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.

Explanation

(a) Because ionosphere cannot reflect e.m. waves having frequencies greater tahn 40 MHz.
(b) The line of sight (L.O.S.) is the distance at which transmitting antena and receiving antena can see each other.
Maximum L.O.S . Distance is
`d_M=d_T+d+R=sqrt(2Rh_t)+sqrt(2Rh+_R)`
Due to curvature of earth, this distance is limited.
(c) Because when we walk, we leave one cell and enters other cell and remain always under mobile network and can talk uninterrupted.