(i) Why is communication using line of sight mode limited to frequencies above 40 MHz?
(ii) A transmitting antenna at the top of a tower has a height 32 m and the height of the receiving antenna is 50m . What is maximum distance between them for satisfactory communication in line of sight mode ?

Explanation

(i) At frequencies above 40 MHz, communication is essentially limited to line-of-sight paths. At these frequencies, the antennas are relatively smaller and can be placed at hights of many wavelengths above the ground. Because of line of- sight nature of propagation, direct waves get blocked at some point by the curvature of the earth.
(ii) Here ` h_T-32 m, h_R=50 and R=6400 km = 64xx 10^5 m`.
Maximum distance between two antennas is
` dm= sqrt(2Rh_t)+sqrt(2Rh_R)`
`=sqrt(2xx64 xx 10^5xx 32)+sqrt(2xx 64xx 10^5xx 50)`
` = sqrt(64xx 64 xx 10^5)+sqrt(100 xx 64 xx 10^5)`
`=64xx 1062xx sqrt(10) +10xx 8xx 10^2 xx sqrt(10)`
` (64+80)xx 10^2sqrt(10) -144 xx 10^2 xx sqrt(10) m=45.5 km`.