A TV tower has a height $$72 \mathrm{~m}$$. The maximum distance upto which TV transmission can be received is (Take, radius of the earth as $$\left.6.4 \times 10^6 \mathrm{~m}\right)$$

Maximum amplitude of message signal in amplitude modulation is 20 cm. If the amplitude of carrier wave is 40 cm, then modulation index of modulated wave is

In detector, output circuit consists of $$R=10~k\Omega$$ and $$C=100~pF$$. The frequency of carrier signal which it can detect is

An optical fibre communication system works on a wavelength of 1.3 $$\mu$$m. The number of subscribers it can feed, if a channel requires 20 kHz are

Ground waves have wavelength