2-47
A28. Natural.
A29. Man-made.
A30. (a) filtering and shielding of the transmitter (b) limiting bandwidth (c) cutting the antenna to the
correct frequency
A31. (a) physical separation of the antenna (b) limiting bandwidth of the antenna (c) use of directional
antennas
A32. Regular and irregular variations.
A33. Regular variations can be predicted but irregular variations are unpredictable.
A34. Daily, seasonal, 11-year, and 27-days variation.
A35. Sporadic E, sudden disturbances, and ionospheric storms.
A36. Muf is maximum usable frequency. Luf is lowest usable frequency. Fot is commonly known as
optimum working frequency.
A37. Muf is highest around noon. Ultraviolet light waves from the sun are most intense.
A38. When luf is too low it is absorbed and is too weak for reception.
A39. Signal-to-noise ratio is low and the probability of multipath propagation is greater.
A40. Frequent signal fading and dropouts.
A41. Fot is the most practical operating frequency that can be relied on to avoid problems of multipath,
absorbtion, and noise.
A42. They can cause attenuation by scattering.
A43. It can cause attenuation by absorbtion.
A44. It is a condition where layers of warm air are formed above layers of cool air.
A45. It can cause vhf and uhf transmission to be propagated far beyond normal line-of-sight distances.
A46. Troposphere.
A47. Vhf and above.
A48. Near the mid-point between the transmitting and receiving antennas, just above the radio horizon.
