Questions on communication systems are generally simple and interesting and so you can easily score marks by attempting them. The following three multiple choice questions were included in Kerala Engineering Entrance 2005 question paper and will be able to answer them in less than three minutes:

(1) If a radio receiver is tuned to 855 kHz radio wave, the frequency of local oscillator in kHz is

(a) 1510

(b) 455

(c) 1310

(d) 1500

(e) 855

Since the tuned frequency is 855 kHz, the receiver is an AM (amplitude modulation) receiver. Modern AM receivers are super heterodyne receivers employing a higher frequency local oscillator. On mixing the local oscillator output with the incoming amplitude modulated carrier, an amplitude modulated wave at intermediate frequency (IF) of 455 kHz (by convention) is produced. Since the intermediate frequency is 455 kHz, it follows that the frequency of local oscillator is 855 kHz + 455 kHz = 1310 kHz.

(2) If *n*_{1} and *n*_{2} are the refractive indices of the core and the cladding respectively of an optical fibre,

(a) *n*_{1 }= *n*_{2}

(b) *n*_{1 }< *n*_{2}

(c) *n*_{2 }< *n*_{1}

(d) *n*_{2 }= 2*n*_{1}

(e)* n*_{2 }= √(2*n*_{1})

Since the optical fibre confines the light signal within the fibre by total internal reflection, the refractive index of the cladding should be less than that of the core. Therefore, *n*_{2 }< *n*_{1} [Option (c)]

(3) A TV tower has a height of 100 m. What is the maximum distance up to which TV transmission can be received? (Radius of the earth, *R * = 8×10^{6 }m)

(a) 34.77 km

(b) 32.7 km

(c) 40 km

(d) 40.7 km

(e) 42.75 km

We have maximum distance, *d* ≈ √(2*Rh*) where* h* is the height of the antenna.

Substituting given values, *d* ≈ √(2×8×10^{6 }×100) = 40×10^{3} m = 40 km.

[The mean radius of the earth is nearly 6400 km. The value is given as 8000 km in the problem to make your calculation simple.

You should remember that the height of the transmitting antenna (or receiving antenna) is the height with respect to the *ground level*. If an antenna is mounted on a mast of height *h*_{1} and the mast is erected on a hill or building of height *h*_{2}, the height of the antenna will be *h = h*_{1} +* h*_{2}]