paper Physics Sample Paper-7

  • question_answer
    A ground receiver is receiving a signal at
    (i) 5 MHz and                
    (ii) 100 MHz
    transmitted from a ground transmitter at a height of 300 m located at a distance 100 km. Identify whether it is coming via space wave or sky wave propagation or satellite transponder. Radius of the earth \[\simeq 6.4\times {{10}^{6}}m,\] \[{{N}_{\max }}\] of the ionosphere \[={{10}^{12}}\,\,{{m}^{-3}}.\]


    Maximum distance covered by space wave communication \[=\sqrt{2{{R}_{e}}h}\] \[=\sqrt{2\times 6.4\times {{10}^{6}}\times 300}=62\,km\] Since, receiver transmitter distance is 100 km, this is ruled out both for (i) and (ii) further \[{{f}_{c}}\] for ionospheric propagation is, \[{{f}_{c}}=9{{({{N}_{\max }})}^{1/2}}=9\times {{({{10}^{12}})}^{1/2}}=9MHz\] So, the signal of 5 MHz \[(<{{f}_{c}})\] comes via ionospheric mode while signal of 100 MHz comes via satellite mode.

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