# Electronics and Communication Engineering - Communication Systems

21.

As the frequency increases, the absorption of ground wave by earth's surface

 A. decreases B. increases C. remains the same D. either (a) or (c)

Explanation:

Because of this reason ground waves can be used for frequencies upto about 1600 kHz only.

22.

The velocity of sound waves in air

 A. is constant at all temperatures B. varies directly as temperature C. varies inversely as absolute temperature D. varies directly as square root of absolute temperature

Explanation:

vT.

23.

The range of a cordless telephone is about

 A. 1000 m B. 500 m C. 100 m D. 10 m

Explanation:

The range depends on quality of instrument and is about 100 m.

24.

An earth mat for a communication tower consists of

 A. one or two radials extending from base of tower at a depth of about 5 cm below ground B. a large number of radials extending from base of tower at a depth of about 3 m C. a large number of radials extending from base of tower at a depth of about 30 cm D. none of the above

Explanation:

The number of radials is large and depth is about 30 cm.

25.

Degaussing in a picture tube means

 A. Adding magnetism in mask and mounting frame B. Removing residual magnetism in mask, shield etc... C. Increasing resistance of coils to decrease time constant D. Decreasing resistance of coils to increase time constant.