A ground receiver receives a signal at 5 MHz , transmitted by a...

Maximum distance covered by space wave communication $=\sqrt{2 R h}=\sqrt{2\left(6400 \times 10^3 \mathrm{~m}\right)(320 \mathrm{~m})}=64 \mathrm{~km}$ Since the distance between transmitter and receiver is $110 \mathrm{~km}$, therefore the given frequency signal cannot communicate through either by ground waves or space waves. For sky wave, the value of critical frequency is $$\begin{aligned} v_c & =9\left(N_{\max }\right)^{1 / 2}=9\left(10^{12}\right)^{1 / 2} \\ & =9 \times 10^6 \mathrm{~Hz}=9 \mathrm{MHz}\end{aligned}$$ where, $N_{\max }=$ Maximum electron density of ionosphere. As the signal frequency $5 \mathrm{MHz} < 9 \mathrm{MHz}$, so the communication is through sky waves.