Umakant Kondapure, Collin Fernandes, Nipun Bhatia, Vikram Bathula and, Ketki Deshpande Solutions for Chapter: Communication Systems, Exercise 3: Competitive Thinking

The maximum frequency of transmitted radio waves above which the radio waves are no longer reflected back by ionosphere is ($N=\text{maximum electron density of ionosphere}$),

For sky wave propagation of a $10 \mathrm{MHz}$ signal, what should be the minimum electron density in ionosphere?

A sky wave with a frequency $55 \mathrm{MHz}$ is incident on $D$-region of earth's atmosphere at $45°$. The angle of refraction is (electron density for $D$-region is $400 \mathrm{electrons} {\mathrm{cm}}^{-3}$ ),

A transmitting antenna of height $h$ and the receiving antenna of height $45 \mathrm{m}$ are separated by a distance of $40 \mathrm{km}$ of satisfactory communication in line-of-sight mode. Then, the value of $h$ is (given, radius of earth is $6400 \mathrm{km}$),

In optical communication system operating at $1200 \mathrm{nm}$, only $2 \%$ of the source frequency is available for TV transmission having a bandwidth of $5 \mathrm{MHz}$. The number of TV channels which can be transmitted is,

Assertion: Electromagnetic waves with frequencies more than the critical frequency of ionosphere cannot be used for communication using sky wave propagation.

Reason: The refractive index of the ionosphere becomes very high for frequencies higher than the critical frequency.

A message signal of frequency $\omega_{\mathrm{m}}$ is superposed on carrier wave of frequency $\omega_{\mathrm{c}}$ to get an amplitude modulated wave (AM). Then, one of the angular frequencies of the AM wave is,

Assertion: A portable AM radio set must be kept horizontal to receive the signals properly.

Reason: Radio waves are polarised electromagnetic waves.