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Give (brief) reasons for the following :...

Give (brief) reasons for the following :
(a) We use 'sky wave' mode of propagation of electromagnetic waves, only for frequencies upto 30 to 40 MHz.
(b) The LOS communication, via space waves, has a (fairly) limited range.
(c) A mobile phone user gets uninterrupted link to talk while walking.

Text Solution

Verified by Experts

(a) Because ionosphere can not reflect
electromagnet waves having frequencies
greater than 40 MHz.
(b) The LOS is that distance at which the
transmitting antenna and receiving antenna can
see each other. Maximum LOS distance is
`d = sqrt(2Rh_(T)) + sqrt(2Rh_(R))`
where R is the radius of earth, `h_(T) and h_(R)` are the
height of transmitting antenna and receiving
antenna respectively.
The distance d is limited, due to curvature of
earth.
(c) While walking when we talk using mobile
phone, we leave on cell and enters other cell
and remain always under mobile network.
Therefore, we can talk uninterrupted.
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Which mode of propagation is used by short wave broadcast services having frequency range from a few MHz upto 30 MHz ? Explain diagrammatically how long distance communication can be achieved by this mode . Why is there an upper limit to frequency of waves used in this mode ?

Perhaps, the most commonly used mode of communication involves use of free space as a communication channel between transmitter and receiver. The various possible modes of using of free space have been described below Ground wave communication : This mode is used for low frequency as at high frequency, losses become large. This is useful for distance of the order of 1 km. Space wave communication: The signals are transmitted directly from transmitter to receiver. There is no limit to upper frequency, but its range is limited to few hundred kilometers. The range depends on the height of transmitting antenna Sky wave communication : The signals are sent towards sky They are reflected back towards earth by ionosphere. Its range can be above 500 km. But, very high frequencies (above 10 MHz) are not reflected back. Satellite communication: The signals that cannot be reflected back by ionosphere are reflected back by satellite. Moon is a passive satellite of earth. The cordless phone installed in a house uses